Eliminating torque roll

wolfy

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I think Dardow just put the link up from twisairs original post.
Would almost appear that the cyclic is being used to manipulate the results a little (bit of right stick before and a bit of left after the gurney flaps are installed) It may be the camera angle though hard to tell.
I still like the idea though I am going to try some on my new build.

wolfy
 

Brian Jackson

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As an academic question, how does this gurney flap differ from a trim tab other than its shape? I have read that a flat trailing edge (instead of a sharp pointed one) produces less drag. Is it something to do with this effect?
 

Doug Riley

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A trim tab is used on a hinged (or torsionally warp-able) airfoil surface to "lever" the 'foil into changing its AOA or camber. As a result, the force generated by the foil increases in the SAME direction that the tab is deflected. E.g., down tab on an elevator levers the elevator surface UP, causing an increase in the elevator's down-load.

The Gurney, IIR, is attached to a non-rotating, rigid foil (such as a race car's spoiler). It adds load in the opposite direction from the direction the flap points. Up-Gurney increases down-load, and vice-versa.

BTW, there's no free lunch. Generating a force by means of an airfoil creates induced drag. The force itself may also add trim drag -- e.g. if the force is a down-load that adds perceived "weight" to the load on the main rotor or wings.

Interestingly, the prop's torque reaction does not, in itself, add to the rotor's load. That's because torque reaction is a pure couple, or twisting tendency. If we counter the couple with a straight-line force in the "down" direction, however, we do add to the rotor's load.

In the same way, the torque reaction from the rotor in a helo does not accelerate the frame in any direction other than rotation about the yaw axis. If we use a tail rotor to counter this torque, however, we're applying a straight-line force that makes the helo want to slip sideways. In contrast, a coaxial helo counters one pure torque with another pure torque. No tendency to slip.

We could counter a gyro's prop torque with counter-rotating propellers and suffer no induced drag or trim drag. But we'd be adding a lot of weight, cost and failure modes for a very small gain in efficiency. Sometimes the cure really is worse than the disease.
 

Brian Jackson

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Thank you, Doug, for that educational explanation. Perhaps I misused the term then. The "trim tab" I was attempting to describe was a simple bent sheet strip I've seen mounted to trailing edges of rudders & HSs. I recall seeing this type bent to ~20-30 degrees. The item being discussed in the OP reminds me of that except it's bent at a right angle. Are these both considered "gurney flaps"? My question earlier was due to wondering if the 90 degree style was somehow more efficient than the shallower-angle style that hangs off the trailing edge.
 

Doug Riley

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Brian, I'm unsure if I understand all of what's going on with Gurney flaps. If they were closer to the middle of the airfoil, they'd be spoilers (in the airplane sense of the word; car-racing "spoilers" are really horizontal stabilizers). To my ignorant eyes, Gurneys appear to be not qualitatively different from elevators or ailerons (i.e camber-altering devices). But there may be more to it than that.

But, yes, the type of trim tab I mentioned is what you're picturing -- either a small plate sticking out aft of the trailing edge, or the trailing edge itself bent over from 0 - 40 deg. The key fact for all trim tabs is that each is mounted on a MOVABLE/WARPABLE airfoil.

These tabs are servos. That is, they work by deflecting the movable/warpable airfoil; the airfoil itself, not the tab, then creates the desired force.

Gurneys are attached to a nonmovable/nonwarpable airfoil. They themselves create the desired force -- download, in the case of racing-car "spoilers."
 
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