One of the opportunities I often take advantage of when photographing helicopters is to compose them against a background that not only gives context, but adds visual energy. Either by composing the elements in the scene, or by taking advantage of the mixture of low altitude and low shutter speeds, each not unusual when shooting helos, to blur the background with motion.
The S-76, above, jumps off the page with its vibrant livery contrasting with the muted colors below it, then I further separated the two by motion-blurring that brown field. As a bonus, the dark lines in the field act to frame the aircraft and offer a sense of direction, funneling the fuselage through the image.
An out-of-focus background helps to bring extra focus to the subject, but because of those low shutter speeds, required to render the blades in motion, the aperture in the lens must be small to restrict the volume of photons entering the camera, and a small aperture tends to render the background in glorious, distracting, clarity.
Well, there's more than one way to reduce that clarity, and motion is one of those ways. (You can read about shutter speed and why you don't want too much of it in a previous article, here.)
Of course, for there to be motion blur, something has to be in motion, and for the blurry thing to be the background, the aircraft must be in motion relative to it, and the camera and the subject need to be moving together. No surprises there, I hope. The effect is more noticeable the closer the aircraft and the passing background are to each other, so lower altitudes are your friend, at least photographically. This red MD Explorer is probably no more than 50 feet above the terrain so, even though we're probably sliding along at only 30 knots, there's plenty of blur in that background (and in the rotor blades, for that matter).
The S300C, below, shows off its three-blade main rotor from, perhaps, 200 feet above this corn field, and the blurred corn makes streaks of color and contrast, emphasizing motion, more so than even the creek bed flowing past the Explorer.
As always, the first goal of a photograph is to prompt a reader or passerby to stop and look. Get their attention. Putting some blur in a photo puts some energy in the photo, and that energy is used to stop a reader in motion. "Look at me; something is happening here!"