Citabria Strut Vibration

[Taildragger]

I have noticed in my Citabria that when at full power and climbing the left strut has a slight vibration to it. It is barely noticeable and I am not too concerned with it currently but I have thought over time this could cause a crack in my wood spar. Has anyone else experienced this? Or have any suggestions as to what it might be that is causing it?

Also are there any preventative maintenance or things that can be done to ensure that the spar does not get cracks in it?

Thanks

 

[VanDy]

I have noticed in my Citabria that when at full power and climbing the left strut has a slight vibration to it. It is barely noticeable and I am not too concerned with it currently but I have thought over time this could cause a crack in my wood spar. Has anyone else experienced this? Or have any suggestions as to what it might be that is causing it?

Also are there any preventative maintenance or things that can be done to ensure that the spar does not get cracks in it?

Thanks

Can you see the vibration or feel it??

I would have it checked out by a mechanic if I was you. The Jury struts keep the lift struts in compression or tension, and lessen the bending loads on the strut. If they are vibrating that load is being sent to three places, fuselage strut fitting, wing spar fitting, and straight up through the jury strut to the spar. There should be inspection panels at both of the spar points that can be taken of very easily so you can get a look inside. A real quick and easy check to see if a wood spar is cracked to push up on the outboard of the wing (where spars are) and listen to hear if you have any cracking or other noises that shouldnt be there.

 

[Silvaire]

I've seen that before and think it's common on higher powered Citabrias. Furthermore, when it comes to cracks induced by vibrations, wood spars are the last thing you need to worry about. Aluminum structures are far more prone to cracking.

In fact I'll go out on a limb (pun intended) and say that virtually every cracked wooden spar found on a Citabria can be traced back to a wingtip having touched the turf at some point in the past due to a full blown or barely averted ground loop which probably never got mentioned anywhere in the logs.

Some Luscombes have a wing strut vibration anomaly that happens on the metal winged models with the single large strut. It only happens when you are flying through rain and it stops if you slow down a bit. Luscombe struts are pretty long, longer than Cessna's.

 

[Taildragger]

Can only see the vibration, cannot feel anything out of the ordinary. And it is only when the engine is at high RPM and usually climbing. I would have never noticed it if I hadn't been flying with one of my old instructors in the back seat and he pointed it out. You really cannot tell unless you focus on watching the left strut for a while and then watch the right strut for a while and then you can tell there is a slight vibration on the left one and the right is solid. It is not an airspeed thing because I can be descending with a reduced power setting and a higher airspeed and there is no vibration. Led me to believe that it might be a bad engine mount but when I checked them they all look good.

Wood spar had no cracks in it and no nails needing to be replaced when it was inspected at last annual inspection last October.

After noticing the vibration in the left strut about a month ago I decided to take take inspection plates off and inspected spar per the AD and did not see any cracks that looked like the examples in the AD. No cracking or other odd noises when I push up on outboard of the wing. I am not an expert on the subject though so might still need an inspection by and IA. Airplane has no damage history and all original fabric and wings.

Thanks for your advice.

Can you see the vibration or feel it??

I would have it checked out by a mechanic if I was you. The Jury struts keep the lift struts in compression or tension, and lessen the bending loads on the strut. If they are vibrating that load is being sent to three places, fuselage strut fitting, wing spar fitting, and straight up through the jury strut to the spar. There should be inspection panels at both of the spar points that can be taken of very easily so you can get a look inside. A real quick and easy check to see if a wood spar is cracked to push up on the outboard of the wing (where spars are) and listen to hear if you have any cracking or other noises that shouldnt be there.

 

[Taildragger]

I've seen that before and think it's common on higher powered Citabrias. Furthermore, when it comes to cracks induced by vibrations, wood spars are the last thing you need to worry about. Aluminum structures are far more prone to cracking.

In fact I'll go out on a limb (pun intended) and say that virtually every cracked wooden spar found on a Citabria can be traced back to a wingtip having touched the turf at some point in the past due to a full blown or barely averted ground loop which probably never got mentioned anywhere in the logs.

Some Luscombes have a wing strut vibration anomaly that happens on the metal winged models with the single large strut. It only happens when you are flying through rain and it stops if you slow down a bit. Luscombe struts are pretty long, longer than Cessna's.

Thanks for the insight. The spar was inspected last October for annual inspection and it checked out good. No cracks and no nails needing replacement. It has no damage history and original fabric and original wings. Fabric is in great condition. I don't do any aerobatics in it and just want to make sure I do any preventative maintenance to ensure that the wood spar always checks out at annual inspection. Do you have any further advice to ensure I prolong the life of the spar/wings?

Also, I have been dealing with tailwheel shimmy when I 3-point land. It is especially bad when I do a 3-point with a passenger in the back. Typically I do wheel landings and ease the tail wheel down and there is no shimmy but occasionally my botched wheel landings turn to 3-pointers that end up with shimmy.

Any suggestions on fixing that and could vibration from shimmy on tailwheel over time cause any wing/spar cracking or damage? I have greased the tailwheel and tightened the nut one cotter key notch.

Thanks!

 

[Skylane81E]

Silly question but,

You say you see but cannot feel the vibe. Is it possible the window is vibrating and distorting your view of the strut?

 

[dell30rb]

I can see the left aileron vibrate slightly in at least one (maybe two) of our club's warrior II's at high power. Can't feel it in the controls. Think its just from normal engine vibration. Most noticeable at FT cruise climb.

FWIW I think a good dynamic balancing would likely take care of your issue. Its something I don't think our club has done. At least I don't recall seeing this done in any of the logbooks I have reviewed for checkrides.

 

[Taildragger]

I can see the left aileron vibrate slightly in at least one (maybe two) of our club's warrior II's at high power. Can't feel it in the controls. Think its just from normal engine vibration. Most noticeable at FT cruise climb.

FWIW I think a good dynamic balancing would likely take care of your issue. Its something I don't think our club has done. At least I don't recall seeing this done in any of the logbooks I have reviewed for checkrides.

Interesting thought. I have never thought about having prop balanced. Is balancing done on fixed pitch propellers?

 

[Taildragger]

Silly question but,

You say you see but cannot feel the vibe. Is it possible the window is vibrating and distorting your view of the strut?

No it is the strut. Not the window.

 

[Silvaire]

...Do you have any further advice to ensure I prolong the life of the spar/wings?...

There is nothing really special that you need to do outside of what you would do to take care of any airplane with or without wood spars. There are a couple of things you need to face up to however as far as Citabrias go. First of all it's somewhat likely that the airplane was at one time or another used for primary tailwheel or aerobatic training and as such, subject to the types of abuse you would expect in that environment. This would include less than graceful landings and ham-fisted loops and aileron rolls.

The second thing as that there is absolutely no way to fully inspect in detail the entire surface of the wing spar without removing the wing fabric. The wing root and lift strut attach points are the critical areas and additional inspection panels can be added to accommodate inspections there but they are not the most eloquent solution. The only REAL solution is to recover the wings and see the spars for yourself and, of course never lend your airplane to anyone after that.

The cracks that exist, as I said earlier are going to be from an over stress of some sort in the past, most likely a wingtip hitting the dirt but could also be from some very badly executed aerobatic maneuvers. The wood spars themselves aren't going to just crack for no reason such as not pampering them in a hangar or such.

There have been a few instances of wings breaking off in flight, one of them happened 10 hours after the AD inspection had been complied with and signed off. American Champion, the current owner of the type certificate would love for everyone to buy new wings from them and have all of the old wood spar wings destroyed. The problem is they are really expensive.

If you know the history of your airplane or if the wings were recovered recently you can be fairly confident of the condition of the spars so long as you haven't whacked a light pole at the fuel station or anything like that. There's really nothing deficient concerning wood spars, it's more about the abuse the typical Citabria has endured in it's life.

As for the tailwheel - if it's a Scott then the first two things to check are that the ears to which the steering springs attach are not deformed (bent upwards) or that the main spring itself has not been bent putting the geometry of the wheel out of kilter. Those are the most common causes of shimmy.

 

[N801BH]

Interesting thought. I have never thought about having prop balanced. Is balancing done on fixed pitch propellers?

Yes on the prop balancing.....

As for the strut vibrating..... The right strut get a slighty contaminated prop blast as the flow is kinda blocked by the right wing as the corkscrew airflow hits the strut ,That flow is decending.... The left strut gets a full blast of the prop since the corkscrew flow is not blocked by the lower engine cowling, that flow is assending... IMHO..

 

[d.grimm]

Check the angle of the tailwheel as pictured. Citabria's
are known for heavy tails and bad springs.
Dave

 

[21541803]

Re: Citabria Shimmey

Also, I have been dealing with tailwheel shimmy when I 3-point land. It is especially bad when I do a 3-point with a passenger in the back. Typically I do wheel landings and ease the tail wheel down and there is no shimmy but occasionally my botched wheel landings turn to 3-pointers that end up with shimmy. Thanks!

Next I got out an old 1950’s auto repair manual that explained king pin front wheel suspension systems and steering castor angle. If you have ever pushed a grocery cart through the supermarket with one of the front wheels shaking side to side you have experienced wheel shimmy and improper castor angle. What I learned from the chapter on steering alignment was basic steering geometry. To measure your steering geometry, drop a line drawn parallel to and through the pivot axis and extend it to the floor and make a mark on the floor where this line hits or use a straight edge parallel to the steering axis shaft. Next drop a line vertically from your tail wheel axle to the floor or again use a straight edge and make a mark on the floor. This will also be where your tail wheel contacts the floor. Now move this line or straight edge that passed from the axle to the wheel/floor contact point horizontally until intersects the pivot axis line at the pivot axis midpoint The line that is parallel to the steering axis must hit the floor ahead of the line dropped vertically from the wheel axle. The angle formed by these two lines is your castor angle. The larger the castor angle the better as far as tail wheel shimmy is concerned. In other words, the farther ahead of the tail wheel that the steering axis line hits the floor the greater the castor angle and the less likely that will have shimmy. To put it another way, the steering axis pin or bolt must be vertical or tilted with the top pointing behind or to the rear of the airplane when the airplane is fully loaded. Emphasis on fully loaded.

When my airplane was empty the steering axis bolt was vertical. When I loaded the airplane, the tail wheel spring compressed and the top of the steering axis bolt was pointing to the front of the airplane. This would put the extension of a line drawn through the steering axis behind the tail wheel contact point. Bad news-it will now shimmy. You don’t want the castor angle to be too large because it will make steering on the ground more difficult. The large castor angle will tend to lift the rear of the airplane slightly as you turn the aircraft. This is called self-centering effect. Having the steering axis bolt vertical or inclined slightly with the top pointing back when fully loaded should be sufficient.

So how do you correct this angle? There are two easy solutions. If your airplane is like most, the spring is bolted at the front to the airframe with a bolt that passes through the spring leaves. The spring then rests on a pad several inches behind the point through which the through bolt passes. Usually the spring is clamped to the pad at this point. You can add a shim between the pad and the spring to increase your steering angle. Or you can take the route I took. I took the spring off and laid it on a piece of poster board and traced out it’s arch. Then I took the spring to a spring shop and had them re-bend the spring until the tail wheel end of it was about 1-1/2 inches below the original. In other words I increased the arch slightly. Walla-no more shimmy when loaded.

One other point. You should carry sufficient air pressure in your tail wheel to keep the tire firmly attached to the rim when it hits the pavement on landing. Due to the small diameter of the tail wheel, it accelerates very rapidly on contact. If you have insufficient pressure in the tire it will slip on the rim and cut the valve stem. You now have a flat tail wheel tire. I know- it’s happened to me twice. I now carry a minimum of 45 pounds of pressure in my Scott tail wheel. The same thing can happen to your main tires but with more surface contact area around the rim it is less likely to happen unless your plane lands at very high speeds.

Tail Wheel with Positive and Negative Castor Angles – exaggerated The terms positive and negative are simply the naming the convention I choose to use as they agree with my text book references.

Gilbert Pierce
Technical Counselor
EAA Chapter 182

Join the Citabria Forum at Yahoo
http://groups.yahoo.com/group/CitabriaPilots/

 

[Dan Thomas]

As above, the caster angle is important. Citabria/Champ tailsprings don't have enough carbon in them to give them adequate temper, and they get sprung easily. They're also not expensive to replace, and Univair has them as well. Watch that little U-bolt that clamps the aft end of the spring pack; it can break and interesting things happen.

Dynamically balancing that tailwheel also helps A LOT to eliminate shimmy. Most tail tires are cheap things, and the wheels themselves are often poor affairs. I have balanced them by holding the wheel on its axle bolt and spinning it using a wire wheel to drive the tire and feeling the shake. A static imbalance will move the wheel vertically only and a dynamic imbalance will make it wobble. Experimenting with some lead stick-on weights in various places to get it smooth will usually cure the problem. Use small blocks of foam rubber to hold the weights in place while testing and then stick them to the inside of the wheel once you find where they should go.

This works for nosewheels, too, especially on the shimmy-prone Cessnas. Cessna has a Service Letter on it dating from 1984 or so. Dynamically balancing the nosewheel will do far more to stop shimmy than fooling with the shimmy dampener.

Strut vibration: Those old wood-spar airplanes have steel struts that are really thin-walled and they rust inside, especially near the lower end. Vibration might be an indication of increasing flexibility due to inner corrosion. There's a punch test recommended for them:

http://www.google.ca/url?sa=t&rct=j...=ws82fChwzap_A215pWX9Yg&bvm=bv.47380653,d.cGE

A pdf file. Takes a minute to load.

Dan

 

[dell30rb]

The advice about the TW shimmy is sound. I have first hand experience with tailwheel shimmy in citabria (decathlon). Your spring is probably 'sprung' and needs to be replaced or rearced by a shop with the right tools.

 

[Skylane81E]

No it is the strut. Not the window.

Roger

 

[flyingmoose]

Well it may not be an issue, but its not normal for it to vibrate. The jury strut is supposed to take care of that. Either way I would keep investigating into it.

 

[Jaybird180]

Thanks for the insight. The spar was inspected last October for annual inspection and it checked out good. No cracks and no nails needing replacement. It has no damage history and original fabric and original wings. Fabric is in great condition. I don't do any aerobatics in it and just want to make sure I do any preventative maintenance to ensure that the wood spar always checks out at annual inspection. Do you have any further advice to ensure I prolong the life of the spar/wings?

You have no idea, unless you're the original owner. Glad you're investigating this.

I've seen that before and think it's common on higher powered Citabrias. Furthermore, when it comes to cracks induced by vibrations, wood spars are the last thing you need to worry about. Aluminum structures are far more prone to cracking.

In fact I'll go out on a limb (pun intended) and say that virtually every cracked wooden spar found on a Citabria can be traced back to a wingtip having touched the turf at some point in the past due to a full blown or barely averted ground loop which probably never got mentioned anywhere in the logs.

Very clever, considering that you can't prove your hypothesis. Not debating it, just that you can't win or lose that argument.

 

[nrpetersen]

What sort of max amplitude? Can you see what the mode shape is (IOW is the motion ~zero at the jury strut?)? Does changing the engine rpm make the vibration go away?

A certain amount of vibration amplitude should be tolerable for a steel strut etc. The amount though may be critical. It is interesting that you seem to get it on one side only. It may be as simple as different torque in the end fittings - or like you have said an early indication of something fishy in the attach plates.

Once the mode shape could be figured out (is it bending like a U or like an S? and in what direction?) you might try to excite the strut on the ground by tapping on it with your finger, and then repeating the same tapping on the other side to see if it is just a slight difference in frequency (which is my guess).

Interesting problem.........

 

[VanDy]

Solid advice on the tailwheel shimmy>

Strut vibration: Those old wood-spar airplanes have steel struts that are really thin-walled and they rust inside, especially near the lower end. Vibration might be an indication of increasing flexibility due to inner corrosion. There's a punch test recommended for them:

http://www.google.ca/url?sa=t&rct=j...=ws82fChwzap_A215pWX9Yg&bvm=bv.47380653,d.cGE

A pdf file. Takes a minute to load.

Dan

Taylorcraft has on AD on their lift struts requiring inspection every 4 years, X-ray or ultrasound, did mine last week. The local shop that did them had them done in an afternoon and was $160. As just a price point.

What sort of max amplitude? Can you see what the mode shape is (IOW is the motion ~zero at the jury strut?)? Does changing the engine rpm make the vibration go away?

A certain amount of vibration amplitude should be tolerable for a steel strut etc. The amount though may be critical. It is interesting that you seem to get it on one side only. It may be as simple as different torque in the end fittings - or like you have said an early indication of something fishy in the attach plates.

Once the mode shape could be figured out (is it bending like a U or like an S? and in what direction?) you might try to excite the strut on the ground by tapping on it with your finger, and then repeating the same tapping on the other side to see if it is just a slight difference in frequency (which is my guess).

Interesting problem.........

My main concern would be elongating of the bolt holes through the spar. It wouldn't happen overnight, of course.

 

[Silvaire]

...It is interesting that you seem to get it on one side only...

Left side, high angle of attack, full power. I've seen this too on a KCAB, it's a subtle vibration. Look at the fabric on the underside of the left wing same scenario, it gets beat pretty good by the propwash.

I'll repeat: this is NOT something that is going to cause cracks in a wood spar. If you want to worry about it causing stresses and something breaking because of it fine - but it's not going to be the wood.

 

[Taildragger]

Left side, high angle of attack, full power. I've seen this too on a KCAB, it's a subtle vibration. Look at the fabric on the underside of the left wing same scenario, it gets beat pretty good by the propwash.

I'll repeat: this is NOT something that is going to cause cracks in a wood spar. If you want to worry about it causing stresses and something breaking because of it fine - but it's not going to be the wood.

Thanks for the advice. The prop-wash theory makes sense.

 

[Taildragger]

Based on attached pics can any of you tell if the tailwheel spring is "sprung" ? I thought everything looked ok on it. And the movement seemed good on it while I had it jacked up. The ears did appear to be slightly bent upwards. The tire is worn pretty bad in the center. It is a Scott 3200

 

[Taildragger]

Check the angle of the tailwheel as pictured. Citabria's
are known for heavy tails and bad springs.
Dave

Dave from what I can tell it does not have a negative or a positive castor angle but a neutral castor angle. I have attached pictures.

Thanks

 

[Taildragger]

If replacing the spring. Is the "Citabria Short Spring Set" on the correct unit to use?

http://www.aircraftspruce.com/catalog/lgpages/tailwheelleafsprings.php?clickkey=9943

http://univairparts.com/pdfs/Catalog_2012_Aeronca.pdf

I have found a few different options. The tailwheel assembly is a Scott 3200.

 

[dell30rb]

Yeah it looks sprung to me...

The red line estimates your angle. The green is about where it should be.

Reference the image of post #12 in this thread.

 

[Silvaire]

I'd say so too but it's hard to tell with that ladder in the background. The ears are bent and the port side spring looks loose but that may be because the tail was pushed over. You really need to take pictures of it after rolling forward and centering the tailwheel.

The tire looks typical for one that has a shimmy issue. We used to have a Maule and were always fighting this type of problem although it had a Maule tailwheel. Whoever said tailwheels were cheap has never had to buy parts for a Scott 3200.

 

[d.grimm]

Taildragger806,
It looks like as soon as you add some people
Or when the tail touched down that any bend in the spring
Will give the angle that shimmies.
I would re-arch the spring( something my Pacer required regularly)
And straighten the tailwheel steering arms and
Tighten tailwheel steering springs, new tire and tube, grease
tailwheel and make sure tire pressure is right.
Tailwheels seem to get neglected maintenance wise IMHO.
Dave