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6.13 Power Requirements

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Overpitching

Overpitching is one reason there is a minimum RRPM limit for engine powered flight or autorotation.

Rotor RPM

104% CF is good, disc size is sufficient, V2 is balancing, drag coning angles are small, RRPM is sufficient.

RRPM starts to decay.

96% CF is reduced, Disc area significantly smaller, Drag is increasing V2 is reduced Loading/coning increases RRPM decrease

86% CF is reduced, Disc area significantly smaller Drag is increasing V2 is reduced Loading / coning increases RRPM decrease

80% CF is reduced, Disc area significantly smaller Drag is increasing V2 is reduced, Loading/coning increases RRPM decrease. You CANNOT recover RRPM below 80% by any means…

When Overpitching Occurs

  • If the pilot mismanages the throttle - Watch your RRPM, develop throttle control as a second nature and reacted instantly to any low rotor RPM warning you may have. RRPM can be down to a critical limit in fractions of a second.
  • If you have a governor - use it.
  • The engine stops and the pilot fails to enter autorotation fast enough; in the climb, at normal profile, you only have 1.2 seconds to do this. Be current in entering autorotation and be quick to do it.
  • Hot, High, Humid or Heavy - The throttle may already be fully open. There are no more ERPM (no more engine power) available to add to RRPM. To pull more pitch in that condition will cause RRPM to decrease as this drag increases with no increase in RRPM available to overcome it. The MAUW is there for your protection. Don't exceed it. Be aware of the density altitude and humidity. Remember that warmer air, more humid air or the air at high altitudes is less dense and you will not have the same power available as in cooler, drier conditions or lower altitudes.  

Ways to recover from Overpitching

  • Add throttle (always lead with the throttle) and lower the lever until you have regained the correct RRPM from ERPM.
  • Flare if it is safe to do so. Flare effects will give RRPM a boost and in conjunction with lowering the lever and adding throttle RRPM can be recovered very quickly

Overpitching cannot happen if you stay in limits. There is a safety margin of power (the 5 min take-off power rating) and more above the 5 min rating which ensures that if the initial limit is exceeded, that it is not the absolute limit. This is not advice that you should exceed it.

The throttle must be fully open to get into an overpitching undesirable aircraft state. If RRPM is decaying and the horn sounds you must enter autorotation. The engine could have stopped. You have no time to investigate. Get the lever down!

Conservation Of Angular Momentum

Coriolis effects all blades, as the blades flap up on an articulate rotor hub, they increase RPM (speed) and accelerate forward; conversely as they extended back down they reduce RPM, decelerate and lag. The principle is also identical to a rotor disc “Coning”, no-matter what type of hub system. Any point on the blade that moves toward the axis of rotation, its velocity increase, RRPM therefore increase.

However this effect is not as prevalent when RRPM decreases due to overpitching. As drag increases it becomes greater than the inertia velocity, slowing the rotor down. In this situation the Coriolis Effect is not capable of preventing or stoping the decay of RRPM.

The rotor disc is capable of over-speeding if the helicopter is flared with RRPM already close to the upper limit due to the Coriolis and Centrifugal Forces (CF), which are caused by inertia of the rotors mass moving inwards.

Minimum RRPM limits

The Coning Angle is the smaller angle between the Tip Path Plane and the longitudinal (span-wise) axis of the blade. Coning angle depends upon centrifugal force and rotor thrust. Centrifugal force always acts parallel to the plane of rotation and acts to reduce the coning angle. Rotor thrust acts to increase the coning angle. As the coning angle increases, the area of the disc decreases. This causes a higher disc loading.

If thrust increases, OR centrifugal force (Rotor RPM) decreases, then the coning angle will increase; conversely if thrust decreases, OR centrifugal force (Rotor RPM) increases, then the coning angle will decrease.

Coning Angle

As RRPM decrease so does the centrifugal force pulling the blade “S” (Surface Area) out from the centre of the disc. Coning angles increase. The disc area becomes smaller as the blade(s) cone up, but the aircraft weighs the same. To support the same weight with a smaller disc area the total thrust needs to increase. Coning angles are also directly affected by the helicopter's weight. The more the helicopter weighs, the greater the thrust required, the greater the coning angles will be (the closer the blades get to the stalling angle).

Disc Area

When viewing the main rotor disc from above, we find the size of the disc area is directly proportional to coning of the blades. The blades do not become longer or shorter, the “S” does not change but the amount of disc area creating vertical thrust does.

Surface Area

As the disc becomes smaller, loading on the blade(s) will increase. Thrust must increase further. In addition, centrifugal force is becoming smaller as drag on the blade increases, reducing RRPM further. This in turn reduces RRPM and disc area (disc size) in a self-perpetuating loop.

Thrust Centrifugal Force

RRPM will decrease, ultimately to a point where V2 is too low for lift to result. Rotor drag will increase exponentially and stop the rotation. The blades will collapse into an upward cone (due to the increased loading) and stop rotating (due to rotor drag so large it cannot be overcome). The blades are now stalled.

As the tail rotor loses effectiveness due to its own low RPM (Main RRPM and Tail RRPM are always linked) it is likely that the helicopter will yaw to the right. All of these things can happen very quickly.

Disc Loading = Gross Weight / Disc Area

Maximum RRPM limits

At 104% RRPM the centrifugal force (CF) acting on the blade (and in turn the attachment bearings which allow blade feathering) is about 4.5 tons.

At 114% RRPM the force exerted is 9 tons! The attachment bearings suffer “Brinelling”.

RRPM Limits

This is one reason why there is a maximum RRPM limit for engine powered flight and autorotation. (The tail rotor ability to develop anti-torque thrust is also dependant on main rotor RPM).