I'm making a soaring version of the Star Wars X-Wing with the foils in the X position.

My knowledge of V-tails is limited, but I understand the basic premise of both upright and inverted forms. My problem is that this is a tailless design with the wings needing to contain both yaw and pitch control surfaces. The A/C is set-up currently with a slight aft CG and a CL at the traditional 1/4 chord point to keep her slightly nose-up in a steady headwind glide. The four wings have a 30deg angle of seperation from the fuselage along the horizontal. Any suggestions as to how to properly mix in yaw and pitch controls?

From : Don Stackhouse

It's going to be tough to get any decent yaw control from this arrangement. The wings' lifting forces have essentially no moment arm to work through in the yaw sense. You would do better with the use of asymmetric drag instead. A split elevon that opens as a drag brake on one side, such as the method used in the Northrop flying wings, is one possibility. Another is to deflect the lower and upper elevons on one side in opposite directions simultaneously, so that they cancel their inputs to pitch or roll, but they create extra drag on that side.

The other problem with using the control surfaces on the wings to provide yaw control is that this will almost certainly eat into the available pitch and roll control authority.

If this is a powered model, you might do better by using separate throttles to the port and starboard power pods, mixed into the rudder channel. Reduce power on one side and increase it on the other for rudder control. For a glider, you could still have the power pods set up as flow-through tubes, but with servo controlled shutters inside that could close to act as drag brakes for rudder and/or glidepath control. You could also mount rudders inside the aft ends of the power pods, out of sight but still effective for control. There might also be merit to using this method for some of your pitch control as well.

The X-wing is an especially difficult subject, not only because of the tailless aspects, but also because of that extraordinarily long nose. This acts like tail surface area way in front of the C/G, almost like putting the feathers on the front end of an arrow. Have you successfully flown it at that C/G location? That's a rather far aft setting for a simple plank flying wing alone. With that long, destabilizing proboscis sticking out in front, frankly I'd be shocked if you are able to get positive static stability in pitch or yaw with that C/G. I expect that for stable flight the C/G will need to be much farther forward. You'll also need to add more reflex to the elevons to maintain flight trim in pitch with this new C/G.

C/G is not just a function of wing alone, it includes the entire airplane including the tail (or lack thereof) and the fuselage. In this case that long nose probably needs to be handled as if it was a canard plus a nose-mounted vertical fin! With the C/G far enough forward it could be made stable, but the C/G required might be VERY far forward.

You might also want to consider using gyros to help the pitch and yaw stability. It may be easier to stabilize this model electronically instead of aerodynamically. However, even gyros are limited by their response time and the response time of the other components in the system. There will be limits on just how much instability they can suppress. You might still need a fairly far forward C/G, even with gyros.

Of course this does open up another possibility for yaw control, although it would probably look a bit weird. The X-wing has a rather prominent nose-cap at the forward end of the fuselage. You could hinge this cap and deflect it side-to-side to act as a rudder. This same technique has been used successfully on pterodactyl models, using the head with that large crest and beak to act as a rudder. There's a scene in the movie "Wargames" where you can see this in action. It's very possible that the real pterodactyls used this effect as well. In your case, this would be a great place to use a profile fuselage, which would minimize the fuselage's destabilizing effects in pitch, minimize the moment of inertia in both pitch and yaw (a big factor in dynamic stability, and very important in your case since the X-wing also has very poor damping in pitch) and maximize the rudder authority available from the hinged nose cap. Of course you would still need to have a very far forward C/G, but at least you would have good rudder authority.

You've picked a very challenging subject. At best it's likely to be a marginal flier, but it's certainly not impossible. If you do get it to work, it's certain to be a crowd-pleaser. Good luck! (you'll need it)

Don Stackhouse
DJ Aerotech