canardlover

Dear fellow designers, I would very much appreciate your input on the use of a flap to be mounted on a canard currently taking shape on my drawing board. The idea is to mount the flapped surface in such a way that its deployment will have minimal effect on airplane pitch. My questions are as follows:
a)where should I place the flap relative to the c.g. (or to the n.p.) ?
b)do you think a combined flap/slat surface would be feasible and more effective ?

My spontaneous thought...... would be to have the 25% chord point of the flapped surface to coincide with the c.g. as depicted but before I proceed please feel free to have your viewpoints on this….Thank you….and…..Cheers/Harald

PS….yes indeed I could combine lowered flaps on the main wing and foreplane via a mixer to achieve this but for certain reasons I will not do that in this case….(please disregard my two empty uploads-dunno how to delete them).…DS

cwrr5

My Feedback: (32)

oh. got pictures now.

Tall Paul

It's the effect of the change in camber with flap, and that change to the pitching moment of the airfoil you have to account for. The flap location isn't really all that relevant.
As what you are doing is outside the box anyway, be prepared for anything, and use the flap incrementally until you determine which way the plane will pitch, if at all, and how severe the pitch might be at differing airspeeds.

cwrr5

My Feedback: (32)

So that would be a third canard on or close to the cg? What are you wanting the flaps to add(or take away)? more lift, more drag, cool factor, etc?

allanflowers

I designed and built a canard with flaperons and leading edge slats a few years back. I found that I could not use the slats, let alone the slats without the pitching moment of the airplane going way beyond what the canard elevator (full flying in my case) could handle. So you are absolutely correct that a flap arrangement had better take into account the pitching action. And, it is not the change in pitching moment of the airfoil section itself but the fact that a canard is a BIPLANE and the rear plane, in my case, suddenly had tons more lift than before.
The Beech Starship, as I recall, has flaps but the sweep angle of the canard surfaces changes to move the canard forward when flaps are deployed, which is clever. I assume it works.
The big problem with your idea is that you are adding a third major surface which will affect the np of the airplane by moving it way forward, just as if you moved the main wing or added surface to the front of it. I suppose you could add a surface that "weathervanes" in such a way that no lift is imparted to the fuselage until you power it into an angle of attack??
Good luck.
Allan
Here's a shot of my canard (currently a hanger queen because it was just too heavy and complicated)

allanflowers

Sorry, I will try again with the pix.
Allan

canardlover

Hi guys and many thanks for your replies....... will try to answer the best I can:

cwrr5: Yes, the reason for the flap is to create drag and produce more lift at lower airspeed to shorten take-off and make approaches steeper. I fly from a rather confined field with big pines and firs. My avatar-canard was tough to land forcing me to approach between two big pines. Any “cool-factor” of the flap would be a bonus…never mind..
Allan: adding a third surface in between two canard wings(b) must NOT in my mind necessarily move the n.p. at all if new wing is properly located. See my attached example with a third larger wing added but with its 25% chord point exactly overlapping the n.p. of the original canard(a)....n.p. stays the same…right..?
Paul: the position of this third wing must be critical to avoid a pitch response assuming the c.g. of the ship is unaltered..right..?
All: as to the pitching moment of the third mid flap my assumption is that it would have a very marked “nose down pitching moment” when flap is lowered and that would further “load down” the already heavily loaded foreplane. To compensate for that I suggested in post #1 to move it a bit forward and coincide with the c.g. rather than the n.p. of the plane. That was my initial TLAR type of reasoning…but shoot it down if you will..!

So the Ferret is your creation Allan.!!….I was so fascinated by it I had to buy a back issue of RCM just for the sake of the Ferret but I´m sorry to hear she is just hanging there now…..more pix..??...pliiiiiis…!

cwrr5

My Feedback: (32)

Quote:
"So the Ferret is your creation Allan.!!….I was so fascinated by it I had to buy a back issue of RCM just for the sake of the Ferret but I´m sorry to hear she is just hanging there now…..more pix..??...pliiiiiis…! "

Heeeey...... I still have my copy!!!!! COOL!!!! I spent HOURS looking at that thing(and drooling). We got us a celebrity here!

canardlover, I gotcha on the flaps. Should work the same whether you have one wing or multiple(???). My question actually would be the "why?"(add a third wing). Besides trying to bash out the theory( [:@] - ouch!), and the inherent "cool factor", what would be the benefit? Would a combination of (larger) flaps and LE slats, in addition to maybe split ailerons(acting as a variable speedbrake, see A-10 warthog) do the trick? I'm very intrested in the outcome of this discussion, I always learn so much when people think "outside the box".

cwrr5

My Feedback: (32)

Come to think of it, my twist would meet those specs! Vertical climb after 5ft t/o roll, vertical landing(rough, but still a landing), with no landing roll.

Mike James

Allen!

Nice to know the creator of that model is on RCU. I was fascinated with it when the article first appeared, and especially happy to see that you bothered to make it look like an actual aircraft, and not just another box. Very cool. Are you still flying it. (Sorry for hijacking this thread... Just wanted to say "Great work", to Allen.)

On the flap issue...
Andy Lennon's book (available through MAN) "The Basics of RC Model Aircraft Design" has some good information on canard arrangements, including canard aircraft with flaps. His math is trustworthy. The book is at http://www.rcstore.com/rs/general/li...id=8&catego=BO

allanflowers

>>more pix..??...pliiiiiis…! <<
I will take a look for more pix. Of course that was before digital photography and I may have to scan some old prints or slides.
I have been taking a look at redoing this plane since I have all the vac-form molds. I have started a CAD model for a new wing and canard but it's pretty low on the priority list. I do have a recent shot of my old tape drawing LO which I will include here now.
Canardlover, keep us informed on your interesting project.
Allan

NFOOTE

I did my senor design project on this same problem.
The problem with putting flaps on the main wing is that it changes the airfoil shape of the main wing with respect to the canard. Deploying flaps causes a change in camber of the airfoil. This changes the stalling characteristics of the airfoil. In a canard aircraft it is important that the canard stall before the main wing and that the main wing go through zero lift before the canard. When flaps are deployed on the main wing, the main wing now stalls before the canard and the canard will go through zero lift before the main wing.
You are also correct in that the flaps also add an additional pitching moment which tends to over-load the already heavily loaded canard. The starship had the moving canard but if you listen to the pilots that flew it the flaps really didn't do anything. The problem with moving the canard forward is that it also changes the neutral point of the aircraft. Thus if the canard moves forward as the flaps are deployed the cg also has to move forward or your static margin goes down, thus it is like a dog chasing its tail. Plus you still have the stalling characteristics to deal with.
The way to get around it is to put flaps on both the main wing and the canard and rotate the whole canard for pitch control. The flaps on the canard have to be deployed at a high rate than the main wing's flaps. This solves both the stalling issues and also the added force placed on the canard.

Nate

Johng

My Feedback: (4)

THis is a cool discussion that has spurred a few ideas that I'll just throw out there in a stream of conciousness:

IF - you are really just looking for a way to get in and out of a tight field there are other solutions. Use the flattest pitch prop that still gives you the up-n-away performance that you want. I've tamed more than one floater by going with a bigger, flatter prop - which generates more drag at idle on approach. It'll improve vertical performance too, which will help avoid the trees on climbout.

Also, concerning landing, you might just want to deploy a drag device to help make the approach steeper. Something like a big 'ol bellyboard speedbrake like on the F-111 or F-100 would allow ou to dive in without picking up speed, while not destroying lift like a spoiler would. Space shuttle approach, baby!

Nate is correct about needing the canard to stall first and still provide lift when the wing is at zero lift. There are other ways to achieve this besides airfoil selection or modification(flaps). You can play with the canard loading (cg location) to get the canard loaded higher, so it must function at a higher angle of attack, closer to stall. This could impact maneverability in aerobatic flight though. Also, a higher aspect ratio wing will stall at a lower angle of attack. And, remember that reynolds number is a big consideration at model sizes - as Mike j can tell you first hand from his long-ez experience. The canard chord can be so small, that it won't provide enough lift to get off the ground. While this can be a headache, you could also tailor it to be a feature that provides stability at approach speed while still being manueverable in up-n-away flight. Of course, this figures into the aspect ratio equation as well. All of these things involve a real balancing act that would take some serious tweaking and forethought.

I might suggest that you consider NACA leading edge droop on the main wing, or even LE flaps to allow it to function at a higher angle of attack. Then use flaps or even slotted flaps on the canard to drive the angle of attack higher. The LE flaps don't add much drag themselves, but they allow the whole plane to get nose high which will be draggy in itself.

canardlover

Thanks all of you for good input, I´m still not sure which way to go but will now take a week off, leave the snow and cold wind in Sweden and go the Canary Island for a week. Will take Andy Lennons book with me and think a bit more before proceeding (thanks for the tip MikeJ)
Nate, yes maybe putting flaps on both surfaces is the easiest way out and in fact that is what Andy Lennon did on his Swan canard featured in Model Builder Oct.-83 where he states:.."...adjust the flap deflection so that when the foreplane flap is at 20deg, the aft flap is at 30 to 35deg deflection........surprisingly, deploying the flaps had no effect on longitudinal trim, the model seems to levitate as it slows down..."
The reason I thought about a third "middle flap surface" instead is special and I will try to explain later.

Johng, yes, low pitch props is an alternative but will detract from normal flying performance so it has a price of course. Must live up to Revver Brother expectations..

Allan, go get yourself a digi camera and show us more details on the Ferret...!..and put the new version of it at the very top of your list...pliiiis..!

....must go pack the trunk now.....again thanks all of and keep the good stuff flowing do not get distracted by me catching the sun..........Cheers/Harald

allanflowers

Here are some Ferret pix.
Allan

allanflowers

a couple more...

NFOOTE

Yep: right on. I should have mentioned it but wasn't thinking. Re plays a huge factor on the characteristics of an airfoil depending on the size and speed of your plane. Good luck finding low speed data... There is some but not a lot and computer programs only go so far.

By the way, with computer radios these days, it makes mixing the flaps between the canard and the main wing a breeze if you go that route.

Good luck on your project,
Nate

allanflowers

After thinking about this thread, I have come to the following conclusion.
Put some kind of flap on the canard surface ONLY. It would probably have to be a dual action thingy in order to provide variable flap settings plus the basic elevator function. Maybe a full flying surface plus a conventional elevator surface?
You don't need a flap or such device on the main wing because it is easy to simply drive it to a greater and greater angle of attack. It will not be able to reach a critical stall point because the foreplane will always stall first.
The canard surface is - in normal flight - already operating at about 130 percent of the loading of the main wing. It is also running at least 2-3 degrees greater angle of attack. This is just the way you have to load a canard in order to insure that it stalls earlier and the ship is pitch stable.
The limiting factor on take-off speed and landing speed on a canard aircraft is the ability of the canard/elevator unit to reach enough lift to compensate for its considerable loading. The lift provided from the main wing is not a issue in terms of minimum flight speeds.
In fact I now remember a canard Viggen (?) built by Col. Art Johnson (I think) that put the flap function on the canard surface. I believe it used elevons or something on the main delta wing for pitch control. This would have been in a magazine at least 10 years ago.
Allan

Mike James

I think you're right, Allen. It's what happens with the canard, not the main wing, that will bite you at takeoff and landing.

Now, every plane will be a little different, and you'll have to do some tests on your particular model to fine-tune it, but...
On the "Avanti", there is a carefully-timed flap deployment sequence, arranged to minimize pitch changes. It uses plain flaps on the inboard main wing, slotted Fowler flaps on the outboard section, and slotted flaps on the foreplane. (Pitch control is via the horizontal stab, not the foreplane.)

Thanks to modern radios and their mixing potential, you could probably work this kind of thing out on any plane, with a few high-altitude tests.

Johng

My Feedback: (4)

This is close to being a circular argument... Of course it is critical to have the canard stall before the main wing - so why add a high lift device to the canard only? An all flying canard or plain flap/elevator on the canard will pitch the plane up to high angle of attack without stalling the main wing as long as the CG is set for stable flight. But I'm guessing canardlover already has that stuff working for him.

Now if you add something that substantially increases the lift or angle of attack capability of the canard like slotted flaps or slats without doing something for the main wing you do indeed risk the main wing stalling first and making a tail first arrival. Flaps on the main wing would not increase the angle of attack that it is capable of( relative to the aircraft datum), but leading edge devices would, thus my suggestion.

I'm just not really following what, besides the obvious, Allan is suggesting???

allanflowers

John,
I agree that one is risking a main wing stall condition with increased foreplane lift. Thus I could just be chasing my tail around the issue. On the otherhand, I remember early "takeoffs" with my Ferret where it simply wouldn't rotate off the ground until the velocity was very high and the end of the runway coming up fast. (If there are engine thrust angle problems in addition, as we found out, cutting the throttle to abort the takeoff resulted in the plane leaping off the ground - but that's another story). In order to warm up for the Ferret, I built and flew a Sig TriStar which, with the intial cg location, didn't want to rotate off the ground either, until the last second at which time the extra drag brought it down very ungracefully and quickly. Even with the cg at the back of its suggested range, no takeoff would occur until the speed was quite high.
Until the foreplane lift reaches a minimum point, elaborations on the main wing are not going to help because the plane is still taxiing on the ground. I think (guess might be a better word) that the limited area/high loading on a "high lift" foreplane will still result in a pitch stable airplane.
Probably the best answer to a low takeoff and landing speed is to build it really light for the wing area. Unfortunately, I suspect that Canard Lover has to deal with high winds in addition to his pine trees.
Allan

destructiveTester

interesting topic.

the canard my father designed last year takes off fine - though it needs a longer run than a conventional planform because the foreplane is naturally more heavily loaded, and is operating at a lower Re than the aftplane...

however it does seem capable of an aft plane stall - It's like both wongs just go and the thing drops out the sky flat - the nose doesn't drop that much.... You are probably going to say that the CG is too far back - but that doesn't seem to be the case - e.g. in low speed cruise I can hold in full up elevator and it does an impression of "having non-circular wheels on the front" - the nose bobs up and down alternately stalling mildly and recovering. It's just in very slow flight that the full both wing stall can occur and very rarely.

The reason why I bring this up is that an aft wing stall is highly undesirable on landing approach! Adding flaps to the canard only could introduce a situation where the stalling speed of the canard is below that of the aft wing - of course dependent on relative incidences, airfoil selection, reynolds number.

JohnG's leading edge flap concept on the aft plane might be just the thing to increase max alpha, max Cl and also aftplane stall speed.

Remember flaps increase camber and decrease the max alpha of stall on a given wing. I have a model which cruises on third power with the body 5 degrees below horizontal in level flight with flaps down and stalls with the body horizontal - the visual cue of body angle as an indicator of airspeed is v. misleading with flaps deployed.

I'd build aft leading edge flaps, standard foreplane flap/elevators (using computer mixing). Another option would be to build oversized ailerons on the aftplane with 2 servos and use mixing to dial in aft plane standard flaps.

Build in fleixibility with your design - and use radio mixing to use whatever works.

Good luck! - and would really like to hear how you get on....

Mike James

When I did the 1/3rd scale Berkut (RC-wise, a long EZ with retracts - See http://homepage.mac.com/mikejames/rc.../berkut01.html ) I had the same issues that Allen has just mentioned. Little or no rotation until a high velocity, lacking elevator authority in general, and the fast landing approaches that go with that. The Berkut setup is the same as Rutan's, with a slotted flap-type of elevator surface on the canard, which I duplicated. Now, assuming the CG isn't simply too far forward...

My solutions were experimental, and were a three-step approach...

First, I tried to fly the airplane with the scale airfoil on the canard. (and scale chord, of barely 4 inches at 1/3rd scale) From past experience, I've learned that one of the "break points" in model aerodynamics (for planes, say between 5 and 15 pounds) is that when the chord of an airfoil drops below about 5 inches, you no longer have the predicted performance for that airfoil. (especially at speeds under about 30 mph) The Berkut would not rotate at all with the scale canard and CG, and ran off the end of a 300 foot long runway, with an OS 1.60 at full power. (Nice engine, by the way)

Step two was to switch to a Clark Y airfoil, increasing the chord by one inch, (to 5 inches) and increase the span about 2 inches on each side. I also increased the incidence of the canard one degree, and "jacked up" the nose gear a couple of inches to put the airplane at a higher angle of attack for takeoff. The airplane would then rotate in about 250 feet, but at a pretty hefty velocity. It had to be flown at full throtte all the time, and would descend in turns, so we had to alternately climb, turn, etc.. We managed to get it down without a crash, but again, at high speed.

At that point there was a huge amount of input from my friends at the field. The most common thoughts were that it was "too heavy", or "needed more power". I believed that it was an airfoil issue. Why? Simply using the "That doesn't look right." rule, it seemed to me that the airplane ought to be able to fly at a lower speed. After all, I did all the wing loading, Reynolds numbers, MAC, CG, stuff before I built it.

So, step three was to switch to an Eppler 197 aifoil, (Thank you, Andy Lennon.) and increase the span 2 more inches total. ( one more inch on each side ) Adding these dimensions, the plane still was very close to scale, and most people wouldn't notice. Knowing that canard airplanes have a wider than normal CG range, and since the plane had been stable so far, I moved the CG back one inch. (not much, on a 1/3rd scale plane) I removed the shims I had used to create the additional canard incidence, putting it back to the original setting. I rotated the nose gear back to it's "fuselage level" position.... That was a leap of faith.

Voila! The plane now rotated at "normal" speed, in about 100 feet, climbed agressively, and had enough power in the mid and low throttle range to "yank" the plane into a forced stall position, where it showed the proper canard performance of settling into a level, stable, "pancake" sort of decent. The plane would go back to normal flight mode by decreasing elevator pitch or adding throttle, exactly as it should. We dead-sticked it, and it landed as well as any similar size plane.

So, this HUGE difference in performance was the result of increasing the canard chord from 4" to 5.25", and increasing the span about 6 inches overall... hardly noticable at 1/3rd scale. If I had taken the advice that it was "too heavy" or "needed more power", I would've been chasing the wrong answers.

Of course, what I did may not translate to your plane, so who knows? But, that's my recent experience, and also my reasoning for thinking that a better canard will solve your problem with no need for flaps. (Or, you could use a speedbrake, instead of flaps, if needed later)