Then there is the turbo-ramjet on the SR71. No traditional bypass, and yet also in afterburner all the time, yet efficient! This very cool video makes even a layman such as myself understand the basics... https://youtu.be/F3ao5SCedIk
> Date: Wed, 4 Nov 2015 09:57:00 -0600 > To: krnet at list.krnet.org > Subject: Re: KR> Jet vs Turbo prop? > From: krnet at list.krnet.org > CC: nerobro at gmail.com > > I don't think that's a fair way of delineating between the kinds of jet > engines. Multi shaft turbines are common in all types of turbine > installations. > > Turbofans are always at least dual, and sometimes three spindle engines. > Though in at least two cases, the "fan" part, isn't coaxial with the gas > generator section. Turbofans are defined by some fraction of the air moved > by the engine, not going through the hot section. Turbofans have even made > it down to "small" sizes, in engines such as the FJ33. > > Turbojets, have all of their air, pass through the hot section. Turbojets > are very uncommon these days in bigger installations. Even fighters are > using turbofans now. Larger turbojets all tend to be multi spool. To be > multi spool, engines need to be of a certain size. Practical limits to > size come into play there. > > Turboprops come in two styles. Free turbine, and .. I'm probably getting > the term wrong here, direct drive. And of the popular models, they're > about 50/50. > > Free turbine are much like a turbofan, in that they have second shaft, > sitting in the exhaust of the gas generator section, that independently > spins the prop gearbox. The gas generator section may or may not be multi > shaft. This sort of turboprop is typically started with the prop > feathered. > > Direct drive tend to be a single shaft. The driveshaft to the prop, is the > same as the shaft that the compressor and turbines are on. This is a > simpler design. But the prop is always a load on the turbine. These > turboprops are started with the prop at it's flattest pitch. > > The number of spools doesn't necessarily directly relate to efficiency. > Turbine efficiency is more directly related to compression ratio. Usually > called pressure ratio in turbines. Higher pressure ratios give you better > thermal efficiency. And bypass ratio, higher bypass ratios mean you're > moving more air for the amount of fuel burned. > > All of this isn't all that relevant in the power regions that say, a KR > could use. KR's need 60-200 hp. Turbines of that size, are very, very > tiny. As combustion chambers shrink, and turbine wheel sizes shrink, > thermal losses become a big factor, also there is leakage around the tips > of turbine blades, and that starts to become a major factor. > > I really do want a 100hp range turboprop... :-) > > O
