RV-8 In Flight Fuselage Noises

Yesterday, with the 8 wound up to fast cruise, I heard (again) a vibration noise coming from the aft end of the air plane. It varied with speed and engine rpm and was bothersome, but not for long. On the next flight, I placed a small towel between the aft seat back and the frame it rests on and the noise is gone. I guess the seat back will need a rubber spacer of some sort to keep it quiet.

The other noise is what sounds like an oil can noise every time coming into the pattern and slowing to approach speed. It always happens just below 100 knots. I do believe it is an engine back fire but don't know for sure.

Can any of you confirm this suspicion? It is at idle power and between 100-90 knots slowing up. The idle power thing is necessary with a fixed pitch prop or this machine will not slow up.

Which is also why it is so difficult to make a 3 pointer with this airplane on a short runway, it does not want to slow down in a timely manner like with a CS prop. I tried the 3 pointers again yesterday and finally said screw this and went back to wheel landings which work just fine.

noise

David,

My 8 makes the same oil canning noise at about the same time you say yours does. It sounds to me like mine comes from the tail. Mine has been doing it since first flight and I have made numerous thorough inspections and found nothing wrong. I think that the rudder skin "oil cans" when I slow down. It does it every time. I can push the rudder skin with my hand and make the sound. 400 hours though....no problem.

David Watson

When I slow my 8 down to stall speed and then increase speed, around 120 kts I get a bang in the aft area that sounds like oil canning. It only does it when I get real slow first.

DEWATSON said:

David,

My 8 makes the same oil canning noise at about the same time you say yours does. It sounds to me like mine comes from the tail. Mine has been doing it since first flight and I have made numerous thorough inspections and found nothing wrong. I think that the rudder skin "oil cans" when I slow down. It does it every time. I can push the rudder skin with my hand and make the sound. 400 hours though....no problem.

David Watson

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Exactly!! Thanks for the information.

Mine to.

Passing through 120 to 130 kts I hear a thump. Actually the lady in back hears the thump and advises me every time.

I remember people talking about the bottom fuselage skins toward the rear
oil canning and trying stop it by adding stiffeners. Not sure this is the same thing.

http://www.vansairforce.com/community/showthread.php?t=80898&highlight=oilcanning+skin

Dave, on the three point landing...lets try an experiment. Put about 50 pounds of baggage in the aft compartment. Go try the three pointer again. Does it make any difference? Also, when you have a passenger, does the problem go away?

I think it may be less a propeller issue and more of a CG issue. Too much weight too far forward. Not exactly sure why that would matter, but its seems like guys with weight forward RV-8s seem to complain about this more than those with batteries and the like in the back.

Also, try coming in a little high with half or full flaps, about 10 knots over stall on short final. Pull power all the way out before you reach the end of the runway. Release a little back pressure to allow the aircraft to drop to compensate for the loss of power, maintaining speed. Approach angle will be steeper but when you flair your speed will bleed off faster as you transition to a nose high attitude looking for the stall. Not sure if I am explaining that right. lol. But it bleeds off your energy quickly IF you do not allow to much speed to build when you drop the nose, so when you flair, what remains tapers off quickly rather than floating you along in ground effect.

Where do you live? I can show you better than I can explain this.

Scott, thanks for the comments on 3 pointing the 8. I will try what you suggest as soon as I get the wheel pants fairings, which were layed up yesterday, off the airplane.
I feel very comfortable wheel landing and would like to feel likewise 3 pointing it.

Just remember, if you try the slightly high approach, do not let speed get away from you when the power is off. In EITHER direction. You are trading one form of energy for another. When you release slight back pressure after the power reduction, don't let the airspeed run away, but keep the sink rate steady right to the flair. If you try pulling back on the stick too early you will run out of energy immediately and plunk it on hard. Its a finess thing. The key with the fixed pitch prop is to find that balance where the energy is expended as you flair right above the runway, but not too soon so that you are stalling out 10 feet high, which is always ugly.

Carrying slight power is ok too, on a slightly flatter approach, but you have to chop it as soon as you are over the edge of the runway. Then just bleed off airspeed in a slightly nose high attitude. Think of it as trying to STOP the aircraft from landing. You are holding that attitude until the airspeed bleeds down and walla, you reach a full stall attitude about the same time as your wheels touch the ground. Its a magical moment.

Best of all, when you do it right, there is far less FORWARD momentum to carry you down the runway, so landing in a short distance is a happy biproduct.

Scott is this what you're trying to describe?

http://youtu.be/MwPozNnEg6g

The 8 is hard to three point well, I'm lazy and stick to what I can do.

Pretty much. Good video! For the new to TD guys: Even if you get a little float in ground effect it still works. Looks like he pulled power right at touch down and transitioned right onto all three wheels. For fix props, you can pull it sooner, relax pressure on the stick a little and it works the same, but with slightly slower roll out. Nice thing is, even if you do not flair and touch down at the same time, you can still hold it off the ground (the "not to let it land trick") and it will just transition into that attitude, coast along a few feet and then settle onto all three wheels.

Try closeing the rear air vent.

I have a noise in my 8 that sounds like propeller tip noise. its louder at high angles of attack and it will go away if I slip the airplane to the side. One day I closed the rear air vent and it went away and of course it comes back when I open it. I don't know if its coming from the prop but its definitely coming through the rear air vent.

Most likely structural vibration. Probably coming from the forward belly skins and or lower IB wing skins. Some RV-8 owners have reported belly skin cracks most likely due to vibration. Can be easily fixed using a structural damping material. Lots of certified aircraft use damping material/composites to prevent skin stress/cracking and reduce noise. Have fixed similar issues in several PA-28s.

http://www.aircraftspruce.com/catalog/appages/dampingpanels.php?clickkey=113451

http://www.earsc.com/HOME/products/DampingandIsolation/DampingComposites/index.asp?SID=150

http://www.earsc.com/HOME/products/DampingandIsolation/DampingMaterials/index.asp?SID=353

flyenforfun said:

I have a noise in my 8 that sounds like propeller tip noise. its louder at high angles of attack and it will go away if I slip the airplane to the side. One day I closed the rear air vent and it went away and of course it comes back when I open it. I don't know if its coming from the prop but its definitely coming through the rear air vent.

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Same noise in my 8 when the vent was open. When I had the floor out, I noticed that the scat tube that supplies air to the vent was not secured well enough to keep the tube from vibrating against the bottom skin. There were significant wear marks in the skin. Securing the tube stopped the noise.

My friend's RV-8 that I helped build would make a vibrating racket from the aft end of the plane that sounded like the tail was trying to come off in flight. Using rudder to yaw left or right would change the pitch and with a certain amount of side-slip held in, the vibrating noise would almost but not quite go away.

It turned out to be the rear of the canopy skirt rubbing up against the top of the turtle deck behind the rear seat... we discovered that as the paint started to wear thru where the rubbing was occurring.

Putting some foam weather stripping in there and it silenced the noise completely.

The noise I hear on every flight when slowing through 90 knots or so is a distinct "whomp" oil can sound. It is not vibration.

The vibration noise heard earlier was the back seat rattling against the fuselage support structure.

David-aviator said:

The noise I hear on every flight when slowing through 90 knots or so is a distinct "whomp" oil can sound. It is not vibration.

The vibration noise heard earlier was the back seat rattling against the fuselage support structure.

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David, if you can locate the section of skin causing the loud oil canning noise you can fix it with some of the adhesive backed structural damping material.

Scat tube maybe it.

I suspect it is the scat tube vibrating against something but I haven't been able to find it yet. I am doing the annual right now and I was thinking of wrapping the tube with some kind of insulation.

bruceg said:

Same noise in my 8 when the vent was open. When I had the floor out, I noticed that the scat tube that supplies air to the vent was not secured well enough to keep the tube from vibrating against the bottom skin. There were significant wear marks in the skin. Securing the tube stopped the noise.

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