Netters This debate about best engines for experimentals, and whether to turbo, whether to Gear drive/PSRU or direct drive has been going on for a while now, and I suspect will probably continue, kind like the Ford/Chevy debate (Chevy is best, LOL ). I am NOT going to take sides of whether a builder should choose auto or aircraft engine, normally aspirated or turbo, or direct drive or PSRU. All these things are in the archives, and found in literal volumes in other writings, both Internet and books like, "Auto Engines for Experimental Airplanes" by Robert Finch, just to sight one example. There are many others.
What all Netters, especially you new members need to take to heart is that engine selection is VERY important. Looking at certified aircraft, you will see that it it the single most important factor when considering an aircraft's present value, how many hours on the engine. Of all the expense of owning an aircraft, once it is completed, the most money you will spend will be for the engine, and its up keep/maintenance. Turbo charging is the cats meow for cheap horsepower, but just ask Orma Robbins about how this "enhancement" comes with its own unique set of problems to deal with and overcome. Also, the article sighted states that turbo charging generally adds at least 50% more power. This is out right fiction! The best that I have seen proven by dyno runs is approximately 40%, and this is with associated engine modifications, AND the use of an inter cooler, which is not mentioned in the article at all. B&M, Vortech, Banks Turbo-charging, and Paxton all report similar values for their "bolt-on" systems. I am not saying that a turbo or supercharger cannot add 50% or more power, but that rather that bolt on systems do not give that kind of increase, and do not want builders running out and buying a turbo for their engine expecting to get a 50% increase in power and torque by just hacking the exhaust in order to add the turbo. The same rules hold true for direct drive vs PSRU. There are definite benefits to a PSRU, but to set one up on a 2.2 to 1 reduction, just to achieve maximum horsepower from an engine from a dyno run, and say that is best does not take into account all phases of flight for the engine, only take off. That is the only time you will use max power. This amount of reduction although it makes the max power available according to the dyno, it does not allow for a reasonable rpm for cruise. This is because the prop will be slowed to 2000 to 2100 rpm, which begins putting it below its cruise efficiency speed. Just compare certified props that are made to run in this range of rpms. They produce max thrust at near redline, and produce best cruise thrust at 75 to 80% engine power. This puts the prop at around 2300 to 2400 rpms on a 2750 redline. This puts the engine in the re drive at 5060 rpms for the 2300, and 5280 rpms for the 2400 rpms at the prop. Now your engine is running just like the Rotax family of engines and can expect the same life, or simply 50 to 100 hour maintenance intervals with a major a max of 500 hours out. It also makes the combination "peaky", where basically you spend literally all your time at or near peak rpm. Robert Finch's book details a lot of engines that have been successfully used in direct drive configuration; the Buick V8, the VW family, the Corvairs, and several others mentioned in his book. In larger aircraft that have more generous weight allowances for the engine, the more complicated and heavier engines have a good appeal. BUT for our applications, in order to stay in the RECOMMENDED weight range of engines AND their output, direct drive offers the best answers, and air cooled the simplest installation. Above all, it takes research and study to decide and engine install, and talking to other actual pilots of those engines. Don't get sucked into the trap of some fancy numbers calculations and good advertising on one web page where one engine is presented as the experimental airplanes dream engine. There are a lot of "assumptions" and over generalizations made at the expense of the builder. No quick answers here. It takes years to build a KR, take enough time to study your engine completely BEFORE spending any money. Colin Rainey brokerpilot9...@earthlink.net EarthLink Revolves Around You.
