Shift of CG

[LARRYEBOYER]

It was suggested to me that my RAF was flying nose high. We decided to change the mast cam setting to #3 from #4. I took off and flew. The nose was lower plus because I shifted the mast back, I expected to have some ground handling issues. I got a bank bag and put 12 lbs of lead sinkers in it and put it in front of the passenger rudder pedals. It handled fine on the ground and seemed fine in the air. I landed and it flaired like I had hoped. (I was having problems in flairing to a full stop landing before.)

A friend showed up and he wanted a ride so we went up. All seemed ok. While flying the controls took on a heavy feel. I brushed it off as passenger weight. When we came in for a landing, the aircraft really flaired nice, but when I pushed the stick forward to taxi, the stick did not go as far forward as before. When I got over to the hanger, I got out and looked at the mast. It had shifted completely back to the #1 position. I have not pulled the magic bushing out, but I can assume I have had a failure. Some one posted that that can create a handling issue. I did not experience any thing scary, but the top push,pull control tubes were tight in the config and the stop settings were wrong and prevented the blades from being pushed forward to the prescribed settings. Not life threatning, but concerning.

[Harry_S.]

Larry;

Have you pulled the mast back and checked the condition of the donut?

I would guess the center hole is a bit aft of center...if that can be determined after and if you can remove the bolt?!

Let us know.


Cheers

[rgraffeo]

Larry,
My rubber bushing was causing the cam setting to change while flying.
I was in #4 & it went to #3.
I didn't notice anything wrong until I landed I looked at the mast. I made marks on the mast to see if it moves.

The bushing was hard as a rock & the bolt was against the mast. The hole in the bushing was torn to where the bolt touched the back of the mast hole.
I didn't experience any trouble flying with the bushing messed up.

[Steve Osborne]

Had the same problem once, but was not the magic bushing. After reassembly from a fly-in I forgot to toque the lower mast bolt tight enough resulting in a concentric spin of 180 degrees from position 3 to 2. Moved it back to # 3 toque to proper specs and have not had any problems since. The mast shift caused a less then -1 to -2 degree fore movement of the gimble head tilt but it still flew and landed normal.

[LARRYEBOYER]

i took the mast off yesterday to inspect all the parts. The magic bushing was like new and pliable with the bolt centered. i agree with Steve. I may not have had the lower bolt tight enough. I was told by another RAF pilot to snug that lower bolt to 60 ft lbs. if you have to add another washer, it is advisable so it is tight.I flew the gyro on the #2 setting and it flew fine. I have to reset the stops to the _1 degree and + 19. Also I think the slant of the rotor head should be checked also.

[Harry_S.]

I know what you're saying Steve, about the cam rotating. I've even used shims, per Paul Bruty and it still slipped. A more refined/definite method of anchoring that cam is needed. I'm thinking about a different shaped shim, right now.

Y'all up North probably won't have the problem with the donut that we do have, down here in the hot and humid South, but...

When that donut goes bad and allows the thru bolt to move to the rear, the rotor force will put even more pressure on the cam to rotate. I've been there...done that.

For those unfamiliar with the mast cam, I'd like to clarify, if you will, your reference to the cam rotating 180*.

The cam itself has only two holes but can be utilized in four different positions, which affects the slant of the upper mast. I haven't made any measurements but all four will be within say, a 1/2" at the bottom location, or say within 10*.

In your scenario, I would say the cam was allowed to rotate about 4-5* actually.

The worst scenario would be flying in the #4 hole and the donut allows the upper bolt to move back to the edge of the donut hole, where it's possible for the rotor and upper mast to rotate the cam to the #1 position. This would not allow a NORMAL deep flare. Been there...done that.

My experience had these warning signs:

Loss of Airspeed.

Loss of Altitude.

Loss of control travel.

Heavy control feel.

If the above conditions happen during your flight, my advice is...increase power to maintain AS and Alt. On your landing approach, make it shallow and do not attempt a deep flare...your rotor disc will be considerably lower over the tail...execute a run on landing with the mains. Been there...done that.


Cheers

[dragonflyerthom]

Great Info Harry S. Seems just another thing that needs to be figured out. I wonder if there are any other remedies available. Hey Aussie Paul what do ya thinki??


Thom

[Aussie_Paul]

Originally Posted by dragonflyerthom

Great Info Harry S. Seems just another thing that needs to be figured out. I wonder if there are any other remedies available. Hey Aussie Paul what do ya thinki??


Thom

When you have found the position that works for you, make a lower cam with the hole in the center. Then make new upper mast plates and shift one of the holes to place the head in your prefered position. This is how I did Hybrid.

With Firebird I am running seperate head plates as in Bensen style. To get the han g test correct will take some trial and error BUT then it will be right.

I have found with Rafs that you do not need 4 positions. Two will suffice.

Firebird will not need after market "stuff" to make it safer, so people will not be adding huge amounts of weight, with long moment arms, having to fit h/stabs etc.

I may need two sets of head plates to cater for fat like me people, and skinny people like Tina. I gotta get a brownie point for that eh Tina?

Aussie Paul.