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Wing construction for ultralight FAR 103 (USA)


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I am in the process of design of an ultralight plane for which I only have some of the skills needed.

My plane will have a 30 foot one piece high wing with a chord of 6 feet and maximum thickness of 9 inches.

It will take-off at about 25 - 27 mph and have a maximum straight level speed of 63 - 64 mph.

Weight is a prime concern but drag is not. I believe that it would be lighter to use the leading edge "D" box spar and the rear spar that the flaps will hinge on as the load elements and attach the wing to the top of the fusalage and with struts at some length of the span for twist prevention and longtitudinal stability. Using 600 - 650 gross weight and a 6 g load factor will wood or wood / foam sandwhich work in this construction. The wing will be fabric covered so the covering is not structural.

Here is what the foil looks like:


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  • 2 weeks later...

I would suggest you start with an NACA airfoil configuration http://www.allstar.f...aero/wing31.htm . Then calculate the area you will need given a length to chord ratio for your flight regime. Then develop an aluminum box to take all the loads and the torsion, Then check for flutter, then see if it will fit into 9inch limitation. Then design the Ailerons that will take control of the aircraft with the fuel moving outward to inward in knowing where the center of gravity of your aircraft is fully loaded. Make sure that it will takr the landing wheel loading and cycles you need if you have wheels


Your wing is very important to your very life so I recommend that you have an aircraft company in Germany or Switzerland that has built 100s of them do the design and build you one and maybe the entire aircraft.

Edited by zorro
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The problem with NACA foil designs is that they are not designed for a maximum speed of 64 MPH and a minimum flight speed of 24 mph.

Most are designed for speeds in excess of 200 mph and are inefficient at speeds below 70 mph. The airfoil above operates well in the range between 24 and 64 MPH according to the NASA Foilsim program and Aeval software.


I would rather not change a design that seems to work well but I could use some help in designing a spar or spar system that will support the wing/aircraft combintion.


The wing won't hold any fuel - the 30 pounds of fuel is located at the center of gravity. I will use Junker style ailerons and ungapped hinged simple flaps. A six foot chord and a 30 foot span provides for lift at 24 mph (14 with flaps down) and low drag co-efficient at the maximum level flight speed of 64 MPH.

I would rather not use aluminum because it will fatigue and need to be replaced. If I use wood "I" beams and protect them from UV and moisture they will last for hundreds of years and be lighter than an aluminum box.


I know very well how important my wing is to my life. As far as anyone in Germany or Switzerland having built even one wing that is designed to operate in the speed range that mine will and although they do have some microlight aircraft their aircraft do not meet the specs for the USA FAR 103 ultralight specifications for weight or speed range.


Thank you for telling me to learn how to design the aluminum box to fit my needs - perhaps that is the most important thing your post said. You either are unwilling to help or don't know how to build the spars any more than I do. Thank you for the concern

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How would I go about calculating the strength of a wooden spar that is 5 inches high, has a web thickness of 1/8" and a top and bottom cap of 1.5" x .75"?

One more at 9 inches high, 1/8" web, and the same top and bottom caps.

The materials are birch plywood (3 ply) for the web and Sitka spruce for the caps.


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  • 2 months later...

Air speed isn't directly the input that decides if a foil fits your use. Reynolds number is more important.


Plywood sold for aircraft construction guarantees some minimum strength, allowing to compute a beam much like is done for metals.


It seems you don't have the necessary knowledge to build your aeroplane. Either learn it, or let knowledgeable people design you craft, or build your craft according to existing drawings - that's already difficult enough.


Many almost-finished aeroplanes can be acquired from amateurs who give up. Cheap, saves time.

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