New to forum

[Ivanstein]

Greetings,

I am a new to the forum and new to ultralight aviation.

I am an aerospace engineer and have decided to design and build my own ultralight. I got into engineering late in the game and have an extensive history in machining and mechanics (mostly automotive amd heavy equipment). So, I have, at least, the basic skills to take this project from start to finish.

I plan on a full metal craft with a semi monocoque fuselage and a rib and stringer wing.

I have been working from my Senior design class text by Raymer and have mostly completed the rough sizing calculations. I have stared into the conceptual stage where I need to start generating basic lay out drawings (read: the fun part).

I am sure I will have many many questions and would be quite interested in finding someone local(ish) who has been down the same road. I am in Wichita KS, and would love to chew the fat at my design firm office (Starbucks) and learn some insider tricks.

Also, if anyone has a 5 axis mill and a CNC lathe with live tooling just sitting in their garage waiting on something to do....


First serious questions:

Do I pick a reasonable gross takeoff weight (W0) considering my own size ( working on reducing that...) Or do I set a gross​ weight based on an average pilot to set wing area for minimum stall speed requirements? If I design it for a 170lb pilot and consider the spare tire I carry as "crash protection" and live with the higher stall speed, is that reasonable?

Next question:
Are there any power plant considerations which will be about 30-35 hp without breaking the bank? I have seen a Yamaha engine showing up on ebay, but cant find any info on it. Also a reasonable Cayuna on ebay, but that seems to be an antiquated engine and parts may be hard to find.

I appreciate any help and would love to find some new friends of like mind to share enthusiasm.

Thanks.

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[rwanttaja]

First serious questions:

Do I pick a reasonable gross takeoff weight (W0) considering my own size ( working on reducing that...) Or do I set a gross​ weight based on an average pilot to set wing area for minimum stall speed requirements? If I design it for a 170lb pilot and consider the spare tire I carry as "crash protection" and live with the higher stall speed, is that reasonable?

Design it for the weight at which you expect it will be operated, and add a 25% margin on top of that. So if you weigh 200 lbs, design it for 250 lbs. It'll thus be happier a lighter weight.

Ron Wanttaja

[cliffo]

I agree with Ron on the weight issue as there is always an issue with going over gross, helmets, clothing, a little carry on kit, etc., etc. As far as engines are concerned no matter what you start out with in 10 yrs. if not sooner it will be way behind in the new designs and technology. I have an old 447, and a rebuilt 40 yr. old Cayuna I went thru for a back-up, doing a couple updates in the process. These older engines are tried and true, and parts are readily available.

[Bill Berson]

Gross weight is your choice, depends on if just one for you or if selling plans to others.
AC103-7 (online) has much info about the weight rules.

[Ivanstein]

Gross weight is your choice, depends on if just one for you or if selling plans to others.
AC103-7 (online) has much info about the weight rules.

Thanks for that. I am currently running calculations for my altitude +1000' which is right at 2300' msl. So, if I am reading the description correctly, the airspeeds can be corrected for a standard atmospheric day at sea level. This will help a bit on the wing area required to meet stall speed requirements, as with the somewhat thinner air here, the q (dynamic pressure) value is lower requiring more wing area to produce the same lift at sea level.

So, apparently I can suffer the higher stall speed due to thinner air. However, that doesn't help with performance at altitude either. I think, with everything engineered, there will be a trade off.

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[Dana]

If you're designing for Part 103 you will want to use the AC103-7 appendix calculations for speeds. Not as actual design values, but as a sizing guide. The calculations in the appendix are very conservative, and accepted as proof of 103 compliance without actual demonstration. So you can have a legal ultralight with somewhat higher performance than the regulations would suggest.

As for engines, the Cuyuna is not a bad choice. Lots of them still around, parts readily available and far cheaper than Rotax.

[Ivanstein]

I have been faithfully crunching numbers on this and have been getting a bunch of really terrible expected wing area results.

In the order of 220ft^2.

This is all driven by, believe it or not, cruise. I assumed a 40kt cruise speed and it turns out that the wing loading should be around 3.5lb/ft^2. My lowest wing area occurs when I adjust the coefficient of lift to match takeoff wing loading to match cruise.

This is all initial sizing, and will be revised as the design progresses. However, I am unsure how the other popular ultralights keep the wing area so low. They all have calculated wing loading of 3.5-4.5 lb/ft^2, but seem to be only 100-125ft^2 area. Most are right around 500lb gross weight.

More investigation is due.

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[Ivanstein]

Picture of some surfacing I have done. The transparent box is the interior dimension for a 20ft enclosed trailer I looked up on line. One of the design goals is to make it fit in one due to the lack of a hanger in my back yard.

Other one is a 4 view. This is a very rough lay out. I have parametrized the wing and flying surfaces to make easy and quick modifications and have formulas to quickly read out areas and tail volume coefficients. This will definitely evolve as time goes on. I am thinking it may end up being a biplane (due to required wing area) but that will be more difficult to stuff in a trailer. However, I should be able to keep the model parameterized with upper and lower wing data and an option to include one or both.

So, as a matter of personal opinion, should I go for a monoplane? Biplane? High, shoulder, low wing? Canard? Possibly a flying wing? Give me your personal favorites!

[Bill Berson]

Wing area is determined for required stall speed, not cruise.

[Ivanstein]

Wing area is determined for required stall speed, not cruise.

Not really. Wing loading is calculated for each expected flight regime. Select the lowest, and size the wing to maintain that wing loading. In this case, because the speed envelope of the ultralight category is so limited, the air density causes the greatest shift in dynamic pressure. Thus, the takeoff condition at ground level has a touch higher q value than cruise at 40kts and 1000ft AGL. So, basically the wing loading at cruise must be lower than at take off (stall x 1.1 kts)

It was rather surprising to see this occur, but after thinking about it, it does make sense.

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