LoonyBob

Budworht Puffin? NPT? Indeed... (picks up hammer and saw)... the main challenge is that the Pelton Wheel turbine is far better suited to the cyclic exhaust puffs of a reciprocating engine; see Napier Nomad. I believe Buchi's original turbine supercharger used an axial flow turbine, but I wot not of any modern units that do. You reenthuse me...

Ok, vacuum is the wrong term; but the mass flow will be negligible, and the recirculation is minor, so the power required to keep it all spinning should be much less.

I don't know that a 150hp RR Dart would be fatter than a 916!

'Course it has to be rotating; by "autorotation", I was referring to the condition whereby the velocity distribution across the disc gives such a low AoA near the centre, that the lift vector drives the blades around - as per gyrocopter, or helicopter in a tizzy.

What about adding a "choke" to the compressor inlet? Once flamed out, if no air was allowed into the compressor, the rotor would be spinning in close to vacuum, so not braking the prop...

Idiot-resistant is the best I can do! The energy to be disappated is calculable, and not particularly huge for a wooden prop on a 150-kt 600kg aircraft - bearing in mind that it's an emergency device, not a frequent-use item. Consider how long a wooden J2200 prop keeps spinning if you close the throttle at 90kts. I'd favour a featherable unit myself, but weight and cost are factors. If it meets an airworthiness design standard, the engine failed criteria will be specified.

Nev, we may be using different definitions of drag coefficient... if the pitch setting is coarse enough to avoid autorotation (partial reverse flow), the mean drag should be of the order of 4~6 times the maximum full-power thrust, which is still less drag than a solid disc of 1.15 times the actuator disc area. With an adjustable pitch prop, which will feature less blade twist, it will be higher; indeed, at several hundred knots, such would likely begin to autorotate; but without autorotation, I can't see how the prop can transfer the momentum.

How about fixed pitch wood & a brake?

Whatever the factor of the rated thrust, the Cd surely won't be more than ~1.15?

Soz for the delay, local electrical storm...

Yep, CS with feathering ability pretty much a must. Or a high-bypass fan!

Where to begin? New principles? I thought the 1950 Rover JET1 used regeneration; I'm sure the 1965 GM Turbine truck did. Good on 'em for having an R&D effort, anyway.

I though heat engines gave a maximum possible thermal efficiency approximating to the expansion ratio by cycle temperature ratios, divided by the absolute ratios. Williams persued this, and ran into stability problems with 3-spool engines. A single turbine ain't gonna cut it...

Because a gas turbine is only burning ~4% of the air passing through it (I believe the more recent airliner burners better this, but nothing small), a lot of the energy in a turboshaft is returned via the exhaust jet; and the efficiency of the jet is directly proportional to the speed, which light light aircraft ain't meant to have.

The Dreher Baby Mamba was inspirational, but a wee bit wee for what we want. I confess to being fascinated by the possibility of affordable micro gas turbines - burning alkylated electrolised hydrogen? - but the challenges are very real.