I usually like them to be at something other than dead side on, although that can work too. I also like to process the image so that I get a lot of detail on the lower side of the aircraft, I say lower, rather than bottom, as that allows for the inverted ones . When we look at an aircraft usually our eyes scan across it, and we tend to effectively change the exposure between the dark bits, and the light bits and build what is effectively an HDR image in our heads. For props I'll usually shoot at 1/160 and for helicopter rotors I try for 1/80 although generally 1/60 is better, and 1/30 would be perfect. I'll also try a smaller helicopter at 1/100 as the smaller helicopters seem to run at a slightly higher RPM. Helicopter rotors are normally run at a constant RPM, the pilot increases the pitch of the rotor blades to increase the lift to climb, or move horizontally from the hover using the collective control. This is unlike normal aircraft, where even if fitted with a variable pitch propellor of some sort, the RMP are usually significantly increased during take off, or when doing many aerobatic maneuvers. For some aircraft you could get the same degree of prop blur at 1/400s during take off, as you would get at 1/160s while the aircraft is at cruising power.