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Old Jul 14, 2011, 02:50 AM
ninja's fly low
Slovakia, east
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Any suggestion then on airfoil that are known to be good for canard config?
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Old Jul 14, 2011, 07:42 PM
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Any suggestion then on airfoil that are known to be good for canard config?
It seems to me that the shape of the airfoil is not as important as the way that they are used on model airplanes. So far my canards have used identical airfoils front and rear composed of flat bottom, symmetrical and totally flat sheet foam. Also I have used airfoils rear and flat sheet up front. The rule to keep in mind with your design, in my opinion, is to make sure that the main wing does not, with it's large area, take command of the model but must use it's lifting force as directed by the canard which controls it's angle of attack. My feeling is to set the rear wing at it's zero alpha angle and the canard at an alpha
of about 0.2. This usually gives a difference of around three degrees. The canard is carrying a heavier load and will stall first unless you have chosen the wrong style of airfoils. You need to choose a canard airfoil which stalls at a lower angle than that of the main wing. Also it is good to know that a high aspect rectangular canard wing as on the Long EZ will have a less gentle stall characteristic. My Twin Duck will have a symmetrical canard airfoil which should stall at a lower angle than semi-symmetrical rear wing.
I am eager to learn more here so please share your thoughts!

Charles
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Old Jul 14, 2011, 07:59 PM
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Please check post 220 here.

http://www.rcgroups.com/forums/showt...613556&page=15

This shows a more conservative forward sweep for the main wing and features large yaw stabilizers.

If you want to view all types of canards in this forum, just go to the Thread name and click on the paper clip. After finding your picture, click on "post" to find the script. This is a super search tool.

Charles
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Old Jul 14, 2011, 10:44 PM
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Originally Posted by canard addict View Post
...It seems to me that the shape of the airfoil is not as important as the way that they are used on model airplanes.
Well, at least that much we agree on.

Airfoils and incidences are both like shoe sizes. What fits perfectly in one situation could be utterly awful in an even slightly different application. You have to look at the needs of each situation individually.

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So far my canards have used identical airfoils front and rear composed of flat bottom, symmetrical and totally flat sheet foam. Also I have used airfoils rear and flat sheet up front. The rule to keep in mind with your design, in my opinion, is to make sure that the main wing does not, with it's large area, take command of the model but must use it's lifting force as directed by the canard which controls it's angle of attack.
What you're referring to is called "static pitch stability", and although airfoils can have some small influence on it, static pitch stability is mostly a function of C/G location, not airfoils.

Airfoil properties do come into play on stall characteristics, but even there they are only one player, not the whole picture. Airplane design inevitably comes down to the complex interactions of a host of different properties, and only by considering all of them together can you determine a reliable answer.

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My feeling is to set the rear wing at it's zero alpha angle and the canard at an alpha of about 0.2. This usually gives a difference of around three degrees. The canard is carrying a heavier load and will stall first unless you have chosen the wrong style of airfoils.
No.

OK, one more time. The incidence between the wing and the fuselage controls the angle the FUSELAGE flies at, NOT the wing.

Even in a canard, where we have to make sure the canard is doing more than its fair share of the work in order to achieve positive static pitch stability, the major part of the plane's total weight is still carried by the wing. The plane will have to be trimmed to whatever pitch attitude gives the wing enough angle of attack ("alpha") to make precisely that amount of lift when the plane is flying at the desired airspeed. This has exactly NOTHING to do with what angle the wing sits on the fuselage at!

Once the wing has found its necessary alpha that it need to do its job of supporting the plane, the angle the fuselage flies at will be the wing alpha minus the angle of incidence between the wing and the fuselage.

Let's use your example to illustrate this. Let's assume the model is an "old school" thermal duration style sailplane, intended to spend most of its life at the minimum-sink airspeed, just a little above stall. Let's say the drag rise leading up to the stall begins at a wing alpha around maybe 5 degrees (relative to the chord line) at our Reynolds numbers (so that's about where the wing alpha for min sink will be), and in addition you have two degrees of washout in the wing. When the Mean Aerodynamic Chord of the wing is at that 5 degree alpha, the root will therefore be closer to six degrees.

Since you chose to build the plane with the wing root set on the fuselage at its zero-lft angle, and because this is a "gasbag floater" of a sailplane you chose a high-lift airfoil, the zero-lift angle is at an incidence of maybe 3 degrees nose-down relative to the airfoil's chord line.Note, there are at least three common ways to measure incidence, from the chord line, from the zero-lift line, and from a tangent to the underside of the airfoil, and there is usually a substantial difference between all of those, unless the airfoil is a flat plate operating at a fairly high Reynolds number.

So, we have the 5 degrees alpha needed to squeeze as much lift as safely possible from the wing, we have one more degree for the difference (due to washout) between the alpha at the MAC vs. the alpha at the wing root,and we have three more degrees due to the difference between the chord line and the zero-lift line on the airfoil.

Adding all that up, is means that when our wing is doing what's necessary to support the plane at thermalling speed (which is what WE decided we wanted it to do), the fuselage will be at 5+1+3 = NINE DEGREES NOSE-UP relative to the horizon!! Very ugly, and very draggy.

Alternatively, consider the case of a pylon racer, one where we decided to maximize speed on the straights, while taking a few lumps in the parasite drag department on the turns. In this case, the necessary lift coefficient on the straights is going to be nearly zero, since we're at only one "G" and at an extremely high airspeed. If we set the wing/fuselage incidence at zero relative to its zero-lift line, the fuselage angle on the straights will be only slightly positive, not the absolute best it could be, but very close to it.

OTOH, if we forgot about the washout and set the wing/fuselage incidence at zero degrees relative to the root zero-lift line, then the fuselage will be flying at about one degree nose-up on the straights, which could be significant at those speeds.

Obviously there are a bunch of factors that come into play on this question, with the most important one being the plane's intended "mission profile".

Now, we've dealt with the wing/fuselage incidence, what about the "decalage" angle (the difference between the wing/fuselage incidence and the canard/fuselage incidence) ?

Again, this depends on a number of factors. There are effects that try to push the nose up, and other factors that try to push it down. C/G location (that's a major one!), the aerodynamic pitching moments of both the wing and the canard, the wing and canard washouts, fuselage aerodynamics, thrust line, control surface deflections, all come into play. The canard must make exactly enough lift, no more, no less, to bring all of this into equilibrium. In the process it will also automatically make whatever amount of lift it needs to support its share of the plane's weight, since that share is determined by the forces it is having to balance out.

If you choose the "wrong" canard incidence, it just means you will have to use some elevator to make up the difference. Elevator alters both the effective chord line (and therefore the incidence angle) and the camber of the canard airfoil. An all-flying tail doesn't change the camber, but it does alter the incidence and therefore the canard's alpha, but the end effect is still the same, the lift of the canard is adjusted until everything is in balance.

So, whatever errors you make in the calculation of the canard incidence will be dealt with when you have to set the elevator trim.

At this point is where the specific quirks of a canard arrangement raise their ugly heads.

Yes, the canard can be adjusted, through a combination of elevator deflection and incidence changes, to make whatever lift and other forces are needed to bring everything else into equilibrium, EXCEPT that there are limits on just how much alpha the canard can handle, and therefore just how much lift it can make.

If the demands on the canard are too great, it will stall before we achieve equilibrium. This often shows up on takeoff run, when the canard stalls before it can lift the nosewheel, so the plane refuses to rotate and lift off. Note, on a nosewheel layout the mains are behind the C/G, so it takes more canard lift to raise the nose for liftoff than what is needed to keep the plane in equilibrium after it has left the ground.

This is why canard aircraft are often prone to having excessively long takeoff runs, followed by a sudden almost "leaping" into the air when the plane finally does rotate for liftoff. There's quite a discussion in the (full scale) VariEze's pilot's handbook about this, and a string of PIO (Pilot-Induced Oscillation) related accidents that made that section of the handbook necessary.

Getting back to the canard lift problem, the opposite situation, too much canard lift, is an even worse issue. This is the case where the canard can make too much lift, enough to make the wing stall before the canard does. In that situation the plane may try to immediately flip over backwards, with typically disastrous results. An in-flight breakup is a common result.

So, for a canard arrangement, you must walk a fine line between making sure the canard stalls before the wing, while still having enough canard lift to operate the plane in all normal flight modes. The boundary between those two extremes can be quite narrow and blurry, which is why developing a good canard aircraft is generally more difficult than developing an equally good aft-tailed layout.

Assuming your plane survives the flight testing necessary to find it, you can adjust where the plane operates between these two extremes by adjusting the C/G location. Moving it forward shifts more load to the canard, which will cause it to stall sooner, while shifting the C/G aft reduces the load on the canard and puts more load on the wing), making the canard stall later. However, C/G location also sets the static pitch stability, so too far forwards could make the plane sluggish on the controls and prone to diving and zooming when going in and out of turns, and prone to porpoising from too much static stability and not enough dynamic stability to control it. OTOH, a too far aft C/G can make the plane statically divergent and impossible to fly.

The end result is that for a canard you have TWO "between a rock and a hard place" criteria you have to satisfy, where in the case of an aft tail there is generally only one (with a few exceptions, such as the Cessna Cardinal).

Quote:
You need to choose a canard airfoil which stalls at a lower angle than that of the main wing.
As I explained above, it's not just the airfoil that determines this, C/G location is at least as important. Where the choice of canard airfoil vs. wing airfoil comes in is when trying to adjust things to satisfy both the need to make the canard always stall first, and simultaneously find a C/G location that results in enough, but not too much, static pitch stability.

Quote:
Also it is good to know that a high aspect rectangular canard wing as on the Long EZ will have a less gentle stall characteristic. My Twin Duck will have a symmetrical canard airfoil which should stall at a lower angle than semi-symmetrical rear wing.
In general probably yes, but this is not automatic. It's very possible to have a symmetrical airfoil that stalls after a semi-symmetrical one.
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Old Jul 15, 2011, 01:35 AM
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Originally Posted by cupsster View Post
Please can anyone explain to me based on design I posted above If I want to incorporate flaps into this kind of a pusher, would it make sense? If they are so close to prop is their effect still present?
Thankx
I think Don already answered your question, but I just want to add that I don't think it is possible to achieve much by adding flaps to a canard design. Normally the flaps don't allow the aircraft to fly any slower because the flaps cause an increased nose-down pitching moment. That increases the load on the canard wing and it generally results in increased stall speed. The stall speed of a correctly designed canard aircraft is determined by the stall of the canard wing. The canard stall occurs when the loading causes the canard to reache its maximum lift co-efficient, CLmax.

To keep a reasonably low stall speed on a canard model, I suggest 1) keep the wing loading low, 2) When selecting the airfoil for the main wing, use an airfoil with small magnitude of aerodynamic pitching moment. If you build a large canard aircraft such as a full size Long-EZ with 8m wingspan, then its its possible to tolerate a larger negative co-efficient of pitching moment. Canard model aircraft on the other hand are likely going to suffer from low reynolds numbers on the canard wing, especially if its a relatively slow flying one. Low reynolds numbers on the canard are a by-product of low speed and small dimensions of the canard wing. The low reynolds numbers causes the canard wing to stall earlier and it limits the amount of elevator authority at low speeds.

Reducing the negative pitching moment can be as simple as reducing the airfoil's camber. Attached are plots I generated in Profili simulation using the E205. The standard E205 has 3.0% camber, my modified version has 1.5% camber and you can see the negative pitching moment (Cm) is significantly reduced.
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Old Jul 15, 2011, 10:48 PM
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Long EZ Project

Hello canard friends....and thank you for the warm welcome to the forum! Nick, I love the video of the Polar Duck! Flies great.... inspiration for me to build a delta wing per Charles' proven design.

I'm about to embark on the Long EZ project (.46 size ARF offered by NitroPlanes, purchased at raidentech.com). Reasonable price, good build quality, but instruction manual contains some omissions. My neighbor, Dave, is close to finishing his yellow Long EZ and I took a few pictures that may be helpful to those of us who are considering this kit. He will be powering his with a glow engine, I will be going electric (I have much to learn about this science!). These kits are available in yellow or white...my plane is white.

I'm not sure what type of covering material was used, but I know it is not monokote. The factory builders did a nice job with the covering, but it is trimmed out very close to the edges. The material draws up good with the application of heat, but if you're not careful, it will draw up too much and expose the wood. I'm planning to cautiously use a low heat for touching up (see photo of aileron covering - result of a bit too much heat). Also, there's an area that can be seen under the canopy that was not covered at the factory. I plan to color match with a similar covering material, or paint to match.

A few necessary modifications.... one to the nose gear. It is much too long and will have to be cut off. In addition, there's no pushrod holes, casing, or rod for the steering function (yet there is a servo dedicated in the tray). It is worth noting the shallow aileron servo areas in the wings. The mounting blocks are too long and have to be cut down to fit and you'll have to use shallow servos as well. If you're planning to power with a glow engine, you can expect some muffler modification - a muffler spacer was needed for Dave's engine to fit properly. Regarding engine (fuel or electric), I am planning to cut the cowl vents for air circulation. In fact, one builder of another Long EZ kit explained how he improvised a louvered vent on the bottom of the cowl to ensure proper cooling of the engine. I'm hoping that there will be sufficient air intake for proper engine operation with the cowl vents opened up on both ends. Prop clearance was another concern. After considering the height, an 11 inch prop (maximum) should be acceptable. With no modification to the furnished landing gear, the distance from the ground to the center of the cowl opening is right at 8.25 inches. Regarding the landing gear....I'm not sure why the designers left such a large space for the landing gear mounting. No concerns regarding strength since the gear mounts to a solid, well build ply plate...but seems unnecessary to have so much space there for the gear. The only other thing to be aware of is the paint quality on the wheel pants - when drilled out to accommodate the wheel hardware, the paint just flaked right off. It appears that the material was not properly prepared and primed. Plan to repaint the wheel pants if you decide to use them. The nose cone and cowl are painted correctly and should not present any problems.

These points will help me as I build my Long EZ and I hope it will help you too of you decide to build this plane. I think it will be a real sweetheart in the air!
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Old Jul 16, 2011, 03:06 AM
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Good pictures Jim!

That nosewheel arrangement looks as if they were allowing for a retract and changed their minds. You might want to have a look at these. Yes, they really are only $8 dollars and I believe strong enough for front-to-back rotation. I'm planning to retrofit them into my 72" Mosquito and also use them in my next project.

On the LongEZ you might have a bit of head-scratching to get one in, especially with steering, but it might work.

Cheers

Nick
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Old Jul 16, 2011, 08:42 AM
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Don, Great discussion! The view points by you and others are what gives
credibility to the thread. To me, different views lead me to reconsider my thoughts based on what great modelers believe to be true. The challenge
to summarize these views and diagnose the results just makes the hobby great.
I like the way you add all extremes together to show how a fuselage can fly at plus nine degrees. I love to adjust my models to fly with all control surfaces in neutral. Getting equilibrium over a wide range of speeds is rewarding. You provoked a thought to add one positive degree of alpha to the twin's main wing
from minus two and to consider the possibility that the large canard may resist stalling. Again, thanks for working to provide us with some of your knowledge.

Charles
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Old Jul 16, 2011, 09:36 AM
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JimRussell, Thanks for the informative post on the Long EZ. The Canard Fairy
was kind enough to bring me a white version of the model which I am proud to
own. It looks true and very well put together. It is actually about a five pound unit contrary to the manual which shows nine pounds. I hope that the published
wing area of 705 square inches is accurate. I feel that a motor at 800 kv and
a weight of 6 to 8 ounces will fly it on 450 watts. The battery IMO should have
five cells delivering 25 amps, but if ballast is questionable, a 4 cell
unit at 30 amps might suffice. An APC 10-5 is a possibility. My Twin Duck will have to be finished first but it will be fun to gather data for the LEZ.

Charles
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Old Jul 16, 2011, 09:24 PM
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Canard in flight

A few photos to share with the group. Designed, built and flown by Charles. A solid performer in the air and a joy to observe in flight!
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Old Jul 17, 2011, 06:35 AM
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I agree with Nick, that the inexpensive Chinese retracts might do the trick for Jim's Long-EZ, but the ones he selected state, "auto 90 degree twist when retracted". They also have a steerable electric nosewheel retract like this one.
The battery pack in my Long-EZ is pretty far forward - not sure if there's room for the nosewheel, but these electric units are the way to go!
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Old Jul 17, 2011, 07:08 AM
What could possibly go wrong?
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Thanks PJ!

On this thread for the DH88 Comet we had a discussion about these retracts and I asked exactly the same question about 90 degree twist. The answer I got was:

Quote:
Nick, that I suspect is Chinese mistranslation for 'they move through 90%', they are simple no nonsense units, no instruction or details and no wire legs
Also, on the reviews for them, there is someone feeling disgruntled because he bought some and then found out that they didn't twist at all. So I think we're safe.

You're right to go for steerable, but on the spec, those ones say "not suitable for a plane heavier than 2.5 kilos, which I think is 5.5lbs. Do they have bigger ones? Incredible prices aren't they!

Cheers

Nick
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Old Jul 17, 2011, 09:03 AM
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Just to confirm, the 54g retracts Nick mentioned do not have any twist and turn action. They simply retract through 90 degrees. They are supplied without legs and without any instructions. They do come with an alternative mounting plate, so the mounting flange can be above or below the unit. There is also a single machine screw, about 12mm x 3mm which, once you work out where it goes, is used to retain the leg.

I have just taken delivery of some (for the aforementioned DH88 Comet) and they appear to be quite robust. From an initial check, their lifting power did seem quite modest but I suspect that the old Rx battery I was using may have been struggling, causing the overload protection to kick in a bit too readily.
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Old Jul 17, 2011, 06:32 PM
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Thanks for the information on the retracts.....you sure can't go wrong with the price, but the fit will be my challenge on the Long EZ. The kit comes with nice wheel pants for the two rear wheels, but nothing for the front. I am considering buying the same type of wheel trim on the front as well (if I can find something similar). I think it would look sharp, not interfere with ground control, and provide some aerodynamics in the air. I also am planning on going with a much lighter (and maybe taller) wheel set.
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Old Jul 17, 2011, 06:41 PM
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It may be that the conventional Long EZ only retracted the nose gear.
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