Solving Model Airplane Design Problems
Case studies on making your model aircraft design fly well
When you design a model airplane, you will inevitably run into problems during early flights of an initial plane design. For this discussion, I will assume that there are no structural issues with your new balsa park flyer aircraft, and we will focus on problems encountered during the initial ground check and test flights. A large part of the satisfaction from designing and flying an original radio control model aircraft is successfully overcoming these challenges. As there are few original aircraft design problems, a review of the experiences of others is always very helpful. A few of my experiences follow.
Large horizontal tail on prototype design Electro Aviator
When I drew up the plans for the Electro Aviator, I was not sure about the size needed for the horizontal stabilizer. I was concerned over the design problem with a tail surface area that was too small, in that the model could display pitch sensitivity. The initial flights went well. Looking at the pictures of the original prototype, the horizontal tail simply appeared too large. The final version of the Electro Aviator used a smaller tail surface, looked a lot better, and flew just fine. This is a good example of applying TLAR (That Looks About Right) to the design of a normal sport flying RC model airplane to achieve good aerodynamic characteristics as well as aesthetic appeal.
Smaller tail surface on second Electro Aviator
I faced another interesting design problem with the Elector Aviator from the use of aluminum Coverite iron-on covering. I always liked the look of a bare metal aircraft fuselage, and the aluminum Coverite fit the bill nicely. I think this version of the Electro Aviator came out with a very attractive appearance.
Do a radio ground check!
However, it became instantly clear during my radio ground range check that the use of this type of metallic covering had just enough interference with my 72 MHz radio to cause severe control jitter when I moved about 15 feet away from the model. This was clearly unacceptable for safe flight.
Aluminum type covering caused radio interference
The only fix was to strip off all the aluminum colored covering, and replace with regular MonoKote. You can see this on the all yellow version of the Electro Aviator. The radio interference problems disappeared. If nothing else, this is a good reminder to always do a radio ground range test to avoid in-flight model airplane design problems. As an aside, I am quite certain that with the newer 2.4 GHz spread spectrum radios this interference issue would not be nearly as much of a problem.
Use of proper wing incidence
I faced an interesting model airplane design problem with my prototype of the Chickadee. The Chickadee was designed for slow flight, using a light weight model with plenty of wing area. As the Chickadee was to be flown indoors, I needed liftoff at a low airspeed as well.
Positive wing incidence of Bleriot monoplane - see the EAA restoration here
The initial test hops of the Chickadee indicated that the airplane would need more airspeed than I desired to lift off. The motor had plenty of thrust, but the aircraft stayed on the ground until it built up quite a head of steam before lift off.
The solution to this design problem was present in a review of full scale aircraft from the early days of flight. As these pioneers of flight had to cope with the under powered engines of the day, they used light weight, high camber wings set at an unusually high positive incidence to achiever safe flight at low airspeeds. I applied this same technique to the Chickadee, by simply increasing the wing incidence by four degrees. This did the trick, and my model airplane design challenge with a low speed lift off was completely solved. Note that I used this same incidence technique for the Blackburn monoplane with very satisfactory results.
Tips for locating the center of gravity
A final model airplane design problem successfully overcome concerned the Finch and locating the center of gravity. When I first designed the nose section of the Finch, it was to provide a necessary mount for the electric motor, as well as space for the lipo battery. It turned out with the Finch prototype was a bit tail heavy. The battery had to be placed as far forward as possible to get the center of gravity in the proper location. Other design schemes, such as building with foam for some of the more complex curved aircraft shapes, will not affect the center of gravity to any large extent. See here how you can even substitute foam for balsa to produce a lighter version of the Finch RC plane (download a free CAD plan).
Finch nose extended one half inch for center of gravity
Follow up designs of the Finch micro model airplane plan corrected this problem. I moved the receiver/servo control brick a bit further towards the nose to assist with moving the center of gravity further forward. In addition, I simple extended the nose of the Finch an extra half inch to provide a more forward location of the battery, thus moving the CG forward as well.
These are just a few examples of the normal electric model airplane design challenges you will face as you prepare your own original aircraft for flight. Design and test flight experiences build over time. You will soon gain a useful series of experiences and design tips that will allow further success on model airplane flight.