Lifting Bodies Are More Than Just Another Pretty Shape - Opinions on Aircraft Design

Lifting Bodies Are More Than Just Another Pretty  Shape - Opinions on Aircraft Design

There are many ways to look at aircraft design. Most of which are field dependent, why design for supersonic flight if your device will never climb above 500'? It would be cool thought, right? I digress, if you are designing for normal flight regimes you will be operating at an altitude with an engine power curve which will vary by altitude and temperature. Now, if you are designing in other regimes, you need to know where you can get small performance boosts where they are available.

Managing wing thicknesses is and use the concept of "wing tip thrust". By making tip vortices more manageable there is a significant reduction in overall wing drag. This "free" reduction in drag is called "wing tip thrust". Like this, there is another simple idea to take into account. If you can design a fuselage that is not pressurized with an essentially flat belly, you can play some shape games. These shape games can effectively make a Clark-Y airfoil. Now it is more complicated than this, but if you begin your mental experiments from this perspective, you can eek out some under-represented performance.

But how do he do it? It is all in the magic. If lift is generated by circulation of a lift envelope, that is created by the difference in pressure created by fluid moving around a shape. Why not generate lift around your fuselage? It will not get you 25% of your lift... but if your fuselage planform area is 15% of your wing area and you can get 40% efficient lift out of minding your p's and q's you could be looking at 6% more total lift. Yeah, you are right, this is all pie in the sky stuff. However, if you could generate 2-3% more lift for the length of your flight, you could reduce the angle of attack on your wings and trim out your drag. Reducing your drag means that you use less energy in the case of electric or less fuel in the engine.

Less fuel means that you can carry more, or fly farther. Nice. 1-2% may not be a lot if you weigh 4oz. . However, if you are looking at varying wind conditions or being able to react to changing conditions and you can play some games about fuselage profiles. Why not, it is not really so hard to get as much as 5-6% out of playing your profiles against each other. That makes for a good amount more flight time in rc. Not the 7min guys, but as you go for 20min... you get 21min. Yeah, it does not sound like much, but if it makes your fun go for 22min... It will be worth it.

Why do I pick Clark-Y? They have flat bottoms, reasonable thickness and a fast taper to the trailing edge. Construction-wise they are easy to construct, because you can lay them on a table. Their CL/alpha is pretty steep, so 1-2deg can get you something and if you can play with what level is, you can get a fuselage angle of attack to a point where you can make some dividends. Most importantly is that they do not separate their boundary layer fast. Most designs will never get to a critical angle of attack, drag will eat you alive if you fly your fuselage at such a high angle of attack.

We make lots of 60" fuselages these days with a mean 12" beam. That gets us an additional 4-5lbs of lift at take off and 1-2 lbs at cruise. Nothing to write home about, NASA will never give me a Ph.D. for it, but it does get me lots of "clever"s and "I would not have thought of that"s. Pretty high praise in engineering circles.