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Coordinated Turns and Adverse Yaw

Does your high-wing plane resist making turns? Does it seem to fly awkwardly through turns? And do you have trouble with runway line-up; you just never can get the nose pointed the direction you want it?

Very possibly, the problems described above come from an aeronautical phenomena called adverse yaw. This can affect any airplane, but is most common in high-wing ``training'' planes, since longer wings and more dihedral generally make the situation worse, as does slower speed. The problem starts as you roll into a turn - in the rolling process, the outside wing, the one going up, has more drag than the inside (going down) wing. This tends to laterally angle the nose of the plane (yaw) away from the turn, which is exactly the opposite from what is desired!

And once you finally horse the plane into the turn, the situation perpetuates itself - the wing on the outside of the turn is going faster, and still has more drag. So you end up with the plane turning, but nose high, skidding into the turn. Controlling the plane is harder - and when you roll out into level flight, you find you've turned past the desired heading! This is called an uncoordinated turn, and full-scale aircraft have a turn indicator that indicates when you're making an uncoordinated, sloppy turn - but usually you can feel it in the seat of your pants. It simply feels ``wrong''.

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Flying full-scale aircraft, you can easily correct for this by applying rudder into the turn. Flying the model equivalents, you may not have the skill or experience to do the same. Having a mixer for rudder and aileron is the easiest fix - you simply dial in some rudder with aileron. So when you give the plane right aileron, the radio automatically gives some right rudder, also.

But for those using non-mixing radios (and there are many of us), there is another method to cure the adverse yaw problem - differential aileron throws. We generally have a sense of symmetry in the way we set up our planes, and its a natural tendency to set up ailerons to move the same amount in each direction. For the high-wing Cessna-type planes, we need to change to a non-symmetrical system. Specifically, we want the ailerons to move UP considerably more than they move DOWN. Doing this will create more drag on the lower (inside) wing in a turn, tending to yaw the plane INTO the turn, and curing the adverse yaw problem! Simple, eh?

So how do we do it? Differential throw (i.e. more throw one direction than in the other) is obtained from a servo which puts out equal throw each direction by using non-symmetrical control link connections, either at the servo or output end of the linkage. This works because we have rotary motion at the servo, essentially linear motion in our linkage, and again rotary motion at the control surface.

By connecting the linkage at other than the ``90 degree'' location on the servo arms, the linear travel of the link will be greater one way than the other, even though servo rotation is the same either way. This is illustrated in Figure 1 for a typical strip aileron setup and is the most common means of adding differential throw. We can also get the differential at the control horn, in the same fashion.

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next up previous
Next: Until Next Month... Up: Front Page Previous: Club Instructors are:

Craig Kulesa
Sun Apr 4 00:24:06 MST 1999