The Harrier

NOV 30 2010
The Harrier: Test Pilot John Farley on the P.1127 and Short SC.1

In 1964 I was a Flight Lieutenant working for the Aerodynamics Research Flight at RAE Bedford (or Aero Flight as it was known) and in our hangar was the Short SC.1 VSTOL research aircraft which had four lift engines and one for propulsion. Meanwhile, at Dunsfold, Hawkers had flown six P.1127 prototypes, with each one incorporating improvements based on the earlier versions. As a result the very first P.1127 (XP831) was looking distinctly old hat so Hawkers were quite happy to release it to the RAE so that the boffins there could compare its vectored thrust principle with the lift engines of the SC.1.

As the junior pilot of four on Aero Flight (and less than a year out of ETPS - the Empire Test Pilot's School) I was naturally pretty chuffed to be asked to go to Dunsfold and bring back XP831 for comparison with the SC.1. Indeed I considered I had blundered into the best job in the UK.

There were so many differences between flying the Short SC.1 and the Hawker P.1127 that the phrase ‘chalk and cheese’ comes to mind. By the time I finish this comparison you may be inclined to feel that it all boils down to ‘Hawkers got it right and Shorts got it wrong’. However, that is far too simplistic a view and involves hindsight - and believe me, in 1964 hindsight about jet VSTOL was in short supply.

At the simplest level both aeroplanes were similar because both were single seat fixed wing jet aircraft that could take off and land vertically. Both could fly on their wings, both could hover, both could transition to and from the hover and both used pure jet thrust to achieve all this.

However, the design teams at Shorts and Hawkers used very different solutions because they started from very different positions. The Short SC.1 was specified, designed and purchased to enable the RAE to do research into jet VSTOL. On the other hand, the Hawker P.1127 was conceived by a fighter design house with a long history of supplying fighters to the Royal Air Force as a possible way of achieving a jet fighter that could land and take off vertically. It was not originally designed to a government specification but to meet a need, as perceived inside Hawkers, that the RAF (and others) needed a VSTOL capability to counter the possibility that conventional aircraft could be grounded by attacking their runways.

This difference in objective was fundamental in determining why Hawkers finished up with a single-engined vectored thrust aircraft, that they hoped they could make work but which had the potential to become a fighter, while Shorts filled a small airframe with four lift engines and one cruise engine to meet Specification ER143T.

One level up from my ‘chalk and cheese’ comment, may I say that it took me very many flights in the P.1127 before I could climb down the ladder without offering up thanks that I had not bent the thing. Yet after shutting down the SC.1, I always felt relief that it had not suffered one of several possible nasty failures!

Why this big difference in how I felt after a flight? The SC.1 had five Rolls RB108 engines - four for lift and one for propulsion. The aircraft was very heavily autostabilised and used full authority autostabilisers in pitch and roll which had priority over the pilot when it came to the reaction controls. Plus there was a manual mechanical back up control mode intended as a last ditch option for emergency use. The pilot controlled the thrust from the lift engines using a helicopter-like ‘collective’ throttle with the left hand. For reasons we shall come to shortly, the SC.1 had very easy handling and later it was established that this good handling even extended to the manual mechanical backup control mode, but the aircraft was a real problem to operate due to very complex systems and the five engines which had to be looked after.

On the other hand, the Hawker P.1127 had a single Bristol Pegasus engine for lift and propulsion plus the aircraft was always mechanically controlled by the pilot and had optional low authority autostabilisers in pitch and roll. Piloting wise, the P.1127 had demanding handling due to two controls for the left hand and intake effects. However it was a delight to operate as it had no potentially dangerous systems and only one engine to be looked after.

To sum up - the Short SC.1 was demanding to operate and easy to handle while the P.1127 was easy to operate but had demanding handling.

I think the differences in operation hardly need explanation. In the case of the SC.1 you were operating a five-engine ‘bomber’ all by yourself. It had none of the benefits of automation that would be available today and so you had five of most things to deal with when it came to starting it up. In the air, after takeoff and getting on to your wings, it was necessary to shut down the four lift engines because they were very thirsty, even at idle. Before landing, the process of restarting them one at a time, using bleed air from the cruise engine, was also easy to get wrong. In some circumstances this had to be done on short finals at below 500 ft. Then there was the issue of the full authority autostabs. These had 100% access to the roll and pitch reaction controls and, unless you kept an eye on a gauge that was quite low down on the right side of the instrument panel, the first indication that they had used up all roll control was when you moved the stick and got no response. Not good.

Compare that to the operation of the P.1127 which was in effect a single seat fighter of the day, say a Hunter, with one extra lever in the cockpit to set the nozzle angle and two extra instruments – neither of which needed much attention. One instrument showed the nozzle angle set (but so did the nozzle lever) while the duct pressure gauge showed that the reaction controls were available (but so did moving the stick). P.1127 handling however was quite another matter.

The reasons why the P.1127 handling was so demanding are rather less obvious. However, I will try to explain them so that you will understand why handling the SC.1 was so easy. As shown in the image below, the throttle box incorporated the throttle and the nozzle lever and was positioned on the left hand side of the cockpit where your left hand would naturally fall when sitting in the seat. The throttle worked as with any jet fighter – forward for more thrust and back for less. An inboard slim nozzle lever set the angle of the nozzles - pull it back and the nozzles were rotated downwards and so the aircraft went slower, push it fully forwards and the nozzles pointed aft making the aircraft a conventional jet.

While this was a brilliantly simple way to achieve the full range of VSTOL manoeuvres, it necessarily posed a piloting trap. Should you move the wrong lever, it might not be possible to recover from the mistake depending on what you were doing at the time. For example, raising the nozzles in the hover would have you dart forward and downwards very rapidly, as has happened more than once in public. The other problem stemmed from the intakes and meant that, if left to its own devices, a P.1127 flying slower than about 100 kt wanted to go tail first. The pilot literally had to use his feet to keep the aircraft pointing into the airflow. This was directly analogous with the need for the pilot of a tail-dragger aeroplane to use his feet to stop it swinging and ground looping when landing, especially in a crosswind.

The reason for this was that the aerodynamic stabilising effects of the P.1127 fin were no different from any other aircraft, so faded away as one got slower. Unfortunately there was a destabilising force that increased as flying speed reduced and so defeated the residual efforts of the fin. This force was called intake momentum drag. It exists on all jet engine intakes and gets greater as rpm is increased. Thus, whenever you were flying slowly and necessarily using jet lift not wing lift, up went your rpm and up went the intake momentum drag.

To understand why this destabilised the aircraft directionally we need to look at the airflow round the aircraft when viewed from above. Below I have tried to indicate that everything is fine when the aircraft is pointing directly into the airflow.

However, what happens if the aircraft starts travelling slightly sideways through the air because of a cross wind or a deliberate move by the pilot, as shown below.

With the total airflow - represented by the blue arrow - now coming at an angle to the nose, we must think about its two components as shown. The green being that part which is straight on the nose and the red that part which is blowing directly across the nose.

The red arrow is of course the troublemaker because its effect is felt at the intake which is well ahead of the centre of gravity and so opposes the fin. Should the pilot allow the aircraft to swap ends and fly tail first you might think it would just be embarrassing for him because in his debrief he will be told to try harder on the rudder. Sadly he is unlikely to actually make the debrief because, at speeds greater than about 70 kt as the aircraft goes seriously sideways, the leading wing will generate much more lift than the other and the aircraft will roll out of control, thanks to what is termed ‘rolling moment due to sideslip’. Such asymmetric lift can easily swamp the aerodynamic and reaction controls.

Clearly some exotic technology was called for to help the pilot keep the aircraft pointing into the airflow. In fact all that was needed was a simple wind vane as seen on any church steeple. It is mounted in front of the pilot and always shows him where the airflow is coming from, as shown on a modern day Harrier below.

I hope that by now you will have appreciated why the P.1127 had demanding handling.

With that in mind, let us consider the SC.1. As the picture of the SC.1 below shows, the lift engine intake momentum drag arrow acts vertically down through the centre of gravity, regardless of which way the aircraft is pointing with respect to the wind, allowing the fin to do its job even as speed reduces. This lack of directional instability made the SC.1 easier to handle in the hover and at low speeds.

The arrow shows the benign direction of the lift engine intake momentum drag in the SC.1.

Add to this that when jetborne the pilot had only one control to operate with the left hand and it becomes clear why the chances of the pilot making a handling mistake in the SC.1 were much less than in the P.1127.

As experience was gained at Bedford with the un-autostabilised P.1127, it became clear that the best way to fly the SC.1 was in the mechanical back-up mode which did not use the autostabilisers thus eliminating the serious problems that could arise with autostabiliser failures and so was very good for one’s peace of mind. That way the attitude control system became just like the P.1127 with the stick position showing how much reaction control authority you had used.

Although handling wise the SC.1 and the P.1127 were very different, make no mistake, both aircraft did great jobs in teaching the UK how best to proceed with the development of jet VSTOL.

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