Home > VansAirForceForums

-Advertise in here!
- Today's Posts | Insert Pics

Keep VAF Going
Donate methods

Point your
camera app here
to donate fast.

Old 04-15-2019, 02:04 PM
RV7ator RV7ator is offline
Join Date: Jan 2005
Location: Boise, ID
Posts: 1,012
Default Solve This Weird Vibration Problem

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
Reply With Quote
Old 04-15-2019, 02:40 PM
Planecrazy232's Avatar
Planecrazy232 Planecrazy232 is offline
Join Date: Aug 2015
Location: Cape Coral, FL
Posts: 445

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


RV6-A "Aluminum Mistress"
Too many hobbies- not enough time.
2016, 2017, 2018, 2019, 2020, 2021, 2022 donor and happy to do it.
Reply With Quote
Old 04-15-2019, 02:48 PM
Janekom's Avatar
Janekom Janekom is offline
Join Date: Jan 2006
Location: South Africa
Posts: 890

My initial thoughts as well but ...... Can be something else.
Jan Hanekom
South Africa
Approved Person nr 325
RV10 - ZU-IBA - Flying and getting lots of TLC
RV10 - ZU-XOX - (Flying)
RV7A - ZU-JRV - (Flying)
RV10 - ZU-JVR - (Written off)
RV7 - ZU-LOL - (Flying)
RV10 - ZU-RVI - (Flying)
RV7A - ZU-MER (Bought back)(Sold again) Flying
RV7A - ZU-JAD (Sold Flying)
Dues paid for 2022 WITH A SMILE
Reply With Quote
Old 04-15-2019, 04:53 PM
RV7ator RV7ator is offline
Join Date: Jan 2005
Location: Boise, ID
Posts: 1,012

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

John Siebold
Reply With Quote
Old 04-15-2019, 05:07 PM
DanH's Avatar
DanH DanH is offline
Join Date: Oct 2005
Location: 08A
Posts: 10,452

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.
Dan Horton
Barrett IO-390
Reply With Quote
Old 04-15-2019, 05:41 PM
mahlon_r mahlon_r is offline
Join Date: Jan 2005
Posts: 1,064

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,
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."
Reply With Quote
Old 04-17-2019, 12:32 PM
breister breister is offline
Join Date: Dec 2006
Location: Atlanta, GA
Posts: 1,231

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?
Reply With Quote
Old 04-17-2019, 02:03 PM
airguy's Avatar
airguy airguy is offline
Join Date: Oct 2006
Location: Garden City, Tx
Posts: 5,662

Originally Posted by mahlon_r View Post
So, if you feel its engine related, you could be feeling a normal engine vibration by transmission... something rubbing or bad mounts.
Good Luck,
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.
Greg Niehues - SEL, IFR, Repairman Cert.
Garden City, TX VAF 2022 dues paid
N16GN flying 950 hrs and counting on 91E10; IO360, SDS, WWRV200, Dynon HDX, IFD440, G5
Built an off-plan RV9A with too much fuel and too much HP. Should drop dead any minute now.
Reply With Quote
Old 04-17-2019, 02:10 PM
F1R F1R is offline
Join Date: Nov 2010
Location: ____
Posts: 922

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!

Last edited by F1R : 04-17-2019 at 02:18 PM.
Reply With Quote
Old 04-18-2019, 07:10 AM
BillL BillL is offline
Join Date: Sep 2007
Location: Central IL
Posts: 6,364
Default 7A Comment.

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 . . . .

Reply With Quote

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

All times are GMT -6. The time now is 04:47 AM.

The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.