On Jun 25, 11:27=A0am, Larry Dighera <LDigh...@[EMAIL PROTECTED]
> wrote:
> On Wed, 25 Jun 2008 10:56:07 -0700 (PDT), papp...@[EMAIL PROTECTED]
wrote in
> <f21210b7-96ff-44c6-9b4b-120e489e7...@[EMAIL PROTECTED]
>:
>
>
>
> >I'm thinking of a clean glider, one that might weigh 1500 pounds and
> >has a glide angle of say 1 in 25. At 50 miles an hour, that would mean
> >in an hour's time it might descend two miles (of course scale it
> >reasonable numbers, I chose those for ease of calculation). That means
> >it's losing about 1500 * 5280 * 2, or about 16 million foot pounds of
> >energy an hour.
>
> >Now if I add an engine swinging an 8 foot diameter prop, maybe as a
> >pusher, the question is, how big an engine for cruise only? A
> >horsepower is =A0550 foot lbs a second, or about 2 million foot pounds
> >an hour. If all of that is correct, it suggests with a 50% efficient
> >prop a little 16 horsepower engine could pretty much keep this thing
> >at constant altitude.
>
> >It p***** the reasonableness test as far as I can see. Any serious
> >disagreements?
>
> It looks reasonable to me, but I'm not qualified to judge.
>
> >For those of you who do things in metric units? I went to school a
> >long long time ago, and here in the US I can buy a little Briggs and
> >Stanton (spelling?) engine with a horsepower rating, not a kilowatt
> >one.
>
> Here's a solution for SI conversions:
> =A0 =A0http://online.unitconverterpro.com/
>
> [rec.aviation.soaring added]
What is the question? Sustainer gliders exist and are available from
most (all?) glider manufacturers. You need to factor increased drag of
the engine mast and maybe other things if a retractable mast, but 25:1
is far from state of the art today. You need to factor engine
efficiency at high density altitudes (most sustainer engines are very
simple and do not have altitude/mixture compensation so this can be a
significant issue) and some ability to climb a little would be nice.
Take a current state of the art sustainer like the ASG-29E for
example, uses a SOLO 2350 engine, 18 hp/13.5 kW. Nominal best L/D
(with engine retracted) is 52:1 with 18m wings.
Practical consideration with modern sailplane design will usual
preclude propellers as large as 8' diameter.
Darryl
(ASH-26E driver)
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