Slotted Flaps Pitching Moment

On Apr 14, 1:59 pm, john smith wrote:
In article ,

"J.Kahn" wrote:
Does the presence of the
slot in the flap move the overall wing CP forward or aft vs the
unslotted flap at a given angle? This is what I'm having trouble
finding out. I would think that the presence of the slot imcreases the
pressure gradient and moves it more aft than the plain one, so that the
pitching moment is increased.

AS Stealth Pilot posted, the purpose of the slot is to energize the flow
over the flap.

A better question is, what are the relationships between center of
pressure (CP) and center of lift (CL)?

Do they differ?

Why do they differ?

Under what conditions do they coincide?

CP is the term most often used, implying that it's the same
thing.

As far as pitching moment, there are other considerations
besides CP movement. In the 150 and 172 the nose pitches UP when flaps
are deployed, not down. The downwash off the flaps strikes the
stabilizer and raises the nose, demanding nose-down trim. The position
of the stab is a bit different in other Cessnas like the 180/185 and
the nose pitches down instead.

Dan

wrote:
On Apr 14, 1:59 pm, john smith wrote:
In article ,

"J.Kahn" wrote:
Does the presence of the
slot in the flap move the overall wing CP forward or aft vs the
unslotted flap at a given angle? This is what I'm having trouble
finding out. I would think that the presence of the slot imcreases the
pressure gradient and moves it more aft than the plain one, so that the
pitching moment is increased.
AS Stealth Pilot posted, the purpose of the slot is to energize the flow
over the flap.

A better question is, what are the relationships between center of
pressure (CP) and center of lift (CL)?

Do they differ?

Why do they differ?

Under what conditions do they coincide?

CP is the term most often used, implying that it's the same
thing.

As far as pitching moment, there are other considerations
besides CP movement. In the 150 and 172 the nose pitches UP when flaps
are deployed, not down. The downwash off the flaps strikes the
stabilizer and raises the nose, demanding nose-down trim. The position
of the stab is a bit different in other Cessnas like the 180/185 and
the nose pitches down instead.

Dan



I realize that I was just looking for the difference between plain and
slotted all other things being the same.

I think I found a clue the answer on the EAA design spreadsheet for
surface sizing. It has a wing moment coefficient box and allows a
factor of "-2" for plain flaps and "-3.5" for slotted. This would seem
to imply that the CM increase for a slotted flap is about 75% more than
a plain one.

John

One more thing: Slotted flaps don't necessarily increase wing area.
Fowler flaps do that. A slotted flap may actually decrease wing area
(by shortening the chord line) as it deploys. It depends on how far
below the chord line the hinge point is. Fowlers, by definition, move
back considerably before their angle increases much. They run on
rollers in a track rather than on a fixed hinge point.

Dan

J.Kahn wrote:
wrote:
On Apr 14, 1:59 pm, john smith wrote:
In article ,

"J.Kahn" wrote:
Does the presence of the
slot in the flap move the overall wing CP forward or aft vs the
unslotted flap at a given angle? This is what I'm having trouble
finding out. I would think that the presence of the slot imcreases the
pressure gradient and moves it more aft than the plain one, so that the
pitching moment is increased.
AS Stealth Pilot posted, the purpose of the slot is to energize the flow
over the flap.

A better question is, what are the relationships between center of
pressure (CP) and center of lift (CL)?

Do they differ?

Why do they differ?

Under what conditions do they coincide?

CP is the term most often used, implying that it's the same
thing.

As far as pitching moment, there are other considerations
besides CP movement. In the 150 and 172 the nose pitches UP when flaps
are deployed, not down. The downwash off the flaps strikes the
stabilizer and raises the nose, demanding nose-down trim. The position
of the stab is a bit different in other Cessnas like the 180/185 and
the nose pitches down instead.

Dan



I realize that I was just looking for the difference between plain and
slotted all other things being the same.

I think I found a clue the answer on the EAA design spreadsheet for
surface sizing. It has a wing moment coefficient box and allows a
factor of "-2" for plain flaps and "-3.5" for slotted. This would seem
to imply that the CM increase for a slotted flap is about 75% more than
a plain one.

John
What's the URL for the EAA design program--(I'm a member) please, thank
you!!--Jerry

Jerry Wass wrote:
J.Kahn wrote:
wrote:
On Apr 14, 1:59 pm, john smith wrote:
In article ,

"J.Kahn" wrote:
Does the presence of the
slot in the flap move the overall wing CP forward or aft vs the
unslotted flap at a given angle? This is what I'm having trouble
finding out. I would think that the presence of the slot imcreases
the
pressure gradient and moves it more aft than the plain one, so that
the
pitching moment is increased.
AS Stealth Pilot posted, the purpose of the slot is to energize the
flow
over the flap.

A better question is, what are the relationships between center of
pressure (CP) and center of lift (CL)?

Do they differ?

Why do they differ?

Under what conditions do they coincide?

CP is the term most often used, implying that it's the same
thing.

As far as pitching moment, there are other considerations
besides CP movement. In the 150 and 172 the nose pitches UP when flaps
are deployed, not down. The downwash off the flaps strikes the
stabilizer and raises the nose, demanding nose-down trim. The position
of the stab is a bit different in other Cessnas like the 180/185 and
the nose pitches down instead.

Dan



I realize that I was just looking for the difference between plain and
slotted all other things being the same.

I think I found a clue the answer on the EAA design spreadsheet for
surface sizing. It has a wing moment coefficient box and allows a
factor of "-2" for plain flaps and "-3.5" for slotted. This would
seem to imply that the CM increase for a slotted flap is about 75%
more than a plain one.

John
What's the URL for the EAA design program--(I'm a member) please, thank
you!!--Jerry

It's on the Sport Aviation page on the EAA website. You don't have to
go through the member login. Different spreadsheets are sprinkled among
the links as you scroll down.

Cheers

John