10/23 Raptor Video

[Andy_RR]

This will also be influenced significantly by the centre of gravity and the CL the foreplane is operating at and therefore it's downwash.

 

[Voidhawk9]

Interesting points, and seems quite plausible to me...
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? Of course, the downwash will be much the same, but the tip vortex will tend to flow upwards, 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?
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...).

But I don't have any letters after my name, so what do I know?

 

[Marc Zeitlin]

So dutch roll combined with pitch instability?... giving that rolling, yawing, pitching sea sick cyclic motion?

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.

 

[pictsidhe]

Smoke might be revealing.

 

[Voidhawk9]

Smoke might be revealing.

Depends where it was coming from, and whether it was generated intentionally or not.

 

[Marc Zeitlin]

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?

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.

Of course, the downwash will be much the same, but the tip vortex will tend to flow upwards...

See:

aerodynamics of wing vortices

specifically the sentence that reads:

"For an observer fixed in the air, all the air within the vortex system is moving downward (called downwash) whereas all the air outside the vortex system is moving upward (called upwash)."​

so while the distal portion of the vortex is moving upward, the medial portion is flowing down, and the average movement of the vortices is downward. If this is correct, then your thesis is moot.

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?

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.

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...).

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.

But I don't have any letters after my name, so what do I know?

I didn't realize that having letters after somone's name was what gave them knowledge or logical thinking capacity .

 

[pictsidhe]

Something funky is going on. To me, it looks like it is losing lift on the RHS. It seems to roll right then pitch down. I suspect that Peter's engineer would have designed an aeroelastically suitable wing. 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.

Paging Mr Wainfan, paging Mr Wainfan!

 

[Topaz]

... 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.

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.

All of this is utter speculation on my part. I'm not trying to say "what happened." What I'm saying is, Occam's Razor applies: The answer is probably something simple and basic like the real static stability margin (not what was calculated) or slop/flexibility in the various control circuits than it is some esoteric aerodynamic effect.

 

[pictsidhe]

Velocity thought that the canard may be stalling. That would get worse with the cheaper further forward.

 

[Marc Zeitlin]

Velocity thought that the canard may be stalling. That would get worse with the cheaper further forward.

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).

Now, if the canard was stalled at the 107 - 112 KIAS that the aircraft was "cruising" at, how in Cthulu's name could it have ever rotated at 100 KIAS or slowed down to land at 76 KIAS without being stalled? Rutan derivative canards, if stalled within a foot of the runway, slam the nose gear down hard enough so that sometimes they break. Raptor, while oscillating a bit, did not come close to that.

So if the canard wasn't stalled, even in ground effect (which gives me, in my COZY MKIV, about 3 - 4 KIAS slower stall speed than at altitude) at 76 KIAS on touchdown, then it certainly wasn't stalled at >105 KIAS in flight.

 

[Kyle Boatright]

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?

 

[Marc Zeitlin]

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?

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...

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.

As I said previously, it's pretty clear to canard drivers that the bobbing on takeoff was a PIO. On landing as well, the oscillations looked like overcontrol (PIO) to me. In between - hard to say. The video didn't show much hand motion on the stick, but it was hard to tell, and the oscillations looked a bit too regular to be PIO to me. But I certainly can't say for sure.

 

[BBerson]

Yes, I just watched the video again and the elevator moves a bunch three times just after liftoff. That bucking stops after 5 oscillations total. Then after several seconds of calm some minor bucking returns. And at the flare the elevator over control movement is clearly visible. Pilot induced oscillation.

 

[Kyle Boatright]

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.

Isn't the hinge line on the Raptor offset further than on most (all?) of the Rutanesque designs, possibly accentuating this characteristic?

 

[wsimpso1]

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?

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.

What I find curious in all of the discussion is this notion that the exact vertical position of the canard matters much, citing the downward flows and the wing tip vortices. Listen to Marc. OK, if you need more justification, we can think our way through what we are talking about here for just a minute:

• The downwash from a 3D foil, if it were a solid chunk of air diverted downward, is a cylinder one span of the foil in diameter. A few inches up or down of a field of air that big sweeping down as it goes aft is pretty much irrelevant. Within the span of the canard, the wing is operating in decreased AOA from this downwash, and moving it up or down a few inches will be largely irrelevant;
• The canard boundary layer that is the wake off the trailing edge of the canard is only an inch or so thick, and travels with the downwash as it moves aft. With canard and wing at same WL hieght, this little narrow wake must be below the wing leading edge unless the airplane is unloaded close to zero g. We did not see any of that on the video either, so this is not close to explaining the behaviour;
• The canard tip vortices are at most a foot or so in diameter in the short distance between canard and wing. So on a 30 plus foot span wing we have maybe a foot wide chunk on each that has canard tip vortices washing it, with part of it reducing AOA and part of it increasing AOA, and the net effect largely cancelled over what 6% of the span. Fails the laugh test too;
• Then there is the fact that all this vortex stuff cycles multiple times a second, not every couple seconds.

No, I do not think canard position has much to do with this. My meager position on this has partly to do with knowledge of airfoil mechanics and partly to do with this airplane being within the ranges of well behaving canard ships.

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.

I do know that the successful canard ships tend to use some combination of farily larger lower extensions of the sails and/or vortillons. The RC tamed its Dutch Roll with such a combo, but the Raptor as is sports small extensions and no vortillons... These are easy to try out. After the engine cooling gets under control...

Billski

 

[Alessandre]

Interesting comment from the video:

Olov Andreen

....
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
same. ...

Broken link attached.

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.

 

[flywheel1935]

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.

How about 'Aussie Augmented Attitudes" (AAA)?


PM has just posted a new YT video, the Audi Engine designer is again offering advice, and guess what, PM is ignoring it , as the "fix" would take too long to implement. (AAA)

Oh, and the 'trim spring' has just stress fractured, guess this is the first of several failures of components that may occur over the next few days and weeks, my moneys still on the PRU belts snapping soon.

 

[Voidhawk9]

Seems to be very optimistic claiming that covering the wheel wells has increased his acceleration (by 2 kts in 1,000ft). Drag is insignificant here, and this 'fix' cannot be permanent without leaving the wheels down.
More likely he has a slightly better density altitude compared to last time.

 

[BBerson]

10/27 video

Raptor Aircraft
It's not a quick fix. It would require removing all kinds of plumbing and then reconnecting. You should realize that this would hardly be a quick fix.

1 hour ago1

 

[BBerson]

He may not be "ignoring", but rather trying one thing at a time? (as some suggested)