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Hello All,

For the last four years I've been the custodian of Dave McMurray's Falco (N155BJ); it's now flying in the UK. It's a fantastic aeroplane and I absolutely love it but I have a small problem and I'm hoping the big brains of the Falco community might be able to help.

Essentially, I have an airfame vibration problem. It's not massive but it is annoying and I have not experienced it on other Falcos I've flown. The vibration has been there for some time (probably for at least 18 months) but I can't remember when I first noticed it. I can't honestly recall if it has been there since I acquired the aeroplane in 2020 but I don't think so as I believe that I would have noticed it from the get go.

The vibration is most prevalent at cruise settings and isn't noticeable at reduced power settings in the circuit. Initially this made me think it might be an engine or prop balancing issue but the engine runs sweetly on the ground even at high rpm and the engine mounts look fine.

A recent flight made me think it might be an elevator/trim issue. Flying at 4,000ft at cruise settings with some forward trim applied the vibration became noticeably worse. I experimented and found that by rolling off some nose down trim the vibration seemed to go away. On landing I checked the trim tab and noticed that there was some play at the hinges (not massive but definitely noticeable). I replaced the hinges and there is now no play in the trim system. At the same time I checked the tension in the elevator cables and reset these to 45 lbs per the Falco construction plans. There is now no play in the elevator circuit (nor is there any play at the ailerons). Both my engineer and I thought that this would solve the issue...Nope.

On the test flight yesterday the vibration was still there but I noticed that it's very much reduced when the elevators are in the neutral position. However, when I eased back on the stick in a climb I noticed the vibration increased, although the same didn't happen when forward stick was applied.

Not sure what to look at next. Has anyone else experienced this problem. I'm all ears.

Thanks

David

Hi David, you do seem to have a perplexing problem.

I am a frequent visitor to your shores (mostly London, Easton Neston and Blackpool) and should be there for Christmas. I would be keen to meet fellow Falcoholics. I am restoring and doing my best to keep flying while doing so a Falco in Cape Town South Africa. Mine has no noticeable vibrations. Can you give us more info as to what altitude, power and rpm you are finding this. I would try to get a vibration meter to determine its frequency and then start hunting at things that are the same freq.

In the meantime, my ten pence of idiocy beckons the following:
Q1 Can I assume you have changed your flexible engine mounts and they are set to the correct torque? If old and rigid they could be setting a dynamic / harmonic vibration, especially if your prop is unbalanced.
Q2 Is there any wear in the control bearings.
Q3 Are your horizontal trailing edges free of any paint stripping off?
Q4 Have you recently balanced the propeller?
Q5 Is there any exhaust staining or oil marks on the horizontal surfaces? Different exhaust set ups might allow the exhaust to effect the local airflow (clutching at straws on this one).

Kindest regards
Alan Evan-Hanes (sn820 ZU-BTT and sn 1425 in bits)
Cape Town South Africa

Hello Alan,

Thanks for the reply. Sorry to be a while replying, there hasn't been a lot of action on the forum so I hadn't checked back in a while. Hope the restoration is going well. I'm up in Scotland so a ways from your usual haunts further south but give me a shout if you're in country.

Many thanks for the suggestions.

1. Engine Mounts. I've thought about this and discussed it with my engineer. He took a look at the last Permit Inspection and said he was happy with them. That said, hardening of the rubber may not always be visible I suppose so it's still on the list as a possibility.

2. Control Bearing Wear. I've been through all the control bearings and cables. The tensions are correct and there is now no play in the control circuits

3. Paint. All the paint is good

4. Prop Balance. James Tseliki had the prop statically balanced in 2019 when it was overhauled but I don't believe it has ever been dynamically balanced.

5. Exhaust staining. Nothing obvious, the system is per the plans so I don't think this is a problem.

My current plan of attack is to look at getting the prop dynamically balanced as, like you, I think this may be the probable culprit. That said I have previous with engine mounts on another aeroplane so that's a possibility too. Looks like I'm going to have to pay my money and make a choice