Could anyone explain the meaning and/or the effect of the gap between a flap and the main wing? I noticed that it differs from airplane to airplane. Last weekend for example I was flying to London and back (living in Europe it is just a small hop) in two different types. The smaller one had no gap at a small flap set, but in total extention there was a gap. The bigger one on the way back had no gap in small offset and in total extention.

Now I was wondering what the effect is of the gap between the flap and the main wing. Is it there so the airplane will still get more lift, but because of the gap it won't slow down that much? Or is it something else?

I'm not positive, but I believe it is to keep the plane from floating too much, but still slow it down for landing.

Arvid,

I think you are talking about slotted and plain flaps...

Slotted flaps (which have a "gap") produce more lift than plain flaps (which don't).

You also get Fowler flaps which are like slotted flaps except they bodily slide back on guides and generally have less angle of deflection than slotted flaps.

check this out

http://www.centennialofflight.gov/es...es/Tech6G3.htm

http://www.centennialofflight.gov/es...ices/Tech6.htm

Flaps inrease lift co-efficient, increase drag, and reduce stalling angle of attack they effectively increase alpha by inreasing the camber of a wing.

Without them 747's would need massively more powerful engines, longer runways, and no cargo or passengers to take off!.

Arvid,

It depends on the particular design of the flap. In some cases, the gap might be incidental, and have little effect. In many cases, flaps are specifically designed with the gap. In these cases, the gap is carefully designed so that the air that flows through the gap keeps the flow on the flap attached, even when the wing is at high angles of attack.

banktoturn

Banktoturn got it. The intention of the gaps is to keep the airflow attached to the flap. This allows the flaps to have higher deflections and the wing to fly at higher angles of attack. This reduces landing speed more than a plain flap would. The downside is the mechanics are much more complicated increasing the risk of failure and weight.

@destructiveTester:
I knew about the function of the flaps, but was not familiar with all the different kinds of flaps and was wondering about the gap. I am working on a Corsair which has a significant gap after extention, so I was wondering. As said by bank to turn, this gap is probably used to let the air stay on the flap while it goes completely down. Thanx for the explanation.

My father just found another site:
http://142.26.194.131/aerodynamics1/controls/Page5.html

Explanation even in more detail including the working of front slots.

The Corsair has plain flaps. The "gaps" between the flaps is necessary because of the gull wing.

I've built a plane with 30% full span split flaps (max deflection 60 degrees), and a plane with 25% full splan slotted flaps (max deflection 40 degrees).

Unfortunately the slotted flap plane only lasted 13 flights, so am not able to comment on max angle of stall with a slotted flap.

Leading edge slots do help max alpha at stall - rule of thumb about + 8-10 degrees - think this topic has been discussed before - I posted a pic of a fixed and retractable slot design a while back.

Certainly the max angle of stall is really reduced by the split flap - it is really disconcerting. My plane flies along with the engine on half power (an SC 53 4 stroke driving a 12 x 6 on a 5 pound plane 10%nitro), and its barely above walking pace, the AofA of the wing is virtually 0 degrees, the flaps on 60 degrees, the plane sinks like a stone. And the stall is unexpected because the AofA of the plane/wing is so low. The effective AofA of the wing is actually quite high of course because of the flap deflection.

I am not sure if slotted flaps increase stall AofA beyond a plain flap/split flap... Any ideas anyone? surely stall is to do with the boundary layer separating on the upper surface towards the leading edge? How would this be affected by the trailing edge flap?

Boiling it down to some very simple generalizations...


Slotted and/or Fowler flaps increase lift (and drag) for approximately the first 20 degrees of travel, and add mostly drag alone, after that. (from 40 degrees to your extreme 60 degrees) Depending somewhat on their chord, they inpart an increased nose DOWN pitching moment. Leading edge devices (slats and leading edge flaps, like on an F-16) increase the camber, and induce a nose UP pitching moment, while lowering the mimimum stall speed.

I put together a section on my site devoted to all this...
See tp://homepage.mac.com/mikejames/rcu_flaps_site/index.html

Thank you all for this input!

@Mike, cool site you've got there. Now we finally can see how they do it in real live and very nice how to make this in RC world!

You're welcome. My pleasure to help, since a lot of people have helped me over the years!