Flap gap/slot seal test on SuperStol

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BDA
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Location: Kenai Alaska

Flap gap/slot seal test on SuperStol

Post by BDA »

I finally got around to testing flap gap seals.
I really believed the slot was the reason the flaps worked so well, so sealing the slot could not help.
But
Slotted flaps were designed to re-accelerate the airflow at high aoa - on a regular airfoil -, so it does not separate because of the added camber.
The slats on a SuperStol do this already.
So - is the slot doing anything beneficial??

Test #1
Full flaps, then Duct tape the slot completely closed, flush to the bottom of the wing.
Test fly for slowest controlable speed
Almost no wind, sock was barely moving

Results
I took off with the flaps down, and taped and stayed just off the runway the entire length of the runway, decreasing speed as I went and increasing power to maintain level flight.
First run I was down to 25 or so on GPS in level flight when I got to the other end. I kept expecting it to break or sink, but it didnt.
Second run, I was around 20 on GPS, very controlable all the way down the runway, with lots of throttle and lots of aoa.

I have done slow flight at these speeds before, but only when its blowing 15-20knts.

It makes sense that keeping high presure air under the wing longer means you go slower.

More testing to come with landing roll distances.
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mac
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Re: Flap gap/slot seal test on SuperStol

Post by mac »

Thanks Shawn. Real clear what you have done. I wonder what your sight picture would be like on a descending final approach? With VG’s in the cove, I feel on my SS picture is noticeable improved, with the view over the cowl better! Would hate to lose that!

Glad you’re back on the forum. Much better location for such info, IMHO. It’s searchable!
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BDA
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Re: Flap gap/slot seal test on SuperStol

Post by BDA »

Thanks Mac,
The VGs made vis over the nose better for sure.
Taping up the slot seems to have taken that away
OR maybe nose is just higher because I am slower.
More tests today.
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BDA
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Re: Flap gap/slot seal test on SuperStol

Post by BDA »

Test #2 today
Wind was completely dead - it was weird for kenai-
so speeds were a little faster and landing rolls a tad longer than yesterday.
#1 flap gap taped closed
#2 taped closed with gurney flaps
#3 No tape - gap open

Long story short - taped gap worked best - landing roll averages were 19’ shorter with no gap
- gurney flaps caused a sharp stall break NO BUENO!
Gurney flaps brought the tail up, and take off was slightly better, was not any slower, not worth the stall

But
If you are not flying on the Hairy edge, hanging on the prop, and being very aggressive with the throttle - you wouldn’t notice the difference.
The biggest difference I saw was the abillity to flare and catch a high sink rate. So this translated to much more accurate spot landings.
Next test will be with an aluminum gap seal so I can test at altitude for stall speed and partial flap settings.
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mac
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Re: Flap gap/slot seal test on SuperStol

Post by mac »

Personally, I like the way my SS stalls now…..which essentially is a non-event. High sink but stable.

The gurney flaps I saw at Oshkosh were on the stabilator (RV-15) and on a certified airplane with a conventional horizontal/elevator arrangement. In both cases, the surfaces were “airfoil” types, not slabs. Small angles, maybe 3/8-1/2”, going both up and down at the trailing edges. Much written about gurney flaps on airfoil sections but nothing about effect on a slab. Since I have slab airfoils on my SS at the horizontal/elevator, won’t be testing, since my gut tells me the benefit might well be negative!

Again, thanks for reporting on all the testing you do. I’ve incorporated several of your proven results and my SS definitely flies better for it.
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BDA
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Re: Flap gap/slot seal test on SuperStol

Post by BDA »

Just to clarify
The taped or sealed flap gap worked very well
Better lift, better stall, better landing roll.

The other test I did was for the Gurney flap doodad, that test did show slight takeoff improvements, but bad stall characteristics.
hiline
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Re: Flap gap/slot seal test on SuperStol

Post by hiline »

This is a very helpful post and information.I'm in the process of installing flap gap seals on my Super stol this week,thank you so much!
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BDA
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Re: Flap gap/slot seal test on SuperStol

Post by BDA »

Hiline - did you install gap seals? How do you like them?

What kind did you use?
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BDA
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Re: Flap gap/slot seal test on SuperStol

Post by BDA »

Update on Flap testing, Last spring I installed and tested Double slotted flaps.
They were great when all the way up because I added a lot of wing area, but as soon as I opened them at all (slots) I got big sink rate. they made a TON of lift and drag, but I think the lift was dumping out through the slots. I will post the double sotted flap testing when I have more data.
But back to the slots - I covered up the first slot about half way with some aluminum strips and duct tape for the stol comp in Valdez. They performed much better with that as a gap seal than without.
Leveled out my sink rate, raised the tail etc.
I had a really good discussion with Frank knapp and Wayne Mackey about flap gaps and keeping high presure air under the wing.
They both said that below 30mph you are just "pushing a cushion of air" in front of you, "stacking" higher presure under the wing, letting it spill through a slot is counter productive for below 30 mph flight.
Pictures to follow.
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