Exploring Aerobatics with Rubber Powered Free Flight Model Airpl

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Crossfire is derived from the control line Crossfire from the 1970s, but this time it is free flight rubber powered, and stick and tissue too. Aerobatics is the new-age of free flight, small space flying is now a reality, real small spaces...

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Aristokrat, obviously aerobatic, also rubber powered and derived from European control line kit of the same name. Aerobatic flight allows small space, real small space, flying.

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My third aerobatic rubber powered design. It took some sleuthing to get it flying right, but now it does as the video stills attest.

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Imitation free flight aerobatic flying model was inspired by a control line model of the same name. It is a neat little flyer if you live in the city or any place with just small areas available.

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Updated from the venerable Z-51, my first aerobatic free flight design, the Apache flies much like that model. Build the wing flat to add some exuberance to the flight. A nice small area FF model.

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The Professor is derived from a control line stunt ship. It took some work, but it is now a reliable flyer of loops, so it works well in small areas. The plan showed the vertical windshield, but you can build something sleeker if you wish.

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Mohawk is my second aerobatic free flight design and has always been a great flyer. You never know whether it will loop, chandelle, or some of each.

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Derived from a European control line model by the same name, this Aristokrat flew right off the board, as they say. Aristkrat flies a large loop pattern, but still is able to stay within small areas such as a city lot or corner of an athletic field.

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Free Flight Aerobatic Design Considerations

by Wayne Patten

It has long been an observation of mine that many, especially new, model builders are largely influenced by their emotions when it comes to choosing a subject. The airplane that looks really cool or the airplane their grandfather flew in WW II motivates them to proceed. If this is your feeling about model airplanes, then you will be better served by the likes of, say, Guillow’s, Dare Design, Diels Engineering, Dumas, Easy Bilt, etc. or even the plastic scale kits.

While I do have some semi-scale designs in the works, scale authenticity is not conducive to the requirements of free flight aerobatic flying. And, let’s face it, by and far, most real airplanes are not aerobatic. So lacking scale authenticity, what does aerobatic flying have to offer?

Dynamic creative fun. In your back yard or other urban and suburban spaces.

Most of my designs revolve around a six longeron fuselage and a W-truss wing supplemented with a side grain punctured gluing procedure. I did not like the idea of a bunch of square box fuselages, so I added taper to the cross-section profile of most designs. Without the above mentioned gluing procedure, the fuselage cannot be assembled. The slight twist will cause the side frames to separate at the joints. The gluing procedure also improves crash damage resistance. The extra longerons together with the laminated balsa nose block largely eliminated solid object impact damage. The addition of a nylon propeller shaft eliminated bent shafts. I was regularly bending 1/16th, 1.5mm music wire shafts. The stick structure has an added benefit In that a broken component can be stripped of covering and dropped back on the plan to effect sound repairs. This includes the fuselage. A good aerobatic flight does not always allow a gentle landing. These designs shrug off rough landings.

But understand this; while crash damage is a large consideration for a model flying low, fast in the roughest of airs near obstacles, crashes are not the norm. The original prototype has over 100 flights and was retired for posterity, not damage. Free flight aerobatics can be flown as, or more reliably than, traditional endurance free flight. Strong words, no? If we have the intelligence to tune for endurance, it would seem that the same intelligence could be used for other flight modes. Just put assumptions aside.

The stick structure also allows the use of domestic tissue as there are few or no compound curves to deal with. This tissue is said to stronger than the traditional esaki and the color range is ever-changing and quite large. I use and supply each kit with polystyrene sheet to fill in wing tips, cheeks, canopies, etc. It helps round out a basically square design and address those compound curves.

Some may balk at the built-up balsa propeller, but the truth is, once it is glued up, the builder is about 20 or 30 minutes from a finished propeller. An extended snap-blade knife and a curved end sanding block will carve off the “steps” and smooth it into a functional propeller. I let the laminated assembly dry for 2 days due to the deep glue joints. Also, the propeller can be built left or right handed. You islanders are not disregarded! Yes, I tried plastic propellers. The wrong pitch, high speed and rotating mass do not fill the aerobatic bill.

Some designs are from my own imagination, a few will be from scale drawings, and quite a few are coming from old control line (C/L) designs. I am re-interpreting a few of the large gamut of designs from the control line community. They range from cute little puddle-jumpers to sleek jet types and anything in between. For the former think Tiger, Avon Racer, etc, for the latter think USA-1, Sagitta, etc. Nobler, Crossfire, Smousen, Imitation, and Aristokrat are ready for release. Some of the originals were actually profiles, but good profiles, you know.

Some may balk at the lack of clear canopy. The crash-resistant structure is often not accommodating to a clear canopy and it is an expense that would add considerably to the price with little return. Remember the control line designs? Most of them had simulated canopies, i.e. painted on canopies. The artwork or other simulation is the way to go. Besides, you can add your favorite pilot to the design.

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It took some work to get this T-tail design flying, but it was worth it.

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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. Think about 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.

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...

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Another one of my model airplanes flying in aerobatic mode rather than the endurance (how long can it stay up) mode of traditional FF.