On March 8th Simon Hargreaves OBE was going to tell us just what it said in the title about his wide ranging career in aviation but sensibly decided to reduce the scope and give more details, concentrating on the STOVL X-35B.
In the Royal Navy from 1975 to 1996 Simon flew Sea Harriers from HMS Hermes with 800 Naval Air Squadron (NAS) in the Falklands war, attended the Empire Test Pilots School (winning the McKenna Trophy) and was a test pilot for the A&AEE at Boscombe Down.
On leaving the navy as CO of 899 NAS he became a test pilot at Dunsfold under CTP Graham Tomlinson and in 2000 joined the Joint Strike Fighter programme flying the X-35 at Lockheed-Martin. He was deputy CTP at Warton from 2002 to 2006, when he retired from BAE Systems and went to Cobham (Flight Refuelling), the Fleet Requirements and Air Direction Unit (FRADU) and Britten-Norman where he is the part-time Director of Flight Operations - with Graham Tomlinson working for him!
He is also Operations Director for Hawker Hunter Aviation, founded by managing Director Mat Potulski, where he flies Hunters. In his spare time he flies the Sea Vixen for Naval Aviation Ltd and flies Hawks as a RN Reserve pilot with 736 NAS.
The Joint Strike Fighter (JSF) programme was a competition for a huge aircraft contract. BAe had been teamed with McDonnell-Douglas but they were eliminated before the hardware stage. Consequently BAe joined the Lockheed-Martin (L-M) consortium with Northrop-Grumman, and engine companies Pratt & Whitney, General Electric and Rolls-Royce. Unusually the work share within the team was determined by who had the expertise and not politically. Real trust was placed in BAe for flight controls, flight and engine control integration and STOVL flight. Rolls-Royce were trusted with the lift fan, roll control nozzles and the articulated rear nozzle. Simon (BAe), on the insistence of L-M, was selected to be the first flight pilot for the STOVL X-35B concept demonstration aircraft. The other JSF competitor was Boeing with the X-32.
There were to be three versions of the JSF: conventional take-off and landing (CTOL), short take-off & vertical landing (STOVL), and carrier catapult launch and arrested landing (CV). The CTOL and STOVL types were broadly similar but the CV version was bigger and stronger (and heavier) to cope with carrier operations. L-M built two aircraft: a CTOL version (X-35A) which would be converted to STOVL (X-35B), and a CV version (X-35C). The STOVL X-35B had a maximum take-off weight of 40,000 lb and it could hover at 29,000 lb compared with the Harrier GR9 at 32,000lb and 22,000lb respectively.
The Boeing X-32 rival was a purely vectored thrust type of unappealing chunky appearance. There was a lot of informal exchange of progress information so each team knew the situation - and the X-35 was behind.
Tom Morgenfeld made the first X-35 flight, in A model configuration, on October 24th 2000 taking off from the Lockheed ‘Skunk Works’ Palmdale facility and flying for 35 minutes to the famous Edwards Air Force Base where the evaluation would take place. The event was “like a football match” with crowds of employees witnessing this very important event as the JSF contract would ensure the future of L-M and partners. The undercarriage failed to retract properly but L-M’s public relations dept edited the film record so it didn’t show.
Simon flew the X-35A as well as flying chase in an F-16. The X-35A was, he said, nicer to fly than the F-16 and was very easy to flight refuel, a procedure used often to extend sortie times in the very tight programme. In 30 days 27 flights were made by six pilots. Mach 1.05 and 5g were achieved, limited only by lack of time. Excellent flying qualities and engine response were demonstrated. The 35A was then hangared for conversion to B configuration. Meanwhile the X-35C was flying at NAS Patuxent River flying mainly FCLP manoeuvres (Field Carrier Landing Practice).
In STOVL mode the engine in the X-35A delivered 20,000 shaft horsepower to the front fan vertically mounted behind the cockpit, by a continuously driven shaft, gearbox and clutch. Once engaged the clutch was locked mechanically. Air to the fan was provided by an intake, with doors, over the fan. An auxiliary intake on top of the fuselage, analogous to the Harrier suck-in doors, augmented the air flow to the main engine at low air speeds. The single rear swivelling nozzle, which weighed about a ton, could deflect at a very high rate downwards by some 98 degrees and in yaw by +/- 13 degrees. It was driven by ‘fueldraulic’ motors which generated heat which raised the fuel temperature. The lower the aircraft the fuel state, the higher the fuel temperature went because of the lower fuel mass heat sink capacity. This could lead to serious problems of cavitation in the ‘fueldraulic’ system. The problem was solved short term by fitting an air scoop and fuel cooler under the aircraft, filling up with refrigerated fuel and returning to base when fuel temperature reached a critical value.
The X-35B also suffered from a ground coupling mode. When the rear nozzle was rotated with the aircraft on the ground, the high angular rate and its large mass swung the aircraft in response. The weight-on-wheels microswitches armed the integrated flight control and propulsion system (IFCPS) as the aircraft rose on its oleos before it had left the ground. Because the aircraft was constrained by the ground the flight control system fought this constraint thus starting a potentially destructive divergent aircraft oscillation.
In the STOVL mode the thrust from the front fan and the rear nozzles was roughly equal at 20,000 lb each. Roll control was from reaction control valves at about half semi-span which added 2,000 lb of thrust. This moment arm was determined by the wing design and led to a deficiency in roll control power which, in conjunction with slow roll valve actuators, caused PIO (pilot induced oscillation) problems. The ground environment was fairly benign and similar to the Harrier due to high mass flows at moderate jet velocities. To achieve the same thrusts with direct lift, like Boeing’s X-32, would have required higher jet velocities and/or higher temperatures and given a bad ground environment with surface erosion problems.
Pitch control was achieved by varying the front fan thrust using variable inlet guide vanes (IGVs) and rear nozzle thrust by varying engine speed (rpm), which of course affected the front fan thrust as well. This interaction had to be computer controlled to maintain constant pitch control response with constant total vertical thrust (lift)!
In the cockpit there was a thrust vector lever(analogous to the Harrier nozzle lever), a throttle and a sidestick controller. The latter was expected to cause problems because many control systems studies had shown that sidesticks and STOVL were incompatible; but there were no problems. The aircraft STOVL control mode was attitude command; no command by the pilot, no change in attitude. Stick-free the aircraft just sat there. The purely manual simple Harrier system was acceleration command; move the stick and the aircraft accelerated about the roll, pitch or yaw axes so needed fairly constant attention.
The X-35 hover attitude was 9 deg nose-up, the ground attitude was 0 deg nose-up leading to a rotation on VTO as the aircraft legs extended and the IFCPS was armed. This was to cause problems.In the decelerating transition at speeds above about 160 kn the airflow could not smoothly turn the 90 degrees downward into the lift fan so the fan would stall; below 160 kn the airflow was satisfactory. Fan engagement gave an almost instantaneous upward thrust of 2,000 lb, its minimum thrust, causing a rapid pitch-up which had to be controlled by a rapid application of nose-down tailplane. However, a few days before first flight it was realised that the satisfactory fan engagement speed was lower than the speed required for tailplane effectiveness which reduces with decrease in speed.. By careful analysis it was decided that if the fan was engaged at between 163 and 161 kn the pitch-up transient could be controlled. This proved to be the case. On the F-35 the side hinged X-35 doors were replaced by a rear hinged door which deflected the air into the fan thus widening the fan engagement speed range.
The initial VTO/hover tests were to be carried out over a ‘pit’, similar to the Dunsfold VTO grid from which the P.1127 had done its initial VTO/hover flying, to keep the aircraft out of the unpredictable ground effects. Due to planning permission problems Lockheed-Martin was constrained in size and position so the ‘pit’ was too small in plan but just about manageable. Much useful ground running on the ‘pit’ was carried out including lift fan engagements, proving reliability, and adjustments to the intake doors to reduce lift fan intake airflow distortions. Force and moment sensors on the undercarriage showed that some 2,000 lb more thrust than that predicted was being generated.No-go ‘pit’ VTOs with a full fuel load were carried out to investigate control responses with the oleos extended and the IFCPS armed but, because the aircraft couldn’t rotate, the hover nozzle positions commanded by the IFCPS gave a forward thrust component which caused the aircraft to shoot off the front of the pit. It was also found that under these conditions the aircraft rolled from wheel to wheel as the IFCPS fought the attitude constraint caused by the ground. This meant that a slow VTO would not be possible.
In the US system the Government is in control, not the contractor design authority as in the UK. Consequently the Government, in this case represented by NAVAIR, could dictate test conditions and they stated that the fuel load for the first cautious VTO would be based on the thrust declared by the engine supplier team. The fact that 2,000 lb more thrust had been measured was not accepted. Also a rapid VTO was essential to avoid the roll mode problem.
The first flight was set at 6 am, not for low air temperature reasons because there was plenty of power, but to ensure light winds to keep inside the 5 kn limitation caused by the ‘pit’ whose small size made it essentially unidirectional. So, when Simon opened the throttle on the first VTO the X-35 took off like a rocket straight to 30 - 40 ft, instead of the agreed 3 - 4 ft, where Simon gained control over the excess thrust. Meanwhile, one of the literally hundreds of NAVAIR representatives involved in the programme was saying, helpfully, “Shut the throttle, shut the throttle!” It became clear to Simon that the IFCPS was not properly sorting out his pitch control and height (thrust) control demands or the pitch control demands of the IFCPS itself so there was a 50 cycles per second hunting motion as the IFCPS computer failed to solve the problem. This motion could be felt by the pilot, seen on the traces but not on the video. Also evident on this first flight was that roll control power was inadequate and PIO was imminent. Interestingly, later at Edwards with lots of space and no pressure this problem diminished considerably. The undersize ‘pit’ was a further problem for Simon because at 40 ft it was out of sight so he had to use distant markers to land, a manoeuvre made more difficult because the IFCPS caused ‘power bounce’ (like the initial Harrier digital engine control system (DECS)) which made it hard to get the power off quickly.
After this “interesting” flight NAVAIR accused Simon of ‘showboating’ and Lockheed-Martin of breaking the agreed flight test rules to gain a competitive advantage on Boeing who had been incrementally building up to the hover and just achieved it that morning . Relations between Lockheed-Martin and NAVAIR hit an all-time low. In one flight Lockheed-Martin had overtaken their rival.Simon played some interesting videos including infra-red shots showing conclusively that the cold front fan configuration prevents any reingestion of the hot rear nozzle exhaust, a problem that plagued the Harrier and many other jet V/STOL projects, including Boeing’s X-31.
The complete X-35 programme was 39 flights and 21 flying hours of which Simon flew 12 covering all the STOVL development flights. The Mission X requirement to combine an STO, level supersonic flight and a VL was flown at M 1.05.
Simon continued by talking about HHA (Hawker Hunter Aviation) and flying the Sea Vixen but space precludes covering these topics properly. However, the editor intends to report them in a later Newsletter.
After a generous questions and answers session the vote of thanks for this outstanding talk, which was packed with easily understandable technical detail and previously unreported information, was given by Martin Pennell.