Some basic questions about canards

[rtfm]

Hi,

I think canard airplanes are aesthetically stunning.  I would love to build one, but knowledgeable voices (for example, Daniel Raymer - as well as many online resources) have me a little worried.  So I have some genuine questions for the folks here.

 

About me:  I have designed a rather unusual Flying Flea which is rapidly taking shape in my Brisbane workshop.  I like working in wood.  I use CAD and CNC to cut all my parts. I have a preference for small airplanes,  Single seat is cool, but dual would be better.

So I would like to design a canard plane which could meet the following criteria.

1. First, I can't afford a big, expensive engine.  I'm looking at something in the 50hp to 60hp range.
2. I would like a canard with a low (35kts - 45kts) stall speed - I'm not aiming for a speed demon although a decent cruise would be nice
3. I like pretty shapes, so I'd be designing something pretty.  Something akin to the Apollo or the eRacer - but smaller, of course.

So, some questions:

1. For those of you who have flown both canards and regular planes - what's the difference?
2. Almost all canard planes have rudders on their wing tips.  However, the Revelaero canards have twin rudders mounted six feet apart on the main wing.  What are the pro's/con's of this arrangement?
3. For those of you who have flown both canards and regular planes - what's the difference?

Thanks in advance for your time folks,

Duncan

[Kent Ashton]

This site is mostly given over to building established canard designs from plans and talking about them.  You are welcome but you might get more advice at HomebuiltAircraft.com. 

If you want to learn about canard airplanes, the Canard Pusher and Cozy newsletters available here are a pretty good start  http://www.cozybuilders.org/

There is lots of canard advice on these to FB groups.  Some good, some of it pretty lame.   https://www.facebook.com/groups/CozyMkIV   https://www.facebook.com/groups/25741482604/

Edited August 1, 2021 by Kent Ashton

[zolotiyeruki]

I'm not an aeronautical engineer, so take my responses with a grain of salt. 

2) wingtip rudder advantages:
--Less interference drag
--Less weight (no tail boom or equivalent structure)
--You get the benefits of both winglet (efficiency) and rudder (control) in a single feature
--Longer moment arm for the rudders.  I didn't realize this one until recently, but the rudder pushes not only sideways, but also aft, so being on the end of the wing improves its effectiveness.
 

[rtfm]

8 hours ago, Kent Ashton said:

This site is mostly given over to building established canard designs from plans and talking about them.

Hi Kent,

Thank you.  However, what about question #1?  What are the differences in takeoff/flying/landing between regular planes and canards?

 

Duncan

[Kent Ashton]

Canard airplanes generally require more runway—they can’t fly until the canard can develop the lift to rotate the airplane to a takeoff attitude.   After takeoff they fly like any airplane but are not usually considered aerobatic although some guys do aerobatics.   On landing, they are slick aircraft without flaps so speed control is a bit trickier but if speed is controlled, they land like most tricycle airplanes.   I would say the main thing for a non-canard pilot to learn is speed control in the pattern.    Ballasting them for a proper CG is more important than most regular aircraft

 

[Marc Zeitlin]

3 hours ago, zolotiyeruki said:

--Less interference drag
--Less weight (no tail boom or equivalent structure)
--You get the benefits of both winglet (efficiency) and rudder (control) in a single feature
--Longer moment arm for the rudders.  I didn't realize this one until recently, but the rudder pushes not only sideways, but also aft, so being on the end of the wing improves its effectiveness.

Very little of this is correct. You may want to take a look at some of the presentations available at:

http://cozybuilders.org/Oshkosh_Presentations/index.htm

There are a lot of misconceptions about how these canard aircraft work, are built, and fly, both among those that love them and among those that dislike them. I'm in the first category, but I still find myself debunking old wive's tales and other incorrect information on a regular basis.

[zolotiyeruki]

5 hours ago, Marc Zeitlin said:

Very little of this is correct. You may want to take a look at some of the presentations available at:

http://cozybuilders.org/Oshkosh_Presentations/index.htm

There are a lot of misconceptions about how these canard aircraft work, are built, and fly, both among those that love them and among those that dislike them. I'm in the first category, but I still find myself debunking old wive's tales and other incorrect information on a regular basis.

I actually have read through a number of your presentations, and found them quite enlightening.  Could you please point out where I've gone wrong?

[Marc Zeitlin]

7 hours ago, zolotiyeruki said:

I actually have read through a number of your presentations, and found them quite enlightening.

Glad you think so.

7 hours ago, zolotiyeruki said:

Could you please point out where I've gone wrong?

I'll give it a shot.

16 hours ago, zolotiyeruki said:

2) wingtip rudder advantages:
--Less interference drag
--Less weight (no tail boom or equivalent structure)
--You get the benefits of both winglet (efficiency) and rudder (control) in a single feature
--Longer moment arm for the rudders.  I didn't realize this one until recently, but the rudder pushes not only sideways, but also aft, so being on the end of the wing improves its effectiveness.

a) There's no reason to believe that the interference drag of a wingtip mounted vertical stabilizer is any less than the interference drag of a fuselage (or boom) mounted vertical stabilizer - in fact, due to the very complex spanwise airflow caused by the swept wing and vortices at the tip, it could be more, less, or the same. Only extensive testing could say.

b) Less weight - well, less than boom mounted vertical stabilizers, maybe, but given the additional structure needed to mount an inward force winglet at the tip of a wing, only maybe. Less than a fuselage mounted vertical stabilizer - doubtful.

c) As I point out in the "Canard Aerodynamics" presentation, the theoretical efficiency gain of the Whitcomb style winglets, upon which the Rutan winglets were based, is never actually achieved because no-one flies these planes high or slow enough to be anywhere near the maximum L/D speed, so the induced drag is a relatively smaller portion of the overall drag, and the reduction in induced drag is tiny, if measurable. And ALL vertical stabilizers (well, almost all, on any plane) will have the rudder integrated with the vertical - it's just a really convenient place to put a rudder.

d) The drag component (aft) of the rudder deflection is somewhere in the neighborhood of 10% of the moment contributed by the lift (inward) component. Drag is not the main component (one common misconception), and adds very little to the rudder effectiveness. So, yeah - it "improves" the effectiveness, but not by much, and isn't the reason for the winglets being on the tip.

Winglets on our planes are on the tip of the wing because with the swept wing, it's the furthest aft they could get so they could be smaller in area; so that to the extent it exists, they could attempt to be Whitcomb winglets to increase efficiency; and because it was structurally simpler (if not necessarily lighter) than inboard boom mounted vertical stabilizers. It's all a compromise, and it's not at all obvious what an optimal solution is - there are a zillion ways to skin a cat.

[zolotiyeruki]

Thanks for taking the time to elaborate.  My comments were intended to be in the context of a comparison to the RevelAero configuration.

It surprises me to hear that the drag component of the rudder is so small, given that its moment arm is so much greater than the moment arm for the inward component.