Aerodynamic design question - strakes and canards

[dcarr4321]

Aerodynamic design question - strakes and canards

On the nose pod gyros has anyone installed a strake or canard? It seems these would work well with pusher type aircraft including gyro's. This would also provide additional lift at higher air speeds thus making the aircraft more efficient.

[mceagle]

A forum search under the word "Canard" should find considerable past discussion on the matter

[birdy]

and if you can't find it, forget it.

[Ga6riel]

strakes are just for blending sections between a section and a body~
my feeling is a canard is not altogether usefull

this would be because of the difference in stall performance, where typically a highly loaded canard has a stall ClMax of around 2, and this to allow it to stall before the mainplane and/or tail, and where tails are usually set at around Cl .8. This arrangement guarantees that the nose of an aircraft drops in a stall, thus recovering airspeed and maintaining control (tail and mainplane not stalled).

Configurations that exploit this best are usually seeking high speed performance, where the higher loading of the canard reduces the area given, relative to a tail of the same performance, hence less area ~ less drag ~ more speed. Well thats the theory, the reality has seen a decrease in performance of all of the surfaces behind the canard. The mainplane is so affected by the downwash of the canard that its Cl Max greatly suffers. Some here would recall the introduction of the much vaunted Beech Starship ~ where are they now?

So therein lies the rub, while canards have been successfull at extending the load CG range, they havnt been accepted as stabiliser forms in the same way that tail surfaces have. And with good reason.

[donshoebridge]

Strakes are low-aspect ratio devices that that do not stall as easy as do tail feathers. On a jet fighter, at high angles of attack, strakes act as large vortex generators, adding energy to the boundry layer of the wings of the fighter, delays a stall condition, and allows the aircraft to turn tighter. Also, because of the typical location for mounting a strake (well forward of the CG), once air gets under it, the center of pressure moves forward and wants to pick the nose up even more, and it snowballs from there. To use strakes, the aircraft MUST be equiped with a horizontal tail and elevator (or full flying tail) large enough the counter the sudden change in pitch attitude.

In a gyro, a strake mounted forward of the CG would make the aircraft act more like a crank-bait fishing lure - the bill would dig into the air. A strake on a gyro would destabilize the gyro in pitch, which is exactly what you don't want. Strakes are highly specialized devices, and takes years to develope and engineer, and they have no place in gyro's. If higer lift at higher speeds is required, add wings and move the back to a location that will cause the gyro to be MORE stable, ie, Carter Copter, Grohen Bros., etc.

[Harry_S.]

I wonder what Burt Rutan would come up with...if he took an interest in pusher Gyroplanes?!

I met him once, when he was a flight test engineer for McDonnell Aircraft. I met him again when he was associated with Jim Bede on the BD-5 project.

I built my BD-5 as far as I could go...and then donated it to a trade school in St. Louis.

I really do wonder what he would come up with?;)


Cheers :)

[bpearson]

Wallis had a couple of adjustable "wings" on the pod on one of his machines. They were quite small and he said that they were used to trim the machine at high speed. He also said they were useless !

Puzzling because his desire to add lightness would lead me to think that if innefective he would remove them. A bit like the H stabs which he says are not of any use but are fitted to nearly all of his machines....... I think.

[donshoebridge]

One thing to keep in mind, when the Wright brothers flew in 1903, their airplane was basically neutral in pitch stability. In a typical canard designed airplane, the canard needs to carry some of the lifting loads to maintaiin static stability. With later models of the Wright flyers, you'll notice that the pilot is sitting upright on the leading edge of the wing. This loaded the canard and made the Wright's airplanes more stable requiring a small amount of positive pitch on the canard to maintain level flight. The EZ series of Rutan designs are no different, a part of the lifting loads are carried by the canard. In fact, the loading per-square-foot is higher on the canard than is on the wings. This insures that the canard stalls first and there is a normal stall recovery. Otherwise, the wing would stall first and you fall tail first - SPLAT!

In a gyro, the same rules would have to be applied, meaning, you would have to move the rotor head aft along with the engine. Since the rotor would never really stall, the canard would always stall first. I know for sure that I wouldn't want to be the first guy to try a stall in it. Hell, I don't even think I'd like to be the 1000th guy to try it. Yikes!

[Ga6riel]

If you look at some of the early film of the Wright Flyer at Kittyhawk it is clearly pitch unstable, moving the pilot forward was the easiest fix they could make.

Tunnel examinations of Rutans revolutionary Longeze concluded that the Cl Max of the mainplane clearly suffered in the wake of the canard, and did much to consolidate canard theory. Although this seems to have been ignored in the Beech Starship, where pilots reported the flaps to be totally innefectual.

Where canards do not stall first, the aircraft pancakes into what is called a flat stall. The have been several canard equiped aircraft that this has occured in, and I can recall the writings of a test pilot who, knowing he had to get more weight on the nose of the aircraft, bravely undid his harness, released the canopy and put his body over the nose to restore the aircraft ballance. In his case, it didnt work out, and the aircraft went in at around 1200 fpm ~flat. He survived with severe spinal injuries.

Now I am somewhat undecided if a gyro would greatly suffer in the same circumstance. However, as we seem to be unable to succesfully enforce the case for pitch stability with a conventional tail, I would be reluctant to reinforce research into canard equiped machines.

[donshoebridge]

Now I am somewhat undecided if a gyro would greatly suffer in the same circumstance. However, as we seem to be unable to succesfully enforce the case for pitch stability with a conventional tail, I would be reluctant to reinforce research into canard equiped machines.

Exactly!!!

[Victor Duarte]

here is an experiment in progress....

[Ga6riel]

has he got any data
or is it just some guy flying around with a canard?

[Victor Duarte]

He is a person that tries several devices with several configurations (like a centrifugal governor on the rotorhead, as seen).
i don't think he collects some data, technically speakins, but his goal is to learn from those experiments, of course.
I don't know what comes from those experiments, but i will ask him when i see him.

[Udi]

Using a canard in a gyro is a very bad idea. Anyone doing it is lacking the basic understanding of how canards work.

Udi

[Victor Duarte]

Udi,
i think there is not a really big risk to put a canard on a gyro..
excepted for self-estime...

[Doug Riley]

As we've said over and over again here, when the canard stalls, it drops the nose. That is a good thing in a FW plane (it prevents or reverses the stall in the main wing). It is a low-G maneuver, however, which is dangerous in a gyro.

Udi's right. Many people claim to be "experimenting" when they are in fact using blind trial and error. That is wasteful and dangerous. An understanding of the present state of the art is necessary in order for experiments to accomplish anything.

[Ga6riel]

Udi's right. Many people claim to be "experimenting" when they are in fact using blind trial and error. That is wasteful and dangerous. An understanding of the present state of the art is necessary in order for experiments to accomplish anything.

thats a fact
we gave up the suck it and see design school between the wars, these days we should be into analytical design

[Victor Duarte]

Udi, Doug,
i have a great respect for your skills.
But i also have respect for this person experiments, even if, like since the beginning of gyros.. they are made with "blind trial and error", but isn't it what has been done most of the times ? empirical.

I saw this thread and persons discussing about canard or not whithout have tried it themselves... i just bring to you someone that has tried it FOR REAL.
Ok you can say "no no, bull****"... fine... but i prefer to ask this person if he has learnt something from that...nothing like real experiments..

My point is not "canards are fine" , my point is that i asked him and he told me that he disn't notice any dangerous effect but also, no remarkable improvement in stability .. he tried it in the priopwash, out of the propwash, and he found his machine was more stable with the elevator out of the prop.

Gents, i need some explanation to make my own understanding..
a canard stalls at speeds that are relatively fast for gyros (let's say 30-50 mph) and with a VR7 air foil (he used a vr7 blade section to build it) it may stall earlier..in fact i don't know, the AoA is fixed.
ok.. let's say that the canard adds some pounds of lift.. if the lift disapears, it is just like if the pilot waight was shifted forward... or am i wrong ?
in this case, the gyro simply would sink, i don't see any low G situation here..
do i miss something ?
Thank you.

[Ga6riel]

Gents, i need some explanation to make my own understanding..
a canard stalls at speeds that are relatively fast for gyros (let's say 30-50 mph) and with a VR7 air foil (he used a vr7 blade section to build it) it may stall earlier..in fact i don't know, the AoA is fixed.
ok.. let's say that the canard adds some pounds of lift.. if the lift disapears, it is just like if the pilot waight was shifted forward... or am i wrong ?
in this case, the gyro simply would sink, i don't see any low G situation here..
do i miss something ?
Thank you.

I think what underscores the observations of many here, is that the romance with the whole canard thing post rutan is over. As noted before, the arrangement of surfaces necesitates that the ClMax (that occurs at or near stall) is highest at the front surfaces, and lowest to the rear. This guarantees the correct attitude for safe recovery being nose down, ~ but that is with a fixed wing.

For a gyro, given any momentum forward (and there is no question there will be some), the canard now has a negative AoA and will actually create a nose down moment that will increase in force, with rather obvious results. For instead of a restorative force, there is a force that further contributes to loss of AoA of the rotor, Im sure you can all visualise what happens next. The so-called bunt with loss of rotor lift, loss of control, and steepening decent, failure of critical components.

I hope this finaly clears this issue up

[Doug Riley]

Victor, I agree with all that Gabriel says.

Shifting weight forward suddenly in a small gyro would not be a good idea. The rotor spindle tends to follow the inclination of the airframe, and so it is roughly the same as throwing the stick forward.

It's important to understand what makes a wing stall. A wing does NOT stall because "the airspeed gets too low." A wing, as such, has no "stall speed!"

Angle of attack determines stall. If you fly a wing at 3 degrees AOA at 1 mph airspeed, it won't stall. It won't make much lift, either.

What we have in real aircraft is a speed below which the wing can't support a certain weight -- without exceeding its stalling AOA. We indirectly arrive at a "stall speed" FOR A GIVEN LOAD. Add more load, and the "stall speed" increases. Reduce the load and it decreases. You must keep this in mind to avoid stalling a FW plane in turns and other high-G maneuvers.

I'm sure that your friend is a curious, adventurous and inventive person. I still maintain that it is wasteful to "experiment" blindly -- that is, without knowing all that has already been done AND having an hypothesis about what you are trying. It's difficult to believe that someone who had done those things would bother with a canard on a gyro.

Maybe I'm missing something. I'd be happy to hear his hypothesis.

[Victor Duarte]

Doug,
a canard is useless, that's what also wen out from his "experiment".
My point was to underline what has been said, not to encourage persons to try a canard.
Personnally, i completely agree with you, i don't see any interest in that.
I just meant that his canard was too small to be really effective, so , dangerous.
Thank you for the explanations.