Stop Rumbling Sounds

A friend was having some problems with a rumbling sound, seemingly affected by airspeed and speed of the engine. He finally traced it down to the floor right and left of center line, just behind the baggage bulkhead! I found that I could touch either side on mine lightly and the other side would oil can - badly! We fixed this quick and simple by riveting on an angle to the center of each side, if you look closely at the attached photo, you can see the fix. This is a photo taken from under the seat area looking backwards.
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Just a note for annual inspection.

Hey Don, that should fix the rumbling but also be sure to add the ends as an inspection point. A stiff member has been added that will now accept longitudinal loads from that part of the airframe. It tends to result in cracks or loose rivets at the end point.

Suggest your friend adds this to the annual inspection to keep an eye on it. This way if it occurs it won't get out of hand and become a surprise one day.

Some 7's have an oil canning on the rear under side of the fuse as well. Solutions were discussed at length 3-4-5 years back in the VAF, and some effected one like your post. They had issues at the ends as mentioned.

Thanks, I will keep my eye on that area. I was not certain this was the best fix, but could not come up with a better idea so used it on mine as well (the photo is my plane)
The oil canning was so bad I worried about cracks from the flexing!
My first thought was to glue something to the inside.

To keep it from causing cracks at the ends of the added angles, the angles should be attached to the two bulkheads at each end. A joggle to lap over the flange at an existing rivet or a 90 bend and riveted to the bulkhead if the flange is on the other side of your stiffener.

Anti-yaw strakes, Don? Will be interested to see how they work out.

I added a stiffener to the belly skin in my RV-8 when I did my build.

I was careful to taper the stiffener to reduce stress at the ends. Should I be concerned enough about cracking around the rivets to crawl back there and tie the ends to the formers?

A related question: I am in phase 1 now and seem to get a roar resonating that comes in right at cruise speed. Any ideas which skin panels to start looking at? I do have bent down exhaust tips.

ILikePike said:

I added a stiffener to the belly skin in my RV-8 when I did my build.
I was careful to taper the stiffener to reduce stress at the ends. Should I be concerned enough about cracking around the rivets to crawl back there and tie the ends to the formers?

Click to expand...

YES, the skin WILL develop cracks at the ends if you do not attach to the bulkheads. I've seen it happen numerous times.

The roaring at cruise might be the canopy skin against the fuselage if there is no felt or something between the two.

Alleviate the stress at the end of the angle.

Jeff

I agree with Mel however the stress relief you added to the end of the angle goes a long way to alleviate the stress at the end of the angle.

To improve your design remove the angle leg before the last rivet. ( soften the end )

I really don't think you will see any cracks in the way you have it in your life time

My View

Joe Dallas

ILikePike said:

I added a stiffener to the belly skin in my RV-8 when I did my build.

I was careful to taper the stiffener to reduce stress at the ends. Should I be concerned enough about cracking around the rivets to crawl back there and tie the ends to the formers?

A related question: I am in phase 1 now and seem to get a roar resonating that comes in right at cruise speed. Any ideas which skin panels to start looking at? I do have bent down exhaust tips.

Click to expand...

Maybe not clear, but my angle is attached to each bulkhead (former).

DonFromTX, I did notice that your external stiffeners were riveted to the formers. This may help keep any cracks from forming???

Jon, I don't think my roar is from the cowl. I built up the edge with micro to make a 3/4 strip for the felt to sit flat on the top skin. It adds stiffness and seals very well.

stiffener alternative - rotate 90 deg

sf3543 said:

To keep it from causing cracks at the ends of the added angles, the angles should be attached to the two bulkheads at each end. A joggle to lap over the flange at an existing rivet or a 90 bend and riveted to the bulkhead if the flange is on the other side of your stiffener.

Click to expand...

ILikePike said:

DonFromTX, I did notice that your external stiffeners were riveted to the formers. This may help keep any cracks from forming???

Click to expand...

There are a couple of loadings that cause cracks. One is the pressure loads from the skin which loads the stiffener, well, like a beam. That loading will concentrate on the skin with no stiffening at the end if not attached to the bulkhead. However on that part of the fuse, there are longitudinal forces stretching the skin between the bulkheads, and attaching to the bulkhead will do nothing for that. There is a lot of skin there, and if the "stiffener" is the same thickness then is is likely of little consequence but would concentrate that stiffer effect at the last rivet.

I avoided (-7) the lengthwise stiffening all together by forming a couple of .025 bulkhead-like lateral stiffeners and did not rivet them into the J channel they do attache to the skin overlap rivet line. They were formed over a buck made to the floor contour, standard flange and just under an inch tall. Three rivets with 3M weather weld - bonding to the floor. Prior to stiffeners, a feather light touch was all that was needed to initiate the oil can and with some down force on the tail it self initiated easily.

YMMV