canard as wing's leading edge AoA modifier

[JeenanRex]

apart from the canard stalling before the wing and thereby preventing actual stall, can a canard be thought of as modifying the incoming air stream at high AoA, so that the angle at which the air stream actually meets the leading edge is at a favorable angle?

 

[Marc Zeitlin]

... can a canard be thought of as modifying the incoming air stream at high AoA, so that the angle at which the air stream actually meets the leading edge is at a favorable angle?

No. First of all, unlike your diagram, the canard (front wing) is always at a higher incidence angle than the main (rear) wing, or else the aircraft will not have pitch stability.

 

Second of all, the canard creates a downwash, the magnitude of which grows as the AOA increases, but the since the relative positions of the canard and main wing (strakes, in this case) change as the aircrafts incidence angle increases, the relationship between the effective downwash from the canard on the strakes is a very complex one.

 

At any rate, the downwash from the canard decreases the effective AOA on the strake area, decreasing the lift produced. Generally, that's not a good thing, and the relative positions of the canard/main wing/strakes have been set to minimize any negative effect. This is one reason why moving the canard either up or down from it's plans position is not a good idea.

[JeenanRex]

thanks....

 

but in case of eurofighter and many other modern aircraft, the canards do move...

 

ok, in the case of eurofighter, the canards are way away from the wing, but in the case of sukhoi 30 mki, it is close to the wing's leading edge and the canards do move in both positive and negative angles

[Bearair]

True, but those are not true canard aircraft. The SU-30 is a conventional fighter with canards added to increase the turning ability of the fighter. They still have a conventional wing/tail arrangement with primary pitch control being provided by the elevators. The canards were simply added as an update to the SU-27 Flanker. Not all the SU-30's have the canards. Canards as an additional flight control surface have been added to many modern fighters and those are actually considered triplanes. The canard maintains stability and controlled flight of the main airfoil at high angles of attack. Not the same as being the primary flight control surface.

 

In the case of the Eurofighter, the main control for pitch comes from the flaperons at the rear of the main airfoil. It's simply a Delta wing fighter. The delta wing dates back a long time and is essentially a flying wing. The addition of canards near the wing leading edges is to have the same effect that the small canards have on the conventional layout fighters. It increases control at higher angles of attack.

 

The Rutan designs don't have flaperons, only ailerons. The horizontal stabilizer is in front of the main body of lift and so is the elevator on the back of the stabilizer. The Eurofighter and the SU-30 have their elevators at the rear of the plane. Also, they are longitudinally unstable by design. If they didn't have a computer controlling the flight systems, they'd tear themselves apart. Most of the GA fleet doesn't have that luxury. We all have a pilot that has to maintain control which is why we fly inherently stable aircraft.

 

We can't all be fighter pilots... (damnit)!

[JeenanRex]

ah... true...

 

then, in the case of an aircraft having the canard not as an elevator plane,it's purpose is to have a higher AoA than the wing thereby stalling before the wing does and hence bring the nose down eventually bringing the aircraft back to stable flight? or is there more to it? please enlighten me upon this...

[Bearair]

The canard on those fighters is there to maintain a lifting surface, and thus control, no matter what angle of attack the wing is at. If you ever watch video of the Flankers doing airshow routines, you'll notice that the plane can have an angle of attack of well over 90 degrees and the canard will be deflected downwards and is pointed in the general direction of flight. This maintains lift and directional control even when the plane is travelling in a direction and pointed OVER 180 degrees from that direction. Along with thrust vectoring, the plane can literally flip around on the pitch axis, change it's vector on the roll axis, and be flying backwards and be controllable. The canards and the thrust vectoring exhaust are what keeps it pointed in a direction until the thrust can push the wing fast enough to develop lift. at low gross weights the plane can just about hover due to excess thrust. Makes for a VERY interesting airshow routine. Keep in mind that the main wing is stalled once it's critical AOA is reached and stays that way all through the low speed maneuvering.

 

So basically the canard develops lift nearly all the time which allows putting the nose wherever the pilot wants it. Think about pitching the plane around in the pitch and roll axes to keep a cannon pointed at a target without worrying about stall speeds. After the shot is made and the inertia bleeds out during the maneuver, you use thrust to regain energy and engage another target or extend and disengage. It puts the pilot in a low energy state until speed can be built up again which makes the aircraft vulnerable, but if it's a dogfight with a single opponent, the ability to do things that were unheard of 20 years ago could come in handy. Do a search on youtube for Flanker or Su-30.

[JeenanRex]

obviously, the canard is computer controlled and hence can be directed appropriately during wing stalls and hence "generate lift". This is very similar to the diagram that i put up earlier, except that i misinterpreted that the downwash from the canard is going to help change the AoA of the main wing and hence maintain lift in the main wing itself. but, now i get it that it is the other way around. That the canard generates the lift at stall and the downwash is not helping the wing at all.

 

now the question is...

 

how is that the canard, being such a small surface, able to generate enough lift, even though there is so much drag being generated during wing stall and hence reducing airspeed?

[Bearair]

The lift that it generates maintains directional control, and that's about it. The rest of the aircraft is held aloft with thrust. A LOT of thrust. Think, 54,000 lbs. (or more) in afterburner.

[JeenanRex]

wow... thats something....

 

one of the reasons designers dont go for a canard based aircraft is because of the downwash that it creates which leads to decreased efficiency of the main lifting surface...

 

is it possible to design and put a canard close to the wing's leading edge so as to reduce the downwash from it, in the sense that the wing acts as the canard's downwash inhibitor?

[coyote]

Sorry to stop the dream and come back to a LongEz issue.

When my speed increases upper than 150 kts, my canard trim is full forward, and i still have to push the stick. I modified the trim to ncrease the stroke, but it is still not enough.

I've been said that i need to adjust my wings washers so that the wing incidence is better fitted with the canard one.

Could one of you tell me if i need to increase or reduce the incidence of the wings ?

[Drew Swenson]

You need a stronger forward trim spring---or an additional one. Or you could put on one of the electric pitch trim motors.

 

The problem is that your spring is not strong enough to hold the position it needs to be in with all of that speed.