Flight tests of the competing Joint Strike Fighter designs are entering the final, crucial phase - demonstration of short take-off and landing capability
Graham Warwick / NAS Patuxent River
Any day now, one or both of the Joint Strike Fighter (JSF) concept demonstrators will complete its first vertical landing - turning a new page in the 30-year history of short take-off and landing (STOVL) combat aircraft. Both Boeing's X-32B and Lockheed Martin's X-35B bring new levels of capability to STOVL, including the ability to fly supersonically and stealthily.
Successful completion of STOVL testing will be the crowning achievement in a concept demonstration phase intended to ensure the winning JSF - to be decided later this year - enters engineering and manufacturing development (EMD) with the lowest possible technical risk. The next few weeks could determine the outcome.
Both teams are trying to achieve in a few weeks what took almost a year to accomplish three decades ago, when Hawker Siddeley took the P1127 precursor to the Harrier from its first tentative, tethered hover through to transition between wingborne and jetborne flight. They have less than two months left to gather data from STOVL testing for their JSF EMD proposals and each has only one test aircraft.
Adequate margins
Boeing began flying its STOVL X-32B in April, but, because of limitations on the demonstrator's propulsion system, has been obliged to relocate testing from hot-and-high Edwards AFB in California to sea-level NAS Patuxent River in Maryland to ensure adequate thrust margin is available for vertical landings. Lockheed Martin plans to begin testing its STOVL X-35B this week with a series of vertical take-offs and landings at its Palmdale, California, plant before moving to Edwards to begin STOVL testing, which is plans to complete at Pax River. Different approaches to testing mean the teams are likely to demonstrate their designs' vertical landing capability within days of each other.
Boeing's STOVL propulsion system choice is direct lift, a modern interpretation of the Harrier's vectored thrust. Hardware changes from the conventional take-off and landing (CTOL) propulsion system include insertion of a Rolls-Royce-supplied lift module with two vectoring nozzles immediately aft of the Pratt & Whitney JSF119 engine, located at the aircraft's centre of gravity.
In STOVL mode, the two-dimensional thrust-vectoring cruise nozzle on the JSF119 closes and the butterfly valves in the lift module open, directing thrust to the lift nozzles. Engine air is ducted to a Harrier-style attitude control system, with forward pitch nozzles under the inlet, aft pitch/yaw nozzles under the tail and roll nozzles near the wingtips. Air is also directed to a jet screen under the fuselage, forward of the lift nozzles. In the hover, this forms an air barrier to prevent hot nozzle gases flowing forward and being re-ingested by the inlet.
Lockheed Martin's chosen STOVL propulsion system comprises a shaft-driven lift fan mounted vertically behind the cockpit, balanced by a three-bearing vectoring nozzle on the JSF119 engine and roll posts installed in the wing. The fan and nozzle are supplied by R-R. The advantage of this approach over direct lift is that the combined fan and engine produce more thrust than the engine alone. The disadvantage is that a clutch is required to engage and disengage the lift fan.
For the X-32B, explains Lt Cdr Paul Stone, a Royal Navy test pilot assigned to the Boeing team, a short take-off (STO) begins with the cruise nozzle open and lift nozzles at 60í down from the horizontal. The pilot opens the throttle and the aircraft accelerates. At about 70kt (130km/h) he hits a button on the throttle. The cruise nozzle closes, butterfly valves open and thrust is redirected downwards - the flow switch takes about a second, says Stone. The pilot pulls back on the stick to rotate and the aircraft becomes airborne. He runs the lift nozzles back to their aftmost setting, 45í down, accelerates the aircraft to fully wingborne flight, then hits the button again to open the cruise nozzle and close the butterfly valves. Doors then close over the lift nozzles. Engine reheat is not used during an STO, says Stone.
The transition to a vertical landing is the reverse of this process, he says. With the aircraft in wingborne flight, the pilot hits the button to close the cruise nozzle, open the butterfly valves and redirect flow to the lift nozzles, which start out in their aftmost position. He then modulates lift nozzle angle - using a thumbwheel on the control column rather than the Harrier's nozzle lever - moving them forward of the vertical to decelerate the aircraft and back to the vertical to hover.
In the hover, the attitude control system is used to manoeuvre the X-32B. Moving the centre stick modulates engine offtake airflow through the "puffer" jets to control the aircraft in roll, pitch and yaw. According to Stone, a propulsion system control algorithm called "constant area matching" ensures that overall engine thrust remains constant as flow varies between the nozzles. To alleviate the suck-down effect as the aircraft nears touchdown, underfuselage lift-improvement devices trap the fountain of air created as lift-nozzle flow strikes the ground and rebounds. On the X-32B demonstrator these devices are bolt-on plates. On the proposed JSF they are retractable.
CTOL to STOVL
For Lockheed Martin's X-35B, an STO begins with the aircraft in CTOL mode. With the engine at ground idle, the pilot pulls the thrust-vector lever (TVL) out of its detent to start the process of switching to STOVL mode. The various doors open, the lift-fan "D" nozzle and engine three-bearing nozzle deploy to 34í and 22í below horizontal, respectively, and the fan clutch engages. It takes about 15s for the fan to spool up to full speed, says STOVL product manager Scott Winship. The pilot then opens the throttle and accelerates to around 60kt before bringing the fan and engine nozzles down to 60í. The aircraft lifts off and accelerates to 200kt in wingborne flight, when the pilot puts the TVL back in its detent and the clutch disengages, nozzles stow and doors close. Reheat is not used during an STO, at least in the demonstrator, says Winship.
Transition to a vertical landing begins with the aircraft slowing below 250kt in CTOL mode. Pulling the TVL out of its detent opens the doors, deploys the nozzles, and engages the clutch to spool up the fan. Again the process takes about 15s. While drag from the open doors helps slow the aircraft, the pilot throttles back and moves the nozzles to the vertical position for the hover.
In the hover, the sidestick is used to manoeuvre the aircraft, the roll posts controlling roll and the three-bearing nozzle controlling yaw. Pitch is controlled by varying the thrust split between the lift fan and engine. Lift-fan thrust is controlled using inlet guide vanes. Combined thrust from the nozzles and roll posts remains constant. Winship says airflow temperatures, pressures and areas were measured accurately on a test stand at Pratt & Whitney, enabling the propulsion control system to calculate thrust precisely in flight.
Build down
Boeing's approach to testing has been characterised as "build down", or "envelope contraction" - starting transitions between CTOL and STOVL mode high and fast and reducing altitude and speed incrementally with each successful test. The increments start large and get smaller as the aircraft approaches fully jetborne flight, says Stone. The X-32B's lift system can be engaged at speeds up to 245kt, but below 60kt is considered jetborne flight and between 60kt and 200kt semi-jetborne, he says.
Testing started at 180kt and had "built down" to 142kt by the time the aircraft left Edwards for Pax River in mid-May. Boeing could not ferry the X-32B cross-country because aerial refuelling was ruled out by a problem discovered during testing of the CTOL X-32A. When the refuelling drogue disconnected from the aircraft's probe it almost hit the air-data booms mounted on the nose. Concerned about possible damage during aerial refuelling, Boeing elected to fly the X-32B across the USA in a series of hops.
After the aircraft's delayed arrival at Pax in early June, with 21h logged in 21 flights, Boeing changed the engine for one having the highest performance levels. Other modifications were made to maximise the thrust margin available, including removal of the inlet cowl and landing gear doors to reduce weight and installing lift improvement devices. The X-32B will have the margin to conduct both vertical take-offs and landings at Pax, says Stone. "Hover will be less marginal than in the Harrier."
Testing resumed
Build-down testing has resumed at Pax; the first vertical landing could come as early as this week. The plan is to conduct the first hovers out of ground effect and free of any interference from the surface, such as hot gas re-ingestion. A range of hover manoeuvres will be tested to check controllability. The next step will be vertical landings on a grid that minimises ground effects, followed by touchdowns on a concrete pad.
Following completion of these tests, it is likely the team will reinstall the inlet cowl and gear doors and remove the lift improvement devices for flights aimed at meeting "Boeing strategic objectives". These include going from a STO to supersonic speed on the same flight, due in July.
Lockheed Martin will begin X-35B testing with vertical take-offs and landings at the Palmdale hover pit because it wants to ensure the aircraft can always be recovered if there is a problem during the STOVL build-down phase. The first test will be a "no-go VTO", says Winship. The aircraft will be loaded to a weight that just exceeds the thrust and the lift system run to full power. The X-35B is expected to rise on its landing-gear oleos, fooling the aircraft into thinking it is airborne and switching the control system to hover mode. This will check the transition between control modes.
Next will be two or three "hops". With weight just below thrust, power will be increased to maximum and the aircraft would become airborne for 5-6s before the pilot throttles back. Again this is to check the transition between control modes. The third step will be a couple of full hover "press ups", says Winship. The aircraft will lift to between 20ft and 50ft (6m and 15m) and the pilot will perform a series of hover manoeuvres to check controllability.
Initial tests completed, the aircraft will complete a couple of CTOL check-out flights before travelling to Edwards to begin STOVL testing. Winship says the X-35B has the thrust margin to perform vertical take-offs and landings at Edwards. Testing planned for Pax will focus on STOs and on demonstrating sea-level VTOL performance. The lift system generates 38,000lb (170kN) of vertical thrust at sea level, he says, while the aircraft weighs 30,000lb dry and carries 7,000lb of fuel. "We can take off vertically at sea level with full gas," Winship says.
Propulsion system performance will be a key factor over the coming weeks. STOVL system hardware and software has been fully tested on engine rigs at Pratt & Whitney and in the aircraft over hover pits at the teams' Palmdale facilities.
P&W delivered three engines to each team. "We have calibrated all three for thrust margin and life left. This allows us to track the performance of each engine and manage the assets so that they always have the best engine," says Bob Cea, P&W's JSF119 programme director.
"But the question is not thrust; it's handling qualities. We have to show how easy it is to operate these aircraft," Cea says. "In Lockheed Martin's aircraft the STOVL engine is responsible for the handling qualities. That is why the software is so critical." In the X-35B, the system must control almost 40,000lb of thrust to within a couple of hundred pounds, he says.
Both JSF teams have completed CTOL and carrier-suitability flight testing, and demonstrating STOVL is the remaining challenge. "What is left to prove is that the aircraft can hover and do short take-off," says Cea. "If the aircraft are close to the test cell, we believe they can do it."
Source: Flight International
