Do you guys think this mod is worth pursuing

ultra thin and ultra stiff carbon fibre sheet running the full length of the flap. Rigidly fixed on the trailing edge of wing. Fixed through a slotted hole in the CF that allows the flap surface to move away from the wing then come closer to the pivot point which forces it up into a curve. Took a bit to sort out the geometry (hence the string) I don’t want to waste too much time if the consensus is that it won’t work. Just looking for an affordable flap mod.

cgoldy

Hmmmm.... is that backwards of the slotted flaps adding air to the top side??? Hmmmm... have seen bottom seals for when flap is retracted...


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Lots of guys will say the mod will block the airflow between the trailing edge of the wing and the leading edge of the flap. Thus negating some of the flap effectiveness. Readers fire away if you wish, but heck, I did learn that here.

My undereducated guess is it'll help on takeoffs and hurt on landings.

go to 1.18 in the video. my first thought is thats the shape of a frank knapp flap.https://www.youtube.com/watch?v=UZLH_2tMoGo

I would think it might help a bit with top end speed. It just turns a slotted flap into a plain flap. You will have better lift with a slotted flap. This should add more drag but reduce lift.
DENNY

Looks pretty Colin, Why would you want to loose the advantages of the slot funneling the air over the top of the flap which reduces the speed at which the flap stalls? The purpose of the slot between the flap and the wing is to accelerate the air flow over the top of the flap thus reducing the speed at which the flap looses lift and turns into total drag.

This may help to explain:
https://aviation.stackexchange.com/questions/30960/how-does-the-slotted-flap-work

Your idea only provides a smooth flow along the lower surface providing ram lift. By blocking the air flow over the top of the flap you will loose the lift once the deflection angle starts moving beyond about 25 degrees. Drag is nice but lift is better.

This should help explain how the air flows and what it does. Notice where and how the streamline flow changes to turbulence. When the flow changes to turbulence, the lift is changing to drag. When the turbulent smoke disappears that part of the wing is doing nothing. At 5:54 you will see what it is doing if using your suggestion. At: 7:59 is what you have now. Notice how the air flowing up through the slot adheres to the top of the flap preventing flow separation.

Any drag produced by the gap on the lower surface with the flaps up at cruise speeds in a Cub is insignificant. It would take a plane which cruises in the 140+mph range when closing that opening could possibly increase the cruse speed by just 1 mph. This was proven on a Cessna 206 by company test pilots. I can name names.

My short answer to your question, fa-get-about-it. Place your efforts elsewhere.

This sort of mod is what Frank Knapp did on Lil Cub to get the flaps to really work. He said he just on a lark duct taped up the gap between the flap and wing and noticed a drastic reduction in stall speed near the ground. His theory is that is helps trap the high pressure air pocket under the wing in ground effect to reduce speed/stall speed.

There's a couple ways you can go about making flaps work harder and from Frank's experience perhaps sealing the slot is one way of doing that. Obviously slotting the flaps is another way.

If there was a safe way to test it on one wing only you'd soon discover any benefit or downside. The added enhancement above of seals in ground effect is interesting.


Edit: Crosswinds STOL used to seal the flap and aileron gaps with uncoated ceconite fabric. Held them on with thin metal strips at the wing and flap LE. Used small (#4?) PK screws.

Edit: Rest of story. One winter I flew my trapping partner's PA-18 N4336Z with their STOL kit and sealed surfaces. Lots of small lakes and tight grassy meadows sitting in terrain bowls. It would want to keep rolling once passed about 20-30* unless lots of counter aileron was applied. I didn't like the down aileron on the down wing. T/O and landing performance was excellent in deep snow with two aboard and gear. We removed the flaps seals for some reason to test but I don't recall the results +- 34 yrs later. We put them back on so maybe there was some benefit.

Gary

Crash said:

This sort of mod is what Frank Knapp did on Lil Cub to get the flaps to really work.

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Look at the video of Frank's airplane in post #5. His flaps also have a curved down airfoil.

BC12D-4-85 said:

If there was a safe way to test it on one wing only you'd soon discover any benefit or downside. The added enhancement above of seals in ground effect is interesting.

Gary

Click to expand...

You could fix the flaps in a down position then tape on a cover for one slot, for a test.

My opinion is still unchanged.

I think Randy from Carbon Concepts has done something like that to he's flaps.
DENNY

skywagon8a said:

Look at the video of Frank's airplane in post #5. His flaps also have a curved down airfoil.

Click to expand...

Talking with him the curved down airfoil is certainly a benefit but he said sealing the flap gap netted him the biggest performance gains.

Stock flap gap isn't of any performance benefit since it doesn't direct the air efficiently. IMO sealing this gap would gain performance. If you want to run a true "slotted" flap it has to be a type like the PSTOL flap where it has a vane to efficiently direct the airflow over the flap element and create a low pressure area where the airspeed is increased flowing through the flap slots.

It appears some further testing is required re what's best during T/O and landing: flap gap open or sealed. I'd also contact Charly Center at Crosswind's STOL for his take on sealed vs not.

Gary

Crash said:

Stock flap gap isn't of any performance benefit since it doesn't direct the air efficiently.

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Study that video I posted in #7 again. Keep an open mind. Then go look at other wind tunnel videos. The flaps on Colin's Cub are double the length of a stock Cub.

Also consider the lift produced via airspeed or effective Reynolds Number over and under the flapped portion of the wing. Prop blast on takeoff increases that. Not so much on landing unless power is added at high AOA or the left edge of the drag bucket. So I wonder if Lil Cub's experience with a lift bonus was during higher power when the benefit was mainly noted....and if so to what degree that benefit may have been noted during a power off stall?

In a wind tunnel they adjust the airspeed and determine the equivalent Reynolds Number. It may be (just a guess) that at low airspeeds the need for a wing cove and gap flow over the flap becomes less beneficial than a sealed wing (?).

Gary

Ok, I haven’t told you the whole story.

1) fitted one only flap gap seal on left wing. Test flew and pulled on flap. Left wing popped. Returned and fitted both seals. Great improvement

2) fitted only one gurney flap on left wing. Test flew and pulled on flap. Right wing popped. WTF! Then realised that because of the balanced flap system on the cub, the extra drag on the left flap stopped it from actuating. Didn’t have enough material to do both flaps but I intend to.

3) fitted my carbon fibre flap booster to the entire left flap. As Pete says, big flaps. Material I used was not stiff enough and the operation was not as good as I could achieve if I used some custom made CF sheet. Test flew and pulled on flap expecting that the flap not to move as with the gurney experiment. But both flaps pulled on equally. So to my simple mind, no extra drag was caused by the flap booster. I perceived (very hard to read IAS at low speeds) a drop of 1 knot stall speed. Indicating 24 knots. ( know that is not accurate but I have to work with what I have) and being only one flap booster installed I couldn’t work out how I could have an improvement in stall speed without some asymmetrical flight characteristics. I removed the booster and flew again in the same conditions and verified results. Remembering that I don’t have a break, just mush.


So thinking if I made a more accurate set and fitted to both wings, might be worth 2 knots.

But maybe not worth the effort but fun anyway

The Age of Experimentation hasn't quite finished...good info!

Gary

cgoldy said:

2) fitted only one gurney flap on left wing. Test flew and pulled on flap. Right wing popped. WTF! Then realised that because of the balanced flap system on the cub, the extra drag on the left flap stopped it from actuating. Didn’t have enough material to do both flaps but I intend to.

Click to expand...

I attribute this to the small amount of lift generated by the gurney flap which kept the left wing flying. I tried a small length of gurney and did find it produced a small amount of lift and a very small amount of drag.

My 206 example in addition to the 1 knot increase in cruise also produced a 1 knot increase in stall speed.

Gary,
Compare the diagram of the lift vs drag on both the hinged and slotted flaps.
https://aviation.stackexchange.com/questions/30960/how-does-the-slotted-flap-work

This backs up what I have stated elsewhere. Slotted provides more lift than hinged. Colin's long flap like mine, only has the inboard section operating in the prop blast. The outboard section with a properly tuned slot provides very smooth airflow off the trailing edge at the maximum angle of attack when deflected to 56 degrees. I tuft tested it.

Yes Pete I agree with the LvsD link so not questioning. Abbott and Doenhoff's Theory of Wing Sections Ch. 8, Fig. 134 confirms that at some Reynolds Number. But Posts #8 and #12 bring to question observations by someone who might have seen a benefit otherwise with sealed flaps.

I'd like to know more...we should know more about this: "His theory is that is helps trap the high pressure air pocket under the wing in ground effect to reduce speed/stall speed." We know and observe that flying in ground effect has consequences like floating, the potential for increased under wing pressure, and changes in wind flow relative to the wings chord. Maybe that's where the rules change if the slow ops by Mr. Knapp have informed.

Edit: I believe another effect of flaps in addition to their own lift and drag is to force air over the top of the wing. They also act as a lower air dam (like Gurney's) which can increase under wing air pressure by resisting flow. The combination might increase lift.

Gary

When you throw an extremely low wing loading into the mix, then consider ground effect which is considered to be about one wing span above ground we can get some different perspectives than that which we get out of ground effect. Most all of the information in the textbooks doesn't include ground effect. Mr. Knapp's very light plane with his special design flap airfoil very likely has low speed numbers which respond differently to a standard configured Cub. His performance attention appears to be directed towards the regime within ground effect. I believe the above video claimed he has a O-360 in his airplane? That takes care of the brute force portion of the performance.

BC12D-4-85 said:

I'd like to know more...we should know more about this: "His theory is that is helps trap the high pressure air pocket under the wing in ground effect to reduce speed/stall speed." We know and observe that flying in ground effect has consequences like floating, the potential for increased under wing pressure, and changes in wind flow relative to the wings chord. Maybe that's where the rules change if the slow ops by Mr. Knapp have informed.

Click to expand...

I would say that is correct. Taking ground effect out of the equation changes things but specifically the sealed flap improves things when you're low and slow.

I would also point out that some of your documentation that you reference is based on non-specific airfoils at higher than cub type speeds. These are all good numbers and good sources of information but overly general and based largely on theory rather than actual testing on the specific aircraft we are discussing.

Crash said:

I would also point out that some of your documentation that you reference is based on non-specific airfoils at higher than cub type speeds. These are all good numbers and good sources of information but overly general and based largely on theory rather than actual testing on the specific aircraft we are discussing.

Click to expand...

True, but there's not much else available that's been peer reviewed and published that I've found. Those studies typically take money. The question and some money has supported airfoil and aircraft specific testing starting in the 1950's. Mississippi State College - Cornish et al.- Texas A&M - Weick et al., and others via military contracted studies involving the Piper Cub. The goals of the first noted were investigating STOL enhancements, and the second flight performance and stability. Little I've found addresses performance in ground effect during takeoff or landing, both with and without flaps or gear extended.

So again rather than offering well informed opinions it'd be nice if some real data were generated and made available....like the tests of VG's and Gurney flaps reported previously here, but now for sealed versus slotted flaps. All I know is the Crosswinds PA-18 I flew one winter with sealed flaps and ailerons was a solid plane near the ground that performed very well on skis. Crosswinds support came from those MSC studies via Gordon Mandell and Cal Center.

Gary