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Old Mar 27, 2011, 11:27 AM
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Remember, dynamic stability (which in my experience is a bigger factor in handling qualities than static stability) is proportional to the square of the moment arm. Even a small change in moment arm can have a big effect.

The separation between them is not the significant measurement. The moment arm is the distance (parallel to the aircraft centerline) from the aerodynamic center of the wing to the aerodynamic center of the stabilizing surface (canard in this case). If the aspect ratios are low, the gap between the wing and stab can be visually misleading.
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Old Mar 28, 2011, 05:48 AM
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John, the rear wing is back to a separation of 2 chords now. Will move the front wing forward a bit to help more. The design did not look right when the top view was drawn. A model must look correct in my opinion. Thanks for your help.
Charles, I know what you mean about looks because subjectively it helps to have confidence in the design. Somtimes the longer I look at a design the more confusing it gets. There are a few factors that need to be weighed up, but I have faith you will contiunue the run of success with canard models. I look forward to seeing your updated drawings.
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Old Mar 29, 2011, 07:29 AM
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Thanks Don and John. Your help is greatly appreciated. Other projects have taken me away from the model for a day or two but after today, I can resume the fun. Side and top views will be shown for discussion. The canard area will need to be high enough to bring the CG position to near center of the fuselage to accommodate the taildragger landing gear. I just do not want another tricycle gear.
Charles
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Old Mar 29, 2011, 08:19 AM
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I can sympathize, I'm partial to taildraggers as well, although generally not with canards.

This is one of those cases where (and I can't believe I'm saying this) using elevons on the wing to assist the elevators on the canard may make sense. You will need to be able to lift the tail and get the plane level during the takeoff run, and doing that with the canard's elevators alone could be difficult. The elevons could give you just the leverage you need to get the back end off the ground.

The tradeoff is that having elevators on the wing hurts your stall speed.
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Old Mar 29, 2011, 05:19 PM
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Thanks, Don, I am leaning toward elevons on front and rear again. They may come in handy when the canard loses it's speed and drops the nose. Tonight
is when I can redraw the views and get some photos to share.
Charles
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Old Mar 29, 2011, 06:42 PM
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That's dangerous. When it drops the nose, the canard has lost some lift, If you resist that using elevators on the wing, you can inhibit the stall break of the canard enough to make the wing stall. Because of the effects of the elevators, you stand a pretty decent chance of getting a leading edge stall on the wing when it does go. At that point the results can get pretty unpredictable.
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Old Mar 30, 2011, 03:53 AM
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If you resist that using elevators on the wing,
That's interesting Don.

I was imagining that, when it drops its nose, the pilot would respond with up-elevator, in this case the panels in the canard go down and, in the wing, they go up. That reduces the angle of attack of the wing, initially. I guess what happens next is that the whole plane tips up. Exactly the scenario Rutan wished to avoid for safety's sake by putting all the pitch control of the Long EZ in the canard.

But, for some model planes, this is where the fun starts. As long as you've got plenty of power to feed in. And a big rudder panel.

Nick
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Old Mar 30, 2011, 06:11 AM
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The problem is that when you deflect the elevators on the wing "up" (which in their case is upwards, not downwards like the canard), you reduce the camber of the wing, not just the effective alpha. You're reducing the wing's lift-making ability at the very point that you're asking it to make as much lift as possible.

Since we MUST make sure that the canard stalls first, not the wing, that means that we have to provide even more stall margin between the canard and the wing than usual, since the wing's camber is now a "moving target". This further reduces the amount of lift we can ask from both the wing and canard, which increases our stall speed.

In Charles' case, his grossly undersized canard (in other words, his planes are essentially tail-less deltas with a small trimming surface on the nose, not really a true canard) gives him a little flexibility, since the canard's ability to grossly mess things up if the wing stalls a little before it is limited.

However, it doesn't end there. If we provide enough elevator authority to continue raising the nose with the elevons on the wing, we disable the nose drop from the canard stall that would normally prevent the wing from being able to stall. Yes, the change in camber at the trailing edge does tend to prevent the flow back there from detaching. Unfortunately, there are limits to how far we can push that. Eventually, if we force the pitch attitude and alpha high enough, the inflow angle at the leading edge of the wing gets to be more than the airflow can handle, and it separates at the leading edge. When that happens, the flow over the entire surface of the wing lets go at once. If the already stalled canard is still making even a little lift, at that point the plane could decide to do a back flip, into an uncontrolled tumble. Things like control surface damage become an issue at that point.

OTOH, if we have enough leading edge sweep in the wing, and a sharp enough leading edge, when the leading edge flow separates we could get a stable vortex over the wing, and large amounts of vortex lift. Of course if the left and right halves of the wing don't do this at the same time and in the same amounts, things could get interesting very quickly.

Stall characteristics are one of those things that even the best airfoil programs are very poor at. Chaos replaces order, and accurately predicting the results of chaos is an oxymoron.
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Old Mar 30, 2011, 07:14 AM
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The canard wing will move forward about half the distance toward the nose.
My control surfaces will move about 1/4 inch at the trailing edges. If the canard has incidence of about two to three degrees and the main wing is at zero, will it be safe with the E-197 modified and E-168?
Charles
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Old Mar 30, 2011, 07:58 AM
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Don
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The problem is that when you deflect the elevators on the wing "up" (which in their case is upwards, not downwards like the canard), you reduce the camber of the wing, not just the effective alpha. You're reducing the wing's lift-making ability at the very point that you're asking it to make as much lift as possible.
This confuses me because with UP elevon, you decrease camber which changes the E214 into more like the E197 which moves the plot to the right and prolongs the stall. If you refer to the plot,less camber takes us toward the E168. It seems that UP elevon decreases lift but delays the stall.
Charles
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Old Mar 30, 2011, 08:13 AM
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Reducing camber reduces the max lift coefficient. Since the max lift coefficent is reduced, the lift you can make at a given airspeed is reduced. This means that your minimum flying speed (stall speed) has to increase.

Delaying the stall can help make sure the canard stalls first. However, if you don't allow that to limit the angle of attack from increasing any further (because you're now using the wing's elevons to pull the nose up even further), eventually you will cause the wing to stall, possibly at the leading edge. That's bad. Very bad.
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Old Mar 30, 2011, 08:56 AM
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Guys, will a more symmetrical E197 and the E168 be a good combination?
I can do ailerons and elevator easier but it seems that Don could be swayed toward mild elevons front and rear.
Charles

Don
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This is one of those cases where (and I can't believe I'm saying this) using elevons on the wing to assist the elevators on the canard may make sense. You will need to be able to lift the tail and get the plane level during the takeoff run, and doing that with the canard's elevators alone could be difficult. The elevons could give you just the leverage you need to get the back end off the ground.
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Old Mar 30, 2011, 09:32 AM
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You're trying to pick out airfoils by themselves, as if they were an independent variable. You need to look at what you want the plane to do, what you want the individual flying surfaces to do, and how they interact with each other, and only then work on finding/designing airfoils that provide the characteristics you need. When I design a plane, I'll do a little airfoil study up front just to get an idea of what's reasonable, then proceed with the rest of the design BEFORE developing the final airfoils. The final airfoil selections/definitions are one of the LAST things to come from the design effort.

Same thing with incidences. You're talking about some pre-set incidence, as if there is some perfect, one-size-fits-all incidence value. It just ain't so. It just means that you will then have to retrim the elevator to get the effective incidence the plane actually needs for the airspeed you want it to fly at. The details of the specific aircraft design will determine what incidences it needs, and what is best for one airplane is not necessarily what works for another.
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Old Mar 30, 2011, 09:46 AM
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Originally Posted by canard addict View Post
Guys, will a more symmetrical E197 and the E168 be a good combination?
I can do ailerons and elevator easier but it seems that Don could be swayed toward mild elevons front and rear.
Charles
Hi Charles, I may not have the same edition of the Lennon's book as you, but I question some aspects of Lennon's analysis of canard airfoils when we look at models with wingspans of 48" or smaller. For such situations, I think Lennon's comments on airfoil selection fail to account for the actual Reynolds numbers that the airfoils experience at the stall speed. When he considers the canard airfoils operating at Re = 100,000 or 200,000 I think that is far from reality for most of our smaller models, although for Lennons larger models maybe its not quite as bad.

What I am trying to say is that airfoils with 12% or greater thickess such as E168 aren't really appropriate for the canard airfoil in the context of a 48" span model. For symmetrical canard airfoils on a model of this size, I think you would get better performance from your own designed duck airfoil. I don't think think the E168 was ever intended for low reynolds numbers less than 150,000. I'd avoid symmetrical airfoils with any more than 10% thickness for this application.

For the main wing on your short coupled model as proposed, I think it would be better to use a more symmetrical airfoil than the E197. The standard E197 probably has almost as much negative pitching moment as your regular flat bottomed airfoils, so I'd expect it to be more compatible with the longer fuselage Goose style design.
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Old Mar 30, 2011, 10:02 AM
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I know that you are right, Don. I chose the airfoils because the 168 has a quick stall and the 197 has a delayed and gentle one. The incidences will have to depend on my intuition with the front wing at about plus two and the rear one at about minus one. The trailing edges can be shimmed for trim if needed. The canard will not be lacking in area and there will be lots of power to prevent stall on maiden. The thrust lines will be at zero reference. With the CG at center, the surfaces should have plenty of leverage. I hope that my configuration will be different from any full scale design although it will be similar to the one that John 235 submitted.
Charles
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