I like the brain trust to comment on a few ideas regarding the source of an unusual, strong, vibration that has annoyed me for 600 hours on a -7.
The set-up: RV-7, IO-375, Whirl Wind Aviation 74RV prop.
The problem: First take-off of the day from a cold start, full power (27.0/2690), accelerating through 105-110 MPH IAS, a sudden onset of strong vibration snaps on like flipping a switch. Felt most strongly through the rudder pedals, it really gets your attention. Here's where it gets weird: I can't reproduce this sudden onset during any phase of flight or subsequent stop-and-goes.
Nothing associated with the engine is rubbing or touching the airframe. The drive train is dynamically balanced (as built so good no weights were required).
I don't think it's an airframe vibration. There's no association with any airspeed or power setting other than the first take-off.
Dialing back rpm after TO reduces the magnitude and frequency diminishes with rpm reduction. The vibration remains as airspeed increases. Eventually, reaching altitude and/or reducing to cruise power (70% or so), things calm down to just an ongoing buzz that's endurable but still not smooth as I know a prop-driven airplane can be.
I wondered if there was some combination of prop pitch associated with power and airspeed that excited some airframe natural frequency. But I'm dropping that concept because a friend who rode with me when the plane was wearing a Hartzell BA remembers the sudden vibration.
I think it's something to do with the vibration isolators. What's installed are what Van's was selling seven years ago. (The invoice does not name the brand.) What I can't wrap my head around is the sudden onset and inability to recreate the vibration. Only under-the-hood temps change during a flight, which will affect isolators to some degree, but I don't know how much. I'm going to chat with Lord about this, if I can get through to a tech.
Any ideas?
John Siebold
The set-up: RV-7, IO-375, Whirl Wind Aviation 74RV prop.
The problem: First take-off of the day from a cold start, full power (27.0/2690), accelerating through 105-110 MPH IAS, a sudden onset of strong vibration snaps on like flipping a switch. Felt most strongly through the rudder pedals, it really gets your attention. Here's where it gets weird: I can't reproduce this sudden onset during any phase of flight or subsequent stop-and-goes.
Nothing associated with the engine is rubbing or touching the airframe. The drive train is dynamically balanced (as built so good no weights were required).
I don't think it's an airframe vibration. There's no association with any airspeed or power setting other than the first take-off.
Dialing back rpm after TO reduces the magnitude and frequency diminishes with rpm reduction. The vibration remains as airspeed increases. Eventually, reaching altitude and/or reducing to cruise power (70% or so), things calm down to just an ongoing buzz that's endurable but still not smooth as I know a prop-driven airplane can be.
I wondered if there was some combination of prop pitch associated with power and airspeed that excited some airframe natural frequency. But I'm dropping that concept because a friend who rode with me when the plane was wearing a Hartzell BA remembers the sudden vibration.
I think it's something to do with the vibration isolators. What's installed are what Van's was selling seven years ago. (The invoice does not name the brand.) What I can't wrap my head around is the sudden onset and inability to recreate the vibration. Only under-the-hood temps change during a flight, which will affect isolators to some degree, but I don't know how much. I'm going to chat with Lord about this, if I can get through to a tech.
Any ideas?
John Siebold