With respect to the winch load values being quoted and using those values in
the previously-posted formula for wingrod sizing, I'm getting some large
wingrod sizes. For example, for a 2M wing, a 150 pound winch load requires
a 7/16" wingrod, and a 50 pound winch load needs a 5/16" wingrod. These are
hefty wingrods.
I'm sure that this formula is correct, but I suspect that the winch load
values tend to get overstated. 150-200 pounds is no doubt a value tha _can_
be reached with a strong winch and "full-pedal zoom launches". I also
suspect that a 50 pound winch load is still quite respectable and would be a
"strong" launch by a typical sport flyer, and even 30 pounds is a good
launch.
I've not measured these values, but plugging known wingrod sizes from planes
that have been flown successfully for years into the formula I've
back-calculated these 30-50 pound winch loads.
Has anyone actually instrumented a sailplane to measure the load aty the
towhook on launch? I'd be interested in seeing those values.
--Bill
>From: "Joe & Jan Wurts" <[EMAIL PROTECTED]>
>To: <[EMAIL PROTECTED]>, "Mark Drela" <[EMAIL PROTECTED]>
>Subject: Re: [RCSE] Joiner sizing
>Date: Thu, 19 Jul 2001 21:32:58 -0700
>
> >I seriously doubt you will see 200 lb pull on your winch.
>
>This number is probably initiated by me. We will occasionally
>(rarely) see this kind of load on an F3B winch. Much more frequent
>on two man F3J towing however. With a little bit of headwind,
>and some lift on tow, the line tension will grow extremely fast,
>frequently far faster than the pilot realizes. This is how most
>planes die on tow. I'd rate the 200 lbs case in the 99th percentile,
>assuming that you have your plane set up well for launch (most
>people have very conservative launch set-ups, which ends up
>precluding the high launch tensions).
>
>I end up designing my planes for the 200 lb load case, and in
>fact, design for deflection limits rather than plane failure. By my
>calculations, the Icon gets to the 200 lb case at a little bit over
>40 ksi compression max. on the upper spar. Using the 100 ksi
>number, this points to about a 500 lb line load failure case.
>Designing to failure at 200 lbs will get you an airplane that is
>pretty flexible on an energetic tow.
>
>All that said, for F3B, the dead air line tension gets to only 50-60
>lbs, and in mild headwinds, about 100 lbs. Still, one thermal on
>tow and the tension can quickly build towards the 200 lb number.
>Smaller airplanes don't have this issue nearly as much due to
>line drag losses reducing the power available.
>
>joe
>
>
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