Ah! Now this puts it all into perspective. And I'd not considered the
wingloading that results from the acceleration and pullout in the zoom phase
of the launch, and that is considerable.
--Bill
>From: tony estep <[EMAIL PROTECTED]>
>To: Bill Harris <[EMAIL PROTECTED]>, [EMAIL PROTECTED]
>Subject: Re: [RCSE] Joiner sizing
>Date: Fri, 20 Jul 2001 10:09:16 -0700 (PDT)
>
>--- 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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