The FAA's old standard for average weight was 170 pounds, but the ASTM standard pilot weight used for LSA is 190 pounds. Playing a little loose with the precise definition of "average," half the population will weigh more than 190. Me, for example, and I'm not all that big by today's standards. Design for heavier pilots, as somebody else pointed out. Not that many pilots weigh 170 or less any more.
Hi Ivan:
I'm following your thread with keen interest. I'm also building my first aircraft and have started my own thread. I'm using Raymer's simplified design excel spreadsheet for the preliminary calculations, and have the student version of RDS and his text for when it's time to get down in the mud. I've got a much simpler design task since I'm building a powered paraglider, and the wing already exists. I'm primarily using the design software to help me select a wing at this point.
Good luck, Michael
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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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!
Last edited by Ivanstein; 09-12-2017 at 01:24 PM.
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)Originally Posted by Bill Berson
It was rather surprising to see this occur, but after thinking about it, it does make sense.
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In level unaccelerated flight, wing loading does not change; it's aircraft weight divided by wing area. Well, it decreases slightly as fuel is burned, but that's it.
For Part 103 ultralights, the FAA has published a set of tables (In the AC103-7 appendix) that essentially give the required wing loading (assuming a 170# pilot) to achieve the mandated stall speed for various generic airfoil types. If your ultralight complies on paper with those tables, no further proof is required, even if the actual pilot is a sumo wrestler, and yes, even if the actual performance is outside the 103 limits. As those tables are rather generous, they're a good place to start your initial wing area sizing.
Thats all correct, except the design is based on the "required" wing loading for best performance at any given regime.
Think of it like this...
At stall, your wing is operating at maximum lift. Indirectly, this is at the maximum coefficient of lift. So, you are using all that wing can possibly create.
Now, give that wing extra speed and you no longer have to operate at maximum lift coefficient. So, in this case you could reduce the coefficient of lift of the airfoil and still have enough lift with less drag due to airfoil thickness or camber. OR...you could decrease wing area and keep the same airfoil resulting in less drag. Either one would be optimal at cruise, but wouldn't get the airplane off the ground at any less than cruise speed. So, it isnt optimal for takeoff.
Another way to think of it is that with higher speed, you can lift more with the given wing and it would be optimal to increase your "cargo" at cruise.
Another idea is, maybe the aircraft can give away a bit of slow speed performance to gain range or loiter performance. In this case the wing may be sized down to be more efficient at cruise to extend range or time aloft but will have a higher takeoff speed and required runway distance.
So, yes, once the wing is designed, wing loading is weight divided by wing area. But during initial sizing, you run scenarios of the optimal wing for each flight regime and the different wing loadings which are best for each. Then you select the lowest required and size the wing for that. If I had assumed stall is the determining factor (which I had, initially) I would have lost a measure of efficiency, which in this category, is very important due to limited weight and flight envelope.
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Last edited by Ivanstein; 09-12-2017 at 01:23 PM.
Hi Ivan:
Raymer has some assumptions about structural weights that I don't think apply well to ultralights. I had to fudge his spreadsheet with values I knew (like the 254 pound empty weight - his W0 was ending up in the 2000-3500 pound range). Thinking about your question, I realized that I have to recalculate his empty weight constant "a." I had a similar issue as you with CL-max, but his simplified spreadsheet doesn't differentiate the CL at takeoff vs. cruise, it just uses CL-max.
Michael
You are correct. Cruise assumes only enough lift to balance drag and with a prop driven craft, the best range comes at best L/D, so drag is the driving parameter. With a high camber airfoil as needed in an ultralight, that is substantial, even at low angle of attack.
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Last edited by Ivanstein; 09-12-2017 at 05:36 PM.
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