±´¶û407ÐýÒí¿ÕÆø¶¯Á¦
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 1 -
INTRODUCTION TO ROTARY-WING AERODYNAMICS
You¡¯re about to embark on an entirely new area of flight: the helicopter.
Although flying a helicopter may not be inherently more difficult than flying a fixedwing airplane (after all, look at all the high-school grads who learned to fly them very
well during the Vietnam era), it can be very different. In fact, it can be so different that,
given the choice, many helicopter instructors (including me) find it easier to teach
someone to fly a helicopter from scratch than to cross-train a fixed-wing airplane pilot.
That being said, however, there are quite a few parallels¡ªparticularly if you look
at the helicopter in the most literal interpretation of its generic name: a rotary-wing
aircraft. Like any other heavier-than-air machine, a helicopter flies because of its wings.
Taken individually, a rotor blade is nothing more than a skinny wing flying at a very high
airspeed; and, taken individually, it obeys exactly the same aerodynamic rules and
principles as any other wing. Like any other wing, it can glide without power if
necessary; like any other wing, it can be stalled.
It¡¯s when we examine this wing in the context of a helicopter, on which it¡¯s
spinning around, that things become a bit more complex; and they become more complex
yet when we start looking at the helicopter as a whole, and the way its various dynamic
components interact both with the atmosphere and with each other.
A LITTLE HISTORY:
The dream of a heavier-than-air machine capable of hovering motionless in the air
has been with us at least since the days of Leonardo da Vinci (if not, indeed, since the
first time some caveman watched a hummingbird). With the appearance of the gasoline
engine in the late 19
th
century, a power source was finally available that was both light
and strong enough to allow for some serious experimentation. By the time of the First
World War, a few experimental test rigs (with rigid rotorlike horizontal propellers) had
actually managed to heave themselves a couple of feet off the ground for a few moments
at a time. Some could even hover, after a fashion¡ªbut, invariably, as soon as they
attempted to move horizontally at any kind of speed, they became uncontrollable.
It wasn¡¯t until after World War I that a Spaniard, Juan de la Cierva, realized that
the blades of any sort of lifting rotor or propeller had to be able to deal with very widely
varying airspeeds as the whole assemblage moved through the air. His innovation of a
hinge, allowing individual blades to flap up and down while rotating, allowed the
construction of aircraft called ¡°autogyros,¡± which flew with a combination of unpowered
rotors and airplane propellers. While capable of flying quite slowly, they still could not
hover, or take off or land vertically.
The step to actual helicopters came with the realization that it would be necessary
to control each blade individually. Both the US and Germany had workable helicopters
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 2 -
by World War II, although their use in combat was negligible, and development of
helicopters has continued unchecked since then.
A SPLIT PERSONALITY:
In a sense, a helicopter behaves like two entirely different aircraft, depending on
whether it¡¯s hovering or flying at appreciable speed (which is called, in helicopterese, in
translation.) When it¡¯s hovering or flying very slowly, you could equate its rotor system
to a group of very small airplanes, flying in very tight circles. Once it gets some speed,
however, it¡¯s more like an airplane suspended beneath a single circular wing (although
one with some odd characteristics). Dealing with the change between these modes is one
of the challenges of helicopter design and piloting. Let¡¯s look at the hovering situation
first.
GOING NOWHERE:
The helicopter¡¯s rotor system is, indeed, similar to a very large propeller, and it
runs at a constant speed. Like a constant-speed propeller on an airplane, the amount of
thrust it delivers to the surrounding air is controlled by varying the pitch of its blades, all
at the same time. This is called collective pitch control, and it¡¯s controlled by a cockpit
lever called, appropriately enough, the collective. It¡¯s mounted along the left side of each
pilot seat, somewhat like a handbrake lever; pulling it upward increases blade pitch.
Of course, if this rotor were just spinning in a rigid horizontal plane atop the
aircraft, and doing nothing more than supporting its weight, the helicopter would be
completely at the mercy of any movement of the air around it¡ªit would drift about
uncontrollably, like a balloon (as, in fact, you¡¯ll probably find it doing on your first
attempts at hovering)! What¡¯s necessary is some way to tilt the rotor disc slightly, so that
a small component of its lift can be directed in any desired direction, whether to keep the
helicopter in one place (hovering), or to accelerate it in a desired direction (forward,
backward, or sideward flight).
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 3 -
This is achieved by changing the pitch of each rotor blade, one at a time, as it
sweeps through the applicable part of the circle. (For example, to fly forward, we¡¯d want
to tilt the rotor disc in that direction, so we¡¯d want to decrease the pitch of each blade as it
sweeps through the forward part of the circle, and increase it as it sweeps through the rear
part.) Since this pitch change occurs once per circle, or cycle, for each blade, it¡¯s called
cyclic pitch control, and the cockpit control that affects it is called the cyclic (pronounced
either ¡°sigh-click¡± or ¡°sick-lick¡±¡ªtake your choice!) and mounted vertically in front of
the pilot. (Actually, since the whole rotor system acts like a big gyroscope, the desired
pitch change is applied 90 degrees before the point at which it finally occurs.)
The way this is done is by means of a device called a swashplate. We don¡¯t need
to go into too much detail here; suffice it to say that it¡¯s a device on the rotor mast to
which the pitch control links of each individual blade are attached. Moving the cyclic in
the cockpit tilts the swashplate in the required direction, changing the pitch of each blade
in turn, while moving the collective moves the entire swashplate up and down, changing
the pitch of all the blades at once. Do you start to understand why a rotor hub looks so
mechanically complex?
LET¡¯S (NOT) TWIST AGAIN...
Of course, keeping the rotor turning means that the powerplant is putting out a lot
of work¡ªand since the helicopter isn¡¯t attached to anything, Newton¡¯s Third Law tells us
that the engine is working just as hard to turn the rest of the helicopter the other way!
Why doesn¡¯t the whole thing just spin round and round? (Early ones did, until they
figured this part out!) Because of the efforts of the antitorque rotor¸ located at the tail of
the helicopter and often called the ¡°tail rotor.¡± This closely resembles an airplane
propeller, and works exactly the same way, producing thrust to offset the torque that's
constantly trying to spin the helicopter around. Like the main rotor, it runs at constant
speed (in fact, it¡¯s geared to it mechanically), so thrust is controlled by varying its pitch.
The cockpit control for this is the pair of antitorque pedals.
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 4 -
ONE GOOD TURN DESERVES ANOTHER...
Just to keep things interesting, the amount of power (and hence engine torque)
required by the main rotor system varies constantly. If you want to climb, for instance,
you¡¯ll pull in more collective. This applies more power to the rotor system and, in turn,
increases the torque that¡¯s trying to turn the helicopter the other way. Thus, in an
American helicopter, an increase in power (collective setting) also requires an increase
in left pedal deflection, and vice versa. (Many European and Russian helicopters turn
their main rotor the other way, so they¡¯d require right pedal with a power increase.
There¡¯s no particular advantage to one way or the other¡ªit¡¯s just one of those weird
things, like driving on the right in England.)
MOVING OUT:
Things become even more complex when we start to fly the helicopter in any
particular direction at a speed faster than, say, a brisk jog. Let¡¯s say we¡¯re flying forward
(the most common direction, after all) at 100 knots.
Now, even in a hover, the helicopter¡¯s little ¡°wings¡± are clipping along at quite an
airspeed. In the Bell 407, for example, at normal RPM the airspeed at the tips of the
blades is almost 450 knots! When we¡¯re moving forward, though, the forward-going
blades on the right side of the helicopter ¡°see¡± a higher airspeed (550 knots at the tip),
while the aft-going blades on the left side ¡°see¡± a correspondingly lower airspeed (350
knots at the tip).
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 5 -
Obviously, the side with the forward-going blades is going to produce a lot more
lift than the one with aft-going blades¡ªand this is why the first attempts at helicopters
would invariably fall over (to the aft-going side) as soon as they moved off from a hover.
It wasn¡¯t until Cierva¡¯s flapping hinge that rotor systems could accommodate this
asymmetric lift.
But wait...there¡¯s more...
Once a helicopter is moving at decent speed, its motion through the air produces
additional lift over most of the rotor disc as a whole. This is called translational lift, and
it¡¯s why it takes much less power to support a helicopter in forward flight than in a hover.
Where you¡¯ll feel this is on both takeoffs and landings. As you lift into a hover, then
gradually begin to gain speed, you¡¯ll feel the helicopter suddenly ¡°come to life¡± and gain
performance as it moves into translational lift. Similarly, as you begin slowing up toward
a hover during a landing approach, you¡¯ll find yourself needing to add quite a bit of
power as the helicopter begins to settle out beneath you. (If this proves to be more power
than you have available at some particular combination of altitude and temperature,
you¡¯re in trouble.) Don¡¯t forget, too, that every power change requires its corresponding
change on the antitorque pedals.
DON¡¯T FLY TOO FAST:
The faster you fly forward, the greater this dissymmetry of lift becomes, and the
more cyclic pressure you¡¯ll need (to the right on American helicopters) to counteract it.
Before you¡¯d run out of control, however, something even more significant starts to occur
at excessive speed. Remember, our forward speed is added to the rotor speed on the
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 6 -
forward-going side, subtracted from it on the aft-going side. At some speed, we¡¯ll reach
the point at which so much speed is being subtracted from the aft-going blades that they
begin to stall.
This is called retreating blade stall, and it¡¯s what sets the absolute upper speed
limit for just about any helicopter (other factors may set lower limits). If you ever are so
foolhardy as to seek it out, the first signs will be very heavy vibration, followed by a
tendency to roll toward the stalling side (left in American helicopters).
Since the air is thinner at high altitudes, a higher blade angle of attack (more
collective) is required to produce the same amount of rotor thrust. This reduces stall
margins, which is why most helicopters (including the 407) have restricted high speed
ranges depending on altitude.
WHAT IF IT QUITS?
This is the question that¡¯s almost invariably asked by the uninitiated¡ªand their
expectation is that if the engine fails, the helicopter will drop like a stone.
Luckily for us, it won¡¯t. Remember, the total aerodynamic force produced by any
wing, whether fixed or rotary, is composed of both a lift and a drag component¡ªand
remember that if the relative wind is coming from below the chord line, the lift
component is directed at least slightly forward. That¡¯s how a fixed-wing airplane
glides¡ªand if you think of rotor blades as little airplanes flying in a tight circle, there¡¯s
no reason they can¡¯t glide in a circle, either!
What¡¯s critical, in an airplane as well as in a helicopter, is to reduce the angle of
attack to prevent the wing (or blade) from stalling. In an airplane, that would mean
lowering the nose; in a helicopter, it means to immediately reduce the collective all the
way to the bottom of its range. Sure, the helicopter will descend¡ªbut it won¡¯t ¡°fall out
of the sky,¡± but descend relatively gently. During this descent, the air flowing upward
through the center part of the rotor, where blade speeds are relatively low, drives the
blades and keeps them turning (while still contributing its share of lift). Out near the tips,
where blade speeds are higher, even more lift is produced, although the blades
themselves are also producing drag. This situation is called autorotation, partly because
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 7 -
the blades are ¡°turning themselves¡± and partly because, as old instructors like to say, they
¡°ought¡¯a rotate if you do everything right.¡± The inner part of the blade is now considered
the driving portion, while the outer part is considered the driven portion.
We¡¯ll cover autorotations in much more detail once we¡¯ve gained a little
experience with the helicopter. For the moment, let¡¯s get into the cockpit and start
looking around.
COCKPIT TOUR:
Right off the bat, you¡¯ll notice something different. All the airplanes you¡¯ve
flown have been commanded from the left seat. The Bell 407, like most helicopters, is
flown from the right.
This is, at least partly, a hangover from the bad old days when helicopters were
even more unstable than they are now. You¡¯ll notice that I didn¡¯t say ¡°less stable;¡± even
the nicest current light helicopters are, at best, neutrally stable. Where an airplane, if
disturbed from trimmed flight, will attempt to return to the trimmed condition, a
helicopter will just go off in the new direction after the disturbance. Most helicopters are,
in fact, unstable¡ªthey don¡¯t even need a disturbance to want to go off in a new direction,
but are constantly trying to do so on their own.
What this means is that they have to be flown every second. For all practical
purposes, you can never let go of the cyclic stick, which you hold in your right hand. The
collective, on (and in!) the other hand, is a bit less critical¡ªwhen you¡¯re not actively
maneuvering, you can let go of it for a few seconds at a time. Therefore, it makes sense
to put the pilot all the way over on the right, so that he or she can get to necessary
controls with the left hand. Not that you want to stay away from the collective all that
long, either¡ªin the event of an engine failure, you need to lower the collective right now
to avoid excessive loss of rotor RPM.
CONTROLS:
As mentioned before, your primary flight control is the cyclic, which isn¡¯t
depicted in Fly! II, the antitorque pedals, and the collective. The latter two are visible in
the view below the main instrument panel, which you can see by tapping [Ctrl+down
arrow].
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 8 -
WHAT ABOUT POWER?
If you have your right hand on the cyclic, your left hand on the collective, and
your feet on the antitorque pedals, how are you going to control the throttle?
Well, in a modern turbine helicopter like the 407, the answer is simple: you don¡¯t!
This wasn¡¯t the case in older piston-powered helicopters such as the 407¡¯s
ancestor, the Bell 47 (the ¡°mosquito carrying a flashlight bulb¡± you see in the opening
credits for M*A*S*H, for example). In those machines, the throttle was controlled by a
motorcycle-style twist grip on the collective, which is why such helicopters (as well as
the iron men and women who flew them) were called ¡°throttle twisters.¡± Helicopters of
this era had mechanical linkages between the collective itself and the throttle, called
¡°correlators,¡± whose purpose was to minimize the amount of throttle-twisting required;
depending on the model of helicopter, the flight conditions (altitude, temperature, gross
weight, etc.) these worked after a fashion¡ªbut there was still plenty of throttle twisting
required.
With the advent of gas turbine power, the helicopter pilot¡¯s workload was
suddenly reduced: these engines have automatic governors or regulators, and when
working properly throttle management becomes almost a ¡°set it and forget it¡± evolution.
In extreme conditions, however, or when maneuvering hard, it¡¯s still possible to ¡°get
ahead¡± of these governors, leading to engine overtemperatures or RPM ¡°sags.¡±
The latest technology, which is installed on the 407, is FADEC¡ªFull Authority
Digital Engine Control. When this is operating correctly, there¡¯s no direct mechanical
connection between the collective and its twist grip throttle and the engine. Instead, an
electronic computer (the Electronic Control Unit, or ECU) measures a whole raft of
parameters including engine and rotor RPM, temperature, altitude, collective position,
etc. and provides not only virtually instantaneous power response with no overshoots, but
protection against overtemperature or overspeed. It also makes engine startup a fully
automatic sequence.
In normal flight, the twist-grip throttle is simply positioned to its ¡°FLY¡± detent
and left there. In case of FADEC failure, the system reverts to manual mode and the
engine can be controlled by the twist throttle.
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 9 -
Let¡¯s take a quick look at the overhead panel, which controls some important
functions, and then move on to the main instrument panel. Tap on to see
the overhead panel.
That big red handle at the aft end is the rotor brake, which is used only during
engine shutdown, and then only after the rotor has slowed down below 40% RPM. Make
sure it¡¯s released before engine start¡ªand when you shut the helicopter down, make sure
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 10 -
the blades are parked at a 45-degree angle to the fuselage. This is to avoid heat damage
to the fiberglass blade if it were parked right above the hot exhaust duct.
Circuit breakers protect all the helicopter¡¯s electrical services. Notice the two
small toggle switches at the front of the circuit breaker panel. These control a couple of
fuel pumps, and the left one is hot-wired to the helicopter battery. This means that if you
don¡¯t make a point of shutting it off during your shutdown procedure, you¡¯ll come back
to a dead batter next time.
NOTE¡ªin the actual helicopter, the overhead panel is mounted in the ceiling, and all
switches are moved forward to turn them on. When viewed on your computer screen, this
means that switch handles move DOWN to turn on, UP to turn off. This applies only to
the overhead panel; switches on the main panel or control pedestal move in the normal
¡°UP=ON¡± direction.
Ahead of the circuit breaker area is the main switch panel, and the most important
switches are in the forward (bottom) row. From left to right, they control the hydraulic
control boost (¡°power steering,¡± if you like), the anticollision light, the inertial particle
separator if installed (this keeps foreign matter, such as snow, dust, or loose grass out of
the engine intake, but costs a bit of power), the generator, and the battery master. The
battery switch is turned on before startup, and turned off last after shutdown; turn it off
prematurely, and you¡¯ll lose such things as all the engine instruments while the engine is
still winding down.
Now hit and we¡¯ll look around the main instrument panel.
MAIN INSTRUMENT PANEL
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 11 -
Across the top of the panel are three rows of caution and warning lights. Most of
the yellow ones are of the ¡°land as soon as practical¡± variety, although ¡°fuel low¡± and
any of the engine or transmission chip lights (meaning that metal chips have been
detected in the oil) qualify for the more immediate ¡°land as soon as possible.¡± So do the
red lights; obviously, if you get the ¡°engine out¡± light, you don¡¯t have much choice. The
one you really don¡¯t want to see is the red RPM light, meaning that rotor RPM is
dangerously low, and you¡¯re approaching blade stall, and this light is reinforced by a loud
horn. Your reaction should be to get the collective down now¸ and establish a steady
autorotation; then, and only then, sort out the problem if there¡¯s sufficient time and
altitude.
The large push-button to the right of the caution and warning lights tests them all
simultaneously.
The main instrument panel itself has three rows of large flight instruments at the
right, and two rows of smaller engine instruments at the left. The first two instruments
down from the top in the leftmost row of large instruments are the two most important
ones for flight.
FLIGHT INSTRUMENTS
Right at the top, in the center of the panel, is the airspeed indicator. You¡¯ll notice
that it reads quite a bit lower than those you¡¯d find in an airplane. It also has not one, but
two redlines. The highest, at 140 KIAS, is the absolute limit for the helicopter, but it
only applies at low altitudes. At higher altitudes and/or temperatures, lower maximum
speed limits apply; they¡¯re placarded in the cockpit.
The other (striped) redline, at 100 knots, is applicable under a number of
conditions, including flight with any (or all) cabin doors removed, flight at high gross
weights, and autorotations. If the placard maximum speed for a given altitude is lower
than 100 KIAS, the lower speed applies.
Not shown is another limitation. Due to the possibility of interference between
the antitorque rotor and the tail boom if full left pedal is applied at high forward speeds,
an automatic device restricts pedal travel at higher airspeeds. If this system is disabled,
maximum speed is restricted to 60 KIAS.
Just below the airspeed indicator is an even more important instrument: the dual
tachometer. Remember, what¡¯s most important in a helicopter is the airspeed over the
blades; obviously, that¡¯s directly related to their RPM. On the dual tachometer, the inner
needle, labeled Np (for Powerplant) shows the RPM of the engine¡¯s output shaft; the
outer needle, labeled Nr (for Rotor) shows the RPM of the main rotor. Both are
expressed in percent of maximum; at 100%, the rotor is turning at 413 RPM (and the
engine output shaft at just over 6000).
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 12 -
In normal operation, the two needles will be ¡°married¡± at 100% RPM. In fact,
you¡¯ll notice that the allowable range for Np is extremely narrow¡ªbasically, 100%
RPM, period.
The allowable range for the rotor is somewhat wide (from 85% to 107% RPM),
but you¡¯ll see that only under very abnormal circumstances, and only when the
transmission and rotor system are decoupled from the engine for some reason (for
example, if the engine has failed). In this case, you¡¯ll see split needles on the tach.
To the right of these instruments are standard flight instruments (artificial horizon, HSI,
altimeter, and VSI). For our purposes, we can consider these relatively unimportant:
since the 407 is not certificated for instrument flight, and since it¡¯s typically flown at low
altitudes, it¡¯s generally operated primarily by the time-honored practice of looking out the
windows. In fact, helicopters in general, and light ones like the 407 in particular, are one
of the last bastions of true ¡°seat of the pants¡± VFR flying. Below the flight instruments is
the indicator for the ADF. You can select navigation information from either the GPS or
the VOR receiver to be displayed on the HSI by using the push-button switch at the right
edge of the instrument panel.
ENGINE INSTRUMENTS
The engine instruments are arranged in two vertical rows just to the left of the
flight instruments, with the three most important ones right next to the airspeed indicator
and dual tachometer.
At the top is the torquemeter, which indicates directly how much power is being
supplied to the rotor. This is your primary power setting instrument, and is affected
directly by movement of the collective.
Next down is MGT, which stands for Measured Gas Temperature¡ªthe
temperature of the stream of hot gas in the engine. While the torquemeter is your power
setting instrument, MGT is your power limiting instrument: at higher temperatures and/or
altitudes, you may reach limiting engine temperature before you have obtained all the
torque you might like. In this case, you¡¯ll just have to be content with what power
(torque) you can obtain. You¡¯ll notice a couple of red markings beyond the redline. The
first, at 843 degrees, is a start limit; you should abort an engine start if MGT goes above
this mark for more than a couple of seconds. The second, at 905 degrees, is an absolute
limit beyond which any engine operation will be recorded immediately as an exceedance.
Below this, and the third of the ¡°really important engine instruments,¡± is Ng¡ªthe
RPM of the gas generator portion of the engine. Like rotor RPM, this is calibrated in per
cent (if for no other reason than that the actual number¡ªover 50,000 RPM at high
power¡ªwould make the gauge pretty crowded). In normal operation, with the FADEC
operating properly, this can be pretty well ignored. If FADEC fails, however, you should
match the setting on the twist-grip throttle to the indicated Ng; markings are engraved on
its bezel ring for the purpose.
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 13 -
All three of these instruments have LCD displays for highly exact readings. In
addition, all of them constantly monitor their values. If any value is exceeded, it will
begin to flash as a warning to the pilot to reduce power immediately; the ¡°check instr¡±
light at the top of the panel will also flash. After a few seconds of flashing (it varies
depending on the parameter), flashing will cease and the date, duration, and value of the
exceedance will be recorded. The digital display in the instrument will display the letter
¡°E¡± and the value of the exceedance for 11 seconds. The ¡°check instr¡± annunciator will
remain on until you push the ¡°instr chk¡± button just below it.
You can tell that there¡¯s been an exceedance (which, in the real world, would be
reported to maintenance) when you first apply electrical power before startup: the
affected gauge will once again display the E and exceedance value, and won¡¯t indicate
correctly until you acknowledge it by pressing the ¡°instr chk¡± button. In the real world,
this information remains stored in the helicopter, and displayed every time it¡¯s powered
up, until cleared by maintenance using a laptop computer.
The remaining engine instruments monitor parameters such as oil pressure and
temperature for both the engine and transmission, fuel quantity and pressure, and
generator output. The top left instrument is a digital clock, which also displays outside
air temperature and system voltage (selected by the red button at the top). Don¡¯t try a
battery start unless you see at least 24 volts, or a hot start is likely.
There are also a few important switches and buttons on the main panel. At the
bottom right, with a red guard to secure it in the ON position inflight, is the master fuel
valve. Normally, it¡¯s turned off after shutdown (which also cuts the left and right fuel
pumps), but in an emergency (such as complete loss of engine control), it provides a lastditch way to shut down the engine. To its left is the FADEC switch; successive pushes
change from FADEC to manual mode, annunciated on the switch. This is normally used
only to deal with FADEC malfunctions. At the extreme right edge are two more
switch/annunciators; one, mentioned above, switches the HSI display from the Nav 1
receiver to the GPS; the other overrides the high-airspeed left antitorque pedal block (for
example, to restore full pedal travel at low speeds if the block fails to disengage). At the
top, just below the red RPM warning light, is a button to mute the various warning horns.
Finally, tap once again on to take a closer look at the
collective. You¡¯ll notice the twist-grip throttle, with its calibrations for approximate Ng
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 14 -
when operating in manual (non-FADEC) mode. Next to these markings is a small, round
silver button. This is the idle stop release. Once the engine is running, the twist grip
can¡¯t be rolled off to the ¡°stop engine¡± position unless this button is pushed and held.
There are two switches on the head of the collective lever. The left one allows
you to turn the landing lights on and off without moving your hand away from the
collective. The right one, spring-loaded to its center OFF position, controls the starter.
For a normal FADEC start, it only has to be held momentarily to the START position;
the starter will then remain engaged, shutting off automatically at the conclusion of the
start sequence. To manually disengage the starter, hold the switch briefly in the
DISENGAGE position.
LET¡¯S FLY!
Control issues:
It¡¯s very difficult to fly the Bell 407 without some form of tail rotor control (either
a ¡°twist¡± yaw axis on your joystick, or (much better) separate pedals. If you don¡¯t have
either of these, the default keys for tail rotor control are and for left and
right, respectively. If your keyboard is laid out with these keys above and below each
other, rather than left and right, you may want to reconfigure them using the Options--
>Keys and buttons-->Helicopter keys menus.
In its default configuration, Fly! II assigns the collective pitch lever to the throttle
on your joystick (or a separate throttle if you¡¯re using one). In an actual helicopter, you
add power by pulling up on the collective. Therefore, for maximum realism, you may
want to use the Options-->Setup Axes... menu to ¡°swap¡± the throttle¡¯s direction of
motion, so that full forward is idle and fully back is maximum power.
STARTUP AND RUNUP:
Bell¡¯s startup checklist ¡°flow¡± goes from a brief visit to the collective, up to the
electrical switches in the ceiling, then down to the panel. Tap to see
the collective. Check that you can move the twist-grip throttle all the way from OFF
through FLY and back to OFF. (Keyboard shortcuts: for OFF,
for FLY. While you have the collective in view, check that the landing lights switch is
OFF, so that full battery power will be available to the starter.
Now tap twice to move to the overhead panel. All switches
should be OFF except HYD (hydraulic control boost) and the anti-collision light. Check
that the rotor brake is released (handle up against the ceiling) and all circuit breakers are
in. Now turn the BATT switch ON.
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 15 -
Tap to return to the main instrument panel, where all sorts of
things should be happening. You should hear the low rotor RPM warning horn (cancel it
if you like by pushing the horn mute button below the RPM warning light, which will be
on). Within 3 seconds, a number of engine and FADEC caution lights will illuminate,
and you¡¯ll hear the (different) engine out warning tone for a moment. A few seconds
later, you¡¯ll hear it again, and the ENG OUT caution light will come on. Mute the horn
once again.
If you haven¡¯t wasted any time getting down to this panel, you may also see the
engine instruments sweep to the tops of their displays, then back down, while
illuminating all segments in their LCD readouts, as part of their power-up self test.
During this test, the Nr indicator (rotor needle in the dual tach) will go to 107%, and the
Np (powerplant needle) will go to 100%. If you want to run this test later (with the
engine running), just hold the LCD TEST button.
Now it¡¯s time to start the engine, which is much easier with FADEC than in past
helicopters. In those, you had to hold a starter button, then manually twist the throttle
open until the engine fires up, then carefully modulate it to keep the RPM accelerating
while avoiding overtemperatures. In fact, that¡¯s still the way you¡¯d start the 407 if you
had a FADEC autostart failure. (If the FADEC fails altogether before takeoff, rather than
just in its start mode, it¡¯s a no-fly item...so ¡°fuhgeddaboutit¡±.
With FADEC, it¡¯s a lot easier. Make sure the collective is all the way down, and
move the cyclic until the CYCLIC CENTERING caution light extinguishes. Go briefly
to the overhead panel and turn on the two boost pump switches at the lower left of the
circuit breaker panel; return to the main panel, check that the red-guarded fuel valve
switch at the lower right is on, then verify that fuel pressure is showing (leftmost row of
instruments, second one down, left display). As long as you¡¯re over on that side, verify
that the FADEC is in AUTO mode, and press-to-test the rudder stop¡ªit should go to the
ENGAGED mode as long as you¡¯re holding the button.
Here we go, and the order is just the opposite from a manual start. First, rotate the
throttle twist grip to IDLE. Then, momentarily move the starter switch on the collective
head to the START position. (You have to do this within 60 seconds of moving the
throttle to idle, or the system ¡°times out.¡± If it does, just roll the throttle to OFF, then
back to IDLE, to reset the timer.)
From here on, the start cycle is automatic. Monitor the engine instruments as the
engine spools up. If it goes over the first triangular red mark beyond the redline, or if the
main rotor hasn¡¯t started to turn by the time you get to 25% Ng RPM, abort the start by
rolling the throttle back to OFF.
In a normal start, however, things will spool up very nicely without further
attention. At 50% Ng, the starter will disengage, and the START light will go out; at
60%, the ignition is no longer needed, and the AUTO RELIGHT light will go out. The
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 16 -
engine should settle down at 63% Ng. Now you can go back to the overhead panel and
turn on the flight instrument switches and the avionics master switch.
Since we don¡¯t have control feel in Fly! II, we can¡¯t really check what the
helicopter feels like with the hydraulic control boost switched off, but we can still do the
preliminary check and, later, verify that it still responds to the controls. Turn the HYD
switch off and verify that the HYDRAULIC SYSTEM caution light illuminates; then turn
it back on and verify that the light extinguishes. More important is the FADEC manual
mode check: push the FADEC switch on the panel, verify that the MANUAL mode light
illuminates, then make a slight adjustment to the throttle twist grip (use the mouse) and
verify that the engine follows you. Return the throttle to idle and re-engage the automatic
FADEC mode.
Almost ready to fly! Roll the throttle up to its FLY detent and wait for both Np
and Nr to stabilize at 100%. Turn the HYD switch off and make some gentle control
inputs on all three controls to verify that there¡¯s no uncommanded movement, and that
you have control. (In Fly! II, you can see the edge of the rotor disk move in the forward
view as you do this.) Turn the HYD switch back on.
LIFTOFF TO A HOVER
The first thing we¡¯re going to do is simply lift off to a normal hover (with the
skids 4 to 6 feet off the ground.) Look straight ahead out the windshield (not at the
ground right in front of you) and slowly start raising the collective. You¡¯ll see the torque
begin to increase.
Before the helicopter even begins to lift off, it¡¯ll get light on its skids, and it may
begin to turn to the right. Gently feed in left pedal until it stops trying to turn; then, as
you gradually continue to add collective, add left pedal at the same rate. When you¡¯ve
reached sufficient power, the helicopter will lift off. Immediately ¡°freeze¡± the collective;
the helicopter will probably rise to just about the right hover height pretty much by itself.
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 17 -
HOVER EFFECTS:
Why does it do this? Because when it¡¯s hovering near the ground (called in
ground effect), the helicopter in effect ¡°stuffs¡± a cushion of air between itself and the
surface; this effect is perceptible up to about half the rotor span above the ground. It
would take much more power to hover out of ground effect, which is why helicopter
performance charts include both ¡°hover in ground effect¡± (HIGE) and ¡°hover out of
ground effect¡± (HOGE) listings. You¡¯ll notice that for a given temperature and gross
weight, the helicopter can HIGE at much higher altitudes than it can HOGE (or, for the
same altitude, it can HIGE on a much warmer day than it can HOGE). On this particular
liftoff, by time you¡¯ve pulled in enough collective to lift off, you probably have enough
to hover a couple of feet off the ground, so the helicopter can seek its own balance
between available power and the effect of the ground cushion.
If you¡¯ve set the various helicopter effects to ¡°realistic¡± in the Options-->Realism->Helicopter... menu, you¡¯ll probably also notice that you¡¯re drifting to the right, and that
it takes a little left cyclic pressure to stay over one spot.
This is an effect of the antitorque rotor. Remember, as the rotor turns to the left
(from your cockpit viewpoint), the whole helicopter is trying to turn to the right; that¡¯s
why you added left pedal as you pulled collective. The tail rotor is pushing the tail to the
right to counteract this¡ªbut in doing that, it tends to push the whole helicopter to the
right as well. Watch an American helicopter in the hover, and you¡¯ll notice that its roll
attitude is very slightly left-skid-down. In the 407 this effect is quite small, since the
rotor mast is installed with a slight built-in left tilt, but it¡¯s still perceptible.
Try to be as gentle as possible on the controls¡ªa helicopter (particularly one with
hydraulic control boost like the 407) is flown with pressures, not movements. In the real
helicopter, you should hardly see the cyclic move at all. In fact, if you watch a good,
smooth helicopter pilot at work, it doesn¡¯t look as he or she is flying at all, but rather very
slowly and gently playing some sort of exotic musical instrument.
For the moment, just try to stay at least near, if over, one particular spot. A stable
hover is probably the most difficult thing to learn at first; it can be taught to some extent,
but then it has to be practiced. The trick is to make, and then remove, very small, smooth
corrections; about half a second before you realize that you need them! (A good exercise
is the old trick of trying to balance a broom vertically above your hand.) It is also
helpful, as mentioned above, not to look at the ground right in front of the helicopter, but
rather out toward the horizon.
It can be frustrating, but also very gratifying. It¡¯s very common for the final leap
to competence to come very suddenly¡ªyou¡¯ll spend hours thrashing around, with sweat
pouring down your face (and remember, you can¡¯t let go of the controls to wipe your
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 18 -
eyes!)¡ªthen, from one moment to the next, ¡°the nickel will drop¡± and the helicopter will
suddenly become stable.
PEDAL TURNS:
Once you have the helicopter reined into a stable hover, try a couple of pedal
turns: apply a small amount of pedal in the desired direction to slowly turn the helicopter.
You may notice that a left pedal turn makes the helicopter sink just a bit, while a right
one may make it rise. This is because a left turn requires more blade pitch from the tail
rotor, and hence more power, which it takes away from the main rotor; a right one
requires less. With practice, you¡¯ll make the almost infinitesimal collective correction
without even thinking about it.
In fact, not thinking is the key to successful hovering (and, to a lesser extent,
helicopter flying altogether). Not that I¡¯m suggesting that you ¡°dumb down.¡± It¡¯s just
that will all the controls affecting not only the helicopter, but each other, and all the
sensory cues coming in at once, no one can think fast enough to handle it all. You need
to practice until you make and remove the myriad control corrections on a subconscious
level¡ªsort of like rubbing your belly and patting your head, writ large. It may be that
the reason it¡¯s easier to hover while looking at the horizon is that you get your visual cues
from peripheral vision, which is processed subconsciously, rather than staring directly at
a target and thinking about it.
HOVER TAXI:
Now let¡¯s try moving slowly around the immediate area¡ªand by that, I mean not
much more than walking speed. Why do I emphasize slowness?
Because we still want to make this a hovering maneuver, which means we don¡¯t
want to ¡°fall off¡± the invisible ground cushion. Move a bit too fast, and the downwash of
air through the rotor can¡¯t replenish the cushion fast enough. Move faster yet, and we¡¯ll
start encountering translational lift before we¡¯re quite ready.
To start moving, apply very gentle pressure to the cyclic in the desired direction;
as soon as the helicopter starts to move, take out at least half of what you just put in. To
slow down, use gentle cyclic pressure opposite the direction of movement.
Moving forward is easy. Moving sideways is a bit less so; because it has
¡°tailfeathers,¡± the helicopter will want to point its nose in the direction it¡¯s moving, so
you¡¯ll need just a bit of opposite pedal. Moving backward is hardest, because the
tailfeathers want to spin the helicopter around; you¡¯ll need to pay close attention, using
either pedal as necessary. Real-world helicopter students spend hours hovering along
taxiway lines and airport markings for practice.
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 19 -
SETTING DOWN:
Let¡¯s end this segment by setting the helicopter back on the ground. Get back to a
stationary hover (or as close to one as you can manage), pick an object out ahead to look
at, and begin slowly lowering the collective. Remember, as you reduce power, you¡¯ll
have to apply right pedal to keep the helicopter pointed in the same direction. If you¡¯re
moving over the ground at all, make sure it¡¯s forward: the helicopter can actually ¡°run
on¡± to its skids quite nicely, but is likely to fall over if there¡¯s any sideways component.
This is a great way to demonstrate the ground cushion, too. If you start from, say,
a 6-foot hover, then make only a tiny collective reduction, the helicopter will just settle a
couple of feet, then stop descending¡ªyou¡¯ve gotten down to where the ground cushion is
thicker. It¡¯ll actually take a very slow, but continuous, downward motion of the
collective to land the helicopter¡ªthe feeling is almost that you¡¯re pushing it onto the
ground. Once the skids touch, smoothly lower the collective all the way to the bottom.
THE HOVERING AUTOROTATION:
It may seem paradoxical to introduce an autorotation this early in your learning
process, but there are some good reasons. One, of course, is that the helicopter doesn¡¯t
know or care how much or little experience you have; it may choose to shut down, for its
own reasons, anytime. Another is that a hovering autorotation is pretty easy, at least
compared to some others you'll encounter.
A final one goes back to my own training, these many years ago. In those days,
my instructor had found a loophole in the FAA regs stating that a student pilot could fly a
helicopter alone, even before any official endorsement for solo, as long as the aircraft was
¡°tethered to the ground.¡± The local FAA office¡¯s interpretation of ¡°tether¡± was any
physical connection, no matter how flimsy, between the helicopter and the ground, so
once I¡¯d mastered (or, more accurately, managed) a halfway decent hover and could
perform ¡°safe, if not elegant¡± hovering autorotations, my little piston-powered helicopter
was equipped with an old Dodge brake drum, tied to one skid by a 10-foot length of
parachute cord. ¡°As long as you don¡¯t pick up that brake drum, you¡¯re legal,¡± I was told,
¡°now get out there and practice.¡± I spent hours practicing hover maneuvers and chugging
along the taxiways of our big old WWII training field in the midwest; by the time my
instructors decided it was time to continue my training, that brake drum was worn down
to a thin, gleaming crescent. It¡¯s still a great way to practice.
Let¡¯s try a couple. This is a good time to enlist the help of a friend, since you¡¯ll
¡°run out of hands¡± otherwise. Lift off into a normal hover, and when everything is stable,
have your friend cut the engine, either by turning off the fuel valve or by hitting [Ctrl+pg
dn].
Two things will happen: the helicopter will settle, and it¡¯ll yaw¡ªfairly hard¡ªto
the left. Why the yaw? Because the tail rotor is still trying to compensate for all the
torque that¡¯s suddenly not there anymore. Smoothly apply right pedal to stop the rotation
and simultaneously smoothly raise the collective to cushion the touchdown.
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 20 -
A WORD ABOUT ROTOR ENERGY:
Obviously, as soon as the engine quit, the main rotor started to slow down
(although you were probably too busy to watch the tach). How fast it slows down
depends on a number of factors, but a significant one is how much energy is stored in the
rotor system as inertia.
A high-inertia rotor system is one that¡¯s quite heavy, often with long blades¡ªin
fact, in some helicopters, additional weight in the form of chunks of depleted uranium is
added to the blade tips. High-inertia rotors tend to maintain their speed longer after an
engine failure. The 407¡¯s predecessor, the LongRanger, had a fairly high-inertia rotor; in
a hovering autorotation from a skid height of a couple of feet, if you just chopped the
throttle, kept the helicopter straight with the pedals, and did nothing at all with the
collective, it would sit down pretty firmly...but not hard enough to break anything. The
JetRanger¡¯s big brother, the Huey, had an even higher-inertia rotor system. I¡¯ve seen
experienced military pilots do a complete autorotation from a couple of thousand feet in
the air, set the helicopter down gently, then pick it up again and do a pedal turn¸ before
those big blades finally ran out of oomph.
The downsides of high-inertia rotor systems is that they¡¯re less responsive in the
air, and they¡¯re heavy¡ªweight that might otherwise go to payload.
Low-inertia rotor systems, on the other hand, are light, responsive, and efficient.
They offer crisp handling and excellent control¡ªbut require a bit more proficiency on
the part of their pilots. Specifically, they¡¯ll ¡°lose turns¡± faster after an engine failure
(although, as we¡¯ll see shortly, they can also regain them faster). The 407¡¯s four-blade
composite rotor is classed as a low-inertia system.
UP, UP, AND AWAY...
With the beginnings of a handle on the hover, let¡¯s finally get the 407 up into the
air and going somewhere. Lift off into a normal hover as before, and turn the helicopter
in the direction in which you want to depart.
Now, apply gentle cyclic pressure to start moving in that direction. As you get to
about 15 knots, you¡¯ll feel the helicopter settle a bit¡ªyou¡¯ve ¡°flown off the ground
cushion.¡± Depending on your hover height when you started, you may need to add a little
collective; don¡¯t forget to coordinate with the pedals.
At around 20 to 25 knots, you¡¯ll feel something different. The helicopter will feel
like it wants to climb (it does!), and it¡¯ll also feel as if it wants to roll to the left (it does!).
You¡¯ve entered translational lift, the condition in which the rotor blades are
affected not only by their rotation, but by the forward speed of the helicopter as a whole.
As we discussed earlier, the forward-going (or advancing) blades are exposed to a higher
airspeed than the aft-going (or retreating) blades. The faster you fly, the more right
cyclic pressure it¡¯ll take.
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 21 -
In forward flight, you¡¯ll find the helicopter much more stable than in the hover; in
fact, it behaves more like an airplane. Use left and right cyclic pressure to roll in and out
of banked turns, just like an airplane; use forward and aft pressure to control your speed.
To climb or descend, adjust the collective (and, of course, the pedals). As in an airplane,
for a given power setting you can fly level at one particular airspeed, descend at a higher
one, or climb at a lower one, so once again the cyclic and collective have to be used
interactively.
GETTING BACK DOWN
Sooner or later we¡¯ll have to get the helicopter back onto the ground. A normal
approach is nothing more than a gradual descent, gradually transitioning into a hover.
First, however, we should get acquainted with something colorfully called the ¡°dead
man¡¯s curve.¡± No, it¡¯s not that nasty switchback where someone went through the
guardrail¡ªit¡¯s more accurately called the ¡°height-velocity diagram.¡±
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 22 -
Here¡¯s a typical H-V envelope; it reads in skid height above ground from bottom
to top, and airspeed from left to right.
We¡¯ll get into energy management in more detail when we discuss autorotations
further. For the moment, it¡¯s enough to know that as long as you operate the helicopter in
the unshaded area of the chart, you should be able to make a safe autorotation (with
average pilot technique) if the engine fails. If you¡¯re in the shaded area (for example, 100
feet off the ground at 20 knots), you can¡¯t make a safe autorotation no matter how good a
pilot you might be.
This isn¡¯t to say, of course, that helicopters are never operated in the ¡°avoid¡±
region; many types of operation (for example, takeoffs and landings from buildings or
elevated platforms, or ¡°external load¡± work like setting powerline pylons) require
extended stays ¡°on the wrong side of the curve.¡± It¡¯s simply a matter of acceptable
risk¡ªas long as the engine is running, you¡¯re fine, so it¡¯s just a matter of your faith in
Bell, Allison/Rolls Royce, and the deity of your choice!
What it means for normal approaches, however, is that you should plan them to
avoid the shaded area.
Take a closer look. You can see that on a takeoff, you should stay below 25 feet until
you reach a speed of 50 knots. Similarly, on an approach, it would be best not to
decelerate below 50 knots until you were down to 25 feet skid height or lower. In the real
world, that¡¯s probably pretty hard to manage unless your landing spot has very wide,
clear approaches (like an airport). Still, you can try to minimize your exposure to the
¡°avoid¡± area.
60 knots is a good speed with which to start a landing approach, since at that
speed you¡¯re well outside the curve. Line up your landing spot and, holding 60 knots at
first, adjust the collective to give you the desired rate of descent¡ªthe chosen spot should
neither move up or down in the windshield.
As you get closer, begin slowing by slight aft cyclic pressure. This will initially
take a further collective reduction to avoid rising above your desired glide path. As you
decelerate below about 30 knots, however, you¡¯ll feel the helicopter begin to settle¡ª
you¡¯re starting to lose the effects of translational lift. It¡¯ll also want to roll to the right,
requiring a slight cyclic correction.
Once you¡¯re below translational lift speeds, you¡¯ll have to start pulling collective
back in. If you¡¯re approaching your landing spot too fast, you may have to apply even
more aft cyclic pressure to reduce speed. Make sure you don¡¯t do this too close to the
ground, as you risk striking the tail rotor if the helicopter is pitched up too high.
Finally, as you settle toward your desired hover height, you¡¯ll need to add more
collective (and left rudder) to establish a hover. Don¡¯t expect everything to come out
right the first time; typically, a student will end up in a hover some distance away from
the desired spot, then hover over to it.
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 23 -
Once over the desired spot, lower the collective to set down as we did before.
STEEP DESCENTS
Now, as we gain experience, we¡¯ll add another maneuver: a steep descent. This is
what you¡¯d use if you had to land in a confined area with tall obstacles. Lift off, climb
out, and establish level flight at 500 feet above the ground and 60 knots.
On our last approach, we made a slight collective reduction to start a very gentle
descent. This time, make a considerably greater reduction (don¡¯t forget the pedals!) to
descend more steeply, still maintaining 60 knots. When you reach 200 feet, smoothly
pull the collective back up to maintain level flight at 60 knots, then add power and climb
back up to 500 feet. Try this several times, making the descent steeper each time.
SPLITTING THE NEEDLES:
There¡¯s no hard and fast dividing line between normal and steep descents, but as
you get into the steeper ones, you may notice something interesting: the rotor and engine
tach needles may ¡°split¡± on the dual tachometer (this is less likely to happen with
FADEC in automatic mode, you may want to try manual just for this demonstration).
What¡¯s happened here is that during the descent, you demand so little power that the rotor
system ¡°decouples¡± from the engine, activating a clutchlike device installed between the
engine and transmission. You can force this condition (starting at a safe altitude) by
using the mouse to roll the throttle below the FLY detent to split the needles,
simultaneously lowering the collective to maintain rotor RPM. Roll the throttle back up
to FLY before you raise the collective to arrest the descent.
Remember: in low-power (or no-power) situations, rotor speed is controlled by the
collective. Raising the collective reduces rotor speed; lowering it increases rotor speed.
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 24 -
QUICK STOPS
Now we¡¯ll try another maneuver, one that most students find a lot of fun: the
quick stop. Let¡¯s say we¡¯re zipping along a taxiway at low altitude and high speed when
a jet pulls out in front of us. We want to slow down fast, but we don¡¯t want to climb.
Establish flight at about 25 feet along an easily-followed reference¡ªa road,
taxiway, or runway. Now, to stop in as short a distance as possible, smoothly and
simultaneously lower the collective to the bottom and apply just enough back pressure on
the cyclic to maintain your altitude, neither climbing nor sinking. You may notice the
dual tach needles splitting during this maneuver; Np will remain at 100%, but Nr may
climb briefly toward 105% or so. As the helicopter declerates and starts to settle, use
forward cyclic pressure to bring it back to a level attitude, and collective as necessary to
hold altitude. Fun, isn¡¯t it? Practice it until you¡¯re proficient at stopping the helicopter in
minimum distance without losing altitude.
PUTTING IT ALL TOGETHER:
There¡¯s a reason we¡¯ve practiced these particular maneuvers in this particular
order: you¡¯ve been gaining the skills you need to perform a safe autorotation!
We¡¯ll start by doing a few practice autorotations to what¡¯s called a ¡°power
recovery¡±¡ªi.e., rather than landing, we¡¯ll terminate the maneuver in a hover. Begin at
500 feet, flying at 100 knots.
Now we¡¯re going to simulate an engine failure by smoothly but rapidly lowering
the collective all the way to the bottom (or reducing throttle to minimum on your
joystick). In addition to various horns and warning lights, which are the least of our
concerns for the moment, you¡¯ll notice a few things: the helicopter will yaw to the left (as
in a hovering autorotation), it¡¯ll tend to drop its nose, and it¡¯ll start a pretty rapid descent.
We¡¯re already at the maximum authorized speed for autorotation, and we want to
get slower¡ªthe speed for maximum glide distance over the ground is 80 KIAS, and that
speed for minimum descent rate is 55 KIAS. We¡¯ll use 80, so apply aft cyclic pressure to
decelerate to that speed. The helicopter will just about hold altitude while you¡¯re
decelerating (which won¡¯t take long), then resume it¡¯s descent. Take a look at the dual
tach; Nr will probably be higher than 100%. As long as it¡¯s below 107%, it¡¯s fine for the
moment. Now hit to pause the simulator while we discuss autorotations.
AUTOROTATIONS AND BANK ACCOUNTS:
I¡¯m indebted to the remarkable Frank Robinson, of Robinson Helicopters, for this
method of explanation. In addition to being a brilliant designer and a very savvy
businessman, Frank is probably the best helicopter instructor I¡¯ve ever encountered.
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 25 -
To keep a helicopter¡¯s rotor turning, and to keep it supporting the helicopter,
requires energy. In normal flight, that energy comes from the powerplant. Where does it
come from during an autorotation?
Actually, it can come from three different sources, and you can compare them to
bank accounts, with the energy taking the place of money. Think of the rotor¡¯s energy
needs as a mortgage¡ªyou gotta pay it, and keep paying it, or you¡¯re in trouble. To draw
the analogy even farther, you can transfer ¡°funds¡± between these ¡°accounts¡± without any
additional charges¡ªalthough if you make the wrong choice at low altitude, at the end of
an autorotation, ¡°there may be a substantial penalty for early withdrawal.¡±
Let¡¯s label the accounts ¡°altitude,¡± ¡°forward speed,¡± and ¡°rotor energy.¡± In
steady forward flight, with the engine running, the balance in all three accounts remains
the same, with constant deposits from the engine matching the constant withdrawals
through the rotor.
Now the engine quits¡ªno more deposits. As in real life, however, the skinflints
at the Rotor Mortgage Company insist that their payments continue (and we sure don¡¯t
want to be foreclosed at this point!), so we¡¯ll need to transfer funds from other accounts.
We have a few bucks saved up in our rotor energy account, but in a low-inertia
ship like the 407, it¡¯s not much¡ªjust enough to keep us afloat while we figure out which
funds to transfer. Since we¡¯re at altitude, the altitude account is flush...so let¡¯s start
¡°transferring funds¡± to the rotor by descending. This is what¡¯s happening when we
bottom the collective¡ªwe¡¯re transferring energy from the altitude account into the rotor
energy account to balance the constant drain from the rotor. If we keep the rotor speed
up, we¡¯re keeping its energy account up, as well as replacing any losses we may have
incurred right after the engine failure.
Obviously, we can¡¯t keep this up forever¡ªsooner or later, we¡¯ll reach the ground.
At this point, our altitude account is pretty well tapped out...but we still haven¡¯t touched
our reserve in the forward speed account. Oh, well...there went the kids¡¯ trip to an Ivy
League college...By reducing our forward speed, we can start transferring energy from
that account into the voracious mortgage...er, rotor. In fact, with a low-inertia rotor
system like the 407s, this speed-reducing maneuver, called a ¡°flare,¡± will actually
increase rotor speed, so we¡¯re building up a little cushion in the rotor energy account.
By the time the flare is complete, we¡¯re within a few feet of the ground (which
means the altitude account is tapped out), and we¡¯ve either slowed to minimal forward
speed, or stopped altogether (which means the forward speed account is broke, too). But
the repo man isn¡¯t here yet¡ªwe still have a decent balance in our rotor energy account.
Now is when we can spend it by raising the collective to cushion the touchdown. With
no deposits, that account will dwindle the instant we start to raise the collective
(particularly since we¡¯re now adding rotor pitch, an expensive luxury)...but before we¡¯re
completely bankrupt, we¡¯ll have the helicopter on the ground.
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 26 -
SAVE AND SPEND WISELY
We can actually manage these accounts during an autorotation, and adjust our
spending plan to best deal with the situation.
For example, if we have a nearby landing spot made (which means you can just
about see it between your feet through the chin bubble), we may want to come down as
slowly as possible. This would dictate a forward airspeed of only 55 knots, so we might
not have as much energy in our ¡°speed¡± account to transfer to our ¡°rotor¡± account during
the flare. On the other hand, we can keep the rotor account as high as possible by leaving
the collective all the way down, and maintaining 107% rotor RPM.
On the other hand, if the only available landing spot is farther away, we may want
to fly at ¡°best glide¡± speed of 80 knots. Our sink rate will be significantly higher¡ªbut
we¡¯ll be coming into the ¡°flare¡± transaction with a full ¡°speed¡± account. This means
we¡¯ll have plenty of energy to transfer into the rotor account at that time, so we might
consider reducing our rate of descent a bit by pulling on just a bit of collective to bring
rotor speed back to 100%--possibly even a bit lower. You¡¯ll notice that the green arc on
the dual tach goes all the way down to 85%...but that doesn¡¯t leave you much for those
¡°unexpected last-minute expenses!¡± Bell recommends no less than 90% for training
autorotations (and remember that the low RPM warning light and horn come on at 95%).
During the flare, we may actually want to lower the collective again to build rotor speed
as high as possible.
TOO MUCH OF A GOOD THING
What we want to avoid is over-flaring. Not only does this risk leaving us up
higher than we want to be with rapidly dwindling rotor energy reserves, but if we¡¯re low
enough it puts the tail rotor awfully close to the ground. As long as the surface is
reasonably smooth, you¡¯ll get a perfectly good autorotative landing by running the skids
on at 10 to 15 knots, and you¡¯ll have much more time to ¡°play¡± the final collective pull
and touchdown.
AUTOROTATION AND POWER RECOVERY
Now do you see why we¡¯ve introduced maneuvers in the particular order we
chose? Seen alone, an autorotation looks difficult and scary...but if you think of it as
nothing more than ¡°a steep descent followed by a quick stop followed by a hovering
autorotation,¡± you can see that it¡¯s just a series of maneuvers in which you¡¯re already
proficient.
We¡¯ll do a few for practice first, leaving off the touchdown at the end. Ready?
Tap to resume the simulation¡ªwe¡¯re in a steep unpowered descent, with the engine
running at 100% RPM but the collective bottomed all the way.
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 27 -
Hold 80 knots. The helicopter will certainly seem to be swooping down at an
impressive rate, but it¡¯s equally certain that it¡¯s not ¡°falling out of the sky.¡± At about 50
feet, apply smooth, but fairly decisive, back pressure on the cyclic to flare. The nose will
come way up, the rate of descent will slow...and you¡¯ll see and hear the rotor speed up to
near its 107% redline (the needles will split for a moment). As the descent stops, apply
forward pressure to level the fuselage, and as the helicopter settles, smoothly raise the
collective again to transition to a hover. Of course, all these large collective movements
will require corresponding pedal inputs. Don¡¯t worry about getting the helicopter to stop
dead¡ªanything below translational lift speed is OK. Do enough of these until you¡¯re
comfortable. Then do a couple of hovering autorotations, just as a refresher.
IT¡¯S THE REAL THING
Now you¡¯re ready for this series of lessons¡¯ ¡°graduation exercise:¡± an actual
autorotation to touchdown. Let¡¯s start at 800 feet and 100 knots, just to give you a little
more room. Ready? Take a deep breath and hit to shut down the engine.
The needles will split right away, and you¡¯ll get the ¡°engine out¡± light.
Simultaneously apply back cyclic pressure to start the speed toward 80 knots, and
smoothly lower the collective. If your reactions are reasonably fast, you probably won¡¯t
get the low RPM light or horn.
Once the descent has stabilized, you can experiment with rotor speed control. A
slight pull on the collective will bring Nr back toward 100%. Ease the collective back
down to build rotor speed back up; you¡¯ll notice that the sink rate increases as you do so.
At about 50 feet, smoothly and firmly apply back pressure to the cyclic to arrest
the descent and reduce your forward speed. Nr should be right at its maximum. As the
helicopter levels out, you should be down to around 15 to 20 feet. Make a definite
forward cyclic correction to level the skids (¡°rocking the ship forward¡± is a good image).
As it begins to settle, smoothly pull in more and more collective to cushion the
touchdown.
It¡¯s important to keep the helicopter coming down (although not too fast) all the
way through the final settle-and-touchdown phase. Pull too much collective too early,
and you¡¯ll run out of blade energy and fall the last few feet¡ªthat¡¯s the ¡°early withdrawal
penalty¡± I mentioned earlier. In a really hard touchdown, it¡¯s not unusual for the main
rotor blades to flex down far enough to chop off the tailboom. It¡¯s much better to settle on
gently, even if you¡¯re still moving forward at 10 or 15 knots.
Bell 407 Helicopter Documentation
Copyright 2001, Terminal Reality Inc. - 28 -
Congratulations! You¡¯ve mastered the most critical maneuver in helicopter
landings. It¡¯s often said that ¡°A good landing is one you can walk away from; a great one
is one after which the aircraft is still flyable.¡± There¡¯s no reason you shouldn¡¯t make
great landings in the 407, power on or power off, from here on.
Ò³:
[1]