Swept wing, canard design

[JaRaMW]

I am thinking of building a model with forward swept wings and canard setup. The advantages of each of those designs are just too seductive. To get an idea, think of a layout similar to the X-29.

Regarding this, I have a few questions:

1. for a forward swept canard, a pusher prop seems more appropriate since a tractor, far ahead of the wing's center of lift, would limit its pitch maneuverability. Thus a pusher closer to the COL sounds like a better idea.
However it is my understanding that pushers suffer from a higher loss of effectiveness due to the disturbed incoming air compared to tractors with the disturbance in the propeller slipstream.
How about an EDF? I've never used one, would they be better suited in terms of effectiveness?

2. The side rudder at the tail, relatively close to the CG in this configuration, results in a short lever arm. What about placing the thing near the nose of the airplane instead (except for the crappy looks)?

3. - this is a general question - what about a forward swept elevator? I am wondering anyway why this design is so rare (if it exists at all). The advantages of higher max. AOA, reduced induced drag (not really a non-issue for low aspect ratio wings like an elevator, isn't it?) and inboard spanwise flow (to keep the wings "clean") makes this look better than the conventional straight or back-swept designs.

[JetPlaneFlyer]

I would guess that with a swept forward wing canard the CG would be pretty close to the longitudinal centre of the fuselage so tractor or pusher would be roughly the same distance from the CG.

The efficiency loss due to disturbed airflow on a pusher are relatively minor. Certainly an EDF would be much less efficient.

There is good reason why all planes have their vertical stabiliser at the back.. the same reason that an arrow has flights at the back not the front. Put the fin at the front and the model will spin around 180 Deg and attempt to fly backward ;)

Swept forward wings (or canards) don’t have inherently lower induced drag. in fact any sweep decreases aspect ratio so tends to increase induced drag, assuming optimum planform in all cases. A swept forward canard would also be very prone to damage on landing.

Swept forward wings do have some potential issues... With a 'safe' stall pattern where the inboard (and rearward) portion of wing stalling first then the centre of pressure of the wing moves rapidly forward as the stall progresses causing a nose up pitch; very dangerous when you are approaching a stall anyway. Swept forward wings are very laterally destabilising which will be likely to make the model want to drop into a spiral when you turn it. This can be addressed by adding dihedral but then inverted performance would be problematic. Also swept forward wings are very prone to flutter and so have to be much stiffer (and so heavier) than straight wings.

It's probably no co-incidence that many (or all?) full size planes that have highly swept forward wings also have computer stability enhancement.

Regardless of all this it would make an interesting model so don’t let me put you off….

Steve

[Airboatflyingshp]

http://www.palosrc.com/instructors/putte.pdf try this and search for Canard Addicts thread in electric power....there's rather a lot there.

[JaRaMW]

Thanks for the input!

I would guess that with a swept forward wing canard the CG would be pretty close to the longitudinal centre of the fuselage so tractor or pusher would be roughly the same distance from the CG.
Yes, but I also want a reasonably long lever arm for elevator and side rudder (if mounted at the nose). In a tractor config, this would either mean motor in the nose -> mass concentration not close to the CG -> low pitch maneuverability or mounting elevator (and rudder?) in front of the propeller, which would not make much sense in this case.

There is good reason why all planes have their vertical stabiliser at the back.. the same reason that an arrow has flights at the back not the front. Put the fin at the front and the model will spin around 180 Deg and attempt to fly backward ;)
I thought the vertical stabilizers were mounted at the tail mainly to prevent the sightseeing pilots from bitching too much. Yes I know, that little distrubance hitting the stabilizer... or de-stabilizer in this case :eek:
But I want maneuverability and efficiency, I'd just fly it on calm days then :D After all, a canard would have the same disadvantage over a tail-mounted elevator, but obviously this can be handled.

Swept forward wings (or canards) don’t have inherently lower induced drag. in fact any sweep decreases aspect ratio so tends to increase induced drag, assuming optimum planform in all cases. A swept forward canard would also be very prone to damage on landing.
It is my understanding that the forward sweep of a wing induces a spanwise flow towards the fuselage as opposed to spanwise flow towards the wing tips for a straight or swept back wing. This causes the boundary layer thickening near the wing tips to be less pronounced, which helps to delay the stall.
Doesn't this inboard flow then also weaken the vortices, simply said by preventing more air from flowing into them?
Another point may be lift distribution: due to this weaker effect of wing tip vortices, the tips could be allowed to create more lift than a non-swept wing and still would not cause drag to rise a lot => better lift/drag or, for given lift, less drag.
These are just my thoughts, feel free to correct me. As far as I remember, the X-29 demonstrated lower induced drag due to forward sweep.

With "lower induced drag in a canard design" I meant that the canard creates lift as opposed to down-lift required by a tail-mounted elevator, so the main wing needs to produce less lift and thus less induced drag.

I don't see much of a problem with the landings... if the wings take all the beating, then at least the fuselage will live longer :D

Swept forward wings do have some potential issues... With a 'safe' stall pattern where the inboard (and rearward) portion of wing stalling first then the centre of pressure of the wing moves rapidly forward as the stall progresses causing a nose up pitch; very dangerous when you are approaching a stall anyway. Swept forward wings are very laterally destabilising which will be likely to make the model want to drop into a spiral when you turn it. This can be addressed by adding dihedral but then inverted performance would be problematic. Also swept forward wings are very prone to flutter and so have to be much stiffer (and so heavier) than straight wings.
Yep, I am aware of the stability issues of such a design but 99.98% of all available models are too tame anyway...

[JetPlaneFlyer]

I thought the vertical stabilizers were mounted at the tail mainly to prevent the sightseeing pilots from bitching too much. Yes I know, that little distrubance hitting the stabilizer... or de-stabilizer in this case :eek:
But I want maneuverability and efficiency, I'd just fly it on calm days then :D After all, a canard would have the same disadvantage over a tail-mounted elevator, but obviously this can be handled.


I'll pick up on this one point as it's the one thing that will mean that the model wont fly, at all..
I'll be clear; if you put the vertical stab at the fronth (no vert tab at rear) then the plane wont fly. It will be violently directionally unstable and will crash.

A canard foreplane is de-stabilising in pitch too but then you have a great big wing at the back (behind the CG) that gives pitch stability ;)

[JaRaMW]

A canard foreplane is de-stabilising in pitch too but then you have a great big wing at the back (behind the CG) that gives pitch stability ;)

Makes sense indeed...

[Brandano]

This does not mean you can't use vertical canards as control surfaces for yaw. You just need a bigger rear mounted vertical surface to compensate for the loss of stability, or a fly by wire sistem capable of coping with instability on the yaw axis, like for example in the F16 CCV/AFTI
http://www.airforceworld.com/fighter/gfx/f16/f16ccv_1.jpg
One advantage of a canard configuration for dynamically unstable planes is that the control surfaces can be left to float freely as a fallback measure in case of flight control malfunctions. They will not produce lift and the plane will revert to a stable, less efficient configuration, controlled by elevons. I believe the SAAB J39 Gripen adopts this safety measure