RADIO CONTROL SCALE AEROBATICS
Eric Hawkinson, 319 Yellowstone Ave., Billings MT 59101
LAST TIME we met, I discussed stall turns and some hints that may be helpful for pilots who are having difficulties with them. Several people wrote that they really enjoyed that column and hoped for more like it. That's good for me, since my primary focus is on flying.
The stall turn is one of those maneuvers that won't usually impress the "locals," even if done well. It's not obvious to the casual observer that there is a fair amount of skill required to do a really nice stall turn.
This time I'll look at some maneuvers that have a slightly bigger "wow" factor. In fact, I'll look at knife-edge flight — the flight regime that usually separates the "men from the boys," to be politically incorrect.
If you watch closely at the field, you might realize that the knife-edge maneuvers really show who has the "touch" and who does not. Plenty of hot-dog "barnstormer" pilots can jam the sticks to the corners and impress the crowds with various snapping maneuvers, with various degrees of precision or predictability.
Some of those same pilots are hard-pressed to perform a perfect four- or eight-point roll. Why? Because any of the maneuvers that have knife-edge elements require that the pilot has a fair amount of skill with the rudder stick.
Knife-edged flight and four-point rolls could realistically be considered the starting points for many of the advanced maneuvers, such as the eight-point, 16-point, or any other hesitation rolls, slow rolls, knife-edge circles, and rolling circles. Any pilot who wants the maximum enjoyment in mastering aerobatic flight had best spend some time on these maneuvers.
Unlike the stall turn, many airplanes knife-edge poorly, and some won't do it at all. The primary factors that will make or break the knife-edge performance are center of gravity (CG) and rudder area.
Plenty of rudder area is almost a necessity for good knife-edge flight. This does not necessarily mean a great deal of deflection; you are better off with a large rudder using moderate throw than with a small rudder that swings 40°. The extreme throws will cause complications, including excess drag and extreme coupling problems. You shouldn't need more than 15–20° of rudder travel for knife-edge flight.
The CG situation is simple: if you have plenty of rudder area and the airplane won't hold knife-edge, you need more power or a farther-aft CG. (Later, I'll look at other design aspects that could be problematic.)
I'll assume that you have adequate power, since the subject is scale aerobatic models, and most scale aerobats will also have adequate rudder. So if you are having trouble holding knife-edge, try moving the CG rearward very carefully. Don't go very far past the rear limit on the plans, unless you have information from a pilot flying the same airplane who indicates that such adjustments are needed — and they are safe.
A good check is to have a more-experienced pilot (if available) check out your airplane. If he or she can fly knife-edge without problems, all you have to do is learn to fly it, and you don't have to mess with setup problems.
Once your aircraft is ready and able, all that's left is for the pilot to master the challenge. That is the point of this hobby, right? Let's get going! (I'm assuming that you have average flying skills, and have spent some time working on the trim of your airplane so that it flies as "true" as you can make it.)
Flying the Knife-Edge
- At a comfortable height for you, fly the airplane at full throttle and stabilize in a straight, level path a couple hundred feet out from the flightline.
- As the airplane passes "center" (the point directly in front of you), roll it smoothly through 90° in your preferred direction. The roll rate should be reasonably fast; this keeps the airplane from departing much from the line you are flying during the quarter-roll. Slow rolls are more difficult than point rolls. Too fast a roll rate will make it difficult to stop at exactly 90° rotation, so use a moderately fast rate.
- If you over-roll or under-roll the model by more than a few degrees, keep rolling until the model is upright and go back to start again. Never attempt to break off the maneuver by pulling elevator! You may not have enough altitude to recover with a half-loop.
- When stopped at 90° (knife-edge), use rudder input to keep the fuselage at the required angle of attack to fly level.
Key points on rudder and timing:
- The rudder input for the first quarter of any quarter-roll from upright will be opposite the aileron you used. For example, if the airplane is coming in from the right and rolling left, you must use right rudder. Put in rudder in the wrong direction, and you will head toward the earth at an alarming rate. Think the maneuver through, and roll out at the first sign of a problem.
- The amount of rudder needed depends on model design, rudder area, CG location, and airspeed. Start by trying half of your rudder stick throw. If the airplane climbs in knife-edge, lessen rudder input. If it loses altitude, increase rudder. There is some "touch" involved here.
- The best entry into knife-edge is one where you start applying the needed rudder nearly at the same time as you start your quarter-roll. This will keep the nose of the airplane from falling during the move. If you wait to input rudder until you are already in knife-edge, the nose will probably have dropped a bit and you'll need more rudder initially to bring it back up slightly past level to hold your altitude. That's okay for first attempts, but as soon as you can, start working on adding opposite rudder right after you initiate the roll.
After holding knife-edge for a few moments, quarter-roll back to level flight using the appropriate aileron, then smoothly release the rudder and relax the aileron. On each attempt, try to perfect the timing of the rudder input and release to make the start and finish smooth. Work toward starting the maneuver before center, with the goal of flying a good four- to six-second knife-edge that is centered on yourself.
Possible Design Snags
When you have rolled to knife-edge and stabilized altitude with rudder, two coupling issues might appear: pitch coupling and roll coupling.
Pitch coupling
- Symptom: the airplane pitches to the top or bottom of the aircraft in knife-edge.
- Causes: vertical CG or stabilizer location, or incorrect decalage.
- Solutions: if trimming for vertical lines and CG is correct, the choices are:
- Manually fly it out by inputting the necessary elevator corrections, or
- Use a mix (mixer) on your radio to add the needed correction.
Roll coupling
- Symptom: the airplane rolls with rudder input.
- Causes: improper dihedral. Too much dihedral and the airplane will roll in the same direction as the rudder input; too little and it rolls in the opposite direction.
- Solutions:
- Reset the dihedral (rebuild or redesign), or
- Hand-correct with aileron input, or
- Use a mixer to cancel the coupling.
Some pilots feel that it is cheating to use mixers to fix coupling problems; I do not. If the airplane is extremely bad, even a mix won't really fix the problem. If the problem is moderate, you can fix it by redesigning and/or reconstructing it, or you can dial up a mix on your radio and fix it.
Since manual adjustment could take many tries and a great deal of cutting-and-gluing, you'll get more flying practice by using a mixer to dial out any unwanted coupling. Manually correcting the flaws is a last choice and takes unnecessary work on your part.
Once again, my scope has outweighed my space. I'll continue this next time by progressing to point rolls. Have fun and fly safely!
Transcribed from original scans by AI. Minor OCR errors may remain.
