e
monoplanes with cantilever wings, offering less resistance to the air
than our biplanes. One of the most difficult problems is to evolve a
high-lift wing which does not impair the aircraft's speed in the air.
For commercial machines we must aim at the largest possible commercial
load, the smallest possible fuel load and, consequently, an engine which
uses fuel economically and, conversely, a lighter fuel. The development
of the engine is receiving constant attention, as are also various
safety devices, among which may be mentioned those guarding against fire
and those varying the lift of wings so as to lower the landing speed and
thus decrease the dangers attendant upon forced landings.

In addition to the high initial cost of machines and engines, their
maintenance also requires the greatest care. Detailed investigation must
be made into all serious accidents. This is now compulsory under the new
Air Navigation Act, and the fitness of pilots is ensured by periodical
medical examination.

Apart from the weather, the safety of an aircraft depends upon its
engine, and perhaps even more upon the installation and accessibility of
engines and their adjuncts, such as the petrol, oil, water and ignition
systems. During the earlier stages of the war the average life of an
engine before complete overhaul was necessary was, of stationary
engines, from 50 to 60 hours, and of rotary engines, about 15 hours.
To-day these figures stand at 200 hours and upwards and from 50 to 60
hours respectively. For commercial purposes this must be further
increased to 300-500 hours as a normal working period.

There are two schools of thought with regard to the efficiency,
reliability and the economy of engines. One school advocates using a
light power plant per horse-power, run normally at about half its
maximum; the other favours a plant of greater weight, more solid
construction and greater efficiency, running at nearly its full
horse-power. The former is more expensive in primary cost and upkeep,
but allows a higher performance and provides reserve horse-power for
emergency; the latter is cheaper, but involves a certain risk owing to
lack of surplus power. We have hitherto shown a tendency to adopt the
former method, the Germans the latter. For commercial purposes a
compromise will probably be found to be best.

Apart from the initial outlay on "air stock," the maintenance, overhead,
fuel, insurance and depreciation charges are very heav