Transient TQ when turning left

[Jeep]

Please help settle a bet.

Why does the TQ transiently increase when turning left in an american/british designed helo.

A. Increase in lift/drag on advancing side

B. Increase in drag on the retreating side due to high angles of attack.

C. Something else.

Facts, figures, drawings and stories appreciated.

PS. sorry if this has been done before.

[paco]

The instructor's hands are on the controls? :O

Phil

[Thomas coupling]

do a search - this has been done lots before.

[helmet fire]

Look for roll induced torque spike

A simple (but not necessarily scientific) explanation is that the angel of attack (AoA) increase on the advancing side creates an increase in drag, and the reduction in AoA on the retreating reduces drag. Because Velocity is squared in the drag equation, the drag increase on the advancing side is greater than the drag reduction on the retreating side, thus the total system sees a drag increase and therefore a torque spike. Roll faster, spike gets bigger!

[NickLappos]

helmet fire,
Unfortunately, your explanation does not say why the effect is noticed in a zero wind hover.

It is not the turn that makes the transient torque, it is the roll rate, BTW. When in a constant bank angle, the torque difference is nil.

We studied this a bit back when we were trying to understand ar combat maneuverability, and documented the effect, but never actually explained it.

[VeeAny]

Just an idea but.

In nil wind if a roll rate is developed towards the retreating blade, does the AoA of the retreating blade not increase, due to an effective decrease in the induced flow velocity as seen at the blade. I am not saying the Induced flow actually changes, but if the blade is flapping down the inflow angle is reduced.

Drag increases with increase in aoa, but the Lift Drag Ratio also changes and the total reaction leans further back on the retreating blade, than it correspondingly leans forward on the advancing blade. Hence Torque Increase.

I've done no calculations to support this, its just an idea.

Nick your thoughts please ?

[B Sousa]

Nick
Im sure you saw plenty of tq changes in the Cobra. It was notorius as was the UH-1M for differences in Tq during right or left turns on climbout...........
One of the first things I was hammered on during transition.

[helmet fire]

As I think I said Nick - not a scientific explanation. And "Roll faster, spike gets bigger". The explanation I gave is consistent with the roll rate observation, and with the steady state angle of bank situation - but doesn't cut it in the hover.

I have not noticed it in the nil wind hover, but will try and remember to next time. BTW VeeAny, a nil wind hover situation has no retreating or advancing side to the disk as both are the same speed.

I have also noticed that the more pitch you are pulling at the time of roll, the larger the effect seems to be, as it does for forward speed. But that is an anecdotal observation, I have not measured it, and will have a look next time to see.

Brian, when teaching UH-1 Gunships, the torque spike was one of the hardest things to get through to experienced pilots transitioning to Gunships as they had rarely , if ever, had to consider it during normal utility operations.

[NickLappos]

helmet fire,

I have a sneaking suspicion that the transient torque could be a tail rotor effect, since it shows up in a hover (iIactually tried in 30 knots rearward flight in a Black hawk, and the torque still jumped up in left rolls. I will have to think some about this!

[helmet fire]

Nick, the last time I went 30 kts backwards in a Black Hawk my copilot went white, the crewie screamed "let me out", the crowd ran for cover, and the last thing I was wanting to look at was the tq gauge!! I guess that's why I am not a test pilot! But your observatoins are very interesting. Is it possible there is some effect in the hover related to tail rotor (otherwise why would it be consistent to the left regardless of advancing/retreating) which also manifests itself in forward flight? Or is there two seperate effects manifesting themselves in the same indications?

the plot thickens.

[Jeep]

So answer D then, a combination of effects?

Helmut: I can understand that the increasing pitch on the advancing side and decreasing pitch on the retreating side will indeed have an effect due to the faster v squared on the advancing side, but is that the only thing that is causing drag to slow the nr? What about the retreating side increase of angle of attack?

I have noticed that in the hover, with nil wind, the tq still goes up when banking left. In that case there is no advancing/retrreating side. How about the force needed to overcome the gyroscopic 'rigidity in space' when banking? Of course I am not serious, but there must be something else.

So far I am inclined to think it is both the advancing side v2 plus the retreating side increase in angle of attack which causes an increase in drag, which in turn demands a tq rise from the HMG.

Nick we are teaching maneuvering flight routinely now at rucker, which is why the question has pitched up once more.

I still havent won my wager though, keep it coming.

[NickLappos]

Jeep,
Unless I am all wet, the effect can't be any type of retreating blade thing, because the effect happens in a hover. It is also symmetrical, in that a right roll drops the torque. I will do some test maneuvers next time I am up, too.

Jeep, if you instruct at Rucker, say hello to my step son, 2LT Steve Irving. He is there right now, initial entry, in tactics in an OH-58D.

[VeeAny]

Apologies for Advancing/ Retreating Terminology, I should have said blade at 270 or 90 degree azimuths (was cutting corners) . Yes in no wind no difference as far as rotor system is concerned. However aoa still increases on downgoing blade , is that a better to say what i was saying.

I suppose my idea fails miserably if you go backwards though.

Having read the extra testing you boys have been up to i'll go with option (d)

V.

[Ogsplash]

Never noticed in the hover but the effect certainly becomes more noticeable with increasing airspeed. In the Seahawk, it is negligible around 40 knots and less but at 160....certainly very noticeable then! Also very noticeable if increasing nose pitch attitude and rolling to the left while in the pitch up.

Have seen it iin every helicopter type regardless of the tailrotor/fenestron/whatever.

[Jeep]

Nick,

I certainly will.

Ogsplash,

It doesnt happen in a lynx, as the hmg allows for some droop (old design).

[MightyGem]

Jeep, and you're an A2!! I thought that you were due back from Rucker
last year?

[Jeep]

Actually old chap I am an A1 and I come back in June this year to blighty. Just trying to settle a bet with some chaps this side of the pond.

So if it happens in hover/nil wind it is not just advancing and retreating side stuff. It must be something to do with which way the blades spin. So that means it is something to do with accelerating a spinning disc in a direction 90 degrees out from the tip path hmmm :) Or tail rotor ......

I cant see me winning money from this one, but at least i might not lose my dollar either.

[VeeAny]

Sorry guys, not trying to hijack the thread can someone tell me how the uk military instructor qualifications work ?

[Simon853]

Coriolis effect?

Si

[407 too]

i have a sneaky feeling this is going to boil down to torque forces generated in the transmission and shaft (flywheel disturbance thing) that the engine has to recover from during this manouver (left) or gains from (right)

i guess i am trying to say it is a reaction to a mechanical force, and nothing to do with the aerodynamics of blades

[Jeep]

407: Nice idea, an increase in rotor profile drag (is it still called that?).

Simon: Finish at CFS as B2, 6 months (army) later upgrade to B1. For those that wish to, there are A2 and A1 upgrades should they wish to 'improve'.

[407 too]

no, i don't think rotor profile drag

i think this would happen to something WTHOUT a rotor.

an engine would have to apply more power to maintain RPM with this force (drag) applied (opposite for right)

[[email protected]]

Jeep, long time no hear - I don't remember this happening in the hover in the Gazelle but I agree about the rate of roll - you remember we used to demo it going from a left turn into a right turn to show the greatest Tq spike. We also taught that it was due to the nature of the governor ie a reset one with no static droop.

[Winnie]

Now this is purely uneducated drivel, but could it not have anything to do with the way the shaft spins? (Direction of rotation). (roll against direction of rotation, as opposed to with the direction of rotation?)

Just like the increase in hover height in a turbine with pedal turns (As explaind by Shawn Coyle).

Just a thought...:confused:

[Jeep]

Crab,

Still alive and kicking matey. I remember the demos well. Static droop allowed in the gem hmg, no static droop in the astazou. The 701 series in the Apache has an impressive tq sqike if you arent careful. The digital control unit, not a full fadec, controls the tq spike when over the 100% mark but its rather easy to get a 30% tq swing when happily winging it around in roll and pitch (for other reasons of course). Hence the questioning of why does it do that. I have heard a few different reasons over the years and thought the experts would give me a defining answer. Of course to your normal 'student' it is easy enough to prove any of the answers to their satisfaction, but to the more enlightened the awkward question can always upset the board work.

Winnie,

I am inclined to think spinning objects/gyroscopic effects are something to do with it. I am waiting for someone to prove it to me :)

[2beers]

Does this happen to all helicopters?

If it does, then shouldn't it be noticeable in autorotation as well? Of course the rotor will spin faster in the turn, but it should wind up faster when rolling right than left if there is some sort of resistance in the rotor-system, gyroscopic or mecanic. Or will it be too small difference to notice?
If there's no difference maybe that shows that the reason for the spike is on the engine-side of the freewheel and not the rotor-side?

Or probably I'm way off, it's known to happen :)

/2beers

[Jeep]

2beers,

I am sure it is happening in auto, should you put a roll input in, but as your NR is allowed to fluctuate in autorotation, the transient rise or fall is masked - the nr moves instead of tq change.

Have another one.

[407 too]

i agree, if it is some sort of mechanical drag, you should see a difference in rate of rpm increase

will give it a try next time i decide to chop the throttle (should i warn the Px first:uhoh: ) :ok:

[[email protected]]

It is the stated fact that this happens in a zero wind hover that is perplexing - the occurence in fwd flight is easily explained by the changes in drag on the advancing and retreating sides as mentioned earlier.

To produce an increase in tq when rolling left (US rotation) the aerodynamic load must increase as drag rises and the Nr tries to decay - therefore the opposite must occur when rolling right.

Any gyroscopic theorists have any data about precession rates with and against the direction of rotation - does a gyro speed up if it precesses against the direction of rotation (spin axis)?

As soon as you remove the advancing and retreating side inequalities from this argument it becomes very confusing - any yaw due to sideslip that might have the effect of increasing or decreasing the Nr would be very small and short-lived in the hover, unless you were prepared to let it sideslip into the ground for the sake of science.

[MightyGem]

Not sure if this will help. Just been flying, EC 135 with full FADEC, 80kts S&L, fairly vigorous left turn to about 30 deg of bank gave a Tq increase of between 3 - 5%. Corresponding drop in Tq for right turns.

Don't ask me what it means. That's why I never got past B1!

[NickLappos]

crab,

It was me who asserted that it is there at all speeds. I did hundreds of maneuvers in 'Hawks and S-76 to understand the nature of maneuverability once upon a time, work that lead to an AGARD tech report and several AHS papers, and that helped lay out the Comanche's main and tail rotor systems. When next in the cockpit, (next week) I will repeat same to confirm.

My speculation then and now is that the engine/drivetrain system, including tail shafting, is all rotating the same way, and the delta torque of that system is a contributer, perhaps via a conservation of angular momentum. The effect is not in pitch in a hover, telling me it is not purely a main rotor effect (not H force, for example) and it follows roll acceleration and rate, even into rearward flight.

I will do some calculations based on inertias and velocities in a few days (when my ears stop ringing from having been boxed!)

[Steve76]

Its been a long time since I studied the aerodynamics; so patience with the lay terms and my ignorance.

Is it possible that the inc. in TQ turning into the advancing blade is correlated to the coupling through the main rotor and the thrust from the tail rotor.

You could also ask the question "why do helicopters turn more easily in one direction, than the other"

Two forces to the right (USA) assisting the turn, only one through the mast for a left hand turn.

Thus it takes more "effort" for the aircraft to change its attitude left as opposed to right. Effort = Torque.

Has anyone noticed at the start of the reality TV show "The Apprentice" when the S76 does the break to the left that the tail "tucks" into the turn? Sloppy flying or something else?

[[email protected]]

Nick, if it is not noticeable in pitch then that must add weight to your drive-train theory since pitch attitude changes are in line with the drivetrain and the Tq spikes are only caused by attitude changes perpendicular to the drive-train.

Steve76 - you have hit on a question that has long been unanswered 'why does the helo turn more easily one way than the other'. I have always found wingovers easier to the right on a US rotation and to the left on a French rotation which correlates to the reduction in Tq when you roll against the direction of rotation. Has TR thrust got anything to do with it? I'm sure it has but not as the major cause.