CG on a Canard

[pat king]

I am working on a Canard design. The airplane has a swept back leading edge and a straight trailing edge. The airplane will not have flaps. The engine is in the back in a pusher configuration. For a conventional airplane I would start with the CG at 20% MAC. Should the canard airplane be any different?

Thanks,  Pat

[Howard Rush]

Way different.  CG will be far forward because the canard destabilizes, rather than stabilizing.  You might make a hand launched glider to find a CG that is stable, but with enough control.  Call me to discuss this if you like.

[Tim Wescott]

What Howard said.

Howard will sneer at this link from Zenith Air for being overly simplistic, but it's worked for me in the past.  You should still consider it to be a starting point, and do glider experiments.

[Tim Wescott]

Just something to think about: any aircraft that is balanced for stability must have a higher coefficient of lift on the front wing than on the rear.  So the canard has to lift more than the wing.

This is why, in the "Mastringer" Ringmaster Canard that I designed several years ago, the stationary part of the canard is thick.

I can't tell you how it works -- I've never built one.  But you might want to keep it in mind.

(You can see the CG on the plan -- that was figured out using gliders.  To repeat: I have not built this plane, so I can guarantee nothing about it's flight performance).

[Dave_Trible]

Tim this looks like the one Doc Holiday built.  I didn't know where the plans came from.  I think he balanced it as indicated.  The first flights were pretty squirrelly ( which surprised me). He had to go to a very slow control mix but it flies pretty well.  It loops and eights about like a normal Ringmaster does. 

Dave

[Tim Wescott]

Tim this looks like the one Doc Holiday built.

I questioned him about that when he posted it here -- he says that he copied Dick Sarpoulis's Wild Goose from Flying Models, with mods to make it look like a Ringmaster.

Since that's pretty much what I did with the exception of the thick canard, I figure that we both just got the same good idea.

[pat king]

Thanks for the input. I am not really a hand launch glider type of guy. I will do a 95 square inch 1/2A version to experiment with under power. Once I have that sorted out, I'll finish the 593 square inch version.
Howard, I will give you a call.

Thanks,  Pat

[Howard Rush]

Pat's idea is extremely cool. 

[Randy Powell]

Built a canard stunter about 20 years ago. As I remember, the balance was somewhere in front of the leading edge of the main wing.

[Tim Wescott]

Built a canard stunter about 20 years ago. As I remember, the balance was somewhere in front of the leading edge of the main wing.

That's typical of a canard.  Makes locating the bellcrank and leadout guides interesting.

[pat king]

The laser cuts are on order for the 1/2A airplane. I will use that one to determine proper CG, elevator size, and if anything needs to be done with the vertical stab. The bellcrank is mounted through the fuselage in front of the wing. The leadout guide is mounted on the end rib. The leadouts are in front of the swept back leading edge.

Pat

[Ted Fancher]

I am working on a Canard design. The airplane has a swept back leading edge and a straight trailing edge. The airplane will not have flaps. The engine is in the back in a pusher configuration. For a conventional airplane I would start with the CG at 20% MAC. Should the canard airplane be any different?

Thanks,  Pat

Pat's original design concept (quoted above) might well have been conceived after reading what amounted to an afterthought comment in a Model Aviation article several decades ago by a highly respected and competitively accomplished stunt champion who opined an almost exact duplicate of Pat's planned configuration as the potential Rosetta stone of stunt model design.  His concept included a CG "conventionally located" at or just forward of the wing's main spar with a conventionally located control system and leadout location combined with an aft mounted engine necessary to achieve the required far aft CG location.  The writer proposed such a concept as necessary to achieve "...the rewards of canard performance improvements [which] could be rich."

Fortunately, Pat posted his intentions and asked the most important question: "Should the canard airplane be any different?"  Double fortunately, the concept has been bandied about at much greater length and depth of inquiry in the interim and, although the canard concept continues to surface periodically more valid assessments have been made and the underlying properties of a canard which make it an unsuitable and, thus, unlikely configuration for conventional aerobatics have become pretty much widely accepted.  While Pat's experiments will be of interest I believe he will ultimately find a remotely successful canard will require a forward swept wing, a tractor engine located much more forward so as to put the CG somewhere forward of the "neutral point" of the total air frame in order to be stable and a fuel tank located at the CG to avoid burning fuel aft of the CG and resulting in stunt ship getting progressively more nose heavy and less responsive as it gets to the parts of the pattern requiring more aggressive response.

Bob Hunt had a design well along that would have addressed many of these underlying configuration issues but we've yet to hear reports of test flights.  Bubba doesn't often make dogs (other than Bronze ones) so I'm looking forward to see what results from his efforts.

Ted

[Tim Wescott]

Pat's original design concept (quoted above) might well have been conceived after reading what amounted to an afterthought comment in a Model Aviation article several decades ago by a highly respected and competitively accomplished stunt champion who opined an almost exact duplicate of Pat's planned configuration as the potential Rosetta stone of stunt model design.  His concept included a CG "conventionally located" at or just forward of the wing's main spar with a conventionally located control system and leadout location combined with an aft mounted engine necessary to achieve the required far aft CG location.  The writer proposed such a concept as necessary to achieve "...the rewards of canard performance improvements [which] could be rich."

I'm going to channel Howard here, from a discussion about canards here some years back.

Basically, you could, in theory, make such a plane fly -- if you had a really high-bandwidth control system that's actively putting in all the stability that you took out by putting the CG so far back.  The airframe itself would be viciously unstable in pitch, but if you could get fast enough gyros and servos you could stabilize the plane.

If you don't do that, and you move the CG in front of the airframe's neutral point to get stability, then you create an airframe that needs a higher coefficient of lift on the forward surface than the rear.  This is a pretty much universal requirement on airframes -- it's just that in a conventional airframe the big wing is the one that also needs the higher C/L, and the little "wing" has the lower.  In a canard, whatever you need to keep the main wing from stalling, you need more of on the canard.  It's why the plane in the PDF that I posted has a thick horizontal stabilizer.

[Ted Fancher]

It should be noted that canards similar to what Pat proposed have been flitting about general aviation facilities for some time.  Similarities include an aft mounted engine and a (highly) swept leading edge of the mainplane/aftmost horizontal surface/wing (Canards are clumsy linguistically!).  They are variously called Variezes and Long Ezes and are early productions of the brilliant Rutans.

Significant *differences* from Pat's planform, however, include a similarly highly swept trailing edge of the main plane (resulting in a comparatively high aspect ratio fully swept wing) and a human pilot located just about as far forward of the ultimate CG location (CG "range" on the man carrying ships) as the engine is aft of that location...essentially balancing one another out.  The combination of the two, a wing swept aft to get its MAC (think average chord) more or less aligned with the spanwise location of the engine--thus approximating the longitudinal location of the center of lift of the swept wing--and the weight of the pilot forward. provides for the necessary result of the CG being forward of the center of pressure/neutral point (known as the "static margin") of the vehicle necessary to produce a stable craft.

Strangely enough (and a head scratcher for us CLPA types looking for tight corners), enlarging the canard surface which provides pitch inputs to our "stunt" ship reduces the static margin (more surface area forward moves the neutral point forward) and will require the CG to be moved further forward as well (to retain a stable platform) thus negating, at least in part, any improved maneuverability gained from the larger surface.  Just a guess on my part but that conundrum might well be the reason for the common use of high aspect ratio canards which I expect would produce more lift per unit of deflection (just like a super long sailplane wing) thus making them a superior means to gain enhanced pitch authority with minimum required static margins.

Now, although the above has been presented as "informed" information it might just be partly or totally hocum.  If so, I've no doubt it will provide the needed impetus for correction from truly "informed" forum sources!

Ted

[Ted Fancher]

I'm going to channel Howard here, from a discussion about canards here some years back.

Basically, you could, in theory, make such a plane fly -- if you had a really high-bandwidth control system that's actively putting in all the stability that you took out by putting the CG so far back.  The airframe itself would be viciously unstable in pitch, but if you could get fast enough gyros and servos you could stabilize the plane.

"snip"

Tim, Is Howard going to produce those with his 3D printer?   Those'll put PJ's VGs to shame overnight!


Ted

[Tim Wescott]

Tim, Is Howard going to produce those with his 3D printer?   Those'll put PJ's VGs to shame overnight!

Howard might try putting me up to designing something (in theory it could be done with a TUT).  I would explain that that would have to wait on my retirement, which I don't think I'll ever achieve.

[pat king]

I have the laser cuts for the small model. If I can get it to do what I want I will finish the 600 square inch airplane. I am not attempting to design a CLPA ship. If I can get the canard to do inside and outside loops, and fly inverted. It will meet my needs. The whole idea is that the airplane will appear to be flying backwards. The leading edge is swept back, the trailing edge is straight. It will be just for fun and to mess with peoples heads.

Pat