While I appreciate the fact that you might be able to design a legal ultralight from scratch , i.e. 253.9999 lbs. empty weight, with a 3.9 lb. / sq. ft. wing loading by using high lift devices to meet the stall speed requirement as defined in Appendix 2 of AC 103.7, but that was not the intent of my post. There are many designs that are sold as "legal" ultralights that clearly don't meet the criteria because they don't have any semblance of a flap or other high lift device and vortex generators are not taken into account in Appendix 2.
For instance the new Belite line has a wing area of about 100 sq. ft. (I am using Belite for demonstration purposes, there are many other designs such as the Max 103 by JDT, and the Legal Eagle ultralights that have the same problem.) At least the Belite is one of the few designs with flaperons, so you could actually use the 2.0 curve, but with a wing loading of 4.5 lbs. / sq. ft., using the Appendix 2 formula, there is no way that it will meet the 24 knot criteria of the table. As the Belite web site says, "FAR 103 COMPLIANCE IS ALWAYS
THE RESPONSIBILITY OF THE OPERATOR, NOT BELITE AIRCRAFT." so if the FAA ever questioned you, you would have to do the technical standards review.
It just seems ludicrous to have "Regulations" that are not realistic in light of the commercial enterprises that are operating under them today. It is clear that FAR 103 was intended for aircraft with wing loadings of less than 3 lbs. / sq. ft. operating with low power 2 cycle engines. But today the average commercial ultralight airplane has a wing loading in the 4+ lbs. / sq. ft. range and 2 cycle engines are disappearing because of pollution standards, so people are turning to 4 cycle engines. On top of that the speed limit is unenforceable even if you could measure it accurately.
I am troubled by commercial operations that appear unable to meed the letter, let alone the spirit, of the FARs. They design ultralight aircraft that would have to go through a Technical Standards Committee review process to demonstrate that their designs can meet the 24 knot stall speed, but they leave it up to the buyer to assume that liability. They even advertise versions with 50 HP engines that are "limited" to a 38 HP "ground limit", whatever that means, to assure that the aircraft does not exceed the 62 MPH ultralight airspeed limit. If you are going to all of the trouble and expense to put a 50 HP engine in your aircraft, I assume you intend to use it because it certainly weighs more than a comparable 35 HP model. How does that satisfy the spirit of FAR 103?