On Wed, Jul 15, 2009 at 1:12 PM, leee wrote:
> The X/YB-35/49s certainly suffered from yaw instability problems; in
> one YB-49 bomb run test it took the pilot four minutes to stabilise
> the aircraft, during which time the bombardier became airsick.
> This compares pretty badly with the B-29, which only took a max of
> 45 seconds to stabilise. The YB-49s weren't fitted with autopilots
> though, which would have helped. The B-2, of course, is fitted
> with a modern FBW FCS, which controls the split aileron airbrakes
> to keep the yaw under control. Incidentally, Edwards AFB is named
> after one of the YB-49 test pilots who died in a YB-49 crash.
I have some comments based on some of the UAV work I've done using smallish
(7-8' wing span) fying wings. This in no way makes me an expert in flying
wing aerodynamics! :-)
We started out with winglets because this is how everyone builds RC size
flying wings.
http://baron.flightgear.org/~curt/UAS/Malolo1/2007-12-12-C/IMG_2217.JPG
As an aside, this airframe was designed to be waterproof (marinized) and
lands in the water next to the boat:
http://baron.flightgear.org/~curt/UAS/Malolo1/2007-12-12-C/IMG_2234.JPG
http://baron.flightgear.org/~curt/UAS/Malolo1/2007-12-12-B/img_5261.jpg
We flew the next prototype (with a much larger and more blended center
section) from a NOAA research ship several hundred miles north of Hawaii
about a year and a half ago.
http://baron.flightgear.org/~curt/UAS/Malolo2/20080331-Malolo2-Launch-2/target2.html
(This is a sequence of pictures so you can click next/prev if you want)
Then we had a stability break through. We moved the winglets inboard to the
wing roots and our yaw stability improved dramatically.
http://atiak.com/ATIResolutionProductSheet102108.pdf
I see that the YB-49 added 4 sets of very small vertical stabilizers
inboard, however they seem very small to me. I think if they would have
made the vertical surface area larger and closer to the centerline, they
would have substantially improved their yaw stability. Oviously those
engineers were just a bunch of hacks. :-) But from looking at the various
designs, I think their must be a desire to have *just* the wing and nothing
else. Tail-less appears to be the goal, and a vertical stab is part of what
we'd call a tail.
Conceptually, with no vertical surfaces, the wing is largely free to rotate
and could fly like a discus or a boomarang. Even winglets at the wing tips
offer a lot less yaw stability than I would have expected. They real key
observation is to move the vertical stabilizers in as close to the
centerline as possible. This "locks" your heading in and helps the wing
track.
This idea shows up in other RC designs ... there's nothing new under the sun
...
http://www.hobby-lobby.com/f-27c_stryker_rtf_electric_plane_85561_prd1.htm
http://www.newcreations-rc.com/Products/thumbs/imd5123%5B1%5D.jpg
Simply moving the winglets inboard turned our design into an *entirely*
different airplane. We couldn't fly inverted before (it would just to a
negative aoa stall and fall out of the maneuver.) Now we can fly inverted,
outside loops, faster rolls, and the aircraft tracks much better. We still
have a lot of forward nose side area though so we do get some adverse yaw in
turns.
Flying wings are really strange beasts to get your head around, but I think
the key to success is to add sufficient vertical stabilizer area close to
the centerline of the aircraft.
Regards,
Curt.
--
Curtis Olson: http://baron.flightgear.org/~curt/
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