Stabilizer Streamline Strut Vibrations

[Steve W]

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.

[svyolo]

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.

[Mark Goldberg]

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

[Steve W]

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.

[svyolo]

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.

[svyolo]

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?

[svyolo]

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.

[Steve W]

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.

[whee]

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.

[Steve W]

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.

[zkelley2]

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.

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.

[Steve W]

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

[whee]

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.

[Steve W]

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

[Bcone1381]

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.

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.

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[Steve W]

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.