On Thu, 27 Nov 2003 at 15:34:14 in message
.net, cddb
wrote:

So somehow, the magic equations seem to explain my confusion...but I
am not quite at peace. Au contraire. I've only talked about L/D,
weight, and thrust. What about drag? I am still claiming that you'll
experience an increase in drag with a weight increase REGARDLESS of
whether or not you have thrust. Thus, due to the increase in drag and
REGARDLESS of thrust or not, your angle THETA will be affected (i.e.
theta will get larger, more negative, airplane will pitch DOWN). In
the case of gliding flight, everybody but me agrees that your glide
angle won't be affected. Obviously, I am wrong but I don't know why.
To reiterate, that's why I've been writing all this stuff, 'cause I
don't get why THETA happens to change only when you have thrust but
doesn't when you don't have any and are gliding.

By rearranging the simple equations I may be able to help you with that
one.

W= weight, L = Lift, D = Drag, t = Thrust, theta = angle of climb (if
negative implies descent, V = forward speed

You started with L = W*cos (theta)
and T
= D + W*sin(theta)

For a change eliminate W from the above
by taking W = L/(cos(theta)

Then T = D + L*tan(theta)

Let K = 0.5*density* wing area

Then T = K*V^2*Cd + K*V^2*Cl Divide both sides by K*V^2

T/(K*V^2) = (Cd + Cl*tan(theta))..........................[1]

Now if T= 0 the left hand side must be 0 and it follows that

Cd = - Cl*tan(theta) this means that theta must be negative and

tan(theta) = -Cd/Cl as one would expect.

The reverse of this is that positive or negative thrust is always needed
_except_ when Tan(theta) = -Cd/Cl

At any other climb angle, thrust or extra drag would be needed for
balance. To glide more steeply at the same Cl and Cd negative thrust
would be needed

Perhaps it does not help after all?

Also in [1] if theta = 0 then

T = K*V^2*Cd which is a necessary condition for steady level flight..




--
David CL Francis