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I came across a Captain Joe video in which he had said that gliders require much more rudder in straight and level flight, as opposed to powered aircraft. Can someone explain the reasoning and physics behind this?

The other thing that is so different to flying a propeller-driven plane is, you really have to use the rudder a lot to fly straight and level. You don't do this much actually in a propeller-driven plane or on the jumbo, I mean only for the landing, really, or for takeoff. But other than that, you don't really use the rudder much.

So, here, I'm constantly in the rudder pedals, trying to keep this little bit of wool [ed.: video panning to the sideslip indicator] actually nice and straight, there we go!

It's more hands-on and I think here you really learn to fly. We realized just today that I didn't use as much rudder as Stefan did, so that just goes to show that in a propeller-driven plane you don't use the rudder as much, only maybe in a turn, and for landing. And obviously in the jumbo, you don't use it at all, just also for takeoff and landing.

I've learned a lot today, especially in terms of, like, you know, using the rudder again and finding thermals.

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The only reason to constantly hold some rudder input in actual straight-and-level flight is to compensate for thrust asymmetry effects like P-factor, rotating slipstream, etc. So, if you were understanding the caption correctly, it was wrong.

On the other hand, if you are making aileron inputs to bring the aircraft back to wings-level after slight disturbance, you'll find that in order to keep the ball or yaw string centered, you need to make much greater rudder inputs along with those aileron inputs in a glider than in an aircraft with a lower-aspect-ratio wing, due to greater "adverse yaw". So to keep the aircraft close to wings-level in air with any amount of turbulence, yes you'll be moving your feet around a lot more in a glider than in an aircraft with a lower-aspect-ratio wing.

(PS -- by "level" in a glider we really mean "nearly" level. In aviation, "straight-and-level" refers to the trajectory of the flight path (i.e. constant altitude), not the pitch attitude of the aircraft. Normally gliders are always slowly descending, unless flying in "lift", i.e. rising air. No one spends any time trying to hold an actual constant altitude in a glider. But a slow glide at minimum sink rate speed or best glide angle speed is a close approximation of that.)

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  • $\begingroup$ That’s what I thought, It seemed a little confusing when I first heard it. $\endgroup$
    – Boeing787
    Commented Mar 29 at 15:49
  • $\begingroup$ I edited the relevant quotes into the question. The OP probably should have done this from the get-go. $\endgroup$ Commented Mar 30 at 9:23
  • $\begingroup$ I saw this same video and had a similar question. I thought he was talking about keeping the plane straight on the yaw axis when he was being towed. $\endgroup$ Commented Mar 30 at 9:28
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Modern sail planes like the one he was flying are built with less yaw stability because it is more efficient to have smaller stabilizers.

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  • $\begingroup$ I dunno, a glider like a Grob-103's vertical fin isn't that much smaller than say a Cessna 152's, is it? And it's acting at a longer moment arm, plus the destabilizing side area of the fuselage ahead of the CG is a lot smaller on the Grob than the Cessna. (On the other hand, the Grob doesn't have a whole bunch of fuselage side area way aft of the CG either -- that actually might be most importance difference between the two in terms of yaw dynamics, other than span and aspect ratio.) I still think span and aspect ratio are ultimately the dominant factors in creating the difference. $\endgroup$ Commented Apr 1 at 21:30
  • $\begingroup$ One more comment, many older gliders had even smaller fixed vertical stabilizers (but larger rudders) than modern ones. (E.g. Schleicher Ka-6 and Ka-2 / Ka-8, and especially Scheibe Bergfalke.) The resulting effects in terms of yaw stability dynamics, especially if the pilot decided for some reason to take his feet off the rudder pedals entirely were-- as you would expect. $\endgroup$ Commented Apr 1 at 21:34

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