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Old 05-23-2006, 02:19 AM
chasingmars chasingmars is offline
ChasingMars
 
Join Date: Apr 2006
Location: Ottawa, Ontario, Canada
Posts: 418
Default Flaps

Ok, I know this thread hasn't seen updates in a bit, but I had to make a few comments from an aerodynamics perspective...

On canards, the big problem with flaps is that extension causes a moment change on the wing, essentially, it causes a pitch down moment, the strength depending on the specifics of the foil and flap arrangement. This is going to effectively increase the loading of the canard to some degree (think of it as moving the center of lift of the rear wing backwards, so for a given cg, the canard is bearing more weight)... with the canard bearing more weight, stall speed for the canard actually increases, and the plane will refuse to fly an angle of attack that stalls the canard.

(In a conventional a/c, btw, the increase in pitch down moment causes increased downwash on the tail, which increases the tails effectiveness and reduces the retrim required, in the Katana's that I, like the original poster, train on, there's very little re-trim on flap changes needed)

Hence, no flaps on most canards, unless you get into heroic measures...

-> think the grizly had some sort of flap arrangement?

Wasn't the Grizly a three-surface design? Presence of the H.tail changes things a lot, three surface designs have some advantages in trim capabilities, which of course, they pay for in extra wetted surface, tanstaafl ('course, on a STOL, this is a good design compromise).

>> On the wing, just like the Starship had/has.

Here's a good example of heroic measures the canard is a "swing-wing" design that pivots forward, to compensate, elegent in theory (main wing CoL moves back, canard lift CoL moves forward, and all remains in balance)... rather complex in reality.

o> LE slats on the main wing would be a better idea due to the reversed
o> change in C/L. This would not push the canard load factor higher as
o> the lift increasing devices are deployed.

> From my understanding LE Slats allow a wings to continue flying at a
> Higher Angle of Attack. Pretty much the opposite of how flaps operate.


Drew is right... LE slats/slots don't get you a higher Cl at a given AoA, but let you continue increasing your AoA before reaching the critical AoA and stalling, getting higher Cl as you go. Problem here is that as you do, you're also increasing the AoA of your (presumably fixed) canard. That stalls at it's critical AoA, limiting the AoA of your wing, and no benefit to the slats unless they are also on your canard (into heroic measures again)... slats are also very sensitive to geometry, and at canard scales, I'd worry about manufacturing variability in homebuilts. If you get into heroic measures again, in this case a variable incidence canard you could get around this by lowering the incidence of your canard with slats extending, but, then you're reducing the lift the canard makes for a given main wing AoA, which for reasonable CG locations, will probably drop your nose, unless the moment change of the slats (typically pitch up) causes you to have a lightly loaded canard where you'd be seeking a new balance between so light that it leads to main wing stall, and heavy enough that the increased main wing Cl max from the slats compensates, while avoiding the prop stike and visibility issues mentioned by Drew.

Combine both to eliminate the moment changes, and in addition to complexity, you still have the problem that the canard will still stall when it's at it's critical AoA, to no great gain for the wing. Basically, you've given yourself a really heavy and complicated landing brake.

Of course, everyone has an idea... so, here's mine:
(and no, I don't think it's really worth the effort or benefit, but if you want flaps on a Cozy LE)

The Me163 Komet, a german rocket intercepter from WWII, has an equivalent problem, as a flying wing design, it is very sensitive to moment changes... it's engineers gave it plain split flaps, nice and simple, but didn't put them at the trailing edge. Instead, they went forward on the undersurface of the wing where their moment change was essentially zero. take this a little further forward to where the moment change compensates for the increase in Cl by slightly unloading the canard to allow for an actual reduction in approach speed, and, perhaps, you'd have a workable high lift system for the Cozy, depending on how far forward they needed to be and how draggy such flaps would be... I'd expect that they wouldn't be that useful on takeoff roll, but might make a difference on landing, how much, I'm not sure...

Canadian regs classify the Cozy IV as a high performance homebuilt essentially because it doesn't have a pair of powerful flaps (which is why I've given this issue some thought), but even with the imposition that brings, I wouldn't think it's worth it though, and there'd be a *lot* of testing to validate the result wouldn't cause a main wing stall.
 


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