Strut Flutter

[n6346m]

Now that I have upgraded to a C-90, I am operating in an areas of flight heretofore unfamiliar to me. One of which is cruising in the 90 to 95 MPH regime. I have been noticing a definite vibration in the airplane right around 90 MPH and it goes away somewhere above that and somewhere below that speed.
I checked everything that I could think of and I couldn't figure out for the life of me what was causing it. Last week I was in the back seat and was looking around while the guy in the front seat was flying. As we entered the 90 MPH vibration speed I noticed the left rear wing strut fluttering to beat the band. It was flexing very noticeably in both parts on either side of the jury strut. As we increased or decreased the speed the fluttering and associated vibration stopped. I saw no give or flexing at the attachment part of the jury strut.
Have any of you ever encountered such a thing and were you able to correct it?
One of the visible differences between my L2-B and an L2-M is that the latter airplane has a four piece "N" shaped jury strut compared to the triangle shape of the other DC airplanes. I wonder if that would help keep the rear strut more stable and reduce the tendency of the rear strut to flex so much.

[magman]

Re: Strut Flutter

Ran into something like this with a Tri-Pacer.

After Droop Tips were installed one of the wings would shake @ some speeds.

After much searching the Drag wires were found to be loose.

Tightening them solved the problem.

[otrcman]

Re: Strut Flutter

Bob,

At least there are two of us with this situation. My airplane is an L-2M, C-85 engine. I have noticed this vibration occasionally at around 90 - 100 MPH, usually descending at around 2000 RPM. I've been faster than that many times without a problem. In each case I've felt a light vibration in the floorboards and then looked out to see the vibrating rear strut. Usually it's been the right strut, but at least once it's been the left. In each case I have chopped the power and the vibration stopped immediately. I've never increased RPM or airspeed to see if it would go away.

So in answer to your speculation about the 4-member jury strut, no that doesn't seem to prevent the vibration.

One other thing: What prop are you using ? I have a McCauley 1B90CM, 74-43. I do have a field approval for that particular prop, so I am legal. My engine and prop are otherwise very smooth and the combination has flown over 2000 hours on my plane.

Dick Fischer

[TimHicks]

Re: Strut Flutter

I had lift-strut flutter one time. I was in a very aggressive slip to landing and I had a student pilot in the right seat.
I felt some vibration and my rightseat said that the right rear lift-strut was vibrating.
My airplane had a A65 and a Sensenich 72x42 wooden prop.
I didn't see the vibrating strut. But he described it just as you did. On both sides of the jury strut. The frequency was about like driving over rumble strips at 45 mph.
It stopped when I came out of the slip.
The slip was a left wing low, nose pointed right, kind of slip.

[Ragwing nut]

Re: Strut Flutter

Our L-2M did it with an 85 on it, but it was the right side and had a door catch attached to the strut, removed it and the vibration went away.

[n6346m]

Re: Strut Flutter

Dick,
I am currently using a 1B90/CM7146 prop. I don't remember if I have the same problem with the Sensenich W72GK-48 wood prop since I am usually on skis when I have that prop on the airplane and I may have dismissed the vibration as ski rigging related. I'm going to check it out when I put the wood prop back on for the winter.
By the way, did your field approval make you cut down your prop, or are you able to use it full length? I would like to use a 74"
prop when I put my floats on the little warbug.

[n6346m]

Re: Strut Flutter

The frequency was about like driving over rumble strips at 45 mph.

Tim, Good way to describe it.

[PA1195]

Re: Strut Flutter

I suggest determining if it's engine rpm or airspeed related. You could fly the suspected airspeed (90?) at various rpm settings in a descent to see if speed is the source. Also fly the suspected rpm range noted in level flight around 90 at various airspeeds both climbing or descending.

Props can be checked for dynamic balance both on the ground and in the air at a selected rpm with specialized equipment. There's sources for this service in Alaska. Example: https://www.youtube.com/watch?v=RyNIPsF-7Jg I've been party to several of these tuning events both stationary and flying and they can diagnose and reduce the vibration issue if it's engine/prop related. Sometimes checking the prop track and re-indexing the prop on the crankshaft can make a difference.

Gary

[otrcman]

Re: Strut Flutter

Bob,

My prop is full length -- 74". I'm aware that the C-85's on other airplanes are usually not that long, but I reasoned with the ACO engineering guy that the C-75 (same mechanics as the C-85, just a lower redline) is legal to run a full 74" and also that the F-19 uses a full length 74" McCauley with an O-200 (at 2750 RPM). He bought my argument and approved the installation. It was the same prop that I had been running on the A-65. But I don't recall ever seeing the strut vibration with the A-65.

I'm sort of a chicken when it comes to structural vibration, so I've never allowed the vibration to continue or gone looking for it. It's so elusive that I have always been caught by surprise when it happens.

Regarding Gary's suggestion to try varying RPM at a constant airspeed to determine what the forcing function is, I can only relate my experience. Each time the vibration has begun, I have chopped the power and the vibration has stopped immediately. That suggests to me that the vibration is related in some way to engine speed, not airspeed. The airspeed wouldn't change noticeably in the time it takes to chop power.

Having been a flight test engineer for most of my career, I do have some knowledge and experience with structural vibration. I'm not an expert on the subject, but have worked around people who are and a little of it has rubbed off. For the strut to vibrate, there has to be a "forcing function". That is to say, there has to be some external input to the strut at the same frequency or a harmonic thereof. That input can be fed through the structure and into the strut or it can be fed through the airflow into the strut. Tim's experience with slipping is probably an example of an airflow induced vibration.

Ragwing's experience was particularly interesting in that he was able to remove a small weight (the door catch) and detune the system so that it did not occur again. Ragwing, was the vibration easily repeatable with the catch in place and then clearly not repeatable after the catch was gone ? Or was the vibration somewhat occasional like mine ?

Bob, I think it's significant that your prop is a different length than mine. The vibration frequency of the strut is what we have in common. Since our struts are the same length, our frequencies will be nearly identical. So we're looking for a forcing function of that same frequency or some multiple. Since your prop is shorter, it's natural bending frequency will be higher than mine. So the prop probably isn't the culprit.

Bob or Ragwing, could you tell me whether you have allowed the strut to vibrate for more than a few seconds ? Say, have you let it go for 10 seconds or more ? My interest here is whether the vibration is divergent or just steady state. A divergent vibration is much more dangerous because it might break the strut in short order. If it's just a steady state vibration, then it would probably suffice to avoid the RPM / speed range where it occurs so that you don't fatigue the strut.

Dick

[PA1195]

Re: Strut Flutter

Naturally any interference fit or motor mount issues should be investigated. Run the engine on the ground in the suspected rpm range and look for unusual movement or temporary metal to metal contact. The strut/wing has to be tight, and the strut clamp fastened properly. Again run it static and look at and feel the struts and wings for vibration. Swap the prop 180* and test after confirming the prop's track and any crankshaft runout.

I flew a C-185 with bad rpm vibration for 5K+ hours. The shock mounted instrument panel was almost a blur and readable only if shimmed at the edges. Gear configuration didn't matter, nor did weight or airspeed. Three engine changes and two props (2 blade Mac and 3 blade Hartzell) made no difference...same with changing rubber dynafocal mounts. I could move the vibration node longitudinally in the fuselage by simply varying rpm. Prop balancing would move the rpm range but not eliminate the problem. Finally a change to an improved motor mount from Seaplanes, Inc. fixed it (http://www.seaplaneswest.com/enginemounts.html)

The point being any engine/prop/airframe can shake due to various reasons. I'd not fly with a vibrating lift strut until the reason was known and eliminated.

Gary

[n6346m]

Re: Strut Flutter

Just to give an update on strut flutter. Since my original posting I have discovered that the flutter only occurs when the back seat is occupied. I tried everything I could when solo and could not duplicate the flutter. It only occurs with aft CG.

[Guest]

Re: Strut Flutter

Just to give an update on strut flutter. Since my original posting I have discovered that the flutter only occurs when the back seat is occupied. I tried everything I could when solo and could not duplicate the flutter. It only occurs with aft CG.

Bob,

Just a curiosity question; did you happen to notice any changes in the flutter when the left aileron was up? down?

Dave

[PA1195]

Re: Strut Flutter

Weight distribution can affect vibration and location. I don't know why.

Aft loading a C-185 on floats I flew would create vibration in the aft cockpit and rear float attach struts and wires. It could be felt by passengers and was not felt when they or gear were absent by me. Separating the flying wires with better insulators helped. It would shake the left water rudder assembly and be visible when looking back out the left window. It was a relatively slow chug-chug vibration.

I'd not fly the Taylorcraft with the struts shaking until the engine and airframe were examined and airworthiness was assured. There are ways to alter or reduce vibration (mentioned above). I'd be examining the prop balance, track, engine and mounts, and the wings and fuselage.

Gary

[n6346m]

Re: Strut Flutter

I'd not fly the Taylorcraft with the struts shaking until the engine and airframe were examined and airworthiness was assured. There are ways to alter or reduce vibration (mentioned above). I'd be examining the prop balance, track, engine and mounts, and the wings and fuselage.

Gary

Gary,
Pretty much did all that when I did the engine upgrade. I put on a fresh overhauled prop that tracks perfectly and it flies hands off. I will try my wood prop next and see if that helps.

[n6346m]

Re: Strut Flutter

Bob,

Just a curiosity question; did you happen to notice any changes in the flutter when the left aileron was up? down?

Dave

Dave,
I haven't tried any aileron input yet, but small rudder inputs seem to make it go away.