Understanding the basics of aerodynamic theory, as you'd imagine, is pretty darn helpful when it comes to designing your own aircraft. Even if you only fly RC airplanes and don't build them, knowing this stuff is also beneficial. It gives you the edge and helps you to diagnose problems.
Question: can you understand aerodynamics without the maths? My problem has always been my mathematical hopelessness - quite bluntly, I absolutely suck at maths. All the way through my school days, I had a passion for learning science but could never get far before the maths held me back. The thing is, I can still design airplanes just fine. I think it's possible to get your head around this stuff without mathematics, you just need to think practically about it.
Here's a rough and ready introduction to the principles of RC airplane aerodynamics, this time mostly focusing on wings and lift - common rules, things to stick to and guides to work within.
Rule 1: most things will fly if there is a) airspeed, b) wing area and c) air. An FT Nutball is a circle that flies. It has no airfoil yet can pootle around quite happily. Even the FT Viggen, which also has not airfoil, can cut through the air just fine and enjoy excellent performance.
The way lift works is actually extremely complicated, but a working theory can provide you with a guide to work to. Wings work by creating differing air pressures. Bernoulli’s Principle shows that the curved surface of an airfoil accelerates passing air over the top of the wing making it thinner. This ‘sucks’ the wing upward.
On the other hand, the bottom of the wing has a higher pressure due to the angle of the wing as it moves through the air. This means that you can design a plane that with a flat wing at a positive angle of attack to achieve lift. The Viggen actually does this with its front canards: they're angled to 'push' the nose up in flight.
It can be problematic to consider these explanations as the complete picture - as mentioned, lift is complicated. For the purposes of basic model aviation, however, it's completely workable to understand it in these terms.
Lift depends on:
- The airfoil shape
- The wing area
- The density of the air
- The speed of the oncoming air
- The angle of attack
Wing to Fuselage Ratio
Yep, that sounds maths-y. All this is talking about, though, is the size of the fuselage in relation to the wing. Their relative sizes can have all sorts of effects of the characteristics of an airplane, but all you really need to know is that, in general, if you increase one you should increase the other. This is because you want to preserve stability. An absence of stability is just asking for trouble really.
Here's a plane I designed and built a while ago. It was small, stubby and (supposed to be) fast. There was a problem though: it was incredibly unstable.
As well as being a little on the heavy side, the short tail resulted in a catastrophically low level of directional stability (on the yaw axis). It just couldn't get going. Here's the thing, if the fuselage had more distance between the wing and tail, this problem would have been reduced. Putting the plane on a diet and perhaps making it bigger in all dimensions to reduce wing loading would also have been a good idea. Here's another small, unstable plane that suffered from the same issue.
Compare this to the FT Scout. The Scout's wing area is greater, It has lower wing loading, and the wingspan is greater. The tail is also further away from the wings due to a longer fuselage. On this three channel model, these dimensions provide way more longitudinal and directional stability.
Here's taking it a step further. This Avro Monoplane model has an even longer wingspan and tail. The stability of this thing is absolutely fantastic.
Knowing what sort of wingspan your plane needs and combining it with a suitable fuselage length can really help you to design a successful airplane. There isn't one single ratio to stick to, but you could use these following guidelines as a (very approximate) cheat sheet.
Extremely Rough and Ready Guide to Stable Airframe Dimensions
- Wingspan should usually be greater than fuselage length
- Increase wing area if you increase weight to keep wing loading the same
- Dihedral/polyhedral is useful for keeping you flying level
- Thinner wings create less drag (think about what a traditional glider looks like)
- Longer tails have better longitudinal and directional stability
- Longer wings have better lateral stability
- Smaller stubbier planes are generally less stable than longer thinner planes.
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Look out for part two of this series coming soon!
Article by James Whomsley
Editor of FliteTest.com