Rearwing canard... puller?

[LedKitty]

Hi gang!

So there I am, sitting in meating after meeting, learning all the data I need to know for my new job when my brain gets full and my mind starts to wander to... what else? Model aviation! :D

So here's one of the things that have been bugging me this afternoon. Now canard aircraft are nothing new, and just about everyone will tell you that they do tend to be a little pitch unstable, just as having any control surface ahead of the CG will make an aircraft unstable in whatever respective plane we're talking about. The whole "throwing a dart backwards thing".

Now I really like the idea of canards because as we all know the wing isn't required to offset the negative lift of a rear mounted stabilizer so positive lift all around, yeeha! BUT! As previously mentioned, we got that pusher prop shoving the whole thing forward, trying to get the CG ahead of what's slowing it down!

So why, may I ask, do we not see more rearwing, canard equipped birds with the prop up front? Wouldn't pulling the thing along offset some of that aerodynamic instability? Plus, having the all that air blown over the canards would provide it with almost vectord thrust, no?

Worried about the CG being too far ahead of the center of lift? Simple! Just put all the fuel, guns, gubbins, etc. in the wings. Move the cockpit a little further aft! Doesn't this make sense? Yet I don't see many (any?) aircraft of that configuration about? :confused:

So surely I've missed something obvious! Please feel free to shoot my silly idea down now :) I need to put this niggling idea to rest!

*flame suit on* :D

Cheers!

Kitty

[Thomas B]

I would never tell you that canards are pitch unstable beause I have never encountered that issue...:)

You could certainly make one that was pitch unstable, but with the least bit of design care it woud not happen.

Having the canard on the front does not detract from the overall stability in yaw or pitch and it does not matter if it is a pusher or a puller. As long as the model has enough lateral area behind the CG, it will ignore the lesser lateral area in front of the CG. Think about a broad tipped hunting arrow. The canard effect of the broad arrowhead is completely overridden by the larger area of the rear fletching on the arrow. Same on a canard and that is why the pointy end stays out in front.

The same goes for pitch stability. It is all about the neutral stability point on the model and you have to have more projected area behind the CG than in front of the CG.

Put the CG far enough in front of the neutral point that the model is stable and it is stable.

Interestingly, Burt Rutan designed a canard that was a pusher AND a puller...the Defiant twin. And, it had a forward mounted "rhino horn" full flying rudder control. It was big enough to induce yaw as needed, but not so large that it detracted enough from the overall stability of the aircraft to matter.

A few aircraft are three surface aircraft and have both a canard and a conventionally located hoizontal tail: The elegant Piaggio Avanti, the Rutan Catbird, the Amsoil Racer. The relative moments and areas combine to make a stable aircraft.

In every case, it is not about exactly where the areas are, but about the overall postion of the relative center of pressure (neutral point) and the center of gravity.

The main reason you never see a tractor single engine canard is the relative difficulty of making the CG work out, with the CG needing to be close to the front of the rear mounted wing. You have placed the heaviest part of the aircraft too far ahead of the needed CG to make things work out well.

The three surface aircraft alow you some flexibilty in placing the CG of the aircraft relative to the heavier parts which can be moved around a bit and put closer to where you need them.

The Brits did do a tractor canard (Miles M39B Libuelle) but the motors were on the front of the wings, near the CG and not on the nose. No reason a tractor twin canard will not work, because it has been proven to work.

[LedKitty]

Neat! Ok, thanks Thomas :) That's sort of what I was thinking. What if the wings had significant forward sweep and the tractor engnie were placed further aft, driving the prop via a driveshaft? Hmm.. this is starting to sound a bit like the Bugatti R100...

Actually, this all started with me trying to dream up a "Crimson Skies" Westland Wyvern :D I figured I'd toss my ideas around here first before taking it to the Fantasy Planes thread ;)

Wheeee!

K. :)

[eflightray]

There is a thread in the Electric Airplane Talk forum called Canard Forum: Show,Discuss, Learn (http://www.rcgroups.com/forums/showthread.php?t=613556) that may be of interest, but it does run to 2400+ posts.

[HerkS]

There is a thread in the Electric Airplane Talk forum called Canard Forum: Show,Discuss, Learn (http://www.rcgroups.com/forums/showthread.php?t=613556) that may be of interest, but it does run to 2400+ posts.


I've looked at that one - exhausting.
Isn't it amazing how much discussion a bad idea can generate???? :cool:

[LedKitty]

How can I not bite? :D

Why is it a bad idea HerkS?


K. ;)

[JetPlaneFlyer]

How can I not bite? :D

Why is it a bad idea .....


K. ;)

I guess because there is no significant advantage in putting the prop at the front. A pusher is just as directionaly stable as a tractor, an aircraft is not a horse and cart ;)
No doubt you could go to all sorts of extremes to design a tractor canard that had a correct CG location but all you would have done is added weight and complexity to resoleve a 'problem' that never existed in the first place.

About the only real disadvantage of a pusher is that the trop can strike the ground on take off or landing so the gear needs to be longer.

Steve

[Brandano]

Well, the one advantage I can think of in a tractor is that if you use a tricycle landing gear you can keep the prop away from the ground in takeoffs and landings, and improve the takeoff distance. Also, the canards would be directly in the wake of the prop and would have a much greater control authority with the motor running

[LedKitty]

That's sort of what I was thinking :) I'm actually now thinking along the lines of a full tandem wing layout, which is basically an evolution of my original idea ;) This moves the center of lift forward which means you don't have to have the CG as far aft. As far as the horse and cart example goes, does not the prop pull the aircraft along (or at least the low pressure area created in front of the prop sucking it along)? Surely the point at which the thrust is transferred to the airframe plays a part... no?

[Thomas B]

Here is the Miles M39B, which has lots of charisma. A twin allows the powerplant weight to be away from the fuse centerline and works much better for what you are talking about.

[biber]

A pusher is just as directionaly stable as a tractor, an aircraft is not a horse and cart ;)It's rather the other way around.
A prop when generating thrust does behave like some horizontal and vertical surface.
It destabilises the aircraft if mounted in front of the CG and stabilises it mounted aft of the CG.
Not that you'd notice it much on a canard, but on tailless designs you can experience it very well.

biber

[JetPlaneFlyer]

Surely the point at which the thrust is transferred to the airframe plays a part... no?
No, thats not how it works. What's important is the thrust vector in relation to the CG, so as long as the thrust vector is reasonably alingned with the CG the model will be pulled, or pushed, straight ahead, it makes no difference. When was the last time you saw a ship with a prop at the front?
Biber brings up a very valid point, due to the 'fin effect' of the prop, a pusher is actually directionally more stable than a tractor and this could be a major consideration on a canard where the fact that the vertical stab cannot be mounted very far behind the CG can make them marginal in directional stability... Score a big plus for canard pushers! (notice that the Miles in the photo above needs three very large vertical stabs!)

Brandano, i agree with the prop clearance argument, in fact i said exactly that in the reply right before yours. I'm not sure I agree with the control authority one though... Do you really want a model whose control authority varies in proportion to throttle position?.. I know I would not. All the gliders I've flown had perfectly good elevator response without any benifit of prop blast over the elevator, no reason for a pusher canard to be any different and in fact i'm not aware of this being a problem in actual pusher canards of which there are plenty... Again; are you trying to solve a 'problem' that does not exist?

[LedKitty]

Again; are you trying to solve a 'problem' that does not exist?

I'm not trying to reinvent the wheel here :) This is about where art meets science. For those that know me, you know that I'm artist as much as a gearhead, so I often get these crazy fanciful dreams in my head and often wonder if there's any way to turn them into reality.

I'm not sure if any of you are familiar with the series of games (computer and tabletop) titled "Crimson Skies". They're set in an alternate history filled with all manner of odd, fantasy aircraft. Some of the designs are just downright implausible, some less so. Some are even based on some of the X-plane designs of the 30s and 40s. It's not really a question of will they fly better (ok, maybe my original queston was), but CAN they be made to fly decently at all?

Alright, so a tractor prop provides no more directional stability than a pusher prop provided the thrust vectors are lined up correctly. Misalignment of one type or the other is more a question of their moment arm in relation to the centers of lift and gravity than whether they are in front, behind or what have you. Fair enough, case closed.

So most rearwing canard designs are pushers because it's the most efficient way in terms of weight to get everthing balanced properly, also quite fair and logical enough. I suppose what I'm looking for now is a configuration that would allow for the greatest flexibility in the placement of powerplants, control surfaces, etc. Not necessarily for efficiency, but to allow for artistic expression. I'm thinking now that tandem wing configuration might be the best way to do this.

*I'm not trying to solve a problem that doesn't exist. I'm hunting for suitable "canvas" to help my creations take flight! :D

Thank you to all for setting me straight and letting me know the "whys". That's exactly why I started the thread in the first place!

Oh, and yes Thomas, the Miles series of aircraft were part of my inspiration :)

Next I'm going to want to look at the XF5U Flying Flapjack. The performance envelope numbers seem almost too good don"t they? ;)

Cheers,

Kitty

[Thomas B]

Couple of other interesting tractor canards (designs only, never built)...but multiengine and not exactly what Ledkitty is speaking to.

[yoyoML]

Take a look at the Rutan Voyager
http://upload.wikimedia.org/wikipedia/commons/thumb/d/d3/Voyager_aircraft.jpg/300px-Voyager_aircraft.jpg
Canard, pusher+puller, extended tail booms for yaw stability...

Crimson skies is a great game. The whole thing is so coherently 1930's style, including the carrier airships.

[Hughbert]

In terms of a model, or fairly light aircraft, a canard pusher may have difficulties in balance when the motor/engine (which can be a substantial proportion of the weight of the aircraft) is placed right at the back, tending towards a rearwards CG - which leads to instability. Then the battery/fuel/other weight needs to be put pretty far forwards in order to counter the weight of the pusher motor and move the CG further forwards again.

The Wright Brothers had exactly this problem - their 1903 flyer had the engine, drivetrain and pilot all fairly far back and it ended up being unstable.

So in that respect, a tractor canard would have a plus - the forward motor would give a forward CG already, sono balancing weight would be needed and you'd have more leniency in where you placed you're battery/other components.

but CAN they be made to fly decently at all?

I would say almost certainly YES. But, there's really only one way you'll find out ;)

[JetPlaneFlyer]

Hughbert,
The rear mounted motors effect on CG is exactly why canards are usually pushers (read the previous posts in this thread).. The proper CG on a canard is typically just in front of the main wing LE, so you need to have the motor mass central to prevent great lumps of tail weight being required back there. usually rear engine with swept wings gets the CG pretty much spot on. Exessivly forward CG on a canard is very bad because the foreplane will not be capable of supporting the weight so the model would refuse to lift off.

This is EXACTLY the reason why there are not many tractor canards... But it could of course be done if you were determined enough especially with electric power where most of the mass is in the battery and this could easily be placed at the rear.

Steve

[Hughbert]

Steve, I was talking in terms of RC electrics, where airframe weight is largely negligible and as such the motor and battery largely govern the CG location - which is what I build (and from her posts I suspect LedKitty does too) :p

All the tail-pushers I have built have needed extreme forward battery positions, and while a canard would allow a CG further back, I imagine the battery would still need to be pretty far forward...

But then I'm still just a student, so who knows? ;)

[JetPlaneFlyer]

and while a canard would allow a CG further back,

No a canard will DEMAND a CG much further aft, there is no comparison to a regular layout pusher as far as CG is concerned.

Putting it in simple terms:
A conventional layout aircraft will typically have it's CG located roughly 25% (on average) along the length of the fuselage, so obviously the heavy items need to be located ahead of this position to achieve balance.. That’s why the engine is located at the front on most normal layout aircraft.
On a canard however the CG will be somewhere around 60-70% of the fuselage length, so some weight needs to be concentrated rear of this point if you want to achieve balance. If you go ant stick a heavy item way up front in the nose (like the motor and prop) then you will need lots of weight right at the rear to compensate. Also because the prop will be so far ahead of the CG it will be highly directionally de-stabilising = very bad.
Actually electric power models are the least challenging in terms of achieving correct CG because most of the power train weight is in the battery and this can be put pretty much anywhere. Compare that to an IC model where most of the weight is in the engine which (unless you use complex drive shaft extensions) has to be where the prop is, also with IC the fuel tank has to be near the CG.

Steve

[Hughbert]

I'm not sure that 'demand' is the correct term.

The limts on CG location are stability and tail-load, correct? In order to be stable, the CG must be sufficiently far forward of the NP, so that is the rear boundary.

But the forward boundary is that the stick force for the elevator cannot be above regulated limits (i.e. the pilot can actually operate the elevators to control the aircraft). I'm not sure in terms of RC what 'stick load' a servo could handle, or what other factors affect the hinge-moment such as location of the canard e.t.c. but I think you could probably get it further forward than in a full-size canard - which would allow a pusher?

[JetPlaneFlyer]

The ultimate forward limit would be when the canard could not generate enough lift (at reasonable take off speed) to raise the nose and hence the main wing wont achieve positive AoA and the model wont fly. Long before that happens you get very undesirable and extremely inefficient flying characteristics because the canard will have to operate at much higher AoA than the main wing resulting in the canard stalling before the main wing can achieve any reasonable lift coefficient.
To some extent this is an inherent 'weakness' of the canard layout but it's exaggerated by too far forward a CG. This is why i said a canard demands a much further aft CG.