Hello all, first post here. I made an account because I have a couple of questions regarding experimental aircraft and modern automotive engines. To frame the question, I feel I should walk you through how I arrived at the question. I have recently become interested in aviation as a hobby and with all new hobbies I have been attempting to absorb as much information as possible. I discovered experimental aircraft after looking at what is required to maintain a certified aircraft. Since I am a mechanical engineer and a hobby mechanic/fabricator and don’t trust even my automotive maintenance and repair to anyone other than myself the fact that there are no restrictions to owners performing maintenance functions on the airplanes in this category seems like a huge plus. While doing more research I discovered that having an aircraft engine overhauled can cost upwards of 30 thousand dollars. I thought that sounds ridiculous considering how old and simplistic the technology seems. This is especially true when you consider that a brand-new gm long block is only $5000 dollars. This got me to the point I was thinking about the possibility of putting LS power into an experimental aircraft. After some research I found the Murphy Moose that has the naturally aspirated LS engine. Due to the higher revs of the automotive engine they used a 2:1 step down gear case to run the prop at the correct RPM.

This finally brings me to my question. From my understanding prop size is determined or rather limited by the speed of the tip. From my research the prop tip must stay under 0.85 of Mach 1. For example, if the given engine develops peak power at 2700 RPM then the diameter of the prop would be at the largest 80” which would result in a prop tip speed of 287 m/s. If all the above is true then why mess around with a 2:1 step down gear case? Just go direct 1:1 and properly size the prop. For an RPM of 4500 the prop diameter would be 48” to have the same 287 m/s speed as the previous example. Is there any issue with reducing the prop size by this much? I would assume that you would need at least 3 blades potentially more to utilize the full power of the engine at WOT, but I cant see that presenting any issues. Is there something that I am missing? The idea of replacing an engine with a brand-new crate engine for $5k as opposed to $30k would lower the cost of flying per hour quite tremendously at least 13 dollars an hour and not needing to buy and maintain the reduction drive would be a huge win as well.