Re: For the real engineers here

[Larry Dighera]

On Wed, 25 Jun 2008 10:56:07 -0700 (PDT), wrote in
>:

>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 550 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 passes 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:
http://online.unitconverterpro.com/

[rec.aviation.soaring added]

[Darryl Ramm]

On Jun 25, 11:27*am, Larry Dighera > wrote:
> On Wed, 25 Jun 2008 10:56:07 -0700 (PDT), wrote in
> >:
>
>
>
> >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 *550 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 passes 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:
> * *http://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)

[[email protected]]

Thanks. As it happens this is a unique high endurance low level and
slow application, and I want to be sure I haven't missed anything
fundamental. It appears I have not (so far), but we all know when a
project is 95% done the most difficult half is still to come.

..



On Jun 25, 2:55 pm, Darryl Ramm > wrote:
> On Jun 25, 11:27 am, Larry Dighera > wrote:
>
>
>
> > On Wed, 25 Jun 2008 10:56:07 -0700 (PDT), wrote in
> > >:
>
> > >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 550 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 passes 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:
> > http://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)

[Ralph Jones[_2_]]

On Wed, 25 Jun 2008 18:27:03 GMT, Larry Dighera >
wrote:
[snip]
>
>>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?)

Briggs & Stratton.

rj