Re: Jabiru 3300
I hope we can keep this thread civil as it is a good discussion. As Tim pointed out your engine is a big investment so folks get pretty passionate about their decisions. I, for one, am glad there are options, the more the better. Which is best is likely to be one of those debates that will go on forever hopefully with ocasional new choices thrown into the mix. I chose the Rotax because it was the most common and best understood power plant for this plane. I also don't like having a reduction unit but it does provide one solution for the following engineering challenge. To optimise the prop based on desired performance envelope will result in the best length, pitch and RPM. To optimise an engine for best HP/weight and other considerations will also result in a best RPM it just wont be the same as the best RPM for the prop. A direct drive engine will compromise these two in exchange for simplicity (a very real advantage in an airplane engine). Consider the following relationship (this has a bunch of hand waving but I believe basically correct). For constant tip speed and static thrust HP needs to increase linearly with the prop RPM at which it is produced. This is to say that if you have 20% more HP at 20% higher prop RPM (and therefor a 20% shorter prop) it should be about a wash in static thrust.
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