Stabilizer Streamline Strut Vibrations

Steve W

Steve W

Well-known member
Mark G had mentioned this during another discussion about broken tail wires.

“”For the hor stab strut of streamline tubing - there are some glue on things that sail planes use to disturb the airflow a little and keep the harmonic and vibrations from developing. One builder glued on some velcro "dots" which helped. But you are kind of in the dark unless you can see what is happing back there. Maybe you can look back and see all the tail surfaces and wires without a camera. Mark””

This post is basically for Mark G and others that have experienced the same problem.

I was up for a short flight today and noticed a speed induced vibration between 140-150 mph. I managed to see the lower strut on the right side vibrating quite violently. I haven’t been able to clearly see the strut in the past but today it was very visible against the white ground. I slowed to the mid 130’s and it would still occasionally vibrate.
Other than installing a jury strut for support has anyone found a solution other than the velcro dots?
Thanks
Steve W P203

The only think different between my previous 26 hours and today was 60 lbs of water in the baggage area. I did notice today there was more elevator deflection in cruise than before which was expected.
 
Last edited:
You have something against velcro dots?

There are several solutions to the problem. More stiffness. Less stiffness. More damping. Less damping. Added mass. Less mass. 1742 different combinations of the previous.

Or velcro dots? Sometimes problems with airplanes don't surface for 5, 10, 20, 40 or 60 years. This applies to probably every aircraft ever built.
 
What seems to help with vibration of the hor stab struts is "turbulator" strips which was recommended by some sailplane guys who see similar vibration issues. It just changes the airflow slightly around that strut so changes the harmonic. These turbulator strips are largely the same as the Velcro dots one builder used which helped him.

Like you noticed - it is usually at certain airspeeds or power settings. Different for different planes. Some Patrols do not have this issue at all. Others do. Mark
 
Thanks Mark and Svyolo.
Has anyone tried a round tube for a strut brace? I saw the turbulator strips in the glider accessories sites. Wonder if they are applied to the length or width of the strut?
No, I have nothing against velcro dots other than they will come off eventually but would like to know how many are to be applied and whether on both surfaces, etc.
 
My local hardware store as good as anything left in a first world place. Their metal section is a lot more extensive than the big box places. Their aluminum section has small aero shaped aluminum tubes, probably the perfect size to fit over flying wires. I am not sure of the brand but most of the stuff they sell comes from K & S I believe. If I have any exposed wires it will be covered with it.
 
You always hate to add drag on purpose but that might be the easiest thing to do to start. Maybe try a little "Gurney flap" on the trailing edge of the strut?
 
The struts are pretty stiff, but they aren't constrained that tightly in rotation. Maybe a really tight interference fit on the strut hole that goes through the stab end of the strut, or even tack weld the bolt in place.
 
I used the RC type streamline strut material over all four wires. I broke a forged rod end in one-two hours of flight. I removed all the covers when I repaired the wire and went with the tab and fork assembly. There is more going on back there than you think.
What I was trying to learn was where to put the turbulator tape and the velcro dots. On top of the strut, the leading edge??
The only other thing I have installed on the strut is protective tape on the leading edge that wraps around to about a half of an inch from the trailing edge.
 
I can’t find the pic but Mike S used 1/4”x1” Velcro strips on his Patrol. 3 on the bottom of each strut.

I’ve looked at my struts for vibration but have never noticed any. Maybe the 4 place isn’t fast enough for it to be an issue. When I do a full power run up the tail-plane shakes so much it is spooky but it is solid in flight.
 
Thanks Whee. The vibration comes right back into the stick and was never like this before. Just a slightly different air flow because or no weight in the back of the plane. Yes I guess the top or bottom surfaces are the places to put the interference. I’ll remove the protection tape first and try that. Then the velcro if needed.
 
whee said:
I can’t find the pic but Mike S used 1/4”x1” Velcro strips on his Patrol. 3 on the bottom of each strut.

I’ve looked at my struts for vibration but have never noticed any. Maybe the 4 place isn’t fast enough for it to be an issue. When I do a full power run up the tail-plane shakes so much it is spooky but it is solid in flight.
Click to expand...

The AoA of the tail strut is quite a bit different from the 4 place to the patrol. The lift strut goes backwards and attaches to the fuselage at the same place as the flying wire under the tail. The velcro is a band aid for a harmonic that should probably be properly fixed.
 
Well, that is kind of what I’m getting at. If the velcro comes off in flight, you can’t pull over to the side of the road and fix it, especially on a long haul. I was wondering what a round tube would do back there in it’s place? Does anyone else have any suggestions on how to permanently fix the vibration?
Thanks
 
Are both sides fluttering or just one side?

Round tube would eliminate any AOA issue with the streamline strut but would add quite a bit of drag. Perhaps a round tube with a streamline cover would work.
 
Seems like only the right side. Thats kind of what I was thinking and then later adjust the angle of the streamline cover more directly into the relative airfow where ever it’s coming from. I’ll have to get back into the plans and figure what the round equivalent is of the streamline tubing. Thanks Whee
 
Steve W said:
Thanks Mark and Svyolo.
Has anyone tried a round tube for a strut brace? I saw the turbulator strips in the glider accessories sites. Wonder if they are applied to the length or width of the strut?
No, I have nothing against velcro dots other than they will come off eventually but would like to know how many are to be applied and whether on both surfaces, etc.
Click to expand...

Apply the turbulator strips lengthwise.

This video is a classic that shows benefits of streamlining a round tube. Start watching at 0:30 thru 6:30. It helps me understand the outstanding benefits streamlined strut.
https://www.youtube.com/watch?v=acySDnGYzVM

A flutter force caused by airflow at a certain speed and angle of attack may be what is inducing the vibration. Resonance of the strut is what keeps it going. A small modification may change things (airflow or resonance of the strut) enough and impact its flutter/vibration.

A little weight change of the strut at a certain point might help or exacerbate it.

Change of the surface from smooth to not smooth....a foreign object like ZigZag tape, or a rough surface like some ome 30 or 60 grit sand paper applied with 3M spray glue is worth a try. If that makes a difference, experiment at reducing the size of the surface material. It might only require a small section to be effective.

Screen Shot 2020-12-01 at 11.55.05 AM.png
 
Thanks Brooks. I have the 9mm tape on order. The only thing questionable from the plans that I can see is there maybe some angling of the tube tipping down in the airstream (trailing edge upward). Mine are parallel to the center of the plane. Maybe that’s an error on my part.
 
And Brooks, yes, I’ve seen the video which made me such a believer in streamlining everything I could. I do believe, however that the streamline stuff I had on the tail wires was a direct cause of the forged end breaking as quick as it did.
I used coverings everywhere I could which helps the final airspeed, shock tubes etc.
 
Streamlining is great but you have to make sure it is online with the airflow. If not then it adds drag and may induce flutter.

For my round shock struts I plane to glue some profiled wood to the back of the struts. This solution could be used on a round tail strut.

Steve, You should call Bob. He’s always been helpful when I wanted to change something.
 
I really wonder what is the axis of excitation. Bending, or twisting. The clevis end would have little to no added stiffness, in twisting. The stab end with the bolt is doing all that work in that axis.
 
Whee, I used plastic streamline covers for my lower strut tubes. I had to shape the ends around the weldments but was happy with the final look. I filled the open ends with some caulking adhesive.
I used the small sized tubing and had it cut to a rough length for shipping. Go to the Aerolite 103 website and see what they have there.
 
Last edited:
Just a pic of the plastic tubing
 

Attachments

  • 4E06988B-4D03-4EBD-9263-3FEEA99A5647.jpeg
    4E06988B-4D03-4EBD-9263-3FEEA99A5647.jpeg
    1.3 MB · Views: 0
A lot of higher performance hang gliders have very slick airfoil shaped external tubing. It forms the triangle that you control the glider with, and supports positive G air loads. The better stuff is pretty thin and low drag. The vast majority of them have what is essentially a full length stall strip or strips on both sides of the tubing, full length. It has been a while but I remember it was there to solve a lot of potential problems, including bending from generating lift and maybe flutter. 10 - 40% MAC from the leading edges.
 
Try taping a 1/16” piece of music wire on one side of the strut. This is commonly done on Pitts aileron interconnect struts and works well. It changes the lift on one side of the surface and the strut will tend to pull slightly in one direction rather than vibrating back and forth
 
Thanks Walt. That’s a great way to troubleshoot. I would make another set if I could determine which way it needs to go to be more inline with the airflow.
 
Start with the tape and if you like the results you can weld the wire on if you want but I’ve never had one that’s taped on come off
 
whee said:
Streamlining is great but you have to make sure it is online with the airflow. If not then it adds drag and may induce flutter.

For my round shock struts I plane to glue some profiled wood to the back of the struts. This solution could be used on a round tail strut.

Steve, You should call Bob. He’s always been helpful when I wanted to change something.
Click to expand...

Regarding Shock Strut Streamlining.....I'll add that on the Patrol Plans (before the Shock Strut went to a round tube) Bob allowed for an 8 Degree twist so that the strut was streamlined with airflow.
Screen Shot 2020-12-03 at 5.09.59 PM.png
The experts say that the ideal length of streamlining is 3.7 times the diameter with thickest part at 37% aft of the leading edge. I spent I think 3 months making these gear leg fairings for the round tube shock strut. It was my first fiberglass project. Only started over once. In the end I learned about vacuum bagging too. Those fairing from aerolite will save you time. Bob has a fairing for the shock strut tube available. Don't make your own. Putting weight in the leading edge like we do our control surfaces would help accommodate flutter.

If you make the lower part of the fairing as a separate piece and attached it to the round lower tube of the shock strut with a round hose clamp, it would be adjustable to align with the airflow.

I might make mine over. Maybe chop off the lower part and rework things. Who knows.....actually....Brooks, don't do that until after its flying.
61844518448__7BA19DCB-3637-4483-8E66-48BD2E7DB12F.jpg
 

Attachments

  • Screen Shot 2020-12-03 at 5.09.59 PM.png
    Screen Shot 2020-12-03 at 5.09.59 PM.png
    152.6 KB · Views: 1
Just an update on the vibration, I haven’t received the turbulator tape yet so I tried the 1” strips like Mike S tried. I flew yesterday for an hour without ballast in the back up to speeds of 155 mph and the vibration is gone. Everything I could see remained calm, struts and wires.
Thanks to all that assisted on this issue. Appreciate the forum and it’s value for the shared info.
 
Sure will Mark. It will be a while. The plane is an hour away and will be sometime before I’m up there again unless the weather warms up.
 
Last edited:
Good to see you got it sorted. I was wondering if that could be the cause of fatigue issues on the threaded portion inboard, if the vibration is left to continue.
 
Thanks for posting this Steve. You put more velcro strips than I imagined. I wonder if fewer strips would accomplish the same thing? Maybe someone will play with it and answer this question. Mark
 
Nev, I think you are referring to the 4 place. The Patrol strut is formed and drilled at each end so no threads.

Mark, Mike Silvernagle mentioned making 6 strips so that's what I used pr side. He may have only added three a side. Unclear on that whether he attached the six to the one strut that was vibrating or three to each.

Mike wrote:
" I ended up cutting 6 pieces of 1/4" wide strips of velcro 1" long and sticking them to the underside of the 4130 streamlined tubing"

If the weather was warmer and I had the opportunity to play with the strips, I'd see what the minimum would be to eliminate the vibration.

Thanks for the feed back guys.
 
While I have no knowledge about this and no expertise - I kind of suspect just one strip in the middle might break up the harmonic vibration. But someone would need to try. Mark
 
This thread might be important enough safety-wise to put somewhere that all builders see it at some point. Maybe a Beartracks article? Sticky in one of the forum categories, like the current one?
 
Correct me if I'm wrong, but I remember some time ago Jared caught an issue during an inspection/preflight on this tail strut? I did a search but didn't find the thread, just wondering out loud if this is related?

Higher time and/or bush fliers seeing any issues?

Andy
 
Mine also broke in flight, there is a different post about that plus and engineering change which fixed the issue.

The beauty of the design is the tail system has redundancy, so wires or struts failing is not a problem.
 
I have also had issues with the streamlined flying wires hitting resonance, they vibrate a lot and you will never know unless you look back when the elevator is unloaded (hands free). You can change the resonant frequency by changing the tension in the wires (I suggest lower, not higher).
 
We first encountered the issues with smaller tires fitted, Goodyear "26s" (which are really 23 inches tall)
 
You must log in or register to reply here.
Back
Top