Aileron Differential

Thanks to Jim Rimmer and the Heart of Texas Soaring Society “Hot Flash”
Reprint from Past Newsletter

The movement of air currents, whether in large air masses or in small areas around the sailplanes we fly, is completely unpredictable. To compensate for this chaotic movement we must constantly react by adjusting the bank and pitch of our plane. The sooner we see the need for a correction, the smaller the correction can be and the more efficiently (less drag) the plane will fly. Small changes in pitch are taken care of by dynamic pitch stability the pilot adjusted with center of gravity changes during initial trim flights. Roll stability of aileron ship are to some degree provided for by swept wings, dihedral, and lateral area inherent in the design of the ship. Larger corrections in bank or the establishment of turns are made by rolling the ship to the desired angle of bank with ailerons. When we use ailerons we want the ship to roll only on its longitudinal axis. The problem is that to raise a wing the aileron increases lift on that wing with the resultant increase in drag. At the same time there usually is a decreased lift on the opposite wing with a decrease in drag.

The descending wing has less drag and moves forward while the rising wing has more drag and moves backwards. This produces a tendency to yaw (turn) in the wrong direction or into the rising wing and away from the intended turn direction. This usually results in a nose high slip with the fuselage side presented to the relative wind with high drag. This is called “adverse yaw” and is fine if you need to lose altitude with lateral fuselage drag as in a landing approach, but bad for beginning a coordinated turn with the fuselage parallel to the relative wind.

A pilot in a sailplane learns to use rudder in the direction of the turn to compensate for adverse yaw. When the correct amount of rudder is used with aileron the turn is said to be “coordinated”. To compensate for this problem and make flying easier, full-sized planes are usually designed with one or a combination of a number of methods to decrease adverse yaw.

Some jet airplanes have only spoilers instead of ailerons. A more common fix for adverse yaw is to mechanically produce differential aileron movement so that there is more up travel than down. In other planes the aileron is hinged towards the top of the wing/aileron joint so that a portion of the leading edge of the aileron sticks down into the slipstream creating drag when the wing is descending to balance the resultant drag from the rising wing. Other planes couple the rudder with the aileron so the pilot does not have to use much rudder in the direction of the turn. The problem with all these “fixes” is that they can only be adjusted to work correctly within a small range of velocities, usually at cruise speed. As the velocity of the ship changes the effect may be too much or too little and the pilot must learn to use the correct amount of rudder for a given amount of aileron application at different velocities. The problem is almost nonexistent with short wings and long tail moments. This is one reason the rudder usually has a longer moment than the elevator. I have a small 2-meter ship with mechanical differential aileron but with rudder mix by a computer transmitter. The ailerons operate with one servo operating cables to both ailerons. With the aileron horn on top and the cable attachment to the hems a little forward of the hinge point there is more up movement than down on each aileron. Changes in the amount of differential movement would require new hems and cable adjustment. This arrangement works fine at normal cruise but on landing the rudder is too effective causing a bad snap roll at slow speeds. To correct this I simply switch out the rudder mix when nearing the pattern. I don’t forget the switch now after being complimented for some interesting aerobatics while on final a couple of times.

How much adverse is enough? One of the best methods of correcting adverse yaw on an WC sailplane is to use differential aileron (more up than down) with a moderate amount of rudder mixing. But how much is the correct amount? First you must be satisfied with something less than perfect except possibly when making smooth, gentle rolls at thermal speed which is how we should be flying most of the time for efficiency. Heavy control usage always increases drag. This amount of rudder mixing for this type of flying will usually be inadequate for heavy control application. When setting up the differential aileron, no down, all up will not be a problem except for being a little slow rolling into turns. Without rudder mixing you are never increasing lift on a wing and are not increasing the possibility of stall which is the beginning of a snap roll. As mentioned before, rudder can cause snap rolls, thus it is always best to be able to switch out rudder mixing for landings so you can use heavy aileron or rudder separately.

Final trim for aileron-rudder mixed ships is always begun with aileron and rudder neutral, then when in flight trim the aileron for level flight. In level flight the rudder or aileron may need to be trimmed to other than neutral because of a warped wing. Construction problems that require other than neutral aileron or rudder for straight level flight should be corrected. If problems are impossible to correct with repairs, the ship can usually be trimmed to minimize the error. In such cases the pilot simply learns to adjust to the ship’s characteristics.

I often hear pilots complain about some characteristic of their ship. Remember that all planes fly slightly differently, and we pilots must simply learn to adjust to these characteristics. In other words: know your ship! Every plane is a compromise, you sacrifice one flight characteristic for one you desire. With high aspect ratios we get better soaring efficiency but slower roll rate and a greater degree of adverse roll. By using thinner airfoils we get better penetration but higher stall speeds and larger thermaling circles. Any change to a given configuration will usually produce noticeably different flight characteristics. You can’t go to the moon in a Cub, but then a rocket ship can’t land at 35 mph on most of our Texas beaches.

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