Jet turbine reliability

On Sunday, June 7, 2015 at 4:15:05 PM UTC+10, Craig Lowrie wrote:
Climb and glide with the Jet is NOT optimal...


Actually my spreadsheet says that climb/glide is the optimal method even with the jet. I was somewhat surprised by this as with piston retractable engined motorgliders it obviously is as the L/D with engine extended is so bad.. Airplane range is a function of a number of things and L/D is a big one (see Breguet range equation). The L/D with the extended jets would seem to be much better but you don't operate it at best L/D for the climb as the jet best rate of climb speed is very much higher than with a piston engine (thrust drop off with speed is much less and power to aircraft = forward speed x thrust). The single engine jet turbos have much lower thrust to weight than a self launcher so use most of the thrust to stay in level flight particularly if you throttle back to be easier on the jet, so climb/glide probably doesn't work so well there.
There is another consideration and that is the specific fuel consumption of the engine. Jets don't do deep throttling well and 70% is about as low as you want to go before the SFC starts to go bad. This is a good reason for two engines on the self launcher. Just shut down and retract one if you need to fly level because of airspace or meteorological considerations.

One benefit of the high climb airspeed is that you aren't waffling around in a high drag configuration near the stall.

Unless you have a very high thrust/weight, 200 knots is way above the best rate of climb speed for a glider.

To make CS22 takeoff performance you a a T/W of about 0.13 to 0.14. So to let you run the engine(s) at say 70% for takeoff and climb the nominal installed T/W needs to be about 0.18 to 0.20 . This gives reasonable takeoff distance with margin over CS22 and an excellent climb rate at around 100 knots for a modern glider.

As my Ventus has the motorglider CM wings I chose to put the fuel in the fuselage rather than in the wings. Operationally easier when operating out of trailer and no mods to the wings. Weights and speeds are all within the Ventus CM envelope.

Whilst a piston self launcher will use less fuel the jet is comparable to aerotow for launch. Not surprising as the efficiency of the jet is 10 to 11% of fuel energy and SO IS A PAWNEE doing an aerotow.

The jet compares very favorably with a car retrieve also and flying back home in your jet is way more fun than a car retrieve.

I'm not the least little bit interested in retrieve only jets. As a friend of mine once said about the turbo concept: 10% of the utility and 90% of the trouble of a self launcher.