XFLR5 can be a bit intimidating and frustrating to learn. Then you will find that occasionally it will do strange things all on its own, which I have never figured out....I do not use XLFR 5 and not sure I would be able to !! Something else to learn !
Look forward to more discussion.
Hi Norman, apologies for the delay but am very grateful for your help.Yes, it's the washout that makes this work. Sweep and taper both cause the circulation to be weakened at the forward end of a swept wing and strengthened at the other end. This results in a shallower slope of the cl over alpha curve at the root of a swept back wing and a steeper curve near the tip. This steeper curve of the lift slope is what causes swept wings to develop a pitching moment in addition to the 2D airfoil pitching moment and tip stall. A small degree of washout can fix the pitching moment problem but only partially alleviates the tip stall problem, thus swept wings usually have some sort of stall delaying device near the tips. The trailing vortex (often called the tip vortex because that's where it usually starts) is part of the wings circulation. Looking at the flow field fro behind you can see that the inboard side of the vortex is going down and the outboard part is going up. The inboard part of the vortex adds to the downwash behind the wing and the ascending part outboard is normally lost. By forcing the vortex to start inboard of the tip a small part of the span (about 22%) is in the upwash side of the circulation. This little piece of wing outboard of the vortex is like a glider flying in slope lift. The lift vector is tilted forward just as a glider diving into an updraft. When you increase the camber of the wingtip the lift there increases but since the lift vector is tilted forward the induced drag on that part of the wing does not increase.
If you force the plane to fly a lot faster than it was designed for the wingtips will start lifting downward. With a swept flying wing the only way to exceed the trim speed is in a sustained dive and even then the plane will pull out by itself because the tips are your pitch control surfaces.
Best of luck! Everything about this design I like -- simple, easy to build, inexpensive, efficient. I presume that cost ($15K) doesn't include the engine...because it's about that same cost.
Here's some more detailed stuff about Bob Hoey's birds.Have you heard of Bob Hoey with his RC birds ? He was onto this proverse yaw and mentions it in a paper. I think it was 1992. The paper is here http://www.johnnyarmstrong.com/soaring-birds-9/birdmodelfeatures-pdf/
He was using 'tip feathers' at different angles of attack for the ' turkey buzzard' and I think rotating tips on the seagull.
I don't know much about Hill's designs. It says something about the success of his ideas that he managed to get financial backing to keep working on the concept for 20 years though (he was the guy behind the Short Sherpa). However it may also says something that nobody carried on with his work after he retired.I think the same as Hill used on his pterodactyl, a swept flying wing monoplane with no vertical fins and I think only wingtip elevons but this was from 1928 ! Does anyone know if this aircraft produced proverse yaw? If this was the only control system then I guess it had to. It seems to be the same system on the Short SB 4 from the 1950's, which Hill was involved with, but this had a fin.
A tail makes the control and stability problems easier to solve and a fuselage is a convenient place to store cargo. Those things make developing a flying wing that has any useful capacity expensive. Now even a hermit like me can have the equivalent to a Cray XMP on his desk and free analysis software similar to what the B-2 was designed with in the late '70s so the development costs have come way down.When you think of the early pioneers like Dunn and his flying wing Biplane which I think was 1912, that's over 100 years of flying wings!
Interesting. I have seen this web site before, but never noticed something.
By careful observation I have seen this on Turkey Vultures, which is the only bird in this area that flys "slow and low" enough to get a good look at the wing tips. I speculate that other birds also have negative twist at the tips, but have been unable to observe it.Left, the lead feather is set at negative 27 degrees (this is not a typo), the second is at -19 degrees, the third at -15 degress and, the remaining stationary feathers are reduced 4-5 degrees per feather until the aft one is at zero. Other combintions of angles have not worked as well as this one.
That's a fantastic picture! Where did you find it? There's been a very fuzzy picture of this plane on the TWITT site for a long time. NX18992 is mentioned on the Mitchell history page but until now I've never seen the N number associated with the plane.
What I'd like to know is what color it was. Was it painted red or some other dark color? Or was it glorious dark varnished wood and fabric? What a sight it would have been sitting out on the lake bed. It was huge, does anyone know what happened to it?