I'm building an 8' span B-2 bomber originally designed - built - flew by Ivan Munninghoff of the U,K,. I'm probably too far along to change any Aerodynamics but I wonder if anyone can shed some light on it's funky aerodynamics. I'm trying to build the plane as close to scale as possiable. Ivan's model has 4 x electric fans - I wanted to save on weight and cost by using a glow engine on the nose, this also helps with the very forward c/g. The fuel tank is mounted directly over the c/g so as fuel is used there is no c/g shift. What's the deal with the razer sharp leading edge, a friend of mine said to use a rounded leading edge fearing the sharp l/e would make the model unstable. According to a book I have - the B-2 is neutrally stable. Also I have 3/4" of washout on the wings. Thanks for reading my Post any help is appreciated.

Hmmm, ambitious stuff!!

As a general rule a sharp leading edge will have a more severe stall point than a rounder l.e..

It might hang on a little bit longer but when it lets go you know about it.

As for 3/4" washout, when you start playing with flying wings you soon learn that there is additional terminology to bring on board. For example the "washout" that you refer to is actually (in my book anyway ) reflexed airfoil.

Is this enough? It is a matter of area and moment more than "enough". It is this that will determine the "stability " of the wing, and the lift moments around the CG have to be fairly exact. If you are building from an existing model then you should be reasonably ok.

BUT I would prefer to find that legendary field of long grass for initial testing...

You may have to add some clear plastic fins to that for stability. The prop has a forward fin effect that may just be enough to have that puppy sliding sideways. This fin effect is why the early Northrop flying wings with props didn't need any rear fins but the later jet version had the four little fins. But in your case with the prop on the nose you'll be adding the fin effect in the wrong place.

Or if you used the split ruddervators or ruddevons or whatever those things are and hooked it up to a heli gyro it may be stable without any fins.

The sweep back of the wing planform requires a lot of wash out to prevent tip stall. Additional wash out is required for the necessary longitudinal dihedral. The combination of CG location and wash out will determine the trimmed flight air speed. Too much washout is like too much up elevator trim and too little is like excessive down elevator trim. The farther aft the CG, the less washout required. However, the CG must be ahead of the aerodynamic center for pitch stability. The swept wing tailless configuration has a force arangement that requires the wing tips to lift down and the rest of the wing to lift up. The amount of twist determines how much of the wing is doing which at the trimmed flight condition. The CG location ahead of the aerodynamic center balances the down force of the wing tips.

Thanks B Mathews for you're reply. Yes I am setting up split rudders for yaw. I'm using a Futaba gya351 for stability.

Excellent, you're way ahead of me obviously....

Ollie very right but I passed over this since I assume you're doing this from plans and the twist would be noted.

If not then you can get the Panknin wing twist calculations for Excel HERE down near the bottom of the page. This work great but you need to know some info about the airfoils your using. Just ignore all the math in the body of the text.

Since it RC you only really need to get fairly close and then some elevator trim can take care of the rest but you'd be surprised at how much you need. It's usually quite a bit.

If you don't have Excel you'll have to get someone to run this for you. Or there may be an online site that has an applet to run this but if so I haven't found it yet.

Good luck

BMatthews,

Unfortunately the Pankin twist formula only works with swept, straight LE &TE, single taper planforms. If the model in question has the planform of a B-2 then following the plans is the only practical alternative. Since the plans may be off a little or the model may not fit the plans exactly, adjustment is likely to be needed. Armed with an understanding of how flying wings are set up equips the modeler to make the proper adjustments.

Oops, that's true all right. Ya caught me not thinking again...

I suppose it could be worth calculating some areas for small segments of the wing and getting an approximation of the equivalent taper ratio and sweep from using the MAC's of each area. It certainly wouldn't be totally accurate but it would be better than nothing if there's no details on the plan....

After all this isn't going to be a sailplane wing where we wouldn't want the trim drag of a displaced surface needed to compensate for an improper twist.

Thanks for the reply Problygo. Yes I have a huge grass feild surrounded by soft weeds that have saved my baken in the past. this aircraft has the wing area to weight ratio of many gliders out there so I think it will fly very slow specially considering the wing is about 3" thick - not counting the fuselage.

The B-2 built by Ivan Munninghoff flew very well but he warns that a stall can require allot of altitude to recover so no low level stalls! Ivans B-2 was "semi scale" The fans required a larger intake than scale and he flattened out the top of the wing so everything was the height as the engine bays. Since I'm deviating from his propulsion I have decided to make the engine inlets / exhausts and fuselage to scale. I also deviated from the airfoil in the center section. Ivans is symmetrical throughout but the real B-2 is somewhat semisymetrical so I made the air foil to resemble the real thing as close as possible. Who knows what will happen? I mean we all know that with enough power / control authority and an FMA Direct copilot you can get a 4'x8' sheet of plywood to fly.

You may not be able to use the Panknin stuff but I do know that if you make the airfoils lifting then you'll need more twist in the wing.

If you insist on this then I'd only use the semi symetrical airfoil in the center area and transition to a full symetricl or even upside down semi symetrical at the tips.

But you're really getting out onto thin ice by doing this. The twist setup IS quite critical. You can fine tune it using some control surface offset but it's got to be within a degree or two of correct. And when playing with the spreadsheet on layouts with minimal sweep like the B2 an airfoil change makes a big difference as the pitching moment changes and this pitching moment is what is so important.

Thanks for the very insightful information Bmathews. Yes the center section is semisymetrical but it only deviates from symmetrical slightly. The outboard wing sections are symetrical and are detachable on my B-2. They have 3/4" of twist. Because of the attach points "Gator RC aluminum tubes" it Will be very easy to turn the outboard wing sections a little at a time until I don't need any trim and fair everything together so it looks "Factory". Is this a good idea? After it's first flights Ivan's B-2 pictures show it with a little up elevator trim.
P.S. the orriginal B-2 Has very semisemitrical outboard wings.

I'd say that's a plan. If you follow the recomended CG then the rest should fall into place.

On the B-2, I believe the leading edges are sharp only at the nose, and at the wingtips. As you move toward the center of the wing half at the LE, the sharpness becomes slightly more blunt. It's hard to tell from photos. I believe the LE sharpness was done for radar signature reasons.

Thanks for the reply Ron S. Yes the leading edge is very sharp from the nose out to about half way down the wing. It's such a sharp "under-cambered" leading edge that it has to have significant aerodynamic benefits but what those benefits are I don't know. Someone on another reply said that it gives the aircraft a definite stall speed. Any other thought you have to enlighten me on this subject would be appreciated.

The sharp leading edge has benefits in reducing the radar cross-section and also in reducing drag in high speed flight.

The sharp edge minimizes the reflection of electromagnetic energy back towards an emmitter. The larger the radius of curvature at the leading edge, the larger the radar cross section will be in the direction normal to the edge.

The old B-58 bomber used an undercambered leading edge for drag reduction at high speed. The leading edge of the B-58 wing looks a lot like the leading edge on a B-2. I suspect that the B-2 designers used a similar leadiing edge because it had good high-speed aerodynamic characteristics and also helped reduce the radar cross-section.

Thanks for the feedback. I'm an aircraft maintenance technician and the majority of the airliners that I work on have under-cambered wings along with an under-camber at the trailing edge. I worked at Northrop/Grumman for 2 years in Palmdale California on the B-2 project. I know a little about low observables from reading books and from T/V but I was working a Grey World project and they didn't tell us anything we didn't need to know about. Have you seen the Have Blue test bed A/C. It also has an under-cambered L/E , maybe for low observable purposes.

I assume you mean Tacit Blue, and not Have Blue. I've seen Tacit Blue, but have never looked at the LE. I will next time I see it. Very strange planform - a loaf of bread with wings!

Have Blue, on the other hand, I'd love to make a model of... I'm partial to airplanes with pointy noses!

Thanks - yes I mean the loaf of bread one. The front of the fuselage looks just like the L/E of the B-2 not really the wings.

CORRECTION!!! The B-2 plans that I have been working from were drawn and built by Chris Golds of the U.K. Ivan Munninghoff also built a B-2 but used 2 x OS 90 ducted fans. Sorry for the Error.