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At this meeting arrangements were made by which
the Aero Club recognizes the Wright patents and will
not give its section to any open meet where the promoters
thereof have not secured a license from the
Wright Company。
The substance of the agreement was that the Aero
Club of America recognizes the rights of the owners of
the Wright patents under the decisions of the Federal
courts and refuses to countenance the infringement of
those patents as long as these decisions remain in force。
In the meantime； in order to encourage aviation； both
at home and abroad； and in order to permit foreign
aviators to take part in aviation contests in this country
it was agreed that the Aero Club of America； as the
American representative of the International Aeronautic
Federation； should approve only such public contests
as may be licensed by the Wright Company and that
the Wright Company； on the other hand； should encourage
the holding of open meets or contests where ever approved as
aforesaid by the Aero Club of America
by granting licenses to promoters who make satisfactory
arrangements with the company for its compensation
for the use of its patents。 At such licensed meet any
machine of any make may participate freely without
securing any further license or permit。 The details and
terms of all meets will be arranged by the committee
having in charge the interests of both organizations。
CHAPTER XXIV。
HINTS ON PROPELLER CONSTRUCTION。
Every professional aviator has his own ideas as to the
design of the propeller； one of the most important features
of flying…machine construction。 While in many
instances the propeller； at a casual glance； may appear
to be identical； close inspection will develop the fact that
in nearly every case some individual idea of the designer
has been incorporated。 Thus； two propellers of the two…
bladed variety； while of the same general size as to
length and width of blade； will vary greatly as to pitch
and 〃twist〃 or curvature。
What the Designers Seek。
Every designer is seeking for the same resultthe
securing of the greatest possible thrust； or air displacement；
with the least possible energy。
The angles of any screw propeller blade having a
uniform or true pitch change gradually for every increased
diameter。 In order to give a reasonably clear
explanation； it will be well to review in a primary way
some of the definitions or terms used in connection with
and applied to screw propellers。
Terms in General Use。
Pitch。The term 〃pitch；〃 as applied to a screw propeller；
is the theoretical distance through which it would
travel without slip in one revolution； and as applied to
a propeller blade it is the angle at which the blades are
set so as to enable them to travel in a spiral path through
a fixed distance theoretically without slip in one revolution。
Pitch speed。The term 〃pitch speed〃 of a screw
propeller is the speed in feet multiplied by the number of
revolutions it is caused to make in one minute of time。
If a screw propeller is revolved 600 times per minute；
and if its pitch is 7 ft。； then the pitch speed of such a
propeller would be 7x600 revolutions； or 4200 ft。 per
minute。
Uniform pitch。A true pitch screw propeller is one
having its blades formed in such a manner as to enable
all of its useful portions； from the portion nearest the
hub to its outer portion； to travel at a uniform pitch
speed。 Or； in other words； the pitch is uniform when the
projected area of the blade is parallel along its full
length and at the same time representing a true sector
of a circle。
All screw propellers having a pitch equal to their
diameters have the same angle for their blades at their
largest diameter。
When Pitch Is Not Uniform。
A screw propeller not having a uniform pitch； but
having the same angle for all portions of its blades； or
some arbitrary angle not a true pitch； is distinguished
from one having a true pitch in the variation of the pitch
speeds that the various portions of its blades are forced
to travel through while traveling at its maximum pitch
speed。
On this subject Mr。 R。 W。 Jamieson says in Aeronautics：
〃Take for example an 8…foot screw propeller having an
8…foot pitch at its largest diameter。 If the angle is the
same throughout its entire blade length； then all the porions
of its blades approaching the hub from its outer portion would
have a gradually decreasing pitch。 The 2…foot
portion would have a 2…foot pitch； the 3…foot portion a 3…
foot pitch； and so on to the 8…foot portion which would
have an 8…foot pitch。 When this form of propeller is
caused to revolve； say 500 r。p。m。； the 8…foot portion would
have a calculated pitch speed of 8 feet by 500 revolutions；
or 4；000 feet per min。； while the 2…foot portion would
have a calculated pitch speed of 500 revolutions by 2 feet；
or 1；000 feet per minute。
Effect of Non…Uniformity。
〃Now； as all of the portions of this type of screw
propeller must travel at some pitch speed； which must have
for its maximum a pitch speed in feet below the calculated
pitch speed of the largest diameter； it follows that
some portions of its blades would perform useful work
while the action of the other portions would be negative
resisting the forward motion of the portions having a
greater pitch speed。 The portions having a pitch speed
below that at which the screw is traveling cease to perform
useful work after their pitch speed has been exceeded
by the portions having a larger diameter and a
greater pitch speed。
〃We might compare the larger and smaller diameter
portions of this form of screw propeller； to two power…
driven vessels connected with a line； one capable of traveling
20 miles per hour； the other 10 miles per hour。 It
can be readily understood that the boat capable of traveling
10 miles per hour would have no useful effect to
help the one traveling 20 miles per hour； as its action
would be such as to impose a dead load upon the latter's
progress。〃
The term 〃slip；〃 as applied to a screw propeller； is the
distance between its calculated pitch speed and the actual
distance it travels through under load； depending upon
the efficiency and proportion of its blades and the amount
of load it has to carry。
The action of a screw propeller while performing useful
work might be compared to a nut traveling on a
threaded bolt； little resistance is offered to its forward
motion while it spins freely without load； but give it a
load to carry； then it will take more power to keep up its
speed； if too great a load is applied the thread will strip；
and so it is with a screw propeller gliding spirally on the
air。 A propeller traveling without load on to new air
might be compared to the nut traveling freely on the bolt。
It would consume but little power and it would travel at
nearly its calculated pitch speed； but