Sure - could be - just never had it in any of the canards I've flown that are similar in waterline position of canard and wing to the Raptor, and my comment was with respect to Mr. Andreen's certainty regarding the canard tip vortices being the cause of whatever it is we're seeing.So dutch roll combined with pitch instability?... giving that rolling, yawing, pitching sea sick cyclic motion?
Probably not, given just a few inches of difference, and the size of the canard tip vortex. I'd be surprise if that was a factor - just a few inches.Marc, all the Rutan derivatives place the canard at the same WL as the wing, whereas Raptor places the canard somewhat lower than the wing, perhaps this is significant?
See:Of course, the downwash will be much the same, but the tip vortex will tend to flow upwards...
See above. The tip vortex from both the Raptor and Rutan derivatives will be at or below the main wing. Even at high AOA's (12 - 15 degrees) the distance between the canard and the main wing is small enough that the vortex will impinge to some extent on the main wing. Unless the size of the vortex is only a few inches in diameter 6 - 10 ft. aft of the canard tip, it'll interact with the main wing.which for a Rutan type will always place it over the wing I suspect. But for Raptor it may flow under the main wing at lower AoA?
Anything is possible. But given that there are numerous canards (e-Go, Ibis) that have canards below the main wing and don't have this behavior makes me think that it's very unlikely to be the cause. Also, it's not like the vortex is some discrete oscillatory "pulse-jet" type thing - it's a continuous flow, of which 1/2 will be flowing downward and 1/2 will be flowing upward. It's hard to see how this could cause the slow frequency oscillations seen. Could be wrong - I'm not an aerodynamicist.It seems to me to be a plausible explanation for the oscillations, as the position of the vortex above or below would affect the lift produced by the main wing, setting up a cycle like we saw (ie vortex under the wing reduces wing lift slightly, tending to raise the nose, at which point the vortex starts flowing over the wing due to the higher AoA, increasing wing lift and lowering the nose a bit...).
I didn't realize that having letters after somone's name was what gave them knowledge or logical thinking capacity .But I don't have any letters after my name, so what do I know?
The lack of rigidity/motions of the ailerons are far more likely to be causing the odd roll oscillations than some aeroelastic effect of the entire wing or tip vortices shed off the canard "by a few inches the wrong way." Not saying that IS the problem, just that it's more likely than more-esoteric explanations. Pitch oscillations could be anything a poorly chosen/executed canard airfoil to overall stability issues caused by the CG being too close to the (actual) neutral point. Or it could be PIO's due to flex or slop in the pitch control system. If there's such in the roll controls, there's no reason to believe that the pitch controls "must" be rigid and slop-free.... The ailerons, however, are doing odd things. If you watch the touchdowns, you can see them bounce. They do not seem too rigid or well balanced to me. The left aileron was up for the entire flight. The right one should have been down the whole flight.
A cursory review of the first flight video disproves that theory. Raptor lifted off the runway at almost exactly 100 KIAS - clearly the canard was not stalled at that point (see 10:30 or so into the video). The rest of the flight was performed at approximately 105 - 118 KIAS - faster than rotation speed. Approach over the threshold was at about 90 KIAS. Then the touchdown, admittedly in ground effect, which lowers stall speed, occurred at approximately 76 KIAS - 30 KIAS slower than the flight "cruise" speed, and 24 KIAS slower than rotation speed (see 12:40 or so into the video).Velocity thought that the canard may be stalling. That would get worse with the cheaper further forward.
Both the GU and Roncz canard elevators used on Rutan derivative canard aircraft are also effectively slotted flaps, and the slot shape and elevator position are important to the functionality and hinge moments. In the interest of speed, Klaus Savier has a highly modified Roncz canard without a slot on his Long-EZ, Determinator, and it definitely decreased drag a lot. But it also reduced the maximum Cl a lot, and his plane lands FAST. NOT a recommended mod for anyone who's not racing, and even for those that are. But that's off topic...Is the pitch bobble possibly because he's turned the Canard into a slotted flap with the offset hinge axis and he's getting a disproportionate amount of authority from the elevator? He applies nose-up elevator and gets an exaggerated effect?
Isn't the hinge line on the Raptor offset further than on most (all?) of the Rutanesque designs, possibly accentuating this characteristic?So yes - the elevator on these planes, as a large percentage of the canard area, are VERY effective at high speeds due to the necessity for the area at low speeds for Cl control. And even at low speeds, these planes are somewhat pitch sensitive (not twitchy, but sensitive) and take some practice to get used to, particularly when close to the ground.
Even the Vari-Viggen had below airfoil hinge line and slotted flap elevators. The reason the canard designers use a slotted flap elevator is because the range of lift needed is larger than you can get with a plain flap elevator and reasonable control forces. Slotted flaps get you there.Is the pitch bobble possibly because he's turned the Canard into a slotted flap with the offset hinge axis and he's getting a disproportionate amount of authority from the elevator? He applies nose-up elevator and gets an exaggerated effect?
In my studies about canard design I always read about the best position for the canard is always above or aligned with the main wing because in high angle of attach the downwash became more apart from the main wing.Interesting comment from the video:
As I see it, it is to lower the canarden at least 100 mm ( 4"). This causes the down-sweep to
end up on the underside of the main wing and will not provide a variable lifting force. The
article below show how it looks for delta wings but the principle for other wing types is the
Broken link attached.
How about 'Aussie Augmented Attitudes" (AAA)?I will point out here that the FPV displayed on the video from the first flight showed yaw and roll and pitch all oscillating together while the slip ball stayed centered - roll and yaw were nicely coordinated while the nose was moving about in all three axes. While the lack of uncoordinated yaw may say "no Dutch Roll" to some, it still says we have all three axes interacting to cause this, much as Dutch Roll can. Perhaps this is different enough to earn its own name, but taming this is what I think Raptor needs. How to do it? Wish I knew for sure.