Vibration in a 7KCAB

T

tim48v

New member
I've been chasing a strange vibration in our 7KCAB for almost a year now and would appreciate any ideas:

It is a low frequency vibration that I feel mainly in the seat and a little in the stick. Seems to be tied to the engine RPM but sometimes appears to come and go. Kind of feels like a twin when the engines aren't in sync.

We've checked the blade angles on the prop; prop tracking and even just did a dynamic balance. All fine.

We installed new rubber engine mounts and checked the torque on all the bolts. No change.

The valves appear normal and oil analysis is normal as well.

I've tightened and added foam rubber to the boot cowl to try and dampen it; no change. I don't see anything vibrating during flight but the vibration seem to be in the lower part of the fuselage.

A local flight instructor who has decades of experience in KCAB's thinks it might be related to the cross over exhaust.

I'm wondering if the prop is mis-indexed to the crank? The drawing in the manual doesn't seem to match the description. The prop is supposed to be 1 bolt hole Counter clockwise from the 0 mark (the drawing shows it lined up with the 0 mark). I assume that the 0 mark is the TDC index; but do they mean CCW when standing in front of the engine or sitting in the plane? (My prop appears to be 1 bolt hole CCW when viewed from inside the plane).

BTW, this is a '67 KCAB with ACA aluminum spar wings. The plane was recovered in '98 and has about 700 SMOH.

Any hints would be greatly appreciated.
 
Tim:

Most low-frequency rumble as you describe it comes from loose belly fabric behind boot cowl that vibrates due to prop wash and/or loose or missing inspection cover same area. There is a metal cross-member that is often broken or missing entirely and is necessary to keep belly fabric taut.

Hope that helps,

RS
 
Robert,
Are you refering to supports 62 and 63 as shown in Figure 12 of the parts manual? I've looked in one of the inspection holes and don't see either of these! BTW, the fabric between stringers 60 and 61 is very tight; even to the point of being slightly concave.

I've also taped all the inspection covers down tight but that didn't seem to make a difference.

Could it be related to rigging when they installed the new wings? (The was done by a previous owner so I don't know if the vibration was present before or not.)

Thanks,


Tim
 
Could be your prop indexing as you suspect. When in the correct position the blade should be in about the 10 o'clock position as the engine comes up on compression, as when you are trying to hand prop the engine.

When indexed IMproperly, the blade will be in the 1 o'clock position and very obviously would be hell to try to hand prop.

Steve
 
The prop indexing seems to be correct.

After Robert's suggestion regarding the belly fabric, I had buddy of mine take a look last night (he's a local tube and fabric expert). He is restoring a Citabria right now and confirms that I'm missing the two cross pieces. He said it won't affect how the plane flys but could cause the drumming that I'm feeling. He's run into this before and has added the supports without opening up the belly.

Wonder if anyone has the exact dimensions of these peices?
 
I've ordered the parts from ACA. They've confirmed that this is probably the cause of the vibration that I've been chasing.

Thanks Robert.

Tim
 
To properly index your prop,bring #1 cylinder up on compression stroke to TDC,the #1 crankpin and#1 bushing will be at the 9 oclock postion standing in front of the plane looking back. Your prop may or may not have a #1 blade,if it does postion the # 1 bolt hole 1 hole counterclockwise from the #1 bushing. If you don't have a #1 blade imagine a line from one end of the prop to the other,it will cross two holes either one can be used for indexing.On the fix pitch prop the #1 is really used by the prop shop for there index mark.You'll see the prop stop in the same position 99.99% of the time. Not being indexed properly can cause excess prop blade stress.

Woodie
N29763
 
I concur with the belly fabric theory because I have experienced it. Those two stringers really seem to take a beating. I have fixed them both TWICE!. Yes it does not affect flight behaviour , other than the vibration that can be felt through the seat becaues it is right above the affected area.
My solution is to repair the stringers in whatever fahion makes your AME happiest and then sew the belly fabric to the stringer. THe sewing will not stop further breakage but it will stop the stringer from coming completely loose and ending up jamming the control stick . THis is the real danger.
The entire attachment of the the metal U channel stringer to the wooden side stringers is very problematic. Wish that there was a final approved repair for this very common problem.
 
We have installed the two new stingers from ACA and the problem appears to be solved. The rear one we were able to install through the inspection holes. So we flew the plane with just that one to see if it would fix the problem. It made it worse! Higher frequency and slightly stronger vibrations. At least I knew we were on the right track. Had to remove the floor boards to install the front stringer.

The new design does not attach to the wooden longitudinal stringers but bolts to the directly to the support brackets that are welded to the frame. No sewing, just 5 fabric pop rivets to hold the fabric tight.
 
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