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6.15 Autorotative Flight

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Autorotation Principle

A rotor faces the upcoming airflow. In doing so, it converts the linear energy into rotary motion, harnessing this effect with its rotors. The energy is collected and stored in the form of momentum. The inertia of the rotors is used to slow its rate of decent (ROD).

We have a pretty good idea of how the Helicopter can not only store energy but also be used to give directional control (as it can tilt its disc) in its travel downwards to the ground.

R22 Autorotation

If the engine should fail in a Helicopter power is lost. Now there is no force to balance rotor drag. The blades RPM will decrease and stop in a very short time.

Autorotative Forces Rotor-Drag

Rotor drag has to be overcome or control of the aircraft will be lost if the blades stop. (Understand that this is not facilitated by the freewheel unit which only removes the drag associated with the now non-running engine.)

Autorotative Forces Thrust

To achieve this, thrust must be reduced. This is why the lever must be lowered if the engine fails. RRPM will then be maintained.

Autorotative Forces Thrust Reduced

With RRPM now maintained, control of speed and direction can be controlled. Most of the forces that the helicopter could use when it had power are still present. The helicopter has a little thrust available but not enough to equal its weight. Because of that the helicopter can only descend.

Autorotative RRPM Maintained

Although this may sound bad, the helicopter still has full directional control with the cyclic. It gives you the ability of tilting the disc.

The pilot can fly the aircraft to a safe site (within limits). How the aircraft is landed is outlined in the section describing 'Engine off Landings'.

R22 Autorotation Descent
RRPM Control in Autorotation

In autorotation control RRPM with lever only e.g. RRPM high, raise lever, check RRPM after lag. Airspeed and direction are controlled with the stick in the usual manner.

Collective Lever UpCyclic Aft
R22 Cyclic Aft

If the stick is moved aft to reduce airspeed the lever must be raised slightly to reduce RRPM through increased rotor drag. If this is not done the main rotor might over-speed. There will be some 'lag' between lever input and a reduction of RRPM.

Collective Lever DownCyclic Forward
R22 Cyclic Forward

If the stick is moved forward to increase airspeed the lever must be lowered slightly to increase RRPM through reduced rotor drag. If this is not done RRPM might become too low. There will be some 'lag' between lever input and an increase of RRPM.

Basic Autorotation

This example demonstrates how to enter autorotation, control the Helicopter for a given airspeed and RRPM (Nr) with how to recover into a climb.

Carb heat should be fully hot well before entry. This prevents engine stall at low RPM due to a sudden change in mixture. This could stop the engine during the autorotative descent.

Start into the wind and check, HASEL.

Height? Minimum 1500
Area clear
Security - hatches and harnesses secure
Engine - T's & P's, Carb heat, Governor off
Lookout - Good sight picture coming on

Collective fully down - smoothly
Right pedal - to correct yaw to the left
Cyclic coming back - slightly as the nose will drop.

R22 Basic Autorotation

Hold the attitude.
RRPM check tacho.
Check ERPM below 80% - ensure needle split.
RRPM adjust (lag).
Collective up slightly to control RRPM (Depends on AUW).

R22 Dual RPM - RRPM
Adjust attitude for 65kt.
Note ROD with VSI.
Note angle of descent.

Before 500' AGL

Recovery
Carb heat to COLD.
Lookout ahead.
Governor back on.
Roll on throttle.
Raise lever to 23".
Keep in balance.
Check RRPM 104%

Climb away
60kt 23"
Look ahead
T's and P's
Fuel
CAT out of yellow

Autorotation Entry

When the engine ceases working, the power required for flight, now autorotative flight, must come from another source. This energy comes from the rate of decrease in potential energy as the helicopter loses altitude. The rotor will initially slow down, feeding on its own energy due to the power loss. Lowering the collective with little or no delay will stop this decay. If RRPM (Nr) is allowed to decay too much, the rotor will stall, allowing the helicopter to assume flying qualities of a rock. The increasing up-flow of air through the rotor system effectively reverses the airflow, tilts the lift vector forward, increasing thrust, which can now be managed by the pilot through small pitch changes through the collective by controlling Nr (in-plane drag). Throughout this procedure, potential energy in the form of loss in altitude is traded off to place kinetic energy in the rotor system.

R22 Autorotation Up-Flow

Now that steady state autorotation has been achieved, the pilot has the option of stretching his glide to a distant landing zone or increasing his loiter time in the air, provided sufficient altitude exists. Just suppose the engine failed and there wasn't a suitable landing site immediately in front of you, but there was one further away. What should you do? Luckily, for pilots in a somewhat stress-inducing situation, the solution is fairly logical and in line with normal reaction - fly at optimum cruise speed (fast). This is called maximum glide range airspeed. It is found at a point tangent to the power required curve from a line extending from the origin. Again, there are trade-offs, and in this case, higher speed and distance over the ground reduces time aloft and rotor speed.

Another alternative on the other end of the spectrum is minimum rate of descent. This occurs at the speed of minimum power required on the power required curve. If there is an available field immediately in front of you, you may use this speed for extra time aloft to ensure crew readiness for landing or make a prudent radio transmission, but there are other factors which enter the ball game as the helicopter approaches the ground.

Engine off Landing (EOL)

Carb heat should be fully hot well before entry. This prevents engine stall at low RPM due to a sudden change in mixture. This could stop the engine during the autorotative descent.

Start into the wind and check, HASEL.

Height - Minimum 1500
Area clear
Security - Hatches and harnesses secure
Engine - T's & P's, Carb heat, Governor off
Lookout - Good sight picture coming on

Into wind
70kt IAS
Correct height
Clear Visibility
Area clear of obstacles
Landing site chosen (Landing surface is suitable)
Governor off (instructor preference)
Check Carb heat to HOT
Verbal warning “Practice engine failure to the hover --- Go!”

R22 Basic Autorotation
Flare and Touchdown

As the ground becomes more in focus, the range of safe airspeed/rotor RPM combinations narrows, and precise management of kinetic energy is necessary. At this point, your new goal is to reduce the kinetic energy along the flight path to zero at the same time ground contact is made, while trading off the stored kinetic energy in rotor RPM for thrust to maintain power requirements for flight before the blades reach a stalled condition. This may seem like a very large chunk to swallow, but if taken in small bites, the process becomes much easier.

From either of the two extreme airspeed range examples previously discussed (max glide/min rate of descent), we will assume a suitable landing zone is now easily within range. If we were at max glide at a high forward speed and associated high rate of descent, it is only logical we slow down.

How slow? Minimum rate of descent sounds logical. But, even at this airspeed, the helicopter's landing gear cannot absorb the amount of energy the helicopter is carrying at ground contact. Therefore, it is advantageous to carry 5 - 10kts extra airspeed over minimum rate of descent airspeed at flare altitude, banking on another trade-off - extra forward airspeed for high rotor RPM.


A nose-up cyclic flare (see figure) at 75 - 100ft AGL increases induced flow. The resulting increase in Angle of Attack (AOA) creates more lift, which decreases rate of descent.


Moreover, the downward shift in relative wind tilts the left vector at blade element more forward, resulting in a larger pro-autorotative force; this increases rotor RPM. Finally, the net rotor thrust is tilted aft, and this decreases ground speed. The flare should be maintained in an effort to reach a point to where forward speed is 5 - 10kts at close proximity to the ground.


At this point, increasing collective, increases thrust and augments braking action, using up part of the stored rotational energy. All that is left is to put in a little forward cyclic to level the aircraft and use that last rotational energy by pulling collective to cushion the landing.

If one chose to arrive at flare altitude at minimum rate of descent airspeed or less, there is little or no forward speed to trade off for this advantageous high rotor RPM. Forward speed is already low, and if too much flare is combined with an improperly timed flare (too high), forward speed may reduce to zero at a high altitude. This condition is known as becoming “vertical,” and since the rotor system already has little stored energy, there will not be enough thrust available with collective increase to slow rate of descent at touchdown to a non-destructive level.

Governor

Not all instructors switch out the governor when making an autorotative approach to power recovery. If the Governor is left engaged, throttle may be reduced to give engine RPM below 80% after the lever has been lowered for entry.

As the lever is raised for recovery, the mechanical correlation effect will increase ERPM, the throttle can also be 'tweaked' to increase. When 80% ERPM is reached the Governor will re-activate and bring RPM up to 104%.

This technique is understood to result in much less wear on the sprag-clutch and the shaft it engages around. It also avoids any risk of incorrect throttle manipulation and low rotor RPM at a crucial time.

The technique may also be used during a 'full on' EOL as a safety measure. RPM recovery is then aided in the same way as above if restoration of power becomes necessary.

EOL in Hover

Note! The lever must not be lowered in an engine failure in the low hover although you have just been trained to do so in forward flight. It is only raised to cushion the landing.

R22 Hover

Start from a stable 2ft hover into light wind or no wind. When ready, close throttle (apply verbal warning).

R22 pedal right

A lot of right pedal will be needed to correct the Yaw to the left.

R22 cyclic right

Removal of tail rotor drift/roll causes Drift to left, so small amount of right stick needed to correct.

collective lever up

After a short pause (counted normally) cushion the landing with smooth action raising the lever.

collective lever down

Now fully down when on the ground.

Advanced Autorotation

Changes in airspeed, RRPM and turns can be used to vary the distance covered over the ground whilst in autorotation. Choose a tree or similar suitable landmark to begin every autorotation. Start above it to show how techniques change the distance covered.

Preparing for Autorotation

Cruising at 2500ft at 70kt and recover at 1000ft AGL

Carb heat should be fully hot well before entry. This prevents engine stall at low RPM due to a sudden change in mixture. This could stop the engine during the autorotative descent.

Start autorotation into the wind and check, HASEL.

Height 2500ft - Minimum 1500
Area clear
Security - hatches and harnesses secure
Engine - T's & P's, Carb heat, Governor off
Lookout - Good sight picture coming on

Clear Visibility
Area clear of obstacles
Landing site chosen (Landing surface is suitable)
Governor off (instructor preference)
Check Carb heat
Verbal warning “Practice engine failure to the hover --- Go!”

Six Autorotation Approaches
Six Autorotation Approaches

1 - Set minimum forward airspeed
Level aircraft
Maintain heading with pedals
ROD 2000fpm & check AOD
1500ft, note distance covered
Select 60kt attitude
Carb heat to cold
Note: VORTEX risk on recovery

2 - 360o at 60kt turn to the right (or best pilot view)
Heading marker
Select 60kt attitude
Balance
Roll on 30o angle of bank (AOB)
Hold attitude & balance in turn
Roll out, note distance covered
Recovery - have 60kt already
Carb heat to cold
Recover

3 - Select constant 35kt attitude
Balance
ROD 1800fpm and AOD check distance covered
Select 60kt attitude about ½ the distance
Carb heat to cold
Recover

4 - 60kt Datum Autorotation
The 60kt datum auto is done first to show distance covered.
(Some Instructors prefer to use 65kt for this demonstration)
Hold attitude
RRPM check tacho
RRPM adjust (Lag)
Lever up to control RRPM
Check ERPM below 80%
Select 60/65kt attitude
Check ROD 1500 – 1600fpm
Note: angle of descent.

5 - Range select 75kt - hold attitude
RRPM – lever
Check RRPM
ROD 1800fpm AOD
Note distance covered
Recovery
Carb heat cold
Flare for 104%
Select 60kt
Lever up for 23"

6 - Extended Range select 75kt - hold attitude
Adjust RRPM 90% (Low Rotor horn off. lf RRPM are reduced below the low warning horn there is no further warning of rotor stall. Even greater monitoring of RRPM (especially with lag) is essential).
ROD 1800fpm, AOD, distance covered.
Note distance covered
Recovery
Carb heat cold
Flare for 104%
Select 60kt
Lever up for 23" (engage governor)

Max Range Autorotation

Reducing RRPM from 104% to 97% (R22) reduces ROD by some 150 fpm. Further reduction of RRPM to 90% only reduces ROD by another 50-60 fpm. Some instructors believe that this small benefit is far outweighed by the danger of having no further stall warning (the horn is already going off or is disconnected by the instructor 'pulling' the circuit breaker) and RRPM perilously close to a stall. Their preference is to use 75kt / 97% for a max range auto. Recovery from this method is also a lot safer and easier to remember under pressure.

Recovery

Simultaneous actions:

1 Look out ahead.
2 Roll on throttle whilst engaging the governor.
3 Raise lever for 23" as selecting 60kt attitude.
4 Wait for airspeed to build and adjust as needed.

collective lever down, roll throttle off

Recovery from low speed or low RRPM autos can be extremely dangerous if done habitually the same as from 60kt (safer at 75kt).

With lower RRPM (max range auto) rotor drag is much higher. To recover, speed is reduced by flaring which also increases RRPM, reducing the rotor drag. Only then can power be applied safely.

Autorotation - Effects of Airspeed on Rate of Descent (ROD)

In the graph ROD reduces until the bottom of the curve is reached. After that point an increase in IAS will result in ROD beginning to increase again. If the power required graph is used (it has a similar curve) then best endurance speed is when the least power is required to fly straight and level, giving BRC speed.

The same applies to best range speed, which explains why the autorotative speed (R22 65kt) is chosen (for best ROD at a speed that gives best range).

Airspeed on Rate of Descent

ROD is affected by speed. Factor X is responsible for the initial reduction in ROD. The gradual increase in ROD after best endurance speed is due to factors Y and Z. Before best endurance speed is reached factor X is greater than the sum of Y and Z. Once that speed is passed Y and Z increase and become greater than X.

As we are considering the effects of adding a horizontal component of ROD airflow through speed, it follows that airflow moving to the disc in the form of ground-relative wind would also reduce ROD up to the same amount of airspeed. This only means something if ground speed is considered, range will be reduced by ground relative wind (if heading into it) as WV must be subtracted from auto speed to arrive at a chosen spot ahead at any airspeed.

That said, wind on the nose can be a large benefit when making a landing as zero GS is AS + WV. Overall, the pilot can harness the results of the graph to reduce ROD and increase the choice of landing sites (within limits). This is explained above, Advanced Autorotation.

The autorotative performance graph for the type of Helicopter being flown is provided in the operating handbook or flight manual. It is also easy to use the power required graph to find least ROD and best range speed. Other factors such as all up weight and altitude (linked to air density) contribute to ROD.

Turns in Autorotation

Any turn will increase rate of descent and RRPM. If the lever is raised to control RRPM during a turn it must be lowered by a similar amount when the Helicopter is rolled out of that turn. The steeper the turns angle of bank (AOB) the greater the increase in RRPM and ROD.