Shutter speed needn't be a tricky problem. In fact, once you do a few simple calculations you will know in advance exactly the amount of blurr you will get in the disc.
Before your flight, ask the pilot (of the target aircraft/helicopter) what his rotor RPM will be in various flight attitudes. e.g. cruise flight, hover etc.
Now do the following equation.
RPM / 60 x 360 / shutter speed = degrees of prop motion during exposure.
suppose the cruise RPM is 2000 rpm for the prop or rotor and a shutter speed of 1/250 per second.
2000/60 = 33.3 prop rotations per second.
33.3 rotations per second x 360 degrees = 11988 degrees of rotation per second.
11988 / shutter speed (250) = 48 degrees of prop/disc rotation during the exposure.
As you can see, you can now calculate the exact amount of motion blurr you desire in your photo. Of course you will also have to take into account your personal ability to hand-hold the camera steadily, and also any inherent vibration or motion between the two aircraft. Also I would strongly suggest NOT bracing your arms, hands or camera against the structure of the helicopter when shooting. The high frequency vibration will transmit directly into your camera and cause blurr.
Also remember that the pros DO
NOT shoot through any plexi at all. I used to wear a safety harness and sit at an open window/doorway for all my air-to-air shoots. There is simply no way to achieve absolute sharpness through glass. One other trick the top photogs use is a small battery operated gyro attached to the tripod socket of the camera. This gyro will allow absolute steady shots several shutter speeds lower than what you can normally hand hold. But they are a $3000.+ option that is beyond most of us.
The best single piece of advice I can offer to you is to sit quietly with BOTH pilots before the flight and have a very thorough pre-brief of what you expect, the positioning of aircraft in relation to each other and the background, and keep full flight safety always in your mind. Remember that flight and personal safety MUST come before anything else.
There's an old axiom that says "It is better to be on the ground wishing you were flying.... than flying and wishing you were on the ground."
Plan the flight and fly the plan.
Hava fun and shoot LOTS and LOTS and LOTS of film.