On Friday 17 Jul 2009, Curtis Olson wrote: > 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_22 >17.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_22 >34.JPG > http://baron.flightgear.org/~curt/UAS/Malolo1/2007-12-12-B/img_52 >61.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-La >unch-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.
Hi Curt, I remember following your Pacific cruise with the Malolo, and also how you found that moving the winglets inboard to the wing roots improved directional stability. The reason that the small vertical stabilisers were added to the YB-49 was to compensate for the vertical area that was lost when the props, and more importantly, their fairings were removed; the propshafts and props were located well above the trailing edge and were faired down to the upper wing surface, providing appreciable vertical surface, but the jet tailpipes exited through the trailing edge. Northrop wanted to redesign the airframe to take the jets but had to make do with using uncompleted XB-35 airframes, hence the need to compensate for the lost vertical area. In the end they actually increased the vertical area by extending the fins below the wing as well. I suspect that a single vertical stabiliser on the centerline would have been the best solution but that, once again, would have required a redesign as the tail fairing, as originally designed, wouldn't have been structurally strong enough to support a fin; in the original design the tail fairing just housed the rear guns and the observer/rear-gunner position, so structurally, all it had to do was support the weight. With your models, which don't rely upon internal frames for strength, it's relatively easy to relocate the v-stabs but the YB-49 team could only attach them where there were suitably strong enough frames i.e. the old propshaft attachment frames. I believe that one of the N-9M 1/3rd scale B-35 development aircraft has been restored to airworthiness and is still flying. I would agree that vertical surfaces are necessary for wings that have relatively simple control systems but as I said re the B-2, if you've got a modern FBW FCS system you can do away with them. LeeE ------------------------------------------------------------------------------ Enter the BlackBerry Developer Challenge This is your chance to win up to $100,000 in prizes! For a limited time, vendors submitting new applications to BlackBerry App World(TM) will have the opportunity to enter the BlackBerry Developer Challenge. See full prize details at: http://p.sf.net/sfu/Challenge _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
