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Need Airfoil help

Old 03-04-2002, 11:56 PM
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flugzoid
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Default Need Airfoil help

Howdy guys, I just picked up my scaled up plans of an ME109. The fuselage is about 39" long and the wingspan is going to be about 43" long (1:9.25 scale). I'm wondering if anyone has an ideas on beginning numbers for this airfoil. I'd like the model to be in the 7-10 pound range, powered with nothing larger than a .60 engine. With the 43" wingspan, the cord tapers from 10" to 5", and the cord depth is 1.5". That's the scale (real aircraft airfoil). I'm looking for ideas on how to make an airfoil that will keep a similar looking shape from a plan view, but will consider (should I consider?) lengthing the wingspan and playing with the % of chord. My terminology's probably all wrong...but any help would sure be appreciated for starting nums.

Thanks guys,
John
Old 03-05-2002, 01:39 AM
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Ollie
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Default Scale ME109

A .60 powered 7 or 8 pound plane will have a nice power loading. However a scale wing of 43 inch span will have a wing loading of 50 ounces per square foot, which is way too high. Send the plans back and have them enlarged another 30 to 40 % to get a reasonable wing loading with a scale appearance. Keep the engine and weight unchanged.

Messing around with the wing span or airfoil without changing anything else won't do the job.
Old 03-05-2002, 02:46 AM
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flugzoid
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Default Need Airfoil help

Roger that Ollie. Hmmm...well, I'm not sure that enlarging the entire wing 30 to 40% will allow the aircraft to look appropriate? I'm going to take a look at that suggestion though, and I certainly appreciate you offering. I guess changing the camber, chord thickness, and extending the wingspan, while keeping the aerodynamically the wing's chord length won't do the trick? Wish I had more experience playing around with different airfoil sections...
Old 03-05-2002, 03:01 AM
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Mike James
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Default Wing loading

I think Ollie means have ALL the plans enlarged... not just the wing.

I do these "pre-paying for plans enlarging" calculations with a simple spreadsheet. This makes the initial planning of the project go easier, and gives you "targets" to hit in your building process.

1. Enter the basic specs of the ACTUAL aircraft (length, span, and known areas) into a column on the spreadsheet, but with the numbers converted to inches. (Makes it easier at the end)

2. Then make several other columns, and simply use formulas to scale the first column's data to say 1/8th, 1/7th, 1/6th, etc.

3. Weigh the things you KNOW about, such as all your hardware, radio gear, engine, wheels, etc.. (or get these numbers from catalogs) Come up with at least a reasonable guess as to the final weight. (some tips on my web site - See "Accurately Predicting Structural Weights" on the "Design and Building Tips" page)

4. Use the spreadsheet to calculate the wing loading at the bottom of each column. The formula is: Weight (oz)/Wing Area (sq. in) X 144 This will give you the wing loading in oz/sq. ft.

Feel free to email me if this isn't clear, or if you need any help.
Old 03-05-2002, 03:07 AM
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Ollie
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Default Need Airfoil help

When I said "send the plans back" I meant everything including the fuselage and tail. The tail areas of the scale configuration are already marginal. If you increase the wing area without increasing the tail areas and moment arm length, you are very likely to run into stability and control problems. Going 30 to 40% larger on everything is much more likely to succeed. The new scale size would be about 1/6.6 or 1/7 rather than 1/9.25.

If you want to keep the size, then put a .30 in it and keep the weight down to about 3.5 pounds. Use contest balsa, lite ply and light weight wheels. Don't load on a thick paint job. If necessary, use mini servos and a small battery pack to meet the weight target.
Old 03-05-2002, 03:31 AM
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mglavin
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Default Need Airfoil help

Ollie

I can relate to scaling up the plans if a .60 is used, but whta if he is uses an engine like a .32SX? These numbers sound reasonable for a 43" WS model...

The weight is not likely to be 7-10lbs. on this model, is it? 43" in wingspan, .32 size engine...
Old 03-05-2002, 04:07 AM
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flugzoid
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Default Need Airfoil help

All EXCELLENT comments gentleman. Thanks very much.
Mike, the spread sheet idea is an excellent one. I'm going to go make one up in just a bit...that is, after I check out your website again.

Ollie: I know you're going to laugh at me when you hear this, but I'm actually going to try and make this "light" model out of fiberglass.... You're right on about enlarging the tail section too. Thanks for clearing up the details of scaling up.

mglavin: Thanks for the suggestion...what sort of wing loading do you look for in a gas aircraft?
Old 03-05-2002, 04:28 AM
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Mike James
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Default Sample of the spreadsheet idea

John,

I emailed you one of these, too.

This is only a sample...Don't build from it! The weights shown were put in as general guesses a few minutes ago, and are "fake", just to show you how the math works. Hope this helps.
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Old 03-05-2002, 06:19 AM
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flugzoid
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Default Need Airfoil help

Thanks Mike. I just got the download and sent you a reply. I will be replicating the format soon.

John
Old 03-05-2002, 10:39 AM
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Ollie
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Default Composite Construction

John,

I'm not laughing! The competition in discus launched R/C gliders has produced composite planes with 60 inch spans and, about the same wing area as your 43 inch wing, that weigh 9 ounces or so ready to fly with 4 servos. The launch speeds are on the order of 100 MPH. Instead of fiberglass, they use foam, kevlar and carbon fiber. You might want to look into similar construction techniques. Carbon fiber has about 5 to 7 times the strength to weight ratio of balsa.

Discovering how to properly apply these materials to scale shapes is a real challenge but the potential is definitely there.

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