RV-8 In Flight Fuselage Noises

[David-aviator]

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.

[DEWATSON]

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

[tracy]

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.

[David-aviator]

Originally Posted by DEWATSON

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

Exactly!! Thanks for the information.

[Paul Tuttle]

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

[billdianne]

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/communit...ilcanning+skin

[wera710]

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.

[David-aviator]

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.

[wera710]

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.

[Sig600]

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.