For the real engineers here

Sure bertie, cuz you know everyone, and you know everything.

You're damn near as smart as LeChaud.

wrote in news:f21210b7-96ff-44c6-9b4b-120e489e7682@
59g2000hsb.googlegroups.com:

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?

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.

http://www.aircraft-spruce.com/da11.html

Even better


Lots of motorgilders have been built with some truly dinky engines and
flown quite well, not to mention the Columban Cri cri...


Bertie

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)

Darryl Ramm wrote:


Practical consideration with modern sailplane design will usual
preclude propellers as large as 8' diameter.

Darryl
(ASH-26E driver)

I took a ride in a Stemme motorglider several years back (50:1, IIRC). One
of it's coolest features, besides going high and fast, was that the propellor
would fold up and tuck into the nose cone when not in use.

John Galban=====N4BQ (PA28-180)

--
Message posted via http://www.aviationkb.com

I saw a clever motorglider a few years ago that had an engine on a
retractable mast - above and behind the pilot (it was a one-place).
The unique feature was that it had a one-blade propeller (there was a
counterweight on the other side), and some means of positioning the
prop after shutting down (so that the engine and prop could fold down
completely inside the fuselage). It was more than a sustainer engine-
I watched the aircraft take off and climb out unaided. Don't know the
type, but have several photos of it somewhere.

Dave

wrote:

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?

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.

well, seems to be correct. still, let me add some annotations:

- i'd calculate directly using power instead of energy. the installed power
you need is simply weight*sink speed/efficiency; in a formula:
P = W*w/eta = m*g*v/(E*eta) with the glide ratio E = Lift/Drag, m the mass
and g the gravitational acceleration

- i prefer SI units, for the simple benefit tp be able to calculate without
conversion factors. this eliminates a quite likely source of mistakes (ask
NASA...). a few years ago, while working in the US, i failed to calculate
the mass of a simple sheet of aluminum (don't laugh!); i had several
numbers for the material's density, but none in the combination of units
for volume and mass that i needed; so i decided it was safer to go via SI
and convert the mass back to ounces...

- the conversion hp-kW is simple: 1 kW = 1.34 hp (= 1.36 german PS) or
roughly 4/3 hp


cheers
uli

On Jun 26, 4:09 am, Uli wrote:
wrote:
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?

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.

well, seems to be correct. still, let me add some annotations:

- i'd calculate directly using power instead of energy. the installed power
you need is simply weight*sink speed/efficiency; in a formula:
P = W*w/eta = m*g*v/(E*eta) with the glide ratio E = Lift/Drag, m the mass
and g the gravitational acceleration

- i prefer SI units, for the simple benefit tp be able to calculate without
conversion factors. this eliminates a quite likely source of mistakes (ask
NASA...). a few years ago, while working in the US, i failed to calculate
the mass of a simple sheet of aluminum (don't laugh!); i had several
numbers for the material's density, but none in the combination of units
for volume and mass that i needed; so i decided it was safer to go via SI
and convert the mass back to ounces...

- the conversion hp-kW is simple: 1 kW = 1.34 hp (= 1.36 german PS) or
roughly 4/3 hp

cheers
uli
I wouldn't argue with you about using SI units in professional
communications -- I do that all of the time -- but in this case I
started out with English units and it was easy to stay within them.
Also, and importantly, the question I asked was more easily understood
by most pilots here, and the more useful answers came back in the same
units. First rule of communication -- speak the language the spoken to
are most likely to understand!

It would have been fun to give the airspeed in furlongs per fortnight,
or for the spectroscopically inclined, nm/sec.

I do appreciate your comments, thanks. Now let's give the thread back
to the little boys with their spray cans.

On Jun 26, 4:09 am, Uli wrote:
wrote:
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?

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.

well, seems to be correct. still, let me add some annotations:

- i'd calculate directly using power instead of energy. the installed power
you need is simply weight*sink speed/efficiency; in a formula:
P = W*w/eta = m*g*v/(E*eta) with the glide ratio E = Lift/Drag, m the mass
and g the gravitational acceleration

- i prefer SI units, for the simple benefit tp be able to calculate without
conversion factors. this eliminates a quite likely source of mistakes (ask
NASA...). a few years ago, while working in the US, i failed to calculate
the mass of a simple sheet of aluminum (don't laugh!); i had several
numbers for the material's density, but none in the combination of units
for volume and mass that i needed; so i decided it was safer to go via SI
and convert the mass back to ounces...

- the conversion hp-kW is simple: 1 kW = 1.34 hp (= 1.36 german PS) or
roughly 4/3 hp

cheers
uli
I wouldn't argue with you about using SI units in professional
communications -- I do that all of the time -- but in this case I
started out with English units and it was easy to stay within them.
Also, and importantly, the question I asked was more easily understood
by most pilots here, and the more useful answers came back in the same
units. First rule of communication -- speak the language the spoken to
are most likely to understand!

It would have been fun to give the airspeed in furlongs per fortnight,
or for the spectroscopically inclined, nm/sec.

I do appreciate your comments, thanks. Now let's give the thread back
to the little boys with their spray cans.