Photographing helicopters air-to-air with sharp details while also having the rotor blades suitably blurred is a challenge for any photographer. The aircraft holding you (the photographer) is often cramped, bumpy, and vibrating, while the aircraft you are trying to capture is also moving, sometimes because of turbulence, sometimes because of mismatches in speed or altitude or pilot comfort levels. Here are some quick tips for capturing great shots in these challenging conditions. Since you have to use a shutter speed that allows the blades to blur (if they aren't blurred, the aircraft will look like it's about to fall out of the sky), your main challenge is to keep the camera steady during each exposure.
When you're shooting on the ground, you might lean against a lamp post or building when your shutter speeds need to be slow. In an aircraft, you don't want to lean against the door frame or seat or anything, really, because those structures are being vibrated by the engine and rotors and air flow around the aircraft. So touch the aircraft as little as possible — let your body absorb as much of the vibration and jiggles as possible. And speaking of air flow, unless your airspeed is extremely low (which is possible in a helo), keep your lens out of the slipstream just outside the door or window. Even if you can't see the vibration through your viewfinder, your lens was not designed to slip cleanly through the air, so it will not be at rest if it's out there. This applies to lens hoods, too. Keep them out of the wind, or take them off (and stow them securely).
Next, if you have stabilization features in your camera or lens, try using them; they should help. But review some shots carefully to be sure the features aren't making things worse. Sometimes the systems will be trying to move the sensor or lens elements or whatever, during the time the shutter is open, and the result is more blurring. A gyroscopic stabilizer can be a big help, not only by functioning as intended — countering unwanted motion in the camera with opposing motion induced by the gyro — but merely by being heavy they tend to dampen smaller motions caused by vibrations. These systems add weight and complexity, with batteries and cables, and that extra weight can be awkward and compounded by G-forces during maneuvers, but they can seriously increase your percentage of good shots. Finally, shoot a lot — digits are cheap, while aircraft operations are not — and check your results closely when you have a lull in the action, such as when the subject aircraft is maneuvering away from you. Zoom in on the display on the back of the camera to make sure your settings and technique are achieving the desired effects. And if you are not getting sharp details in the subject aircraft with suitably blurred blades, improve your technique or go ahead and increase your shutter speed. Coming back with usable, if not ideal, imagery is better than coming back with nothing but blurry ones. Oh, and fly safe. Listen to your pilots, ask them for what you'd like, but accept what they're willing to do. No image is worth injury or death.