Hammer wrote:

Quickdraw1 wrote:So on the flip side of that is for the pilot that wants low and slow, flap gap seals are a disaster no matter what stol kit you have.

Absolutely...I'm touching down around 90 mph, burning up 1,800 feet of runway at sea level while smoking my brakes, and only able to get 250 fpm descent with all 40 degrees hanging out and the power chopped. Must be why I can't get into any of the backcountry strips.

Where did that blanket generalization come from, anyway? I hear it repeated again and again, but it's simply not supported by experience...well, not mine anyway. Being able to slow down and loose altitude has just never been any kind of issue for me. If there's a strip my flap gap seals are keeping me out of, I haven't found it.

Would I be able to drop a little steeper or land a little slower without them? Maybe, but the wing flies so well with them that I don't see the point in changing it.

I'm not advocating for or against them, just saying that the old chestnut about how they ruin STOL performance is wildly inaccurate in my experience. And ya...I've flown a couple airplanes aside from the one I own.

+1

I have them on my 180. The plane had them on when purchased. They are unfortunately riveted in place. I am not sure how much they help the cruise speed but I feel like my bird climbs much better than most 180/185s. I have read through this thread several times trying to see if I should bite the bullet and remove them. Before the addition of the Sportsman it seemed the plane did not fly well below 55 mph. Since adding the Sportsman I have no problems flying 50 mph approaches as long as I am light. The plane touches down predictably where I want at a slow speed. I am more concerned about having enough runway to take off versus having enough for landing - I am not sure the flap gap seals affect the take off distance as drastically. For now there aren't any potential strips that I would land or take off from where the limitation is the presence of the flap gap seals. I guess if I were competing in a STOL contest it would make sense to remove them.

Josh

This is an interesting - and long - thread, which finally inspired me to comment.

Coming from (the competitive side of) soaring I learned quite a bit about aerodynamics as it is the only aspect of design defining the performance of a glider. Having a background in experimental physics helps me to reconcile theoretical concepts with empirical data.
As has been pointed out flap gap seals would be - in theory - detrimental to the effect of Fowler flaps as those flaps need the airflow through the gap to improve the slow flight/STOL characteristics of the wing.
Why do Fowler flaps - in some cases - still work with the gap seals installed?
a) because they probably don’t seal well.
High performance gliders with camber-changing flaps use gap seals on all control surfaces. A wide Teflon tape bonds to flap and wing, inhibiting any flow through the gap without affecting flap deflection, and pieces of closed cell foam seal both (spanwise) ends of the flaps against the wing.
In addition we use thin Mylar foil to cover both the top and bottom slits between wing and flap to not disturb the laminar flow which in modern airfoils extends to more than 90% of chord (into the flap in some cases).
Those are true gap seals, no cross flow!
A gap seal like this on a 70+ year non-laminar airfoil would - in general - improve performance somewhat in 0º (perhaps 10º) flap settings.
A bolted - or riveted - on strip of metal is not going to seal anywhere close to that.

b) because no two wings are the same.
60+ year old airplanes have dinged wings, repaired and adjusted in any which way, adjusted flap hinges etc. etc., which generate their particular flow regimes which could affect flight charactersitics in specific/extrem situations.
Add the difference in airflow imposed by a Horton, a Sportsman STOL kit - basically modified airfoils - and you end up with a rather individual wing which obviously defies generalisation as far as its performance under very special circumstances (eg. very low airspeed, high AoA, etc…) is concerned.


My conclusion: if the wing has gap seals and flies the way you like it - leave them on.
if there are no gap seals I wouldn’t put them on and expect better slow flight performance. They might just work as intended and stop the gap flow which the Fowlers need.

Disclosure: I have a Sportsman and VG’s on my 170B and they perform the way I expect/understand them to. No gap seals ever.

Another aspect that people tend to overlook is the flap gap seals only "seal" when the flaps are retracted. Once extended there is room for air to flow over the flap. It's inaccurate to say that they convert a fowler flap into a split flap, though they may reduce the efficiency of the fowler flap. Or not...a lack of flap efficiency is not how I would describe the flight characteristics of my airplane.

If I could remove and install them in an hour I'd run experiments, but they're riveted on, and I've never experienced any of the negative effects people attribute to them.

As for reducing drag...no idea how well they work. Hard to imagine they make much difference on a big-tire 170 with bubble windows, but they might on faster airplanes.

I have had two 185's, one with flap gap seals, one one without. The Flap gap seal I had on my 1st 185 was part of the Robertson STOL kit. It was defiantly faster. How did it effect the STOL handling, apparently not at all, since its could get in absurdly short. Representative of other aircraft with other kits, absolutely not. It did however, make the aircraft 3-5 knots faster.

The flap gap airflow is critical. Just to illustrate how little it takes to modify the airflow. Go up to any FEDEX Caravan and look at the outer leading edge of the flap. You will notice what appears to be a de-ice boot. Not a boot, but rather a flow modifier. There are about 20 molded wedges on the upper surface of this boot. This was added to the Grand Caravan over the regular Caravan and 675 Caravan. It made the flaps have more stable airflow through the slot, especially at near stall speeds. It allowed for a 600 lb gross weight increase with no other changes to the wing. Additionally, you can fit it to a Standard or 675 Caravan and gain a nearly 400 lb gross weight increase through paperwork via Wipaire.

So fiddling with that airflow through the slot can and does have effects, more than one might think.

dogpilot wrote:I have had two 185's, one with flap gap seals, one one without. The Flap gap seal I had on my 1st 185 was part of the Robertson STOL kit. It was defiantly faster. How did it effect the STOL handling, apparently not at all, since its could get in absurdly short. Representative of other aircraft with other kits, absolutely not. It did however, make the aircraft 3-5 knots faster.

The flap gap airflow is critical. Just to illustrate how little it takes to modify the airflow. Go up to any FEDEX Caravan and look at the outer leading edge of the flap. You will notice what appears to be a de-ice boot. Not a boot, but rather a flow modifier. There are about 20 molded wedges on the upper surface of this boot. This was added to the Grand Caravan over the regular Caravan and 675 Caravan. It made the flaps have more stable airflow through the slot, especially at near stall speeds. It allowed for a 600 lb gross weight increase with no other changes to the wing. Additionally, you can fit it to a Standard or 675 Caravan and gain a nearly 400 lb gross weight increase through paperwork via Wipaire.

So fiddling with that airflow through the slot can and does have effects, more than one might think.

Curious I have the Robertson kit with no flap gap seals, though it's a 206

Honestly, I could not tell you one way or the other on the Robertson mod. My original 185, I received in Honduras. The Robertson Kit was done when it was new. I never saw any other mods in the books. I did enquire about it and was told it was part of the 185 mod kit by the original recipient of the 185. I could not say one way or the other if it was genuinely part of the original mod kit. On a detail note, all the paint matched and it had not been re-touched and original when I received the aircraft (if I am vague about the passing of ownership, it was a gift to me, I was doing "stuff" down there then). I must say attaching the flaps was a PIA with the gap seal mod.

Frankly, since Sierra Industries seems to have changed hands a bit and who exactly owns the Robertson STC at the moment is a bit confusing. Last time I looked, they no longer had the 185 mentioned on their web page. That was several years back and I never looked again. I was mildly interested in a kit for my 2nd 185, since I rather liked the mod. It is beyond a total PIA to retrofit so it was more of a casual interest. From the Robertson kits I have eyeballed on different aircraft over the years, there is a significant variation between aircraft types.

Canalguna wrote:

dogpilot wrote:I have had two 185's, one with flap gap seals, one one without. The Flap gap seal I had on my 1st 185 was part of the Robertson STOL kit. It was defiantly faster. How did it effect the STOL handling, apparently not at all, since its could get in absurdly short. Representative of other aircraft with other kits, absolutely not. It did however, make the aircraft 3-5 knots faster.

The flap gap airflow is critical. Just to illustrate how little it takes to modify the airflow. Go up to any FEDEX Caravan and look at the outer leading edge of the flap. You will notice what appears to be a de-ice boot. Not a boot, but rather a flow modifier. There are about 20 molded wedges on the upper surface of this boot. This was added to the Grand Caravan over the regular Caravan and 675 Caravan. It made the flaps have more stable airflow through the slot, especially at near stall speeds. It allowed for a 600 lb gross weight increase with no other changes to the wing. Additionally, you can fit it to a Standard or 675 Caravan and gain a nearly 400 lb gross weight increase through paperwork via Wipaire.

So fiddling with that airflow through the slot can and does have effects, more than one might think.

Curious I have the Robertson kit with no flap gap seals, though it's a 206

This is interesting. Every 206 I've seen has gap seals and I assumed they were factory on the 206 with the shape it has. Now I need to look through my manuals...

A1Skinner wrote:

Canalguna wrote:

dogpilot wrote:I have had two 185's, one with flap gap seals, one one without. The Flap gap seal I had on my 1st 185 was part of the Robertson STOL kit. It was defiantly faster. How did it effect the STOL handling, apparently not at all, since its could get in absurdly short. Representative of other aircraft with other kits, absolutely not. It did however, make the aircraft 3-5 knots faster.

The flap gap airflow is critical. Just to illustrate how little it takes to modify the airflow. Go up to any FEDEX Caravan and look at the outer leading edge of the flap. You will notice what appears to be a de-ice boot. Not a boot, but rather a flow modifier. There are about 20 molded wedges on the upper surface of this boot. This was added to the Grand Caravan over the regular Caravan and 675 Caravan. It made the flaps have more stable airflow through the slot, especially at near stall speeds. It allowed for a 600 lb gross weight increase with no other changes to the wing. Additionally, you can fit it to a Standard or 675 Caravan and gain a nearly 400 lb gross weight increase through paperwork via Wipaire.

So fiddling with that airflow through the slot can and does have effects, more than one might think.

Curious I have the Robertson kit with no flap gap seals, though it's a 206

This is interesting. Every 206 I've seen has gap seals and I assumed they were factory on the 206 with the shape it has. Now I need to look through my manuals...

They are factory on later ones. I don't know when Cessna started adding them to the 206, but they weren't on the earlier ones from the factory.

Cary

It doesn't seem they were removed at somepoint either in my plane

All but one of our Cessna airplanes of various years and models (though none later than 86) were equipped with Robertson STOL kits. None of those airplanes had flap gap seals. Most of these airplanes were RSTOL equipped when new.

My old 170B had flap gap seals. They were added by the previous owner, a former WW II P-51 driver, who picked that 170 up new in Wichita. I would note that he tended to be rather “direct” in our discussions. When I was having a pre buy done on the airplane, I asked him how the flap gap seals worked. His response was a rather significant rant, laced with frequent obscenities, suggesting to me that he wasn’t impressed. Also, it’s worth noting that this guy and that airplane were well known in Alaska aviation circles, and he was known to be REALLY handy with that plane.

He’d had those gap seals installed at the same time the plane was painted. I never really considered taking them off. If it’s been easy, though, I would have.

FWIW.

MTV

Robertson STOL did not include flap gap seals, that would be counter productive for STOL. Robertson did however produce flap gap seals as a speed mod. My 185 had them until we took a box cutter to them and removed them. Excellent improvement in low speed landings.

Robertson STOL kit was made up of the following:

1. Leading edge cuff for pre-1973 Cessnas. Also came with a hangar rash tip bumpers for the leading edge which I always thought was kind of odd.

2. Drooping ailerons connected to the flaps.

3. Stall fences between the flaps and ailerons.

4. Aileron gap seals

Optional for the earlier 180’s was a manual rudder trim

For models with fixed elevators and electric flaps:

5. Flap dump switch on the yoke

6. Automatic elevator trim actuated by the flaps that does not use the trim tab.

Cary wrote:

A1Skinner wrote:

Canalguna wrote:

dogpilot wrote:I have had two 185's, one with flap gap seals, one one without. The Flap gap seal I had on my 1st 185 was part of the Robertson STOL kit. It was defiantly faster. How did it effect the STOL handling, apparently not at all, since its could get in absurdly short. Representative of other aircraft with other kits, absolutely not. It did however, make the aircraft 3-5 knots faster.

The flap gap airflow is critical. Just to illustrate how little it takes to modify the airflow. Go up to any FEDEX Caravan and look at the outer leading edge of the flap. You will notice what appears to be a de-ice boot. Not a boot, but rather a flow modifier. There are about 20 molded wedges on the upper surface of this boot. This was added to the Grand Caravan over the regular Caravan and 675 Caravan. It made the flaps have more stable airflow through the slot, especially at near stall speeds. It allowed for a 600 lb gross weight increase with no other changes to the wing. Additionally, you can fit it to a Standard or 675 Caravan and gain a nearly 400 lb gross weight increase through paperwork via Wipaire.

So fiddling with that airflow through the slot can and does have effects, more than one might think.

Curious I have the Robertson kit with no flap gap seals, though it's a 206

This is interesting. Every 206 I've seen has gap seals and I assumed they were factory on the 206 with the shape it has. Now I need to look through my manuals...

They are factory on later ones. I don't know when Cessna started adding them to the 206, but they weren't on the earlier ones from the factory.

Cary

They are factory on my 65 P model. I doubled checked the parts manual and they are listed as part of the wing assembly, and not an option. So yes, they were on at least some the early 206s from the factory.

Cary wrote:Or to put it in real world terms, just about no detectable difference, which is pretty much what I experienced when I had the Knots 2 U flap gap seals installed many years ago (hard to believe that this thread is 8+ years old!). The increase in cruise speed was what I wanted and what I got, along with a noticeable increase in climb rate, without any noticeable change in landing or take off performance. My P172D's wing is stock, except for really droopy tips, which in my experience with umpty-ump 172s makes for somewhat more solid handling at slow speeds, doesn't reduce the stall speeds by any discernible amount, but makes the "break" more like a "mush".

Cary

Ptrcisely my experience with Knots2U flap gap seals on my 172E.