Gums,
Thanks for coming back - I can offer some information on the F-35B lift system that may be of interest to you.
First, it might be of help to outline the way the F-35B achieves 'powered lift' flight. Power is extracted from the main engine by a shaft which drives the lift fan. The lift fan is aligned vertically, so generates a forward pillar of cold gas. The aft 'lift pillar' is generated by using a three Bearing Swivel Module (3BSM) located between the main engine's aft turbine stage and the nozzle. The 3BSM rotates through 90 degrees to generate the aft 'pilar' of hot gas. (By the way, both the 3BSM and the lift fan were designed by Rolls Royce). Both 'pillars' can be controlled in thrust and direction to control the aircraft.
The lift fan drive shaft runs between the first stage of the engine and a clutch/gearbox assembly on the aft side of the lift fan. The shaft is fixed to the LP shaft of the engine and rotates all the time. When going into 'powered lift' mode, the clutch is engaged, the shaft spins up the lift fan, and when the fan has fully spun up to shaft speed, the two are mechanically locked. The clutch is then disengaged. Going from "powered lift' to normal flight is the reverse procedure. 'Powered lift' mode is selected by a switch on the left hand control in the cockpit. This selection initiates the process I've just outlined, plus a lot of other stuff in around 15 seconds. This includes:
- Upper lift fan door opens
- Lower lift fan doors (2 off) open
- Upper auxiliary air intake doors (2 off) open
- Aft 3BSM doors (2 off) open
- 3BSM swivels downwards
- Roll post doors, one in each wing lower surface, open and roll post nozzles move downwards to clear wing aperture. (The roll posts are fed by bypass air from the main engine and provide roll control as well as around 2000 pounds of thrust)
- Inboard weapons bay doors are partially opened to help control flows of hot and cold gas around the aircraft
- All control surfaces are moved to optimise lift system thrust by controlling movement of air around the aircraft.
- Aircraft flight control software transitions from normal wing borne control laws to powered lift control laws
- Pilot controls change their function from 'wing borne' to 'powered lift' - in powered lift, pilot has no control over aircraft pitch attitude. Right hand 'inceptor' now functions as a vertical rate demand input (fore and aft control movement) and lateral rate input (side to side control movement). Left hand inceptor now functions as a fore and aft rate input using fore and aft motion.
Note - this list is not exhaustive. What (I hope) this lot puts across is that the transition to and from powered lift mode is a seriously complex process, and there are no 'standby' or secondary' drives or options available. The main point is that once you have committed to sucking half the power out of the main engine forwards to the lift fan, you have to stay on two 'pillars' of gas. There are numerous sensors, interlocks, fault detectors and so forth built into the powered lift system, and the F-35B development programme was (quite understandably) driven by the customer to ensure that the pilot would either be prevented from engaging power lift in an unsafe condition, or would be prompted to switch back to wing borne flight as soon as an issue arose. However, sudden failure of the lift fan will cause the aircraft to pitch nose down very quickly, and I believe that the F-35B seat system is equipped with an automatic ejection feature.
The final point I would offer is that the F-35B has a wide powered lift flight envelope, all the way from zero knots to somewhere over 200 knots. The aircraft was required to be able to conduct landings in the powered lift mode all the way from a 'near conventional' to a full vertical recovery. I'm not surprised to hear that Eglin based aircraft are doing a range of rolling landings. For Dave, I am not sure that an RVL uses less fuel than a VL. I am fairly sure that for the Harrier/Sea Harrier, the most fuel efficient way to land was a conventional rolling landing with the nozzles aft (I am sure that a PPruner out there will correct me on this in the likely event that I'm wrong). Basically, the more time you spend not using the wings for lift, the more fuel you use.
The SRVL method for carrier use is driven solely by the Uk's desire to bring back heavier loads at higher temperatures and lower pressures than was called for in the JSF Requirement Document (the JORD). Fuel economy doesn't as far as I know, have anything to do with it. Again, I'm happy to be put right on that.
Hope all this stuff helps, best regards as ever to all the very bright and amazingly hard working Brits who worked with their equally talented US counterparts to make all this stuff work. I'm not sure they always get the credit they deserve.
Engines