Tried Gurney flaps yet? What's your conclusion?

[Keith Renecle]

Hi Ken,
So my elevator trailing edges are 4 mm thick, so top and bottom protrude 3 mm, or just less than 1/8" and like I said, the effect is amazing. I guess that this is showing up some kind of basic error on the model if it makes that much difference. I'd just like to know what is actually happening.......technically. I also want to use the Eagle Tree data recorder to check how much power a flight uses and then remove the strips again to see what the difference, if any, there is. This is so interesting!

[Ken Culbertson]

Hi Ken,
So my elevator trailing edges are 4 mm thick, so top and bottom protrude 3 mm, or just less than 1/8" and like I said, the effect is amazing. I guess that this is showing up some kind of basic error on the model if it makes that much difference. I'd just like to know what is actually happening.......technically. I also want to use the Eagle Tree data recorder to check how much power a flight uses and then remove the strips again to see what the difference, if any, there is. This is so interesting!

I have not logged anything, too much trouble on my ESC, and my "scientific" method is to measure battery head room after each flight.  Before the strips I was averaging 30-32% remaining.  With the Gurney strips on the two flights I measured, the headroom on both was 30%.  I don't get concerned unless it gets under 20.  Both flights were in about a 10mph wind so it will use more in really light wind but sill, I don't see any downside.  My question list keeps growing.  Does size matter (amount of elevator covered)?  What is the optimum height?  Is distance from the TE (like Bob's application) make them better?  I am guessing the answer to all of them depends on the plane.  In my case, how much more stable can it get?  I was able to hold it steady within a couple of inches on a domed circle in wind at 4 different heights.  I haven't been able to do that since I was in my 30's!  Now back to the only maneuver that fails to recognize that I am the one flying the plane - the Evil Reverse Wingover.  The Gurney's didn't help it at all....boo. 

[Keith Renecle]

Ha-ha.....the Evil Reverse Wingover! That is always the most difficult one for me too. As we follow the F2B rules here in Darkest Africa, the square eight has the highest K-factor and I've always believed that the reverse wingover should have that. It takes a lot of practice, and I have a natural aversion to that! Everything on your model must be working in sync to hit a good wingover.....including the pilot! Then too, it's the first maneuver and in a serious competition, the nerves kick in, so yes, your name of "Evil Reverse Wingover" is spot on!

I was also thinking that I should try putting a triangular wedge in front of that flat 3 mm bump on each side to see if it changes anything.

[Dave_Trible]

Something you might try:  tape a piece of crepe paper combat streamer to the TE of each elevator- 1 foot long is probably enough.  Go fly and compare.  It should feel about the same.

Dave

[Ken Culbertson]

Something you might try:  tape a piece of crepe paper combat streamer to the TE of each elevator- 1 foot long is probably enough.  Go fly and compare.  It should feel about the same.

Dave

Interesting because that was my reaction when I was asked how it felt.  It felt like the plane was towing a streamer except the corners were better.

Ken

[Keith Renecle]

Something you might try:  tape a piece of crepe paper combat streamer to the TE of each elevator- 1 foot long is probably enough.  Go fly and compare.  It should feel about the same.

Dave

Thanks Dave, that will be interesting too, so I'll try that soon.

Regards,
Keith R

[Paul Van Dort]

3D printed Gunrey flaps. 5 gram each. I can go down to 3 grams I reckon.  Next weekend test time!

[Doug Moon]

Hello Bob H,
I see yours are set off the TE of the elevator and others are putting theirs right onto the back of the TE. Is there a reason yours are offset and should the be or not??

[Igor Burger]

3D printed Gunrey flaps. 5 gram each. I can go down to 3 grams I reckon.  Next weekend test time!

I would say keep them heavy, it can be that problem of the model is dynamic disballance of flaps to elevator making positive feedback to controls in corners (making it hard to keep straight after corner) instead of some aerodynamic black magic. Alex S. has it also on one of his models on WCh in Poland and I had also such model which I needed 5 g weight on each elevator 

[Igor Burger]

Something you might try:  tape a piece of crepe paper combat streamer to the TE of each elevator- 1 foot long is probably enough.  Go fly and compare.  It should feel about the same.

Dave

Right, if stab/elevator does something wrong aerodynamically somewhere around neutral, then ANYTHiNG what can change it will help (turbulators, VGs, sharp LE, blunt LE  etc ...) I even know guy who makes -1 deg one left elevator and +1 on right elevator to avoid it to be "too precise"  ... so that if somethig bad happens, then only on fraction of tail 

[Brett Buck]

Right, if stab/elevator does something wrong aerodynamically somewhere around neutral, then ANYTHiNG what can change it will help (turbulators, VGs, sharp LE, blunt LE  etc ...) I even know guy who makes -1 deg one left elevator and +1 on right elevator to avoid it to be "too precise"  ... so that if somethig bad happens, then only on fraction of tail 

   That's very interesting - it has the net effect of the Gurney flap or other elevator-centering/stabilizing devices, without showing it. A very long time ago, I had a problem with elevator "flutter" on a really poor design (that I came up with...) and ended up putting a prety large aluminum wedge-type device with two vanes feathered in opposite directions. This would likely do the same. It solved the flutter problem - until I folded the stabilizer in inverted level flight in extreme turbulence!  No great loss, the airplane was awful otherwise, and butt-ugly.

   Not to be a wise guy, but the very little experimentation I have done with elevator wedges/Gurney flaps had been very negative - the airplane felt like the elevator was frozen solid, or the controls were bound up, because it took immense effort to break it out of neutral. The issue I always fight with my airplanes (particularly the last 3, which were nearly identical), is getting it "freed up" around neutral, not getting it to track, even without any sort of centering device.

    I tried running the CG back, but then the control pressure VS deflection seemed screwed up, a ton of force to get it to move at all, then, it lightened up as soon as I got any motion. I think that is the intended effect, to try to get some self-centering effect, but (again, not to be a wise guy) I haven't had that problem on any of piped-era airplanes, you let off the pressure, it stops dead, with nothing.

  I note again - I am running a different tail airfoil and planform than most, it is not a flat stab with a pointy LE, it is an airfoiled stab with the high point at the hinge line, and otherwise looks like a stretched-out version of the Green Box Nobler tail airfoil. Contributor or not, I have no idea, but tracking and centering has never been a problem and all the tricky devices like trip strips, turbulators, VGs, elevator wedges, either have no effect or have negative effects.

    I will freely admit that I have have not done enough testing to prove my point, and I could easily by missing something, but so far, my elevator wedge experience has been generally negative, and doesn't seem to solve any problem I have.

        Brett

[Dave_Trible]

Purely an educated guess on my part but I expect the problem encountered/benefit discovered is far more pronounced with ball joint "sticktion" controls and electric airplanes without the benefit of vibration to free the elevator so it can seek its own neutral in the slipstream air.  It's totally up to the pilot to find the sweet spot following a sharp turn and also groove in level flight.  These devices should aid the elevator lock in the right place.

Dave

[Igor Burger]

   That's very interesting - it has the net effect of the Gurney flap or other elevator-centering/stabilizing devices, without showing it. A very long time ago, I had a problem with elevator "flutter" on a really poor design (that I came up with...) and ended up putting a prety large aluminum wedge-type device with two vanes feathered in opposite directions. This would likely do the same. It solved the flutter problem - until I folded the stabilizer in inverted level flight in extreme turbulence!  No great loss, the airplane was awful otherwise, and butt-ugly.

Funny, exactly that was my problem, it was piped model and my flaps started to flatter at some RPM. So I installed 5g weights on both flap tips to "detune" it to lower RPM. It was sucessfull, but model did not fly well. It has mixed feeling of nose heavy/tail heavy. I tried all possible, but only the same weight on elevator helped (on trailing edge of elevator). Funny was that model after installing them (it was 10grams at tail) had feeling of more nose heavy CG position (which was actually sveral mm back).

[Dennis Nunes]

Good morning,

Well, I though I would add my $0.02 to this discussion and share my experience on this topic. See the attached PDF file.


Enjoy,
Dennis

[Crist Rigotti]

Thank you Dennis.  Very informative!

[Paul Van Dort]

Today I tested the gurney flaps. I was disappointed. These are MY findings with MY stuntship. My findings cannot be generalised... : Although these flaps represent about half an ounce extra tailweigth, the ship felt less responsive. loops become bigger and corners need more steering. In the wingover, when there is no pressure on the elevator, some flutter was noticeable acoustically. The little slop in the elevator horn created a vibration noise. The landing was more difficult than normal and the glidepath was shorter. I removed the gurney flaps and had 7 enjoyable flights without them...

[Ken Culbertson]

Today I tested the gurney flaps. I was disappointed. These are MY findings with MY stuntship. My findings cannot be generalised... : Although these flaps represent about half an ounce extra tailweigth, the ship felt less responsive. loops become bigger and corners need more steering. In the wingover, when there is no pressure on the elevator, some flutter was noticeable acoustically. The little slop in the elevator horn created a vibration noise. The landing was more difficult than normal and the glidepath was shorter. I removed the gurney flaps and had 7 enjoyable flights without them...

Interesting.  I wonder if the shape of your elevator with the balancing tabs has anything to do with it.  I can't see the Gurney things causing flutter in a normal application.

Ken

[frank williams]

I think T-strips work for us. Here are some of my thoughts on the subject of T-strips and Gurney flaps.

[Crist Rigotti]

Thank you Frank!

[Ken Culbertson]

I think T-strips work for us. Here are some of my thoughts on the subject of T-strips and Gurney flaps.

Thank you SO MUCH for the technical info. After reading some of it (I am more interested in the conclusions than the math) it appears that a full span T shape that is approximately 1/8" tall, is as thin as possible and perhaps does not have a tapered ramp (no bump before the ridge) is the best starting point, and maybe ending point as well.  I don't know if having it 90 degrees to the flap surface matters which would make the flap a flat "Y" shape on a tapered elevator.  I read that the flap will effectively increase the moment which may explain the better corners and perhaps the better locking (Instantly Fancherized).  The optimal height is probably greater than the 1/8" but the added drag would scare my batteries.  They were OK with the 2% increase from the ones I put on.  I suspect that the drag increase with height is not linear.

Some are trying layers of tape and having some success.  One I know of uses a thick elevator TE which may contribute.  Somehow, after looking at all of the examples I wonder if a couple layers of tape is producing the vortexes sufficiently large to have the necessary "wow" effect. 

Ken

[Dennis Nunes]

I think T-strips work for us. Here are some of my thoughts on the subject of T-strips and Gurney flaps.

Hi Frank,
You posted some excellent "thoughts" as to why Gurney flaps, T-strips, Elevator Wedges, Trip Inducing Plates or whatever you call them, do what they do. Thank you.

I was curious as to the planes you tested on. Were they IC or Electric? The plastic L-shapes of 3.2mm (.125") and 4mm (.157") sounds interesting. For my electric model the original "built-in" plates were .625" tall, with a .25" thick elevator, put the top of the plate at .188" above the surface of the elevators. But, I did put a fillet between the elevator's surface and the front of T-strip on both sides. They looked great. However, on my electric plane they produced a tremendous amount of drag and brought my remaining battery capacity to near zero.

Once I sanded the T-strip off, my lap times went from 5.35 seconds down to 5.00 seconds and the remaining battery increased to 18-20%, changing absolutely nothing, which was significant. So there is a limit one will experience with electric models if they are too tall. Where that "magical" height is I'm not sure. So far the .020" high full length strips of tape has worked remarkably well.


Dennis

[Ken Culbertson]

So there is a limit one will experience with electric models if they are too tall. Where that "magical" height is I'm not sure. So far the .020" high full length strips of tape has worked remarkably well.

Dennis:  I am still fishing for that magic height and whether or not they need to be full span.  My gut tells me that that they need to be high enough to create a vortex in front of the flap TE as well as behind it which gets better with height but not so tall as to kill the battery.  I have only tested a 1/8" above the TE full span "T" so far and I did not experience the huge battery drop I was expecting.  I went from 32% to 30%.  My lap times went from 5.6 to 5.9 (65' lines, active timer).  Aside from the expected benefits, I did not see any downside to the increased lap time.  If anything, it was performing better.   I can't put a finger on it but I think that the timer was reacting to the increased drag.  Increasing my base RPM had no effect.  What I added in base it seemed to take away in boost.  My question to myself is do I need to experiment further. #1 Hunting is gone.  That is why I tried them in the first place.  #2 Corners are tighter with the same control input and still smooth and locked on exit even better than before and #3 I am not overtaxing my batteries.  So perhaps it is time do a victory lap and recertify the pilot on his new and improved toy.

Ken

One thing I noticed in going through Frank's stuff.  The flaps were always directly attached to the TE with nothing in front.  No plate, no wedge.  Could stuff in front of the vertical flap be breaking up the forward vortex and creating unwanted drag?  Just a thought. 

[fred cesquim]

again Bob Hunt leading innovation, thank you!
i tried and improved a lot indeed!
next plnae will have them bench added and painted along the finish.

[Matt Colan]

Awesome work Frank! Gives a lot to ponder!

[fred cesquim]

Hi Fred:

Thanks for the kudos, but this one is not my innovation. I saw these devices at last year's Nats on several West Coast airplanes and decided to try them. They have worked amazingly well for me on three of my planes, and I intend to add them to all my lanes as I am able. I believe the credit for this idea in our realm goes to Chris Cox.

Later - Bob

Not your creation but you made it popular here in this forum and everyone that tried loved! Same thing with other developments you made like Foam Wings, Tuned Piped engines, E-Power...the list goes on. These things existed before but you put them to good use and made them user friendly by thinking outside the the narrow minded c/l "circles". Thank you for improving our sport and sharing with us.

[Brent Williams]

I experimented with this concept over the weekend by adding some 3/32 basswood square strips to the full length of the trailing edge of my elevator, top and bottom.  I used rubber cement to glue the strips.  It definitely helped my plane.  It grooved better in level flight and it turned and locked much better.  The added drag did consume more battery. 
Bottom line is that the strips were a very noticeable improvement for this plane.

[Ara Dedekian]

      Many of you likely know of Kermit Weeks, founder of Fantasy of Flight and his videos. His latest one is an update on the ME 108 Taifun restoration. There's an interesting segment on the reversed edges of the fuselage skins that could be related to this discussion. The segment starts at 3:25 and goes to 3:45. I don't know how to attach the video so if interested, go to, 'Gerd Muehlbauer visits the ME 108 at FOF' on YouTube.

    Ara

[frank williams]

Hi Ara,
I went to the video and saw the segment you were talking about.
Very Interesting.
I assume the skin panel edges were able to perform a function like vortex generators and bring more energetic air to the surface and help prevent large scale flow separation.
That plane seemed to have very good performance for the time.
Frank