motorgliders as towplanes

On 17 Mar, 16:16, Darryl Ramm wrote:

Gravitational potential energy gained by soaring (climbing in lift) or
by winching/towing/motorglider engine is the power source for gliding
flight.

So what's the power source when climbing in a thermal, when
gravitational potential enegry is decreasing?

If you take a 500m launch, you have 4,905kJ/kgof potential energy to
play with - no more and no less. As we all know, that's about enough
energy to keep a glider flying for ten minutes or so. For flights of
any longer, another power source is needed, and though gravitational
potential may be used as a store from time to time, it is only the
store, not the source.

Ian

On Mar 17, 11:13*am, The Real Doctor
wrote:
On 17 Mar, 16:16, Darryl Ramm wrote:

Gravitational potential energy gained by soaring (climbing in lift) or
by winching/towing/motorglider engine is the power source for gliding
flight.

So what's the power source when climbing in a thermal, when
gravitational potential enegry is decreasing?

If you take a 500m launch, you have 4,905kJ/kgof potential energy to
play with - no more and no less. As we all know, that's about enough
energy to keep a glider flying for ten minutes or so. For flights of
any longer, another power source is needed, and though gravitational
potential may be used as a store from time to time, it is only the
store, not the source.

Ian

Say what? Gravitational potential energy *increases* not decreases as
the glider climbs.

And that "store" is the transfer mechanism that allows the glider to
glide. There is no other mechanism. With no gravity, even with an
atmosphere (which would be difficult to arrange), your glider could
not glide. Even if you could arrange to launch the glider with some
airspeed drag would eventually slow it down to a dead stop. Any air
currents would move the glider around but you would not be able to
soar/glider based on those. (OK some forms of dynamic soaring might be
possible).

When you are climbing in a thermal the source of power (or increase in
energy, if you prefer but power is a perfectly correct term) is the
force of the raising airmass lifting the glider against gravitational
pull. The energy gained is just the difference in gravitational
potential energy between the different altitudes, the average power
spent doing this is that difference in potential energy divided by the
time to climb. Yes the force to deliver that energy/power came from
somewhere i.e. the glider had a tiny but real effect on the raising
air mass.

Want some ball park numbers? A 400kg (880 pounds) glider climbing in
a strong thermal at 5 meters per second (~ 10 knots). Is gaining
energy at a rate = g * M * v ~ 9.8 m/s2 * 400 kg * 5 m/s = 19 kW or 26
horsepower.

Guy's this has wandered into junior-high school level physics. Time to
either let Bob's chickens' contribute to this thread or let it die.

Darryl

On 17 Mar, 19:04, Darryl Ramm wrote:

Say what? Gravitational potential energy *increases* not decreases as
the glider climbs.

Sorry, typo.

And that "store" is the transfer mechanism that allows the glider to
glide.

No it's not. The gravity force is necessary, but it's perfectly
possible to soar for extended periods and distances without adding to
or drawing from the potential energy store. It happens any time we fly
level - along a wavebar, running a ridge, following a cloud street.

There is no other mechanism. With no gravity, even with an
atmosphere (which would be difficult to arrange), your glider could
not glide.

You miss, as so many people do, the point. Because the force of
gravity is necessary for a glider to work, people assume that gravity
somehow "powers" the flight. Which it does not.

Guy's this has wandered into junior-high school level physics.

No, it has wandered into junior high school level misconceptions about
physics!

Here's another one for you. Does a glider turn (normally) by (a)
rolling (b) pitching (c) yawing or (d) other?


Ian

A glider in steady free flight is always descending through the air in
which it is flying. If you can fly in air that is going up as fast or
faster than the sink rate of the glider, then you can gain potential
height energy. Isn't that what soaring flight is all about?

Derek Copeland
(Qualified gliding instructor with Gold C and 3 Diamonds BTW)

At 22:07 18 March 2009, The Real Doctor wrote:
On 17 Mar, 19:04, Darryl Ramm wrote:

Say what? Gravitational potential energy *increases* not decreases as
the glider climbs.

Sorry, typo.

And that "store" is the transfer mechanism that allows the glider to
glide.

No it's not. The gravity force is necessary, but it's perfectly
possible to soar for extended periods and distances without adding to
or drawing from the potential energy store. It happens any time we fly
level - along a wavebar, running a ridge, following a cloud street.

There is no other mechanism. With no gravity, even with an
atmosphere (which would be difficult to arrange), your glider could
not glide.

You miss, as so many people do, the point. Because the force of
gravity is necessary for a glider to work, people assume that gravity
somehow "powers" the flight. Which it does not.

Guy's this has wandered into junior-high school level physics.

No, it has wandered into junior high school level misconceptions about
physics!

Here's another one for you. Does a glider turn (normally) by (a)
rolling (b) pitching (c) yawing or (d) other?


Ian

Having read all 170 posts in this thread I am still happily ignorant of the
theory of flight!

Jim

On 19 Mar, 07:45, Jim White wrote:
Having read all 170 posts in this thread I am still happily ignorant of the
theory of flight!

Aircraft stay up because they're afraid of the ground.

That said, you might find it interesting to get hold of the TV
programme and video which the Open University made on gliding
mathematics some years ago. The course in question, "MST207 Applied
Mathematics" is defunct, but a fair number of copies of the AV
material lurk around gliding clubs.

Ian

Ok I'll bite.....

Turning flight is accelerated flight, so the forces acting on the glider
are "unbalanced".

Since I am a firm believer that you can't change gravity (although some
of you out there try to in your analysis), gravity remains constant. If
we keep airpseed constant then drag should not change either. So we have
to change lift in order to get acceleration.

So I say we have to change the direction of lift, inward toward the center
of the circle. (centripetal force). This should be accomplished by
rolling.

Since we have added a "Horizontal component of lift", total lift must be
increased. This normally requires additional nose up force on stick or
trim. (angle of attack) You might call this pitch, but I don't think
the actual pitch attitude changes, just the stick forces and angle of
attack.

Yawing can be conidered to be necessary, or at least correction for
adverse yaw.

But to rephrase the question,

Q) What force causes a glider to turn.
A) Lift

Cookie

Here's another one for you. Does a glider turn (normally) by (a)rolling
(b) pitching (c) yawing or (d) other?


Ian

At 12:30 19 March 2009, Bob Cook wrote:

Clearly not a geologist then. I wonder why they do gravity surveys if it
is a constant?

Ok I'll bite.....

Since I am a firm believer that you can't change gravity (although some
of you out there try to in your analysis), gravity remains constant.

Further to earlier post. Gravity is measured in Gals (in honour of
Galileo), 1 Gal = an acceleration of 1 cm/sec2. Average gravity at the
surface is 980.654321 Gals (or so). At the poles it is more at about 983
Gals and on equatorial mountain tops only 977 Gals - a difference of about
0.6%. There are significant local variations. Now if we start to take
height into account...

By the way, what happened to the discussion about slow tows?


At 14:15 19 March 2009, Big Wings wrote:
At 12:30 19 March 2009, Bob Cook wrote:

Clearly not a geologist then. I wonder why they do gravity surveys if
it
is a constant?

Ok I'll bite.....

Since I am a firm believer that you can't change gravity (although
some
of you out there try to in your analysis), gravity remains constant.

Dear Big,

You are on to something here. Get one of those geo surveys, and fly your
glider only over areas of higher gravity. This extra gravity will give
your glider extra power, and you will be the best glider pilot!

Cookie

At 14:15 19 March 2009, Big Wings wrote:
At 12:30 19 March 2009, Bob Cook wrote:

Clearly not a geologist then. I wonder why they do gravity surveys if
it
is a constant?

Ok I'll bite.....

Since I am a firm believer that you can't change gravity (although
some
of you out there try to in your analysis), gravity remains constant.