canard lift question

[moonbase1]

I am building a canard glider and I have read that I need to generate lift from my stabilator. Is this necessary? Correct me if I’m wrong, but isn’t the whole point of control surfaces to change the angle of the wing? If the control surfaces were only movable two-dimensional objects, wouldn’t they still be able to leverage the wing?

I would rather not create lift from my canard stabilator because I will be operating at a wide verity of speeds and I don’t want to hassle with the added dynamics different amounts of lift coming from the front of my plane at different speeds. If the lift curves of the wing and stabilator were proportional through this wide variety of speeds, then it wouldn’t be a problem. Is there any way to achieve this? Come on all you veteran aerospace engineers out there, help a student out!

[Majortomski]

Ok back to basics.
In the most general of cases
ALL conventional airfoils generate a downward pitching moment. Without a stabilizing force these airfoils will tumble, nose down.

To fix counter this, in a conventional airplane the horizontal stabilizer IS pulling downward. And again in the most genral of terms the downward force of the stabilizer is only balanced against the downward pitching moment of the airfoil at ONE airspeed. A good designer can tailor the stabilizer to provide the proper down force over a significant speed range without retrimming, this is a neutrally stable airplane.

What triggered Rutan to explore the canard in the 70's was a bit of common sence. If the stabilizer is pulling downward with 50 pounds of force then the wing must not only lift the weight of the airplane it must now also lift this additional 50 pounds just to fly level. This forces us to a larger wing, with more induced drag.

in the case of the canard the force to counteract the downward wing pitch is now UP, the canard is now aiding in lifting the airplane instead of fighting the wing. So the wing can now be reduced in size by a proportionate ammount, and therefore less induced drag from a smaller wing. Voila a more efficient airplane.

Your job is to tailor the size of the canard's lift to fit the forces on the main airfoil.

When Beech built the Starship, they couldn't get a canard the right size to fit all flight conditions. The result was a swing wing canard that went aft in cruise flight to minimize its lift, then swung forward of straight to provide enough lift when the flaps were down and the aircraft slowed down.

HTH

Tom

[Ollie]

I am building a canard glider and I have read that I need to generate lift from my stabilator.

Ans: A canard glider has a small forewing and a large aftwing. The two wings lift vectors sum drag vectors equal the weight vector. You have to take the forces and their directions to able to do the trig. (or geom.) math to doing vector addition. The torques of the forewing and the aft wing have the sum of the torques so the total torque is zero

Is this necessary?

Ans: Yes.

Correct me if I’m wrong, but isn’t the whole point of control surfaces to change the angle of the wing?

Ans: No, the purpose of control is point the whole glider.


If the control surfaces were only movable two-dimensional objects, wouldn’t they still be able to leverage the wing?

Ans: Your question is naive.

I would rather not create lift from my canard stabilator because I will be operating at a wide verity of speeds and I don’t want to hassle with the added dynamics different amounts of lift coming from the front of my plane at different speeds.

Ans: Naive

If the lift curves of the wing and stabilator were proportional through this wide variety of speeds, then it wouldn’t be a problem.

Ans: The polar lift curves are about coefficients. The lift force is equal to atmospheric density, times speed squared, times lift coefficient, times area.

Is there any way to achieve this? Come on all you veteran aerospace engineers out there, help a student out!

Ans: Read and study "Model Aircraft Aerodynamics" by Martin Simons
http://www.amazon.com/exec/obidos/tg/detail/-/1854861905/104-1036173-7186343?v=glance

Also, "Basics of R/C Model Aircraft Design: Practical Techniques for Building Better Models"
http://www.amazon.com/exec/obidos/tg/detail/-/0911295402/104-1036173-7186343?v=glance
Clue:
Why don't canard's do some maneuvers? Read the canard Chapter on design wings so the forewing stalls before aftwing stalls and the aftwing goes into zreo lift before the forewing goes zero lift. Why unavoid sudden lost of control for both conditions?

[moonbase1]

Perhaps I need to clarify. The canard boom will only support the elevator/stabilator. The rudder/fin will be mounted on the top of the wing.

[davidfee]

moonbase1,
There must be a misunderstanding as to what you call "lift" from the canard. The canard is a stabilizer, which means it's purpose is to counter the pitching moment of the wing. With the stabilizer in front of the wing, that generally means positive lift in the vertical direction.

-David

[Ollie]

The canard neutral point (all over aircraft aerodynamic center) is ahead of the wing's aerodynamic center. For stability, the neutral point (NP) must behind the Center of Gravity (CG). The tail's (forewing's) aerodynamic center is well ahead of the CG. Therefore, when the vertical forces from the wing and tail (forewing) must add to over all weight. The clockwise moment (around the CG) of the tail (forewing) is due do tail (forewing) lift. The anticlockwise moment (around the CG) of the wing is due to wing lift. The sum of the moments of the lifts and the moments of the airfoils are equal to zero moment over all.

[davidfee]

Good points.

[biber]

From an aerodynamical point of view most of the planes are just multiple winged systems except lifting bodies deltas and other tailless designs (btw, canards have tails, don't they?). The only real difference between canards and the common tailed designs is the area ratio of the forward and aft wings.

To avoid excess induced drag its no good idea to have the hor. stab. (either tail or canard forewing) producing much lift since the sum of lift distributions of all involved lifting or spoiling wings is the counting. This overall lift distribution should be ellipitical shaped. With two involved wings of very different wingspans it's very very hard to get close to that even at only one single airspeed. Not to mention over a wide range of speed...
For giving a picture how such an attempt could look like see the following link:
http://www.fva.rwth-aachen.de/mainframe_projekte_FVA27.htm
Look closely at the main wing root section. It tapers towards the fuselage! And if one could only build it strong enough it would taper much more towards the fuse than it is shown there in the pics. Thats the try to avoid generating lift with the main wing where the forewing already adds lift to the overall lift distribution, so that it comes out at least close to an elliptical after all.

Most of what seems to be an advantage of the canard design at first turns then out to be a drawback at last.

The reason why rutan canards perform well is not that they are canards. He could have built a common design also with same stallspeed, similar characteristics, and better performance, if it was built up that clean and light and with sense. It was just too easy to build a much better performer than a Chessna 172 or something.

But, moon, that shouldn't discourage you in going on building it, can be a fun thing. Just don't estimate a new world sensation in performance to come out. Yes, there are reasons why canards are such rare things...

biber

[moonbase1]

Thanks for the info guys, it has all been a big help. That is quite the ship biber! For the forewing, is a stabilator the right choice? Or would an elevator type control be the best choice for the canard?

The reason I plan to mount the vertical stabilizer on the wing is to ease the build of the canard boom. I had planed to make it a stationary fin with no movement just to help stabilize the craft. But would it be worth it to strengthen the boom and incorporate a full working vertical stabilizer with rudder?

I will have ailerons on the wing, but they will be rather small. Would it help to design the forewing with two countering elevator surfaces (I’m sure this has a name but I don’t know what it is!)

Thanks again for all your help. The information I gain from your experience may mean the difference between the success or failure of this project!

[biber]

What do you mean with stabilator and elevator type control? Does stabilator mean full flying stabilizer?

Since the forewing won't even have a wingspan close to the mainwing there won't be much benefit of two countering elevator surfaces.

To get used to the specialties of canard designs and to test your actual design idea I would recommend building some balsa gliders and tossing them around, modify and toss again.

biber

[moonbase1]

Your right biber, I mean full flying stabilizer. I guess it wouldn't be a bad idea to build a couple of balsa test gliders!

[macr0t0r]

Canards: different, cool-looking, efficient....and a royal pain to set-up right.

If you want to fly at a wide range of speeds, then you need to be able to actuate the whole canard (i.e. full-flying). A full-flying canard is the easiest, while some designers made the TRIM adjust the canard incidence, but have elevator control use standard control flaps on the canard.

If you mount the vertical fin on the main wing, then a rudder will have almost no affect. There will be very little leverage between the rudder and the center of gravity. The best way to get yaw-control on a canard is to mount the fins on the wingtips, then deflect only the rudder you want to yaw to. Example: to yaw to the right, you would deflect the right fin (causing drag on the right wingtip) yet leave the left fin straight (minimal drag). The drag will greatly help in yawing the craft.

Ailerons on the canard will have no effect. Why are you making the ailerons so small on the wing?

Anyhow, good luck on the project. If you are like me, you'll learn a great deal just through trial and error. Post your results!

- Jim

[vintage1]

I always think the best way to get to grips with CG/wing/tail is to consider two exactly equal and completely symmetrical-in-every-sense wings connected by a boom.

Which way will it glide? Forwards or backwards? where should the CG be?

Assuming it has a bit of longitudinal dihedral, it would glide equally well forwards or backwards, depending on whether the CG was ahead of, or behind, the mid point of the two wings...which probably explains the antics of the original 'flying flea'
;)

Not sure where the fin would go tho :D

[Ollie]

http://members.optusnet.com.au/~ksi_phil/cp43-11.htm
Solitaire
http://www.sailplanedirectory.com/zwfmot.htm

[moonbase1]

Sorry for the absence, but I have made some progress! I finished the full flying stabilizer, and I think it turned out great! The extra balsa strips I put on the LE allowed me to shape it quite aerodynamically without giving up much strength. Below is the finished product. I will also post this on my "rocket powered canard" build thread.

[macr0t0r]

Nice workmanship! You wouldn't happen to have any drawings or diagrams of what you are cooking up, would you? You got me curious.

- Jim

[markdrela]

moonbase1,
There must be a misunderstanding as to what you call "lift" from the canard. The canard is a stabilizer, which means it's purpose is to counter the pitching moment of the wing. With the stabilizer in front of the wing, that generally means positive lift in the vertical direction.

I wouldn't say it that way. On a canard configuration, the canard is a moment-trimming surface, but it's not a stabilizer. It's actually a DE-stabilizer, just like any aerodynamic surface placed in front of the CG. The aft wing is the stabilizing surface.

[davidfee]

I wouldn't say it that way. On a canard configuration, the canard is a moment-trimming surface, but it's not a stabilizer. It's actually a DE-stabilizer, just like any aerodynamic surface placed in front of the CG. The aft wing is the stabilizing surface.

You're absolutely right, of course. Sorry about the way I was so careless with my words. Your explanation is much more accurate and gives a better mental picture of what's going on.

thanks,
-David :)