[from Air International - F-35 Lightning II 46 page Publication c.Early 2011-2?]
Mark Ayton spoke with Peter Wilson, a former Royal Navy Sea Harrier pilot and now STOVL lead test pilot at NAS Patuxent River
"...Pilot’s View
The author was keen to hear what the F-35 is like to fly particularly at take-off which always shows dramatic acceleration. Peter Wilson explained: “The take-off itself is unremarkable, in afterburner the aeroplane accelerates dramatically, but it’s comparable with legacy fighters, and very weight dependent.”
Both the F-35A and the F-35C can carry more than 50% of their empty weight in fuel internally which gives an enormous variation of acceleration.
One very notable system on the F-35 is the side stick located on the right side of the cockpit. The mechanics of the side stick are well balanced with just the right amount of movement (about 1½ inches or 38mm) according to Peter Wilson who said: “You first notice this when using the stick to rotate and bring the nose up to establish an attitude at which the aeroplane’s going to climb away. The aeroplane feels absolutely rock solid, the handling feels precise.”
A very distinct feature of the F-35 is noise both inside the cockpit and out. “From the cockpit it’s not especially loud but it doesn’t sound like any other aeroplane that I’ve flown,” said the lead STOVL pilot.
The ride quality of the F-35 is also different, especially the precision with which the pilot can manoeuvre the aircraft using the side stick to put it exactly where he or she wants. “It’s most noticeable when you’re trying to do a tightly controlled formation task, like air refuelling. I’ve plugged into a tanker many times with a remarkably high success rate, higher than I would have had on the Harrier, and with a different technique. The pilot formates the air refuelling probe directly onto the basket of the tanker, sits behind it, and just plugs it when it’s steady and level.
Coming in to land is also precise. “Even in a cross wind it’s easy, the aeroplane points its nose in to wind very nicely and reduces side slip,” said Peter Wilson.
Symbology in the helmet-mounted display allows the pilot to see the aircraft track, confirming that he or she is aligned with the runway even if the nose is not because of crosswind. The side stick is extremely precise for both flaring (the technique used to gradually reduce the descent rate) the aircraft and adjusting any drift, but even if he or she does not make any correction the aircraft will land and straighten itself up “beautifully” according to Peter Wilson. “It’s the easiest aeroplane I’ve ever landed and really does look after you. When I tell you how easy it is to land, in the back of my mind, I am thinking ‘isn’t that going to be great for the young pilot who has worked hard throughout the mission and needs to get home when he is tired’,” he added.
To date all conventional landings have been carried out manually with the stick. An automatic system on the throttle allows the pilot to select the APC (automatic power control) mode that controls the angle of attack flown on final approach during which the throttle moves up and down in response to the changes. At touchdown the throttle automatically goes to idle, the pilot applies the brakes to stop the aircraft and exits the runway. “Once on the ground, I do not have any flap levers to move or any flight controls to reposition, and if I want to get airborne again all I have to do is put the power up and initiate the rotation,” said
Peter Wilson.
The throttle commands thrust and not the rpm of the engine, so at idle the engine is providing 10% of the thrust available and when pushed forward to the mil stop it provides 100% of the available thrust or full mil power. The throttle gives a linear variation of the percentage of thrust available with its position, which makes it subtly different to use. One hundred percent thrust means just that, with no variation (which can be the case with a legacy aircraft), so the pilot knows when the engine is providing all of the power that it can.
Landing Vertically
One of the most fascinating aspects of the whole programme is the way in which the F-35B achieves a vertical landing. When preparing to transition from conventional to STOVL mode the first thing the pilot must do is confi gure the aircraft to be able to fly at slow speed. This process is called conversion and from the pilot’s perspective it starts when the aircraft is moving at 250kts (460km/h) or less at which point he or she simply presses a button.
“Seconds later, assuming all has gone well, you are in the mode that allows the aircraft to go to the hover,” said Peter Wilson.
Nine external doors open in sequence taking about 8 seconds, after which the propulsion system (not to be confused with the engine) starts to spool up. The clutch engages to spool up the lift fan located behind the cockpit (which takes about 5-6 seconds) and the control laws change to make use of the propulsive effectors that have just been brought to life. The aircraft is now in STOVL mode and ready for a vertical landing. “You feel a little tingle in your back through the seat and it sounds like a very large mosquito buzzing behind your head,” said Peter commenting on the lift fan.
The lift fan nozzle and main engine nozzle move independently as per the control laws of the aircraft (the aircraft is programmed to position the nozzles where the force is required). Peter Wilson says the varying pitch of the engine can be clearly heard from the cockpit as the thrust changes during low speed manoeuvring.
Commenting on the hover, Peter Wilson told AIR International: “It is absolutely astonishing, the aeroplane is rock solid in the hover, and holds its position extremely accurately without pilot input.”
The aircraft can be accurately moved left to right, fore and aft, and up and down by 3ft (1m) at the preferred position of 100ft (33m) above the ground before descent. Control of the F-35B is governed by something called the unified control law, which was developed during research at Boscombe Down in the UK with the Vectored-thrust Aircraft Advanced Control (VAAC) Harrier in a project funded jointly by the UK and US as part of the Joint Strike Fighter programme.
And perhaps the real testament to the unified control law is the experience of pilots who had never before flown a STOVL aircraft. Having practised in the simulator, they have been able to step into an F-35B and complete a vertical landing with relative ease.
To descend from the hover and land, the pilot has to push on the side stick until he or she feels a stop, and hold it there until the aircraft detects the landing, at which point it returns the propulsion system back to idle and moves the nozzle to the correct position, allowing the pilot to taxi forward with nothing else to do. “The precision with which you can land is amazing – on the spot plus or minus 12 inches, every time consistently,” said Wilson.
Nine Hops
During STOVL testing in February 2010, Peter Wilson flew nine sorties from NAS Patuxent River in about four hours, all of which were less than 5 minutes in duration. Each sortie carried a relatively low fuel load allowing Peter to take off, and fly around for a brief period to ensure the fuel was at the right level in preparation for a landing test. “The highlights on the day were the take-offs. I took off as slow as 50 knots [92km/h] with the STOVL mode engaged, accelerated out to the normal pattern speed of 150 knots [276km/h], turned downwind, and positioned ready for a vertical landing,” he said.
Some of the vertical landings required extreme nose-down attitudes on the aircraft at various weights and phenomenal descent rates. Recounting the landings, Peter Wilson told AIR International: “I was trimming nose down to make the nose gear hit first rather than the main gear coming down as fast as I could, given the control law of the aeroplane. When the nose gear (underneath the pilot’s seat) hits first at that sort of descent rate it gets your attention because it’s a pretty heavy landing and a remarkable experience in the cockpit.”
F-35B Take-off Options
The F-35B STOVL variant has a range of take-off options using different modes to suit the basing. Take-offs from a ship, with either a flat deck or one with a ski jump, are also possible with a mode for each scenario. These are short take-off scenarios that can be achieved at speeds as low as 50kts with a deck or ground run of no more than a 200ft (60m). In the same mode, a take-off as fast as 150 knots is possible if the weight of the aircraft requires that speed. If the aircraft is light it can take off at a slow speed and faster when heavy.
Take-off at speeds as low as 5, 10, 15, 20kts (9, 18, 27 and 36km/h) are also possible, each of which is effectively a vertical take-off while moving forward. There are different ways of rotating the aircraft in STOVL mode, including the usual ‘pull on the stick’. Other ways are by pressing a button or programming a ground distance required after which, the aircraft control law initiates the rotation and selects the ideal angle for climb-out.
F-35Bs BF-01 and BF-02 are the only B-models currently undertaking STOVL testing and therefore performing take-offs in STOVL mode. Peter Wilson commented: “We have found a remarkable similarity between BF-01 and BF-02 which gives us the confidence to move on and get more aeroplanes [BF-04 followed by BF-03] into STOVL mode very soon.” At the time of closing for press in mid-April the first vertical take-off had not taken place...."