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 17 Mar, 17:30, Bob Cook wrote:

I don't like your use of the word power below however. *Force, energy and
power are not the same. *People often confuse them.

Mind you, it's amazing how many people think the power source for
soaring flight is gravity.

I don't. I used "power" because I meant "power"!

Ian

Darryl Ramm wrote:
On Mar 17, 8:53 am, The Real Doctor wrote:
On 17 Mar, 12:30, Bob Cook wrote:

OK, all this talk about the aerodynamics of towing brings me to something
about the aerodynamics of gliding!
The following question came up at a recent club meeting. I believe the
question was originally on the Bronze Badge Exam.
Q) What force provides the forward motion necessary to move a glider
through the air?
a) lift
b) centripetal force
c) gravity
I contend that it is a flawed question, and that none of the choices for
answers are (completly) correct!
It's a flawed question because, as Newton pointed out, NO force is
required for steady motion.

Mind you, it's amazing how many people think the power source for
soaring flight is gravity.

Ian

No force? Newton's first law tells you that for constant velocity the
forward component of lift must match the drag (or the glider would
accelerate or decelerate, and if so then the second law tells you how
much).

There is drag force so a forward force is required to counter that or
the glider would decelerate, and something has to overcome that force.
And that is the forward component of lift. Answer (a) is clearly the
best answer and this question as is I think is a suitable test of a
basic understanding of what is going on.

Gravitational potential energy gained by soaring (climbing in lift) or
by winching/towing/motorglider engine is the power source for gliding
flight. Thankfully we have several ways to charge that power source.

Darryl
Yebbut...imagine a glider magically inserted into earth's atmosphere
after global warming has removed all grabbity. What gets it moving?

Bob - going to gather my chicken's eggs, now - W.

All of the energy to maintain a glider in steady flight through still
air is derived from gravity - there is no other source of energy.

Potential energy of the glider is converted to kinetic energy in the
form of forward motion in order to overcome drag. In a 1000 lb
sailplane with a 40:1 glide, the wings will develop about 25 lbs of
forward thrust to overcome drag and about 1,000.3 lbs of total lift.
The glide angle is about 1.5 degrees.

I don't understand why people have a problem understanding this - it's
a very similar problem to a ball rolling down an inclined plane.

Just basic physics!

Mike

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

In message
, The
Real Doctor writes
On 17 Mar, 12:30, Bob Cook wrote:
OK, all this talk about the aerodynamics of towing brings me to something
about the aerodynamics of gliding!

The following question came up at a recent club meeting. *I believe the
question was originally on the Bronze Badge Exam.

Q) What force provides the forward motion necessary to move a glider
through the air?

a) lift
b) centripetal force
c) gravity

I contend that it is a flawed question, and that none of the choices for
answers are (completly) correct!

It's a flawed question because, as Newton pointed out, NO force is
required for steady motion.

Mind you, it's amazing how many people think the power source for
soaring flight is gravity.

Ian

If gravity isn't acting why does the glider need to produce lift with
it's wings?

--
Surfer!
Email to: ramwater at uk2 dot net

Indeed this has degraded into junior high physics and if I remember
most junior high students got poor grades in physics.... I think I'll
pull out the chickens...

If Bob's chicken is sliding down a hill.. what force causes it to
continue sliding?

It is gravity. There is no forward component of lift that contributes
to forward motion. By definition lift is perpendicular to the
direction of motion and drag is parallel. How can lift make anything
move forward if it acts perpendicular to the line of motion? In steady
state gliding flight, Net Lift is equal to Weight times the cosine of
the gliding angle. This value will "always" be less than weight.
Please don't try to sound smart and point out the obvious answer that
if the angle is zero, lift equals weight. True, but then it fails to
satisfy the condition: Steady state gliding flight.

As for the thread that this originally morphed into. Why does my
glider handle so poorly when being pulled slowly on tow? The answer
lies in the dynamics of the tow. It's not obvious when the towing
force is steady state. The clue is in how the glider behaves when the
tow force is changing because of gusts and the fact that you are
trying to maintain position behind the towplane during a climb. The
wing will see a greater variation in angle of attack as a function of
changes in rope tension at low airspeed than it does at higher speed.
Combining this effect with low dynamic pressure results in reduced
aileron effectiveness and therefore poor handling.

-Kevin

On Mar 17, 2:41*pm, KevinFinke wrote:
Indeed this has degraded into junior high physics and if I remember
most junior high students got poor grades in physics.... I think I'll
pull out the chickens...

If Bob's chicken is sliding down a hill.. what force causes it to
continue sliding?

It is gravity. There is no forward component of lift that contributes
to forward motion. By definition lift is perpendicular to the
direction of motion and drag is parallel. How can lift make anything
move forward if it acts perpendicular to the line of motion? In steady
state gliding flight, Net Lift is equal to Weight times the cosine of
the gliding angle. This value will "always" be less than weight.
Please don't try to sound smart and point out the obvious answer that
if the angle is zero, lift equals weight. True, but then it fails to
satisfy the condition: Steady state gliding flight.

As for the thread that this originally morphed into. Why does my
glider handle so poorly when being pulled slowly on tow? The answer
lies in the dynamics of the tow. It's not obvious when the towing
force is steady state. The clue is in how the glider behaves when the
tow force is changing because of gusts and the fact that you are
trying to maintain position behind the towplane during a climb. The
wing will see a greater variation in angle of attack as a function of
changes in rope tension at low airspeed than it does at higher speed.
Combining this effect with low dynamic pressure results in reduced
aileron effectiveness and therefore poor handling.

-Kevin

Sorry back to the hen house for you. Lift is *not* perpendicular to
the direction of motion. Lift is perpendicular to the airflow. That
small difference is going to get you in trouble in these arguments.

The chicken sliding down the hill is propelled forward by the
component of the force pushing up on it from the sloped surface in
response to gravity pushing it down onto that surface. Gravity
operates vertically down through the center of mass and without that
sloping surface the chicken would not move forward. There is a
component of this force vector pushing forward and the chicken moves
forward down the slope.

This is exactly analogous to the lift vector pointed forward on the
glider and the forward component of that vector providing the force to
overcome drag in forward flight. Gravity operates on the glider though
it's center of mass, without that lift vector pointed forward the
glider would not (continue to) move forward.

Gravity, without the thrust vector leaning forward, does not explain
how a glider glides through the air, however Gravity is what provides
the power/energy to do this (or store for that energy if you prefer).

The above discussion applies equally to chickens and African or
European swallows.

However I actually think you are onto a good line of reasoning about
the on-tow issue.

Darryl

Jim White had it right, way back several weeks ago. What makes it work is
MAGIC.



At 22:41 17 March 2009, Darryl Ramm wrote:
On Mar 17, 2:41=A0pm, KevinFinke wrote:
Indeed this has degraded into junior high physics and if I remember
most junior high students got poor grades in physics.... I think I'll
pull out the chickens...

If Bob's chicken is sliding down a hill.. what force causes it to
continue sliding?

It is gravity. There is no forward component of lift that contributes
to forward motion. By definition lift is perpendicular to the
direction of motion and drag is parallel. How can lift make anything
move forward if it acts perpendicular to the line of motion? In steady
state gliding flight, Net Lift is equal to Weight times the cosine of
the gliding angle. This value will "always" be less than weight.
Please don't try to sound smart and point out the obvious answer that
if the angle is zero, lift equals weight. True, but then it fails to
satisfy the condition: Steady state gliding flight.

As for the thread that this originally morphed into. Why does my
glider handle so poorly when being pulled slowly on tow? The answer
lies in the dynamics of the tow. It's not obvious when the towing
force is steady state. The clue is in how the glider behaves when the
tow force is changing because of gusts and the fact that you are
trying to maintain position behind the towplane during a climb. The
wing will see a greater variation in angle of attack as a function of
changes in rope tension at low airspeed than it does at higher speed.
Combining this effect with low dynamic pressure results in reduced
aileron effectiveness and therefore poor handling.

-Kevin

Sorry back to the hen house for you. Lift is *not* perpendicular to
the direction of motion. Lift is perpendicular to the airflow. That
small difference is going to get you in trouble in these arguments.

The chicken sliding down the hill is propelled forward by the
component of the force pushing up on it from the sloped surface in
response to gravity pushing it down onto that surface. Gravity
operates vertically down through the center of mass and without that
sloping surface the chicken would not move forward. There is a
component of this force vector pushing forward and the chicken moves
forward down the slope.

This is exactly analogous to the lift vector pointed forward on the
glider and the forward component of that vector providing the force to
overcome drag in forward flight. Gravity operates on the glider though
it's center of mass, without that lift vector pointed forward the
glider would not (continue to) move forward.

Gravity, without the thrust vector leaning forward, does not explain
how a glider glides through the air, however Gravity is what provides
the power/energy to do this (or store for that energy if you prefer).

The above discussion applies equally to chickens and African or
European swallows.

However I actually think you are onto a good line of reasoning about
the on-tow issue.

Darryl

At 18:16 17 March 2009, The Real Doctor wrote:

Ian,

YOU did use the term "power" correctly.

What I didn't like about it was that the question I posed referred to
force. (OK, so you gave us some additional information)

I agree that a sailplane, in gliding flight, in still air, has no
"power" at all. (Although, as you said, some wrongly believe that
gliders are "gravity powered")

By some of the responses, I think I am correct in assuming that some
confuse power, energy, and force.

So I again ask, (not to Ian, but to some of the others who answered my
question with "gravity",) how can gravity alone, a force which acts
vertically downward, impart forward motion to a glider or anything for
that matter?


Cookie



I don't. I used "power" because I meant "power"!

Ian