Gurney Flap mod for our planes

[Mark Goldberg]

I mentioned this in a posting about Bob and his condition - but it is worthy of having its own thread. A builder in Alaska told me about a mod to straight flaps that he felt would help a lot. Basically, it is a little angle on the bottom trailing edge of the outboard end of the flap. Maybe 1/3 the total span of the flap in the area outboard of the tail surfaces. This builder told me it made normal, straight flaps behave like Fowler flaps. He had seen reports on the effect of this on a Cessna 195. Turned ineffective flaps into very useful flaps.

Bob gave me his thoughts and suggested we find someone to try it out - and he gave me some details of what he thinks would be a good starting point. If anyone would like to try it out - contact me and I will provide Bob's guidance on how to do it. Basically, this can be tried out with double sided tape to see its effectiveness. Mark

[Gerhard Rieger]

It makes good sense, I still have a long way to go, would like to know the outcome. Thanks Mark

[TimTall]

I'm nowhere near ready to try this on the Bearhawk but I'd love to try it on the Murphy. They are straight flaps and they aren't that effective so it might be a good test bed.

[svyolo]

https://soar.wichita.edu/bitstream/handle/10057/3715/t10084_Davis.pdf?sequence=3

Interesting

[Mark Goldberg]

Thanks John. Most of the literature I have glimpsed at talks of a 1% to 2% downward lip. Of the chord. At first I thought they were referring to the chord of the FLAP, but they talk about the chord of the whole wing (62"). We think that would be excessive. The Cessna Caravan I looked at had a downward lip of maybe 1/2". Bob was thinking to try it with a downward lip of 1/4" to 3/8". No more. Hopefully someone will be willing to try it out and report. Contact me if you are flying a Bearhawk and want to experiment with this. Mark

[svyolo]

A couple of things caught my attention. One was a lot more drag at low AOA (cruise), and the lift coefficient really tapered off much more sharply above approach AOA. The last one means that when you flare, you don't get much more lift. Also quite a bit more pitching moment, making pitch (flare) authority even more of an issue.

It did show much more lift at approach AOA, which means you can fly slower at the same attitude. That part I definitely like

[robcaldwell]

If the Gurney Flap could produce similar results of the Fowler Flap, that would be interesting.

Gurney Flap: https://en.wikipedia.org/wiki/Gurney_flap

Fowler Flap: https://en.wikipedia.org/wiki/Flap_(...s)#Fowler_flap

[Aero_tango]

Hi Mark,
I’m actually using them (gurney flap) as a rudder and aileron trim tab on my BH. They are extremely effective. Mine are 3/8 “, which is about 2.5% of the aileron cord. Easy to try, just need to tape them down with some aluminum tape, then rivet in place once you find the right length. Look at my video (23:30).

Never taught about what you are proposing. Worth to try for sure.

I would like to try, but haven’t much time available in the next couple months:-(
I can certainly be a Beta tester, to confirm initial findings...
Mike

[zkelley2]

Certainly worth a try.

http://www.dept.aoe.vt.edu/~mason/Ma...A2007-4175.pdf
This study has some good data. They don't have a Reynolds number that equates to us with our airfoil and stall speeds, but there's enough of them I think you can still see a trend towards ours. It looks like a 1.5% chord tab(.95") placed around 95%(61.75") is an ideal lift to drag compromise.
With a 1.5%, you'd only increasing the Cd by .01 while increasing the Clmax .39

That of course is not with our airfoil, so, ymwv.

[way_up_north]

Ive got a really stupid question.....since I know zip about aero design...

What if the Gurney flap was on a piano hinge ....flat during cruise.....as the aircraft flap activated and moved through its range it moved the gurney flap into position.. ...

I ask this as I would think the Gurney flap would always be working...even when the aircraft flaps were retracted...so you would get extra lift but sacrifice a little airspeed

This is an over complication to what you guys are trying to test...but I was thinking if the flap was a success and did act like a fowler flap...but effected cruise airspeed..it could be a work around...

[Rollie]

I'll try it. Just tell me what to do.

[Rollie]

Today I had a chance to try out these things. I did what Bob suggested, only putting them on the part of the flap outboard of the elevators. I had to make them two part due to the length of metal I had left laying around my shop.

For flight testing I went up before mounting them and did a few stalls both power off and power on then I started a glide at 50 kts with full flaps and noted my rate of descent. I also tried to compare cruise speed before and after but it was a hot afternoon with a lot of thermals, even with the autopilot on holding the altitude the speed fluctuated too much to verify whether I lost any speed with the gurney flap mod.

At 2500 MSL power off stalls were at 38 kts before and 37 after.
Power on stalls before were 37 before and 35 after and I had to put it in such and extreme nose high attitude after they were installed that I'd say you would never get there by accident.

Power off glide from 2500 MSL stock, full flaps, engine idle was about 500 fpm. with gurney flap mods same configuration it was 1000 fpm.

I'll fly with them again and see if I can see any benefit to keeping them on. My initial opinion is that the change in stall speed and cruise is minimal, it is harder to pull the flap handle and that the only use I see for them would be in a STOL contest. I don't see keeping them on all the time because in the event of an engine failure, if you need to get into somewhere tight and need all your flaps, that high sink rate isn't going to be very easy to arrest without power.

I stuck them on with double sided gorilla tape and they stayed where I put them. I'll try flying with them a couple more times before I peel them off. I didn't try glides at different flap settings with them on so I'll try that and see how that goes.

Also since it was noticeably harder to pull the flap handle I figured it was more stress on the flaps so I slowed down more than I usually do to lower the flaps.

[svyolo]

Rollie;
Nice job. I read a some material on this, and your experience kind of verifies what I thought. Lift goes up, but so does drag. The killer is the increase in forward pitching moment. The tail needs to generate more downforce to counter it, and thus the wing needs to make more lift to stay aloft.

I think in a STOL contest you would run out of pitch authority/rate. You would only try it once as prop strikes are uneconomical.

[zkelley2]

500 to 1000fpm in the power off glide? That's a lot more drag. Did you try different airspeed? I'm sure your L/D max changed.

[Rollie]

I didn't vary the speed a lot. I started at 50 and also tried about 46-47 kts with the same rate of descent.

[Rollie]

I forgot to mention that I installed VGs a couple weeks ago. So all my testing is with VGs installed along with the gurney flap.

[Battson]

Rollie - when you fly next, I would be interested to know whether the nose attitude (sometimes called deck angle in this context) has changed with the gurney flap.

For instance, when you are set up for approach at 38kts is the nose angle to the horizon might be 10 degrees with flap installed. Hopefully its easy to measure with your panel instruments.
If you repeat the test once you remove the gurney flap, I would really like to know how it changes the deck angle / nose attitude for the given approach setup.

[Rollie]

I'll do that. I hadn't thought of it before but it would be useful info.

[Bdflies]

1000 fpm descent! YEOWW!!
What really does interest me is your VG's. I’d really like to know your impressions of that installation. What manufacturer? How did you determine placement?
Maybe a VG thread, instead of drifting that far on this one...

Bill