motorgliders as towplanes

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 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) *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.

Depends what they are doing.

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

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

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