Andy asked: > What about the idea of VGs ONLY in the wing root area? This > might make the stall speed and characteristics a tad better at the lower > limit, without significantly changing the rest of the wing behavior. If > the root is the only area that actually stalls, holding off that stall as > long as possible seems desirable. > > Even if the (partial) stall becomes more abrupt, in this case I would > think it would just lead to a more pronounced mushing, and the extra > knot or two of "non-mush" airspeed might help keep folks out of > trouble at the lower end. > > Just an idea...I'm no aerodynamicist either! > > Andy
Andy, Interesting thoughts. Let's think it through more, though. If you put vortex generators on the wing root to delay the wing root stall, then the smooth air over the empennage allows the tail to go down farther increasing the angle of attack of the entire wing. Now, the entire wing, not just the wing root, is very close to the stall. If there's an additional disturbance such as a gust, wind shear, control movement, you could stall the entire wing. Much of the idea of having ONLY the wing root stall is that the rest of the wing is flying normally with smooth air and the ailerons work normally. If you get the main part of the wing closer to the stall, there's increased chance that a lowered aileron will stall that wing rather than raise it. As I discussed, I've seen reports that the airfoil used on the Coupe has undesirable stall characteristics. Fred Weick was THE premier researcher on aircraft stalls of his time. He was assistant director then director of the NACA full size wind tunnel and was able to do extensive research on the subject. Though it's true that vortex generators weren't available to investigate back then, I WOULD NOT add them to a Coupe's wings as the Coupes stall behavior was very specifically designed. Possible results of vortex generators on a Coupe: 1. abrupt, unexpected stalls 2a. loss of "characteristic inability to spin" resulting in spin 2b. inability to recover from that spin Please be sure to get FAA approval and a REALLY GOOD aerodynamicist DER involved in your experiments and flight tests. This is not one to mess with sloppily. I'm sure they'll require rigorous (and expensive) data collection. (The Ercoupe was one of THE most flight tested light aircraft ever.) Do your experiments at altitude and be sure to wear a parachute. ___________________________ Also, allowing the wing to go deeper into the slow flight regime could have another interesting effect. That class of airfoils (low-aspect ratio, thick, "Hershey-bar" wings) develop great lift at airspeeds over 90. Tough they glide well enough at the right speed, their glide ratio needs to be higher than thinner wings. For example, the Coupe's best glide ratio is around 75-85 mph compared to about 50-70 for a Cessna. At lower airspeeds, the thick wings get into a high sink regime. Slowing a Coupe to minimum flying speed with power off can give really dramatic sink. I don't think I'd want to extend my Coupe's flight into even lower airspeed ranges. In flight (except when doing stall testing and practice), the slowest I'd want to go would be max range speed (or minimum power airspeed) around 65-70. This is slower than best glide ratio airspeed (or maximum range airspeed). I personally only **used** minimum flying speed AFTER I'd leveled out of the flare around 1'-5' above the runway. At this point, flying level just above the ground, it's great to slow the plane as much as possible before touchdown. _______________________ Here's something you might try. At the end of your flare, as you are flying level a yard or so above the ground, have your power set to about 200 rpm above idle. I usually made this adjustment without looking during the flare, doing it by sound after experimenting at altitude. As I normally flew the last part of final at idle, I would increase power during the flare **just enough that I could hear the small difference in rpm**. Along with this slight rpm (power) increase, hold the nose up to the runway far-end-lights and keep it there, slowly pulling back the yoke as the airplane slows - doing all this about a yard or so high. This gives some slipstream blow that retards the wing root stall and also blows down the tail better. This allows even lower minimum airspeeds and an even slower touchdown with low in-ground-effect sink. This results in a can't-feel-it squeeker touchdown most of the time (as long as the runway is long enough because it does lengthen the float). Good luck, Ed
