Assuming the plate is fixed and perpendicular to = the wind=20 what you would like to know is the force against it. Since force = =3D=20 Pressure/Area we first need to find the pressure.

The dynamic pressure energy of the airstream at = 200 mph is=20 found by 1/2pV^2 where p is air density and V is velocity. I am = going to=20 assume sea level air density even though we know it will be less at=20 altitude. So that is one square foot of air =3D 0.00234 slugs = (sorry I'm an=20 old English units guy). Velocity (ft/sec) =3D 200mph = *5280/3600 =3D=20 293.3 ft/sec

That gives us a dynamic Pressure =3D=20 1/2(0.00234*(293.3)^2 =3D 0.5*(0.00234)*(86034) =3D 100.66 lbs/ft^2 or = 100 lbs per=20 square foot. Dividing by 144 gives us lbs per square inch. = So=20 pressure =3D 100.66/144 =3D .699 lbs/sq inch or about 19 inches of = H20. =20 But, sticking with psi we have

That pressure is acting on 15 sq inches so the = total force=20 on that plate =3D 0.699*15 =3D 10.485 lbs of force.

That is assuming all the energy in the air is = converted to=20 dynamic pressure which would probably only happens at the very center of = the=20 plate, the air near the edges would tend to flow (at least = partially)=20 around the plate, so the velocity/pressure distribution would not = be even=20 across the plate. It would be maximum in the center and taper off = near the=20 edges. So this means the actually force would undoubtedly be = less=20 than 10 lbs because of that and the fact that the air at altitude will = be=20 considerably less dense, depending of course on your=20 altitude.

Then there is the fact the plate is moving = through=20 the air means that the pressure in back of the plate is going to be less = than if=20 it were standing still. But I am not = going to even=20 attempt to get into quantifying those factors. But, the = force=20 will still likely be less than 10 lb

You might try searching the web for things like = - air=20 resistance, plate plate, dynamic pressure, etc

Hope this helps

----- Original Message -----

From:=20 Steve = Thomas=20

To: Rotary motors in = aircraft

Sent: Thursday, October 18, = 2007 2:35=20 PM

Subject: [FlyRotary] Math = Help

Not being much of a math or engineering whiz, can = anyone help=20 me calculate the force the relative wind would put on a device that = hangs in=20 the breeze on my airplane? Assuming a 200 MPH airspeed, and a = flat plate=20 of 15 sq. inches, what would be the amount of force on the plate in=20 pounds?

Best Regards,

Steve

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