Re: [Flightgear-users] Zephyr UAV Sets Record

[Curtis O. <cur...@gm...>]

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.

Actually, they look a bit over-sized for that size of wing and much
> smaller ones, with a smoother transition to the wingtip join i.e.
> with a couple of inches radius would probably be more efficient,


Certainly for the purpose of tip vortices, they were way over sized.


> weigh less and cause a lot less drag.  For stability, you could try
> adding couple of smaller fins at the fuselage/wing join line and
> set them as far back as you can securely fix them, so that they
> overhang the trailing edge.  You could experiment with canting them
> outwards, out of the vertical, as well, to give the prop a bit more
> space.


I didn't post pictures, but that is exactly what we did, with dramatic
improvement.

The Northrop flying wings had problems with yaw too, developing in
> to a yaw and roll oscillation that could take the pilots several
> minutes+ to control.  From their cockpit viewpoint, the pilots
> could see that the wingtips were actually tracing out a circle.  As
> the wing yawed, one of the tips would be moving forward and the
> other backwards, which meant that the one moving forwards would
> gain a little extra lift while the one moving backwards would loose
> a little, adding the roll.  Apparently, sea-sickness could be a
> real problem at times.


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.
-- 
Curtis Olson: http://baron.flightgear.org/~curt/