I have a RV-7A with 65 hrs on it. It has an XP-360 with 8.5:1 pistons and 1 mag and 1 electronic ig. The prop was tested at 0.08 cps and balanced to 0.04 cps at 60hrs. I am getting breakage of the alternate air cable near the air inlet. My airbox is also experiencing breakage (lost 3 rivets). There must be some type of high frequency vibration. I have an MT prop and typically run at 8,500ft at 21mp and 2370rpm. Should I be running at a different rpm? Should i go lower to 2300 rpm or go higher at 2,400rpm. Does anyone know which rpm band is best to operate at. (I vary the rpm typically throughout the flight from 2,350-2,380)
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Alternate air breakage
- Thread starter Dean
- Start date
dhall_polo
Well Known Member
typical
I don't think this is about picking your favorite RPM. You're relatively new RV is still in the break-in period, and you should expect to find things that need to be beefed up or tightened. I'd say that the FAB is a notorious weak point. The typical failure is the top plate on the fab box, cracking around the bolts where it's connected to the carb. (Note that I'm talking about the vertical induction FAB.)
IMO, the FAB should be closely inspected every oil change, especially for the first 200 hours. I ended up adding doublers and bigger washers, and I also added some bolts to help compress the filter against the FAB. We all build the FAB so that the filter is tightly sandwiched, but fiberglass can change shape with heat, so the filter can end up being a little lose after break-in. If there's any movement, eventually the fiberglass will fail.
Don
I don't think this is about picking your favorite RPM. You're relatively new RV is still in the break-in period, and you should expect to find things that need to be beefed up or tightened. I'd say that the FAB is a notorious weak point. The typical failure is the top plate on the fab box, cracking around the bolts where it's connected to the carb. (Note that I'm talking about the vertical induction FAB.)
IMO, the FAB should be closely inspected every oil change, especially for the first 200 hours. I ended up adding doublers and bigger washers, and I also added some bolts to help compress the filter against the FAB. We all build the FAB so that the filter is tightly sandwiched, but fiberglass can change shape with heat, so the filter can end up being a little lose after break-in. If there's any movement, eventually the fiberglass will fail.
Don
Your engine draws air in through the airbox twice per crankshaft revolution. At 2400 rpm (for example) that's 4800 times per minute, or 80 cycles per second. Not an extremely high frequency vibration, but the pulses are large. At that rate it doesn't take long to accumulate a lot of fatigue cycles. Problems with the filtered airbox disassembling itself seem to be somewhat common, especially on O-360 engines. The cure is probably less about picking an operating speed than stiffening the structure to better withstand the vibration.
