Advice on canards

[rosco]

Here is a design that I have 'knocked up' and I would like to ask a couple of you some questions...

Does the canard have to have a positive angle of attack?
I'm hoping for this plane to fly from 60 to 100 mph(I understand that speed will have somthing to do with it)

Do you think the canards that I have large enough? (wouldn't it just be the same as a 'normal' plane as its just a lever in front of, instead of behind it)

Thanks for any input.
cheers
rosco

[Brandano]

The canard has a positive AOA in flight. the purpose of the canard is the same of that of the tail (or of the airfoil reflex), to counteract the natural nose-down pitching moment of the airfoil. The canard is is usually thinner and at a higher rigging angle than the wing so that it will stal first, and reduce the possibility of the wing stalling

[vintage1]

For high speed rig the canard zero zero..the trim will take care of the slight positive incidence needed.

Its plenty big enough..you can fly without it at all using elevons!

Use one of the CG calculators to get CG right. It will be in an unexpected place!

My guess is somewhere around the leading edge of the rear wing.

I suggest notching the main wing rear to give plenty of prop clearance to reduce noise and improve efficiency.

[Ollie]

Before high speed rig you must take off from the ground. After your high speed rig I think that you want to a good landing too. At low speed the fore wing must lift before stall and the aft wing stalls first!! At high speed, the fore wing must have positive lift before the aft wing goes in zero and negative lift other wise the canard tucks-under!! Be careful to coordinate the two wings aspect ratios, select of two airfoils , two wings stalling angle of attacks, two wings so that don't tuck-under at high speed. It is a complex canard design problem.

Read about chapter 22 of "R/C Model Aircraft Design" by Andy Lennon.

The aft wing flys in the downwash from the fore wing. The effective angle of attack of the aft wing shifts in spanwise direction from fore wing span to aft wing span.

[allanflowers]

You NEVER want the front plane (the canard surface) to stall after the main wing (sorry Ollie).
It would be best to rig the canard two degrees or so greater angle of attack than the main wing. It is advisable to use a similar airfoil on each. Otherwise you want a thinner/easier stalling airfoil on the canard. About 20% area of the main wing on the canard surface is probably okay. The bigger the canard, the more forward you will need to move your cg.
The important thing is to run the wing loading on the canard at 130-140 percent of that on the main wing to make sure it stalls first. Also remember that this is a BIPLANE... the front (canard) wing is doing it share of lifting )actually more than its share. The stall point of the canard surface will determine the minimum takeoff and landing speed of the plane. If you take off with too low a speed, the canard suddenly stalls, picks up tons of drag and down you go. You want a long runway for safety until you know what you are dealing with. Maybe the thrust angle on the prop will have to be tipped UP to keep it from gluing the plane to the ground during your takeoff roll. My canards tended to stick until I tried to abort, dropped the throttle - THEN they would jump in the air once the prop force was reduced (my main landing gear might have been too rearward, making this worse).
Notching into the wing for the prop is a great idea because it is always hard to get the cg forward enough with that engine hanging back there.
Most canards are placed pretty high on the fuse to reduce some of the backwash issue on the main wing but each design is unique.
Still I would highly recommend making a half-size simplified chuck glider to test your calculated cg and, if you can swing it around fast on a string, you can check for any porpoising problems too.
Good luck.
http://i9.photobucket.com/albums/a53/allanflowers/ferretA.jpg

[rosco]

Thanks Allan
Chuck glider it is then...
calculated C.G. seems right! :)

[rosco]

Here is the a test bed that I had cobbled together. I have made it , so that I can get a feel for this...
http://www.rcgroups.com/forums/showthread.php?t=677858

This is a 'destroyable' prototype, of the prototype in the above link. Its an old flying wing of mine with a fuse and canard. The canard will do the work of the elevator and the wing part will only be ailerons.

I have a couple of questions for anyone who would be nice enough to chime in.

1.On a canard, does the elevator work opposite to a conventional plane?

2.The main wing is a meter wide (about 3 feet)...How much throw on the elevator should I have?

3.What types of reactions should I expect the plane to do, like porposing etc?

Thanks for any knowledge.
cheers
rosco

[JetPlaneFlyer]

Yes, an elevator mounted on the foreplane works opposite to what you would find on a conventional aircraft... 'Down' elevator movement will produce a nose 'up' pitch.

Providing you have the canard and main wing relative incidence angles set correctly and the CG somewhere near then, at a guess, 6mm each way should be enough. If you have dual rates use them so you can change sensitivity during the flight if required.

It looks to me that the size of the tip mounted fins could be a bit small. The model may be marginal in yaw stability. If it tends to yaw sideways or 'Dutch- Roll' then add larger fins... or to be safe make them larger now.

[vintage1]

I also think that adding the forward fuselage ahead of the CG will need counter balancing with bigger fins.