Flap design question

I have been looking at some of the pics people at Oshkosh have been
posting and was wondering
about the design of the flaps on some of these planes. Instead of the
whole trailing edge of the wing
hinging down like most planes I am familiar with, only the bottom skin
of the trailing edge hinged down
while the top surface was stationary. I was wondering what the
advantages and disadvantages of this
flap design are.

--
Chris W

"They that can give up essential liberty
to obtain a little temporary safety
deserve neither liberty nor safety."
-- Benjamin Franklin, 1759 Historical Review of Pennsylvania

Hi,

They are called "split flaps". Without going into a long winded
explanation, the basic advantage is that they tend to be structurally
stronger for a given level of mechanical complexity than many other more
complex flap configurations. As a result, they can be generally extended at
a higher speed than plain, Fowler, slotted, etc. designs implemented on
"Experimental home-builts". If you crawl around aircraft at "AirAdventure",
Sun-N-Fun", or one of the other major EAA fly-ins you can see that the
extension mechanisms tend to be simpler and more direct than other designs.
Also, since they don't interrupt the top surface of the airfoil there is no
"air leakage" from the bottom of the airfoil to the top of the airfoil at
the nose of the flap. When they are carefully designed and fabricated, the
smooth mechanical interface with the wing will produce less parasitic drag
when they are retracted than plain flaps.

The whole purpose of "flaps" is to increase the effective camber of the wing
which increases lift at slower speeds. This, of course, increases the
induced drag and decreases the critical angle of attack of the now
"modified" airfoil. Many general aviation flap designs can "droop" far
enough where a point is reached where the combination of induced drag and
parisitic drag is greater than the additional lift gained and then the
hyper-extended flap acts as a speed-brake and permits a greater nose-down
angle without gaining speed.

In general, the disadvantage of a simple split flap is that the amount of
additional "lift" versus "drag" that is created when they are extended is
less than with other more complex designs.

If you are interested in seeing some "numbers", there are studies published
comparing the effects of different flap designs when used with specific
general airfoil designs. Do a Google.com "advanced search" with "split
flaps" as an exact phrase and "effect", "effects", and "comparison" in the
"any of these words" modifier field. You will find a ton of information
with graphs.

J


"Chris W" wrote in message
...
I have been looking at some of the pics people at Oshkosh have been
posting and was wondering
about the design of the flaps on some of these planes. Instead of the
whole trailing edge of the wing
hinging down like most planes I am familiar with, only the bottom skin
of the trailing edge hinged down
while the top surface was stationary. I was wondering what the
advantages and disadvantages of this
flap design are.

--
Chris W

"They that can give up essential liberty
to obtain a little temporary safety
deserve neither liberty nor safety."
-- Benjamin Franklin, 1759 Historical Review of Pennsylvania