Pratt & Whitney’s F135 propulsion system recently powered the Lockheed Martin F-35B Lightning II short takeoff/vertical landing (STOVL) fighter through the first in-flight engagement of its STOVL propulsion system. In the cockpit, at the controls during this historic flight was Graham Tomlinson, F-35 Lead STOVL Pilot and an employee of BAE Systems. Graham shared his thoughts on the significance of this major milestone, the performance of the aircraft and the performance of the Pratt & Whitney F135 engine.
1. As part of the first F-35B in-flight STOVL engagement, what specific criteria were you evaluating?
Graham Tomlinson: In the cockpit the task is to make sure that the “throttle and stick” responses are as predicted through these first conversions and our first experience of flight in STOVL mode. This really means comparing the responses to the training we’ve done in the simulators. All the responses did match extremely well – pitch, roll, yaw, and thrust - so it was a very easy and satisfying task for me. And the things the simulator can’t reproduce – noises, vibrations etc – were pleasantly subdued in the real thing.
But the key criteria were engineering responses, checked and analyzed on telemetry by the control room team. The pilot only gets a general impression; the engineers can check the detail. We were especially interested in the propulsion system response through the conversions, spooling up the liftfan and radically changing the thrust center of the engine and liftfan combo. We’ve done this static on the ground but never before with the aerodynamic effects of 200 knots (on the doors, on the intake flows, on the liftfan exhaust etc).
And after conversion in STOVL mode this again was our first flight opportunity to check the total thrust response and the balance of thrust from liftfan and core engine which are now not just thrust, but primary aircraft controls. These key parameters, plus the roll bleed posts off the core engine, are absolutely critical to STOVL flight control. The engineers were very pleased with the results.
2. Were there any surprises during the 14-minute STOVL segment of the test flight, not just in aircraft/engine performance, but also how it felt?
Graham Tomlinson: I hadn’t realized some of the limitations of motion simulators. When we make small acceleration requests the simulator gives a modest kick-in-the-pants. In flight, the kick-in-the-pants feels stronger (which all pilots will love!).
The other expected effect, but still nice to confirm in flight, was the smoothness of the ride. The liftfan is right behind your back, and you can hear/feel it humming away, but both the noise levels and the vibration levels were just fine. It’ll undoubtedly get louder as we slow down to the hover, but I was very pleased with the subdued levels at the speeds we’ve tested so far.
3. How would you characterize the performance of the Pratt & Whitney F135 engine and how does its performance differ from other fighter aircraft engines?
Graham Tomlinson: So far in the JSF test program the F135 has lived up to our lofty expectations; it has been there when we want it, and has been no trouble at all. That’s a remarkable achievement for an engine which has to reconfigure from a 40K pound thrust category in Max AB conventional mode, to a 40K pound thrust category with the liftfan spinning in STOVL mode. For me the best thing has been that we simply haven’t had to worry about it!
4. What was going through your mind when you engaged that clutch for the first time?
Graham Tomlinson: The flight test team is super-professional. We’re all thoroughly practiced and trained; we can do the job in our sleep. And that means that for this first button press there was zero stress, we were just going the through a routine test we’d seen a hundred times before. And that’s a good thing too, as pilots’ brains are well known to shrivel to the size of a pea in flight!
5. What is the best part of your job as the F-35 test pilot?
Graham Tomlinson: Being part of the team, bringing back the results and the data we need, and the satisfying feeling when the team makes progress. We’ve just made a huge step forward.