Torque rise phenomenon
 

In the type of helicopter I fly, there is a phenomenon of torque rise in which during take-off the torque rises by itself without any collective lever input. The stages at which it happens distinctly during a normal take-off, is once at roughly about 40-50 kmph and another at about 80 kmph. The amount of torque rise is quite high, about 5-10% (per engine) in each instance and if uncorrected the total torque rise is in the range of 10-20% per engine by the time the nominal take-off speed of 120 kmph is achieved.

The pilot therefore, is forced to glance inside to keep a check on the torque and reduce the collective lever to ensure that the torque remains same as was used to initiate the take-off (hover torque +4% per engine). It also forces the pilot to keep glancing inside during the critical phase of take-off. It especially becomes critical when operating in the regime of Take Off rating, when, if uncorrected, there are chances of over torquing. (There is a voice warning for that though)

Though, I am not convinced for the reason cited for this phenomenon, it has been attributed to flexing of airframe with increase in forward speed, in which the all composite body of the aircraft gets slightly compressed due to aerodynamic forces on it, which in turn leads to an un-intended movement of control rod (without any feedback on the collective lever) leading to increase in pitch on the blades resulting in increase in torque.

Few Questions:

1. Does this sort of torque rise occur in any other helicopter? or atleast anything similar.

2. Though it would be difficult to explain until the airframe is studied in detail, if such a thing of control rods actually moving due to flexing of airframe can occur, can some design modification be done to prevent this un-intended input on control rods?

3. Could this be happening due to any aerodynamic 'actions' on the rotor itself? (which in all probability would be increase in rotor drag, leading to the FADEC compensating by increasing torque by itself to maintain R RPM)

Is this a certified helicopter? Sounds a bit odd to me.

The AS 350 and 355 will reduce torque with an increase in altitude (and vice versa - pay attention on descent).

On take off "falling off the bubble" by accelerating out of ground effect will cause the pilot to increase the collective setting causing a torque increase to arrest the sink rate. Could this be the cause of the increase at 40 -50 kmph?

All the engines are doing is changing their power output automatically to maintain the required NR. They don't really care what position the collective is in. During those stages of flight that you mentioned, if the aerodynamic forces on the main rotor are causing it to accelerate one way or the other, the engines are just going to automatically compensate accordingly (to maintain a constant NR), even though the collective position remains unchanged.

Same reason why if your MGB is in the process of seizing that you'll see the torque steadily rise to maintain NR despite the collective position remaining unchanged. Eventually when the engine power maxes out you'll then see NR start to droop. I don't think what you are seeing has anything to do with control rods moving in response to airframe flexing.

The FADEC is just a constant speed N2 governor, all it really cares about is maintaining the correct engine output speed. Sure, it knows the position of the collective control (I'm guessing there is a collective position sensor in the flight control system somewhere), but this is just an "anticipation" function so that when the collective moves, and at what rate, it knows to give the engine an extra squirt of fuel (or not), as determined by the control software. Collective movement is just fine tuning the metering of the fuel, a collective in fixed position is not influencing the FADEC to do much at all.

Normally you would expect total torque to reduce at higher speeds with fixed collective due to tail rotor becoming much more effective.

Perhaps in your case it is a control mixing issue. Could it be that the forward cyclic required at higher speeds somehow results in more overall (collective) pitch going on to the blades?

Black Fox - it might help if you told us what the helicopter type is.

The lower speed torque spike might be explained by the immediate precursor to ETL which is a higher rotor drag due to the tip vortices - many don't notice this but do see the reduction in rotor drag as ETL is achieved because the aircraft wants to climb. Max power required for a cushion creep style take-off (limited power) is always just before the onset of ETL.

The higher speed torque increase is more difficult to explain, especially without knowing what the aircraft is.

PS - by take off, we assume you mean transition to forward flight since the take off is what gets you to the hover (unless you are doing a running takeoff).

Tail torque
 

Are you moving the pedals? You can easily change the torque requirement in the conditions you describe. But why not just tell UA what type you are talking about. A type qualified FI would probably tell you in a heartbeat what is going on

Black Fox - do you have a SAS in the yaw channel? Some aircraft have it to give yaw stability in the hover and it kicks out above a certain airspeed.

This could cause a yaw input that you don't see in the cockpit which could give a torque spike.

Strange that you still have that problem with the ALH. I thought the problem had been resolved after the mod.
Are you flying the earlier models or the newer ones?

OP: "....attributed to flexing of airframe with increase in forward speed, in which the all composite body of the aircraft gets slightly compressed due to aerodynamic forces on it, which in turn leads to an un-intended movement of control rod (without any feedback on the collective lever) leading to increase in pitch on the blades resulting in increase in torque."

Check tie rods and gearbox mounts etc, sounds disturbing.
Torque should decrease with fixed lever as speed increases to 120kph

With a constant collective setting you should experience a decrease in torque as airspeed increases as total rotor drag decreases with speed, in addition to less pedal needed. So what you are explaining sounds odd.

Maybe TQ increase due to "ram air effect" with increased airspeed..
It happens in fixed wing having to reduce TQ on the take-off run as airspeed increases !!

Zac, there ain't no ram air effect for most helos as the intakes are hidden away to avoid ingesting FOD in the hover.

Thanks AC,, guess I should stick to FW and butt out.

zac
in a helicopter the engine is governed so the power the engine makes is the power required to turn the rotor system (MR and TR) at the same RPM, if the lever isn't moved then normally less power is required to turn the rotor and the governing system would reduce the power output of the engine(s) accordingly.

finer points that vary this may be if the helicopter had been hovering at a very good Lift Drag ratio and it gets much worse but that's the other way around normally OR some helicopters run different RPM often commanded by a TR pedal position sensor, if it commanded an RPM rise then more power may be required.

Neither of these sounds like what is happening here. It sounds like this helicopter is broken (more pitch being added due flexure of something), (or has a pusher prop with varying pitch !)

Pls OP tell us more . What type?

Perhaps Black Fox will get back with his aircraft type ...otherwise we can't be much help.

Or AnFI will continue to patronise like this And then come up with random stuff like this ???????

which ones????

Oh dear AnFI - another corking post!

Gee Crab thanks for your 'helpful contribution' please don't insult me anymore otherwise I'll get in trouble. Unless you're not actually being sarcastic?

Lala it's difficult to answer you because I'm not sure what you are saying, the bit you know you think is condescending and the 2 bits you don't know you think are wrong? They are not wrong, and the answer to your question is Airbus do it, maybe others. If you dig you can find the pedal position transducer output in the VEMD.

Surely that would be to avoid Nr changes, not create them, in a similar way to how a collective position transducer works?