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?

AnFI - I fly an Airbus helicopter and the yaw system has a position transducer and a yaw force link - both of which are there to coordinate the AP computer's output and are exactly NOTHING to do with Nr.

Do please explain why you would command an Nr change with a yaw sensor instead of a collective one.

I think LAla's point was that you were explaining basic governing as if you were addressing a 5 year old and not a helicopter pilot - then you make a statement about not moving the lever and the power decreasing without any qualifying comments about having increased speed.

The EC130 has a yaw anticipator which is designed in conjunction with ASI input to droop the Nr by 8 rpm on acquisition of effective translational lift on departure.

What that has to do with this thread or the price of fish is not clear to me, but maybe it'll help get the thread back on track.

Punto - it isn't relevant since presumably it would give a Tq decrease rather than the spike that the OP specified.

Out of interest, do you know if this arrangement between the Yaw channel and the FADEC is to reduce noise and vibration in the cruise or to provide more TR thrust in the hover?

Is this the only airbus helo with this facility?

Punto
my statments were clear and factual and relevant to the thread, the price of fish is factors that change the power required without moving the lever. I don't think crabs display of sarcasm and ignorance is much help and risks a divergence

Crab: "Do please explain why you would command an Nr change with a yaw sensor instead of a collective one."
I'd have thought that was fairly obvious (and anyway it's not a yaw sensor, it's a pedal position transducer, do keep up) and I wouldn't want to condescend to you anymore than necessary, but I am happy to explain it to you like the 5 yr old you mention if neccessary?
and
"....without any qualifying comments about having increased speed." the whole thread is about the power change required as the speed increases, if you weren't so focussed on trying to get at me all the time you might notice that. I refer you to my earlier statement "Torque should decrease with fixed lever as speed increases to 120kph"

and Crab you will note that the esteemed 212 has just learned that Pedal Transducers ARE used to alter the Nr, that's the great power of pprune.
Anyone can learn something here, even you !

Lala, there is something wrong with this helicopter. If the transducer were the wrong way around there would be an Nr increase. I don't know what is wrong with this helicopter, but those are factors that would change the power required without moving the lever (... as speed is increased), just most of the factors should produce the opposite effect, which makes the op's claim more disturbing. An uncommanded pitch increase is therefore the most likely culprit.

Black fox stated
All composite body and ... FADEC.
Can't be that many types?
Guimbal comes to mind?
Any others?

SLB

So we have to refer to your earlier posts constantly to clarify the statements you make in the later ones???

come on then, let's have the reason....

TR or MR pitch???





SLB - it would appear that the EC130 meets all the criteria

@SLB: I think this is about the indian BK117 look-alike, the infamous HAL Dhruv...

HAL Dhruv which was a technology share from the EC665 Tiger in the MR area.

Most if not all FADEC equipped helicopters have position sensors in the YAW and COLL channels.

The data from the sensors is input into the "mapped" model in the EEC to adjust fuel flow accordingly very accurately without the need for a "governor reset" correction although that function still exists.

A well sorted unit is impeccable with the downside being that it can lull you into a false sense that as the Nr is so stable the rotor must have a lot of mass and therefore inertia.

The reality may not be as you suspect.

RVDT - I guess these sensors act as anticipators so that any yaw or collective input is used to predict an increase or decrease in rotor drag (either main or tail) and adjust the fuel before the change happens. Reduces or eliminates transient droop then?

if so, this is different to Puntosaurus EC130 step change in Nr and not what Anfi described as yaw transducers altering Nr, yes?

Condescending and supercilious - nice mix AnFI:)

You say transducer, I say a sensor....let's call the whole thing off....(there's a song in there somewhere) :)

You've missed the point again it's not whether its a sensor or transducer it's whether it's a YAW sensor or a PEDAL POSITION transducer (or sensor if you prefer). The difference is obvious (5yr olds: one is the rotation about the normal axis and the other is the control for the pitch of the TR). I said what I meant, and I was right and you were wrong as usual, stop harassing me from your position of relative inexperience and ignorance, it is getting tedious.

No Lala
The H130 has Pedal Position sensors (or transducers) and alters it's Nr because of the inputs recieved therefrom. The offical reason is some nonsense about being quieter (and it is very quiet), but the real reason is likely to be to produce more TR thrust in the hover.

so you've basically copied Lala's suggested reasons - great depth of knowledge - well done! The major selling point of the 130 (apart from the size of its cabin) is the very low noise signature - that will be why it is done.

A yaw pedal position transducer can be called a position sensor since it provides the AP (or FADEC) with the actual pedal position which will be representative of TR pitch.

A yaw sensor would not be in the yaw control run but would be an output from the yaw rate gyro/horizontal gyro and be used for rate damping or yaw synchronisation functions.

You must have to wear sunglasses to prevent your (self-assessed) glittering intellect from blinding you;)

crab you seem to be slowly understanding what i said but you are so damn rude i just dont understand where that comes from
when I told you there were Airbus products with PPI that command Nr change you (and several others) didn't believe it, now you find that that is true you accuse me of poor knowledge and are sarcastic again about sunglasses etc I answered Lala's question, yes he was right about that part.
You don't understand the difference between a yaw sensor and a control position transducer, I explain it to like a 5yr old and then you tell me it as if I didn't know in the first place. Your constant personal attack, almost always where you turn out to be wrong diverts threads from the topic and makes discussion really unpleasant. Are you a rantallion? Can't we just stay on topic?

WhoKnows: "@SLB: I think this is about the indian BK117 look-alike, the infamous HAL Dhruv..." probably right, black fox gives India as location
worth noting 4 out of 7 Ecuadorian Dhruvs crashed, what a total waste of time carrying 2 engines, simple reliable performance is the best safety asset, certainly at least in this type of arena. (composite saving 50% of weight etc etc)

SLB: unlikely to be Guimbal, black fox says torque rise on both engines

Err no - you were quite insistent that it is a transducer and not a sensor - that is not the case as I patiently explained - a yaw transducer and a yaw position sensor do the same thing - they give a signal proportional to the yaw pedal position. And they don't directly command the Nr change - that is what the FADEC does since it, and not the transducer/sensor/transmitter:ok:

and here we are back on your favourite argument which you get all threads around to eventually - talk about not staying on topic:ugh:

PS - you really need to get a sense of humour;)

I indeed overlooked the 2 engines phrase.
So probably a Dhruv although there is 1 EC130 on the Indian register: VT-GVO.
Now we have sorted that out, are any of the previous posters who wanted to know the type, familiar with a Dhruv?

SLB

crab you are very confused. Your transducer and sensor point is irrelevant (the sensor IS a transducer duh) YOU said YAW sensor but it does NOT sense YAW it senses PEDAL POSITION duh, wake up

Me "The H130 has Pedal Position sensors (or transducers) and alters it's Nr because of the inputs recieved therefrom." it: the 130 because of the inputs recieved from the transducer. Of course the Pedal Transducer doesn't directly alter the Nr, there are components in between.

You are a waste of time, what do you think about the Dhruv?
The twin concept depends on logic of your level for it to exist.

Tedious? Look in the mirror, AnFI.

For you to opine that crab@ speaks from inexperience and ignorance, even with the defining 'relative', is exceptionally offensive. Knowing the backgrounds of you both there is no doubt in my mind who has a more logical, sound and knowledgeable approach to helicopter operations. And it isn't you.

Could you give us just the slightest of hints as to the level of this prats experience John? Not talking about you crab. ;)

@SLB: See post #9 [;