Torque Spikes

[CPGAH]

I am fairly content with why, the Torque will rise when rolling right (CW rotors) in forward flight.
I am less clear as to why the torque reduces less significantly when rolling left in a similar manner. I have drawn many vectors but just as many blanks.

Any answers that don't involve calculus would be greatly appreciated.

[Aucky]

Stab in the dark, not entirely sure what your asking, do you mean whilst maintaining level flight in an in trim condition turn? If so, in fwd fight the CofG would usually be a little behind the rotor mast due to the balance between the forward thrust at the head and drag on the airframe (reduced by a horizontal stabiliser).

If the CofG is behind the mast then as you roll right you will need to add a little collective to maintain level flight and you will need a little right pedal to maintain the trim (CofG falling slightly into the turn behind the mast pushing the nose left) - both of which add TQ in CW system.

When you roll left you will need to add a little collective again to maintain level flight, but you will need a little left pedal as the CofG aft of the mast will try to push the nose slightly to the right. So pedal is reducing TQ (CW system) but collective input increasing TQ.

As I said, not sure if this is what your asking...

[CPGAH]

Aucky, I was interested in the transient Tq spike whilst rolling around the longitudinal axis, not the increase reqd to maintain S&L.
As far as I understand, when rolling Rt, the Tq will rise due to the difference in V of the advancing and retreating blades (assuming fwd flight). The Lift increase on the advancing side will be multiplied by the V squared which will be greater than the reduction of Lift on the retreating side. this leads to a net increase in Lift (whilst rolling), and therefore a net increase in Drag too. Hence a short lived Tq Spike. Add to that a bit of right boot to maintain balance as the ac wants to yaw left in sympathy and you have a noticeable Tx Tq inc.
It looks like the opposite should be true when rolling left. Indeed it does, but every time I observe it it seems a much smaller change. there must be some other force masking the effect. I can't think which one though...

[JohnDixson]

Remember that the rotor has gyroscopic properties.

[CPGAH]

JD, That sounds more like phase lag than gyroscopic. the rotors are not a solid body. Perhaps the rotor head could display gyroscopic rigidity and precession but I don't think the whole rotor disc does...

[Arm out the window]

I don't think there's a simple explanation for this one using arrows and relative airflow vectors - I'd tend to fall back on the idea that the rotor disc as a whole is, as you indicate, made up of a number of flexing, flapping, twisting objects whirling at great speed through a fluid, and is furthermore asymmetric in its travel through that fluid (ie advancing and retreating blades), so 'it just does' would be a fair enough comment!

Maybe a better way to look at it is that, because of this asymmetry, it would probably be strange to find that the magnitude of the torque spike or drop was equal for rolls either way. It's not a see-saw that goes up and down the same in both directions, so it doesn't make sense to assume things should be equal.

[RVDT]

Discussed previously here (http://www.pprune.org/rotorheads/160583-transient-tq-when-turning-left.html).

Apparently it is evident in a nil wind hover as well!

[sycamore]

CP,try rolling left and right from a trimmed condition ,and holding pedals `fixed`; and then see what the sideslip(ball) variation is.....

[[email protected]]

If you just roll left and right there won't be any side slip.

[sycamore]

Crab, perhaps to make it explicit,try each seperately....

[army_av8r]

torque spikes are caused by numerous factors, i will assume we are talking about clockwise rotors based on first post. we will assume we are traveling at 80Kias. at this airspeed, the rotor is tilted forward and has some coning angle. because of this, the front half of the rotor disk "sees" a relatively horizontal(clean) flow of air. the rear of the rotor disk sees a more vertical airflow(or induced flow). so in straight and level flight, more induced drag exists over the rear of the disk. when we command a RIGHT cyclic input, the blade over the tail increases its mechanical angle and AOA. the blade over the nose does the opposite and reduces its angle. because the total drag is higher in the rear AND we commanded a larger amount of lift(and Drag) in the rear. the total rotor drag INCREASES. this is sensed by the governor as a droop and fuel is increased. now, for the the LEFT cyclic... first we must discuss blade efficiency as it relates to drag. as angle of attack is increased the profile drag increases in a parabolic manner. for twice the angle, you get four times drag(give or take) so when you turn LEFT you are actually reducing the angle over the tail(in the high drag zone) which means if you cut the angle in half you cut the drag by 4. you still have increased drag over the nose though, and because of this, the torque will decrease but not as much as it increases. factors that affect this are GW(coning), DA, Airspeed, and the RATE and Magnitude of the cyclic movement. ok hands hurt. im done!

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armyav8r - read the linked discussion RVDT has posted and you will see this was done to death - it is present in a still air hover according to very knowledgeable people like Mr Lappos.

[Peter3127]

I'll ask the stupid question ....

For ACW rotor systems do all tailrotor drive shafts (not the TR, the shaft) go the same way? And for the same MR direction is the effect the same in the hover regardless of the shaft direction? (Testing the drive train influence).

Having just written that, and considering the more rigid nature of the TR system, is the roll producing a gyroscopically produced TR Tq that has to be reacted the usual way and hence causing a Tq spike at the MR shaft? Being in the vertical plane and much larger in second moment inertia etc than drive shafts it would be a likely culprit for horizontal plane torque inputs when subject to roll rates.

Time to dig out the Dynamics texts, or a bottle of wine. My brain hurts already.

:ok:

[army_av8r]

When we say that torque spikes occur at a hover, are we saying that the torque goes up for left cyclic and down for right cyclic? what about fore and aft at a hover.(assuming CCW) are we also assuming the other controls remained locked in place. obviously moving the other controls could account for increase decrease in torque at a hover. I will check tonight. going to start from 15 foot hover and "ONLY" move the cyclic left and note any change in torque. then repeat for right. im at a loss right now because i have always taught students that transient torque due to cyclic movements only occur with forward speed/wind. just when I think I have it figured out... i learn something new!!! im curious to see these results tonight!

[CPGAH]

Likewise, the hover in still wind, and indeed, lappos' aft ward test flights have gone against my understanding... When this fog clears, I too shall try it out in the hover...

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Peter - there might be something in that - when you roll right in a CW rotor you are rolling opposite to the TR thrust which is to the left - perhaps this phenomena doesn't occur in a notar.