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Old Jan 26, 2004, 11:42 AM
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My first aircraft design and I've got some questions!

Hello everyone. I've decided to design my first park flyer plane. It's an acrobatic sport scale model of a very famous plane. At this time, I would like to keep the model of plane out of this.

Anyway, I'm looking at trying to determine what airfoil would work the best as well as find the definitions of these airfoils or a program that generates them.

I'm an AutoCAD guy from way back so if the program could kick them out in DXF or DWG format would be the best.

Thanks for all of your help and I'll keep everyone informed as the design progresses. I'm sure I'll have a lot more questions.

Thanks!

Kevin
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Old Jan 26, 2004, 12:05 PM
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Punta Gorda, FL
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The airfoil selection depends on the range of angles of attack or the range of coefficients of lift that the designer desires. The range of air speeds and airfoil profile drag polars bear on how much power is required. The wing loading determines how much lift the wing and its airfioil must generate which in turn determines the range of airspeeds that the plane can fly. All of these things bear directly or indirectly on airfoil selection. Many of these considerations will be in conflict with each other and the compromises that resolve the conflicts depend on the priorities of the designer's objectives. Unless the designer has a clear idea of these objectives and priorities and can quantify them the design decisions can't proceed rationally.

Many of the design decisions concerning configuration are already made when a scale subject is chosen. With out knowing the specifics of the scale configuration the airfoil selection becomes somewhat of a crapshoot.
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Old Jan 26, 2004, 12:11 PM
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aghhhhhhh.....I'll do some more research on-line.
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Old Jan 26, 2004, 12:24 PM
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for a park flyer, something symmetrical if you want it to look like the original. Doesn't need to be, at park flyer speeds thickness is important, since speed isn't needed.
15% wouldn't be too thick..
This 'un uses a Speed 400.. the designer also has a 280 version.
Either works OK.
http://www.angelfire.com/indie/aeros...emto400-02.jpg
.
Keeping the scale look will hurt you. The wing will be too small at park flyer size, unless you can find a lot of lightness to install.
Look at what Hobby Lobby has for park flyers.. that's the best clue for what can work, see what does work.
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Old Jan 26, 2004, 12:26 PM
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Ollie is correct of course. You need first to decide what flight envelope you're aiming for.

However unless you're really keen to get involved in some seriously complex mathematics you could always do what most model designers do. Find a model of similar type and size that is known to fly well and copy the airfoil it uses. At park flyer size/speeds the airfoil really isn't very critical and it's basically impossible to find accurate data for any airfoils.

If you really want to play with airfoil designs then I recommend Profili 2 ( www.profili2.com ). It contains a couple of thousand different airfoils, many of which will be completely impossible to tell apart in performance terms when used on something like a park flyer. It will export to DXF.

Steve
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Old Jan 26, 2004, 12:31 PM
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Thanks everyone. It's an acrobatic plane with a fully symetrical wing. I want to keep the speed down (hence parkflyer) and it's in the geared speed 300 range (stickmount). I would compare it to a stevensaero Park cap as far as the performace that I'm wanting (and I own one of those). How would you determine the airfoil on an existing plane? It's not a plane that would be used for 3D flying but more scale like acrobatic show stuff. Plus just great flying around the field. I'm going for a sport scale plane. I have come up with some internal controls for the tail feathers that I think will add some nice touches.
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Old Jan 26, 2004, 12:36 PM
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Also, what is the best way to determine the thickness of wood needed? I'm an architectural guy and have span charts to tell me how far a 2 x 10 YP #2 will span but I haven't found anything on balsa. I plan to try to stay consistant with what other kit designers are doing, but didn't know if there was some "rules of thumb"

Thanks again.

Kevin
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Old Jan 26, 2004, 02:08 PM
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It isn't that bad Kevin.

Begin by considering if you want to be able to fly inverted as well as upright. That will help determine how much mean camber of the airfoil is allowable. The plane has to be able to take off and land. How slow, in feet per second is the desired airspeed? This will allow the maximum upright coefficient of lift to be calculated from the gross weight and wing area. If the plane is to fly upside down, what is the minimum desired air speed while inverted? Presumablbly it won't have to land upside down so the minimum airspeed inverted can be higher than upright and this will allow some positive amount of mean camber. This will allow the a negative limit on the airfoil coefficient of lift range to be calculated.

The scale configuration and wing area will then determine the wing chord. The wing chord and minimum airspeed will allow calculation of the minimum reynolds number. that will determine if data is available to find the drag of various airfoil candidates. Next consider the configuration of the fuselage and tail. This will determine the parasitic drag and how much is left in the drag budget to allocate to the airfoil drag. The maximum desired airspeed and the airfoil profile drag will determine the power required for level flight. Next consider the desired rate of climb, the available power and the gross weight together with the maximum lift coefficient of the wing as determined by the stalling speed, will determine the rate of climb.

There is more but this should give you some things to consider to get you started and reveal where some of the conflicts are so that you can begin to set priorities.
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Old Jan 26, 2004, 02:27 PM
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So it sounds like I need to design the fuse and tail feathers (tail fethers will be flat with rounded edge) and estimate it's weight before I need to worry about the wing design. The wing span will fall in the mid 30" wingspan from what the other guys have done for the size that I'm wanting. Once I get those numbers and an estimated weight (let's say 17 oz ready to fly with all equipment) then I can better pick out an airfoil.

There's more to this plane stuff than I thought but I'm not giving up until it's flying and flying well!

Thanks again for your help Ollie. I'll post some pics as I get the design under way.

Kevin
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Old Jan 26, 2004, 02:48 PM
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How about an aspect ratio or average wing chord? If the wing is tapered, the root and tip chords? Inverted flight capability? Stalling speed in feet per second?

First comes the performance envelope which helps set the weight target. Then comes the structural design to meet the weight objective. This is where really important trade off among ease of construction, cost and performance occur.

Everything affects everything else! It's deliciously complex.
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Old Jan 26, 2004, 02:56 PM
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I'm just starting on the design so I don't have that info yet. I have the paper 3-view drawings but I haven't had a chance to pull them into CAD yet and do any scaling on them. Hope to get this information soon.

Kevin
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Old Jan 26, 2004, 03:17 PM
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You don't need a CAD program to do some scaling. If the desired wing span is 30 inches and the measured wing span on the paper three view is X inches, just divide 30 by the X measurement in inches and you get the scale factor. Use it to multiply the chords measured on the paper three view to get the model's chords.
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Old Jan 26, 2004, 03:22 PM
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Yep, but I've got a $200k house that I'm finishing right now that has to be completed in a few hours so my time is a little limited right now to do that.

I'm going to estimate somewhere in the 7" - 8" range.
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