Radio Control: Sport-Aerobatics
Ron VanPutte
IT DOES all of us some good to be called on the carpet once in a while. It was my turn on the carpet when I received the following letter from John Banas (Warren, Mass.): "As a mechanical engineer and avid student of aerodynamics, I am writing because of your comments on aileron differential in the Sept., 1977 issue of Model Aviation."
"Pattern aircraft which do not roll axially are a problem to all of us at one time or another; but this non-axial roll has been in many articles and in many flying field discussions erroneously attributed to the phenomenon of adverse yaw. By some loose definition it could be called 'adverse yaw', i.e. (1) it is yaw, and (2) it is adverse in that it is not helping in any way.
"In any case, true adverse yaw is a result of differential drag, and it can be said without question that if non-axial rolling does exist, and can be remedied by aileron differential (whatever be the preferred method of obtaining it), then it was not adverse yaw ruining the roll axiality.
"The logic behind this is the fact that true adverse yaw can be remedied by aileron differential, provided the roll does not exceed 90°. Beyond this point, the relatively more deflected aileron is now in exactly the opposite situation that the flyer had originally intended, as is the relatively less deflected aileron. From this point on, true adverse yaw will exist in greater magnitude than would occur if no differential yaw was employed.
"Case and point: If more deflection up on the right aileron in a right roll alleviates the so-called 'adverse yaw', then once the aircraft is inverted, the wing on the right has less deflection up than the wing on the left. Exactly the opposite situation now exists as when the aircraft was upright. The bottom line is that aileron differential, though it may improve axiality when properly applied, does not eliminate true adverse yaw per se, unless the aircraft is not rolled more than 90°.
"Hopefully, this explanation will clear up a common misinterpretation and a well intended but poor application of aerodynamic principles."
My comment is that I was caught trying to give a simple explanation about a very complex dynamic situation. It is definitely true that aileron differential will cause exactly the opposite effect, when the aircraft is rolled inverted and elevator is applied to maintain constant altitude in the roll. However, the use of aileron differential does improve axial rolls, since it alleviates adverse yaw at a very critical time — at the initiation of the roll, before the aircraft can develop substantial roll rate.
Anyone who has flown light aircraft
RC Aerobatics/VanPutte continued from page 21
is very familiar with the necessity to coordinate the application of aileron and rudder. If one attempts to roll most light aircraft by using aileron, the nose will generally yaw in the wrong direction before it rolls. Some airplanes, like the J-3 Cub, will end up in a skid with one wing barely higher than the other.
The important fact is that adverse yaw causes the nose of the airplane to move laterally in the opposite direction from the roll before the airplane really begins rolling. Consequently, the roll is begun with the aircraft in a yawed condition. This is what makes the axial roll look bad. Aileron differential helps make axial rolls look better by reducing the initial adverse yaw.
There are many other sources of yaw in a roll, but all of them require that the airplane must be rolling in order to generate the yawing moment which causes the yaw.
The most obvious of these causes is the spanwise variation of wing velocity due to roll rate. This changes wing angle of attack along the span (see Figure 2), so that even with perfectly balanced ailerons there will be a yawing moment. The local wing sections near the tips have higher velocity because of the roll rate and therefore higher lift; the variation in lift distribution causes a yawing moment about the center of gravity. At low roll rates this is small; at higher roll rates it can be significant.
The fin and rudder will sense the local flow due to the rolling motion (see Figure 1) and will generate forces that can either assist or oppose the roll-induced yaw. The direction depends on the sign of wing sweep, fin position, and other details of the airplane. In any case, these are roll-dependent aerodynamic effects and are not related to aileron differential per se.
I would also like to point out that in many aerobatic airplanes the incidence and rigging give some coupling that helps axial rolls, and sometimes differential is deliberately reduced or reversed to obtain the desired coupling. There is no single cure; the solution is to understand the multiple sources of yaw and to adjust rigging, differential, and piloting technique to suit the airplane.
Most contest fliers have practiced coordinated use of rudder to eliminate the early yaw and get an axially clean roll. This means that a small amount of rudder is applied at roll initiation to keep the nose pointed where it should be. The timing is critical — too much rudder before the roll can cause a sideslip and an unclean roll.
If your model tends to yaw off at roll start, try a little rudder and see if that cleans it up. If not, experiment with different amounts of aileron differential, and if necessary check the rigging for wing incidence and thrust-line misalignment.
A final point: when evaluating axial rolls, watch the airplane from some distance so the entire motion can be seen. Up close the small yawing motions are exaggerated and you may think the roll is worse than it really is. Two of the most common sources are the vertical fin and the wing itself. The simplest one to visualize is the yaw generated by the vertical fin.
Visualize the airplane rolling to the right (clockwise as viewed from behind the airplane). The vertical fin will be moving to the right as it rolls (see Fig. 1), generating an angle of attack relative to the local airflow, which is a function of how far a particular point on the fin is from the axis of rotation. This angle of attack will cause a force on the fin in the direction opposite to the way that it is rolling; for a roll to the right, the force will be to the left. The force will result in a yawing moment, which causes the nose to move to the right. Since the airplane is rolling to the right, this yaw to the right is called proverse yaw (as compared with adverse yaw). Since any yaw tendency will make axial rolls look bad, many aircraft designers try to minimize fin proverse yaw by lowering the aerodynamic center of the vertical fin. Placing a sub-fin below the fuselage and having the rudder extend to the bottom of the sub-fin helps move the vertical fin aerodynamic center down. The Dirty Birdy is a good example of this technique.
The yawing tendency of the wing as it rolls is a little tougher to visualize. Again consider the airplane rolling to the right (Fig. 2). The right wing will be moving down and the left wing will obviously be moving up. The roll velocity will cause an increase in the local angles of attack on the right wing and corresponding reductions on the left wing. The actual angles of attack will be a function of how fast the airplane is rolling, the aircraft speed, and the distance the particular wing station is from the axis of rotation.
When a wing generates lift, there is also drag generated due to the lift; this drag is called induced drag. When the right wing moves down in the roll, the lift is increased on that wing because of the angle of attack change which is caused by the roll rate. The increase in lift on the right wing causes an increase in induced drag at each spanwise station on the right wing. In the same way, the decrease in lift on the left wing causes a decrease in induced drag at each spanwise station of the left wing. The effect of these drag changes is a nose-right yaw moment. Like the vertical fin effect, this is also proverse yaw. However, unlike the vertical fin, there isn't much which can be done to reduce this yaw. Consequently, the proverse yaw generated by the wing will always cause problems in axial rolls.
The bottom line is that there will always be some yaw during axial rolls, but that aileron differential helps to cut down the initial yaw and makes the rolls look better. magazine office and I have received many letters (Editor: And phone calls!) requesting that we send copies of the schematic. I hope something like that doesn't happen again, because who knows how many more people would have built the tester had the schematic been available with the article. By the way, I learned that the 74121 integrated circuit (Radio Shack #276-1814) is not available at all Radio Shack stores and may have to be purchased from another source.
A few days ago I received a letter from John Haumersen, an old friend who used to fly with me when I was stationed at Andrews AFB, Maryland (near Washington, D.C.). John currently lives in Bettendorf, Iowa, but he is a retired Army Colonel who was a tank corps commander. Anyway, here is an excerpt from John's letter:
"My main purpose in writing is to report on my use of your equations for locating the CG of an aircraft with varying geometry. I've had a lot of fun using those equations on models for the past couple of years finding that they always worked out, even improving the CG location on some kits. Then about eight months ago I got a T.I. SR52 computer and programed it to solve your equations. Works like a charm!
"I was so pleased that I sent the program in to T.I. and it was accepted for publication. Now any of our modeling friends with access to a T.I. SR52 can obtain the program from T.I. One fellow has already written me that he has used the program successfully on several planes.
"I've tried to make it back to Toledo for the show to see old friends but I missed last year. This coming year I'm going to make or bust! Maybe I'll get a chance to see you there. I look forward to it!"
The equations he talked about were published in the April 1976 Model Aviation.
They work very well for initial center of mass determination for a new airplane. If there is sufficient interest, I'd be glad to republish the equations in a future column.
Ron Van Putte, 12 Connie Drive, Shalimar, FL 32579
Transcribed from original scans by AI. Minor OCR errors may remain.
