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Thread: Vortex Generator - not enough elevator authority

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    Vortex Generator - not enough elevator authority

    I installed a VG kit in Feb 22 on my PA-12, it has wing and horizontal stabilizer VG's. Ever since, I do not have enough trim/elevator authority to get the stick back in flare, it's like pulling a donkey. And I slam off the mains on landing when solo. Anyone else have this problem? I spoke to micro aerodynamics and they are going to give me a call back.

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    frequent_flyer's Avatar
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    Quote Originally Posted by Poptart&Joey View Post
    I installed a VG kit in Feb 22 on my PA-12, it has wing and horizontal stabilizer VG's. Ever since, I do not have enough trim/elevator authority to get the stick back in flare, it's like pulling a donkey. And I slam off the mains on landing when solo. Anyone else have this problem? I spoke to micro aerodynamics and they are going to give me a call back.
    You seem to be describing an unacceptable stick force gradient rather than lack of elevator authority.

    I'm no expert on the PA-12 but where is the CG when solo? Also, are you at the trim limit and/or elevator limit in flare?

    My wild speculation is that the stabilizer is more effective because of the VG and not so easily overpowered by the elevators.

    My FX-3 Carbon Cub only has wing VG so will be interested to hear what others have experienced after adding stab VG.
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    stewartb's Avatar
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    VGs allow the wing to fly slower. If your tail isn’t keeping up you need bigger elevators, or toss some weight in the back.
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    sjohnson's Avatar
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    Can't speak to the PA12, but on my PA 18, adding VGs and big tires made it virtually impossible to get into a 3-point attitude at VG-assisted touchdown speeds. Adding an elevator gap seal fixed it.

    You can get a certified gap seal from Cub Crafters, or do your own with two pieces of polyurethane tape for each gap. Adhere two wide pieces lengthwise, one up and one down, with about a 1/2" or so of adhesive exposed on each long side, sticking one exposed length on top of the stabilizer and and the other on the bottom of the elevator, so that the tape passes through the hinge line. Try it first with blue masking tape or paper to get the hang of installing it (but obviously don't fly with blue tape).
    There are three simple rules for making consistently smooth landings. Unfortunately no one knows what they are.
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    skywagon8a's Avatar
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    Quote Originally Posted by Poptart&Joey View Post
    ....I do not have enough trim/elevator authority to get the stick back in flare, it's like pulling a donkey.
    Are you trimming nose up for landings or are you just forcing the stick back? If you are trimming, does your stabilizer trim all the way nose up? In the full nose up position the leading edge of the stabilizer is supposed to rest on the top of the top longeron. Sometimes the tube which connects the stabilizer to the trim screw jack is installed upside down. If this happens there is not enought nose up trim.

    It's a quick check. Just trim it full nose up and take a quick look. It's obvious if it's not right.
    N1PA
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    JP's Avatar
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    I added horizontal stab VGs about ten years ago to the -11. I noticed much more elevator authority. The tail is certainly far more responsive in slow flight. And slow flight in an -11 is spectacular to begin with with the VGs. You really have to work to stall in most attitudes. Stick force varies depending on trim.
    JP Russell--The Cub Therapist
    1947 PA-11 Cub Special

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    stewartb's Avatar
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    The OP needs to describe his -12. I’m guessing it’s got a 320 on a long mount?
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    frequent_flyer's Avatar
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    Quote Originally Posted by stewartb View Post
    VGs allow the wing to fly slower.
    That would seem to be the primary purpose of wing VG but what is the primary purpose of VG added to the stab? Is it to increase stabilizer effectiveness or elevator effectiveness? I would assume stab effectiveness increases but elevator effectiveness decreases particularly if there is free airflow through the elevator hinge line.

    Any reputable references to stab VG theory?
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  9. #9
    stewartb's Avatar
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    Given where you install tail VGs? They energize the elevator.
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    skywagon8a's Avatar
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    Quote Originally Posted by frequent_flyer View Post
    That would seem to be the primary purpose of wing VG but what is the primary purpose of VG added to the stab? Is it to increase stabilizer effectiveness or elevator effectiveness? I would assume stab effectiveness increases but elevator effectiveness decreases particularly if there is free airflow through the elevator hinge line.

    Any reputable references to stab VG theory?
    Think about the purpose of a trimming stabilizer. It is nothing more than a trimming surface which due to it's size doubles as a horizontal stabilizing device. The elevators are used to alter the trimmed attitude. When the elevators are moved to their maximum angle the airflow is disturbed at the hinge line to the extent of disturbing the boundary layer and the effectiveness of the elevator. The Vgs do their thing of altering the boundary layer flow aft of their location which is just ahead of the hinge line, thus restoring the boundary flow to the elevator's surface.
    N1PA
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    frequent_flyer's Avatar
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    Quote Originally Posted by skywagon8a View Post
    Think about the purpose of a trimming stabilizer. It is nothing more than a trimming surface which due to it's size doubles as a horizontal stabilizing device. The elevators are used to alter the trimmed attitude. When the elevators are moved to their maximum angle the airflow is disturbed at the hinge line to the extent of disturbing the boundary layer and the effectiveness of the elevator. The Vgs do their thing of altering the boundary layer flow aft of their location which is just ahead of the hinge line, thus restoring the boundary flow to the elevator's surface.
    I had not seen any pictures or drawings of stab VG installation. If they are installed just forward of the hinge line then, yes, they would only affect flow over the elevator. Thanks.
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  12. #12

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    Couple inches ahead of aft portion of horizontal.

    Were the VGs installed and the bottom of the horizontal stab?

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    Gordon Misch's Avatar
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    Were the VGs installed and the bottom of the horizontal stab?
    Yes, this.

    Also, as Stewart suggested, when my -12 had O-320 with long mount and -18 tail feathers (no VGs) it would not flare well while solo without some power. Now, with short mount, Crosswinds Stol kit and VGs it flares just fine without power and without touching the trim from cruise.
    Gordon

    N4328M KTDO
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    Photo of Poptart- VG

    Quote Originally Posted by Poptart&Joey View Post
    I installed a VG kit in Feb 22 on my PA-12, it has wing and horizontal stabilizer VG's. Ever since, I do not have enough trim/elevator authority to get the stick back in flare, it's like pulling a donkey. And I slam off the mains on landing when solo. Anyone else have this problem? I spoke to micro aerodynamics and they are going to give me a call back.

    i'm full nose up trim, and still fighting to get the stick in my lap. vgs on the wings and the horizontal stabilizer. I'm coming in at 60 over the numbers (same speed as pre VG) and also the same speed as a friends PA12 who has the VGs also.

    i'm slamming the mains.

    this never happened pre-vgs- I had plenty of stick left on landing, until touch down and roll.
    A 30k hour pilot went up solo (also owns a pa12) and is saying out of stick, its not right.

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    Click image for larger version. 

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  16. #16

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    O235 no tail feathers, OG 12

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    Are you saying you can’t arrest your descent at all? Or that you’ve run out of elevator authority? Cause a three point done right is ‘out of elevator’ also. Just needs to be at 6” height or so. No disrespect meant, just trying to understand. I added their VG’s last fall and did pre and post testing - nothing like that happened to mine.
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  18. #18
    KJC's Avatar
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    I had a -12 with O-320, long mount and micro VG’s. Absolutely no issues at all. I wonder if you broke the bungee spring or it’s not installed. It a spring and a 1/16 cable from the top of the elevator horn, over a pulley and attached to the stabilizer yoke. Look in the parts book. It shows it.

    Also I’d do a serious FOD check front to back. I have a feeling that the Vgs aren’t the cause of this.
    Last edited by KJC; 06-01-2022 at 07:35 PM.
    PA-12 N418BS

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    I agree with KJC. My plane is a NorthStar, and it doesn’t have the spring assistance on the elevator. It’s heavy all the time while landing, but it still works as designed.

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    Balanced elevators with a portion added outboard ahead of the hinge line have more authority in my experience. Standard elevators have less and the stick force increases more with elevator deflection. Seal the elevator-hinge gap as suggested plus the rudder-vertical stabilizer for more authority at low airspeeds.

    Gary

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    Use power on approach. I had a nose heavy 320 -12 w/non balanced tail. You could cheat it with power and make it work. But, it was work. I agree with others that something is not sorted. The VGs are an improvement in my experience, not a hinderance.

  22. #22
    skywagon8a's Avatar
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    Quote Originally Posted by Poptart&Joey View Post
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    When this picture was taken, was the trim in the full nose up position?

    IF the answer is yes, then the trim system is not installed correctly as the stabilizer is not resting on the longerons as it should be.
    N1PA

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    Steve Pierce's Avatar
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    The stabilizer yoke V should be pointing down. Check the stabilizer and elevator up and down throws per the Type Certificate Data Sheet. Would be the first 12 I have seen that doesn't get full elevator travel.

    Also where is your CG when this occurs?
    Steve Pierce

    Everybody is ignorant, only on different subjects.
    Will Rogers

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    skywagon8a's Avatar
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    Quote Originally Posted by Steve Pierce View Post
    The stabilizer yoke V should be pointing down. Check the stabilizer and elevator up and down throws per the Type Certificate Data Sheet. Would be the first 12 I have seen that doesn't get full elevator travel.

    Also where is your CG when this occurs?
    Also, the tabs on the cross tube which connects to the yoke should be up rather than down. I have found this error. Ask Seaworthy about his.
    N1PA

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    stewartb's Avatar
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    My question is whether the elevator travel is restricted or the elevator response is diminished, making the pilot wish for more travel. Big difference, and there’s nothing about a VG install that would change the elevator travel.

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    Quote Originally Posted by Poptart&Joey View Post
    I installed a VG kit in Feb 22 on my PA-12, it has wing and horizontal stabilizer VG's. Ever since, I do not have enough trim/elevator authority to get the stick back in flare, it's like pulling a donkey. And I slam off the mains on landing when solo. Anyone else have this problem? I spoke to micro aerodynamics and they are going to give me a call back.
    I am trimming full aft against the stop. Here's a pic and one area where the vg's were not installed exact

    Click image for larger version. 

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  27. #27
    KJC's Avatar
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    I don’t know how you screw up vg placement especially with the stickers they give you, regardless, the thing would probably fly fine with them on sideways. Do this:

    1). Check for fod in the bellcrank behind the rear sear under the cloth baggage. Make sure everything is lubed and free travel.

    2). Check and make sure the trim is actually moving under an air load. With a single grove trim it may be slipping and you just turning the handle and it’s not moving.

    3). Put the tail on a sawhorse and pull all 4 stab inspection covers. Check the trim yoke and make sure the v part is facing down. Check the bungee spring and pulley and make sure it is pulling the elevator up as the stab is trimmed nose up (leading edge down).

    4). Check the elevators for full deflection. Check the type cert. for degrees of travel because I don’t recall the exact numbers 25/30 degrees??

    A stock O235 with no flaps shouldn’t have this problem at all. I’m thinking something changed and I doubt it’s the vgs. Even with an O320 long mount and flaps, I never used full nose down trim and I always keep the plane in trim for the speed.

    Let’s see a pic of the ones on the stab.
    PA-12 N418BS
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    skywagon8a's Avatar
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    Let’s see a pic of the stab through the front slotted inspection plate with the stab in the full nose up position.
    N1PA

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    I think you are just experiencing the new normal with the VGs. They are allowing the airplane to fly slower Go work the MCA. Get a feel for the elevator load at level mca. Then add power for a slight climb. MIght clear things up a bit.

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    Steve Pierce's Avatar
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    What is MCA?
    Steve Pierce

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    Minimum controlled airspeed.
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  32. #32
    Steve Pierce's Avatar
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    I am not good at all these abbreviations.
    Steve Pierce

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    Will Rogers
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    aflyer's Avatar
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    They are called TLAs.

    Three Letter Acronyms

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    Steve Pierce's Avatar
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    I have to use the Urban dictionary a lot too.
    Steve Pierce

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  35. #35
    skywagon8a's Avatar
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    Quote Originally Posted by Steve Pierce View Post
    What is MCA?
    Slow Flight
    N1PA
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    Joey indicates that it flew fine before VGs, and no other changes were made. If it were my airplane and I was sure the installer had not left lead in the engine comartment or made any other changes, the VGs would come off for a test flight.

    She did have the wing painted after installation, which could be a source of reluctance - but the philosophy should never be "add a mod, and if it doesn' work well, add more mods."

    Opinion. Like Steve, I too hate acronyms. They force us to do strange things, like sending notices to missions instead of airmen.

  37. #37

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    Quote Originally Posted by Poptart&Joey View Post
    I installed a VG kit in Feb 22 on my PA-12, it has wing and horizontal stabilizer VG's. Ever since, I do not have enough trim/elevator authority to get the stick back in flare, it's like pulling a donkey. And I slam off the mains on landing when solo. Anyone else have this problem? I spoke to micro aerodynamics and they are going to give me a call back.

    Well - it appears the bungee/spring is busted- and the timing was such the VGs likely masked it - degrees of travel is accurate at 25, upping to 27. Also found a leak in static affecting ASI- standby- will have update when fixed.

  38. #38
    akavidflyer's Avatar
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    Quote Originally Posted by Poptart&Joey View Post
    Well - it appears the bungee/spring is busted- and the timing was such the VGs likely masked it - degrees of travel is accurate at 25, upping to 27. Also found a leak in static affecting ASI- standby- will have update when fixed.
    Buddy had his spring break on his 12 and it jammed the elevator almost completely (he could move the stick fore and aft about an inch). He was able to land it hot using trim and throttle.

  39. #39
    stewartb's Avatar
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    If the elevator bungee is broken wouldn't aft stick become easier?
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  40. #40

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    Curious what your thoughts are?

    Quote Originally Posted by stewartb View Post
    If the elevator bungee is broken wouldn't aft stick become easier?

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