View Full Version : Gurney Flaps (again)

04-05-2004, 10:55 PM
This topic has been discussed before, and I read all the responses, but, I have a few questions.
I did a Google search and read most of the information I received. Most of the wind tunnel test and research done on Gurney flaps were done on "go fast" planes. I'm not really interested in increased lift on a Cub wing in level flight. I'm interested in slow flight, as in when landing.
1. What does it do when installed on the Cub flap when it is deployed?
2. Does it give more roll control if it is installed on the ailerons?
3. Would it give you more pitch control if they were installed on the elevators?
All comments are welcome. Thanks.

Jerry Burr
04-05-2004, 11:51 PM
Hi T.J. I know this is a zoomy go fast, but a Spitfire has them on the top and bottom of the trailing edge of the elevators. Jerry.

04-06-2004, 11:29 AM
I read in the older posts that you were going to try them on your plane. Did you get around to it yet?
I went by Waynes and we talked slats and Gurneys. I was impressed with the slat video he has.

Steve Pierce
04-06-2004, 12:12 PM
I looked at a Mk48 Sea Fire (Navy version of Spitfire) today and didn't see anything on the elevator except what appeared to be a trim tab. What do Gurney flaps look like?

04-06-2004, 12:29 PM
I read that a gurney flap is essentially a flat tab extruding into the airstream on the lower side towards the trailing edge of a wing. Is this the proper definition?

04-06-2004, 02:25 PM
More or less. They are mounted at the trailing edge of an airfoil, perpendicular to the chordline of the airfoil on the high pressure side (which is why they are on the top side on automotive spoilers and usually on the bottom side on wings). Their height is usually made from 1.0% to 2.0% of the chord. So on a Cub with it's 63" chord, they would usually extend downward between 5/8" and 1 1/4". If I were doing one, I'd probably make it somewhere between 0.75" and 1.0" in height. If you mount them with rivnuts, you can remove and swap them out easily. When removed, they look like a long piece of aluminum 90 degree angle.
All the best,

ex-fr8 dog
01-04-2005, 03:46 AM
Bell 407 has a Gurney on the vertical fin, red line IAS 140 kts. not just go fast airplanes.

Jerry Burr
01-04-2005, 08:43 AM
Most of the Bells also have one on the horizontal stabilizer (s). It's a aftermarket mod. Don't tell Crash. Jerry. :D

Bill Cangero
02-03-2005, 08:22 AM
I remember reading about an outfit in New Jersey that tows banners with Super Cubs. Supposedly they had STCs for the Gurney flaps.

Can anyone find them?

06-18-2015, 02:57 PM
Gurney Flaps Research, Design, Installation and Flight test
OK I've been hearing about these flaps long enough. My curiosity got the better of me so here goes.
I have learned second hand that a certain member of SC.org from Maine uses them on his ailerons while in the contests at Greenville. His report is that the ailerons become heavier with them installed. That leads me to believe that they are producing extra lift. I have flown both the 185 & 206 with drooping ailerons and they are heavier while producing the extra lift. That tells me that when ailerons produce more lift they are heavier. Thus the gurney produces lift.
Next the following data was reviewed:




Now when reading this type of information my eyes and brain cloud over before I finish page one. After absorbing this data and performing some extensive engineering calculations while consulting with my PWAG (Pete's Wild A$$ Guess) I came to the conclusion that a section of 2024-T3, 48" long by .025" thick with a 17/32" flange was the correct size and materiel. [I have a sheet of 2024-T3 x .025" in stock and also have a 4 foot shear and brake.] The 17/32" dimension seemed to be the most efficient (least drag/most lift) according to the above studies. 0.832% of chord (63-3/16") = 17/32" The protrusions of the Dragonfly wing trailing edge were too much work for a simple experiment. This angle was installed on the lower trailing edge of the outboard 4 feet on the right flap using the latest high tech fastening materiel (Duct tape). 8"' lengths of yarn were taped every foot top and bottom along and just ahead of the Gurney. My flap is 110" long and I had previously tuft tested the right flap so had an airflow history. I chose the outboard section outside of the prop wash in order to maximize the effect for flight test purposes.

The first take off was done with one notch (20 degrees) of flaps. The extra lift or drag of the Gurney raised the right flap and drooped the left flap 2" lower. This is due to the flap cable set up in the Cub which allows one to go down while the other goes up while partially deployed.

In normal cruise, smooth air, 2500 feet the plane was left wing heavy by 1/2" aileron deflection (Gurney was producing that much lift). Stall speed power idle Flaps 0- 42 mph IAS, Flaps 56- 32 mph IAS.
Dive to 140, ball was out one ball to the left.

Landed, removed Gurney and fly again. Wing heaviness was gone. Stall speed power idle Flaps 0- 44 mph IAS, Flaps 56- 32 mph IAS. Dive to 140 ball was out 3/4 ball to the left. Less drag without Gurney.

Since this clean 2 mph stall reduction was with Gurney only on the right side, is it safe to assume that a full Gurney installation will produce a 4 mph stall reduction? Maybe, certainly more than 2 mph. That's a 4.7% - 9.5% reduction in clean stall speed, not bad!

Cruise speed was the same with and without the Gurney. 102-105 mph. 20.5" mp @ 2400rpm, 8.2 gph.

Elevator trim was 1/8" more nose up with Gurney installed. This indicates that the Gurney was moving the center of lift slightly aft.

The yarn showed a little turbulence behind the wing with the Gurney and smooth air flow without the Gurney. This verified a small drag component.

It was noted that the directional stability was a little better with the Gurney installed. This was an unexpected welcome discovery.

Do the Gurneys work? Yes.
Are they a worth while modification? Yes for a small (2 - 4 mph) reduction in stall speed for Cubs with no flaps.
Will I install them on my Cub? Maybe, I might use a 4 foot section on the outboard ends of both flaps for improved stability purposes.

06-19-2015, 05:54 AM
Thanks Pete:lol:


Dave Calkins
06-21-2015, 11:06 AM
Pete and others. There is another thread here at sc. Org where I posted findings and photos of my efforts with Gurney flaps. If I could find that post, I would link yu to it. It was an unflapped exp Cubby. A sweetheart to fly!

They work. They increase stick forces when added to ailerons. I removed them from ailerons as it took the sweetness out of flying that cub because the Gurneys are pushing the aileron trailing edge up.

Ps. One-sided testing as described above is not the best way to test ( i am trying to state this nicely, please take that gently)


Dave Calkins
06-21-2015, 11:09 AM

there is great discussion there also. From our own Jim C. He knows.

Also I see that Pete (skywagon8A) commented there. My apologies. C U

06-21-2015, 07:02 PM
Willyb are you telling stories:cheers.

06-21-2015, 07:56 PM
I think this is what they are supposed to look like, the plane they were on never got to compete as it was TOO windy on Saturday for safety and that was about the only plane flying and it had gone home on Sunday by the time the competition took place. 206392064020641

06-22-2015, 05:42 AM
....Ps. One-sided testing as described above is not the best way to test ( i am trying to state this nicely, please take that gently)

That's OK Dave. Didn't you ever carry cargo on one float? Snow shoes tied to a jury strut? It depends on what you wish to learn, whether to test on one side or two. I have done one sided tests in the past for FAA certification purposes. For example did you know that the Seabee had to demonstrate one flap down and one flap up take off and landings in a cross wind? I did these same tests for the Twin Bee with a four foot longer flap. I did the spin tests for Lake when they certified the aux fuel tanks in the floats. The spins were done with fuel in one float only. Another time I hit a bird with one float on my Lake. While it was removed for repair I decided to find out how it handled with just one float. It was normal except being 5 mph slower. When I removed the second float the speed went back to the same as when both floats were installed. An interesting observation.

So, One sided testing is sometimes done. The particular question that is seeking an answer is evaluated beforehand. In the case of the Gurney flap this was the easiest method to determine the amount of lift and drag produced by a 17/32" x 4 foot angle mounted mid span on a Cub wing.

06-22-2015, 10:22 AM
I remember Jerry Burr telling us at lunch in LHV that if you want to see how well you wing mod works to take it off one side and see which wing stalls first


Dave Calkins
06-23-2015, 10:46 AM
I remember Jerry Burr telling us at lunch in LHV that if you want to see how well you wing mod works to take it off one side and see which wing stalls first


I remember his statement differently.

Also, in Petes test, he mentioned that he had to hold some amount of aileron to counteract the increased lift of the gurney'ed wing. I would prefer to answer the question more succinctly than that.

I mean absolutely no disrespect to Pete Skwagon. He has done more in aviation than I have. And I have a small brain rattling around in a large head

06-23-2015, 12:54 PM
There was no disrespect noted on my part. Sometimes I run off at the mouth or typing fingers more than necessary with extraneous rambling. My test was only to prove to myself and willyb whether or not the Gurney does anything yes or no. There really wasn't any intent to find an optimum dimension or shape. The only instrumentation was an airspeed indicator and a slip ball. Everything else was seat of the pants and eyeball observation. My only input as to size and shape were those two documents which I noted. It may have helped if I had a force gauge to measure stick pressures. The 1/2" aileron deflection mentioned was only to show how little aileron was required to offset the lift generated. Actually I was pleased that there was no penalty noted. Since my runway (pond) is a mile long it was perfectly safe to make a one sided installation for an immediate analysis of go or no-go. Actually my Cub will fly very slow, 28 mph IAS, in ground effect with nothing more than extended wings and flaps. Any slower would be dragging the air rudder in the water.

The flaps which you tested in the other thread appear to be between 3/4" & 7/8". Is that close? If so that likely explains your top speed loss when compared to my test with the smaller flap.

I think that a small Gurney would be helpful in those situations when a Cub has half of a moose in the baggage pushing the CG at or beyond the aft limit since the Gurney tends to move the center of lift back.

Dave Calkins
06-24-2015, 11:05 AM
My Gurneys were about 3/4"

Droop ailerons work waaaayyy better.