Torque variations in a turn

[Curyfury]

Having flown mainly in robbies, exceeding torque ratings wasnt really a concern since there was no gauge to directly monitor. However, i recently rode in an astarb3 and the pilot demonstrated how sudden lateral cyclic inputs can cause a spike in torque, with right cyclic causing a larger spike. The flying handbook also mentioned aft cyclic reduces torque and foward increases torque.

 

I understand what torque is, but why are varying cyclic inputs causing such variations?

[Eric Hunt]

The cyclic inputs moving the disk around will cause drag changes, and the engine then has to put out more/less power (torque) to keep the rotor RPM constant. Aft cyclic, flaring, reduces the induced flow and drag, so the Tq will drop. Forward cyclic is the opposite, so revs want to drop and Tq will go up.

[iChris]

However, i recently rode in an astarb3 and the pilot demonstrated how sudden lateral cyclic inputs can cause a spike in torque, with right cyclic causing a larger spike. The flying handbook also mentioned aft cyclic reduces torque and foward increases torque.

 

I understand what torque is, but why are varying cyclic inputs causing such variations?

 

As written in Eric Hunts post above, torque changes with roll are related to the variation in induced flow through the rotor. It’s really not surprising because a rotor turning edgewise through the air is a complex mix of airflow asymmetry. In the case of the AS350 in straight and level forward flight, a right roll would increase the angle of attack over the tail, leading to the rotor’s precession up on its left side. This results in an increase in induced drag over the tail for right rolls vs. left rolls. The reverse is true for U.S. turning rotors, an increase in induced drag over the tail for left rolls vs. right rolls.

 

Below are graphical representations (counterclockwise turning rotor). The first based on Sikorsky S-67 flight test data presented in a technical paper by Ebert and Lappos. In the Ebert and Lappos paper, they concluded, whatever the source of the torque variation, the effects are certainly real. The latter two, based in part, on a computer modeling analysis and further investigation by R.W. Prouty concluded, the effect is the assumed induced velocity distribution combined with coning.”

 

 

 

Edited March 11, 2016 by iChris

[Curyfury]

Call me dumb, but i still dont get it. I sort of get why it changes with forward and aft cyclic inputs But not right and left. If someone can dum it down for me, id appreciate it.

[iChris]

Call me dumb, but i still dont get it. I sort of get why it changes with forward and aft cyclic inputs But not right and left. If someone can dum it down for me, id appreciate it.

 

 

To start a roll to the left, the pilot must increase the lift on the rearward portion of the disc in order to make the rotor flap up on its right side. Since the lift vector in this location is tilted aft, the increased lift results in more induced drag. The corresponding decrease in lift for the blade over the nose, however, results in only a small decrease in induced drag since the lift vector here was tilted only slightly aft in the beginning. Thus the total required torque for the entire rotor increases. The engine governor tries to maintain the rotor speed and as it responds to the requirement the pilot observes an increase in torque.

 

Starting a roll to the right requires more lift on the blade over the nose, but since the lift at this point has little rearward tilt, the effect is small. But at the rear, the decrease in lift and induced drag is significant causing the blade to flap down on the right side, and thus the pilot observes a decrease in torque.

 

In affect, torque is really the measure of drag on the rotor blades.

Edited March 12, 2016 by iChris

[Curyfury]

Thanks! I think i understand now.

[ridethisbike]

If you need the visual in a piston helicopter, keep a close eye on the manifold pressure when doing these maneuvers with no collective input. Might be small with the lateral movements, but the fore/aft movements will result in a noticeably different manifold pressure for the same collective setting.

 

As stated, it all has to do with drag.

[Hvozer]

I fly with uh-1 and uh-60ies it is quite observable changes with cyclic inputs I agree with all you but there is something different with sudden forward cyclic when performing a vertical take off. Even if you have horizontal velocity after quick or sudden forward cyclic input rotor blades start facing more vertical angles so induced flow increases and drug increases but we observe that torquemater decreases. There must be something much more effective than drug. In this case my opinion is after sudden forward cyclic input we create negative G power so mast bearing friction drops suddenly which is very high just before the movement. This is just my opinion. I wonder about your opinions about this . Thank you

[Eric Hunt]

 

 

This is just my opinion

Correct. Just yours.

[iChris]

I fly with uh-1 and uh-60ies it is quite observable changes with cyclic inputs I agree with all you but there is something different with sudden forward cyclic when performing a vertical take off.

 

Even if you have horizontal velocity after quick or sudden forward cyclic input rotor blades start facing more vertical angles so induced flow increases and drug increases but we observe that torquemater decreases.

 

There must be something much more effective than drug. In this case my opinion is after sudden forward cyclic input we create negative G power so mast bearing friction drops suddenly which is very high just before the movement. This is just my opinion. I wonder about your opinions about this . Thank you

 

In a no-wind vertical climb, the inflow due to the climb aids in the makeup of a vertical induced velocity vector. Thereafter, any forward cyclic input in order to gain airspeed or any horizontal wind, modifies that vertical flow to a more horizontal flow vector. Consequently, any reduction in the induced flow velocity or any reduction in the induced flow angle, represents a reduction in induced drag, torque, and yields a more vertical lift vector.

 

This post was in regard to normal 1G (positive G) maneuvers, in particular, the turn maneuver. However, what you’re referring to is a zero G maneuver were lift is momentarily reduced to zero and therefore induced drag is also at zero (induced drag is a byproduct of left). That would leave us with only the profile drag of the rotors and the frictional drag of the transmission and drivetrain. In any case, it’s the reduction in the induced drag which is key to the decrease in torque.

Edited November 16, 2018 by iChris