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Aerobatic FAI F3C Contest > XCell whum...whum...whum Noise

Four Stroker
Senior Heliman
Location: Atlanta

I have an XCell 46 graphite with T&T head and original plastic grips. I have flown with Hi Products AD46 600 blades and DY Products F50 600 blades. There is a pronounced resonance in FFF only of about 5 to 10 Hertz with both sets of blades - very loud. I was about to tear down the heli and look for the usual bent shafts and bad bearings until I flew last week on a nice warm day. The problem completely went away.

Have any of you seasoned (old) guys ever encountered this type of resonance ? The T&T damping is much softer than the original (can be tightened up with shims). I assume that the only thing temp. would effect is the plastic blade grips. Is this a damping and blade grip phenomena ?

Some of you FAI guys should know something about this !

04-14-2008 03:44 PM

cannibal440
Senior Heliman
Location: cookeville, tennessee USA

make sure no bearings are sticky or notchy. ive heard that noise before. are you tightening the blades enough?
loosen the rods on the swash and see how freely everything moves. if it feels different in one spot go up the head till you find something. could be a bad o-ring on the dampener too.

tracking always right on or change often?

No i havent flown before but I did stay at a holiday inn last night.

04-14-2008 05:22 PM

HOMEPAGE

Four Stroker
Senior Heliman
Location: Atlanta

Tracking is perfect. New bearings. New O-rings.

04-14-2008 06:20 PM

ErichF
Key Veteran
Location: Odessa, FL 33556 (Tampa Area)

Check your tail rotor system. Tail rotor effectiveness, and therefore gain, are affected by temperature changes. Your gain in FFF may have been slightly too high for the conditions.

My Caliber 90 will sometimes make an oscillating humming sound when it's haulin' boogy. The changeover to a BLS251 eliminated that for the most part.

04-14-2008 10:31 PM

Four Stroker
Senior Heliman
Location: Atlanta

Thanks Erich;

That could be the problem. I changed out my ancient 9205 for a 9253 and ran the gain up experimenting. I forgot that I did this last fall! The 501 was set at 50% which is way too much for a 9205 and maybe high for a 9253. I used to run it at 30% in idle up. The frequency is about right for tail rotor wag. It wasn't visible from the ground.

Anyone have a rule of thumb for 501/9253 combo ?

04-15-2008 02:38 AM

Four Stroker
Senior Heliman
Location: Atlanta

This turned out to be a rather obscure problem. The main gear was warped. It felt fine on the bench, but the bevel gear got extremely crunchy when the heli was picked up be the head. There is enough axial slop in the bearings (new) to do this. A warped MA main gear is no big surprise. Test flown today.

05-11-2008 01:02 AM

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Aerobatic FAI F3C Contest > XCell whum...whum...whum Noise

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