VECTORED THRUST AIRCRAFT ADVANCED CONTROL – VTOL JET FLYING FOR BEGINNERS

I have said in a previous article that at one point I wanted to be a jet fighter pilot and that although the physiological effects are still a little challenging, really the dream has never faded.??Well in this case it was a real surprise, and certainly not something I had expected to happen whilst on the Test Pilots’ course.??All in all it was a bit of a special day, because it was one where I flew three different aircraft, but there is more of that to come and I cannot say, even today, that the VAAC Harrier was the most memorable!

The VAAC Harrier was an early product of the Joint Strike Fighter VTOL programme which had its beginnings in the mid 1980s (I remember seeing some of the conceptual designs when I was a you Midshipman in the MOD).??Everyone was aware that the original Harrier flight control system, where the pilot had to control both the jet thrust and the jet nozzle angle, was a little complex and prone to a few, perhaps many, human factors problems.??It was probably why the Harrier pilot training pipeline was felt to be the toughest and why Navy Harrier pilots didn’t hesitate to tell you why theirs was an even more skilled job.

As a result the UK and US funded a development programme to be delivered in the UK to solve the control problem and thus the VAAC Harrier was born.??Based at Boscombe Down the team were always after willing subjects to give it a go and for some reason, in 1999, they decided that a bunch of trainee helicopter test pilots should add to the data.??As a result, on a warm and rather hazy summers day, we prepared for our trips with the project pilot.

So that the data could be useable we were given a brief on exactly how the system worked and what we were trying to look for.??As helicopter pilots our control philosophy is slightly different to a fixed wing pilot.???To go up we raise the collective lever and keep the cyclic where it is (unless we want to reduce speed) whereas a fixed wing pilot will pull back on the stick and push the throttle forward (opposite in a sense).??Thus we were to give a different view on how easy, or hard, the aircraft was to fly based on our own training and experience.

Strapping in to the front seat I was immediately struck by how small the cockpit was and that the view around the front of the aircraft was not particularly good (in comparison to a Gazelle of course).??The obligatory oxygen connectors were all set and my legs were suitably attired with the obligatory leg restraint cords.??Once the engine was started we then set off to the runway for the obligatory rolling, rather than normal, take-off and we rocketed skywards to see how things were up and away.

For a helicopter pilot looking through a HUD is a new experience but one which you quickly get used to.??I had played enough computer games to understand the symbology so was relatively happy flying around at about 300 knots and using the rather limited 45 degrees (I think) angle of bank.??All of this was to get use to the feel of the controls, which were all rather conventional at this point, and of course the amount of control input required to make the aircraft manoeuvre.

It was then back to the airfield to do some transitions to the hover, landings and take-offs to see how the development system worked.??There was a special landing spot at the airfield to which some extra line up markers??were added, because the hover height was markedly higher than most are used to.

Two control modes were looked at during the sortie.

The first mode was called unified.??In this mode the??pilot pulls back on the stick to go up and pushes forward to go down, regardless of airspeed or groundspeed. When in forward flight??if the pilot centres both the stick and throttle, the aircraft holds the existing speed, bank attitude and climb or dive angle. When in the hover, centralising everything maintains the existing hover height, position and heading.???I found this mode a little confusing since it felt wrong to be pushing the stick forward in the hover to land,??given that doing that in a helicopter would lead to the aircraft moving forward but it did seem to work once I got over my instincts.

The second mode was called??Fusion. This gave automatic selection of nozzle angle but where forward flight is is similar to a conventional fast jet, but when operating at slower speed in the circuit the pilot uses the throttle to control height and the stick to control speed through attitude much like a helicopter.??This of course made me feel quite at home and certainly was the most intuitive mode to use from my perspective.

In addition to these two modes there was a fine tuning mode for hover operations called Translational Rate Command (TRC).??This is a familiar mode to a lot of helicopter pilots and effectively allows the pilot to ‘beep’ a speed and direction using a multiway switch on top of the throttle.??When in the hover the aircraft moves to the selected angle and speed??which the aircraft will hold. It was all very intuitive and is a feature on many modern helicopters.??Interestingly this is quite similar to the Hover mode we now have in Norwegian AW101, so it must have been good!

Once we had completed our hover landings that was really the end of the sortie since we had run out of water.?For those who didn’t know the main limiting factor for Harrier hover time is the water they spray into the rotating jet nozzles to increase the thrust.??This is so important to the activity that the pilots have a special gauge in the HUD to show it draining away.??No water, no hover and probably a very heavy landing.

So having answered the debrief points on the flight, which was also capture on HUD video we helicopter pilots went away happy that we had added something to the JSF programme.??But to this day I don’t know which mode the project went for in the end but there is no question that the VAAC system did make the aircraft much easier to operate.??I am sure it will have more than a little to do with the Shipboard Rolling Vertical Landing (SRVL) trial that is about to take place in the autumn, but I’ll leave an F35B pilot to wax lyrical about that.??But of course he wont be able to say its difficult any more, will he?

(Thanks go to the sadly missed John Farley whose words helped me remember some of the technical details in this article).