Wednesday, July 15, 2009

Chapter 5:Cranfield's Highest Classroom

MS 760 Paris G-APRU tucked away in the Flight Hangar at the College of Aeronautics, Cranfield in 1967

The Paris was used to introduce students to the art of flight testing jet aircraft. The nose probe with vanes to measure sideslip and angle-of attack give the clue that this is not the normal trainer or business jet.

Credit: Terry Fox (Air Britain)

A quiet background whine was the only noise in a small classroom where, in company with two other students, I was recording the indications given by a series of panel-mounted dials. With the last value written down, I clicked my microphone on to report: ”Readings complete.”
A patch of sunshine slid slowly across the panel. With a few seconds grace before my next task I shifted slightly in the seat to ease the discomfort of parachute and seat harness and snatched a quick look away from the instruments. Instead of the green lawns of the College campus, a limitless vista of cerulean blue surrounded us, with a floor of fair-weather cumulus nearly two miles below. To my right, in the foreground a wingtip, tipped by a streamlined fuel tank, dipped towards the earth as the aircraft turned.

For this was no ordinary classroom. This was Morane Saulnier Paris G-APRU, a twin-jet four-seat trainer used by the Department of Flight at the College of Aeronautics at Cranfield to give practical flight experience to postgraduate students dealing with the Sciences of Aerodynamics, Aircraft Design, Electrical Engineering and Propulsion. The only external clue to its task was a four-feet long nose probe sporting flight test yaw, pitch and sideslip vanes.
This spring day in 1967, the purpose of our flight was to determine the maneuvering stability of the Paris and the effect of pulling various amounts of g-force. At twenty thousand feet the aircraft was turning to starboard to come back on a reciprocal track towards Cranfield. The run just completed enabled datum conditions to be measured at 300 knots in steady trimmed flight. Elevator angle and the setting of the adjustable tailplane, airspeed and our altitude had been measured. Fuel state had been noted so that weight calculations could be made subsequently and the Hussenot A13 paper trace recorder had been run briefly to obtain the datum conditions and to enable a visual check on its operation.

The Paris was now heading south, with the sun blazing down through the Perspex canopy and throwing the instruments into sharp relief. Ron Wingrove, the Department test pilot, was setting up the aircraft for the next run as we drifted over a layer of thickening cloud spreading from the south west. The engines were whining at 20,000rpm. Airspeed was steady on 300 knots and we were trimmed straight and level.
In the right hand seat next to the pilot, I was monitoring the flight instruments. Peter Cox and Colin Harris in the rear pair of seats were responsible for measurement of tailplane and elevator angles, and checking the stick force from the SFECMA-instrumented control column. They also had the controls for the A.13 recorder. The extensive instrumentation fitted to this aircraft enabled the dials on the panel to read out specific parameters which could be changed depending on the test programme required. In this way six parameters out of a total of thirteen were selected for display on the six panel-mounted micro-ammeters. These thirteen parameters for which circuitry was installed permanently in the aircraft included those needed to assess handling qualities and to measure performance.

A disembodied voice intruded on our small world. This was the radar controller at Bedford, informing us of conflicting traffic a nine o’clock. I looked to my left to see them, two USAF Phantoms out of Alconbury, streaming smoke and climbing fast. In a flash they were past us and we were clear to begin.

Colin Harris in the left hand rear seat switched on the A.13 recorder. A green light on the control box started to flash, indicating that the paper was moving through the recorder. Operation of the recorder time-base was confirmed by a red light blinking twice per second. With both lights signifying that the system was functioning satisfactorily we heard the confirming message in our earphones:”Recorder On”

Wingrove pulled the Paris up into a climb, leaving the throttles in the cruise position. The engine oil system on the Paris was not cleared for operation under negative-g conditions. To ensure maintaining positive g Wingrove put the stick smartly over to the left, the port wing went down and we rolled past the vertical, still under positive g. A continuing pull put us in a steep dive as we rolled out, aiming to pull out at our original trimmed condition of 300 knots and 20,000 feet.
As the wings came level the nose was pointed down towards the cloud deck. The stick came further back and during the pull-out the accelerometer reading crept higher. We were breathing more heavily under the strain and the g-force made it difficult to write down the instrument readings, but the blinking lights conformed that the recorder was working satisfactorily. The nose probe crept up past the horizon, pencils returned to their normal weight and the aircraft whistled up into the heavens again.

We repeated this roller-coaster exercise during a series of pullouts at increasing values of g-force. The next step was to record the same parameters again, this time in level flight but using progressively tighter turns to achieve higher values of g-force. When this was finally completed, it was time to return. By now we were twenty miles north of base. Steering 190 degrees we began the descent, airbrakes out and throttled back. The last sunshine glinted off the starboard tip tank, then the sun dimmed and was snuffed out as we slipped down into the cloud. Mist started to form on the canopy and only the faint hiss of the air conditioning system broke the silence.

We slid out of the clouds, Wingrove snapped the airbrakes in and eased the throttles forward to check our descent. The runway lights were glowing ahead and slightly to our right. At one thousand feet we ran in towards the field from the East, overhauling at a safe distance a CSE Twin Comanche from Kidlington practicing an ILS approach. Over the center of the filed we entered the break to port, buffeting slightly as the airbrakes came out to kill our speed. Slowing to one forty knots and the flaps and gear were lowered.
“Romeo Uniform. Finals. Three greens”
We rolled out of the descending turn to whistle down a flat approach to the runway. A slight check back on the stick and the tires sqealed as we touched down. Mission complete. Total time of flight:twenty five minutes.
As we slowed, Wingrove switched the pressurization off and opened the canopy to let in a blast of fresh air, spiced with jet fuel fumes. We turned off the runway and taxied back to the ramp, past the pair of Doves used for basic flight test work by the department.

Our day’s work was not yet finished. While the aircraft was prepared for the next sortie, the paper trace from our flight was unloaded from the recorder for the results to be analyzed and discussed. Another three students were already being briefed for their flight, to carry out the same exercise at a lower altitude. Instructors Harry Ratcliffe and John Quick were kept busy overseeing the various stages of this operation.
The room fell silent as we three students drank coffee while waiting for our A.13 trace to appear. Australian Dave Ferry, a pipe-smoking naval aviator, was quietly working at the back of the room, analyzing his records from the previous flight. Our crew today came from different backgrounds. Colin Harris was a Flight Lieutenant from the Royal Air Force. Peter Cox came to Cranfield from Hawker Siddeley Aviation, while I came straight from Southampton University.
Flight testing was a team effort. The sortie just completed was but one chapter which began some months previously when we were introduced to flight testing using the propeller-driven Doves. Techniques of measuring thrust and drag were taught. The Doves grew feathers, or rather wool tufts to investigate airflow behavior during stalling, and trailed static bombs around to measure pressure errors within the pitot-static system. Slowly the mysteries of basic practical aerodynamics were revealed.

We then progressed onto the Paris for investigation of the stability and control of an aircraft capable of flying both higher and faster than the Dove. Dutch Rolls were now seen in terms of lateral derivatives and time-vector methods rather than the mental image of the worse-for-wear Dutch sailor previously imagined. The configuration of the Paris jetwas constantly being changed during this investigation. Flights were made with the tip tanks full of fuel, the next flight with them empty to vary the rolling inertia of the aircraft. In further investigations of longitudinal stability it was a case of:”An aft c.g to be measured today. Can we have the two heaviest students in the back seats, please.” Ballast weights were installed on some flights to further shift the c.g.

Many graphs were plotted and slowly the reasons for the aircraft’s flying characteristics became clearer. At the end of today’s flights the results of the various tests would be combined to show how this particular aspect of aircraft stability was affected in practice by changes in airspeed and altitude. The effect of c.g shift on the stick force per ”g’ would be calculated to ensure that reasonable stick forces resulted from all aircraft loadings within the given operational limits. The control system of every aircraft must be designed so that at forward cg it does not take an excessive amount of force to pull out of a dive, or conversely at aft cg the forces do not become so light that the pilot can overstress the aircraft.

Despite the camaraderie between the young students on the Flight Test Course, there was an underlying seriousness. Starting as academic exercises, our test points in the aircraft were showing that the results were not just points on a graph, but represented hard physical facts. A miscalculation could mean the loss of a pilot or an aircraft. The history of flight testing has been written the hard way, with aircraft coming apart in the air or slamming into the ground because of design deficiencies.
At last our paper trace was unloaded and brought into the office. As slide rules came out and we started to decipher the wavering lines on the paper trace, the rising whistle of the two Marbore turbojets outside the classroom told us that another flight was on its way. A few minutes later the Paris was airborne once more, climbing until it was lost from sight, leaving a fading whine from the jets echoing across the field as it embarked upon another sortie as Cranfield’s highest classroom.


dave said...

I would love to hear more about your testing in GAPRU as i am the current owner for the past 10 years. I started a ground up restoration on the Parisjet and am about 30% complete.

Dave Bennett
skype name Davebennett1 or Parisjet

Sky Captain said...

Flight Lieutenant Ron was one of the guys that taught me to fly at Cranfield back in the eighties. I know Angus has passed on, but what happened to Ron and Harvey?