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Old Jan 12, 2011, 06:11 AM
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Charles

Thanks, I'll start drafting the plan next week.
Both sections at minus 1.5 degrees from the fuselage datum line and 1.5 degrees down thrust?

So the main wing zero lift line and the fuselage datum line is parallel is a good starting point for a tractor. it's good to know. I didn't even think about the zero lift angle. said "a little knowledge is a bad thing". my other canards were pushers and I design with a looks about right to me incidence and because the canard before had all moving canard I didn't have to think about canard incidence.
And best thing about it is I've got someone to pass the buck if it doesn't work properly.

Tony
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Old Jan 12, 2011, 07:49 AM
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We have another case of "the tail wagging the dog" going on here.

Incidence is something unique to each aircraft design. Yes, you can get away with about the same numbers on a general class of aircraft (and aft tails are somewhat more forgiving in this respect than canards), but the forces involved vary from airplane to airplane. Just because certain numbers worked on one of Chuck's airplanes does not mean those numbers are appropriate for yours.

C/G sets static stability. Once that is determined, then the balance between things like the pitching moments due to C/G, lift moments and aerodynamic pitching moments for the wing and canard, moments due to the fuselage and prop, and the desired trimmed airspeed all add up to determine what the final lift of the canard needs to be at zero elevator deflection, and therefore its required incidence.

The same is true of thrust line. Up or down thrust depends on the vertical and longitudinal location of the C/G, and the thrust angle you need to make the thrust line pass through it, assuming you don't have airframe interactions to deal with in addition to that. Right thrust depends on the inflow angle in the prop that results from the downthrust plus the plane's pitch attitude, and the resulting P-factor you have to counteract.

The incidence of the canard changes when you make an elevator input, regardless of whether it's an all-flying tail, or a canard+elevator. The basic incidence with zero elevator deflection that you build it at strongly influences the pitch trim in normal flight, but has very little to do with what happens at stall, with a large elevator deflection.

The best thing to do is make sure your canard incidence and thrust line are easy to adjust. The wing is less of an issue. If the plane is going to be large, complex and expensive, or there are other issues that make it impractical to experiment with these settings, consider building a smaller, cheaper, "breadboard" test model, perhaps a profile model just to keep things simpler.

Your Vvt is about double the usual number. However, the vertical tail moment arm is relatively short, which hurts dynamic stability in yaw, so having some extra might be good. Also, the long nose with significant side area, and the destabilizing effects of the tractor prop, will offset some of that Vvt of the fins alone. Overall your effective Vvt will be somewhat lower. OTOH, if you still have too much Vvt after adding the effects of those other factors, you can expect some spiral instability, particularly in cruise where the dihedral effect of the swept wing will be minimal.

The tractor prop right in front of the canard means you will have dramatic changes in elevator authority when you change power settings, and this will also alter your pitch trim and static pitch stability. It's likely that at high power settings you will be able to stall the wing long before the canard. You might be able to do some unconventional maneuvers, such as back flips (not the same as loops!). However, you might also find it difficult to avoid doing them unintentionally! You had better build your control surfaces, linkages and servos strong enough to withstand flying backwards.

OTOH, if you cut the power completely on final approach, it's quite likely you will not have enough elevator authority to flare for touchdown. You will want to keep enough power on to get at least to a "zero thrust" condition with the prop. The tricky part will be getting the power low enough for a decent glide slope on approach, while still getting enough airflow over the elevator to flare for touchdown.

The other option that would help some of these issues would of course be a pusher prop (I can't believe I just said that!!). However, going that route will cost you quite a bit of prop efficiency, which means more power and bigger batteries to get the same flight time and in-flight performance. That means more wing area to keep your stall speed down, which means a bigger airplane, which means more power, which means even bigger motor and batteries, and so on. It's easy for that sort of thing to snowball on you.

Other possible issues to discuss, but those are good for a start.
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Old Jan 12, 2011, 07:58 AM
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Harpoon Perdu

Incidence angles: I have the theory that the elevator trim makes a massive difference to the canard airfoil and angle.

First of all, as I think Don would say, get the CoG right, so that you load the canard slightly more. Then use a main wing airfoil that's close to symetrical so that zero lift is zero AoA, with no down-pitching moment. Then, with a flat bottomed airfoil for the canard, give it an incidence of no more than 1 deg relative to the wing. In straight and level flight, as the plane slows down, the canard will stall first and, as it speeds up, it will start to lift first (which is also important). I believe you will end up trimming the elevator with the panel deflected down a little (depending on speed). This will give the canard airfoil a camber, and a cambered airfoil has lift when the AoA is less than zero, doesn't it? The canard in this configuration will have an incidence of more than 1 deg relative to the wing. Before take-off on the knee-trembling maiden flight, trim the elevator this way.

I am not at all sure that my physics is right in the above paragraph, but can say for certain that my planes have flown well, excluding pilot error

Here is the plan I used for my simplest Starship..

cheers

Nick

PS This crossed with Don's post, above. But I don't think anything he wrote makes me want to change any of it!
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Old Jan 12, 2011, 08:24 AM
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Originally Posted by Perdu View Post
Charles

Thanks, I'll start drafting the plan next week.
Both sections at minus 1.5 degrees from the fuselage datum line and 1.5 degrees down thrust?
Well, if you like an airplane that putters around with the fuselage's nose pointed up several degrees.

Quote:
So the main wing zero lift line and the fuselage datum line is parallel is a good starting point for a tractor.
Well, um, NO.

What airspeed do you want to fly at? What angle of attack ("alpha") do you need for the wing to make the necessary amount of lift at that airspeed? What incidence angle between the wing and the fuselage do you need to make the fuselage level when the wing is at its required angle of attack? For example, if at your intended flying speed the wing needs an alpha of 1.2 degrees relative to its chord line to support its share of the plane's weight, then you need an incidence angle between the wing's chord line and the fuselage of that same 1.2 degrees.

The camber of each flying surface determines its aerodynamic pitching moment. You need to know the pitching moments and lifts of all the flying surfaces, the moments about the C/G that those lifts create, and if anything is well above or below the C/G you might need to figure in pitching moments due to drag as well. Add up all the moments and figure the canard lift, canard alpha, and therefore the canard incidence you need to bring all of those moments into equilibrium with each other.

John Roncz had an outstanding series of articles on preliminary aircraft design, in the EAA's "Sport Aviation" magazine from the Feb. '90 issue to Feb. '91, which included Excel spreadsheets to help figure all of this out. It was oriented around a full-scale single-engine tractor arrangement, but applies equally well to canards. The articles give an outstanding plain-English explanation of all these issues, where they come from and how to deal with them. You might check at your public library, or you might be able to get reprints thrugh the EAA.
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Old Jan 12, 2011, 10:47 AM
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Tony and Don
Quote:
Quote:
Originally Posted by Perdu
Charles

Thanks, I'll start drafting the plan next week.
Both sections at minus 1.5 degrees from the fuselage datum line and 1.5 degrees down thrust?

Well, if you like an airplane that putters around with the fuselage's nose pointed up several degrees.
What a great discussion!! I would bet on one degree and not several. At minus 1.5 degrees on the main wing, you will be sitting on zero alpha with the canard at alpha one. I have found that the canard must have complete control of the main wing's angle of attack and to never let the main wing lift before the canard. To me, Tony's model should fly similar to my Delta Duck except that instead of a symmetrical airfoil carrying the main load he has what could be called a high lift one. BTW, as discussed a few pages back, the large delta section in the main wing's center will have turbulent lift beyond it's stall angle. I am excited about your beautiful design, Tony.
Charles
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Old Jan 12, 2011, 02:21 PM
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Don

Sorry about that, reading back
Quote:
So the main wing zero lift line and the fuselage datum line is parallel is a good starting point for a tractor.
my language/grammar is lousy I meant to say for my proposed canard. I know it would be different for another plane.

I was thinking about a pusher but reading your and others posts I thought a tractor would be more efficient and because it would have no undercarriage it would have to be a long nose and a folding prop.

I suppose the brief is:
More agile (from my last canard which was a 5 AR)
Cruise about 40 mph
Belly landing and 3 channel (I can't use the rudder any more)
Aesthetically pleasing.

Tony
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Old Jan 12, 2011, 02:57 PM
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Perdu Tony
Quote:
So the main wing zero lift line and the fuselage datum line is parallel is a good starting point for a tractor.
That gets my vote. Tractor power is a refreshing change in my shop after many pushers. I enjoy elevons on both wings and feel they are a MUST on the main wing for your style model. If you are concerned about yaw stability, just add a bit of heighth and extra area to the rear of the vertical stabs. If you find that more lift is needed for the main wing, a shim can always be added under the trailing edge but I feel strongly that it will not be needed. Your model should get into the air OK and then the fun starts with final balance and trim adjustments.
Charles
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Old Jan 12, 2011, 04:12 PM
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Charles, that is NOT a good setup for maximizing performance.

The wing, even in a canard, is still the primary source of support for the plane's weight, and therefore makes the majority of the lift. In order to make that lift, it must have a positive angle of attack relative to the zero lift line.

If the fuselage datum line (which is generally very close to the fuselage's zero lift line) is parallel to the wing's zero lift line, then the only way the wing can make positive lift is if the fuselage is also at a significantly positive angle of attack as well. This causes unnecessary fuselage drag.
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Old Jan 12, 2011, 04:21 PM
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Tony, with the canard layout you're looking at, there really is no good solution that avoids significant compromises.

Yes, a pusher arrangement causes significant efficiency losses for the prop, among other things, such as vibration, vibrational stresses, higher and more annoying noise, safety issues on launch, etc..

Yes, a tractor prop on the nose, right in front of the canard, can cause static and dynamic stability problems in both pitch and yaw. In addition, it makes it nearly impossible to make the canard stall before the wing at all flght conditions and power settings.

In addition, your swept wing and canard certainly look very cool, but the sweep is unnecessary at the speeds this aircraft will be flying, and creates other problems of its own.

Of course, anything will fly if you put a big enough motor on it.

No matter what you do, there are problems, probably significant ones. However, I do think your plane looks really cool.
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Old Jan 12, 2011, 05:45 PM
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Don Stackhouse
Quote:
If the fuselage datum line (which is generally very close to the fuselage's zero lift line) is parallel to the wing's zero lift line, then the only way the wing can make positive lift is if the fuselage is also at a significantly positive angle of attack as well. This causes unnecessary fuselage drag.
Don, I would never disagree with you or any other professional on full scale design where drag is certainly an important factor. A little drag from a nose up attitude or a thick airfoil seems welcome to me on our high powered models which need to slow down on the thin air under low thrust conditions.
Your analysis is always great and has been the basis for discussion all along.
I'm sure that each viewer appreciates your contributions.
Charles
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Old Jan 12, 2011, 05:59 PM
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Delta Duck 2

Cabin fever from snow and ice has had me in the shop for three days. I hate to finish her because it will give me idle hands, there is no more room in the garage and the maiden flight will be delayed because of winter weather. The motor is a Grayson Hobby 2217-06, 1500 kv, 3s-2100 battery, 8-4 APC-E, pulls 27 amps at 290 watts.
Charles
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Old Jan 14, 2011, 06:32 AM
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Don

I know it's a sweeping statement but do you think a tractor prop on the nose and a fuselage mounted canard in the prop wash will always have serious problems.

Thank you for the John Roncz series reference I'll have to try to find it online, a can't read well without my text to voice program.

Tony
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Old Jan 14, 2011, 07:06 AM
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Tony

I didn't have any problems with this arraingement:
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Old Jan 14, 2011, 08:02 AM
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... do you think a tractor prop on the nose and a fuselage mounted canard in the prop wash will always have serious problems....
It's really a matter of "which problem?"

No matter what you do on this one, nose-mounted tractor or aft-mounted pusher, you will have problems.

A nose-mounted tractor is destabilizing in pitch and yaw. However, the big problem is that the amount of propwash over the canard varies with the power setting and airspeed. With the throttle completely back and the prop windmilling, you might not have enough elevator authority to flare for touchdown on landing. At full power it might act like vectored-thrust, with so much elevator authority that you can flip the plane around completely backwards in flight.

Note, Nick and Charles have been experimenting with what amounts to a tailless delta arrangement, where much, if not most, of their elevator authority comes from someplace other than the canard. Their canards are undersize enough that they are not a main determinant of pitch stability, which is part of what keeps them out of trouble. They are also trying to deliberately flat-spin their planes, which means they are deliberately stalling the wing. You could do that here as well, but that would require downsizing the canard and using elevons on the wings for much of your elevator authority. Even so, if you have elevators on the canard, you could still have some "interesting" stall characteristics.

The traditional alternative is to use an aft-mounted pusher arrangement. However, that typically results in an increase in vibrations and vibration stresses, and a loss of propeller efficiency that typically can be anywhere from moderate to severe. In the case of your arrangement I would expect the losses to be more than moderate, but not extremely severe.

Another option would be to mount the prop in a slot in the middle of the wing. Yes, it's been done before, but I strongly recommend against it. The vibrations and efficiency losses from a pusher arrangement are bad enough, but running the prop in a slot takes that problem to a whole new level !!!

Another arrangement would be to put nacelles in the wings where the fins attach, with tractor props ahead of the wing leading edge. Would look unusual but probably not as cool (my opinion, you might feel differently), requires two motors, props and ESC's, bigger problems if you have one of those motors quit, but it would avoid the efficiency issues. In fact it could help reduce your induced drag if you can get counter-rotating props and run them turning tops-outward. The swirl in the propwash will resist the rotation of the tip vortices.

No matter what you do, there will be compromises.
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Old Jan 14, 2011, 10:35 AM
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Don

Twin props sounds like an interesting idea. Remember this one, unearthed by Dereck early last year and published in QEFI magazine

Plans are available from Traplet. It wasn't originally designed with any control surfaces in the canard. Maybe due to the prop wash issues we've been talking about. It just had elevons.

On the other hand, the Starships all work perfectly well, including gentle loops and they don't have any pitch control except for the canard out in front. My Polar Duck has a tractor prop mounted on a rear pylon and it gets all its pitch authority from the canard at the front. I don't think my little extra panel behind the prop makes much difference at all.

cheers

Nick
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