So, WHAT is(are)Your question(s)/concern(s)???? - Wil Taylor

It is a little hard to explain but here goes. The place I work insists on flying their test hops(part of which is checking degrees of roll at certain airspeeds 200-550 kts with flight controls at null postion) with 4 wing pylons(2 on each wing)installed. The test book says don't do the roll test on asysmetrically loaded aircraft. 3 of these pylons are 6 inches wide. The other one is about 40 lbs heavier and has an aerodynamic type fairing at the top of the pylon where mounts it to the wing. The pylon is 9 inches wide at the widest part of the flared type fairing, otherwise the shape is exactly the same as the other 3 pylons. The aero pylon is installed on the outer part of the the RIGHT wing. We have had about a 90% of these test flights fail for excessive LEFT roll(more than 30 degrees in less than 6 seconds) at 500kts and 550kts. Every aircraft with the aero pylon on the RIGHT wing has rolled LEFT at the high speeds but a few did not roll out of the limits but were very close to failure limits. At low speeds the aircraft tends to roll right but within limits and at 400kts the left roll appears but within limits. The greater the speed the harder and quicker the left roll is. We had one fail for RIGHT roll, but the aero pylon was installed on the LEFT wing. The roll rate failure is not related to flight control rigging, I am the guy who has to sign off these rig checks and have been doing them for almost 20 years now. I am absolutely convinced the pylon miss match is the problem but looking for someone other backing or ideas.  

Sounds like the Pylon is a factor... but...

What airspeed is the aircraft trimmed for [3-axis], when these tests are conducted????

From experience I discovered the following [for a mechanical flight control system acft...]:

If an asymetrically loaded aircraft was trimmed for (say) 300KT, then accelerated or decelerated from this trim-airspeed condition, then the acft would tend to roll in opposite directions, IE: @ low airspeed it would roll into the "heavy" wing; @ higher air speeds roll away from the "heavy" wing. This is purely a fact of the aero-trim forces increasing or decreasing with airspeed.

Regards, Wil Taylor