odd ball idea...forward canard horizontal stabilizer?

[Blll]

Hi all...

Here is at least one of an apparent infinite number of oddball aerodynamic ideas I _havent_ seen discussed before....

How about using a forward canard wing _at the front_ of an otherwise normal pusher gyro....

Is that even "doable"?

It would be very nice if it provided a horizontal stabilizing force by providing an upwards LIFT...thereby increasing the overall efficiency of the gyro...

Of course all your centers of lift, drag, vectors, thrust etc etc need to be reevaluated...

And the forward canard may have to have an adjustable AOA relative to the keel/direction of flight...and it might be a fixed relationship to the rotor AOA so some dual linkage of canard and rotor might be possible....

I can think of a few theorectical advantages and disadvantages...but I dont want to bias anyone before they can think about it and post......

Any ideas or comments?

take care

Blll

[Doug Riley]

This suggestion comes up a couple times a year. A forward tail surface is de-stabilizing: as its angle of attack increases, its lift increases, raising the nose, increasing the angle, and so on. The Wright Bros' Flyer was unstable for just this reason. Putting a vertical fin forward would cause the same problem, but in the yaw axis. Tail-first arrows flip the minute they're released.

The Rutan designs work by moving the CG forward enough so that the canard stalls before the nose rises much. This drops the nose and un-stalls the canard. We don't want the nose dropping suddenly on a gyro.

[eruttan]

In a ruttan et al design the horizontal stabalizer is the main wing, from a "pointing into the wind" point of view.

The canard is a elevator and stall limiting device. It is in no way a horizontal stabilizer.

[BenMullett]

...I had thought that a canard would be entirely unsuitable on a gyro, except for the following evidence:

1) Ken Wallis uses pitch-adjustable canards to trim his high speed machines
2) I just found a report in Peter Lovegroves' "Gyroplane Miscellavia" Volume 5 of a model using a substantial canard that reportedly flies better than others without.



All the best, Ben.

OK, picture scanned in from Vol 5 - hope it helps.
I've had no luck with simulation though (see below). :eek:
Peter Lovegrove's books are available through the Brtitish Rotorcraft Association - www.gyroplanes.org

[Screw]

Screw-In

Something Doug didn't mention (Yes, this subject come up quit a bit), a canard is a fixed wing attached to the nose. a full tilt rotor is floating. As the nose is raised, the rotor doesn't. This will cause pitching and occillation problems.

Lets go a little further....eruttan (Who are you?) noted that the canard is a stall limiting device. A rotor doesn't stall, but it's rpm does determine lift. Let's say you have a pusher gyro with a fixed angle of attack on the rotor to cure the occilation problems. You use a canard for pitch control. A canard only works on forward airspeed. You'd have to have an exponential control inputs for the canard based on airspeed. Alot of input at slow speed, a little input at higher speeds.

Ron Herron has a tractor with a fixed angle of attack on the rotor and a horizontal stab with elevator. This works well because the prop blast over the horizontal makes it more effective.

A canard out in the wind is solely dependant to forward airspeed to work. You could loss control of the pitch at slow airspeeds.

Screw-Out

[ymmv]

John, I'm not so sure about this. In his Little Wing video, Herron sort of goes out of his way to demonstrate that the stab on his LW-2 gives ample control with the engine idling and the airspeed very low. But it's on a longer lever arm than a canard would be.

[mcbirdman]

[/QUOTE]Ron Herron has a tractor with a fixed angle of attack on the rotor and a horizontal stab with elevator. This works well because the prop blast over the horizontal makes it more effective.[QUOTE]


Actually the main reason that it works well for more reasons than that. The arm of having the tail that far back means there is a lot more leverage available that simply won't be matched by a canard on the nose. It is an added benefit that the tail also follows through the prop blast. The drag of the tail being in the back also helps keep it there. A canard in the front probably does the opposite as mentioned above.

[Screw]

Screw-In

ymmv, re-read my post. I agree his works well for several reasons but I just named one. As far as the moment arm goes, I agree to a point, however even if the moment arm were equal, the prop wash over the surface increases the leverage.

Canards and gyros are like chocolate and onions.

Screw-Out

[mcbirdman]

"even if the moment arm were equal, the prop wash over the surface increases the leverage."

Actually prop wash does not improve leverage. The length of the arm does. The length of the arm is - leverage. Prop wash over the tail surface increases LIFT in one direction or the other or nuetral.

That is why you can put a tail behind a cabin that has an engine behind it blocking the air. When the engine is running there is airflow/lift. When the engine quits however the tail being blocked cannot be as efficient as a tail on the aircraft way out with a long arm - even if the tail surface is small. It is the leverage the small tail can create in the smooth air that can out perform a big tail with a short arm hidden from the air. In other words, a smaller tail with long arm can simulate a big tail with short arm. However it doesn't help when the large tail is further handicapped by the airflow blocked by the cabin. My personal taste says the tall tails look disporportinate.

jtm

[eruttan]

If a canard is a stall limiting device on a fixed wing what is it on a gyro?

It a very strict sense you cannot put a canard on a gyro, because it cannot limit stall.

We can argue about what a canard is but let us just call it a wing forward of the CG.

Now the real question is what can a wing forward of the CG do that helps us?

As Screw pointed out you can use it to controll pitch if you fix the rotor pitch.
You could do other things. The carter copter is using such a device in a very interesting way.

But no matter what you do with it it will never be a horizontal stabalizational force.

[BenMullett]

...there are seemingly too many discrepancies between the fixed wing canard and the part-gyro-rotor canard for me to be able to make it work - at least in simulation.

Have made an X-Plane 6.51 sim of a Bensen with a canard, which I will upload to their site if there is any interest in it.

It "can-'ard-ly" fly ! :D

Pity. Characteristics include needing to ride the stick like a one-wheel-bike.

Take-off is a monster (even compared with a standard Bensen sim) and it will flip inverted and pound your head on the runway if you don't watch it as it lifts off.

Once in the air it is possible to find a stable groove, and once set up for landing it is OK to land, but re-trimming between flight modes (don't ask about gusts) is hazardous.

Yes, I fooled with the CG. Yes, I tried an all-flying Canard. Yes, I banged my head on the ground.

Yes, I know that sims are not reality, but X-Plane can beat the pants off pages of theory in my experience - even with rotorcraft (ask Jay Carter), so do try it (download for free) at:-
www.x-plane.com and www.x-plane.org


All the best, Ben

PS no connection with Austin Meyer's Laminar Research, just a happy user. ;)

[raghu]

Ben, the fly in the ointment wrt. canards in rotory wings is the difference in lift slope between the rotor and canard. In a FW the lift slopes are of the same order so you can get the CG ahead of the aerodynamic centre quite easily to arrive at static aoa stability. In contrast in a rotary wing aircraft the rotor has a much shallower lift slope than the canard and getting the ac behind CG can be very tricky ( if not nearly impossible).

To get this configuration to be stable you need to make the canard work really hard. Given that the rotor is flying at ~9 degrees at cruise, you would have to set the canard incidence to be > 9 degrees at cruise. You may manage to get things stable at say 11, or 12 degrees but then you get very close to canard stall. If the stall of the canard is not too sharp at least in theory you MAY have a viable configuration. Play with the incidence of the canard to find out.

An alternative is to use a HS and canard configuration. The HS would primarily give stability while the canard is there for damping. Having said all this it is probably worth asking why anyone would want to use a canard rather than a stab in the first place.

[C. Beaty]

Tail first flying machines don't work any better than tail first arrows; a conclusion that wouldn't be hard to draw for anyone that has watched films of the Wright Brothers trying to control pitch attitude of their early Flyers. The necessary skill must have been something akin to riding rodeo bulls and broncos.

The class of aircraft popularized by Bert Rutan is something entirely different. Variezes and similar are in fact tandem wing aircraft where, as Raghu mentioned, the CG can be located ahead of the center of lift.

[Udi]

Although any horizontal lifting surface forward of the CG is destabilizing, forward winglets may be designed to improve high-speed performance in a gyro. During high-speed flight, the rotor AOA becomes smaller and, as a result, the gyro is flying in a more nose-down attitude. A large stab (in the back) is resisting this nose down tendency, keeping the nose up by exerting down force. This down force is adding to the total weight that the rotors have to lift.

If small forward winglets were designed to provide the needed nose-up lift to counter the low rotor AOA, the stab would operate at a lower negative AOA and, as a bonus, the winglets would support some of the gyro weight, allowing the rotors to slow down and further improve gyro efficiency. This is similar, in principle, to the function of the wings on the Carter Copter.

There is one caveat though. A larger rear stab is required to maintain the same level of static and dynamic stability. A larger stab means more drag. But I think that overall, a well-designed system may be beneficial. The emphasis is on "well designed". Don’t try this at home, kids!

Udi :cool:

[BenMullett]

You are dead right, the difference in lift slope is major - I have yet to try a narrow delta, which might be closer in lift slope, since that needs a non-standard foil created to give the right lift response, but even Boeing put a tail on their canard-rotor!

The pure canard/rotor feels like a non-starter, but that model report triggered me. We have learnt a lot at model scale recently, and it is so easy to try something in X-Plane.

And yes - Georges Chaulet tried a tandem gyro model which flew, but the second rotor suffered from flying in the wash of the first - the CG was well forward of the middle point between the rotors to make it work - so there doubtless will be some 'interesting' features to a tandem canard bi-rotor, which might fly but it is hard to see why one would want it...

All the best, Ben

PS, the tiny 'canards' on the high speed Wallis machines are there for trim purposes only - they are all-flying surfaces that are used to trim th fuselage for best penetration. Ken voiced an opinion that if he needed some nose-up trim at speed, he preferred to add lift at the nose rather than downforce at the tail.

[raghu]

Tail first flying machines don't work any better than tail first arrows; a conclusion that wouldn't be hard to draw for anyone that has watched films of the Wright Brothers trying to control pitch attitude of their early Flyers.

Just to point out in case some folks misconstrue the import of CBs mail , the canard aircraft that the wright brothers flew can quite easily be made stable. It has nothing to do with the canard as such but more to do with an ill placed CG positio . The wrights where offcourse aware of this but they felt the instability gave them better control. Essentially, any two fixed wing surfaces placed one behind the other can be made stable. A single surface wing (like a tail first arrow) by nature results in unstable configurations unless of course you reflex the trailing edge of the airfoil upwards or resort to a sweep back.

I say this because since the times of the wright brothers people have mistakenly assumed canard aircraft to be unstable by nature. Not so, in canard FW aircraft the primary lifting surface (wing) provides both the lift and the stability while the canard allows the configuration to be trimmed. In a more conventional tail-in-the-back set up the primary lifting surface (wing) provides lift and the tail at the back performance both the satibilizing and trimming function.

[raghu]

ben, you can use a square low aspect ratio stab ( AR =1) it does not have to be a delta. Any of the standard aerofoils in X-plane can be used. Though in case xplane does not model explicitly the low aspect ratio non linerities this wont work. But then again I guess even the delta will not work.

Also, you will ( at least at low speeds) almost never manage to get the slopes of the rotor and canard similar. But you dont need to. You can compensate by playing with the incidence. Essentially, at say cruise speed the trim should be such that the canard sees an aoa of much more than 9 degrees. This way it is more loaded than the rotor and if correctly done any change in aoa due to a gust will have a bigger impact on the rotor than the canard and the response will be stable.

[C. Beaty]

No argument, Raghu, that load bearing airfoils in tandem can be made stable. Furthermore, it's not particularly relevant whether the big one is in front or in back or even if they're both the same size.

But what needs to be clearly defined here is that an unloaded airfoil forward of the CG results in divergence. It tends to flip the aircraft.

[Doug Riley]

Raghu, are you suggesting deliberate use of a canard stall to create a stable pitch response to an updraft? I.e. move the CG forward enough so that the canard is nearly stalled all the time, and does indeed stall when the aircraft's AOA increases in the updraft?

If forced to use a canard (against my better judgment) instead of a rear HS on a gyro, I think I'd rather try to flatten out its lift curve and make the stall a non-event by using a very low aspect ratio canard and perhaps an extra-large leading edge radius...

[raghu]

Raghu, are you suggesting deliberate use of a canard stall to create a stable pitch response to an updraft? I.e. move the CG forward enough so that the canard is nearly stalled all the time, and does indeed stall when the aircraft's AOA increases in the updraft?



Not quite Doug! The reason for the higher AOA has nothing to do with trying to use the canard stall, that just is a undesired side effect that makes the configuration less safe and harder to work. For any two surface (rotor/wing) aircraft to be stable the fore surface must be loaded higher than than the rear surface.

Here is why. Consider two lifting surfaces (wings or rotor). Assume they both have the same slope and both have zero lift at zero AOA. Say for the sake of argument both surfaces have the same area (they dont have to be). Also, assume that both surfaces are rigged with identical incidence.

Now lets try and make this arangement stable. To trim the CG would have to be placed such that the sum of the moments is zero. In this example this would be the mid point between the two surfaces. If both surfaces are rigged with the same incidence to the fusalage (or some refference), you will find that you can trim but YOU WILL NOT BE ABLE TO MAKE THE AIRCRAFT POSITIVELY STABLE; you will only get neutral stability.

This is because any gust disturbance has the same effect on both the fore and aft surface and their is no resulting pitching moment.
This is true even if the two surfaces have different areas. Try it by playing with different numbers on paper.

The only way to make stable and trim is by having the aft surface operate at a lower AOA ( or be loaded less) than the fore surface.

The intusion is that when there is a disturbance due to a gust the AOA of both surface will change by the same amount, but because the aft surface has a lesser incidence than the fore surface, the change in AOA has a larger impact on the aft surface than on the fore one. Vola you have a stable response.

Thus, the aft surface must have a lower incidence than the fore surface ( assuming identical lift slopes). This is the basis for making both canards and conventional designs stable. This is why you will find stab set at negative incidence and canards set at positive incidence.

Note, the slopes of the two surfaces dont have to be the same as in the example. You can still make the aircraft stable- it just need the fore surface to be more heavily loaded or the rear one to be less lightly loaded.

[raghu]

CB, I agree! My post was really just to point to others in case they misinterpret your mail. Often the blame of the wright flyer is attributed to the canard rather than the CG. This is clearly, well, a canard :-) !
......

Oh, though you would like to know.... in the interest of marital harmony I resort to the library rather than tap into my better half.