motorgliders as towplanes

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
org |

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 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 |

At 19:48 19 March 2009, Martin Gregorie wrote:
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.

But it *does* make it the right answer to the written test when that test
is given by that same authority. For the test, concentrate not on what is
necessarily true, but on what the FAA wants for an answer. After the
written is out of the way, just learn to fly the aircraft.

Jim Beckman

On Fri, 20 Mar 2009 14:30:04 +0000, Jim Beckman wrote:

But it *does* make it the right answer to the written test when that
test is given by that same authority. For the test, concentrate not on
what is necessarily true, but on what the FAA wants for an answer.
After the written is out of the way, just learn to fly the aircraft.

Yes, and I learnt it as the answer to that Bronze question. It just
annoys me that official publications should enshrine myths as some sort
of truth.

Its no better than introductory texts showing packets of air dividing at
the LE of a ring and meeting up again at the TE, when a photo taken in a
wind tunnel shows clearly that doesn't happen. Far better to publish the
photo than a bogus diagram.


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

On 21 Mar, 16:48, Martin Gregorie
wrote

Its no better than introductory texts showing packets of air dividing at
the LE of a ring and meeting up again at the TE, when a photo taken in a
wind tunnel shows clearly that doesn't happen. Far better to publish the
photo than a bogus diagram.

Oh, that one drives me mad. Not only does it not happen ... there is
no conceivable reason why it should happen. And yet many many websites
and books cite it as fact. Grrrrrrrrrrrrrrrrrr.

Incidentally, if you don't have a copy of "An Album of Fluid Motion"
by Milton Van Dyke, get it. You'll love it. Every gliding club should
have one.

Ian

On Sat, 21 Mar 2009 12:28:59 -0700, The Real Doctor wrote:

Incidentally, if you don't have a copy of "An Album of Fluid Motion" by
Milton Van Dyke, get it. You'll love it. Every gliding club should have
one.

Another recommendation:

http://www.av8n.com/how/ gives a good description of how an aeroplane
works. A lot of it is similar to "Stick and Rudder", so its written for
pilots rather than aircraft designers. However, section 3 has excellent
descriptions and diagrams showing how a wing works. It was recommended to
me by a professional aerodynamicist who is a model designer and has flown
gliders.


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

On Sat, 21 Mar 2009 20:01:23 +0000, Martin Gregorie wrote:

On Sat, 21 Mar 2009 12:28:59 -0700, The Real Doctor wrote:

Incidentally, if you don't have a copy of "An Album of Fluid Motion" by
Milton Van Dyke, get it. You'll love it. Every gliding club should have
one.


Here's a link to animated visualizations of the flow past a wing:
http://www.av8n.com/irro/ - click the flag of your favored language to
see the visualizations.

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

That is a VERY good reference!!!

Mike Schumann

"Martin Gregorie" wrote in message
...
On Sat, 21 Mar 2009 12:28:59 -0700, The Real Doctor wrote:

Incidentally, if you don't have a copy of "An Album of Fluid Motion" by
Milton Van Dyke, get it. You'll love it. Every gliding club should have
one.

Another recommendation:

http://www.av8n.com/how/ gives a good description of how an aeroplane
works. A lot of it is similar to "Stick and Rudder", so its written for
pilots rather than aircraft designers. However, section 3 has excellent
descriptions and diagrams showing how a wing works. It was recommended to
me by a professional aerodynamicist who is a model designer and has flown
gliders.


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