In a multi-engine propellor-driven aircraft, propellor synchronization occurs when the two propellors are turning at the same RPM. Propellor synchronization eliminates beating. The purpose is for comfort of the crew and passengers.
The claim I'd like examined is that out-of-sync propellors increases the likelihood of airsick passengers:
- "that god awful sound is enough to make passengers and pilots alike very ill after 15-20 minutes" (from the airliners.net forum)
- "I don't have passengers to make sick" (from the jetcareers.com forum)
- "If it doesn't work it can either be slightly annoying or bad enough to make passengers sick" (from the airliners.net forum)
I believe that un-synced props would be annoying or distracting, but I am doubtful that they can cause airsickness. I can imagine several mechanisms for how this could be true, but I haven't found any evidence other than anecdotes in forums.