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How much extra lift can I expect installing gourney flap?

How much extra lift can I expect installing gourney flap?

(OP)
I have a pre-designed airfoil. It is designed to make 50lbs of lift at 100 mph at 7deg at sea level. It is not a high camber wing. It measures roughly 22 wide with 17" chord 5% camber and 125 chord thickness. I want to install a gourney flap and am wondering how much extra lift I can expect with what size of gourney flap?

RE: How much extra lift can I expect installing gourney flap?

(OP)
Thanks. I installed a 3% GF and did a little seat of the pants testing and the wing definataly lifts alot more. I was thinking of going to a 5% or 8% GF but from the data that was shown there is not alot of extra lift to be had there. Drag is not as much of a concern in this application as the top speed is only 100 mph. Although eccessive drag would be a concern. I think at this time the standard wing is around 28lbs of drag at 100 mph with 50 lbs of lift. Does anyone have any knowledge with larger GF?

RE: How much extra lift can I expect installing gourney flap?

Whilst  not really a Gurney flap. you could look at the airfoil performance curves for sailplanes with 90* landing flaps. Shrader, PIK, Sweizer, all made aircraft with a camber changing flap that would deploy to 90* for approach control.Selig had some of this data, but I do not know how to find it on his website.
B.E.   

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them.  Old professor

RE: How much extra lift can I expect installing gourney flap?

allblowdup,

      Are you running endplates on your wing?  If not, what you have is a wing with a very poor Aspect Ratio (AR=Span/Chord).  Your AR is about 1.3 which means the lift induced drag will be nearly as great as the lift (you reported 28 lbs. to 50 lbs.)  You can do much better than that with the same wing.  If you have not already done so, you should add end plates to prevent the wing tip vortices from giving away -lift and inducing drag.  These end plates should be as tall as legally (according to your rules) and practically possible.

      The relationship between AR and Induced drag is:  Cdi=Cl*Cl/(pi*AR*e    (e is typically.91)  So you can see that doubling your AR has the potential to halve your drag.  It will also improve your downforce.

      Just a thought.

 P6


     Change in your endplate design has the potential of being far more important than the Gurney lip.

RE: How much extra lift can I expect installing gourney flap?

(OP)
Thanks, yes the wing does have end plates that extend 1.5" above and 1/2" below the wing.should i make it taller or extend lower?

RE: How much extra lift can I expect installing gourney flap?

on a chord of 17", those are pretty minimal endplates ... it'd be nice to seal the lower end plate against the body (or maybe leaeve a 1/4" gap ...)

RE: How much extra lift can I expect installing gourney flap?

(OP)
This actually a canard wing on the front of a race boat. So there is only a tube that sticks out of the boat with the wing attached to it. We can rotate them 30 deg while we race to lift the nose of the boat. I made new end plates todat that are 1" below and 2" above on both inside and outside of the wing. They work much better. Oh they are also canted 12 deg to keep them from hitting waves.

RE: How much extra lift can I expect installing gourney flap?

do you really use them at 30degs ? or this that a sort of aero- braking setting ??

the inner endplate is probably working more as a splitter, keeping the airflow around the nose from affecting the wing.

it's great to get the practical feedback, it'd be nice to see something of what's happening ... tufts probably won't work well.

increasing the camber of the airfoil would increase the lift (and probably the drag too if that's important).  are you allowed a fixed L/E slat ? (that'd keep the airlow attached at higher pitch angles).

RE: How much extra lift can I expect installing gourney flap?

(OP)
Actually we only rotate them to approx 12-15 deg up and 12-15 deg down depending on whetehre we need lift or down force the hull lifts by itself and in windy conditions we need alot of lift when traveling with the wind and some down force when going into the wind. Not sure what you mean by a O/E flap

RE: How much extra lift can I expect installing gourney flap?

a L/E slat is a fixed vane ahead of the wing L/E (Leading Edge) helps to keep the wing from stalling ... not really applicable for your app (seeing as you pitch up and down).  also camber wouldn't work well (in part 'cause it biases the wing to produce lift up (or down) at zero incidence).

  

RE: How much extra lift can I expect installing gourney flap?

(OP)
Yes and i think the higher camber wings have more abrubt stall characteristic to them.

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