motorgliders as towplanes

Derek Copeland wrote:
In free unaccelerated flight with no thrust, i.e. no aerotow, winch, or
turbo, a glider IS essentially gravity powered. The resultant force of
gravity plus wing lift, angled very slightly forward, opposes drag. Thus a
glider runs down a very slight slope through the air. The less drag there
is, the flatter the glide angle becomes.

Nicely worded answer.

On several occasions I've had (non glider pilot) friends ask me why does
it help when we make our gliders heavier with water ballast. Seems
counter-intuitive.

I'm thinking that a proper explanation is in terms of the gravitational
force in a similar fashion to what you describe. Higher mass = higher
gravitational force (F=MA). Hence the glider is "pulled down the slope"
by a higher force. The glide angle is no better, but we can glide
faster at essentially the same glide angle which is an advantage (normal
caveats about thermal climb ability trade-off). A more complete answer
might also discuss the higher drag at higher speed interplay, but that
could probably be left out as a simplification. Perhaps a further
discussion of the classic experiment where in a vacuum a feather and a
rock will fall to earth at the same rate because the acceleration of
gravity is a constant (I know, but within limits it *is* a constant).
But in the presence of air the "air-drag" on the feather is relatively
high compared to the relatively low gravity "down-pull" due to its low mass.

Comments on this explanation are welcomed/sought. I thought I would
find a well worded description of this in Reichmann but it isn't there
that I can see. TIA

Regards,

-Doug

On Mar 18, 8:20*am, Doug Hoffman wrote:
Derek Copeland wrote:
In free unaccelerated flight with no thrust, i.e. no aerotow, winch, or
turbo, a glider IS essentially gravity powered. The resultant force of
gravity plus wing lift, angled very slightly forward, opposes drag. Thus a
glider runs down a very slight slope through the air. The less drag there
is, the flatter the glide angle becomes.

Nicely worded answer.

On several occasions I've had (non glider pilot) friends ask me why does
it help when we make our gliders heavier with water ballast. *Seems
counter-intuitive.

I'm thinking that a proper explanation is in terms of the gravitational
force in a similar fashion to what you describe. *Higher mass = higher
gravitational force (F=MA). *Hence the glider is "pulled down the slope"
by a higher force. *The glide angle is no better, but we can glide
faster at essentially the same glide angle which is an advantage (normal
caveats about thermal climb ability trade-off). *A more complete answer
might also discuss the higher drag at higher speed interplay, but that
could probably be left out as a simplification. *Perhaps a further
discussion of the classic experiment where in a vacuum a feather and a
rock will fall to earth at the same rate because the acceleration of
gravity is a constant (I know, but within limits it *is* a constant).
But in the presence of air the "air-drag" on the feather is relatively
high compared to the relatively low gravity "down-pull" due to its low mass.

Comments on this explanation are welcomed/sought. *I thought I would
find a well worded description of this in Reichmann but it isn't there
that I can see. *TIA

Regards,

-Doug

Remembering that the power for the glider is coming from the
gravitational potential energy, so it is correct that a higher mass
glider has more energy and this is where the increased L/D does come
from. However you can't do an analysis quite like that to explain the
results. A good discussion of this for L/D for powered aircraft is is
in "Mechanics of Flight" by Phillips, if you read the "Power Failure
and Gliding Flight" chapter you will get a pretty good picture of what
happens, even if not really discussing glider wind loading.

Google books has extracts on line at

http://books.google.com/books?id=6-_...nics+of+flight

and available from Amazon.

http://www.amazon.com/Mechanics-Flig.../dp/0471334588

It's expensive but very good.

Darryl

On Mar 18, 6:45*am, Derek Copeland wrote:
In free unaccelerated flight with no thrust, i.e. no aerotow, winch, or
turbo, a glider IS essentially gravity powered. The resultant force of
gravity plus wing lift, angled very slightly forward, opposes drag. Thus a
glider runs down a very slight slope through the air. The less drag there
is, the flatter the glide angle becomes.

Both airbrakes and large positive flap angles increase the drag, so the
glider has to run down a steeper slope to maintain speed.

Airbrakes both reduce lift, by disrupting the airflow over part of the
wing, and increase drag, so the answer to that question is obvious. *

Large amounts of positive flap increases lift, but also increases drag to
a much greater extent.

Derek Copeland

At 12:30 18 March 2009, Bob Cook wrote:

Well, at least I did learn a lot from the previous thread, thanks mostly
to
Ian who helped me "tidy up" some of my conceptions.

Yes this is Jr High physics. *I think if we took the final exam, Ian
would
get an "A", while I would get a "C" and most of the guys would do
worse!

Since everybody liked my "gravity" question so much, here is more from
my pet peeve department.

Q) Two identical gliders on final approach. *Glider A has spoilers
closed.
Glider B opens spoilers. *Glider B will make a steeper approach because
"spoilers reduce lift". *True or false? *And why.

Q) Two identical gliders on final approach. *Glider A has flaps
retracted.
Glider B has flaps extended. *Glider B will make a steeper approach
because "flaps increase lift". * True or false? And why.

Cookie

Actually the amount of lift in both cases is unchanged (once the
approach is stabilized) Total lift equals the weight of the glider -
otherwise the glider would experience a vertical acceleration. There
are some vector effects from the angle of the glideslope, but I'm
pretty sure they are secondary in most cases. What changes is the lift
coefficient. In the case of spoilers the parts of the wing that are
not affected by the spoilers operate at a higher lift coefficient to
hold the glider up - this produces more induced drag, on top of the
drag of the spoilers themselves.

With flaps extended the lift coefficient only goes up as the speed
goes down. Again the main effect on glideslope is the drag of the
flaps, not a change in lifting force.

For the technically inclined, the lift formula is 1/2pV^2SCl, where p
(rho) is the density of air, V is velocity, S is wing area and Cl is
lift coefficient. Flaps let you achieve a higher Cl at a lower
airspeed by changing the characteristics of the airfoil, spoilers
force you to fly at a higher Cl because the effective S goes down. If
you are already at max Cl for the wing and deploy spoilers you will
accelerate downward until you generate enough additional airspeed to
stop it.

9B

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

On 18 Mar, 00:45, Nyal Williams wrote:
I'd like to turn this around since I'm not a physicist or an engineer.
What force causes a ball to roll down an inclined plane?

Well, not gravity, or at least not just gravity, because gravity moves
things downwards, not along. So the answer is really that two forces
are involved:

1. Gravity, less the vertical component of the reaction force, moves
it down and

2. The horizontal component of the reaction force moves it along

Ian

On 18 Mar, 12:30, Bob Cook wrote:

Q) Two identical gliders on final approach. *Glider A has spoilers closed.
*Glider B opens spoilers. *Glider B will make a steeper approach because
"spoilers reduce lift". *True or false? *And why.

Sort of true. All other things being equal, spoilers reduce lift, but
the system adjusts itself, or is adjusted, so the the amount of lift
increases again (if you're accelerating towards the ground you're
Doing It Wrong!) but with increased drag.

Q) Two identical gliders on final approach. *Glider A has flaps
retracted.
*Glider B has flaps extended. *Glider B will make a steeper approach
because "flaps increase lift". * True or false? And why.

Same sort of thing, really. Vertical equilibrium (ie constant descent
rate) is reached with a higher drag, so more energy lost, so the
descent rate is higher.

But I se the paradox you're getting at - one device decreases lift,
one increases it and yet they both have the same effect. The key is in
drag: air brakes force you and flaps permit you to fly in a higher
drag regime, and that's what makes the approach steeper. Lift never
makes a difference - because it acts, by definition, at right angles
to the flight path it never does or needs work.

Ian
Ian
Cookie

On 18 Mar, 13:45, Derek Copeland wrote:
In free unaccelerated flight with no thrust, i.e. no aerotow, winch, or
turbo, a glider IS essentially gravity powered.

Not true. A glider can fly perfectly happily while increasing its
portential energy - exactly the opposite of being gravity powered.

Ian

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

On 17 Mar, 18:33, Mike the Strike wrote:
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.

And how often do we fly in still air? For that matter, how often do we
build gliders on the top of hills, fly them to the bottom and then
abandon them. A very little energy may be stored as gravitational
potential, but gravity certainly isn't the source of the energy!

Ian

On 17 Mar, 21:41, KevinFinke wrote:

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

Can't be gravity, or not just gravity, because chickens, like apples,
fall straight down when gravity gets hold of them.

How can gravity push something forwards?

It is gravity. There is no forward component of lift that contributes
to forward motion.

What do you think overcomes the horizontal component of drag, then?

In steady
state gliding flight, Net Lift is equal to Weight times the cosine of
the gliding angle.

No it's not.You forgot something!

Ian