wrote:
Could someone clarify something for me concerning LSA's. The websites
that have the detailed LSA aircraft limitations listed say that the
plane must have a maximum stalling speed of 51 mph at the maximum gross
takeoff weight WITHOUT the use of high lift devices.

I plugged the numbers for a Sonex into the John Roncz spreadsheets. (
Max Gross TOW of 1150 lbs, stall of 46 mph ) and it reports that I need
a wing area of 180 sq. feet. The Sonex only has 98 square feet of
wing. What am I missing?

Thanks
Neal


You are not missing anything Neal. The Sonex stall numbers are plainly
bogus. I will venture that Monnet is using indicated stall speed, not
true. That way he can say anything he wants because who knows what the
installation errors are. The calculated stall for the Sonex is 51-52
mph clean at 1100lbs, using the formula that is the basis for
certificated aircraft and assuming a Cl max of 1.6-1.7 for the NACA
64-415 airfoil.

The proof of the pudding? Monnet has contradictory information right on
his site. Go to the Xenos part of the site and look at the claimed
stall, which in this case DOES agree with the standard calculation.
With 158 sqf of wing area it is 44mph, which is right on the money.
Whereas the Sonex stalls clean ONLY 2 MPH FASTER with 60 sqf LESS wing
area! How can this be??? It's magic! Nah, he just fudged the numbers.
Airfoil differences do not account for this as the variation in Clmax
values of a point up or down have surprisingly small effects, maybe 1
mph for each point above or below 1.6.

For when you don't have software available, just a calculator, memorize
the simple formula in your head and apply it to any aircraft's specs you
come across to assess their accuracy:

Sea Level Stall in Kts = Sq root of: [(295 x Gr Wt) Divided by (Clmax x
Wing Area)]

Multiply by 1.15 for mph.

Assume 1.6 for the Clmax. Almost all airfoils are between 1.5 and 1.7,
which gives a 1 mph or so variation up or down relative to 1.6.

That is not to say that the Sonex is not a very clever design.

John