I know I might get rediculous drag:

the induced drag C_Di = C_L^2/(Pi*AR)

On Sun, 23 May 2004 10:51:27 -0700, wrote:


Kevin,

There must be some typo in your formular. I looked up my book (Design
for Fly) and is something like:

V=16.2*sqrt(WL/Cl)

The book also states that the Cl for a Fowler flap could be 2.8
compared with 1.4 of no flap. So if I take your number of +1.0 for the
LES(leading edge slats), I think a Cl of 3.0 seems reasonable with
some flap.

Also remember my gross is reduced by almost 20% (not considering the
increase due to the additional slats because I don't how much more
weight) and the increase of the wing area by about 10%. For stock
C150, the wing loading is almost exactly 10 and V stall clean is
47kts. This will give a Cl of only about 1.2. Oh well. Anyway if we
use Cl=3 and wing loading of 7 let's see what we get:

V=16.2*sqrt(7/3)= 24.7 kts

Wow. Geez! I'm sure at 30mph the formula might break (or Cl will
change) but anyway, I still think 40mph is not that unreachable.

Jizhong

On Sun, 23 May 2004 16:58:25 GMT, Kevin Horton wrote:

On Sat, 22 May 2004 21:04:20 -0700, jizhonghe wrote:

Great. Thanks for the clarificaiton. I always thought they just give you a
low number. Now I know at least they are honest albeit misleading. So I
looked up the C152 POH and the CAS stall clean is actually 47kts (54mph).
I should re-adjust my goal. I think a CAS of 40mph should be quite
respectable for a little bush plane. And the square of (54/40)^2=1.83 with
the light weight should be able to reduce the T/O run by half.


The stall speed basically depends on the wing loading and the maximum
coefficient of lift that is achieved.

The stall speed in equivalent airspeed (which can be considered to be the
same as calibrated airspeed for low speed and low altitude) is:

VS = 0.8379 * sqrt(wing loading/CLmax)

VS is in knots,
Wing loading is in lb/sq. ft

I don't have data for the C150 at hand, but looking at the C182Q POH for
an example, I get wing loading of 2950/174 = 16.95 lb/sq ft. The forward
CG stall speed at 2950 lb is 54 kt CAS. This requires a CLmax of 1.72,
which is about what I would expect for a flapped wing with no leading edge
devices.

Several references indicate a well designed slat might give about an extra
1.0 CLmax, so you might be able to get the C182 CLmax to about 2.7. This
would give a stall speed of about 43 kt CAS, or about 80% of the original
value. You think you can get the C150 stall speed from 54 mph to 40 mph,
which would be a reduction to 74% of the original value. This seems
unlikely from just adding slats. You would need to also make a big
improvement to the flaps. These mods add weight, and they would require a
lot of knowledge of aerodynamics and structural engineering to actually
achieve the predicted performance, and to have a strong structure.

References:

Fluid Dynamic Lift, Hoerner
Theory of Wing Sections, Abbott and Doenhoff

Good luck.