Slotted Flaps Pitching Moment

[J.Kahn]

Been researching this but can't find a direct answer:

Is the increase in nose down pitching moment from deployment of flaps
any more for a single slotted flap vs a plain flap, all other factors
being the same? If a design that used a plain flap was switched to
slotted while keeping the areas etc, the same, is more tail volume
required to compensate?

John

[john smith[_2_]]

In article >,
"J.Kahn" > wrote:

> Been researching this but can't find a direct answer:
>
> Is the increase in nose down pitching moment from deployment of flaps
> any more for a single slotted flap vs a plain flap, all other factors
> being the same? If a design that used a plain flap was switched to
> slotted while keeping the areas etc, the same, is more tail volume
> required to compensate?

As I understand aerodynamics, the sloted flap is a means of increasing
the wing cord by using less material.
It is the increase in the cord width which allows the center of lift to
move aft, resulting in a pitch down moment.

[Stealth Pilot[_2_]]

On Fri, 13 Apr 2007 19:58:24 -0400, john smith >
wrote:

>In article >,
> "J.Kahn" > wrote:
>
>> Been researching this but can't find a direct answer:
>>
>> Is the increase in nose down pitching moment from deployment of flaps
>> any more for a single slotted flap vs a plain flap, all other factors
>> being the same? If a design that used a plain flap was switched to
>> slotted while keeping the areas etc, the same, is more tail volume
>> required to compensate?
>
>As I understand aerodynamics, the sloted flap is a means of increasing
>the wing cord by using less material.
>It is the increase in the cord width which allows the center of lift to
>move aft, resulting in a pitch down moment.

for the slotted flap that makes sense.

the position of the lift is an average of all the pressures on the
wing. if you increase the camber by bending down a part of the back of
the wing you change the gas flows which changes the pressure
distributions which moves the resultant back toward the new activity.

a wittman tailwind has a simple flap about 4~5 inches wide which
causes more of a pitch change than a slotted fowler flap on a cessna
150.

I would think that the factors are camber, aerofoil section
characteristics, whether the "tail volume" was already marginal and
how much trim effectiveness there is in your system.

it would probably need to be tested but this should be none too
traumatic because you can pull the flaps off in a hurry if you need
to.
One thing you would need to look at is whether your max flaps speed
changes with the new flaps and whether you have upset the flutter
dynamics of the wing.

Stealth Pilot

[J.Kahn]

Stealth Pilot wrote:
> On Fri, 13 Apr 2007 19:58:24 -0400, john smith >
> wrote:
>
>> In article >,
>> "J.Kahn" > wrote:
>>
>>> Been researching this but can't find a direct answer:
>>>
>>> Is the increase in nose down pitching moment from deployment of flaps
>>> any more for a single slotted flap vs a plain flap, all other factors
>>> being the same? If a design that used a plain flap was switched to
>>> slotted while keeping the areas etc, the same, is more tail volume
>>> required to compensate?
>> As I understand aerodynamics, the sloted flap is a means of increasing
>> the wing cord by using less material.
>> It is the increase in the cord width which allows the center of lift to
>> move aft, resulting in a pitch down moment.
>
> for the slotted flap that makes sense.
>
> the position of the lift is an average of all the pressures on the
> wing. if you increase the camber by bending down a part of the back of
> the wing you change the gas flows which changes the pressure
> distributions which moves the resultant back toward the new activity.
>
> a wittman tailwind has a simple flap about 4~5 inches wide which
> causes more of a pitch change than a slotted fowler flap on a cessna
> 150.
>
> I would think that the factors are camber, aerofoil section
> characteristics, whether the "tail volume" was already marginal and
> how much trim effectiveness there is in your system.
>
> it would probably need to be tested but this should be none too
> traumatic because you can pull the flaps off in a hurry if you need
> to.
> One thing you would need to look at is whether your max flaps speed
> changes with the new flaps and whether you have upset the flutter
> dynamics of the wing.
>
> Stealth Pilot

Well, as I understand it the slot is simply to energize the upper
boundary layer to keep the flow on top attached at higher angles, same
as a leading edge slat. Now the leading edge slat, when deployed, moves
the CP forward and decreases pitching moment. 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.

John

[john smith[_2_]]

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?

[Peter Dohm]

"john smith" > wrote in message
...
> 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?

There are a lot, if not all, of the old NACA reports online these days on
the NASA Technical Reports Server at:
http://ntrs.larc.nasa.gov/search.jsp?N=279&Ne=25

I got to the following by simply entering the search term: slotted flap
http://ntrs.larc.nasa.gov/search.jsp?N=279&Ntk=all&Ntx=mode%20matchall&Ntt=s
lotted%2Bflap (If that URL wraps, you may have to cut-and-paste or run the
search again.)

If you don't find the info on NTRS, the next easiest method would probably
be to built a couple of mock-ups and project each of them well outside of a
car at a time when there is no wind; such as is frequently the case in early
morning. (As you may have guessed, I am too lazy and cheap to built a wind
tunnel--and I suspect intuitively that this is the sort of test in which
interference from the floor and ceiling of the tunnel would have a major
influence.)

Peter
(Not an aerodynamicist)

[[email protected]]

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

[J.Kahn]

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

[[email protected]]

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

[jerry wass]

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

[J.Kahn]

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