Osborne Tip Tanks on an older 250

[William Hughes]

I have searched the board looking for information on the moment arms for fuel in the Osborne tip tanks, and I have the ICS POH that has a marked up C of G envelope diagram that applies to a 2800 to 3000 installation. Mine are now installed and the test flight went well - one aborted takeoff when the engine failed to make takeoff rpm! I also had the aircraft re-weighed and the empty weight and balance done from scratch.

I have been running various scenarios using that information. With just me in it and full tanks it is inside the envelope. With me, the wife, and two guests and full tanks it is still in the envelope.

But with myself and the CFI in the front seats, full tanks put it about 1/2" to 1" ahead of the envelope until about an hour into the flight. If I put 200 lbs into the baggage compartment that puts it right on the line at start. That, however, seems a bit nuts.

Am I missing something here?

[JIMICS2452]

On my installation for my 1960 250 the paperwork shows the tanks fuel at the same arm as the main fuel tanks, 90".