In a VTOL aircraft including a lift fan, said lift fan intaking air from above and exhausting said air downward to produce vertical lift, and powerplant means which include a cruise engine and a VTOL engine;
- said cruise engine including a gas turbine engine whose output is mainly shaft work, driveshaft means, a front fan, and flow valve means downstream of said front fan;
- said front fan being much smaller than said lift fan;
- said gas turbine engine having a first combustion chamber and powering said front fan through said driveshaft means;
- said front fan producing lightly compressed air to be known as “blower air”;
- and said flow valve means being in a first position, which routes most or all said blower air to exhaust in a backward direction to produce thrust for level flight;
- said VTOL engine including said cruise engine, boost compressor means powered by said driveshaft means, compressed air duct means, second combustion chamber, and a lift fan turbine, but said flow valve means being in a second position, causing most or all said blower air to enter said boost compressor means;
- said boost compressor means having a higher native pressure ratio than said front fan's pressure ratio;
- said compressed air duct means conveying the main output of said boost compressor means to said second combustion chamber, where fuel and ignition are added to create working gas;
- said working gas going to said lift fan turbine;
- said lift fan turbine powering said lift fan during VTOL flight;
- said flow valve means physically located in series between said front fan and said boost compressor means;
- said flow valve means being at least in part substantially annular;
- ducting means connecting some discharge of said boost compressor means to intake means of said gas turbine engine;
- mechanical variability operating on said ducting means;
- said variability choosing:
- (a) ambient air during said cruising flight, or else
- (b) said some discharge from said boost compressor means during said VTOL flight;
- said choice (b) supercharging said gas turbine engine, thereby increasing by at least half again its power, in order to help drive the added load of said boost compressor means on said driveshaft means;
- said gas turbine engine having at least two spools;
- one said spool being for a high pressure gasifier;
- a second said spool being a low pressure spool with at least two low pressure turbine stages and connected to said driveshaft means;
- and flow alteration means operating on a later stage of said low pressure turbine stages to reduce by at least half its work of expansion during said cruising flight, in order to better match the lighter load on said driveshaft means when said boost compressor means are blocked off by said flow valve means in said first position.