Radio Control: Sport/Aerobatics
I want to share with you several letters I have received from readers. Usually my mail contains questions, and I can answer most of them, but there were two letters that arrived recently with questions that have me stumped. It doesn't do my ego a bit of good to admit that I don't know all the answers to questions about model aircraft aerodynamics, but if any of you can help these modelers out, it'll make me feel better.
The first letter is from Cal Malinka (Rialto, CA), who wrote: "Read your article in the January 1982 issue of Model Aviation with interest. I would like your help in correcting a roll–yaw aerodynamic problem that I have encountered. The Top Flite Contender is one of the best sport-flying models I have owned. It has one fault, however. As you covered in your article, when a rudder command is given, the airplane rolls in the opposite direction! Per your article, this indicates the wing should have more dihedral (the wing per plans has no dihedral). Some years back, I built a .60-size Contender with 2 in. dihedral in each wing panel. This worked fine as far as the roll was concerned; it did not roll at all when given a rudder command, but it did pitch down very severely. It was scary how much up elevator was necessary to overcome the down-pitching effect. Can you give me some ideas on how to correct this?"
"I would like to get a .40-size Contender kit and build it for sport flying, if I can correct the bad aerodynamic tendency I have described. As your article mentions, I should enlarge the fin underneath the fuselage and extend the rudder below the fuselage centerline. Is that all that can be done?"
"I once used a Contender to investigate the left yaw or 'P factor' effect to try to determine how much contribution the 'spiral slipstream' has. I enlarged the ventral fin underneath the fuselage as an experiment and noticed no difference in left yaw during takeoff. I'm convinced this effect is mostly gyroscopic effect when a tail-dragger lifts up—there's hardly any yaw tendency in a tricycle-gear plane. Unfortunately, I was too dumb to notice what effect the large ventral fin had on the roll effect when giving a rudder command."
The strange pitching effect which sometimes occurs when the rudder is deflected has been around for some time. The first time I heard about it was about ten years ago when Don Lowe asked me what caused it. I didn't have a ready answer for Don at the time—and still don't. In most control problems, one is able to uncouple the symmetric control and aircraft motion (pitch) from the nonsymmetric controls and motions (roll, yaw). For example, you don't expect an aircraft to roll or yaw when you deflect the elevator. In the same way, rudder deflection usually doesn't cause the airplane to pitch. Unfortunately, sometimes it does.
There are probably many factors that cause the airplane to pitch when the rudder is deflected, including:
- the effect of the wing and fuselage sidewash on the horizontal tail in a sideslip,
- the effect of propeller wake on both the wing and horizontal tail in a sideslip,
- the direct effect of the change in airflow over the horizontal tail due to rudder deflection.
I really don't know why some airplanes pitch when rudder is deflected and others don't. If someone has good information on the subject, how about sharing it with the rest of us? It would be great if someone had a systematic process for eliminating the unwanted pitch when the rudder was deflected. Then we wouldn't have any excuses for lousy-looking point rolls. (On the other hand, then we'd have to blame it on our lousy flying. Hmmm.)
How about that second letter containing a problem for which I don't have a surefire answer?
It came from Charlie Rector (Fredericksburg, VA) and said, "I am writing to see if you can help with a trim problem we have run across a couple of times for which we can find no explanation or solution.
"Two aircraft, a Sig Kavalier and an RCM Trainer 40, are extremely sensitive to elevator trim—so sensitive that one click on the trim lever will change the aircraft's attitude from climb to dive. The Kavalier was built from a Sig kit and the Trainer 40 was one of the Name Brand (?) prefab kits. Both fly well and exhibit no bad habits other than sensitive trim. We have other Kavaliers and Trainer 40s in the club, and their response to trim is what you would normally expect. We have tried shifting the CG forward and increasing the incidence in the wing—and that did little good. We tried desensitizing the elevator movement, and the only thing we accomplished was that we ran out of up elevator on landings. Neither radio has an excessive amount of servo movement on trim.
"I have been in RC for around 20 years, so I have learned a little about trimming our aircraft, but these two have me (and the rest of the club) stumped. I have no problem flying either ship," he continued, "since I normally trim my aircraft to descend slightly, so I am always holding a little back pressure on the stick. However, these aircraft belong to novices, and they have a very difficult time just flying level. If you can shed any light as to the cause and cure for the problem, please let me know."
Boy, he really stuck me in a box, didn't he? When I started reading the letter, I was sure that it would be simple to suggest several probable causes that would lead to an easy solution. However, Charlie systematically covered almost all of the fixes that I would try. I have only one left; if that doesn't work, I'm out of the solution business. How about the rest of you? Remember that these are airplane designs which have been flown successfully by hundreds of other modelers, so it can't be a problem inherent in the design.
While the rest of you are mulling over the problem, I will have written my last solution to Charlie Rector in hope that it works. Just in case it bombs out, how about writing to me with your solution? (Also, just in case my solution works, I'm going to give it to you now.)
Quite a few years ago I remember that Len Purdy had a similar problem with one or more of his Lanier aircraft designs. He squared off the trailing edge of the elevators and virtually eliminated the trim sensitivity, while retaining most of the control effectiveness in moderate-to-large control deflections. A sharp trailing edge on an elevator will not necessarily make the elevator trim smooth and linear. The way that the trailing-edge sharpness is obtained often causes what is called airflow separation from the elevator on one side or the other, depending on whether the elevator is trimmed up or down. This, in turn, causes large changes in the pitching moment acting on the airplane. If the trailing edge of the elevator is squared off, the elevator trim sensitivity decreases because the airflow separation is triggered at the trailing edge. I hope this was the right solution to Charlie Rector's problem; I'll let you know. In the meantime, please send me any solutions you would like to share with the rest of us.
The last letter I'd like to share with you contained a solution rather than a problem, and it should be of substantial general interest. It came from Dick Snyder (Dallastown, PA), who wrote, "Meant to write a few months ago when you mentioned high fuel consumption when using a pipe. We went through that a couple of years ago. OS Max, Supertigres and Webras would just get through the Expert pattern. If you made long turnarounds, you might flame out about Spin time or before. Then we started using 5% nitro fuel (Cool Power) and not only did the fuel mileage increase, but the power did too."
RC Aerobatics/Van Putte
"RPMs on the ground were about the same, but in the vertical maneuvers there was a lot more power," he said.
"With the OS Max, I can fly a full Masters pattern and three more maneuvers before I have to come in, and there's usually an ounce left of the original 16. So, if you're using the higher nitro fuel (I am — 10% RVP), try cutting back. I rushed out to Sears and got some of that filled epoxy (see February column — RVP). It works great. I've often wondered why someone didn't do that. It's not really expensive, either."
After receiving the nice comment about the Sears epoxy, I was feeling pretty good until the other night when I got a long-distance call from Bud Wolfe (Scottsdale, AZ), who told me that the people at the Sears store had never heard of the product and it wasn't even listed in the regional distributor's record books. So apparently the Sears filled epoxy isn't available everywhere. However, I have purchased it in Dayton, OH, Washington, DC, and here in the booming metropolis of Fort Walton Beach, FL. Since Bud went to the trouble of letting me know he couldn't buy it where he lived, I went to the local Sears store, bought two packages of it, and sent them to him. Now he has one package to use and one to take to his local Sears and show them what he was talking about.
Ron Van Putte 111 Sleepy Oaks Rd., Ft. Walton Beach, FL 32548.
Transcribed from original scans by AI. Minor OCR errors may remain.
