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Mar 09, 2004, 11:05 PM
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Trikster's Avatar

Starting my first scratch built, need some help...

I am planning on putting together one of Chris3D's Tiger Moths and/or Pitts. This will be a scaled up version from his RFFS design and be assembled out of Depron. My question lies in this...

I have a bunch (30) of the J250 motors from All Electronics. I wish to run these DD (the cost of adding a gearbox and the Depron makes it almost as much as a NIB white Tiger Moth from GWS) using either the 4025 or 4040 GWS props. I also have the 3030 and 3020 props if that would make a better choice. My main issue is, what should my wingspan be? I am thinking in the 18 - 20 inch range, but since I am a foam kit builder, I know nothing about figuring out scratch dimensions.

Any help would be appreciated.


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Mar 09, 2004, 11:44 PM
It's the last plane honey
SlowRider's Avatar
So that's why you were looking for the foam... Can't wait to see it.

Mar 09, 2004, 11:45 PM
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Mar 10, 2004, 07:45 AM
Team 30 Micro EDF
Mike Taylor's Avatar
The motor itself weighs 1 5/8 ounces. Figure the weight of the rest of your gear, pick an acceptable wing loading, and let that determine your needed wing area, and work back from there to your wing span...
Mar 10, 2004, 10:23 PM
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I appreciate the advice Mike, but I have only built kits in the past. I have no clue what you just said...
Mar 10, 2004, 11:19 PM
Foam Junkie
Originally posted by Mike Taylor
The motor itself weighs 1 5/8 ounces. Figure the weight of the rest of your gear, pick an acceptable wing loading, and let that determine your needed wing area, and work back from there to your wing span...
Square foot = length X width / 144
Wing loading = Weight / Square Foot

Generally the bigger the wing the more weight it will support at a slower speed.

A heavy wing loading needs more speed to keep it flying and that means more power. It will have a higher stall speed so your landings will need to be a little hotter.

A light wing loading like 4 oz per sqft for example will allow the aircraft to lumber along very slowly and use less power to do so.

Im no expert but this is the way I understand the concept. Someone correct me if I am wrong.
Last edited by BGR; Mar 10, 2004 at 11:21 PM.
Mar 11, 2004, 12:05 AM
Team 30 Micro EDF
Mike Taylor's Avatar
OK, let's try again...

Look at the weights and wing areas of your other planes. Figure out the square inches, divide by 144 to get the number of square feet. Then figure out the ounce pers square foot. You should get some idea of how much weight a square foot will hold up. Heaveier weights per square foot of wing area will fly faster than lower weights.

Find the one you like and pick that as your target loading. Figures for park flyers are perhaps 6 oz/sq foot for the sake of argument.

Now weigh the motor, gear, batteries, and take a guess at the airframe weight. Add that all up. Using this weight, figure out how many square feet (or square inches) you will need to keep that wing loading.

Once you have the area your plane needs you can divide that in half (for your biplane). Pick an aspect ratio (like 6:1 is common).

Area = span X chord (and span = 6 X chord for 6:1 AR)
therefore, Area = ( AR times 'X')('X'), or
Area = X squared times AR
X squared = Area / AR

If each wings area is 100 sq inches, then
X squared = 100 / 6, or
X squared = ~16, and X = 4
Your wing would then be 4" chord, and 24" span...

It's just algebra...
Mar 11, 2004, 12:08 AM
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The math part is easy, but without the equations it was difficult. Thanks Mike and BGR, I will work with that and go from there.

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