Flying the Avro 504
The Avro 504 was the main primary training aircraft for the Royal Air Force (and initially the Royal Flying Corps) from 1915 until 1927. Many of these aircraft also found their way into civilian hands and were used by ‘barnstormers’ to give short pleasure flights to fare paying passengers. The Shuttleworth example was used for such flights between Luton and Dunstable for a charge of 5s (25p) and was later used in the film ‘Reach for the Sky’, the story of Douglas Bader. However, the most interesting aspect of the Avro 504 for the pilot is the fact that the aircraft is powered by a rotary engine.
The rotary engine became prominent with the introduction of the 50hp Gnome in 1908 and was the standard power plant for many of the earlier WW1 aircraft. Indeed, the Shuttleworth Collection operates several other rotary engined aircraft, including the Blackburn Monoplane, the Sopwith Pup, the Sopwith Triplane and the Bristol M1C. In the early days of aviation, conventional engines of the time were unsuitable for use in aircraft. They were often very heavy, due to the requirement for a large flywheel and they were also prone to overheating due to the fact that for aviation use they had to run almost continuously at full power. In the rotary engine, the engine crankcase and cylinders rotate around a fixed crankshaft. This automatically provided a flywheel and since the cylinders themselves were now rotating, an element of cooling was provided even when the aircraft was stationary. Grouping the cylinders in one plane around a single crank gave a very short and therefore light power plant. The problems with the arrangement included large gyroscopic forces on the aircraft and engine bearers as the aircraft was manoeuvred caused by gyroscopic precession of the heavy and rapidly rotating engine. Also, fuel and air had to be got into the engine, and this was done through the hollow crankshaft from a primitive carburettor. In some designs, oil was also fed through the crankshaft and hence these aircraft have a total loss oil system, the oil mixing with the petrol and burning in the engine. Castor oil is used as it is not diluted by petrol and it is the burning of this oil that gives the characteristic smell when rotary engines are running.
The Avro 504 is powered by a 110hp Le Rhone rotary engine and it is the handling of this engine that makes the aircraft challenging to operate for the modern day pilot. The engine controls comprise 2 levers mounted side by side on a graduated quadrant on the left side of the cockpit. The longer lever (known as the ‘block tube’) is connected to a simple block tube carburettor. This controls the fuel air mixture entering the hollow crankshaft and subsequently reaching the cylinders. However, life is not that simple! There is a second, shorter lever, known as the ‘fine adjustment’ that can further effect the amount of fuel in the mixture at a particular block tube setting. This essentially controls a fuel tap upstream of the block tube carburettor, but a tap that is capable of very fine adjustment. For each block tube setting there is therefore a small range of ‘fine adjustment’ settings over which the engine will still run. These are bounded at one end by the lean cut (too little fuel, engine will cut out) and at the other the rich cut (too much fuel, engine will stop). The latter case is far more serious, as it takes a long time to clear a rich cut with the attendant over fuelling. This would certainly be longer than the time available in a glide if the engine failed below a couple of thousand feet.
So there we have it. Not an easy engine to keep running! So how do we do it in practise? At the Collection we have developed certain rotary engine handling procedures to minimise the risks and preserve the life of these rare engines. However, the bottom line is that the aircraft must be flown in such a way that it must always be able to glide land back on the airfield should the engine stop. Come with me on a flight in the Avro 504 and I will tell you how it works in practise.
The rotary-engined aircraft are always positioned at the take off point, as they are impractical to taxy over any significant distance. Having strapped into the 504 and checked the flying controls and cockpit switches I ask the engineers to turn on the oil supply to the engine. A syringe is then used to prime the engine by injecting a small amount of fuel into each cylinder through the exhaust valve. At this point both block tube and fine adjustment levers are still closed and a pump in the cockpit is used to pressurise the fuel tank to 2.5psi. I check that chocks are in position and also that I have 2 men holding back the aircraft by the tailskid. I then set the block tube lever to a position on the graduated scale that I know from experience roughly equates to satisfactory low power. After turning on the ignition, I call contact and the groundcrew swing the propeller. It is a large prop and a relatively heavy engine and swinging the prop is not an easy job. This difficulty is compounded by the single skid that protrudes beneath the propeller. As the engine fires, it runs on the injected prime charge and then begins to stop as the prime is exhausted. The aim is then to ease the fine adjustment lever forward and catch the engine. The exact position of the levers depends on various factors including ambient pressure, temperature and fuel tank pressure and must be found by trial and error on the day. The rate of movement of the fine adjustment lever when ‘catching’ the engine is also critical. Once idling, this low power warm up must be completed within about 40-50 seconds after which it is time to establish the full power settings for the conditions and complete a full power check. Slowly I advance the block tube to a position known to give close to full power (which, unfortunately is not fully forward –that would be too easy!) and the engine again starts to cut as the fine adjustment setting is now too lean for the increased block tube setting. Slowly I increase the fine adjustment setting, looking for more than 1000RPM but accepting up to 1150RPM with smooth running and no sign of a rich cut. The airflow from the propeller now drives a small propeller on the strut that powers a Rotherham fuel pressure pump. I now no longer need to provide fuel tank pressure by hand pumping but I must monitor the fuel tank pressure gauge to ensure it remains reasonably constant or the running positions of the engine control levers will change. So now I have established the full power setting for the day and am almost ready to go. The bottom line is that I must be airborne within 2.5 minutes of first starting the engine or it will overheat with possible damage. To allow the groundcrew to remove the chocks, I must momentarily go back to the slow running position on both levers (memorised from before) remembering to move the fine adjustment lever first to prevent a rich cut. I wave the groundcrew and chocks away and I can further reduce power at this stage to help in chock removal by operating the ignition cut out blip switch on the control column. It is essentisal to blip when removing the chocks or the aircraft will quickly over run the groundcrew with the attendant risk of injury. However, we do not encourage over blipping at high power today as it can put shock loads on a rare and valuable engine. Then, as the aircraft starts to move (quite rapidly – things will happen quickly now!) I carefully go back to the lever positions that were found to give full power on the chocks.
As the aircraft accelerates, the engine RPM increase from the static power case and the increased centrifugal force can richen the mixture going to the cylinders. I therefore smoothly ease back the fine adjustment on the takeoff roll to make sure that the engine is not approaching a rich cut. The rest of the flight, until the landing approach, is then flown at essentially full power, although I continually ‘feel back’ the fine adjustment lever to ensure that I am erring on the side of over leanness and not over richness. I also reduce power when diving into the display line to prevent engine overspeed.
You will notice that I have not said too much so far about the handling characteristics of the aircraft. As a trainer, it is relatively easy to fly, although that is in comparison to its fighter stable mates and to the standards of its day, not a modern light aircraft with a tricycle undercarriage. In the Collection, the Avro 504 is used to give pilots training on and confidence in handling a rotary engine before they graduate to our other rotary-engined types, some of which can be quite a handful! However, if a reasonably experienced pilot, qualified on tail wheel types, can control the engine successfully he can fly the Avro 504!
The aircraft swings to the left on the TO roll and when combined with the engine handling distractions this caused RFC and RAF flying instructors to leave the teaching of takeoffs until well into the syllabus of the day. Due to the low weight and high drag of these aircraft, it is also imperative that the nose is lowered and a forced landing area picked ahead if there is any sign of engine failure on the climb out. This is because speed will wash off rapidly after an engine stoppage and flying speed must be maintained for a safe landing without stalling. This was a point that would be continually re-emphasised to trainee pilots of the era, although there were still unfortunately many such accidents. Once safely airborne, if left unattended for any length of time, the aircraft will deviate although she is perfectly controllable. The roll control is sluggish but adequate although the aircraft requires attention to maintain balanced flight directionally. This is a common feature of many early types. Since the engine is either operated at full power or idle, the change in propeller slipstream over the tail surfaces means that pitch and yaw control vary greatly between these 2 conditions and subsequent response must be anticipated. During the display itself, I use manoeuvre to control speed and always have a landing plan should the engine fail.
To land the aircraft, it is positioned for a glide landing directly into wind. The fine adjustment is then closed, effectively cutting the fuel supply off. In this situation the engine continues to rotate (due to the slipstream and flywheel effects) but is producing no power. Once certain of landing within the airfield boundary, the aircraft can be side slipped as required to increase the rate of descent. Just before touch down, I reset the block tube and fine adjustment levers to the full power condition, (remembering to move the block tube first and easing up the fine adjustment very slowly), and then use the ignition cut out or ‘blip’ switch to control the power. This ensures that power is available should a landing swing develop or an overshoot be required. The aircraft is landed on 3 points and has a very short ground roll from its approach speed of 55mph. However, the bungee attached undercarriage offers little damping and bounces are possible. These can be difficult to deal with, as the immediate response of a more modern engine is not available to cushion them, and so they should be avoided if possible. After landing, the engine is normally stopped (or stops of it’s own accord as the pilot runs out of hands to control the aircraft and reset the engine ground idle lever positions!) and the aircraft towed back to the flight line. Ideally, the engine is run at low power for about 30 seconds after landing to allow temperatures to stabilise. This ‘cooling’ run can be usefully used to clear the runway.
So that is the Avro 504 and a description of rotary engine handling in general. Some of the rotary-engined aircraft will be flying today provided conditions are suitable and hopefully you will now have more of an appreciation of what is going on in the cockpit. Operating such aircraft today will never be entirely without risk but we believe that the procedures developed by the Collection will allow you to see these magnificent machines in their natural element for years to come. And whilst you are watching, remember that there are very few places in the world (and none other currently in the United Kingdom) where such aircraft are regularly displayed.
Enjoy the show!