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Old Jan 16, 2011, 03:30 PM
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Charles

Quote:
A tricycle LG would work well with nose steering.
That's the problem, I can't use the rudder stick for rudder or steering.
Quote:
Does the sweep here refer to the canard?
Don't know, I think he was talking about the wing. see Don's post #4959

Tony
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Old Jan 16, 2011, 04:10 PM
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Quote:
Originally Posted by canard addict View Post
...Tony, It seems that Don likes the power up front better.
OK, one more time...

There is no perfect solution for this airplane.

Pusher means efficiency losses, possibly severe, among other things.

Tractor opens the possibility of handling problems on this particular airplane, possibly severe.

Prop-in-slot means for-certain major efficiency losses, noise, vibration, etc..

Props on nacelles on the wings means failure mode issues.

Prop on a pylon means thrust line issues.

So, which set of problems do you prefer? In this particular case, I suspect pusher will be the easiest to make work effectively, all you have to do is shoehorn enough raw power into it to overcome the efficiency loss. That's a lot more straightforward to do than sorting out a bunch of really tricky handling problems.

The use of landing gear does complicate things. The mains need to be just behind the C/G. If you put them further aft, you will need miles of runway to get enough speed to rotate for liftoff, since the elevators will not be getting any help from the propwash, and the too-far-aft mains would shift more load to the nosewheel. When the plane finally did get enough elevator authority to lift the nose, it would tend to leap into the air suddenly, perhaps uncontrollably, as the weight shifted from the wheels to the flying surfaces, causing a possibly violent pitch-up.

However, with the mains near the C/G, the prop will be far aft of them, which means the problem of ground clearance when the nose is up for touchdown and for liftoff becomes worse. It would be a good idea to have a small ventral fin to help protect the prop. Also, this problem will restrict the prop diameter. This is a classic problem with pusher installations, the inability to have enough diameter, and more efficiency loss, over and above that caused by the distortion and turbulence from the airframe into the prop. Quite a few pushers paid big penalties for this. Mounting the motor higher in the fuselage or even on a pylon is one way around it, but then you're back to having thrust line problems. Having extremely tall landing gear is another solution, but that opens its own "Pandora's Box" of new problems.

Quote:
..Does the sweep here refer to the canard?...
Both the wing and the canard. Each introduces its own set of additional aerodynamic problems.
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Old Jan 16, 2011, 04:13 PM
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There is no reason you can't get away with coupling the nosewheel steering to the aileron channel. It's unconventional, but it will work. We do it all the time on our Roadkill Series models, where ailerons, rudder, and nose or tail wheel steering are all done by one servo.
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Old Jan 17, 2011, 02:14 AM
What could possibly go wrong?
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E.D.F.?


Nick
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Old Jan 17, 2011, 06:50 AM
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EDF would be a logical way to go. However, the efficiency of a ducted fan is generally far below that of a well-designed prop, largely due to the reduced diameter. The duct design is very critical and somewhat tricky, and has to be optimized for a particular airspeed and power, with losses at other airspeeds and powers.
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Old Jan 17, 2011, 07:59 AM
What could possibly go wrong?
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Don
Quote:
the efficiency of a ducted fan is generally far below that of a well-designed prop,
Yes, I've avoided EDF for that reason. (and also because I like to fly slowly) But technology marches on and I've seen some pretty successful EDF park flyers at the model aero club.

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Old Jan 17, 2011, 08:16 AM
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Ducted fans are a bit of a contradiction. Basically they improve thrust at the low-speed end of their operating envelope, by reducing the tip losses. At higher speeds, the drag of the duct eventually becomes greater than the savings from the reduced tip losses, at which point you would be better off without it.

However, that's in comparson to a free prop of the same diameter. Generally it isn't possible to make them as large as the diameter of a free prop on the same installation. The inlet and outlet diameters and shapes are also critical, with several conflicting factors that have to be juggled, and that change significantly with airspeed and power setting.
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Old Jan 17, 2011, 08:36 AM
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Charles
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My D Squared canard build log shows how a torsion main gear is built into the wing. Just go to post #1 here, last paragraph and find what page the build starts on.
Thank you, sorry but I can't find it, could you link it for me.


Don

Quote:
There is no reason you can't get away with coupling the nosewheel steering to the aileron channel.
Thanks for that, I thought the ailerons would lift the wing.

Tony
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Old Jan 17, 2011, 09:03 AM
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At the speeds where you need the nosewheel steering it should not be a problem.
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Old Jan 17, 2011, 09:52 AM
Jets are for kids-of all ages
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I've done the nosewheel to the ailerons on a Foam Board Long EZ and it worked just fine, It takes very little nosewheel/aileron for tracking centerline if you line up properly and soon as it lifts off it is all natural. (Works fine)

Roger
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Old Jan 17, 2011, 10:06 AM
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Charles

Quote:
My D Squared canard build log shows how a torsion main gear is built into the wing. Just go to post #1 here, last paragraph and find what page the build starts on.
Tony, The landing gear details are shown on page 53, post 787 and on page 60, posts 892 and 900. The long torsion link makes the gear almost damage free from hard landings. It is the best I have ever had.
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Old Jan 17, 2011, 10:18 AM
What could possibly go wrong?
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Landing Gear

Tony

Here's the one I did for my Long EZ. I hope the picture's helpful...

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Old Jan 18, 2011, 05:24 AM
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Thanks Guys for the help with the landing gear.

Nick you live in the UK, where do you buy your steerable noselegs? I search for one but only one I can find is without a servo saver. Do you use a servo saver?

Tony
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Old Jan 18, 2011, 06:07 AM
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Don, you described scenarios that actually do happen. Over many years designing and building the models, I experienced all of them. I feel that it is next to impossible to design perfect plane. Every design is the compromise.


Quote:
Originally Posted by Don Stackhouse View Post
OK, one more time...

There is no perfect solution for this airplane.

Pusher means efficiency losses, possibly severe, among other things.

Tractor opens the possibility of handling problems on this particular airplane, possibly severe.

Prop-in-slot means for-certain major efficiency losses, noise, vibration, etc..

Props on nacelles on the wings means failure mode issues.

Prop on a pylon means thrust line issues.

So, which set of problems do you prefer? In this particular case, I suspect pusher will be the easiest to make work effectively, all you have to do is shoehorn enough raw power into it to overcome the efficiency loss. That's a lot more straightforward to do than sorting out a bunch of really tricky handling problems.

The use of landing gear does complicate things. The mains need to be just behind the C/G. If you put them further aft, you will need miles of runway to get enough speed to rotate for liftoff, since the elevators will not be getting any help from the propwash, and the too-far-aft mains would shift more load to the nosewheel. When the plane finally did get enough elevator authority to lift the nose, it would tend to leap into the air suddenly, perhaps uncontrollably, as the weight shifted from the wheels to the flying surfaces, causing a possibly violent pitch-up.

However, with the mains near the C/G, the prop will be far aft of them, which means the problem of ground clearance when the nose is up for touchdown and for liftoff becomes worse. It would be a good idea to have a small ventral fin to help protect the prop. Also, this problem will restrict the prop diameter. This is a classic problem with pusher installations, the inability to have enough diameter, and more efficiency loss, over and above that caused by the distortion and turbulence from the airframe into the prop. Quite a few pushers paid big penalties for this. Mounting the motor higher in the fuselage or even on a pylon is one way around it, but then you're back to having thrust line problems. Having extremely tall landing gear is another solution, but that opens its own "Pandora's Box" of new problems.



Both the wing and the canard. Each introduces its own set of additional aerodynamic problems.
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Old Jan 18, 2011, 06:41 AM
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landing gear design

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Originally Posted by Perdu View Post
Thanks Guys for the help with the landing gear....Tony
A couple more pointers.

Find the longitudinal and vertical location of the C/G and plot it on you side view. Draw a line downward from it at about 5-10 degrees aft of vertical. The axles of the main wheels will be somewhere along this line.

Draw a line forward from the bottom tip of the prop, angled about 15 degrees below horizontal, like the attitude relative to the ground that the plane will have when fully stalled. Note, 15 degrees is more than the stall angle of most 2-D airfoils at our Reynolds numbers, but we need to allow extra because of the reduction of the lift curve slope caused by your wing sweep (same reason the Concorde needed such tall gear). Draw a parallel line below it at a distance equal to the ground clearance you want to maintain for the prop at liftoff or touchdown. Where this last line intersects with the first (almost vertical) line is where the contact point of the main wheels with the ground should be.

The nose wheel should be as far forward as you can get it and still have adequate structure inside the fuselage to attach it. The wing should be near a zero-lift angle when the plane is resting on all three wheels. This prevents gusts from upsetting the plane while taxiing.

This gives you a landing gear with adequate handling properties on the ground and for liftoff and touchdown. However, in your case you may find that it results in a landing gear that is unreasonably tall, unless you make your prop unreasonably small. At that point you may have to consider mounting the motor higher in the tail cone, or find some other alternative. Airplane design is inevitably a compromise. When you design an airplane that looks like a jet but then try to fit a prop to it, those sorts of problems are to be expected.

Good luck.
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