Vortex generators for aviation are used to prevent what is called as boundary layer separation, as Larry referred to. At high angles of attack the relative wind strikes the leading edge at a more severe angle causing it to deflect up and pass over the wing but with an increasing distance. The angle also causes a turbulence in the air flow that destroys lift, starting at the trailing edge and moving forward to the leading edge. This is in effect the wing gradually stalling, and is why you must lower the nose to begin flying again in stall recovery, to restore proper airflow over the wing. The small vortices as they are called caused by the angled deflectors attached to or near the leading edge disrupt the smooth flow of air causing a small vacuum to occur next to the skin of the wing. This has the effect of "sucking" the boundary layer down to stay in contact with the wing when the angle of attack would normally cause the air to deflect and go high over the wing, losing lift in the process. That is how they can lower the stall speed, effectively lowering the landing speeds and increasing control effectiveness at slower speeds. They are also small enough as to not significantly impact the cruise speeds of most aircraft that I have read about use them. Naturally there will be a point in speed where they will become significant, but for most aircraft in our realm minor if any effect.
Turbulence created for intake manifolds is done to prevent separation, better atomization or mixing of the air/fuel mixture. This is especially important when the airflow is very fast or the engine is cold, when it is easy for the fuel to separate out of the mix and pool other places in the intake manifold. In early cars they had rough spots in the intake below the carb, in late models they have smooth throttle bodies, but rough intake castings to get that swirl for better mixing since in most cars the fuel is introduced at the intake valve by a fuel injector. If you make your own intake manifold for your aircraft engine, you may want to make sure you think about some of this. No heat at all, and no mixing action can lead to a big difference in fuel distribution across the cylinders. If you fly at any altitude higher than 1500MSL than you are going to get cooler air through the carb than on the ground plus a density change. Just some things to consider.... Colin & Bev Rainey KR2(td) N96TA Sanford, FL crain...@cfl.rr.com http://kr-builder.org/Colin/index.html
