--- Bill Harris <[EMAIL PROTECTED]> wrote:
> how do you calculate the winch load from the speed
> of the
> plane?
>
> --Bill
>
The lift on a wing is:
L = (.00237/2) * CL * S * V^2
where .00237 = density of air
S = area in sq ft
V = velocity in ft/sec
CL = coefficient of lift, which during the launch is roughly 1.0
The tension in the line = the lift generated by the wings, so you can
solve for V, which is where I get the 125 mph for my 2-meter plane to
create 150 lbs of line tension. As Joe Wurts points out in a recent
post on launch tension, a wind gust or a strong thermal will add lift.
However, the Brits measuring launch tension on their F3B planes got
nothing over 40 kg, about 88 lbs. This jibes with my 72 mph conjecture
(for a 3M plane) based on the maximum speed at which my winch can pull
in line when unloaded.
Now 2 guys pulling a line in via a pulley certainly don't make the
plane fly faster than 72 mph. However, if the wind up there is 20 mph
and the plane is hanging there with no forward velocity, just kiting,
and the line is stretchy, and there's a thermal, and the flaps are down
allowing the CL to rise to, say, 1.2, and then they run like hell --
who knows? Due to the plane's inertia, the tension in the line can
spike to a level much greater than that sustainable by wing lift.
The line stretches in response to the increased tension. Then the pilot
resolves this unstable situation by pointing the nose straight down.
Let's say that there's 150 lbs of tension in the line, and that the
plane weighs 5 lbs. That's 30 g. So the plane accelerates downward with
an initial acceleration of 30 times the acceleration of gravity (!!!).
The acceleration declines linearly as the tension is taken up, until it
equals the acceleration of gravity. By that time the plane can be going
_fast_ (see http://www.mvsaclub.com/zoom.htm)
Now when you pull the nose up you can generate a lot of wing-busting
force. Joe's "layups.xls" spreadsheet allows you to design for maximum
load both due to line tension and to the high-speed pullout.
If the plane doesn't explode, you also generate a lot of altitude.
Dennis Phelan sez he has measured F3B launch altitude at about 1000
feet when everything goes just right.
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