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Gurney or traditional?

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Daleandee

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I've been reading about the Gurney flap and seen an interesting application of it on a Sonex as trim for the rudder:

1587522349678.png

Would this be a better approach than a traditional fixed trim tab in a rudder trim application? I'm pretty dense so please respond with large letters written in crayola colors ... 🤔
 

TFF

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It’s how they slow the Cessna Skycatcher down so it does not bust LSA speeds.
 

Victor Bravo

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please respond with large letters written in crayola colors ... 🤔
I'll leave that part to Armilite.

As far as a trim tab for the rudder, both a Gurney Flap and a traditional trim tab would likely work.

However, for it to work best you need to be able to adjust and fine tune it... and it is a LOT easier to adjust and fine-tune a traditional "beer can" tab than it will be to adjust the size of a Gurney Flap.

A full-length Gurney flap will create a much larger amount of force than a typical small U-bend-em tab. Probably way too much force.

A short length of Gurney flap will likely create more drag than a 20 degree tab.

So I vote for the old traditional way.

Start with a 3 x 4 inch piece of .032" aluminum, stuck onto the rudder with plain old duct tape the the first try. Start with a 15-20 degree bend at the mid point of the tab, leaving the bent half (2 x 3 inches) hanging behind the mid point of the rudder. That is conservative for a reason, and if you need to bend the tab much more than 30 degrees then it's time to put on a slightly larger tab.

Once you have fine-tuned the tab to where you want it, then you can stick it on with really really good double-stick "carpet tape", painmt the tab to match, and apply a layer of the best quality/thickest clear packing tape over the leading edge of the tab to keep it from peeling off of the rudder at speed.

Oh, and make the tab with rounded corners and smooth rounded edges. The suggestion of .032" metal is not for strength, it's so the metal tab's edge is thick enough to not slice you open like a razor. And the suggestion of round corners is because... well, don't get me started.
 
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Mcmark

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Most of the aerobatic airplanes are using wedges. make it long to start. Use dbl side tape and trim to achieve desired result. Paint and use tape or glue to affix to surface.
 

wsimpso1

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You are getting good advice. Gurney Flaps work, but they are usually draggier than a conventional fixed tab or a wedge. If your rudder trailing edge is too thin (many metal control surfaces are), a Gurney Flap of less than 0.2% of chord can have almost no drag, but work well. Make it deeper than that and it sticks above the boundary layer and causes drag. The pictured one almost assuredly adds measurable drag to that airplane.

A number of homebuilt airplanes with no on-center feel to controls have been given on-center feel by putting a small T section in between the skins - a double Gurney Flap if you will. You want less than 0.3% chord of total width to keep from adding drag, but that is enough to center the surface, and you can still reduce one side to neutralize the rudder.

Dan Gurney was a very successful race team owner and driver in Indy, F1, and international sport cars. A shame that he is known for two very minor inventions, the Gurney Flap and the Gurney Bump. The Gurney Flap was a way to increase foil down force without violating max wing area rules, which were determined not by total surface area, but by planform area. Usually they were changeable, and teams carried a bunch of different ones to help tune the cars for different tracks. Teams knew that more downforce increased cornering speeds but decreased max speed, and they would seek lowest laps times with the balancing act of wing adjustments and Gurney Flap sizes. The Gurney Bump was a teardrop shaped bulge put in the roof of closed cockpit racecars Dan drove - he was tall and needed the added headroom. The LeMans winning MkIV on display at The Henry Ford was driven by Dan Gurney and A.J. Foyt and has a prominent Gurney Bump on the roof...

Billski
 

Hot Wings

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Had it stuck in my mind that the G-flap actually has less drag for the increase in lift than a tab so I looked at my stash of info. Not so clear cut, but for the application of this thread - a trim tab - I'd have to agree with VB.

Use a normal tab - with the same caveats with regard to cuts and snags.

Reason: While G-flaps are powerful tools for tuning lift without the drag of a conventional flap they are not very effective at zero lift in creating any force, other than drag. Once the airfoil starts to develop lift then they start to work as desired.

So......if the rudder is just flopping in the breeze, as normal stick free control surfaces do, there will be no lift. We need something to actually redirect the air, rather than just stir it up.

For a wing heavy condition - then I'd use a G-flap.
 

TFF

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As a Gurney fan, I think he will always be known for one week in 1967 where he won LeMans in an all American team and the next weekend wins the Belgian GP in an All American car of his own brand. Actually I think he will be remembered as a really nice guy. Dan Gurney for President.
 

Daleandee

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Most of the aerobatic airplanes are using wedges.
First ... thanks to all that responded! There is some excellent information here. FWIW when I was initially doing this the wedge approach was what I used as some of the other builders were using these and they blend well and don't project past the end of the rudder. But seeing the Gurney flap had me intrigued and some of the information I found led me to believe that it might have less drag than a conventional tab. I didn't know the answer so I thought I'd ask where some pretty smart folks hang out.

Thanks again,

Dale

PS: VB ... I really didn't have anyone in mind with my "crayola colors" comment. Almost hurt myself when I made the connection ... :popcorn:
 

Turd Ferguson

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Cessna used a Gurney tab on the "B" model Caravan flaps as one item in the package of tricks to keep the stall speed under 61ks for certification.

A lot of Tailwind guys use a wedge on the rudder.
 

Marc Bourget

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Billski said: "If your rudder trailing edge is too thin (many metal control surfaces are), :

How's that fit with the defined knife-edge trailing edge and the "Kutta Condition"? :^)

[I've opened this thread up with that query! - :^) ]
 

wsimpso1

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The boundary layer becomes thick enough by the trailing edge that you can not actually satisfy the Kutta Condition anyway. But if you try, and get really thin trailing edges, they can be weak to the point of being fragile, they are dangerous to folks walking around the airplane, and the control feel will usually lack on-center feel that many of us find desirable.

Many simple aluminum control surfaces have either a third ply of aluminum between the skins at the trailing edge or they have a keyhole shaped piece or a little T section. This strengthens the trailing edge, thickens it enough to give on-center feel, and makes any injuries from the inevitable collision less severe.
 

Lendo

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I remember reading that a thicker rounded training edge behaves as if it's sharp anyway.
George
 
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