Posted 26 December 2014 - 16:18
Flying the Pup
Richard Shuttleworth purchased the Collection's Pup in 1936. It was orignally being constructed as a single-seat Scout-Pup was a later name-but the 1918 armistice intervened and it was converted, along with nine other aircraft still on the Sopwith production line, into a Dove. This was a civilian, two-seat variant of the military Pup. Shuttleworth reconverted the machine, returning it to its single-seat configuration.
I learned of the Pup's existence when I was still a young aero modeller. Although I designed a small, rubber-powered version of the machine, it never came to fruition. But I did witness flights of several other model Pups and all showed the characteristics of a delightfully stable machine, bouncing along on light, thermally air. This was a vision that has stayed throughout my acquaintance with the aircraft and it hasn't burst yet.
My first flight in the Collection's Pup was on the morning of a Sunday airshow at Old War den in 1995. The air was light, bouncy even. There was puffy cumulus at 2,500 feet and a light, south-westerly drift of warm summer air. I was not disappointed. The light and responsive controls, coupled with a fair field of view and a comfortable cockpit gave the impression of an ideal Sunday flyer. But then, as I looked for ward along the single Vickers machine-gun, I realized that this delightfully pleasant little aircraft was also, in the right hands, a potent war machine. 'In the right hands' is the key, for although the Pup is an enchanting machine to fly, it does have some interesting take-off and landing characteristics to fly it effectively, a mastery of the delights of operating a rotary engine is essential.
So, let's look at the engine. The Le Rhone in the Pup is a nine-cylinder, four-stroke motor, aspirated by two valves in each cylinder head, and controlled by a single push/pull rod. The associated rocker has an exhaust valve at one end and an inlet at the other. The advantage of the single push rod system is lightness, which is essential for the rotary. The disadvantage is that it is impossible to overlap the exhaust and inlet valve timing, so perform ance is limited. Nevertheless, it does provide more than adequate power for the Pup.
All rotary engines are characterised by a hollow crankshaft which is attached to the air frame. Around it rotate the cylinders and crankcase, the propeller being attached to the latter. The result is a light power plant that does not require liquid cooling. But it can overheat on the ground with prolonged running and the gyroscopic precession of the rotating mass causes some not insignificant problems for the pilot. The handling of the engine is also specific-any pilot errors in this area can quickly lead to power plant failure.
The pilot of the Pup is presented with a cockpit containing few flight instruments and, relatively speaking, many engine-associated controls, systems and indicators. The fuel is transferred to the engine by air pressure generated by a cockpit handpump and/or a Rotherham propeller pump. The latter is fitted in the main propeller slipstream on the port forward wing strut and the former is fixed on the right side of the cockpit. At the base of the cockpit handpump is a small tap. Setting it perpendicular to the feed pipe isolates the pump; turning it parallel with the pipe allows the tank to be pressurised by the pilot; finding the correct position, at about 45° to the pipe, vents the air system to atmosphere. The various positions of the tap can be used in flight to control tank over-pressure, tank under-pressure and to possibly cure an air leak. After flight, the tap is used to equalise tank pressure with ambient. There is also an air pressure gauge to monitor the fuel tank pressure-the maximum is 2.5 psi-and a Jones valve, which acts as an engineer adjustable pressure relief valve.
Oil flow to the engine is total loss; an on/off tap, which is inaccessible to the pilot, is fitted in the line under the tank. Oil flow is confirmed by an oil pulsator fitted to the lower left of the instrument panel. The oil meniscus moves slowly up and down in sympathy with engine rpm when the engine is running, thus confirming that oil flows. However, given the blue mist that can readily be seen emitting from the engine, and the pleasant odour of castor oil in the cockpit, the pilot is in no doubt as to whether or not oil flows to his rotary! Nevertheless, the pulsator may have one use. In certain circumstances e.g. failure of the rpm gauge, engine rpm can be checked by timing the rate of pulsation and comparing the result to a calibration chart. But, as with the oil flow, in practice, the pilot's perception is as good an indication of engine rpm as any-in this case it is achieved by ear and airframe feel.
A fuel on/off tap is located in the fuel line, forward, on the lower left side of the cockpit. Fuel flow to the engine is pilot-controlled by two, coaxial levers fitted in a quadrant marked from one to ten on the left cockpit wall. The larger, outboard lever, referred to as the blocktube or fuel/air lever, is directly connect ed to a blocktube carburettor, which is located on the pilot end of the extended hollow crank shaft. It controls airflow to the engine and to a certain extent fuel flow. The inboard, smaller lever, referred to as either the petrol lever, the fine adjustment lever or the tampier filter tap lever, directly controls fuel flow into the blocktube.
Contrary to popular opinion, rotary engines of the two lever type, such as fitted to the Pup, can be modulated over a range of rpm. In practice, this is about 700 to 1,150 rpm on the ground, noted on the cockpit rpm gauge, which equates to about 50 to 100 per cent of available power respectively. The approximate lever positions for 700 rpm are 3.0 on the blocktube lever and 2.5 to 3.0 on the petrol lever. For 1,150 rpm, the approximate posi tions are 7.0 and 3.5 to 4.0 respectively. For a given blocktube position, if the petrol lever is set too far forward (too rich), or too far back (too lean) the engine will cut. A rich cut should clear in about thirty seconds, which is disastrous in low-level flight; a lean cut can be cured in about five seconds. The trick for safe operation of a rotary is to know when the engine is running rich or weak and what to do if it is.
Finally, the engine suite is completed by a single ignition system with magneto on/off switch on the instrument panel and a 'blip' but ton on the control column, which cuts the ignition when depressed. Although the engine can be modulated by judicious use of the blip switch, it is not recommended as a primary engine control. The engine is 'shock' loaded every time the blip switch is pressed and con tinuous flight with the ignition switched off leads to oiled and fuel-fouled plugs. Better to use the petrol lever to modulate the engine over its lim ited range-or if even less thrust is required, to shut the engine down completely by selecting the petrol lever to the fuel off position.
Prior to flight, normal checks of locking wires, wire tensions, control integrity and airframe condition must be made. Perhaps the most important check is that of the under-wing hoops. They are fitted to the lower wing spars, via metal cups, and serve to protect the lower wing aileron horns in the event of a wingtip striking the ground. They give as good a hint as any that the ground-handling of the machine is special.Cockpit entry is via a single footstep in the left fuselage side and a hard point on the wing-root. When seated, the pilot notes a good field of view for an aircraft of the period, with a gap cut into the centre panel of the upper wing centre section to improve the view forwards and upwards.
Rotary engine time is a precious commodity - time between overhauls is measured in a few tens of hours. So, given the propensity to overheat on the ground and the wish not to waste precious engine time, the Collection parks the rotary-engined aircraft at the mar shalling point of the runway-in-use prior to flight. Following strap-in, the pilot immediately carries out normal pre-take-off vital actions as his hands will be full once the engine is running. The harness is secured, the flight con trols checked and goggles are positioned and secured-the cockpit environment of the Pup is windy, get into the air with loose goggles at your peril.
When settled and prepared, the pilot calls ready to start to the groundcrew. The fuel is confirmed on, the ignition off and the engine levers are set closed. The groundcrew turns the engine oil on and confirm the same with the pilot. Each engine cylinder is then primed in turn by depressing the respective exhaust valve and injecting a measured amount of petrol by syringe. Meanwhile, the pilot pumps the fuel tank to a pressure of 2 to 2.5 psi and checks that the pressure is maintained. The groundcrew turns the engine over several revolutions to distribute and mix the fuel. When the groundcrew calls ready, the pilot sets the blocktube to about 3.0, rechecks fuel tank pressure, holds the control column fully back, calls "Contact" and sets the ignition on. The groundcrew confirms that there is one of their number holding the tail down and that chocks are in position.
The propeller is swung. If all has been set and primed properly, the engine bursts into life with its characteristic staccato crackle, the air frame twists in opposition to the torque and a cloud of blue smoke is quickly carried away in the slipstream. The pilot waits for the prime to burn off then, as the engine dies, he advances the petrol lever slowly towards the expected running position. This will vary according to ambient conditions but will never be more than about 0.5 of a division from position 3.0. The engine is warmed at about 700 to 800 rpm for about fifty seconds as the lever positions for smooth running are essayed.
The engine will go from rich to lean and vice versa with a petrol lever movement of between one-eighth to one-quarter of an inch around position 3.0. A lean engine exhaust sounds light but rough, a rich exhaust note is harsh and heavy. With practice, the two states can be heard and felt through the airframe and if the pilot is diligent, they can be noted as a slight drop in rpm as the over-rich or over-lean state is reached. A rich cut is cured by closing the fuel lever and awaiting engine pick up-it should pick-up within thirty seconds in the air, but it will stop and remain stopped on the ground. A lean cut is remedied by slightly advancing the petrol lever and the engine should pick up within a few seconds, either on the ground or in the air.
At this point the pilot cannot fail to be impressed by the smoothness of the rotary when compared to radial engines. This is probably due to the big end, the largest mass in the engine, being stationary in the rotary engine, whereas it rotates in the radial.
After a fifty-second warm-up, full power is tested. The blocktube lever is advanced to about position 7.0. As there is not enough fuel flow to maintain running, the engine lean cuts. The petrol lever is then advanced slowly until the engine picks up and the lever posi tion will be between 3.5 and 4.0. Again, the rich and lean positions are noted and this time the lever spread is about a quarter to half an inch. The maximum rpm is noted, normally about 1,050 rpm, but provided it is above 1,000 rpm, the flight can go ahead. Time at high power is minimised, about thirty seconds is reasonable. Power is then reduced to minimum by first retarding the petrol lever to cause a lean cut, then resetting the blocktube to the slow-running position, then resetting the petrol lever as appropriate.
Following a quick cockpit check and a confirmation that the fuel tank air pressure is sufficient to commit to flight , the chocks are waved away. The blip button is now used for the first time. As the low power setting of 700 rpm provides about fifty per cent of maximum thrust, if engine power is not killed when the chocks are removed-given that the machine has no brakes-the machine will jump forward and strike the groundcrew. When the groundcrew is clear, the blip button is released and the blocktube advanced, followed by the same with the petrol lever, both being set to the positions for high power noted during the ground run. The aircraft accelerates briskly and the pilot must guide the machine between two limiting handling areas. If the tail is raised too quickly, even with full right rudder applied, gyroscopic pre cession will cause the machine to yaw about 30° to the left. Following that, if right yaw is rapidly applied to control the situation, the propeller will certainly strike the ground. Conversely, if the tail is kept on the ground too long, the aircraft will take to the air close to the stall, with all the associated dangers. The tail must be lifted slowly and progressively as the aircraft accelerates.
As airspeed increases, the propeller unloads and the engine rpm increases. Fuel arrives at the engine via the hollow crankshaft and is fed to the engine via tubes running along the sides of the cylinders. The increased centrifugal force produced by the accelerating engine will enrich the mixture. Therefore, if the petrol lever is not retarded slightly on take-off, the engine will suffer a rich cut and the pilot will get to practise Engine Failure After Take-Off drill.
Climbing at the best climb speed of 65 mph, the delights of the Pup become clear. The controls are light and effective, the machine is relatively stable in pitch and roll, but it does wander a little in yaw. With four ailerons and no ameliorating devices, adverse yaw is apparent, but is easily controlled with the effective rudder. Gyroscopic precession is always there-there is no getting away from it with a rotary engine-but it is not excessive. At medium to high airspeed it is no worse than that of a Griffon-powered Spitfire, but the rudder power, although effective, is a lot less.
The stall qualities are also good, but they must be tested with the engine off. The fuel lever is retarded and the pilot tips forward in the seat as drag takes over from thrust. There is slight but adequate warning of the coming stall in the form of airframe buffet and stall break occurs at about 35 mph indicated. After aerodynamic recovery, the petrol lever is advanced and the engine bursts into life with associated torque roll, precession turn and airframe acceleration. But, as experience is gained, the apparently random movement of the aircraft becomes predictable and is easily contained with slight pressure on the relevant (aerodynamic) control. As successive manoeuvres are attempted, the pilot must take special heed of the engine note. As speed is increased, for similar rea sons to those on take-off, the petrol lever must be retarded to prevent a rich cut. As alti tude is increased and the air gets thinner, the petrol lever must be further retarded to pre vent a similar occurrence. Conversely, the petrol lever must be advanced as airspeed and altitude reduces to maintain smooth run ning and to prevent a lean cut.
A descent with engine on is only possible in a fast dive, so entry to the circuit pattern must be made engine off. The petrol lever is retarded and the engine stopped, although it does windmill in the slipstream. The machine glides well at 65 mph and a forced landing pattern is set up. The ideal turn-in-point is about 800 feet, a half mile or so from the air field, in the late downwind position. A constant aspect turning approach is then made, aiming to land about a third of the way into the airfield and keeping the speed slightly fast. At this stage, there is no substitute for a little excess kinetic .energy; excess energy is easily lost in side-slip, but without an engine, it is impossible to gain. So, in the latter stages of the approach, heavy side-slip is used to bring the touchdown to a point 100 yards or so into the field. Just before touch, the engine levers are reset to the running positions and as the engine picks up it is killed on the blip button. It should then be ready for instant use if it be necessary to help stop a swing during landing.
To prevent a swing starting during the landing run, the landing must always be made into the wind. It must also be a three-pointer. Wheeled landings invariably lead to a ground loop to the right, owing to the gyroscopic pre cession of the rotating engine causing the nose to go to the right as the tail is lowered. This, coupled with the narrow forward under carriage and the lack of airflow over the rud der, gives the pilot no chance of recovery (hence, under-wing skids).
While the Pup slows after landing, the blip button is released and as the engine fires, the control levers are retarded to achieve 700 rpm or so. The aircraft deceleration is pro longed by blipping, but the aim is to allow the engine to run at low power for about thirty seconds to temperature stabilise the cylinder and crankcase. The ground roll-out distance is not critical at Old Warden, however, great care must be taken not to allow the concen tration to drift-until she stops, the Pup is still waiting to swing and to catch the unwary. The machine is guided off the duty runway and the engine is shut down by closing the petrol lever and fully advancing the blocktube. When stopped, the silence is deafening. The engine clicks and snaps as it cools, the air-frame dies. The ignition is checked off and fuel tank air pressure released with the tap at the base of the cockpit pump. The pilot vacates the air craft to a quiet hissing noise as the tank pressure blows off.
So, in the euphoria of a flight in the Pup, what have we forgotten? We didn't check the tank pressure once in flight, nor did we look at the oil pulsator. Of the latter, we were in no doubt that oil was flowing as a constant stream of castor oil fumes continuously invaded the cockpit. As to the former, if air pressure had fallen in flight, the engine would have died through lack of fuel. Should this have happened, a check of the air pressure gauge would have confirmed the problem and a few pumps on the cockpit handpump would have rectified it-we make a mental note to bring the pressure gauge and the pulsator into the cockpit scan on the next flight.
I've now flown several sorties in the Pup and what a marvellous machine she is. The handling qualities are superb for a machine of her era… but there are enough interesting characteristics to require a certain amount of piloting skill to achieve safe flight.It's no wonder the Scout pilots of the RFC and RNAS held the machine in high regard. As for me, well, the vision still hasn't burst!