motorgliders as towplanes

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

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.

The Real Doctor wrote:
On 18 Mar, 22:34, 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?

No, I'm being serious in a usenet-is-not-reallife sort of way. I
introduced this, you see, as a common misconception in the gliding
world. Lots of people think that gliders are "powered by gravity", and
they aren't!

So what does power gliders? Seems like you are the only one who knows the
real answer.

The Real Doctor wrote:
On 19 Mar, 00:03, KevinFinke wrote:
Ian, "The Real Doctor" *Out of curiosity, what exactly do you have a
doctorate in?

Numerical solution of non-linear diffusion problems.Which is actually
a lot more interesting than it sounds.

I didn't know doctorates were awarded quite that way. I always thought they
were awarded in broad fields like math or physics or computer science.

Did you develop some new numerical techniques for such problems? Was your
work done in a math or computer science department?

Bob Cook wrote:
Gravity is a downward force. Things move downward due to gravity,
including gliders.

Aren't there some things that move upward due to gravity? Even upward and
sideways too?

But the question was not "what gives a glider motion?", it was "what
gives a glider FORWARD motion?"

Sorry, can't do it without LIFT. (don't forget drag either)

Three forces act on a glider. Not one, not two, THREE.

But isn't there only one potential energy field operative? Not two, or
three, just one? Didn't Aharonov and Bohm show that potentials are more
"fundamental" than forces, so that we should really be considering the
system using potentials?

Is somebody keeping score on the pedantry? I hope I got at least a couple
points!

Jim Beckman wrote:
At 18:22 17 March 2009, Bob Whelan wrote:

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

It won't move. Why would it?

Jim Beckman

Indeed. But I find it easier to magically insert the glider than to
magically turn off gravity. That's why my chicken has a soft landing
place for her eggs.

And being 100% non-facetious in this paragraph, it appears some folks'
misconceptions may have been clarified to various degrees as a result of
this thread...put me in the camp that believes clear thought is a good
thing. Once a person grasps the essential role of gravity in powering
sailplanes, the lifty bits become distinctly easier to grasp, despite
the pesky intrusion of (ahem) plane geometry into resolving resultant
forces.

Regards,
Bob W.

Bob Cook wrote:
Since I am a firm believer that you can't change gravity (although
some of you out there try to in your analysis)

They're just using thought experiments:

http://en.wikipedia.org/wiki/Thought_experiment

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

Bob Cook wrote:
Not many took a stab at the "spoilers and flaps" questons!

Probably because it was a no-win situation.

Spoilers do not "reduce lift". Spoilers increase drag. As drag
increases, glide slope steepens.

Spoilers redistribute lift, but not reduce lift.

You're claim conflicts with that in the FAA "Pilot's Handbook of
Aeronautical Knowledge":

"Found on many gliders and some aircraft, high drag devices called
spoilers are deployed from the wings to spoil the smooth airflow,
reducing lift and increasing drag."

http://www.faa.gov/library/manuals/a...apter%2003.pdf

Flaps do not "increase lift". Flaps increase drag. As drag
increases, gilde slope steepens.

Flaps change the coeffecient of lift, but not lift.

Again, from the PHAK, same chapter:

"Flaps are the most common high-lift devices used on aircraft. These
surfaces, which are attached to the trailing edge of the wing, increase
both lift and induced drag for any given AOA."

(But I see you're being pedantic. Now you make a distinction between
"lift" and "coefficient of lift".)

Another question:

Q) Two gliders, one is 40:1 racer and glider two is 20:1 trainer.
Both weigh 800#

Glider one has twice the lift of glider two. True or flase and why.

Another no-win situation, since you don't indicate what they are doing.
Are they sitting stationary on the ground? Then both have zero lift. Are
they turning and if so, are the turn radii and descent rates different?

On 19 Mar, 09:19, Doug Hoffman wrote:

I thought the best L/D stayed about the same. *It just occurs at higher
speed with ballast.

Yup. I think this is a good one to think about in terms of angle of
attack.

The only two things - normally - you can change to change lift are
lift coefficient (via AoA) and speed. If you generate the extra lift
needed to keep a heavier glider up by flying faster and keeping CL the
same then CD will also be the same. D will therefore increase by
exactly the same proportion as L and L/D stays the same.

Ian