[Ernie Martin]

Take a look at a thread titled "Different engines possible?". BTW, it's always a good idea to do a Search of this forum to see if your question has been answered before.

The following are excerpts from that thread, where someone asked a question similar to yours.

Roger answered, in part: I've always had a bit of a problem with various "rocket scientists" scabbing "better ideas" onto Skymasters. Inevitably the scabbed on projects either fail to live up to the hype, or end up literally crashing and burning into yet another pile of metal.

I added: I have an MS in engineering from Caltech, spent 30 years in the aerospace industry and have consulted for both GE and the FAA. And one thing missing from any discussion in this Message Board -- not just this thread but the whole Message Board -- is "Systems Engineering". The aircraft as a whole is a "system". Everything is interelated. It was designed as a system. Change one item and it affects everything else. Often adversely. Change the engine and it affects the loads on the airframe, on the prop, on the prop governor -- maybe even the harness.

None of us has the money or resources to make sure that a change in one area will not adversely affect another area. So it doesn't get checked. Or it gets checked poorly.

I read this thread when it started and I wanted to jump in and say "are you crazy?" I didn't because I'm one of the site administrators and did not want to discourage full discourse. But now that Roger voiced his views I wanted to say "save not only the money but also the grief".

Before I went into engineering I spent five years in a business that souped up cars for racing. The typical client had limited resources. So he changed the carb from a 2-barrel to a 4-barrel. Never was the desired performance achieved. The compression ratio, cam timing, valve size and exhaust system -- which were designed for the 2-barrel -- remained, so there was now lots of fuel but the combustion efficiency sucked. Even clients who modified the whole engine ended up unhappy. Yes, the engine was a rocket but the drivetrain (transmission, rear axle, etc.) was designed for the wimpy engine, so elements of it kept failing.

Your safest aircraft is a well maintained certified aircraft. Limit your changes to the avionics suite.

Further questions and responses dealt with engines that were STC'd for the Skymaster (such as the Riley Rocket and AirScan Skymasters), and I was asked whether I would find these acceptable. My response follows: A different engine with significantly greater horsepower or one with radically different characteristics (read: diesel powered and water cooled) would be acceptable to me only if there have been dozens of such installations spanning several years and they have been without trouble.

Two other thoughts.

First, it's possible that I'm more conservative than most, because a) I fly mostly my family and principally over water, and b) I spent 19 years on communications satellites, where the designs had to be perfect since you can't fix a satellite once in orbit.

Second, my opinion is based not just on good engineering principles, but on hands-on observations. When you look in the area of the rear engine, you see how cramped and well designed are all of the elements associated with the engine. The engineering effort that went into the design of all mounts, control cables, hoses, etc., must have cost millions in 1960s dollars, and I just cannot believed that kind of robust design can come from an STC. Just think of the consequences if a throttle control cable fails on take-off or landing, because the STC design gave short thrift to that element. And you, being an early user, are the guinea pig.

Not for me. I'm not a test pilot.

In the thread "Another question about alternative engines" I posted an image of the baffling used to keep all of the cylinders at the proper temperature, to give you an idea of the huge design effort spent by Cessna just in baffling. Take a look at the image and ask yourself if an STC would have an equally robust design.

My apologies for the long answer.

Ernie