Our models are so overpowered, why not just make all the wings thick and be done with it?
A few days ago I got an email from Dave G. containing the following statement:
“I think you are overlooking the effects of wing thickness. Thick wings have higher drag but that is not an issue with RC planes due to high thrust to weight [ratio]. Planes that have thick wings seem to have benign stall characteristics. [For example] Kadet Senior, Eratix, Twist.”
This is a great topic for an article! Thank you, Dave, for writing. Keep your emails coming, folks!
Wings that are about 12% thick are capable of producing the highest amount of lift. Most full-size airplanes use airfoils that are this thick, like the famous Clark Y airfoil.
The thicker a wing, the more drag it produces as the airplane flies through the air. Drag is always bad, isn’t it? Well, not exactly.
Aerobatic airplanes use thick wings for a couple of reasons. First, they need to be really strong while keeping their weight as low as possible. The thinner a wing is, the harder it is to make it strong. As a rule of thumb, cutting the thickness of a wing in half will increase its weight by about 50%. The thick wings on aerobatic airplanes are partially there to save weight.
The second reason is that aerobatic pilots want their airplanes to fly at the same airspeed whether they are pointing up or down. It makes flying their maneuvers a lot easier, and it looks better for the judges.
A thick wing has a sort of built-in automatic speed cruise control. You see, the drag on an airplane increases as the square of the airspeed. If you double your airspeed, the drag increases four times. If the drag is high to begin with, then increasing the airspeed by only a small amount will cause the drag to grow so much that it keeps the airplane from speeding up any more. This is exactly what an aerobatic pilot wants.
Dave mentioned the Eratix 3D and Twist 3D model airplanes. These model airplanes are designed to be all out aerobatic performers. They definitely fall under this category.
I’m very familiar with the other model airplane that Dave mentioned, the Kadet Senior. I have owned one for at least 15 years. I do not believe it is advertised as a trainer, but it has all the flying characteristics of one.
Trainer airplanes need to be easy to fly and land. They also need to be strong enough to survive bad landings. The Kadet Senior has a low wing loading and a strong landing gear. The wing is huge, rectangular, and thick. I’m sure the thickness of the wing helps with making it strong, but I do not think that is the primary reason why it is so thick.
Thick airfoils have very gentle stall characteristics. The airflow starts to separate from the top of the wing starting at the trailing edge. As the angle of attack of the wing increases, this separation bubble slowly inches its way forward. At some point, the lift generated from the wing stops increasing and it is said to be stalled.
Other Types of Models
So what’s left? What types of model airplanes are better off not using thick wings? Easy. Non-aerobatic, non-trainer types. Doh!
More seriously, racing airplanes have thin wings. They need to go as fast as possible. They also do not want to stall at the turns, so in reality they have to find a balance.
Park flyers can benefit from thin wings. Then they won’t be affected so much by the wind. They can fly faster if needed for better penetration. Note that I’m not saying that they have thin wings!
Gliders are all about efficiency, so their wings are usually about 12% thick.
Small and Slow Model Airplanes
Small model airplanes that are designed to fly slowly have a lot of trouble being efficient flyers. When the angle of attack of their wings increases, they stall early and abruptly. A way to increase their maximum lift is by making their wings thin, no more than 6-8%.
There’s another wrinkle to this whole issue. The reason why slow and small model airplanes have early and abrupt stalls is because of the way the air flows over them. The air does not have much energy as it flows over the wing, and therefore it is easily diverted from where we want it to go.
A way to overcome this problem is by stirring up the air. If we cause the air to become turbulent, then the model will fly as if it were a bigger model. The stall will then be much nicer, and the maximum lift will be higher.
There are many ways of introducing turbulence into the air flow. These are the so-called vortex generators. Some of these, for example, are stepped vortex airfoils. Modelers call then Kline-Fogleman airfoils.
Vortex generators add some drag, so they need to be used carefully. But they let thin wings have nice stall characteristics. They also let thin-winged airplanes fly more slowly than they would otherwise. The main challenge then becomes making thin wings strong and light enough.
Weight is the biggest enemy in model airplanes. If you do not like how a model airplane handles in the air, adding more power is usually not the answer. Wing loading is primarily what determines how well a model airplane handles. An overweight hog is always a pig to control. Adding more power is not really a cure. It will still fly lousy.
A model airplane with thin wings and a low wing loading is hard to beat in the air. Try it sometime. That is exactly what I did with my ModiFly design, and it is a joy to fly. Penetration into the wind is great. Less damage in a crash. Always easy to fly.