Temperatures well below zero call for ski undercarriages and special precautions in starting and running engines
AIRCRAFT ENGINES ARE CAREFULLY COVERED to protect them from the cold. Few heated hangars exist at the various landing grounds and the aeroplanes often have to remain in the open. The engine oil is drained out while it is warm and is heated before being poured in again. In addition, other precautions are taken. Precautions have also to be taken to prevent moisture from getting into restricted spaces on the aircraft, because if the moisture froze it might do considerable damage.
ONE of aviation’s greatest problems is winter, with its attributes of snow, ice and long periods of temperatures below zero. If Arctic air routes are to be regularly used in future inter-continental flights, this problem is one which must be faced not only by pilots but also by aeroplane designers. Canada, where winter flying is as important as summer flying, provides an excellent field for experiment. The atmospheric temperatures there vary daily and with the latitude, so that it is necessary always to be prepared for the worst—to allow for extremes. It is not uncommon to experience a variation of 60° Fahr, on the ground in twelve hours. A similar variation may be encountered on a short flight of one or two hundred miles.
Skis are necessary for landing in most localities of Canada, and their design must incorporate many special factors. Weight is one of the most serious considerations, as the skis are about three times as heavy as the gear they replace on an aircraft weighing 5,000 lb. During the past fifteen years various shapes and sizes of ski have been tested for manoeuvrability, shock absorption, wing loading, power loading, ground drag and other factors. The modern streamlined ski, however, is so designed as to have slightly less air resistance than the replaced wheel and the ground drag has been so far eliminated that the ski gives a positive lift to the machine. Modern skis are rather heavier than the older type but the extra weight is counteracted by their performance.
The ski comprises a built-up wooden shoe (coated on the underside with duralumin), an oleo pedestal and cam-type trimming gear. There is, in addition to the main skis, a small ski attached to the tail skid to prevent the rear fuselage from becoming damaged by sinking into the snow.
It sometimes happens that flights to the northern parts of Canada necessitate a take-off fit only for wheels and a destination with ground fit only for skis. This indicates that there is a need for a ski-wheel combination; but here again the weight factor is an important one and experimentation is proving necessary.
Emergency is often overcome by ingenuity. Some time ago a rescue aeroplane set off for a glacier from an airport below the snowline. Spring had already come and there was not a sign of snow on the airfield; but skis were necessary for landing. To overcome the difficulty, the firehose was turned on the field until the whole ground was a sea of mud on which the ski-plane could slither to a take-off. Other structural differences necessary for winter flying are comparatively slight. The airframe undergoes great strain, and particularly the undercarriage and attachments. These must be strengthened as much as possible. Infiltration of snow or rain into the aerofoils, fuselage, control leads, electric generators, venturi tubes and so forth must be prevented. Damage will inevitably occur if sufficient moisture collects and freezes in a tube or other restricted space. The structure generally is studied from this point of view. The airframe is weatherproofed, engine-driven generators are used, and venturis are usually replaced by a vacuum pump.
Large inspection ports or panels for easy access, the provision of towing and tying-down rings, large oil drain cocks and control locks all assist the maintenance work. The personnel have to work in difficult conditions, sometimes where there, is no heated hangar, and the craft cannot be moved about as though it were, on “tarmac”.
For engine operation in winter conditions preheating is necessary if the temperature has been below 0° Fahr. It is not sufficient to drain the oil tank immediately after flight and to refill it with hot oil immediately before starting. Not all the oil is drained from the engine, and the residue on the cylinder walls, bearings and other surfaces requires special heating. The engine of a 200 horse-power seven-cylinder machine using a moderately heavy oil, after having stood in a temperature of -10° Fahr, for twelve hours, would not turn even with a thirteen-stone man hanging on the airscrew tip.
The usual method of heating is by a fabric cover directing warmth round the engine from a wood or petrol stove. These covers vary and range from a circular tent, using the airscrew as a centre pole, to one resembling an engine cover with a sleeve attachment, in which is inserted a tin pipe to heat into the cover. They must be kept as compact as possible to restrict outside radiation and weight.
Fire danger, although present, is not excessive if precautions are taken to ensure there are no leaks in the fuel system. The engine is stopped by the main fuel cock, a clearance of two or three feet is allowed between the heater and the engine, and an operator generally stands by with an extinguisher. Lubrication in sub-zero weather resolves itself into two phases — starting and prolonged running. A light oil, solving the starting difficulty, would probably break down under the strain of running and a compromise is therefore necessary.
Not long ago a moderately heavy oil was used, and all tanks and exposed piping were insulated with asbestos and felt. This was a laborious and wasteful process. Moreover it was not particularly effective in low temperatures and was liable to cause overheating when the engine was warm.
Now lower-viscosity oils are used and are most successful. Starting is easier and there is less
wear in the engine. Although oil consumption is slightly increased, fuel consumption tends to decrease. Insulation, or lagging, is now unnecessary, and initial lubrication is improved in temperatures above -40° Fahr., although it is still necessary to protect all exposed parts from the direct blast of the slipstream. An oil with a pour-point of -40° Fahr, would, of course, eliminate many of the difficulties of sub-zero operation.
Carburation must be considered from the same viewpoints as lubrication— namely, starting and prolonged running. The most satisfactory method of getting the engine to fire was found to be a direct injection of the normal fuel in the form of a finely atomized spray into the blower section of the radial engine or into the induction manifold of a V or in-line type. Highly volatile fuels such as ether tend to damage the engine if not handled carefully and, although normal fuels do not vaporize readily in low temperatures, the fine spray overcomes this difficulty. Another method which is sometimes used, but not with such success, is the “choke”, as in a car — a petrol-soaked rag held over the air intake.
There is always the possibility of freezing or clogging in the carburettor when aircraft are operating in conditions of snow or rain or even through a wide variation of temperature such as is encountered in Canada. To combat this there must be a controllable alternative warm air intake. The position of this is important. If it is on the side or underneath the fuselage, it is liable to become blocked with snow picked up by the airscrew during ground running or while taking off.
Frostbite is one of the greatest handicaps to the maintenance of the engine, for operation is often necessary from bases where no hangars are available. Working i n mitts is impossible. A frame and canvas structure mounted on skids with an opening at one end large enough for it to cover the engine and having enough room inside to turn the airscrew round completely is the makeshift “nose-hangar” found most useful. Work is inevitably slowed up in a limited space such as this but, with a stove, a lantern and a small bench, it can be accomplished in relative comfort.
Special maintenance for winter flying is necessary, as the engines are being severely treated. Before winter operations are begun all parts and controls not lubricated automatically from the engine are cleaned and treated with non-freezing oils and greases. Accessibility and low maintenance requirements are of primary importance to the mechanic in ski-plane design.
PREPARING A JUNKERS Ju 52 AIRCRAFT tor flight. Although the oil is drained off when an aircraft is left in the open, a certain amount remains on the cylinder walls and on the bearings. This oil becomes so thick that the engine cannot be started until it has been warmed up. The elongation of the engine cover down to the ground enables hot air from a stove to be led into the cover, where it rises and warms the engine.
Aircraft on skis are difficult to handle on the ground. This work resembles the handling of a seaplane where forethought and common sense are necessary if damage is to be avoided. Pilots must remember not to turn sharply, either with the engine or without, as the skis stick and wrench the undercarriage. Taxying is hazardous. The ground must be carefully examined for rocks, stumps and other obstacles just under the snow. If the surface is of glare-ice, it is easy to overshoot; if the surface is rough, and a strong wind is blowing, it is easy for the aircraft to be blown over when attempting to turn or taxi across wind.
The length of the take-off or landing run varies directly with the surface and this changes every few hours. Wet or deep snow offers greater resistance than dry snow and sometimes it may prove impossible to take off at all. In deep snow, once the aeroplane has started, it is advisable to keep going to avoid being stuck.
If the aircraft is left standing on the snow or ice for a short period, poles should be placed under the skis to prevent the machine from being frozen in. Frozen lakes and rivers offer added difficulties. In certain conditions slush is found on top of the ice, even in very low temperatures, and it is best, whenever possible, to taxi ashore.
If, however, freezing-in does occur, the skis have to be chopped clear and jacked up. The rough ice has to be melted off with a blow torch. Anchorage of the wing is the chief means of securing aircraft in the open. This may be achieved with ice-toggles, sacks frozen to the ground or poles loaded with rocks. Alternatively, if the wing or wings are reasonably close to the ground, a snow wall can be built in front of the aeroplane.
The Danger of Frostbite
Enclosed, heated cockpits are essential for the winter pilot. It is almost impossible to write or to use a wireless key if mitts must be worn, and an aviator’s efficiency depends largely upon the conditions with which he has to contend. It is necessary in the north to wear, in addition to the ordinary aviator’s uniform, clothes such as the people living in the district wear. Yet every pilot should be given an opportunity to acclimatize himself to the weather conditions. He should also be taught how to take care of himself in the severe cold; otherwise casualties will occur through frostbite, snow blindness or exposure.
A good example of knowing what to do was given by Sir Hubert Wilkins, the Arctic explorer. On one of his North Polar flights he was compelled to make a forced landing on treacherous ice, as his pilot’s hands were frozen. Sir Hubert stepped from the aeroplane on to a spot of thin ice and went through. When he had come to the surface and had crawled ashore, he at once stripped off his clothes and, to the astonishment of the aviator, ran vigorously up and down the ice, halting only to rub his fur-lined garments in the snow. He had learnt from the Eskimos the trick of restoring circulation and preventing freezing after immersion.
Map-reading in the winter is difficult, as prominent landmarks such as lakes, ponds and rivers are often indistinguishable from the rest of the area, especially when there is a heavy snowfall. Another danger is in landing on unmarked snow in bright sunlight or if the sky is overcast and there are no contrasting objects. All these factors make the task of judging the surface doubly difficult, and it is a good rule to maintain the aircraft at a flat angle with the throttle partly open until the aviator is certain of the ground level.
Winter flying in Canada does not present the problem of ice forming on the wings to the same extent as it is encountered frequently in Europe. This danger occurs only at freezing-point.
Nor is winter flying entirely one of handicaps. Winter weather, at any rate in Canada, is usually reliable. It is clear with excellent visibility and its cold is of definite assistance in that the cold air is heavier and denser and therefore gives greater lifting power.
MAINTENANCE WORK ON AIRCRAFT in the open is made difficult by the danger to engineers of frostbite; it is not possible for them to do their work when wearing mitts. For engine adjustments a canvas cover, which will allow the propeller to be turned, is generally rigged round the nose of the aircraft. With a stove and a lamp the engineer is thus able to do his work in comparative comfort.
Special maintenance for winter flying is necessary, as the engines are being severely treated. Before winter operations are begun all parts and controls not lubricated automatically from the engine are cleaned and treated with non-