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Old Nov 24, 2010, 11:37 AM
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Don Stackhouse's Avatar
United States, OH, Bradford
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Could be a combination of high thrust line plus blanking of the rear stab+elevator by the prop.

Nick, refresh my memory - exactly what is your little aft "trim tab" elevator controlled by, the throttle or the elevator stick?

My guess is that you might get away with a fixed position of that elevator, although it would take some experimenting to find the right setting. At most, you might need to couple its servo to the throttle channel, and do all your pitch attitude commands with the canard elevator alone. Let the aft elevator do absolutely nothing, other than to correct for trim changes caused by power changes.

When power comes up, so does propwash, exactly in concert with it, since the propwash and the nose-down moment from the thrust are directly connected to each other. If the thrust changes, so does the airflow over the aft elevator, so the correcting force it provides changes with the thrust. As the thrust decreases through zero and into the range of windmilling drag, the airflow over the aft elevator continues to decrease, reducing its correcting force. Properly done, the correcting force changes naturally in lockstep with the amount of thrust.

However, if you have the aft elevator coupled with the canard elevator, then the elevator position in a given flight condition is not necessarily tied to just the power setting anymore. It's now helping the canard elevator to trim the other forces on the airframe. If you then change the airflow over it through a power change, those other equilibriums also get disturbed. In addition, some of the correcting force for the nose-down effect of power is now coming from the canard. When you pull the power back, the blanking of the aft elevator by the windmilling prop takes away its correcting force, but the portion of the correcting force that you are still getting from the canard does not go away. This, coupled with the nose-up moment from the windmilling prop, pulls the nose up. In other words, pretty much the same thing you suspect.

For experimental purposes, I suggest putting the aft elevator on a separate servo, connected to a separate channel on the transmitter. Set it for takeoff at an approximate deflection that is in the same ballpark as you were getting at that power setting with the previous setup. Get the plane in flight, trimmed there with the canard elevator. Try varying the power, and trim out any pitch attitude change with the aft elevator. See if there is a position for it that gives you little or no pitch attitude change for throttle changes from idle to full throttle. If there is a variation, note the aft elevator position for idle and for full throttle, then plug the aft elevator servo into the throttle channel, with the aft elevator linkage adjusted to give you that same total range of elevator movement.

It is possible that the "perfect" correction from the aft elevator is non-linear. However, with a little perseverance and luck, you might find a linear range of movement, or just a constant position for it, that is close enough for practical use. At that point, you can fly the plane with the canard elevator, and let the aft elevator just do what it needs to, on its own.
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Last edited by Don Stackhouse; Nov 24, 2010 at 11:52 AM.
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Old Nov 24, 2010, 05:48 PM
What could possibly go wrong?
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Trevor and Don, Many thanks for that suggestion.

At present I have the rear elevator on a seperate servo, which is connected in parallel to channel 2 with the canard elevator but with less travel. I'll try the idea of putting it on a spare channel and linking it to the throttle with a variety of settings.

Very interesting!

Nick
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Old Nov 24, 2010, 08:11 PM
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Don Stackhouse's Avatar
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Nick, the other thing you may find is that it needs to be slaved primarily to the throttle, but also needs a little added in from the elevator channel as well, to correct for changes in the authority of the "trim tab" as airspeed changes.

Still, with the right "recipe" of inputs from the correct channels, it should be possible to develop something that corrects for thrust line effects with little or no attention from the pilot.

Whenever you depart from the fundamentals (such as having a thrust line that does not pass through the C/G), a certain amount of quirky little interactions and complexities are to be expected.
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Old Nov 24, 2010, 08:20 PM
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It seems that you have the great slow fly floater action that I am enjoying now at moderate power with my Duck. The wing is very sensitive to battery position going from a bomb to floater within 1.5 inches of movement. If you like the nose up gentle landing approach then maybe the battery position is perfect. The full power down thrust must be adding to the main wing loading to keep it from a rapid climb. I assume that the aft stab pushes the rear end down in the prop blast if it has a negative angle. The overall resultant forces are apparently giving you the requirements to ROW. With power off, the wing loading decreases and the drag onto the motor increases which calls for nose up rotation and climb. Do I see a bit of down flap reflex on the canard? What if you reflex the front flaps just a bit upward and the rear flaps a bit downward?
That should not upset the ROW and might do some good in flight. Just something to think about. You have done a super job with ROW and landings plus a great fuselage pontoon!! Is the short fat wing fun or what?
Charles
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Old Nov 26, 2010, 03:48 PM
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Delta Duck Changes

The 30 inch wing span was pushing my eyesight and the large 9.75" tip chord may have caused a whip stall when pushed in a tight turn. A 36" span with a 7.25" tip chord may fix both problems? Area was added to the vertical fin and rudder to control the extra 52 square inches added to the wing. The positive incidence of the canard will be held to one degree instead of a generous two to control the climb rate. The CG will be moved rearward. The down thrust will be reduced slightly from 2.1 to about 1.8 degrees.How about some predictions on the new flight characteristics regarding tip behavior, three point landings, spins, axial rolls, snap rolls, high and low speed stalls. The controls are two sets of elevons and rudder.
Charles
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Old Nov 26, 2010, 03:52 PM
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larger rudder.
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Old Nov 26, 2010, 04:19 PM
What could possibly go wrong?
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Quote:
How about some predictions on the new flight characteristics?
Honestly Charles, I can only guess and I'll look forward to some better informed predictions from some of your other contributors. It looks like one of my favorite flying wings. I think less wing loading is always going to give you a wider envelope, and I think the bigger ws will give you more positive control for roll. Higher aspect ratio? I'll be interested to hear. For pitch, I found that, when I gave my canard a little more moment, I could sustain high AoA very easily. Also, I'm playing presently with control surfaces right in the propwash, as you know. Why not build something more like this, perhaps even with the wing, the canard and the motor all exactly in line?

Best of luck, I'm watching

Nick
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Old Nov 26, 2010, 07:11 PM
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Charles, The delta duck wing update looks very good to me. I would also go for the higher aspect ratio main wing if building one for myself. I don't know a lot about the flight performance of the Delta Duck Mk I, so my comments could be wide of the mark. I expect the higher AR will help make more precise turns and it will probably also slow down better. There will be fractionally less loading on the canard wing due to the CG change. I am looking forward to see what effect the change will have on the spin behavoir. I'm going to guess you will get more dramatic spins with the updated version. The higher AR should make it easier to push the main wing into a deep stall.

Its great to follow your adventure with this model.
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Old Nov 26, 2010, 11:14 PM
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Nickchud
Quote:
Why not build something more like this, perhaps even with the wing, the canard and the motor all exactly in line?
It worked very well with Rutan but I want the canard high to protect it from dragging the ground in a cross wind. The low wing is now my favorite for smooth axial rolls.

John 235
Quote:
There will be fractionally less loading on the canard wing due to the CG change. I am looking forward to see what effect the change will have on the spin behavoir. I'm going to guess you will get more dramatic spins with the updated version.
The CG has moved rearward by 5/8" since the main wing carries more load. I feel that the canard will require less incidence due to the wing's extra lift. The flat spin could be better if the new CG is closer to the center of mass. If a version 3 is built, the vertical tail may be moved forward to enhance the spin. One thing for sure is that the axial roll and the spin will slow down due to the longer wing.
Charles
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Old Nov 30, 2010, 05:19 PM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
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Some canards

With special thanks to Charles, John235, Don, Captain Canardly and many others, here are some canard planes that definitely fly, well flew. If they're not still flying, they're awaiting refinements.

Charles, well done for keeping us all going. Canards are a fascinating subject, endless possibilities.

You'll perhaps be pleased to hear that I went for an eye test today!

Cheers

Nick
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Old Nov 30, 2010, 10:41 PM
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I do have a couple sets of (twin motors) to engage! Nice collection nick!
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Old Dec 01, 2010, 10:17 AM
Visitor from Reality
United States, VA, Arlington
Joined Dec 1996
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Hi Nick
Nice collection - especially the Moggy Minor in the background! What year is she - yeah, it's nowt to do with canards, but my first car back in 1970 was a '58 Minor four door, in dark green. Great car, I passed my driving test and two weeks later, drove it from Hull to the NW 0f Scotland to catch the Stranrar ferry to N. Ireland. If I ever win the lottery, am getting another!

Nick's models set me to thinking about a tiny homebuilt - in the mid 1980's, I helped park a two seater version into a hangar at our base's airshow. It was tiny, though reputedly larger than the single seat version. I think we put it under a desk

http://en.wikipedia.org/wiki/Rutan_Quickie

Further digging offered that the 'Dragonfly' looks similar:
http://www.davemorris.com/Dave/dfly-faq.html

Suspect 'Quickie' may have been BARF'd at some point. The owner of the fullsize I helped park did offer that it was really better suited to long, smooth tarmac runways than the grassfields aircraft that size can usually live with just fine.

While scratting for inspiration vice working seriously, these links turned up:
http://mickaircraft.com/LSA/LSA.htm
http://www.lotusespritworld.co.uk/EH...lassfibre.html (scroll right down to the page bottom)

Off to do something else!

D
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Old Dec 01, 2010, 11:19 AM
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Hi Dereck

Right first time... It's 4-door '58 Minor. Goes very nicely. The brakes aren't much good, but who needs brakes when you've got a good loud horn like that one?

The Quickie not being good for rough grass runways looks like support for Charles' point about mounting canard wings up high. My Starship has no retracts but it does have a canard sweep mechanism. I've learnt to come in to land with the canard forward as designed and, just on landing, to sweep it back. This usually keeps it safe.

I didn't know Colin Chapman ever met the Rutans. Small world isn't it?

Thanks for your comments.

Cheers

Nick
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Old Dec 01, 2010, 12:30 PM
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The reason the Quickie isn't such a good choice for grass strips is because the main landing gear isn't well suited for that abusive environment. The canard spar is the shock absorbing system, and the wheels are too small to roll easily over the much rougher ground of a sod runway.
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Old Dec 01, 2010, 12:54 PM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
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Landing gear for an abusive environment....... Now that's my number one puzzle.

I always try and land like a crow, just flare and stall a couple of inches from the ground. But 50% of the time I stall too high or hit the ground too fast.

Hence my interest in floats and skids. The Delta Duck and ChuddleDuck worked well with 3" wheels and springy thin piano wire. The Polar Duck is brilliant in the current snowy conditions. Winds are 10-15mph so I can land with very low ground speed, but I can't really test the various pitch control options. At present I favor an almost fixed position for that little rear elevator. It's fixed with a little bit of down trim and it's linked to the main elevator but it only travels about 10mm total. Linking it to the throttle was unnecessary and seemed to reduce the canard authority. Possibly a return to elevons is worth a try. I haven't yet managed an outside loop, which surprises me, but I haven't tried it with this latest arrangement, due to the strong winds. There's a danger that the original Polaris has the best answer.... forget the canard. But I want to try all the options for pitch control from the front before I do that.

Nick

PS Dereck:
Quote:
my first car back in 1970 was a '58 Minor four door, in dark green. Great car, I passed my driving test and two weeks later, drove it from Hull to the NW 0f Scotland to catch the Stranrar ferry to N. Ireland.
I drove to Stranrar in 1970 in a 1955 Singer Roadster 4AD with a bunch of student friends to tour N. Ireland. We could have been on the same trip! We spent the night before sailing camped by Loch Lomond. Someone lent us a frame tent, we'd never seen it before and we didn't try and erect it till after midnight. You will guess, correctly, that we slept in the open air.
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