propwash2

My Feedback: (3)

I have been watching video footage of real Rafale jets maneuvering and noticed that the canards pitch down substantially when the jet is put into a steep climb or tight turn. As I am building a turbine Rafale I am interested in this observed control integration and whether I should incorporate it in my models control mixing or just do the conventional positive canard movement with up elevon mix. Is the reason the canards pitch down to keep them at a lower AOA and thus generating maximum lift in high alpha maneuvers?

perttime

I was just looking at a picture of the experimental X-31 jet at high AoA and the canard was pretty close to horizontal. I suppose they want the canard to keep flying, for control, whatever else is going on.

Doing the same thing in RC might not be practical. Those fly-by-wire jets have the big multiple computers constantly evaluating what exactly each surface needs to be doing. Also, the airframes are not stabile like RC models: the computers make them flyable.

da Rock

It would be nice to find a written explanation of what you guys have observed. Without that we're left to speculate. so...............

If the airplane is flying at a high angle of attack, it's probable that the canard would be stalled out if it was locked into that AOA as well. Wings usually have camber altering surfaces like flaps and slats that permit them to function usefully at higher AOA. Canards and stabilators usually don't have those devices and have non-changing profiles. Their profiles (airfoils) are often symmetrical due to their functions, and symmetrical profiles have even less range of AOA between stall angles. So it's a given that the canards need to be kept "flatter" to the airflow to keep them producing lift efficiently. They're also operating in lots less favorable RE which would add to their problems.

It would lead that if they were to continue to contribute efficient pitch control, that they'd be operated in an efficient range of AOAs. And that'd mean that their control system would work them the way we see them being worked. And with computer aided control systems, that's not too hard to do.

But that's just a guess. The "true" answer will need to come from someone with inside information on those fighters.

So basically, just what perttime said...............

Shoe

Here's my speculation: The main purpose of the Rafale's canards is to work in conjunction with the elevons to control the aircraft longitudinally by generating pitching moment. The moment that the canards contribute to the the total aircraft pitching moment is due not only to the lift and moment they generate directly, but also due to their effects on the main wing. At high angles of attack, I would expect the canards to exert more overall longitudinal control influence by changing the lift and pitching moment of the main wing than through the forces and moments they generate directly. The canard scheduling you observed on the Rafale suggests the effect of canard deflection might reverse sign beyond a certain angle of attack. For these reasons, I think it would be rather challenging to determine right mix of canard and elevon deflection to best achieve the desired longitudinal response at high angles of attack (particularly challenging to determine through flight testing). If you don't have a way to feed angle of attack into your control surface deflection mixing, I doubt that you could get predictable response by simply reversing canard deflection beyond a certain stick deflection. Without AOA feedback, I can imagine your jet pitching one direction if you pulled back on the stick slowly, and the other direction if you pulled back quickly (probably not a desirable flying quality). Best of luck! I would look to people with Rafale/Gripen/Mirage experience for how they scheduled their canards as a starting point.

BMatthews

I'm with the general flow of the others and echo that you shouldn't be trying this in a direct ativated style of control.

The full sized fighters typically fly with a neutral or negative stability margin these days and rely on the computers to control the surfaces to catch and limit the divergences that would occur if the flying was left up to the pilot alone. Without the various sensors and on board computer to make sense of it all you don't dare try to copy the full sized CG location and control system. Far better to keep the CG at what is normal for single ended control inputs (our typical RC style of flying) for forget about the aft CG closed loop arrangement.

However another option occurs to me. You could look at the idea of hinging the canards at the 23 to 24% point and then mass ballance the surfaces to a degree where they balance "just" in front of the hinge pivot and let the airflow of the moment "trial" the surfaces to whatever forms a low drag sort of positioning. A bit of grease in the hinge system would act as a viscous damping agent to avoid or at least limit any flutter but with the surfaces set up and balanced as indicated this should not occur I would think.

With such free flying surfaces when you're flying under high AOA on the main wing the canards would tend to at least operate as you've noticed by "flying" at a far lower angle of attack and coincidentally they would be directing air into the separated bubble when things get that far gone and may help with very high AoA flying by doing so.

Of course this is all guessing. I'd like to think that it's EDUCATED guessing but it's still guessing. A cheapie test model would be a nice way to try the idea out before commiting it to a fancy scale model. Something very cheap and dirty with a pusher glow prop engine and square fuselage that only looks Rafale'ish in a "stand way back and squint" sort of way...

perttime

On the electric side people seem to do lots of flat foam pushers to test their ideas.

I was thinking that it might be interesting to assign a couple of different mixes to the canard: no canard movement, up elevator = down canard, up elevator = up canard.

edit: I must have been thinking of some sort of a "canards as reverse flaps with elevator to canard mix" : when you are at higher AoA, flip a switch to get the canards to lower AoA. Better experiment on something cheap, though...

guille2006

BEWARE!!!!!!!!!!!!
ALL PLANES MENTIONED ARE STATICALLY UNSTABLE!!!!!!!!!!!!
YOU'RE GOING TO BUILD A STABLE ONE SINCE YOU'RE NOT INSTALLING A FLY-BY-WIRE SYSTEM.
In many cases, those canards are intelled in order to creta a turbullence over the LERX and lift is sustantially increases (su-30 is the typical example)

Crominator56

Lots of talk about stable vs unstable, all very true: here is what it means in practical terms:
If the design is statically unstable, meaning the CG is aft of the mystical Aerodynamic center, meaning that when the nose pitches up (as in combat maneuvers) the tendancy is for the nose to pitch up further. So then canards would actually require a downward force to prevent the aircraft from continuing to pitch up into a stall. This may explain why they look like they are at such a low angle of attack. They may be trying to pitch the nose back down.


As a side note, planes like the F-16 (conventional config) require lift on the tail to counter this pitch-up tendancy, which actually adds lift when you need it most.

cheers

John

perttime

Ahh, but the canard planes also have control surfaces at the rear.
The canard just adds a few things that the computer can do, in addition to using the tail surfaces.

propwash2

My Feedback: (3)

Thanks for the great input - all of you. As my model will have a positive CG position I'll keep it simple and just have them pitch up with up elevon - I'll keep them fixed for the first flight and slowly mix it in as she goes. As brought up by Crominator56 - I wonder how efficent the canard setup really is ? - as although you can fly with a twitchy aft CG position to control it you are having to
use the canard to fight the pitch tendency ( opposing forces). Anyone have a informed opinion on how these new canard fighters
(Rafale/Eurofighter) stack up against conventional setups like the F15/F16/F18 in maneuverability?

guille2006

Cannard is as eficient as any "conventional" tail... maneuvrability depends a lot of static margin (distance between CoG and Center of pressure) and how big your control surfaces are. Just analyse the "free body diagram" af a plane to get it...
I'd rather make a comparison between Rafale/Eurofigther against Su-27 / Mig-29; those russians are most maneuvrable than others (and I'm not pointing to the canard+trhust-vector-control ones).

Johng

My Feedback: (4)

I'm with Bmatthews on this. The 'conventional" configured fighters of the US behave similarly, in that the control surfaces seem to be showing activity out of line with the maneuver conditions. This is bacause these planes are statically unstable. That means that when pitched up, the airplane tries to contiue pitching up. So, the Rafale canard shows "down" deflection during high alpha flight since the rest of the airplane is trying to continue pulling up. I have not observed the Rafale, but I have seen the same thing on F-16 and F-18 in high-alpha flight.

This type of thing could be emulated on a model by using gyro technology on the pitch channel, borrowed from the heli boys. Heading hold would be able to fight static instability. However, it would take some math or risky experimentation to figure out how to set the control power and gain on the gyro to not have a real bad time. Plus elevator control response would be significantly different, as it's only a gyro, not a full flight computer.

=skywalker=

My Feedback: (7)

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