This post is one in a series on the development of the F-35. Click here for a Flash graphic with system cutaways. Click on Read More to read the entire post on that system and check out next week's special coverage of the program in the Dec 12 edition of AW&ST.
Not surprisingly, the greatest development challenges for the STOVL F-35 have involved the one truly unique aspect of its design: the 19,000lb-thrust shaft-driven Rolls-Royce lift fan, its driveshaft, clutch, gearbox, inlet, nozzle and doors. So let's look at them one at a time.
Weighing 2,700lb, the lift-fan module turns 29,000hp of shaft power into 19,000lb of vertical thrust from a 50in-diameter two-stage fan with a pressure ratio of 2.1 and intake flow of 520lb/sec. Driven by bevel gears, the counter-rotating stages are titanium blisks, their wide-chord blades attached by linear friction welding. First-stage blades are hollow to save weight: plans to make the second stage hollow were dropped due to cost.
The lift fan has gone through several design iterations to reduce weight and increase durability. Rolls has identified potential for 5-10% thrust growth within the existing fan diameter, mostly through improved 3D aerodynamics on the blades and “tip treatment” of the first stage – a lip or plenum that would increase efficiency in the high flow distortion at the blade tip. There is no funding yet for lift-fan growth, however.
Production F-35Bs are to be fitted with a modified lift-fan driveshaft with flexible couplings to cope with greater-than-predicted length variations. (See separate post.)
Cooling and wear have been the main design issues with the clutch, which is designed to last 1,500 engagements. The dry clutch generates a lot of heat during the 9-12sec it takes to engage, which is removed by cooling oil. But flight tests have shown the clutch generates heat even when not engaged, as the driveshaft is always turning.
To remove heat generated by drag between the clutch plates when disengaged, a passive cooling circuit has been added to provide cooling air in up-and-away flight, when the forced-cooling fan used in STOVL mode is turned off. An overtemperature sensor has also been added, to alert the pilot to climb above 10,000ft until the clutch has cooled.
Relevatively few flights and not all clutches experience plate drag. Pratt says the “trigger mechanism” is close to being identified and may not require a redesign, simply an adjustment of tolerances. (See separate post.) The torque that has to be transmitted via the clutch plates during spool-up, before the mechanical lock takes the load, is minimized by throttling the lift-fan flow - an operation known as "turndown". The variable inlet guide-vanes are closed to unload the first stage. This makes it easier to spin up the fan, but results in a massive torque split on the gearbox input pinion, between the upper and lower bevel gears driving the first and second stages.
A gearbox rig failure in 2003 caused a redesign, but is now regarded as a good thing, as it resulted in new design tools and a better understanding of the system, says Rolls. The lift-fan system is desiged to operate for 200h of the 4,000h life of the STOVL engine (the CTOL/CV engine has an 8,000h cold-section life requirement).
Inlet and door
Graphic: Lockheed Martin
Tasked with turning airflow through 90° and into the lift-fan without excessive losses or distortion, the inlet was completely redesigned from the X-35B concept demonstrator to the F-35B. The X-35 had bifold doors. These worked for the short demonstration, but produced large areas of flow separation which excited a 2/rev resonance mode that gave the lift fan only minutes of life in some modes, says Lockheed.
While the lift fan was redesigned to increase its strength and move its resonance modes outside the running range, dozens of door configurations were tested. In the end, Lockheed had to flare the inlet bellmouth and use a large aft-hinged “57 Chevy hood” door that turns the flow down and into the fan.
Still seeing distortion at the fan, designers had to “pooch out” the inlet lip, slightly changing the outer mold line – a significant change in a stealthy aircraft. Operation of the door was modified to open it to 65°, from 35°, during a short take-off. There was small drag penalty, but inlet distortion was reduced and pressure recovery increased for better operability and more thrust.
In 2010, operation of the lift-fan door was further modified to help overcome flutter of the auxiliary air-inlet doors (see post). Variable-area vane box nozzle
The X-35B used a telescoping vectoring engine nozzle (TEVEN) to control lift-fan thrust, but to reduce weight this was replaced on the F-35B by the variable-area vane box bozzle (VAVBN). This works like a venetian blind, with vanes that are modulated to both vector thrust for pitch control and back-pressure the two-stage fan to control surge margin.
A shortfall in performance in early tests was traced to interaction between the turbomachinery and the vanes, because the flow leaving the lift-fan did not have enough room to turn before entering the nozzle. The result was a substantial redesign that cambered the lift-fan centerbody and reshaped the airfoil sections of the struts supporting the lift-fan, and the vanes themselves, to smooth the flow through the VAVBN.
Back to F-35B Graphic or on to Swivel Nozzle