For the real engineers here

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]

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)

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)

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