Curtiss XP-55 Ascender canard fighter (Model CW-24)

Actually, most of you are wrong on the aerodynamics/controls of the XP-55; as was I until I read the excellent books recently published on the subject. It was basically designed as a statically stable flying wing. The canard itself, being designed to weather vane when there weren't any pitch control inputs. However, when a pitch control input was used it would move the control surface and also cause the vehicle to be unstable in pitch. So they they were basically trying to have a statically stable platform that was dynamically unstable in pitch. This was so they could have the best of both worlds; stable in cruise flight, unstable while maneuvering. They were trying to design the F-16 before they had the technology to handle it. Now, the Hurricane was unstable as well, but it was quite marginal in that regard. It seems they were going for greater instability for greater maneuverability with the XP-55.

Unfortunately, they were unable to overcome the excess friction/lag in the control system, which caused the nose to porpoise in pitch. They also had issues with the small size of the canard, in the sense that it didn't have the control power to get the aircraft out of a deep stall. It was far more advanced then I had originally considered.
 
XP-55 suffered numerous balance and control problems.
It was essentially a flying wing with a small trimming canard on the nose.

After test pilots XP-24B complained about poor yaw stability, engineers added fixed fins both above and below the engine cowling. Then they moved rudders farther outboard, which also moved them slightly farther aft. Still, neither of these "fixes" moved the lateral centre of area far enough aft to significantly improve yaw stability. XP-24B still had a very short tail moment arm. Poor yaw control was one of the reasons they lost a prototype in a flat spin.
Rutan solved his problem by moving his vertical fins and rudders all the way out to the wing tips, the farthest aft possible. Rutan also used his vertical fins as end-plates/Witcolm winglets to reduce wing tip vortices.
Flat spins and un-recoverable stalls are often also caused by centre of gravity too far aft of the centre of lift.
Curtiss engineers tried to move the centre of lift farther aft by enlarging wing tips and sweeping them even farther aft.
The primary reason for XP-54's swept wings was to improve balance. For comparison, look at the contemporary Japanese Luke, Shinden and Senden pushers. They all mount their engines between main wing spars to move the centre of balance forward. However, this configuration required lengthy/problematic drive shafts to spin the propeller aft of the wings' trailing edge.
Unfortunately, Curtiss engineers did not understand how wing sweep affects stall characteristics. Most airplanes stall wing root first. This helps maintain roll/aileron control part way into the stall. Unfortunately, on swept wings, stall turbulence quickly migrates to the outer wing panels, reducing (desired) nose down tendencies. Rutan solved this problem with vortilons. Vortilons are small wing fences on the underside that resemble engine pylons on jet airliners. Military swept wing airplanes use a variety of fences (MiG 15), fins and dogs tooth leading (F-4 Phantom) edge modifications to limit turbulent migration.
 
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Just to clarify, Burt Rutan solved stability problems with his canards by moving the centre of gravity well forward thus requiring the canard to provide a significant amount of lift. Just look at the flat-bottomed, cambered airfoil used on Long-Eze canards.

Also note how Long-Eze clusters variable loads like fuel, passengers and baggage at the Centre of Gravity/wing root leading edge. The primary reason many of Rutan’s canards have LERX, strakes, etc. is because they are a convenient place to stow fuel near the C. of G.
Also remember than most of Rutan’s canards are optimized for cruise efficiency. That mission limits their STOL and aerobatic performance.

And to clarify my point about stalls in swept wing airplanes. Some Long Ezes suffered from stalled wing tips (nose down pitching moment) while wing roots were still lifting (nose up pitching moment).
After a Velocity kitplane (4-seater cline of Long Eze) “mushed” (stage stall) into a lake, they fitted vortilons to all future kits. Fortunately, the “mushing” Velocity descended so slowly that the pilot survived landing in a lake!
 
riggerrob said:
Just to clarify, Burt Rutan solved stability problems with his canards by moving the centre of gravity well forward thus requiring the canard to provide a significant amount of lift. Just look at the flat-bottomed, cambered airfoil used on Long-Eze canards.

Also note how Long-Eze clusters variable loads like fuel, passengers and baggage at the Centre of Gravity/wing root leading edge. The primary reason many of Rutan’s canards have LERX, strakes, etc. is because they are a convenient place to stow fuel near the C. of G.
Also remember than most of Rutan’s canards are optimized for cruise efficiency. That mission limits their STOL and aerobatic performance.

And to clarify my point about stalls in swept wing airplanes. Some Long Ezes suffered from stalled wing tips (nose down pitching moment) while wing roots were still lifting (nose up pitching moment).
After a Velocity kitplane (4-seater cline of Long Eze) “mushed” (stage stall) into a lake, they fitted vortilons to all future kits. Fortunately, the “mushing” Velocity descended so slowly that the pilot survived landing in a lake!
Click to expand...

That's all fine and well, but hasn't anything to do with the XP-55. The XP-55 was not a canard aircraft in that it's forward control surface did not carry any of the aircraft load in stable flight as canard aircraft do. The forward control surface was not for trimming, it was only for maneuvering control; dynamic stability, or instability, in this case. If the pilot wasn't pitching the aircraft, the only contribution from the forward control surface was weight and drag. This was neither like the J7W1, SS4, or the Rutan designs.

Yes, Curtiss swept the wings for the same reason Jack Northrop's wings were swept; stability. Because the XP-55 was a flying wing, only with a fuselage and a pitch control surface for maneuvering on the nose.

Unfortunately, there was too much friction in the control system, leading to lag responses and porpoising. The aircraft also had drag and cooling problems. They really needed the help of the NAA engineers in that regard.
 
Dear sundog,
I am not trying to disagree with you.

Rather I am trying to explain stability and control problems with WW2 vintage canards like Ascender and the Japaneses Shinden.
Those problems were not solved until decades later and even Burt Rutan admits that he "lucked out" on some of his canard configurations.

Are you saying that Ascender's canard functioned more like the "rhino rudder" on Rutan's Defiant? .. in that it free-floated until the pilot pressed on a rudder pedal?
 

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