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In this answer to a question about controls, Peter Kämpf mentions the Klemm 35:

...the rudder has two narrow strips which are deflected slightly to the left and the right, respectively. This increases the force gradient over deflection for the first few degrees when the rudder is moved away from its centered position.

How does this interact with left-turning tendency coming from the engine slipstream? Normally, small movements of the rudder are needed to counteract the effect of the slipstream on the fin, so increasing the force required for small movements would seem to hinder rather than help the pilot.

On the other hand, I'd expect the right-pointing strip to be in the lee of the fin (as the slipstream passes left-to-right over it), which would reduce the force on it, letting the rudder move a little to the right on its own, which is exactly what you want.

What actually happens? Does the neutral position of the rudder change at different power settings when flying straight and level? Does this arrangement reduce your foot workload compared to other aircraft?

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  • $\begingroup$ @rbp I don't understand your edit. Why only left? if it turns right it would not apply? (en.wikipedia.org/wiki/Adverse_yaw) $\endgroup$ – Federico May 2 '16 at 13:37
  • $\begingroup$ Except for some very old planes, all singles props turn CW and have a left turning tendency. See paragraph 3 in Phliar Phil's answer $\endgroup$ – rbp May 2 '16 at 15:36
  • $\begingroup$ Perhaps you forget that I fly an aircraft with a CCW prop, which has a right-turning tendency. $\endgroup$ – Dan Hulme May 2 '16 at 15:43
  • $\begingroup$ How could I ever forget about you @DanHulme? Your plane is a 1930s model, which i put into the category of "very old." Its the same thing in Helis. Right pedal as you add power, unless you're in a Eurocopter, then its left. "step on the down-traveling pedal?" $\endgroup$ – rbp May 3 '16 at 5:13
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Adverse Yaw created by the propellor will vary based on aircraft and design. Lots of modern single engine aircraft do things to counter act adverse yaw but if we take a pretty neutral aircraft design and apply the tabs you reference all you will see is a slightly more aggressive rudder. Since there is a tab in each direction in a centered situation they will cancel each other out however they will cause the rudder to have a more drastic impact at smaller angles of deflection.

What actually happens? Does the neutral position of the rudder change at different power settings when flying straight and level? Does this arrangement reduce your foot workload compared to other aircraft?

The neutral position of the rudder will always change a bit as the power settings vary. For this you have rudder trim. Generally speaking a lot of GA planes have a bit of left rudder trim always in to counteract P-Factor some even have permanent design features to do so. It should also be noted that P-Factor is most prominent at high angles of attack and high power settings (takeoff and climb out) generally this also means lower airspeeds. Once you settle into cruise its effects are decreased. I have never felt that it adds to my work load when flying but Ercoupe pilots may have a different opinion on that.... The design you mention does not really reduce workload, for that you would need something like a yaw damper some new planes like the Cirrus SR22 have them.

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First, "adverse yaw" refers to the increased drag of a down aileron (and reduced drag on an up aileron) -- which mean that as you deflect the ailerons to roll left (in order to turn left), the adverse yaw makes the nose yaw right -- exactly the opposite of what you want. Therefore you balance any application of aileron with just the right amount of rudder to counteract the adverse yaw. This is called "coordination".

For any control surface (like rudder or aileron) there is a small "dead zone" around neutral, where movement of the surface does not have an appreciable aerodynamic effect. Those narrow strips reduce the size of that dead zone and improve control. Therefore they help, not hurt.

What you're talking about is what the FAA (and others) call "left-turning tendency". When flying straight and level, the engine power affects the amount of rudder required, and significantly so. At high power more right rudder is required, and especially so at low speeds like climbing. At low power settings less right rudder is required, especially at high speeds -- so in a descent many aircraft require left ruder to straight. (Of course this is most applicable to single-engine airplanes where the fin is in the propwash.)

To help the pilot with the forces required, surfaces have trim tabs -- see Why Pitch Trim Up/Down & Roll Left/Right switches on yoke or control stick?

(Incidentally, many older small English engines like the dh Gipsy engine, https://en.wikipedia.org/wiki/De_Havilland_Gipsy_Major, turn the opposite direction so airplanes like the Tiger Moth that use that engine have a right-turning tendency and require left rudder.)

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    $\begingroup$ You might want to add that on smaller airplanes those trim tabs are just fixed strips of metal at the trailing edge which can be bent (while on the ground) to trim the zero point of the control surface. $\endgroup$ – Peter Kämpf Aug 30 '15 at 7:33
  • $\begingroup$ Thanks for the interesting tidbit about the airplane I fly. I feel like your second paragraph starts to answer my question, but I still don't really understand how (if at all) the self-centring changes the need to use rudder at different power settings. $\endgroup$ – Dan Hulme Aug 30 '15 at 22:32
  • $\begingroup$ It doesn't change the need at all -- no matter what improvements you make, you will need different amounts of rudder at different airspeeds and throttle settings. Unless you fly an airplane that has equal amounts of fin (vertical stabilizer) above and below the centerline. $\endgroup$ – Phliar Phil Sep 10 '15 at 2:41

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