Author Topic: Gurney flaps (Read 3495 times)

Brett Buck · « **on:** June 11, 2012, 05:36:23 PM »

Has anybody, to our collective knowledge, tried a deployable Gurney flap on a stunt/combat airplane? Is the boundary layer so thin that it would take micrometer deployment accuracy to keep it from screwing you up?

Brett

Howard Rush · « **Reply #1 on:** June 11, 2012, 06:18:15 PM »

I put a fixed Gurney flap on a combat plane. About the only effect I saw was a whole lot of pitching moment. Boundary layer is fairly thick at the back of the wing. I posted some stuff awhile back on both issues.

Brett Buck · « **Reply #2 on:** June 11, 2012, 11:38:16 PM »

Quote from: Howard Rush on June 11, 2012, 06:18:15 PM

I put a fixed Gurney flap on a combat plane. About the only effect I saw was a whole lot of pitching moment.

That's what they tell me. Could it be that you ran out of control authority because of the pitching moment and thus couldn't get any advantage from the increased camber?

I was just doing a thought experiment on how one might implement a movable Gurney flap - like a single plate that moves vertically out of either the upper or lower surface as the controls move, and is maybe flush otherwise.

BTW, I have tried a rudimentary self-actuating LE slat. You don't want that.

Brett

Howard Rush · « **Reply #3 on:** June 12, 2012, 12:42:26 AM »

Quote from: Brett Buck on June 11, 2012, 11:38:16 PM

That's what they tell me. Could it be that you ran out of control authority because of the pitching moment and thus couldn't get any advantage from the increased camber?

I would have pushed it to stall in both directions, taking whatever measures it took to get there, so I doubt it.

I'd just have a slab about 1/4" high, center it at neutral, and have it stick out all the way at full control. That is a wimpy version of the flaps on the aircraft held by the clown in the left margin.

Roger on the HP slat.

Tim Wescott · « **Reply #4 on:** June 12, 2012, 08:41:45 AM »

You mean a Zaparka flap (see what Wikipedia does for all human knowledge?).

I suspect that it would engage with a "bang", or at least that it would have an action that would be very soft around neutral, and come on line over a small range as the angle of the flap approached 90 degrees. I also suspect that in order to really enjoy its effects you'd want it to deploy fully before you reach full elevator deflection yet not go past 90 degrees, which would complicate your mechanism.

The Wiki article also mentions that the flap dimensions are dependent on boundary layer size, so you'd have to experiment somewhat.

What I found interesting was that a double Gurney flap does good things on thick airfoils -- I'm wondering what one would do on a flapless design (probably not nearly enough to justify not having real flaps).

Tom Niebuhr · « **Reply #5 on:** June 12, 2012, 11:08:52 AM »

Indy cars are running about 230 mph on the straightaway. They use the "Wickerbill" or Gurney flap. In this case they are about 1/8" high. Velocity is part of the equation, so I wonder if a double Gurney flap would have any drastic effect at all at our lower velocity. Satisfactory protrusion height will require a bit of experimentation.

I will be trying P.J. Rolands vortex generator on my next stunter. I suspect that this might be a more favorable approach. Although in both cases it remains to be seen for any particular airplane.

Howard Rush · « **Reply #6 on:** June 12, 2012, 12:38:41 PM »

A virtue would be the absence of hinge moment.

dirty dan · « **Reply #7 on:** June 13, 2012, 12:41:20 PM »

My opinion here is that after all these years I would still be in favor of Dan Gurney for president.

Remember, his one and only campaign promise was to pass a nation-wide law requiring all vehicles to run open-exhaust on one specific day each year. Of course today we would have to have an exemption for cars such as Howard's Pious, what with the truly sad noise one of these cars would make even with gas engine engaged and no muffler fitted.

Wait! Let's require monster megaphones to replace the mufflers...

Dan

Shultzie · « **Reply #8 on:** June 13, 2012, 05:07:17 PM »

Quote from: dirty dan on June 13, 2012, 12:41:20 PM

My opinion here is that after all these years I would still be in favor of Dan Gurney for president.

Dan

Dan and Mike Potter...If you guys are lurking? Do you remember my pristene old dark Green 67 Mercury Cougar...MIKE! JUST A KUDO' FOR DOING SUCH A GREAT JOB OF REPLACING THAT TIMING CHAIN for us....those eve's spent in our old garage in our first little $24,000.00 Boeing Bedroom starter home at 42nd Ave S. near TJ high school?
NUTTIN WAS COOLER THAN THE SOUND OF AN OLD 67 V8 MERCURY COUGAR!!!

L0U CRANE · « **Reply #9 on:** June 14, 2012, 01:55:06 AM »

Hmmm, I seem to remember a Chapparal - or "Flap-paral"* - at the Nuerburgring about 1964 or 1965. Big bore Chev V-8 and automatic trans. Sounded different; great but different...

Buncha guys named 'Giovanni' played different tunes in red cars...

And, yes, IT WAS IN THE EARLY 1960'S... and I was there. And if it wasn't Dan in it, it was most likely Jim Hall.

* - Chapparal with a wing mounted on two posts, behind the driver, that 'deployed' to downward AoA on braking. Also added grip whilst deployed.

phil c · « **Reply #10 on:** June 25, 2012, 08:13:55 AM »

`Gary James has some info that squaring off the rear of the airfoil to 2-4% of chord does positive things for performance. According to the Xfoil testing it generates a vortex along the trailing edge the increases lift and reduces separation at higher angles of attack.

Reminiscent of the Lou Andrews advice on the Barnstormer to make sure that the trailing edge of the fixed flaps should be square with sharp edges. No tapering allowed.

PJ Rowland · « **Reply #11 on:** June 26, 2012, 01:35:11 AM »

I want to read this :

Storms, B.L.; Jang, C.S. (1994). "Lift Enhancement of an Airfoil Using a Gurney Flap and Vortex Generators". Journal of Aircraft 31 (3): 542–547. DOI:10.2514/3.46528. Retrieved 2007-07-05.

However I cannot find the article.. I'm hoping Howard has a copy.

I do not understand enough of the Gurney Flap idea beyond appearing to increase camber - Do not we acheive similar with the Flaps moving the opposite way? Increasing Camber.

Tom : I look forward to your results, Glad your VG Kit arrived.

Douglas Ames · « **Reply #12 on:** June 28, 2012, 07:42:05 PM »

Quote from: PJ Rowland on June 26, 2012, 01:35:11 AM

<snip>
I do not understand enough of the Gurney Flap idea beyond appearing to increase camber - Do not we acheive similar with the Flaps moving the opposite way? Increasing Camber.

Here's Wiki's explanation - http://en.wikipedia.org/wiki/Gurney_flap

In regards to a stunter you'd need a secondary flap and hingeline with pushrod operated linkage to it at a higher ratio.

If you fiddled with the geometry I think you could get the Gurney Flap at 90 deg to the airstream when the main flap is at 30 ?

Just a thought.

TomLaw · « **Reply #13 on:** June 28, 2012, 08:16:11 PM »

http://www.dept.aoe.vt.edu/~mason/Mason_f/AIAA2007-4175.pdf
http://www.scribd.com/doc/60104227/Gurney-Flap

Howard Rush · « **Reply #14 on:** June 28, 2012, 09:09:42 PM »

Quote from: Douglas Ames on June 28, 2012, 07:42:05 PM

Here's Wiki's explanation - http://en.wikipedia.org/wiki/Gurney_flap

In regards to a stunter you'd need a secondary flap and hingeline with pushrod operated linkage to it at a higher ratio.

If you fiddled with the geometry I think you could get the Gurney Flap at 90 deg to the airstream when the main flap is at 30 ?

Just a thought.

That's a clever mechanism.

That would have the unfortunate side effect of a heap of hinge moment. PJ and I use the secondary flap on a small chunk of span, but drive it the other direction to relieve hinge moment. You might be able to reduce the size of the main flap a lot and make it OK. You could even eliminate the main flap.

FLOYD CARTER · « **Reply #15 on:** July 13, 2012, 01:52:19 PM »

Howard's flap thingie seems to be just a dynamic trim tab, to reduce flap actuating force. Seems to me that we actually want an increasing restoring force with control movement. Doesn't that provide pilot feedback?

Floyd

PJ Rowland · « **Reply #16 on:** July 13, 2012, 07:12:48 PM »

Floyd, we use it to reduce the forced feedback from the flaps.

Howard Rush · « **Reply #17 on:** July 13, 2012, 10:10:12 PM »

Quote from: FLOYD CARTER on July 13, 2012, 01:52:19 PM

Howard's flap thingie seems to be just a dynamic trim tab, to reduce flap actuating force. Seems to me that we actually want an increasing restoring force with control movement. Doesn't that provide pilot feedback?

Yes, it would. More to come on this.

Peter Germann · « **Reply #18 on:** April 29, 2014, 10:03:29 AM »

Tested a Gurney flap today, glued to TE:
10 mm (0.4") wide, protruding 3 mm (1/8") top & bottom
No effect felt on lift and/or pitch moment
laptime 0.3 sec slower (5.2 up to 5.5) when running same governed RPM
Peter Germann

Curare · « **Reply #19 on:** April 30, 2014, 11:29:55 PM »

This may not be of any particular use, but after Peter Germann's comments I was reminded of something. I have fitted a form of Gurney flap to overly slipper F3A models.

I had a model that halfway through the schedule would overspeed through a series of vertical downlines to the point that when I got a specific maneuver (I think it was cuban eight or something) I felt like the model was doing 300 miles an hour, and I had no time to think.

What started as a joke degenerated into a fixture of the model. I taped two popsicle sticks to the trailing edge of the wing, close to the fuselage. these were aligned with the trailing edge, but had the faces perpendicular to the wing centreline, so in effect, I had 3-4mm protrusions above and below the trailing edge. These were fixed (not attached to the ailerons but a fixed part of the wing). The effect was striking. Downline speed was much more predictable, and no change in trim was noticed.

After that I made a set of carbon 'popsicle sticks' and they've been fitted to the plane ever since.

With that said, from Peter's experiments and my, I believe a large trailing edge thickness may go a long way into stopping a model overspeeding, or at least creating a more constant speed, no?