按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
circle； but not to any point lying to the left of the tangent
_A_ _B_。 Finally； when the wind had a greater
speed than the aeroplane； as in Fig。 4； the machine could
move only in directions limited by the tangents _A_ _C_
and _A_ _D_。
Matter of Fuel Consumption。
Taking the case in which the wind had a speed equal
to half that of the aeroplane； Mr。 Lanchester said that
for a given journey out and home； down wind and back；
the aeroplane would require 30 per cent more fuel than
if the trip were made in still air； while if the journey
was made at right angles to the direction of the wind
the fuel needed would be 15 per cent more than in a
calm。 This 30 per cent extra was quite a heavy enough
addition to the fuel； and to secure even this figure it
was necessary that the aeroplane should have a speed of
twice that of the maximum wind in which it was desired
to operate the machine。 Again； as stated in the last
lecture； to insure the automatic stability of the machine
it was necessary that the aeroplane speed should be
largely in excess of that of the gusts of wind liable to
be encountered。
Eccentricities of the Wind。
There was； Mr。 Lanchester said； a loose connection
between the average velocity of the wind and the maximum
speed of the gusts。 When the average speed of
the wind was 40 miles per hour； that of the gusts might
be equal or more。 At one moment there might be a
calm or the direction of the wind even reversed； followed；
the next moment； by a violent gust。 About the same
minimum speed was desirable for security against gusts
as was demanded by other considerations。 Sixty miles
an hour was the least figure desirable in an aeroplane；
and this should be exceeded as much as possible。 Actually；
the Wright machine had a speed of 38 miles per
hour； while Farman's Voisin machine flew at 45 miles
per hour。
Both machines were extremely sensitive to high winds；
and the speaker； in spite of newspaper reports to the
contrary； had never seen either flown in more than a
gentle breeze。 The damping out of the oscillations of
the flight path； discussed in the last lecture； increased
with the fourth power of the natural velocity of flight；
and rapid damping formed the easiest； and sometimes
the only； defense against dangerous oscillations。 A
machine just stable at 35 miles per hour would have
reasonably rapid damping if its speed were increased to
60 miles per hour。
Thinks Use Is Limited。
It was； the lecturer proceeded； inconceivable that any
very extended use should be made of the aeroplane unless
the speed was much greater than that of the motor car。
It might in special cases be of service； apart from this
increase of speed； as in the exploration of countries
destitute of roads； but it would have no general utility。
With an automobile averaging 25 to 35 miles per hour；
almost any part of Europe； Russia excepted； was attainable
in a day's journey。 A flying machine of but
equal speed would have no advantages； but if the speed
could be raised to 90 or 100 miles per hour； the whole
continent of Europe would become a playground； every
part being within a daylight flight of Berlin。 Further；
some marine craft now had speeds of 40 miles per hour；
and efficiently to follow up and report movements of
such vessels an aeroplane should travel at 60 miles per
hour at least。 Hence from all points of view appeared
the imperative desirability of very high velocities of
flight。 The difficulties of achievement were； however；
great。
Weight of Lightest Motors。
As shown in the first lecture of his course； the resistance
to motion was nearly independent of the velocity；
so that the total work done in transporting a given
weight was nearly constant。 Hence the question of fuel
economy was not a bar to high velocities of flight； though
should these become excessive； the body resistance might
constitute a large proportion of the total。 The horsepower
required varied as the velocity； so the factor governing
the maximum velocity of flight was the horsepower
that could be developed on a given weight。 At
present the weight per horsepower of feather…weight
motors appeared to range from 2 1/4 pounds up to 7
pounds per brake horsepower； some actual figures being
as follows：
Antoinette。。。。。。。。 5 lbs。
Fiat。。。。。。。。。。。。。。 3 lbs。
Gnome。。。。。。。 Under 3 lbs。
Metallurgic。。。。。。。 8 lbs。
Renault。。。。。。。。。。。 7 lbs。
Wright。。。。。。。。。。。。。6 lbs。
Automobile engines； on the other hand； commonly
weighed 12 pounds to 13 pounds per brake horsepower。
For short flights fuel economy was of less importance
than a saving in the weight of the engine。 For long
flights； however； the case was different。 Thus； if the
gasolene consumption was 1/2 pound per horsepower hour；
and the engine weighed 3 pounds per brake horsepower；
the fuel needed for a six…hour flight would weigh as much
as the engine； but for half an hour's flight its weight
would be unimportant。
Best Means of Propulsion。
The best method of propulsion was by the screw；
which acting in air was subject to much the same conditions
as obtained in marine work。 Its efficiency depended
on its diameter and pitch and on its position；
whether in front of or behind the body propelled。 From
this theory of dynamic support； Mr。 Lanchester proceeded；
the efficiency of each element of a screw propeller
could be represented by curves such as were given
in his first lecture before the society； and from these
curves the over…all efficiency of any proposed propeller
could be computed； by mere inspection； with a fair degree
of accuracy。 These curves showed that the tips of
long…bladed propellers were inefficient； as was also the
portion of the blade near the root。 In actual marine
practice the blade from boss to tip was commonly of
such a length that the over…all efficiency was 95 per cent
of that of the most efficient element of it。
Advocates Propellers in Rear。
From these curves the diameter and appropriate pitch
of a screw could be calculated； and the number of
revolutions was then fixed。 Thus； for a speed of 80 feet
per second the pitch might come out as 8 feet； in which
case the revolutions would be 600 per minute； which
might； however； be too low for the motor。 It was then
necessary either to gear down the propeller； as was done
in the Wright machine； or； if it was decided to drive it
direct； to sacrifice some of the efficiency of the propeller。
An analogous case arose in the application of the steam
turbine to the propulsion of cargo boats； a problem as
yet unsolved。 The propeller should always be aft； so
that it could abstract energy from the wake current； and
also so that its wash was clear of the body propelled。
The best possible efficiency was about 70 per cent； and
it was safe to rely upon 66 per cent。
Benefits of Soaring Flight。
There was； Mr。 Lanchester proceeded； some possibility
o