Solve This Weird Vibration Problem

[RV7ator]

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

[Planecrazy232]

Try tapping your brakes when this happens? Could be tire balance.

[Janekom]

My initial thoughts as well but ...... Can be something else.

[RV7ator]

No, it's definitely not tires. It's engine related.

John Siebold

[DanH]

Download a smart phone app called "Resonance". It was intended as a structural analysis tool for the floor in a building, be it seems to be useful for chasing any vibe. The key is the ability to identify the five most powerful frequencies.

[mahlon_r]

If the dynamic balance was good when you didn't have to add weights, that tells you that the engine and prop are NOT vibrating abnormally. So, there is no abnormal vibration from them. Guaranteed. If you can't see it with the dynamic prop balancer it isn't there. If the engine was shaking, the prop balancer would show it to you and you wouldn't be able to balance it away. So, if you feel its engine related, you could be feeling a normal engine vibration by transmission... something rubbing or bad mounts.
Good Luck,
Mahlon

[breister]

Back to wheel balance. While very unusual, it could be the NOSE wheel. Tapping the brakes won't stop that.

Can you mount a GoPro or similar camera to watch the nose wheel following takeoff?

[airguy]

Originally Posted by mahlon_r

So, if you feel its engine related, you could be feeling a normal engine vibration by transmission... something rubbing or bad mounts.
Good Luck,
Mahlon

Or a resonant vibration in a leg or gear fairing - I had that once on my left main, one rivet had worked its way out and another was loose, and at a certain rpm range the prop pulse would start the wheel pant vibrating in resonance with the prop. A little glass work and all was fine.

[F1R]

The only$0.25 I can add.
If you are feeling it on the rudder pedals and floor boards perhaps check your exhaust pipes and hangars for contact on the bottom of the firewall.

Also I once had a wing root radius fairing that would buzz( actually it sounded like a cow in a barn ) Yawing left or right would change the frequency slightly. A small patch of 100mph Tape on the nose of the fairing solved it. It took a few flight hours to ID/ locate and solve. Perhaps check the various fibreglass fairing to AL interfaces for signs of vibration. Gear leg fairings and wheel spats come to mind.
The rubber strips that seal the wings to fues on both sides also.

Good Hunting!

[BillL]

I recall a friend saying he took off in formation and the wingman reported his wheel pant keep oscillating in flight. I don't recall the details, so it might not apply. Just a thought . . . .