I am pulling the wings on one of my 206s and have the chance to remove the flap gap seals. I have a lot of time in many 206s but all with flap gap seals. I used to have relatively nice long runways to operate off of and ROC was the critical factor. Now I have a little short grass strip and the STOL performance is critical. All my wings have the Sportsman and Wing-X so I get pretty good performance out of them now but want that little edge since we go out at gross weight. Do the flap gap seals improve climb that much? I can see where the flaps 20 take-off airflow would be improved. On a wing without Flap Gap Seal is Short field take-off improved that much? Are any of these measurable?

contact 182 STOL...Bill Reid...he has pretty good argument against flap gap seals....

I have heard and read that for stol performance you don't want the flap gap seals. Aeleron seals are OK. Atleast on the Cessna wing.

I have a friend who flew a C-175 tail wheel calling on ranchers selling feed, over 4000 hrs landing on any flat surface. He installed flap gap seals and removed them because of the decrease in performance doing short field work. If you are interested I can put you in touch with him.

OK, I don't have an answer to the original question (is STOL performance better without flap gap seals?), but can anyone here explain from an aerodynamic perspective why having a gap between the flap leading edge and the underside of the wing would make a wing develop more lift in high angle of attack flight (i.e., short field takeoff)? This proposition just seems a bit backwards, but if someone has a good engineering explanation, I'm all ears.

The gap and radius fairing on the fowler flap allow the airflow to stay attached to the top of the flap generating lift when the flaps are deployed. The gap takes the high pressure air from the bottom of the wing and blows it over the top of the cambered surface of the flap sucking the air from over the wing down. This can keep airflow attached longer at lower airspeeds and higher angle of attacks. A plain flap deflects the air moving past the bottom of the wing down with a little draw down of the air over the top of the wing. Split flaps just deflect the air the bottom of the wing down and creating mostly drag.
I know that my Cessna fowler flaps would work better without the gap seals but they are factory installed and I can do all the drilling and rivitin but what would be the measurable change for all the effort? I saw a post of 20% but I think thats a bit much. Right now at SL, 0 wind, 80 deg f, gross, flaps 20, grass, 800ft takeoffs. My engine and prop stomps. Cargo doors removed, deflector (spoiler) installed. 20% would be 160 ft shorter. 10% 80-100 ft shorter would be worth it.

Shydive
I always thought that was probly the explanation. But I did think high pressure was on top of the wing and low pressure underneath?????
But just thinkin as I'm readin your post............ scary, I know
Say you are full flaps, and relatively higher angle of attack now that air(lift) that is going over the flap,is doing more to slow you, than lift you??????

n2485q wrote:contact 182 STOL...Bill Reid...he has pretty good argument against flap gap seals....

Van Pray aka Skydive 206 grew up next door to Marshal Quakenbush and SPORTSMAN STOL "factory".
I think Cal City Skydive was one of SPORTSMAN STOL first customers (pre STC) . I love SPORTSMAN STOL
for what it does and doesn't do to Cessna's. FLAP GAP SEALS on FLAPS are not part of or never have been on or with SPORTSMAN STOL -Flap Gap seals go with horton,mid America, owl etc. Adding flap gap seals to airplane with or without STOL kit KILLS 20% of your lift at takeoff in a cessna . When Cessna engineers designed the "Fowler flap" that comes to the rear and down .Take the aileron all the way down to the stop and measure the angle -set the flaps at that angle (on 182 it's about 17 degrees) and set your flap accordingly for max performance. I have Mechinical flaps and have a block of wood (aerospace quality) to set my flaps at 17 degrees. Look at the flap on Cessna when it's down = looks like a small airfoil -just like the wing . If you add flap gap seals air doesn't get to top surface of flap to provide LIFT . Pipers and low wing dirt darts have a DRAG flap with no additional lift.

First time poster, but I have been lurking for a while. My job requires a bit of wind tunnel testing various airplanes and I have been involved in a bit of low-speed airplane configuration work (flap gap and overlap at the various design angles) so I'll share a few things that I have learned.

Most of what I've read here is spot on with my experience. The gap seals on the flap most likely disrupt the airflow though the flap gap and thus degrade the performance of the flaps. That airflow is key to the flaps working well. We spend a bit of time tailoring the gap to get the best flap performanc. Having a gap seal for that slot is great for cruise as there is definitely air leaking from the bottom of the wing to the top (= drag ) Their effect is probably worse for the take off configuration when the flap is relatively close to the seal, again more drag as the flap could end up being stalled. So you will have longer takeoff runs. At landing positions, there is probably less effect for two reasons. One, the flap is further from the disturbance created by the seal and two, it is good to have the flaps partially stalled to get the necessary drag with the associated lift for a nice steep approach. Ideally, it would be great to have a retractable seal like the one that is on the F8F.

Also, split flaps don't just create drag. They keep the airflow on the top of the wing attached when it might otherwise start to separate. Cessna 400 series airplanes have split flaps, with the exception of the 441. The engineers at cessna are, or maybe were not dummies. They are a relatively good solution for some circumstances.

BTW I have a 182 but it came with the flap and aileron seals on it. I probably wouldn't purchase the STC on my own, but they were already on the airplane when I bought it. It is more of a cruiser machine than a back country machine. One can hope though.

Van, we ran our 206 at the DZ in Alaska without the flap gap seals. We operated out of a tiny grass strip and some other remote spots for tandems here and there. There is a noticeable improvement for STOL without the seals. We didn't have the Sportsman or Wing-X though.

with my 182TN, i dont think it makes a ton of diff either way...it does, though, make u realize that the more HP hooked to the prop that u have, seems to make the most diff up here in idaho...lowest i operate out of is 4740' alt...
my 540 is turbo-normalized with the cuff,fences,and vg's...comes off is less than 450', less i'm heavy...not as strong of pull as a straight turbo like the 206's run, but at 29" MP, it gets it done pretty good...my old 182 with the 470, no stol or etc, did ok also, but the takeoffroll tripled, not too fun anywhere in the middle-fork at less than 2000 long, and if u r heavy without HP, stay home! if i couldn't have my current 182, a 206T would be the ultimate choice for me for back country haulin'...jo

If you know how to search around here you might find a longer version of my empirical testing of Flap Gap Seals.

Short version is do NOT use them for back country type work. Bought my plane without them. Added them. Nearly wrecked the plane. Took them off. They are STILL off.

Z - your spell checker does not like back country spelled as one word.?!?

At least I was fortunate enough to have been told by Vern Miller, (deceased), that if I was intent on trying flap gap seals then at least throw away my Horton's that came UNINSTALLED when I bought the plane and purchase a new set of Knots 2 You flap gap seals. They can be removed with close to 60 screws. The Horton set would have been riveted in place.

What I really lost was a high rate of decent. Was unable to hardly get 500 ft/min.
Was used to easily going above 700 ft/min. Could earlier do a hard slip with full flaps close to 1,400 ft/min. Took some minor finesse when close to the ground.
Caused me some minor damage by having to do an old 170 "over the nose" slip into a NO-Go around strip on the north side of the Klamath.

I fly a 53 170-B with an old "Bush" 180hp conversion. MT Prop. (Thanks again MTV) and a long list of other STCs.

Chris

PS: If you wish to see the test repeated, and you are willing to do all the screw work, then we just may be able to arrange to repeat the test. Just NOT at a strip with a short STEEP approach.

nmflyguy wrote:but can anyone here explain from an aerodynamic perspective why having a gap between the flap leading edge and the underside of the wing would make a wing develop more lift in high angle of attack flight (i.e., short field takeoff)? This proposition just seems a bit backwards,

I'll add something that might not have been described luridly enough previously. I'm not an aero engineer, so any aero's out there can feel free to correct me.

The cross section of the gap between the flap and the wing (at moderate flap extension) is a narrowing, funnel shaped gap. It also is a directional gap.

So as high pressure air flows into the gap on climbout, where you are going 55-65 miles an hour... the air is ACCELERATING through that gap. So the air being squirted out of that gap (slot) may be 80 or 90 miles an hour, which means you have more actual energy in the air than if it were free stream.

The shape of the gap is critical, because it squirts that air rearward along the upper surface of the wing, not upward "off" the wing surface (like through the aileron gap). So you get the benefit of higher energy airflow across the top of the wing, which means more downwash off the wing (lift) and less airflow separation (less stall).

I'm guessing the Cessna flap's rearward "Fowler" travel is not really affected by the slot that much, but it adds effective wing area up to a point.

So you get one big benefit (the increased camber or effective airfoil curvature), one medium size benefit (slot squirting high energy air along the otherwise weak upper surface flow), and one small benefit (a couple percent increase in planform wing area).

Skydive206 wrote: I saw a post of 20% but I think thats a bit much. Right now at SL, 0 wind, 80 deg f, gross, flaps 20, grass, 800ft takeoffs. My engine and prop stomps. Cargo doors removed, deflector (spoiler) installed. 20% would be 160 ft shorter. 10% 80-100 ft shorter would be worth it.

I expect you are plenty aware of this, but since you already have WingX and Sportsman, there is not likely any mod that will reduce your ground roll by 20% (short of a PT-6 or major weightloss that is...) In the realm of performance mods, 1 plus 1 rarely equals 2. And even though you are not adding a mod, the concept is the same.

I had read a Cessna report that stated the change from plain flaps to paralift ( I think that's what they called their fowlers?) yeilded a 10% difference. But I am foggy as to what wing that was on. I would guess it was a report on 170s.

Any PT-6 updates?

Take care, Rob

I am finding it hard to get quantitive proof of gains or losses for the Cessna flap gap seals. 20% is huge numbers in our game. 160ft reduction in takeoff roll? 640ft packed full of meat. Im rotating at just under 65 mph breaking ground under 70. 14 mph decrease in liftoff speed. If it did I would have the most bad ass 206 around. Ive spent a lot of money for just 3-5 % gains. I see that the sellers of flap gap seals advertise 50 to 100 fpm gain in rate of climb. Thats a huge number on its own. I havent found any Cessna sales info on the flap gap seals, this factory mod could not have changed takeoff performance that much our it would have changed the performance numbers. But Cessna also says that the Cargo door deflector doesnt change performance numbers. The loss is at least 100 fpm. The turbine is moving fwd but slowly. Carving the Cowl now.

The increase in climb performance is figured flaps up with the gap sealed. When you are in the takeoff configuration you will not see that performance increase. Seems that for shortfield or backcountry work you will not immediately retract the flaps as the wheels leave the ground. If you have the flaps in the takeoff configuration until you have cleared your 50' obstacle, you will see no benefit.

182 STOL driver wrote:

n2485q wrote:Pipers and low wing dirt darts have a DRAG flap with no additional lift.

Not true - Piper flaps (ignoring snark about " ... and other dirt darts") are not "fowlers" but they still add significant lift .... which is why deploying the Cherokee flaps significantly reduces stall speed (3 mph per notch per the book). Third notch (40 degrees) adds a lot of drag, but it still reduces stall speed by another 3 mph.

Skydive206 wrote:I am finding it hard to get quantitive proof of gains or losses for the Cessna flap gap seals. 20% is huge numbers in our game. 160ft reduction in takeoff roll? 640ft packed full of meat. Im rotating at just under 65 mph breaking ground under 70. 14 mph decrease in liftoff speed. If it did I would have the most bad ass 206 around. Ive spent a lot of money for just 3-5 % gains. I see that the sellers of flap gap seals advertise 50 to 100 fpm gain in rate of climb. Thats a huge number on its own. I havent found any Cessna sales info on the flap gap seals, this factory mod could not have changed takeoff performance that much our it would have changed the performance numbers. But Cessna also says that the Cargo door deflector doesnt change performance numbers. The loss is at least 100 fpm. The turbine is moving fwd but slowly. Carving the Cowl now.

Oh man... I bet 99.56 % of the guys in here have no idea how much work making a custom plug for a cowling really is...

Looking good..... so far.

Ben.

Hey Skydive 206... are you VP Senior or Junior? I got some memorable stories about Sr. from back in the Cal City days.

He used to throw human bodies down past our gliders, and those were the good stories

Im Jr. The target had that nice big black dot to generate thermals. I had my glider permit at 14 in Cal City and still use my and lift hunting skills today.