The Case for Free Flight Aerobatics

by Wayne B. Patten

I wish I could say that I am an expert in free flight aerobatic model designing and building, but I assuredly do not dare say that. Ignorant that I am, I still might be the world’s leading proponent for the discipline, perhaps the the world’s only proponent.

Read any FF experts opinions on the subject and you will be told how to make a stable flying model that is light and can resist the vagaries of the atmosphere we have here on Earth. You will learn how to maximize the flight time and how to build nice and light and strong. I have to say there isn’t a thing wrong with what they are advising.

But it is not the whole picture, either.

I learned this at a church youth model airplane building activity using the AMA Delta Dart 40 count kit package. At one particular session I went to retrieve the rubber motors for the youth who had their airplanes finished only to not be able to find them. I had a spool of my personal rubber with me, so I cut motors from that and sent the kids off to fly in another part of the hotel room. Finally getting a moments break from the twenty or so kids I was working with, I looked around the room partition that separated the two areas only to see the Delta Dart doing several loops over the kids heads rather than a climbing spiral that it normally would perform. The room supervisor was right there so I commented that that was not the way that they were supposed to fly, but I guessed that it was okay for in here. The supervisor responded, “Oh no, it’s perfect, they are only using one corner of the room and having a blast.” Hearing comments from the kids like, “It’s diving on us, it’s diving on us!” I had to agree.

I am going to stop here for a moment. I suspect that the century and a half of deeming free flight only capable of endurance flight trim has simply been an entrenched assumption. Consider this: If a free flight model can be tuned to avoid maneuvers, then it should also be possible to tune it to maneuver. Some would say that that is asking for a crash. Ask, or not, endurance FF designs crash anyway. What’s to lose?

The Delta Dart flights revealed a potential that is necessary for successful FF aerobatics. That is the ability to pull out of a loop slightly higher than entry altitude. I wondered how to pursue this characteristic and began designing prototypes to that end, using the P-51 Mustang fighter as my reference. The second prototype had the flight characteristics, but lacked crash resistance (and more on this later). The third prototype had both with a modification that my intuition hinted at. Mind you, this whole project came out of intuition, not mathematics or logic. In fact, as near as I can see, it is difficult to use logic. I will attempt to in the next paragraphs.

In general, with traditional, that is, endurance mode, FF balance is about 20 to 33 percent aft of the wing leading edge. Wing tip wash-in mollifies stall. Dihedral is used to limit roll axis variations. Rudder is tuned to keep the model stable in yaw and spirally resistant. Tailplane is tuned for glide and turn. Down and side thrust limit power climbing. Motor torque is supplied in moderation. If a given model is well tuned it will even fly out of sight. A school yard is considered a small flying field, save the modelers that fly indoors.

I do not know of any way to incrementally move from endurance mode to aerobatic mode. But since they are sort of opposites, why don’t we just reverse what we can and see what happens? Balance? Put it back near the MAC. Dihedral? A little if you want, or build the wing flat and and have slight anhedral. Motor power? All the torque the airframe can tolerate, climb be damned. Wing incidence? We use 3º positive since we still need lift, forget wash-in. Tailplane? 2.5° negative was a workable starting point, experience has revealed that tailplane incidence depends on wing location. Tailplane is sized to “let go” at low end of flying speed. Rudder? Big enough to handle the motor torque, offset as needed.

How does all this work? The good fortune of my first two designs, the Z-51 and the Mohawk, created a false sense of mastery. The following designs would not fly when the parameters of the first two were applied to them. In time I was able to figure out that wing down wash has a direct effect on the tailplane. Of the two Z-51 models I am currently flying, the one with anhedral flies with the most exuberance. It will often exit a loop fifteen to twenty feet, 5-6 meters off the ground. Its wing developed a warp and it still flew well rolling to the right and landing normally. The Mohawk flies larger loops and can break into chandelles (sideways loops, really), climbing for the entire motor run. Since I mentioned a wing warp, I will add an aside; the wing design is certainly different, but it is not hard to build. I apply the tissue, spray the water on and lean the wing up against something to dry. It seldom dries with any warps. It has proven to be a light, strong and readily repairable structure.

There is an inter-coupling of features I have found necessary for FF aerobatics. First the motor cross-section is determined by how much torque the airframe can handle. This essentially dictates the all up weight. Coupled with this is a two point propeller requirement. The propeller needs to be pitched just below the pitch that would cause the airplane to stall in a vertical climb. This keeps propeller rotation speed as low as is possible. Coupled with this is a propeller with as low a mass as is possible. The low speed and low weight keep the gyroscopic forces as low as is possible. This reduces the resistance to axis change, which is needed in aerobatics. Rather than being resistant to change, we want to readily induce change in a constant and consistent manner. Read the previous sentence several times to break the old FF engram.

Looking at a simple loop, the initial high torque gets the aircraft going vertical, slightly slowed by gravity which extends the climb just slightly as the tailplane has less influence. From about 45° inverted there is acceleration which increases tailplane influence, in turn tightening the loop radius. This enables the aircraft to exit the loop higher than it entered. The increased speed of the dive, augmented by the high pitch propeller, increases tailplane influence and wing lift.

I mentioned moving the balance point back to the MAC earlier. In truth, a normal balance point will fly aerobatics alright. A rearward balance point adds something; the ability to roll without a nose drop. I have seen the Z-51 do a 360° roll, getting blown sideways two or three meters and fly on unperturbed. I have also seen this plane be rocked violently side to side by the wind and fly on unperturbed. You can be sure that I have reservations in accepting the presentations of established FF gurus.

If I have any reason to pursue FF aerobatics it is this: I think it has the potential to lift FF from the ranks of a few dedicated die hards to the realm of a world wide STEAM hobby. As a rubber powered model, it is too light to do any damage when it strikes something, or even someone. Add to that the factor that only a small space is adequate for flying. Add to that the quietness of rubber power. Add to that the restrictions RC is now facing with the FAA. Add to that the limited workspace and tool inventory needed for construction. Add to that the fact that most of the world’s populace are urban and suburban residents. Add to that it is a rewarding challenge for those burned out on video games.

Maybe FF aerobatic times are near...