Gurney Flap

[BobMo]

Has anyone experimented with the Gurney Flap? Particularly, the addition of the angle with 1.5 percent of cord. This seems to be the recommended ratio. Would be interested in results for a wing without flaps. Trying to lower stall speed for a LSA.

Thanks,
Bob

[Ron Blum]

Bob: I've used Gurney flaps a lot. They're used to fix problems. For some of the OEMs they're used to correct minor roll issues as the flaps are repositioned and to make control surfaces more dynamically stable (and center better ... which also increases hinge moment/control forces). I doubt you'll change stall speed significantly. The Gurney makes 2 changes: 1) it thickens/blunts the trailing edge (good for some high-speed airfoils) and 2) changes the camber/mean chord line. They normally don't add a lot of drag because they are in the boundary layer flow (or removing some of it). A better (newer) or higher lift airfoil or more wing area is a better/more efficient answer if it's a new airplane.

If it's an existing airplane, go for it. It's an easy modification: thin aluminum angle, double-sided taped to the lower surface and covered in Mach 1 tape (aluminum tape). If it doesn't work, you've spent $20 in materials and a little flight time ... well worth the learning experience.

[martymayes]

Cessna used a Gurney flap on the "B" model Caravans to help reduce the stall speed as the original configuation didn't meet the reg. requirements for stall speed. Have no idea what percent of chord they used but from memory I think it hung down appox. 3/4" to 1". They also used vortex generators on the leading edge radius of the flap. From my understanding, those 2 aerodynamic changes plus increasing the idle speed of the engine brought the airplane into regulatory compliance. Not sure how much reduction they needed so I can't tell you how effective the gurney flap by itself was. If there is a "B" model Caravan at your airport, check it out.

[Matt Gonitzke]

I was looking at a C162 last night, and it had them on the elevator trim tab.

[Ron Blum]

I was looking at a C162 last night, and it had them on the elevator trim tab.

If they are on both sides (top and bottom in this case), it is to center the elevator surface and/or increase stick force gradients (both will happen, but you may only be needing one of those characteristics). Many Cessna Citation rudders will also have them ... for the same purpose.

[Max Torque]

There's a NACA report or two on Gurney flaps that has lots of good info. (Had it saved, but external drive crashed.) In a nutshell, they're actually quite effective and with a low drag penalty for what you're looking at. Vortex generators - properly placed - and Gurney flaps would definitely be a help. Harry Riblett has an excellent book about airfoils (GA Airfoils - sold at EAA store) that has all sorts of good info in it. Harry has helped many people with airfoil related problems. Theory of Wing Sections by Abbott and Von Doenhoff is another excellent book (but a dry read).