Tandem-wing control arrangement?

[cluttonfred]

Not for the first time, I am drawn to the French Mauboussin M.40 Hémiptère tandem-wing single-seater as a possible inspiration for a homebuilt design. The original used an interesting combination of elevators on the front and rear wing acting contrary to one another but only the rear ones acting as elevons. It seems to me that a simpler arrangement would be to use the rear wing control surfaces alone as elevons, freeing up the front surfaces to act as flaps. A spring bias mechanism in the control system or movable tabs on each wing connected to the flap lever could be used to counteract the pitch change due to deploying the flaps, effectively pushing forward on the stick automatically. What would be the pros and cons of such a system in a tandem wing design? Cheers, Matthew

[rwanttaja]

Not for the first time, I am drawn to the French Mauboussin M.40 Hémiptère tandem-wing single-seater as a possible inspiration for a homebuilt design. The original used an interesting combination of elevators on the front and rear wing acting contrary to one another but only the rear ones acting as elevons. It seems to me that a simpler arrangement would be to use the rear wing control surfaces alone as elevons, freeing up the front surfaces to act as flaps. A spring bias mechanism in the control system or movable tabs on each wing connected to the flap lever could be used to counteract the pitch change due to deploying the flaps, effectively pushing forward on the stick automatically. What would be the pros and cons of such a system in a tandem wing design? Cheers, Matthew

Probably the hardest thing about implementing such a design is figuring out where the CG is supposed to fall. The center of lift should be *between* the two wings; considering the relative size of the wings, I'm guessing it's just behind the cockpit. If this is the case, the CG probably is somewhere around the aft spar of the forward wing. 'Course, I'm a Space Engineer, not an Aero Engineer, so I could be wrong.

If this is the case, it seems to me that the pitch change with flap deployment should be minor. I'm inclined to NOT add gadgets to the control system unless absolutely necessary. Don't think I'd add a spring bias to it, at least at first.

The other factor, of course, is the relatively small size of the aircraft and the huge wing area: What makes you think you *need* flaps? :-)

Ron Wanttaja

[cluttonfred]

Ron, you're right that the M.40 has an awful lot of wing at 138 sq ft for such a little plane, but apparently it was built quite stoutly: 600 lbs empty and 880 lbs gross. Then again, it was meant for operation from the little grass airfields of 1930s France so a big wing was important for getting in and out of little airfields. If you haven't already, check out this 1936 article from Britain's FLIGHT magazine, including some interesting points about the lift and drag of the tandem wing combination vs. a conventional single wing: http://www.flightglobal.com/pdfarchi...0-%202533.html Burt Rutan's original Quickie is at the other end of the spectrum in terms of tandem wings--great speed out of a little engine and tiny wings, but not suitable for short grass strips.

[rwanttaja]

Ron, you're right that the M.40 has an awful lot of wing at 138 sq ft for such a little plane, but apparently it was built quite stoutly: 600 lbs empty and 880 lbs gross.

Hundred pounds lighter than a Fly Baby, 15% more wing area. I reeeeealy don't think you'll need flaps. :-)

Second page of the article says it has a 507-pound empty weight and a 770-pound gross. That gives an even lower wing loading.

I see also that the original had a 40 HP engine. Put an O-200 on it and name it, "Otis".

Ron Wanttaja

[cluttonfred]

On the CG arrangement, there are lots of schools of thought and, of course, dozens of variations on the tandem-wing theme with their won particularities and quirks. One system I have seen used with success on Mignet-type tandems is relative wing loadings. As long as the other factors are not extreme--relative angles of attack of the two wings, variation in airfoil, etc.--when the front wing is loaded substantially more than the rear wing, say 30-50% greater wing loading, stall behavior is benign and may even result in an aircraft "characteristically incapable of stalls and spins" as the FAA likes to say. That would, of course, be a design goal if it did not compromise performance too much. Imagine a side-by-side or staggered two-seat version of the Hémiptère with a bubble canopy and tri-gear powered by an Aerovee or Rotax 912 at about the same wing area (as we've said, there's already plenty) but a gross weight of about 1,000 lbs to allow for the second seat.