PStol flaps, effects and airflow

[Farmboy]

So an interesting topic came up today that I thought I’d share, as most of you have a better understanding about it.

The addition of pstol (fowler type) flaps onto an existing taildragger, and perhaps other types, significantly changes the attitude of the aircraft on approach.

To do this, it appears that the aft trailing edge/rear spar/etc is essentially carrying more load, as the resultant lift from the flaps is so much more effective. Is this true?

If so, this doesn’t mean the center of lift has moved aft, but it does seem that the required downforce on the tail will have increased?

So now if you install these flaps on a taildragger that uses a trim tab instead of a trimmable stabilizer, will the tail be less capable to handle or utilize the benefits of the Pstol flaps? Would you think there is a chance of running out of elevator?

Steve, what’s your first hand experience in your cub - can you get to a point of low elevator response, and does the position of the stabilizer change it, other than stick forces?

Inquiring minds...

Pb


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[Crash Jr.]

It's not so much that the lift from the flaps alone is more effective but the flaps are making that whole area of wing more effective. The pstol or a fowler flap in general is not only increasing the camber of that section of wing (like a barn door flap) but actually increasing the chord of the wing which in effect moves the center of lift rearwards. In addition, the high speed airflow between the vane and the flap elements on the PSTOL flaps increases the low pressure zone above the flaps which increases lift in that localized area which increases lift and also moves the center of low pressure (lift) rearwards on the wing which increases the nose down moment.

That's the technical side. I'm sure those with practical experience can chime in on the rest of it.

 

[Bearhawk Builder]

I was just talking to Bob Barrows about this subject and he feels that at least on his designs a larger tail surface would be beneficial if not required. Maybe that's the topic that came up Peter? Lot's of good effects of fowlers but with some design limits in reference to the rest of the plane. They sure seem to work well on the SC.

 

[Farmboy]

I was just talking to Bob Barrows about this subject and he feels that at least on his designs a larger tail surface would be beneficial if not required. Maybe that's the topic that came up Peter? Lot's of good effects of fowlers but with some design limits in reference to the rest of the plane. They sure seem to work well on the SC.

Yup his designs would be a good example. Would a trimmable stab help in this respect, or not make any/enough difference. Need more experienced input.


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[stewartb]

Semi-Fowler'd Cessna 182s got a wider tail in 1965 to improve elevator authority in the flare.

Exp Cubs with slats got extended airframes to improve trim and elevator authority even with big tail feathers. Those same exp Cubs have moved past Keller flaps now, too. I sure like mine.

 

[Crash Jr.]

Feedback from some customers has been that with the larger (extended-experimental) flaps there can be some blanking of the tail or at least heavy buffeting with the PSTOL flaps. That's why the extended tails came into vogue was to get the horizontal tail surfaces into a bit cleaner air.

 

[The Kid]

I wonder how many sets of the P-STOLS Airframes has sold for certified cubs? Anyone have an about figure?

 

[RaisedByWolves]

I wonder how many sets of the P-STOLS Airframes has sold for certified cubs? Anyone have an about figure?

Mine was sn 253 which I’d imagine means 126 sets. I heard they are delivery into aug now


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[mam90]

Maybe relative, maybe not. Gulfstream’s have conventional trim tabs on the elevators, as well as large Fowler flaps. The stab is not trimmable, but does re-position with flap extension.....

 

[stewartb]

Feedback from some customers has been that with the larger (extended-experimental) flaps there can be some blanking of the tail or at least heavy buffeting with the PSTOL flaps. That's why the extended tails came into vogue was to get the horizontal tail surfaces into a bit cleaner air.

The airframes were extended without regard to double slotted flaps, which for most of us were an option. Lots of guys weren't willing to pay the price or weight penalty and stayed with "standard" flaps. They're stretched even further now with no intention of using double slotted flaps. The evolution of these exp Cubs is very interesting to watch.

 

[The Kid]

What I was getting at was what percentage of certified cubs that have these bad boys? I don't know how many certified cubs there are out there but has to be thousands doesn't it? It must be way under 5% I'd say, but growing. I like what Airframes is; SUPER CUB CITY!

 

[Farmboy]

Ironically I just found that Elliot Sequin of Wasabi Flight Test recently posted video 6 of a flight test program for a builder that decided to modify his Lancair by adding a Fowler type of flap. So far, per the latest video, it appears that they have run into or close to an elevator/stabililzer stall scenario. The tufts underneath the tail clearly show a need at minimum for VG's, and perhaps larger tail and control surface. End result is the flaps have not yet helped the wing design of the Lancair.

Totally different airfoil and environment that what we are discussing, but interesting video none-the-less.

pb

 

[KevinJ]

I’d say it’s a fraction of a percent of cubs that have them. I’d also be willing to bet that the cubs with them accumulate more hrs than the average.

 

[stewartb]

Funny, I’d say the opposite. Commercial guys won’t spend the money, and they don’t need to.

 

[KevinJ]

Funny, I’d say the opposite. Commercial guys won’t spend the money, and they don’t need to.

Ultima Thule boys put many hrs on them?

 

[stewartb]

I wouldn’t know. Alaska’s a lot bigger than Ultima Thule.

 

[Crash Jr.]

It's definitely not limited to one group or another as to who buys the flaps. They've certainly been popular but it's accepted that only a fraction of all PA-18 owners will buy any particular mod. There's just a lot of cubs out there and among those, few get flown regularly and among that small group only a fraction of owners fly enough to justify the cost of a spendy mod.

As for the commercial guys getting PSTOL flaps; the ones that can afford them or justify the cost/benefit have them already or have them on order. They're no less popular among commercial flyers than they are among recreational pilots. Lots of part 135 guys getting their orders in right now so they can write them off on 2020's taxes. I would say the above comment is correct, the customers for the PSTOL flaps are the fraction of cub owners that fly a lot, have the money to spend, and can appreciate what they money buys them. They're not for everybody, just like 35's; but those that have them love them.

 

[mike mcs repair]

..Lots of part 135 guys getting their orders in right now so they can write them off on 2020's taxes. ..

tax write off:

it's not based on the ORDER DATE....

you only get to write them off, ONCE you put them IN SERVICE.......

 

[TurboBeaver]

Just had a friend in Idaho get his Cub back from a two year restoration, that included the PSTOL flaps, he loves the plane now, and confirmed the lesser nose angle with them on final: but simply commented
" They were NOT worth the $8500"
I like the idea alot, but when I redo my wings will simply extend the Cub flaps out
to 90" and move ailerons out to end of wings. There is another crowd that likes
6' flaps and then longer ailerons out to the end better yet. So probably should go fly that combo before deciding.[emoji848]


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[Farmboy]

Just to really mess with you guys and hope you can keep up, here’s some cross-mixing of threads.

Over on The Big Slip thread, this was just posted :

with Keller flaps my BCSC Super Cub slips very poorly, running out of rudder and becoming an clumsy sideways flying manoeuvre if you want to keep it in a straight line.
so I keep the flaps off or only at one notch when needing to slip into somewhere, hand dropping off throttle onto flap lever as I straighten out of the slip and then gently pull full flaps, takes some practice to keep the airspeed alive as once you pull the keller flaps at low energy (low airspeed) your speed bleeds off quickly.

So I find this interesting, and along the same vein as the topic here. It seems there is some data that some airplanes may need an increased tail section, or extended as Stewart mentioned, with the installation of Keller/Fowler flaps.

Does anyone else have data points on this?

Thanks,
Pb


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[stewartb]

With slats and Keller flaps I can't imagine needing to slip. One technique some guys with similar airplanes as mine use to slow the approach is to stand on a rudder with wings level on final to create more drag and slow it down.

 

[john schwamm]

Just had a friend in Idaho get his Cub back from a two year restoration, that included the PSTOL flaps, he loves the plane now, and confirmed the lesser nose angle with them on final: but simply commented
" They were NOT worth the $8500"
I like the idea alot, but when I redo my wings will simply extend the Cub flaps out
to 90" and move ailerons out to end of wings. There is another crowd that likes
6' flaps and then longer ailerons out to the end better yet. So probably should go fly that combo before deciding.[emoji848]


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Turbobeaver,
Yes, you need to fly them to see, they are worth it. Land slower, stalls slower, flat deck angle. What is not to like. Trust me, they are worth it. Just does not want to stall. Fly one and see.
John

 

[TurboBeaver]

Have not flown them John, but want too.
How would you compare PSTOL to standard old Sullivan 90" conversion???
I really love the huge difference the Sullivan's flaps made on floats over standard 5' flaps.
I suspect you have flown most of the combo's???
I am all ears [emoji848]
E

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[stewartb]

May as well add 9' and 12' Kellers. And now? Rev 3 vented flaps. Reports say they don't provide as much drag so landings are a little less steep but the added lift improves takeoffs. That sounds like a perfect match for floats.

 

[john schwamm]

Only flown stock and the Keller/Airframes. Could not be happier with the Airframes. The reduced stall speed by at least 5mph alone is magic, and the docile nature at slow speed is far better than stock. And that slow is no longer nose high.
John

 

[Airguide]

Picked a set last week that had been on order for a few months, 258&259 certified. Anxious to get them installed on my cub, Several thousand hours in Cubs mostly on Alaska Peninsula doing Part 135 and guide stuff with full tanks and one person and gear wish I would of had them some days!


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[Farmboy]

I didn’t mean to have this tangent off into a “value-added” discussion as it wasn’t the point.
I was just exploring thoughts of these on something other than a SuperCub, and potential side effects.
The hard part is it would be easy to snowball into way more of a project than it started. And if you need a bigger or longer tail, how you quantify what size that would be.
Theoretical questions of course, but my mind was wandering.


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[Airguide]

Oops sorry to go off course I thought you were asking Steve about his experience relating to Cubs. My experience with Airframes I don’t expect anything but good to happen when I get them installed.


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[Steve Pierce]

All I know is I feel pretty comfortable in my Cub and and I dam sure don't want to go back to stock flaps. As to how they work, no idea.


Stewart, I read that the 182 got a wider tail because pilots kept pranging the nose gear through no fault of the airplane. My 520 with a 3 blade prop and small horizontal has not problem with having enough elevator.

MCS, depends on the date you pay for them, not a thing to do with installation date. I am married to my accountant. 8)

 

[stewartb]

Imagine how good you'll be after TWO weeks!

Pstol flaps are a great mod. If I'd had them on my old -12 (especially on floats) I might still own that airplane.