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Question about additional horizontal fin

RVBottomly

PATRON
Asotin County Washington (KLWS)
I saw an interesting PA 12 the other day. One thing stood out: it had extra horizontal fins on the fuselage just ahead of the trim opening.

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PA 12.jpg

It had vortex generators and bigger gear, but otherwise didn't seem heavily modified.

Does anyone know why those fins would be added?
 

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I believe it is part of the vortex generator kit. Some have the small vg’s on the underside of the horizontal stab and other kits have the fin you show in the picture
 
Correct. This is part of the Cub Crafters VG STC. The entire thing, both strakes, clamps etc, including attachment hardware weighs in at almost 2.5 pounds.
 
That kit was developed by Boundary Layer Air, which later sold the STC to Cub Crafters. The “strake” forward of the horizontal stab. helps keep the airflow “organized” across the underside of the stabilizer AND elevator, while the VGs mounted under the tail only affect flow over the elevators.

MTV
 
That looks to be a highly modified 12. Cub gear/tail, flaps, left side door, bushwheel/garr arrow tailwheel, suspect bigger engine. DENNY
 
That kit was developed by Boundary Layer Air, which later sold the STC to Cub Crafters. The “strake” forward of the horizontal stab. helps keep the airflow “organized” across the underside of the stabilizer AND elevator, while the VGs mounted under the tail only affect flow over the elevators.

MTV
Mike,
Thanks. I wonder if my Cub could benefit by the addition of VGs under the trailing edge of the horizontal stabilizer as well as the strakes. Or would that be redundant? I have temporarily removed the strakes to access the rear fuselage inspection holes, which were partially covered by the strakes, and was amazed at how heavy the installation is. Each strake weighs almost a pound and there is a lot of attachment hardware for them inside, nuts, screws, washers, clamps and bracketry securing it all to the upper longerons. Probably it all adds over two pounds.
I have also wondered how much weird air comes from having the gap between the root of the horizontal stabilizer and the fuselage on a Cub.
 
Hi MTV. I just can't let your comment go unanswered. The BLR kit was developed primarily to attempt to cure the moose stall, which was the rage of the day. BLR couldn't limit the elevator travel as it is a primary control with limits found in the TC. The next easiest would be to add to the surface area of the stabilizer, to simply overpower the elevator. It works. However if you have a heavy engine / prop the last thing you need are a pair of fins holding the tail up on landing. You suggest that the strakes keeps the airflow organized over the bottom of the stabilizer and elevator. Do you have data showing the bottom of the stabilizer completely stalled with or without the strakes? I have video of the same in a fully developed spin and the air flow over the stabilizer bottom is quite normal. Because of this continuity of airflow the vg's along the stabilizer trailing edge are doing their job along the entire span of the tail. Not just 2 or 3 feet in the center. I might add that during testing the number of vg's under the stabilizer were reduced from the original array, as they were not needed to do the job. Less vg's less maintenance. I don't mind you hyping your favorite brand, just don't diss the rest with unsubstantiated claims. Show me the data. Jerry
 
The lift produced by the tail is in a downward direction. The vortex generators on the bottom allow the tail to continue generating lift in a downward direction at slower speeds. I’m aware that the strake is part of the boundary layer mod and I am assuming that the stc requires that they be there, I also assume they aid in the tails developing downward force at slow speeds.
 
Hi MTV. I just can't let your comment go unanswered. The BLR kit was developed primarily to attempt to cure the moose stall, which was the rage of the day. BLR couldn't limit the elevator travel as it is a primary control with limits found in the TC. The next easiest would be to add to the surface area of the stabilizer, to simply overpower the elevator. It works. However if you have a heavy engine / prop the last thing you need are a pair of fins holding the tail up on landing. You suggest that the strakes keeps the airflow organized over the bottom of the stabilizer and elevator. Do you have data showing the bottom of the stabilizer completely stalled with or without the strakes? I have video of the same in a fully developed spin and the air flow over the stabilizer bottom is quite normal. Because of this continuity of airflow the vg's along the stabilizer trailing edge are doing their job along the entire span of the tail. Not just 2 or 3 feet in the center. I might add that during testing the number of vg's under the stabilizer were reduced from the original array, as they were not needed to do the job. Less vg's less maintenance. I don't mind you hyping your favorite brand, just don't diss the rest with unsubstantiated claims. Show me the data. Jerry
,

Im not questioning the efficacy of the VGs under the tail. I’d suggest you visit with Penney Nixon, who did some pretty extensive flight test on various mods on the Super Cub, including the VG kits. I believe he very much liked the BLR kit. I’m not a Flight Test Engineer, or even a test pilot, Im certainly not qualified to explain the aerodynamics of these things, but he is.

Years ago, I also visited with Bob DeReusch (I’m quite certain I butchered the spelling of his name) at Boundary Layer Research. We discussed this at length, but that’s also been 25 or 30 plus years ago, so my memory may not be perfect. I don’t take Prevagen. But he did show me several videos of the BLR kit, including of the tufted tail of a Super Cub. It was impressive. Have you seen those videos?

I for one prefer the BLR kit. But diversity is good.

Good to see you post on here. Please post more frequently.

MTV
 
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That looks to be a highly modified 12. Cub gear/tail, flaps, left side door, bushwheel/garr arrow tailwheel, suspect bigger engine. DENNY

LOL. You are right, of course. It's just that I've never actually seen an unmodified PA 12, so I already don't know what to look for.
 
Untitled.pngThis is the best I can do with windows 10. MTV, You were a lot braver than I in trying to spell Bob's name. I am the last person they would be willing to share info with. Take Care. Jerry
 

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, I’d suggest you visit with Penney Nixon, who did some pretty extensive flight test on various mods on the Super Cub,
MTV

He was one of our flight training instructors in Naknek, when he was Commander of the King Salmon air force base back in?? late 80's or early 90's...

he got all excited when some of our January caribou hunters reported guys in all white on snow machines.... he said they were "sneaking up" to "attack the base"..... training games...
 
Penny Nixon flew the Supercub moose stall test flights in what, 1994-95? I don't recall Micro VGs being around back then. BLR had very little if any presence at that time, either.

Does BLR use a tail strake on Cessnas? Any other aircraft types? I recall that early Micros didn't address the tail, or was it just the fin? My Cessna's Micros include stabilizer and fin VGs. Heck, I've read VGs don't help a Cessna, anyway, so I guess it doesn't matter. (Wrong!) ;)
 
Hi stewartb. I have the first Micro STC for the Piper Cub. It is dated Jan 4 1993. Obviously our testing was complete at that time. Early Micro's did address the tail because the stock tail would not stall the improved wing. You are correct BLR didn't come in until a couple of years later. Later changes to Micro were primarily influenced by marketing. Jerry
 
My PA11 had an early MicroVG’s kit on the wings. it worked.

As I recall, Penny Nixon was a Husky Driver maybe? :pop::pop:
 
After seeing those strakes on a couple 4 place cubs recently I asked Kirk Ellis about them. Apparently he is just now trying them and says they make a noticeable improvement in handling. Didn't really get more detail than that. Must help with heavier tailed Piper aircraft and pitch control.
 
Cubcrafter strake and VGs installed on my plane before I bought it. Went out to do stalls shortly after purchase. Plane wont stall. What happens is the wing is solid as a rock no instability. No pre-stall feel. No wing drop as occurs with stall. The plane drops like a rock in nose high attitude and remains that way until corrected. This was done with half tanks, power on and off and as I remember no flaps, just me the pilot and trimmed as I fly it. Might have tried all the flap settings but can't remember. I was thinking that as the angle of attack for the horizontal increased that tail strake would get hit by the relative airflow thus pushing the horizontal up and in some way counteracting maximum elevator authority. I am not an aeronautical engineer. Must say aflter all that testing it made me feel a lot more confident in the plane.
 
Penny has a PA14 that’s been in his family for a long time....as well as a 180. Super interesting person to spend a few hours with.
 
Hi stewartb. I have the first Micro STC for the Piper Cub. It is dated Jan 4 1993. Obviously our testing was complete at that time. Early Micro's did address the tail because the stock tail would not stall the improved wing. You are correct BLR didn't come in until a couple of years later. Later changes to Micro were primarily influenced by marketing. Jerry
I had an early Micro VG kit for my cub, did not have VG's under tail on mine on that kit, and on wings did not go inboard of fuel tanks back then. Micro sent me later version when we recovered the wings and now have them all the way inboard, and under the tail too.
John
 
I had an early Micro VG kit for my cub, did not have VG's under tail on mine on that kit, and on wings did not go inboard of fuel tanks back then. Micro sent me later version when we recovered the wings and now have them all the way inboard, and under the tail too.
John


make a difference??in anything that you could notice??
 
Did a lot of testing, before and after install on original Micro VG set back in the day. Basically about 20%+ reduction in stall speeds back then. And much firmer feel on controls at slow speeds.
Made so many changes to the cub when we put the revised set on hard to tell. Added big tailfeathers, flaps to fuselage, Airframes/Keller flaps. Doubt I could tell anything different after all those mods with just a few more VG's. The flaps are the biggest improvement to me on a cub, expensive but big change in stall speeds and attitude. Really almost just sinks now at slow speeds, hard to stall.
John
 
Doesn’t Charlie Center’s 180 hp mod also require the tail strakes?

After all, that is a PA12.

Or am I wrong again?
 
When I talked to Charly about -12 mods he had a relationship with Micro and used slightly larger than normal Micro VGs. I don't recall anything about a strake. He does have the STC for the beefed-up tail yoke and recommends that for high power -12s. That adds a lower strut to the stabilizer, not a strake.
 
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Stewartb: that’s what I remember. Wasn’t there a airplane magazine with his green and white 12 on the cover with a photo of the tail showing the strake and the mini-struts to the front of the horizontal? So was that was a combination of Charlie’s struts and VG’s strakes?

When I talked to Charly about -12 mods he had a relationship with Micro and used slightly larger than normal Micro VGs. I don't recall anything about a strake. He does have the STC for the beefed-up tail yoke and recommends that for high power -12s. That adds a lower strut to the stabilizer, not a strake.
 
Memory fades, but comes back in very short bursts.....

One of the things I recall about talking to Bob at BLR about the strakes was the turbulence that comes off the inboard end of the flaps in a stock Cub. He actually fabricated temporary extensions to see what extending the flaps in to the fuselage did to that turbulence. He found that doing so actually increased that turbulence that spun down the side of the fuselage and affected the horizontal stabs.

As I recall the discussion, that was what led to the strake initially, at least....to better organize the flow over the inboard sections of the tail. I can't recall if there were other things the strakes address, but I believe that was what originated the idea of the strakes. His before and after strake videos were quite convincing as well.

MTV
 
As a further point, not directly related to VGs, Penny Nixon’s flight tests accomplished something else as well. These tests were done under contract to the US Department of Interior’s Office of Aircraft Services and the Alaska State Troopers Aircraft Division. The purpose, as noted earlier in this thread, was to evaluate stall characteristics of Super Cub aircraft. He used at least two different Cubs, one Trooper Cub, one OAS.

The planes were flown with different size tires. Part of these tests were to evaluate whether “large tires” were a contributing factor in unusual or modified stall behavior of Cubs. At the time, certain quarters in the FAA had concluded, with zero aerodynamic or flight test basis, that “large tires” were the root cause of the infamous “moose stall”.

Nixon’s flight tests, completed by a certified Flight Test pilot, demonstrated clearly that increased tire size simply slowed the airplanes cruise speed, but had no negative influence on stall characteristics. Which, being actual flight test data, contracted by another govt. agency, shut the FAA up at least about how tires contributed to stall/spin accidents.

MTV
 
The Cub I saw used belonged to Gene Zerkel at AK Aircraft Sales. I hung around there a fair bit and was curious why there were hundreds of pieces of yarn taped to the Cub and why they were swapping tires. I spoke with Mr. Nixon a few times about the flights. I don’t recall VGs being part of the tests. I recall they chose it because it conformed to the type certificate with no mods. It was all about tires, and his conclusion was that while big tires affected CG, they did not cause moose stalls. It seemed to me that the tundra tire worksheet came out after that, but I don’t know if it was the result of those test flights.
 
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Stewartb: that’s what I remember. Wasn’t there a airplane magazine with his green and white 12 on the cover with a photo of the tail showing the strake and the mini-struts to the front of the horizontal? So was that was a combination of Charlie’s struts and VG’s strakes?

Yes, winter 2000/2001 issue of Northern Pilot magazine.
Cover photo shows added strut, but there's no strakes.
Text of article mentions an added horizontal stab "lift strut that connects to the jackscrew assembly".
Also mentions using oversized VG's from MicroAero on the wings,
squared-off rudder & elevator, & a Crosswinds STOL kit (cuffs, fences, & modified tips).

IMG_20220131_071903269.jpg
 

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