Slotted Flaps Pitching Moment

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