motorgliders as towplanes

On 18 Mar, 01:38, Darryl Ramm wrote:

As I explained, Gravity provides the energy...

Then you will need to explain how gravity provides the energy when the
glider is climbing.

Ian

The Real Doctor wrote:
On 18 Mar, 01:38, Darryl Ramm wrote:

As I explained, Gravity provides the energy...

Then you will need to explain how gravity provides the energy when the
glider is climbing.

Just curious, but are you being pedantic?

On Mar 18, 3:34*pm, Jim Logajan wrote:
The Real Doctor wrote:

On 18 Mar, 01:38, Darryl Ramm wrote:

As I explained, Gravity provides the energy...

Then you will need to explain how gravity provides the energy when the
glider is climbing.

Just curious, but are you being pedantic?

Just an extreme case of rasterbation. If he keeps it up he will go
blind.

Darryl.

On 18 Mar, 22:59, Darryl Ramm wrote:
On Mar 18, 3:34*pm, Jim Logajan wrote:

The Real Doctor wrote:

On 18 Mar, 01:38, Darryl Ramm wrote:

As I explained, Gravity provides the energy...

Then you will need to explain how gravity provides the energy when the
glider is climbing.

Just curious, but are you being pedantic?

Just an extreme case of rasterbation. If he keeps it up he will go
blind.

Go on then. Explain how "gravity provides the energy" when a glider is
climbing ...

Ian

On Mar 19, 1:18*am, The Real Doctor wrote:
On 18 Mar, 22:59, Darryl Ramm wrote:

On Mar 18, 3:34*pm, Jim Logajan wrote:

The Real Doctor wrote:

On 18 Mar, 01:38, Darryl Ramm wrote:

As I explained, Gravity provides the energy...

Then you will need to explain how gravity provides the energy when the
glider is climbing.

Just curious, but are you being pedantic?

Just an extreme case of rasterbation. If he keeps it up he will go
blind.

Go on then. Explain how "gravity provides the energy" when a glider is
climbing ...

Ian


A glider "climbs" when you pull back on the stick and converts kinetic
energy to gravitational potential energy, but that does not get you
far since you can't create within that closed system. The glider also
"climbs" - (maybe you should think of "lifted" if "climb" confuses
you) by a rising air mass and that does increase the glider's
gravitational potential energy. You can then utilize that energy to go
places. There is no other coupling between raising air and the glider
somehow magically using that to get energy go places. Are you confused
by the case of flying in zero sink? That's no different the raising
air just happens to match the sink rate, gravity is still required/is
the coupling mechanism. And a glider while being lifted in a thermal
or wave etc. is still expending gravitational potential energy to
maintain forward flight, it's just being lifted faster than it
descends.

Replace drag with the effort of running, and potential energy with
kinetic (but it lets me invoke chickens again).... If a chicken runs
backwards in a stationary train it is expending a certain amount of
energy (glider sinking in still air). As the train picks up speed and
exceeds the chicken's speed the net speed of the chicken moves forward
(glider is now being lifted in lift), and the chicken gains an
increase in net energy however the chicken is still expending the same
energy to walk to the back of the train (the glider is still using
gravitational potential energy to fly). Don't like that, think of a
ball rolling down an infinitely long inclined ramp and the ramp being
raised faster than the ball falls. What way does the ball move? to an
observer on the ground? Is the ball giving up gravitational potential
energy to slide down the ramp relative to an observer on the ramp?
(yes). Is the ball gaining net energy? (yes).


So how many ways do people need to keep answering the same pedantic
question you keep asking? Do the ultimate thought experiment, turn off
gravity and the glider will just float along with moving air currents
but will be unable to glide anywhere.


Darryl

On Mar 19, 9:39*am, Darryl Ramm wrote:
On Mar 19, 1:18*am, The Real Doctor wrote:



On 18 Mar, 22:59, Darryl Ramm wrote:

On Mar 18, 3:34*pm, Jim Logajan wrote:

The Real Doctor wrote:

On 18 Mar, 01:38, Darryl Ramm wrote:

As I explained, Gravity provides the energy...

Then you will need to explain how gravity provides the energy when the
glider is climbing.

Just curious, but are you being pedantic?

Just an extreme case of rasterbation. If he keeps it up he will go
blind.

Go on then. Explain how "gravity provides the energy" when a glider is
climbing ...

Ian

A glider "climbs" when you pull back on the stick and converts kinetic
energy to gravitational potential energy, but that does not get you
far since you can't create within that closed system. The glider also
"climbs" - (maybe you should think of "lifted" if "climb" confuses
you) by a rising air mass and that does increase the glider's
gravitational potential energy. You can then utilize that energy to go
places. There is no other coupling between raising air and the glider
somehow magically using that to get energy go places. Are you confused
by the case of flying in zero sink? That's no different the raising
air just happens to match the sink rate, gravity is still required/is
the coupling mechanism. And a glider while being lifted in a thermal
or wave etc. is still expending gravitational potential energy to
maintain forward flight, it's just being lifted faster than it
descends.

Replace drag with the effort of running, and potential energy with
kinetic (but it lets me invoke chickens again).... If a chicken runs
backwards in a stationary train it is expending a certain amount of
energy (glider sinking in still air). As the train picks up speed and
exceeds the chicken's speed the net speed of the chicken moves forward
(glider is now being lifted in lift), and the chicken gains an
increase in net energy however the chicken is still expending the same
energy to walk to the back of the train (the glider is still using
gravitational potential energy to fly). Don't like that, think of a
ball rolling down an infinitely long inclined ramp and the ramp being
raised faster than the ball falls. What way does the ball move? to an
observer on the ground? *Is the ball giving up gravitational potential
energy to slide down the ramp relative to an observer on the ramp?
(yes). Is the ball gaining net energy? (yes).

So how many ways do people need to keep answering the same pedantic
question you keep asking? Do the ultimate thought experiment, turn off
gravity and the glider will just float along with moving air currents
but will be unable to glide anywhere.

Darryl

I use walking down the up escalator as my analogy - works with people
who spend a lot of time at the shopping mall. I have demonstrated it
to onlookers on rare occasions, but this usually upsets the mall cops.

Of course if you think about the analogy the energy for the whole
operation comes from the electric motors that drive the steps and
carry everyone's sorry butts to the next floor up. In soaring the
power is provided by the sun heating the air that rises (or forms
pressure systems that creates wind that flows over mountains). Gravity
and aerodynamics are just the way we turn that energy into a combined
forward and downward motion.

9B

9B

On 19 Mar, 17:57, wrote:

Of course if you think about the analogy the energy for the whole
operation comes from the electric motors that drive the steps and
carry everyone's sorry butts to the next floor up. In soaring the
power is provided by the sun heating the air that rises (or forms
pressure systems that creates wind that flows over mountains). Gravity
and aerodynamics are just the way we turn that energy into a combined
forward and downward motion.

Give the man a coconut!

Ian

On 19 Mar, 16:39, Darryl Ramm wrote:

A glider "climbs" when you pull back on the stick and converts kinetic
energy to gravitational potential energy,

Technically that's a "zoom", not a climb.

The glider also
"climbs" - (maybe you should think of "lifted" if "climb" confuses
you) by a rising air mass and that does increase the glider's
gravitational potential energy.

But people keep telling me that it's conversion of PE to drag which
keeps the glider flying. How can that be happening when the PE is
increasing?

So how many ways do people need to keep answering the same pedantic
question you keep asking?

Well, some accurate ways would make a good start!

It amazes and depresses me how prevalent the nonsensical believe that
"gravity powers gliders" extends in the gliding world.

Do the ultimate thought experiment, turn off
gravity and the glider will just float along with moving air currents
but will be unable to glide anywhere.

Cars won't be able to work either, since no gravity = no weight = no
friction at the tyres. Would you say that gravity powers cars?

Ian

PS Thanks for your concern about my confusion. I don't really go in
for arguments from authority, but would it allay your concern to know
that I have taught and examined fluid dynamics at two major UK
universities for over twenty years?

The Real Doctor wrote:
It amazes and depresses me how prevalent the nonsensical believe that
"gravity powers gliders" extends in the gliding world.

We all owe you a debt of gratitude for showing us how ignorant we are.

Cars won't be able to work either, since no gravity = no weight = no
friction at the tyres. Would you say that gravity powers cars?

I think pedantic trolling powers cars:
http://blog.modernmechanix.com/2008/...-makes-85-mph/

I don't really go in for arguments from authority, but

I removed the contradiction in order to protect small children.

Ian, "The Real Doctor" Out of curiosity, what exactly do you have a
doctorate in?

Aside from that.... In order to seek clarity in all of these
discussions I suspect that we have a mis-understanding because we are
trying to discuss these using two different reference frames. If
that's the case, then that would explain a lot.

I hope that we are all in agreement about the three forces acting on a
glider. For simplicity they are lift(L), drag(D) and weight(W=mg). As
has been corrected by Darryl, I agree that it is correct that lift, by
definition, is perpendicular to the airflow. However, for a glider in
steady state gliding flight, airflow and direction of motion are
parallel. Any body have any problems so far? I'm hoping this will get
me out of the hen house...

If we align the axis system such that weight is vertical and the
descent angle is theta. The equilibrium equations a

Vert. Axis 0 = L*cos(theta) + D*sin(theta) - W
Horz. Axis 0 = L*sin(theta) - D*cos(theta)

I'm guessing this is the source of the Lift providing the horizontal
motion argument. Clearly there is no gravity term in that component.
But the motion isn't strictly horizontal or vertical with these
equations. It is both, and therefore I would advocate a simplified set
where the direction of motion is the basis for the axis system.
Therefore....

If the axis system is aligned along the lift vector the equations
simplify to: (For the sliding block this tends to be the convention
that most books I own present) Replace L with N for Normal.

Lift Axis 0 = L - W*cos(theta)
Drag Axis 0 = D - W*sin(theta)

Any objections so far? I sure hope not. I can't imagine how....

The nice thing about convention 2 is that the lift and drag vectors
are isolated variables in the equation, and the weight is already
known so it's easy to solve the other values.

L = W*cos(theta) and D=W*sin(theta)

I can even rearrange the equations in set 1 and get the same
relationships. So, what in the world am I missing when I say Lift =
Weight * cos (glide angle)? Ian, you are the real doctor. I'll confess
my ignorance. I don't want to guess, cause I just don't know what
answer you're looking for, but what did I forget?

The other advantage of using convention 2 is in describing the motion
of the system. The object is constrained to the plane, and therefore
you can get rid of the "vertical" axis in this example and look at the
equation with one dimension. Because the lift force or normal force
constrains the object to the plane, you'll have no accelerations or
displacements in this direction, for a steady state example. In this
case that would be it's glide path. The only equation left is D = W*sin
(theta) So again I argue, Lift, because it is perpendicular to the
direction of motion, can not provide the motive force! The motive
force is governed by a balance between gravity, drag, and the glide
angle. Don't get me wrong, I'm not saying lift isn't important. It is
very important to making the glider stay on a glide path. Maybe this
is just a chicken before the egg argument. I can see the circularity
of the discussion. Why do those chickens keep coming up....

This would be a whole lot easier to explain with pictures. So I'll
cite a reference...If anybody has a copy of the BGA Manual: "Gliding:
Theory of Flight", please reference the discussion of forces on flight
in Chapter 4. The book goes through a very good explanation of how
gravity provides the motive force for gliding. It's an excellent book
and I highly recommend it. If only it had a discussion of forces on
tow....

-Kevin

PS I think we need some good flying weather so that we all get out of
the house and away from the computer....