motorgliders as towplanes

Weight is a force????? Did I miss something in HS Physics? Weight = Mass.

Mike Schumann

"KevinFinke" wrote in message
...
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....

Dear Big,

You are on to something here. Get one of those geo surveys, and fly your
glider only over areas of higher gravity. This extra gravity will give
your glider extra power, and you will be the best glider pilot!

Cookie

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.

Ian,

I was trying to point out a misconception that the "horizontal component
of lift" in a turn is somehow balanced by some other force.

If it were, the glider would not turn.

A turn is an acceleraton, requiring unbalanced forces. Lift is greater
than the sum of gravity plus drag.

Cookie

At 18:25 19 March 2009, The Real Doctor wrote:
On 19 Mar, 12:45, Bob Cook wrote:

Q) A glider is in circling flight. =A0The glider circles because there
is=
a
horizontal component of lift. =A0This horizontal component of lift is
balanced by an equal and opposite force, centrifugal force.

True or False and why?

I hope you're not getting hung up on the old centripetal/centrifugal
debate. Centripetal force is just as real in a stationary axix system
as centrifugal is in a turning one!

By and large (ignoring a few second-order effects) what you wrote is
fine.

Ian

OK. badly worded question.

Here I was trying to point out that DRAG is the major difference between a
40:1 glider and a 20: glider. Not lift, not gravity.


Drag determines the direction of flight.

More drag= steeper glide (spoilers, flaps)

Less drag = flatter glide

Cookie



At 18:23 19 March 2009, The Real Doctor wrote:
On 19 Mar, 12:15, Bob Cook wrote:

Spoilers redistribute lift, but not reduce lift.

They reduce the lift over part of the wing. Whether the overall lift
increases, decreases or stays the same depends on what the pilot does.

Flaps change the coeffecient of lift, but not lift.

If you keep everything else the same then they do change lift. Every
tried dumping your landing flap just before the flare ...?

Q) =A0Two 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. =A0True or flase and why.

Depends what they are doing.

Ian

Mike Schumann wrote:
Weight is a force????? Did I miss something in HS Physics? Weight = Mass.


Yeah. This is often a confusing issue. If one is dealing with just the
metric system then I think the terminology is clear and unambiguous. A
kilogram is clearly a unit of mass, and a newton is clearly a unit of
force. Here in the good old USA we have the pound-force and the
pound-mass. Oh boy. I was taught that the unit of mass in English
units is the slug. But I have yet to see a bathroom scale with a
readout in slugs. :-)

When people talk about "on the moon we would weigh less" they have to be
referring to force (not mass) and indeed our bathroom scales would
reflect force and not mass if used on the moon.

I just wish the US would perform its conversion to metric units for
*everything*. The sooner the better. But that job is being handled by
our highly efficient government. Don't hold your breath. :-)

Regards,

-Doug

p.s. The US auto industry voluntarily converted everything to hard
metric years ago because we have to work with suppliers from around the
world and sell product the same. We couldn't wait for our government to
get that job done. Remember in 1978 when the US government was
orchestrating the conversion? The interstate freeways even had all of
the kph speed limit signs installed but covered. At the last minute
some senator decided he didn't like the idea of converting and
everything was scuttled and here we are.

Bob,

If we are gonna play what if.....

What if there was no lift? (like a brick,)

Sure it would move because of gravity, but in what direction? Down! Not
forward!

So tell me again how gravity imparts a forward force to a glider?


Although "no gravity is not possible, "no lift" is easily
demonstrated:

Next time you fly, try this. Push forward on the stick. Keep pushing
until your angle of attack goes to zero (no lift). Tell me which way your
glider flies!

Cookie


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

Ian,

If we draw vector force diagrams of two identical gliders, one with flaps
extended, and the other with flaps retracted, we can easily see that
"lift" is essentially the same in both cases.

The direction of flight will be steeper with flaps extended, and drag will
be signifigantly greater.

As for spoilers, vector diagrams will also show lift is not reduced
(except for a tiny, tiny amount) but drag is greatly increased and the
fligth path is steepened.

Remember were are talking about stabilized unaccelerated flight here.

Sudden application or retraction of flaps might have MOMENTARY changes in
lift, but not once stabilized unacellerated flight is resumed.

Misconceptions commonly found in publications and professed by many
"experts":

Gravity powers a glider and provides forward motion.
Flaps increase lift
Spoilers reduce lift
Centrifugal force turns a glider
Aircraft climb due to increased lift
Aircraft descend due to reduced lift

Any more?

Cookie



At 01:00 20 March 2009, Bob Cook wrote:
OK. badly worded question.

Here I was trying to point out that DRAG is the major difference between
a
40:1 glider and a 20: glider. Not lift, not gravity.


Drag determines the direction of flight.

More drag= steeper glide (spoilers, flaps)

Less drag = flatter glide

Cookie



At 18:23 19 March 2009, The Real Doctor wrote:
On 19 Mar, 12:15, Bob Cook wrote:

Spoilers redistribute lift, but not reduce lift.

They reduce the lift over part of the wing. Whether the overall lift
increases, decreases or stays the same depends on what the pilot does.

Flaps change the coeffecient of lift, but not lift.

If you keep everything else the same then they do change lift. Every
tried dumping your landing flap just before the flare ...?

Q) =A0Two 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. =A0True or flase and
why.

Depends what they are doing.

Ian

This one is for Ian so you others don't read this, OK?

Ian,

I have invented a "gravity powered machine". It uses weight(s) as fuel.
Once the fuel is exhaused, the "used" fuel is fully recoverable and can
simply be put back in the "tank", and the machine will start running
again!

Here is how it works.

Take a bucket and place 100 steel balls into it.
Take another bucket and place 100 steel balls into it, but also place 1/2
of a steel ball into it. These steel balls are the "fuel"

String a rope over a pulley from the peak of the barn. Tie the buckets to
each end of the rope, about 10' high. Naturally the heavier bucket will
fall to the ground, lifting the light bucket. Remove one steel ball from
the heavy bucket making it now the lighter bucket. Now the other bucket
will fall to the ground, remove one seel ball, etc. The machine will go
up and down until you run out of steel balls.

But don't worry, just pick up the steel balls and place them in the
buckets again, have fun all day with gravity power!

(Same way a glider is powered by gravity!)


Cookie






At 18:38 19 March 2009, The Real Doctor wrote:
On 19 Mar, 17:04, Jim Logajan wrote:

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

The sun.

Also, initially, internal combustion engines, stretched rubber or, if
you buy an Electrostart and choose your contract carefully, nuclear
fission!

Ian

Bob Cook wrote:

If we are gonna play what if.....

What if there was no lift? (like a brick,)

Sure it would move because of gravity, but in what direction? Down! Not
forward!

That is correct and demonstrates a useful "thought exercise".


So tell me again how gravity imparts a forward force to a glider?

By itself it cannot. The presence of gravity *and* air are required for
gliding (forward) motion.


Although "no gravity is not possible,

Sure it is. Place your glider in outer space. Likely? No. Possible?
Certainly.


"no lift" is easily demonstrated:

And lift(as we are using the term) without gravity is not possible.

Regards,

-Doug

Doug,

I was about to give you a "gold star" but then found some more
misconceptions.

There is gravity in outer space. Anywhere there is matter, with mass,
there is Gravity. The moon is held in orbit by the earth's gravity.
Earth in solar orbit, etc.

Lift would be possible without gravity, (if it was possible to have no
gravity.) If you placed an airplane in air, but no gravity, and provided
a propelling force (engine), the wing could certainly produce lift without
gravity . Without gravity, lift would be unbalanced, resulting in your
aircraft doing successive loops!

Air (or other "fluid" if we are taking this into outer space), is needed
to provide lift. But the forces involved in gliding flight are still
three.....lift, drag, and gravity.

Although thought provoking exerceses, better to say "what is" than
"what if.".....

Cookie





At 01:39 20 March 2009, Doug Hoffman wrote:
Bob Cook wrote:

If we are gonna play what if.....

What if there was no lift? (like a brick,)

Sure it would move because of gravity, but in what direction? Down!
Not
forward!

That is correct and demonstrates a useful "thought exercise".


So tell me again how gravity imparts a forward force to a glider?

By itself it cannot. The presence of gravity *and* air are required for

gliding (forward) motion.


Although "no gravity is not possible,

Sure it is. Place your glider in outer space. Likely? No. Possible?
Certainly.


"no lift" is easily demonstrated:

And lift(as we are using the term) without gravity is not possible.

Regards,

-Doug