On Tuesday 26 August 2008, Curtis Olson wrote: > On Tue, Aug 26, 2008 at 12:58 PM, LeeE wrote: > > Winglets are primarily used to improve the efficiency of the > > wing by effectively increasing it's aspect ratio without > > increasing the span (by much). They can also be used to change > > the wake characteristics too, but they don't do anything for > > longitudinal stability, as you've found out:) > > Well winglets on a flying wing do provide some level of yaw > stability. And you need something or you end up with a long > skinny discus. The thinking is that if you need vertical stabs, > why not kill two birds with one stone and put them out on the > ends of the wings. As we found out though, an inboard stabilizer > with the same area is far more effective for yaw stability. Big > winglets seem to hurt roll performance by quite a bit. The > flying wing with winglets had a big problem with tip stalling and > snap rolls, even well above stall speed. Flying inverted was > also impossible. When I push the nose "down" to keep it above > the horizon, the wing would do an inverted stall, snap roll and > it would tumble out of inverted flight. > > When the winglets are removed and replaced with inboard vertical > stabs, inverted flight became easy and stable, roll rates > improved dramatically, the tip stall/snap roll problem was > significantly improved, and the wing turned into an entirely > different airplane. [Snip...]
Heh - they'll have a little effect, but not much, and it's probably as much due to producing drag behind the aerodynamic center as it is to aerodynamic control. If you look at the wing in plan-view, imagine drawing a line from the aerodynamic center of the aircraft to the aerodynamic centers of the winglets. You can then split that line into it's two vectors - one along the axis of the aircraft and the other at right angles to it. Because the wing is slightly swept, there'll be a component behind the Aerodynamic center, but it'll be much smaller than the component at right angles to it - so not very efficient for longitudinal control. Using inboard vertical fins is equivalent to locating the winglets further back, out on trailing booms (extend the line from the aero-center of the aircraft through the inboard fins out to the span at the wingtips). I'm not too surprised by the bad performance inverted with the winglets - they've got appreciable camber and must produce quite a large low-pressure region between the wing and the winglets, meaning when it's inverted you'd need a higher (-ve) AoA than you'd expect. Not only that, but it's possible that with the amount of (-ve) AoA needed you'd be bringing the leading edges of the winglets past the vertical and instead of the pressure wave moving up the winglets and off their ends, it could be moving down them to the wing/winglet join region. Did you just re-position the winglets inboard or did you make up some symmetrical aerofoil fins? If you were using the winglets, with their camber, you could try swapping them, so that the camber is reversed, just for comparison. > It's fun stuff to play with in the RC world when I can have my > feet on the ground. :-) If you are playing you bet your life, > you have to make sure you get it pretty close to right on the > first try. > > Curt. I bet it's fun :) LeeE ------------------------------------------------------------------------- This SF.Net email is sponsored by the Moblin Your Move Developer's challenge Build the coolest Linux based applications with Moblin SDK & win great prizes Grand prize is a trip for two to an Open Source event anywhere in the world http://moblin-contest.org/redirect.php?banner_id=100&url=/ _______________________________________________ Flightgear-users mailing list Flightgear-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-users
