Vertical Stabilizer flutter

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aabreu
Posts: 7
Joined: Sun Nov 01, 2020 12:14 pm

Vertical Stabilizer flutter

Post by aabreu »

Hi. New to the forum.

I'm part of a flying club that owns a Pietenpol. We are located in Toledo Ohio. KTDZ.
I've flown it for over 10 years. It's amazing how bulletproof the design is. However, we have run into things over the years.

My club members are curious if other Pietenpol fliers notice the vertical stablizer flutter during climbout. It is concerning.
In fact, one of the bracket at the root of the stabilizer broke due to fatigue. The replacements appear to be holding up. My theory is that this flutter caused the failure. I was grateful the stab is held on by four brackets. Glad I even noticed it before trying to take off again.

We are contemplating building another stablizer and moving two of the wires towards the leading edge of the stabilizer to minimize the deflections.
Has anyone else modified the location of these wires.

Would appreciate to read about other's experiences and thoughts.

Thanks a bunch,
Andy Abreu
Light Sport CFI
Toledo Buzzards Light Sport Aircraft Club
N6186L
KTDZ
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taildrags
Posts: 436
Joined: Tue Mar 07, 2017 10:39 pm

Re: Vertical Stabilizer flutter

Post by taildrags »

Andy; what engine is on the airplane? And what do the windscreens look like? I'm just wondering if the vertical stabilizer vibration is in sync with the propeller pulses. Similar question with the windscreens, wondering if they might be aggravating or creating the propeller stream getting to the VS. Personally, I have not noticed anything like that on my airplane, but I did have one of my tail brace wires break in the past.

There are plenty of videos out there that pilots have shot in all flight regimes, looking back at the tail, including many recent ones. I have some of Mike Cuy's where he's demonstrating his smoke system, but there are plenty of others out there.

Oscar Zuniga
Medford, OR
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Richard Roller
Posts: 128
Joined: Mon May 22, 2017 11:14 am
Location: Olathe, Ks.

Re: Vertical Stabilizer flutter

Post by Richard Roller »

I fly N34KP, which has been flying since 2000. I've never had any problems with vertical stab flutter or vibration. I am curious about your statement where you are considering moving one set of wires forward. I'm curious if the airframe is built to the plans in that respect. Do the forward set of wires go to the forward spar of the horizontal stab?
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taildrags
Posts: 436
Joined: Tue Mar 07, 2017 10:39 pm

Re: Vertical Stabilizer flutter

Post by taildrags »

Richard: I don't know if all Grega GN-1s are setup like this, but Ernie Moreno's is. See pic.

-Oscar
tail01.jpg
aabreu
Posts: 7
Joined: Sun Nov 01, 2020 12:14 pm

Re: Vertical Stabilizer flutter

Post by aabreu »

Hi Everyone,
Thanks for the input.
One of the guys in the club "Glen" went out and tightening the wires to see if that made a difference in flight.
Here is a video of the stabilizer dancing around.
https://youtu.be/Z1WhuC3XpLQ
Needless to say, he made one go around the patch and put her back in the hangar.

Glen tore off the fabric and found some failed glue joints.
I have some pictures in my text messages showing where the glue joints broke if anyone is interested.
Seems like the fabric was the only thing holding the stab together. :!:

Not sure what we plan to do next. Repair the glue joints? Build another stab? Relocate the wires forward? Are design changes required to move the wires forward? We plan to discuss it tonight.

Another thought. At high AOA, there is allot of turbulence hitting the tail. I wonder if adding some vortex generators on the center section of the wing could tamp down the turbulence back there.


Andy Abreu
N6186L
Toledo Ohio
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Richard Roller
Posts: 128
Joined: Mon May 22, 2017 11:14 am
Location: Olathe, Ks.

Re: Vertical Stabilizer flutter

Post by Richard Roller »

Is it a Pietenpol or a Grega?
aabreu
Posts: 7
Joined: Sun Nov 01, 2020 12:14 pm

Re: Vertical Stabilizer flutter

Post by aabreu »

It's a Pietenpol.
aabreu
Posts: 7
Joined: Sun Nov 01, 2020 12:14 pm

Re: Vertical Stabilizer flutter

Post by aabreu »

Here is an edited version of the video.

https://youtu.be/4DpfgoyMYpY

Yikes.

Andy Abreu
6186L
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taildrags
Posts: 436
Joined: Tue Mar 07, 2017 10:39 pm

Re: Vertical Stabilizer flutter

Post by taildrags »

Andy; your pilot should consider himself lucky to still be around. That thing could have flown apart at any moment, especially if there had been bumpy air. As far as a fix, I would say that 90+ years of experience of Air Campers flying safely and successfully without structural failures of that sort would point to nothing more than a simple repair & reinstall, or replace with a new VS, and go fly again. It may be that a glue joint or three broke loose and that's what precipitated the fluttering... not the other way around.

I repaired my VS after it broke in a nose-over incident. It's flying without any problems such as your video shows. Some pix of my repairs here- http://flysquirrel.net/piets/repairs/repairs.html , about the middle of that page. Fun and educational, not at all difficult.

Oscar Zuniga
Medford, OR
Air Camper NX41CC, A75 power
Spark6
Posts: 22
Joined: Tue Jul 07, 2020 11:55 am

Re: Vertical Stabilizer flutter

Post by Spark6 »

That video is alarming, to say the least! Good on you guys for grounding the airplane and figuring it out. Was there any play or movement noted in the V-stab during y'all's preflights leading up to this? Or did the movement only show up in flight?
- Barrett
aabreu
Posts: 7
Joined: Sun Nov 01, 2020 12:14 pm

Re: Vertical Stabilizer flutter

Post by aabreu »

Hi guys,

Spark6, I think the vibration has been there for a very long time. Years.
I suspect this caused the forward brackets to fatigue and break.
Never noticed anything unusual about the vertical it during the pre-flight. Only during flight. The fabric does allot to keep the structure together. Thank god. Wonder what other surprises are lurking underneath the fabric. :shock:

Taildrags... Thanks for posting the pictures for building a new vertical. That will come in very handy. We decided to purchase the wood kit for a new vertical. Saving the old vertical as a template for the holes. Reuse the hinges.

May try to disassemble and reglue the broken one back together... but more likely we will just hang it on the hangar wall for decoration.

Be safe.
Andy Abreu
N6186L
tom kreiner
Posts: 62
Joined: Sat Feb 25, 2017 9:49 am

Re: Vertical Stabilizer flutter

Post by tom kreiner »

Andy,

Does anyone in your chapter have a set of plans? On page 7 of mine, the wires from the Vertical Stab to the Horizontal have a common point at the Vertical, one going towards the leading edge, one going to the hinge edge of the Horizontal.

Your plane, in contrast, has two unique wires, one towards the front of the Vertical and the other towards the rear. While I don't know what the exact effect of this may have had on the life of your Vertical Stab, there could have been a destructive vibration that was caused by the two sets of wires.

Please chime in if y'all have seen other Piets with this same split wire setup, as I've only looked at around 30 or so Piets, and I can't recall seeing the wires split on any other aircraft.
aabreu
Posts: 7
Joined: Sun Nov 01, 2020 12:14 pm

Re: Vertical Stabilizer flutter

Post by aabreu »

Hi Tom Kreiner

Yes we have a set of plans. We bought a wood kit and plan to remake the stab to the plans. We'll see how it goes.

Your probably talking about the picture of the yellow Piet.
Take a look at the video above. Our Piet is blue. The wires are per the plans.

Be safe
Andy
donbrewer
Posts: 23
Joined: Sat May 27, 2017 2:36 pm

Re: Vertical Stabilizer flutter

Post by donbrewer »

Hi Andy, Regarding your vertical stabilizer flutter [vibration], perhaps I may make a couple of observations after watching your video. The vibration appears to occur at a high rpm and during climb out. When you reduced power, the vibration seems to stop as seen during your descent and approach. The airspeed difference between climb out and descent is not more than @ 20mph, so it appears airspeed itself is not causing the vibration. You said you found broken glue joints in the stabilizer. I do not think the glue joints broke and caused the vibration. I think the vibration caused the glue joints to fail. What caused the vibration? I believe you should look closer at your engine and airframe. I see the horizontal stabilizers moving a bit but are held steady by the wires. I feel the vibration is through the airframe and horizontal stabilizers. a couple of things I can suggest: 1. Check the engine mount rubber shock dampers for hardness and wear. 2. Check the propeller tracking as this can cause big time harmonic vibration of the engine and airframe. I do believe you need to rectify the cause of the vibration and not the effect. Cheers, Don Brewer
aabreu
Posts: 7
Joined: Sun Nov 01, 2020 12:14 pm

Re: Vertical Stabilizer flutter

Post by aabreu »

Hi donbrewer

I agree. The vibration does happen at high angles of attack and high power settings.
At high angles of attack, there is allot of drag created by the wing.. and associated turbulence. My guess is this the cause. I'm not sure how to minimize this other than reducing the angle of attack and climb rate. I wonder if vortex generators would help somehow.
Another theory I have is the prop. We make our own propellers. We should have swapped it out with Prince prop we have to see if it went away. Too late now. Once we replace the stab, I'm thinking the wobble will not be noticeable. I've seen videos where pcs of string are taped to surfaces to see the air flow. Maybe we can experiment with it.

Fly safe,
Andy
doug bryant
Posts: 18
Joined: Mon Feb 08, 2021 7:42 pm

Re: Vertical Stabilizer flutter

Post by doug bryant »

i flew my first piet approx 125hrs with my home made prop with no issues. hopefully you have this corrected by now.
B3E892A3-3293-4F91-806C-8CDC69EBBB9E.jpeg
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