motorgliders as towplanes

Darryl Ramm wrote:
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.

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

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.

OK. Thanks for that. Is a proper explanation that complicated? I was
hoping to avoid a $140 answer. :-)

Regards,

-Doug




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

You can change the reference or axis system all you want, But then you
also change the direction of "down".


Gravity acts downward and no other diredtion.


Lift plus grivity act together and form a resultant force parallel to the
direction of flight. Drag acts in the opposite direction at equal
magnitude.

Gravity (alone) is not the force that provides forward motion to a
glider.

A simple three vector diagram will show this.


Cook

At 00:03 19 March 2009, KevinFinke wrote:
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....

Not many took a stab at the "spoilers and flaps" questons!

Ian pretty much got it right, and at least saw the paradox in my
questions, How can extra lift (flaps)increase glide slope while, reduced
lift (spoiler) also increases glide slope?

THere are two misconceptions in the above question.

The answer to both original questions is FALSE.

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

Spoilers redistribute lift, but not reduce lift.

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

Flaps change the coeffecient of lift, but not 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.


Cookie



At 22:17 18 March 2009, The Real Doctor wrote:
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

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

Ian,

Misconceptions sure die hard. Many "Cannot handle the truth".

Here' s another one.

Q) A glider is in circling flight. The glider circles because there is a
horizontal component of lift. This horizontal component of lift is
balanced by an equal and opposite force, centrifugal force.

True or False and why?

Cookie

Bob Cook wrote:
You can change the reference or axis system all you want, But then you
also change the direction of "down".


Gravity acts downward and no other diredtion.


Lift plus grivity act together and form a resultant force parallel to the
direction of flight. Drag acts in the opposite direction at equal
magnitude.

Gravity (alone) is not the force that provides forward motion to a
glider.

Maybe we are wordsmithing/semanticizing a bit. Take away gravity and
air movement and place a glider somewhere up in the still air (I know,
suspend disbelief for a moment) and let go of the glider. No initial
motion is given to the glider, it is just "suspended in air". How much
lift and/or forward motion do we then get? None. Add gravity to the
same scenario and the glider will then move forward (after an initial
drop). So perhaps gravity *is* the sole force required for forward
motion. Of course we need air as well. But still air is a gaseous
mass, not a force or even a source of force. The reaction force caused
by the combination of gravity (sole source of force) and the presence of
air (air is not a force) leads to the forward motion of the glider.

Regards,

-Doug

I just gotta laugh. When Internet "arguements" get heated or go nowhere,
people resort to a comparison of "qualifications".

As if the guy with the longer resume is somehow always correct.

BTW, my qualificatons: Airport Bum.

Cookie



At 00:03 19 March 2009, KevinFinke wrote:
Ian, "The Real Doctor" Out of curiosity, what exactly do you have a
doctorate in?

I think you would find that a glider flying though air would stop moving
pretty quickly if you could turn off gravity! There would still be lift
and drag until it stopped, but nothing to drive it forward.

Derek Copeland


At 12:00 19 March 2009, Bob Cook wrote:

Gravity acts downward and no other direction.


Lift plus grivity act together and form a resultant force parallel to
the
direction of flight. Drag acts in the opposite direction at equal
magnitude.

Gravity (alone) is not the force that provides forward motion to a
glider.

A simple three vector diagram will show this.


Cook

Dude!



You can pretend there is no gravity. You can pretend there is no air.

But there IS gravity, and there IS air.

Gravity is a downward force. Things move downward due to gravity,
including gliders.

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.

Answer to question again: The resultant force of lift added to gravity,
balanced by drag.

Cookie


At 12:53 19 March 2009, Doug Hoffman wrote:
Bob Cook wrote:
You can change the reference or axis system all you want, But then
you
also change the direction of "down".


Gravity acts downward and no other diredtion.


Lift plus grivity act together and form a resultant force parallel to
the
direction of flight. Drag acts in the opposite direction at equal
magnitude.

Gravity (alone) is not the force that provides forward motion to a
glider.

Maybe we are wordsmithing/semanticizing a bit. Take away gravity and
air movement and place a glider somewhere up in the still air (I know,
suspend disbelief for a moment) and let go of the glider. No initial
motion is given to the glider, it is just "suspended in air". How much

lift and/or forward motion do we then get? None. Add gravity to the
same scenario and the glider will then move forward (after an initial
drop). So perhaps gravity *is* the sole force required for forward
motion. Of course we need air as well. But still air is a gaseous
mass, not a force or even a source of force. The reaction force caused
by the combination of gravity (sole source of force) and the presence of

air (air is not a force) leads to the forward motion of the glider.

Regards,

-Doug

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.