Does any backcountry aircraft use a true fowler flap?


I know of a few aftermarket designs which have included slotted or double slotted flaps. This isn't what I am looking for.


For a long time now, I have been toying with some concepts to reduce the nose-high attitude at stall with our Bearhawk, to give better forward visibility during a true max-performance landing. I also want more lift so we can fly slower. The flaps look like the best bet here - benefits are ripe for the picking, and changing the flaps isn't a colossal modification. The Bearhawk flaps are honestly 'bolt on'.

Looking at some data sourced from an old Naval Aviation book, I found the following data which suggests that fowler flaps are greatly superior to both slotted and plain flaps in terms of reducing the AoA at stall, and for increasing the lift too. Apparently a win-win. They basically redouble the same benefit over again, compared to no flaps (basic section).


Before I go any further with my hair-brained investigations, like starting an engineering design and dragging up my old university texts, I wanted to see if any other light aircraft use this kind of flap. Anyone know of any??

Obviously it's not all goodness and sunshine, otherwise more aircraft would be using them. I am guessing weight of the rails, complexity, and extra drag would be some of the main issues.

But.... an extra 35% lift and 4 degrees lower nose attitude sounds pretty darn good!
If I can shave off just 4 kts at the stall, I would be landing at a cub-like airspeed... and cruising at 130KTAS... so in my head it's worth investigating at least a little bit.

Which have double slotted?

Cessna uses fowler flaps quest kodiak does, look up sherpa aircraft they use fowlers too.

cbfraser wrote:Which have double slotted?

There are a couple of outfits setting double-slotted kits for Super Cubs, and other aircraft which can use the same wing like the PA-20. STOLperformance is an example


The bright orange Super Maule posted in the JC thread also seems to have them installed.

cstolaircraft wrote:Cessna uses fowler flaps quest kodiak does, look up sherpa aircraft they use fowlers too.

Sorry to nit-pick, but the differences are relevant here.
Cessna claims flaps on some of their common aircraft have "some fowler action", like the 100 and 200 series aircraft, but all that I have seen aren't true fowlers, they are slotted in every sense.

True fowlers extend backwards and "significantly" increase the area, before dropping down. They also form an unbroken camber with the top wing surface. Slotted flaps let some air over the top, and don't significantly increase the wing area.
Do you know of a Cessna which uses a true fowler?

Thanks for reminding me about the Sherpa!!
Not much chance I can see the Sherpa in person to have a look over their designs.... Although, this looks like 70% fowler + 30% slotted.


This photo here is a great illustration, you can really see the increased wing area. This is what makes all the difference.

cstolaircraft wrote:Cessna uses fowler flaps

I was thinking the same thing and while a google search does indicate the same Batton's description of a true fowler seem to prove other wise. I also thought the Helio Courier had flowers but don't know if they are true fowlers. The SuperSTOL claims to have fowlers.

Battson, Get to designing...I'd love to install them on my BH when I start working on the wings in a year or so

The Cessna high wing airplanes (170B, 180, 185, 206) have what are considered "Semi Fowler" flaps. They do in fact meet the definition of the Fowler design, ie: they both deflect and increase the effective wing area, they are mounted on tracks, and move aft, thus increasing the wing area as they deploy. . They do not employ several flap sections, however, as do many transport aircraft flap systems in their very complex (read heavy) flap systems.

Take a look at what Doug Keller is doing with the flaps on the Super Cub clones: http://www.performancestol.com/about_us.htm

These are, by his definition, double slotted flaps, but they appear to be quite effective.

MTV

Battson wrote:

cstolaircraft wrote:Cessna uses fowler flaps quest kodiak does, look up sherpa aircraft they use fowlers too.

Sorry to nit-pick, but the differences are relevant here.
Cessna claims flaps on some of their common aircraft have "some fowler action", like the 100 and 200 series aircraft, but all that I have seen aren't true fowlers, they are slotted in every sense.

True fowlers extend backwards and "significantly" increase the area, before dropping down. They also form an unbroken camber with the top wing surface. Slotted flaps let some air over the top, and don't significantly increase the wing area.
Do you know of a Cessna which uses a true fowler?

Thanks for reminding me about the Sherpa!!
Not much chance I can see the Sherpa in person to have a look over their designs.... Although, this looks like 70% fowler + 30% slotted.


This photo here is a great illustration, you can really see the increased wing area. This is what makes all the difference.

I do have to agree that cessna fowlers dont add much but they do add some.

Experiment with making some tracks for the fowler flaps you would like to have. test them on the bench then when perfected put them on the plane and tell the bearhawk guys what you did I think you idea would be a big hit. Maybe add some retracting slats too?

whee wrote:

cstolaircraft wrote:Cessna uses fowler flaps

I was thinking the same thing and while a google search does indicate the same Batton's description of a true fowler seem to prove other wise. I also thought the Helio Courier had flowers but don't know if they are true fowlers. The SuperSTOL claims to have fowlers.

Battson, Get to designing...I'd love to install them on my BH when I start working on the wings in a year or so

The super stol does have fowlers

Piper's fat winged airplanes lift well and make good slow power/pitch approaches but the nose is high and the guy in back can't see much. Cessna used a thinner, bigger, faster wing but it didn't slow down as well until they put the fowler flaps on it. I always thought a combination of the two would work well. Never saw anything like that until a guy came down with a new Cub, a Savage I think, that had slats and effective flaps. I really liked it, but I don't think they were fowler type flaps.

At 1:20 in this video you get a glimpse of the Kodiak flap action. I'd hate to muddle up the cleanliness of the BH by adding tracks that are always hanging out.

C-130 is a backcountry aircraft... there aren't many others that can haul 35,000 lbs into a 4000 ft strip at 4000 MSL. And I might be understating the performance of the latest model. It has Fowler flaps.

whee wrote:At 1:20 in this video you get a glimpse of the Kodiak flap action. I'd hate to muddle up the cleanliness of the BH by adding tracks that are always hanging out.

Yeah I hear you loud and clear, Jon.
But... the only way to achieve this with an existing Bearhawk wing, will be using tracks, like the first Sherpa picture above. I would be willing to accept it for a big improvement in performance...

I don't think a little fowler / slotted flap like a Cessna uses would be worth the effort, for only a little extra wing area. Those big Sherpa flaps hanging waaaay back are a big part of why is can fly so darn slow, for it's size.

My two issues are whether the rear spar can handle all the torque, I'll need Bob's advice; and then whether I can recycle the existing flap drive or if I would need to go electric.

Electric...eww...please don't.

Adding huge flaps like the Sherpa would be worth the couple mph of speed loss.

Found an article by Russ Erb that says by his calculations the stall speed would only be reduced 1 knot by adding fowler. He also said Bob had planed on using fowlers so maybe the rear spar is already engineered for them.

whee wrote:Electric...eww...please don't.

Adding huge flaps like the Sherpa would be worth the couple mph of speed loss.

Found an article by Russ Erb that says by his calculations the stall speed would only be reduced 1 knot by adding fowler. He also said Bob had planed on using fowlers so maybe the rear spar is already engineered for them.

Yeah... I know....

That could be a gold-strike, if Bob already has plans then it might be halfway done already!

As a fellow engineer, I'm sure you'd agree it's basically impossible to calculate this stuff accurately... But I agree the span of the BH flaps isn't exactly "wide span".

There is at least one BH wing project out there with a Fowler design. Perhaps he will chime in. He spent a serious amount of effort and coin on the project so far.

As for load calculations, I think they might be more straightforward (and accurate) than you might think. A lot of work has been done in this area.

lesuther wrote:There is at least one BH wing project out there with a Fowler design. Perhaps he will chime in. He spent a serious amount of effort and coin on the project so far.

As for load calculations, I think they might be more straightforward (and accurate) than you might think. A lot of work has been done in this area.

Do you know who it is? It's a pretty tight community, hopefully I can make contact. Thanks very much for the intel!

I think the mechanical engineering side of the calculations would be very straightforward (load, stress/strain, etc.). It's the aerodynamic calculations which I don't put any weight in.

There's not many personally available computational engines around for a 3D flow model which work without a huge amount of validation (the old BS in, BS out theory). Sure you can do it for 2D sections, but the assumptions you are forced to make throw the benefits we're talking about here into the margin for error.


My next question is - does anyone actually believe the potential benefits are big enough to make it worth trying?? Has anyone got experience of the benefits from these aftermarket cub flaps? MTV?
If someone else it trying it on a BH, maybe I wait and see what they learn...

If a Piaggio Avanti (or whatever they are calling it now) landed at Johnson Creek would that count? Don't laugh, I saw the picture of the MU2 there.

The Piaggio does have a true fowler flap. It is not commonly used in the aviation world. Like you said, most airplanes have slotted flaps: Bearhawk, Pipers, or slotted flaps with fowler-motion (Cessna 100 series).

Just a thought, why not keep what flaps you have and think about how to get more out of them? If you have enough room in the flap cove you could put some vortex generators on the flaps themselves. Just enough clearance so that in your flaps up case they are not getting smashed. These would be smaller than what is normally available out there for purchase but you could make some simple ones to give it a try. You're experimental (N registered in New Zealand, right?) so you could give it a try pretty easily. If it works, then you could possibly get your flaps to deflect another few degrees to keep the L/D where it should be for a nice approach. What is the flap angle for landing on a Bearhawk anyway?

Boeing used this on some of their machines to get a better deck angle for approach and landing.

It looks like you are a handy guy so it would probably worth a shot.

Battson wrote:My next question is - does anyone actually believe the potential benefits are big enough to make it worth trying?? Has anyone got experience of the benefits from these aftermarket cub flaps? MTV?
If someone else it trying it on a BH, maybe I wait and see what they learn...

I fly with a high time cub pilot with both standard flaps and the double slotted Keller, somewhat regularly. This is in a 180hp "experimental" Super Cub build (but about 99% identical to a certified). The Keller flaps reduce stall speed by a minimum of 5mph and significantly reduce the AOA. The pilot states that the controls seemed to gain a bit of feel/authority with slightly less mush (because reduced AOA maybe? More clean air over the control surfaces? I don't know).

YMMV, but I've heard nothing but good reviews of the double slotted setup...