motorgliders as towplanes

Bob Cook wrote:
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.

You are picking knits and missing the overall points. Clearly I meant
place it in space where gravity is negligible.


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!

Another knit. This discussion is about *gliding*.


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.".....

OK Bob. You want to see things just one way and apparently nothing else
anyone says is going to sway you. No problem here. Have fun. I think
I am done with this.

Regards,

-Doug

Cookie...are you drinking enough? The weather in your part of the world
must be bad as you do not seem to leave much time for flying.

Jim


At 01:30 20 March 2009, Bob Cook wrote:
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

On 20 Mar, 01:39, Doug Hoffman wrote:
Bob Cook wrote:

Although "no gravity is not possible,

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

Or just fly a vomit-comet style ballistic trajectory.

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

Aircraft in a 90 degree bank can still produce lift ...

Ian

On 20 Mar, 01:00, Bob Cook wrote:
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.

It all depends on your reference frame. To someone on the ground, an
unbalanced sideways force gives rise to the necessary acceleration.
But to an observer moving with the glider - the pilot, say - there is
no sideways acceleration /of the glider/ (the rest of the world may,
of course, be doing something). To the moving observer, an equal an
opposite centrifugal force provides the necessary balance.

We engineers like modelling with moving reference frames and
centrifugal forces because it turns dynamics problems into statics
problems, which are generally simpler. Physicists, and particularly
school physics teachers, traditionally get terribly upset by the idea
of centrifugal force.

Ian

On 19 Mar, 19:48, Martin Gregorie
wrote:

There is a question in the UK Bronze badge written paper about the
proportion of lift provided by the top and bottom surfaces of a wing
that's just as wrong. The so-called "correct" answer is 70/30, but as a
wing is a device for imparting momentum to an air mass its a meaningless
question.

Oh no. Not that one.

At the surface of the wing, it exerts a force on the air mass. A long
distance away (typically 2 chord lengths) it's a momentum change. In
between the effect of the wing is a pressure change /and/ a momentum
change.

Overall, the integrated pressure across the top surface is about 70%
of the total lift force, and the intergrated pressure across the
bottom surface is about 30% of the total lift.

Significance? Irregularities on the top surface will reduce lift by
more than the same irregularities on the bottom surface. Hence top-
surface-only airbrakes: they are more effective there than underneath,
because they destroy more lift, necessitating a bigger and draggier
change of AoA to compensate.

Ian

On 20 Mar, 01:15, Bob Cook wrote:

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.

What happens if you extend the flaps on a glider while keeping
everything else (AoA, airspeed) constant?

Ian

On 19 Mar, 19:41, Jim Logajan wrote:
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.

So you were being pedantic after all.

No I wasn't. Unless you think that "the sun" and "gravity" are somehow
the same power source.

Don't feel bad about this. The misapprehensions run very deep, and a
huge number of gliding instructors really don;t understand this stuff.
Not that it matters, really, because you don't need to know
aerodynamics to fly a glider!

Ian

On 20 Mar, 07:30, Jim White wrote:
Cookie...are you drinking enough? The weather in your part of the world
must be bad as you do not seem to leave much time for flying.

Dunno where he is, but I'm in Scotland. Need I say more?

Ian

On Fri, 20 Mar 2009 02:39:17 -0700, The Real Doctor wrote:

On 19 Mar, 19:48, Martin Gregorie
wrote:

There is a question in the UK Bronze badge written paper about the
proportion of lift provided by the top and bottom surfaces of a wing
that's just as wrong. The so-called "correct" answer is 70/30, but as a
wing is a device for imparting momentum to an air mass its a
meaningless question.

Oh no. Not that one.

At the surface of the wing, it exerts a force on the air mass. A long
distance away (typically 2 chord lengths) it's a momentum change. In
between the effect of the wing is a pressure change /and/ a momentum
change.

Overall, the integrated pressure across the top surface is about 70% of
the total lift force, and the intergrated pressure across the bottom
surface is about 30% of the total lift.

Significance? Irregularities on the top surface will reduce lift by more
than the same irregularities on the bottom surface. Hence top-
surface-only airbrakes: they are more effective there than underneath,
because they destroy more lift, necessitating a bigger and draggier
change of AoA to compensate.

True enough, but the point I was trying (badly) to make is that the lift
is due to the whole wing section and shouldn't be apportioned to the two
surfaces as a number taught to neophytes. Apart from anything else this
breaks down when you consider the pressure distribution across the top
surface. Why not also teach an arbitrary percentage of lift generated by
the LE suction spike at high Cl?


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

Doug,

You're the one who took the coversation into aouter space, not me.

Yes, I only look at this one way. In physics, its black or white,there is
pretty much not a lot of "gray area".

But I think you got my point that the question on the Bronze Badge exam is
flawed, and none of the answers are correct.

Cookie



At 04:17 20 March 2009, Doug Hoffman wrote:
Bob Cook wrote:
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.

You are picking knits and missing the overall points. Clearly I meant
place it in space where gravity is negligible.


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!

Another knit. This discussion is about *gliding*.


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.".....

OK Bob. You want to see things just one way and apparently nothing else

anyone says is going to sway you. No problem here. Have fun. I think
I am done with this.

Regards,

-Doug