what stall speed do I want?

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DavidBooks

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Feb 7, 2021
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Howdy do. I'm designing a part 103 ultralight and at the moment I'm attempting to design the wings. I need to find the wing loading, but the only problem is that I need to have an ideal or wanted stall speed in mind for my aircraft. I plan to have a Hirth F-33 on it, and the structure will be mostly wood and somewhere in the range of x-250 lbs in weight. (It'll be somewhere in the range of 422lbs full weight) I have no prior experience with aircraft and I cannot find any reliable sources on the f-33's performance in flight. I need to have a stall speed in mind or I will not be able to calculate the wing loading. what would a reasonable stall speed be to aim for in this situation? Thank you.
 

Marc Zeitlin

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I need to have a stall speed in mind or I will not be able to calculate the wing loading. what would a reasonable stall speed be to aim for in this situation? Thank you.
Well, since the Part 103 regulations require a stall speed below 24 KCAS, you should shoot for something less than that so that when you don't reach your desired performance (because nobody ever does), you'll still be legal.
 

Vigilant1

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Part 103 itself bounds your answer. The maximum allowable power off stall speed allowed by that reg is 24 kts.
 

User27

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At 422lb, and with a stall speed of 24KCAS your wing area will be around 145 ftsq for a CL of 1.5
Wing loading will therefore be around 3 lb/sqft.
An Aerolite 103 has a wing area of 124sqft
 

Bille Floyd

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Howdy do.
... I plan to have a Hirth F-33 on it, ...
Your First post ; (WELCOME TO HBA) !!!

The F-33, puts out 28Hp, and is 40Lb with the redrive , (it's air cooled) ; the
Polini Thor 202, does 33Hp , and weighs 37Lb W/re-drive, (it's Liquid cooled).
The 202 is newer technology.

Bille
 

Dana

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Hi David, for a Part 103 ultralight, the FAA's Advisory Circular AC103-7 appendicies are a good place to start. Appendix 2 has a nomograph for determining the stall speed; if you work backwards from the required 24 knot stall speed with your target weight, you will get the required wing area.

The nice thing about the 103-7 calculations is that they're very conservative, and accepted by the FAA as proof that the ultralight is legal, even if its actual stall speed is higher. And you only have to assume a 170# pilot, even if you actually weigh more.

So, assume the maximum 254# empty weight, 30# of fuel (the max allowable), and a 170# pilot for 454#. Work out the wing area based on the airfoil you're using, and you get the required wing area / wing loading for a legal ultralight, and design the structure based on that. At 422# you will have a slightly lower wing loading and stall speed. You can always go larger on the wing area if you want, you might get better climb performance but the lower stall speed will reduce its ability to handle wind. The best handling ultralights tend to be right at the legal limit.
 

DavidBooks

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Hi David, for a Part 103 ultralight, the FAA's Advisory Circular AC103-7 appendicies are a good place to start. Appendix 2 has a nomograph for determining the stall speed; if you work backwards from the required 24 knot stall speed with your target weight, you will get the required wing area.

The nice thing about the 103-7 calculations is that they're very conservative, and accepted by the FAA as proof that the ultralight is legal, even if its actual stall speed is higher. And you only have to assume a 170# pilot, even if you actually weigh more.

So, assume the maximum 254# empty weight, 30# of fuel (the max allowable), and a 170# pilot for 454#. Work out the wing area based on the airfoil you're using, and you get the required wing area / wing loading for a legal ultralight, and design the structure based on that. At 422# you will have a slightly lower wing loading and stall speed. You can always go larger on the wing area if you want, you might get better climb performance but the lower stall speed will reduce its ability to handle wind. The best handling ultralights tend to be right at the legal limit.
Alrighty then, will do. Thank you very much!
 

Riggerrob

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Stall should be as high as you think you can get away with legally.
EAA said the FAA doesn't check stall speed.
I disagree.
Stall speed determines how badly you will injured during a crash. The slower you arrive at the scene of the accident, the fewer injuries you will suffer.
I disagree with many other statements above that encourage you to plan you airplane, heavier, faster, etc. than the air regulations. Those air regulations are written in blood. Also consider how conforming to air regulations will reduce your risk of dealing with gov't lawyers. I have 5 years experience with Transport Canada's lawyers and they were the most miserable years of my life! Legal hassles cost far more in terms of wage loss, mental health and delayed surgery than the King Air crash that started the whole miserable lawsuit.
A smart designer tries to make his airplane lighter, land slower, climb faster, etc. than the minimums specified in regulations.
 

BBerson

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Human powered aircraft are the slowest and lightest but don't meet any transport safety standard.
I prefer the Canada ultralight regulations.
 

Dana

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I disagree.
Stall speed determines how badly you will injured during a crash. The slower you arrive at the scene of the accident, the fewer injuries you will suffer.
I disagree with many other statements above that encourage you to plan you airplane, heavier, faster, etc. than the air regulations...
A smart designer tries to make his airplane lighter, land slower, climb faster, etc. than the minimums specified in regulations.
True to an extent, but at ultralight speeds that logic breaks down. The early ultralights were very light with very low stall speeds but it severely limited where and when they could fly, it's why Part 103 allows ULs to fly a half hour before sunrise and a half hour after sunset, when the winds are calm. Nowadays people want an ultralight that can fly in the breeze at noon, you need a higher stall speed to do that safely. Either way, you're limited to 24kts max under 103, which is still pretty slow.
 

Protech Racing

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If you fly at 4 # per ft , it will be very close to 103 legal and still dense enough to fly most days .
 

Protech Racing

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Most UL crashes are departure stall, flip and hit the ground inverted , followed by approach stall and fall. Actual stall speed and impact speed dont seem relevant. to me.
 
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