motorgliders as towplanes

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

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

On 19 Mar, 12:45, Bob Cook wrote:

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?

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

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

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 Mar 20, 1:32 am, The Real Doctor wrote:

Physicists, and particularly
school physics teachers, traditionally get terribly upset by the idea
of centrifugal force.

Ian

Physicsists make equally snarky comments about engineers, Ian. My
engineering dynamics professor at UC, Berkeley was adamant in
opposing the perpetuation of the centrifugal force myth. It's phony
physics and can lead to seriously erroneous conclusions.

Myles

On 21 Mar, 05:21, Myles wrote:
On Mar 20, 1:32 am, The Real Doctor wrote:

Physicists, and particularly
school physics teachers, traditionally get terribly upset by the idea
of centrifugal force.

Physicsists make equally snarky comments about engineers, Ian. *

Indeed. Bu hey, the mathematicians despise us all.

My
engineering dynamics professor at UC, Berkeley was *adamant in
opposing the perpetuation of the centrifugal force myth. *It's phony
physics and can lead to seriously erroneous conclusions.

It's a perfectly useful tool if applied correctly. That normally means
within a moving axis system, and getting there is not always simple.
You always have to decide whether it's going to be easier overall to
use the difficult model with the simple setup (stationary axes) or the
simple model with the difficult setup (moving axes).

It's the same in fluids - normally we model a glider by holding it
still and letting the air move past, but that's not always the best
way, or the easiest way. Mind you, I'm a typical lazy engineer, so for
me best = easiest in about 99% of cases.

Ian

Myles wrote:
On Mar 20, 1:32 am, The Real Doctor wrote:

... My engineering dynamics professor at UC, Berkeley was adamant in
opposing the perpetuation of the centrifugal force myth. It's phony
physics and can lead to seriously erroneous conclusions.

Yeah, yeah, centrifugal force is a reaction to a centripetal force, I
know the difference,.... I don't care. The layman understands the former
term and not the latter. To the typical student, I'll use the term
"centrifugal". If he's a physicist, I'll say "centripetal". :-)

Tony V.

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 Thu, 19 Mar 2009 13:09:30 -0500, Jim Logajan wrote:

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."

Printing it in some official or semi-official publication doesn't make it
right.

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.


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