Horsepower required for level flight question...

Hi all...

Was reading about a glider the other day....

It has something like a sink rate of 125 feet per minute at a 400 pound total
wieght when its going about 35 mph...

Now given that 1foot pound/sec = .00182 HP and the glider is being powered by
gravity....

125 * 400 / 60 = 833

833 * 0.00182 = 1.5

So, are those calcs right and it would only take 1.5 horsepower to keep that
glider going in level flight at 35 mph?

If thats the case..it makes me wanna get a glider, a weed wacker with a prop on
it and do some CHEAP, long, slow flights

take care

Blll

Good observation.

Take my Nimbus 2C glider for example. The flying weight, less water
ballast, is about 1000 pounds. The Lift over Drag is 47:1 at 51 knots. (At
1440 pounds with water ballast, the L/D rises to 49:1 at 59 knots)

Divide 1000 by 47 and get 21.3 pounds of thrust needed for 51 Knot level
flight. Your weedwhacker engine would easily do that.

Gliders with small "sustainer" engines are widely available from Europe.
They call them "turbo" gliders as opposed to the "self launcher" gliders
with bigger engines. The engine can be retracted into the fuselage behind
the wing. If needed, the engine can be extended and air started with just
airflow through the tiny prop.

However, with 49:1 glide ratio, if well flown, you won't NEED the little
engine.

Bill Daniels


"BllFs6" wrote in message
...
Hi all...

Was reading about a glider the other day....

It has something like a sink rate of 125 feet per minute at a 400 pound
total
wieght when its going about 35 mph...

Now given that 1foot pound/sec = .00182 HP and the glider is being powered
by
gravity....

125 * 400 / 60 = 833

833 * 0.00182 = 1.5

So, are those calcs right and it would only take 1.5 horsepower to keep
that
glider going in level flight at 35 mph?

If thats the case..it makes me wanna get a glider, a weed wacker with a
prop on
it and do some CHEAP, long, slow flights

take care

Blll

So, are those calcs right and it would only take 1.5 horsepower to keep that
glider going in level flight at 35 mph?


-------------------------------------------------------

Sounds about right but... (you KNEW there was a but, right?) :-)

You want thrust, not horsepower. Reality tends to differ from the
conversion-factor solutions due to velocity and prop losses. (Remember the NSU
'Prinz'? Ever see the little ducted-fan power pod that used the tiny Wankle?
Something like 6hp and TINY. With even a whiff of green air it could keep you
up all day after a winch launch.)

And of course the weght is no longer 400 but something more (ie, engine,
controls, fuel, mounting, etc).

But you're on the right track: Lift vs Weight, Thrust vs Drag (Notice that
horsepower is not mentioned :-) Once airborne, it doesn't take much to keep a
high-lift, light weight, aerodynamically efficient airframe flying. The tricky
bit is getting airborne to begin with :-)

(Then comes those horrible inverse equations dealing with drag (square it) and
power (cube it) any time you want to get there in a hurry.)

----------------------------------------

You did good, Bill. Your logic mirrors that of the Wright brothers which puts
you head & shoulders above 99% of the crowd.

-R.S.Hoover

You did good, Bill. Your logic mirrors that of the Wright brothers which
puts
you head & shoulders above 99% of the crowd.

-R.S.Hoover

Well...

I think you just insulted the Wright brothers

take care

Blll

Hi Guys

Thanks for the replies Bill and Vee Duber!

Given that gliders are sooo darn efficient fuel wise and that it would take a
pretty minimal engine for one to self launch (20hp is my WAG for a single
seater).....why do we not see more airplanes that a cross between a glider and
a conventional plane?

Glider pilots hate engines? Powered pilots want barrel rolls and roaring
engines? Never the twain shall meet?

Seems to me if cost and the amount of time airborne are your primary desire
drivers then a "overpowered" glider would be the ticket...

Am I right in the main disadvantage of a powered glider is long take off rolls
and lack of fancy aerobatics? Other considerations I am missing?

And another question just popped into my head...anyone ever seen a canard
glider?

take care

Blll

Long wing spans are harder to land in a cross wind.

Larry

"Boelkowj" wrote in message
...
Long wing spans are harder to land in a cross wind.

Larry

Oh, I dunno, I have landed a glider in 20+ knots x-wind. My old Archer only
had a 17 knot demonstrated x-wind. Then again, with a 35 knot touchdown
speed less 20 knot headwind, just ignore the runway and land into the
wind. - only takes about 50 feet.

Bill Daniels

"BllFs6" wrote in message
...
Hi Guys

Thanks for the replies Bill and Vee Duber!

Given that gliders are sooo darn efficient fuel wise and that it would
take a
pretty minimal engine for one to self launch (20hp is my WAG for a single
seater).....

More like 50 - 60 HP to get a decent ROC.

why do we not see more airplanes that a cross between a glider and
a conventional plane?

We do, they are called "Touring Motorgliders" and are widely available.
See Grob 109 or the Super Ximango. They use Rotax 912's.


Glider pilots hate engines? Powered pilots want barrel rolls and roaring
engines? Never the twain shall meet?

Aerobatic gliders are very common. Engines make aerobatics less capable
because of the weight and reduced load factors. Silent aerobatics in a
glider are a wonder to behold.

OTOH, Bob Carlson of Albuquerque has an ultralight sailplane with two model
airplane turbojets on it - goes like a scalded cat. We call it the "Dog
Whistle" behind his back. Bob has an airshow act doing aerobatics in it.

Self launch gliders are getting more popular all the time. In Europe, over
half the new gliders sold have self launch capability.


Seems to me if cost and the amount of time airborne are your primary
desire
drivers then a "overpowered" glider would be the ticket...

If you want air time and low cost, go for a pure sailplane. Most of my
buddies are out on 300 mile, 6 hour flights every weekend. 6 hours from a
$20 tow is less than $4 an hour. Pure gliders stay in the air so long they
have to be plumbed for relief tubes.

BTW, I always thought of an X-15 or the Space Shuttle as an overpowered
glider.....

Am I right in the main disadvantage of a powered glider is long take off
rolls
and lack of fancy aerobatics? Other considerations I am missing?

The disadvantage is cost, maintenance and complexity. The number of bells
and whistles in a motorglider darn near equals a twin.


And another question just popped into my head...anyone ever seen a canard
glider?

Yup, Burt Rutan's "Solitare". Probably his worst design which is why you
haven't heard of it. Canards are very inefficient at low speeds and high
AOA - they make lousy gliders.


Bill Daniels

BllFs6 wrote:

Am I right in the main disadvantage of a powered glider is long take off rolls
and lack of fancy aerobatics? Other considerations I am missing?

4 to 6 seat capacity comes to my mind (needs/wants)...