The
aircraft propeller looks like a simple mechanism to the uneducated individual.
To the educated, an aircraft propeller represents the highest sophistication in
aerodynamics, mechanical engineering and structural design. This report will
touch on the history of the propeller, from early pioneers/experiments,
advancement during/after the war, all the way up to current applications of the
propeller. The creation of the propeller can be traced back to Leonardo da
Vinci. Da Vinci’s “helical screw” helicopter is believed to be the
ancestor of the air propeller and the helicopter rotor. The first idea of a
propulsive airscrew, however, belongs to J.P. Paucton, a French mathematician.
Paucton envisioned a flying machine that had two airscrews, one for propulsion
and the other for sustaining flight. The idea of using an airscrew for
propulsion was utilized during the late 1700’s to early 1800’s. Only after
experimentation did the inventors conclude that more propulsive power could be
obtained by merely straightening out the surface of the airscrew blades.
Attempts to utilize the “straight blade” propeller were made by balloonists.
These contraptions were quite strange and hardly fulfilled their purpose of
actually propelling the balloon. The basic propeller had evolved from the simple
concepts of da Vinci, and was slowly becoming an effective means of aerial
propulsion. To reach the next plateau of flight an increased knowledge of the
propeller would be needed, and the mysteries of the propeller and mechanical
power would need to be solved. These substantial tasks remained for aviation’s
pioneers to tackle during the 19th century. Throughout the 19th century,
aviation pioneers explored and tinkered with the concepts of flight to design a
viable airship. Some pioneers tried to transform the balloons into navigable
cigar shaped airships by experimenting with sails, propellers, and paddlewheels
but all produced limited results. Other experimenters, who were convinced that
man flight should have wings, worked to establish basic principles in
aerodynamics, flight stability and control, as well as propulsion. Controlled
mechanical flight came on August 9, 1884. Charles Renard and A.C. Krebs flew the
airship “La France” on a closed circuit from Chalais-Meudon to Villacoublay
and back in 23 minutes. The airship “La France” was powered by a 9
horsepower electric motor that drove a 23ft diameter propeller and reached a
speed of 14.5 mph. This flight was the birth of the dirigible, a steerable,
lighter-than-air ship with adequate propulsion. Another important milestone in
aviation, was the understanding of aerodynamics. Sir George Cayley, a British
theorist, was acclaimed as the father of aerodynamics. He established a solid
foundation of aerodynamic principles that were essential to the success of other
pioneers. In 1875, Thomas Moy created a large model that had twin 12ft
propellers with 6 blades each! Interestingly enough these blades could be
adjusted to produce maximum thrust under certain conditions, an early
recognition of the need for changing blade pitch. Without a doubt, the most
expensive and spectacular project of its time was that carried out by Sir Hiram
Maxim. His numerous experiments with propellers, culminated in the construction
of a huge, four-ton biplane in 1890. This contraption was powered by two 180hp
steam engines that each drove propellers 17ft, 10inches in diameter and weighing
135lbs. The two-blade propellers, inversely tapered and squared at the tips 5 ½
ft wide, were made of American Pine, planed smooth, covered with glued canvas
and stayed to the propeller shafts with steel wire to handle the high thrust
loads. These massive propellers produced 1,100lbs of thrust each during full
power while rotating at 425rpm. Maxim’s jumbo creation didn’t last long
however, it jumped the test track and suffered extensive damage. Hands down, the
most influential aviation pioneers were the Wright brothers. They had concluded
that a propeller was simply a whirling wing, but didn’t have the appropriate
information to consult when comprehending the fundamental principles of blade
shape and motion. This dilemma made designing the propeller one of the Wright
brothers most challenging problems. Despite the lack of previous information to
consult, the brothers were able to learn, through investigation and trial/error,
that large propeller diameters would produce high thrust for a given power
input. The brothers also determined that high torque produced by large, slow
turning blades adversely affected the flying qualities (p-factor). On their
first aircraft, they utilized 8 ½ ft propellers installed behind the wind to
minimize airflow disturbance, incorporated counter-rotating propellers to
eliminate the problems associated with torque, and gained thrust efficiency by
reducing the blades’ rotational speed using a chain and sprocket transmission.
The Wright brother’s propeller was 66% efficient which was much higher that
any other propeller of the time. The foundations of a disciplined approach to
propeller design evolved soon thereafter. With the advancements and refinements
made by early inventors, engineers could use those test results to design
propellers with better performance and structural reliability. These
advancements led to the development of the first generation of well-designed
propellers. One of the first designs was the “Integrale”, developed by
Lucien Chauviere, the world’s first industry standard propeller manufacturer.
By 1910, the number of propeller producers multiplied, and numerous advancements
were made. While most of the manufacturers were focusing on wooden propellers, a
few visionaries were experimenting with metal propellers and variable pitch
blades. Geoffrey deHavilland, an English engineer, tested propellers whose
aluminum blades could be adjusted to change their angle. At the same time,
German pioneers Hugo Junkers and Hans Reissner experimented with lightweight
metal propellers. The first U.S. propeller production facility was the Requa
Gibson Company founded in 1909, which was headed by Canadian engineer Wallace R.
Turnbull. Turnbull tested and confirmed that the large, slow-speed propellers
produced higher thrust efficiencies than those compared with smaller, high-speed
propellers. More importantly, Turnbull confirmed the universal law of
aerodynamics: the efficiency of any aerodynamic device rises as the amount of
air it acts upon increases and the velocity of that air decreases. These
theories were expanded during WWI. The war brought much advancement to the
propeller. Stronger materials were created through “bonding” which made
propellers compatible with the larger, more powerful engines. Propeller
balancing techniques were developed, which greatly smoothed out the ride.
Experiments with variable pitch blades were introduced as well. Two major
breakthroughs occurred after the war: the once piece metal propeller, and the
ground adjustable pitch propeller. The metal propeller allowed operations in all
climates, whereas the wooden prop would fail in extreme conditions. The metal
propeller could be made thinner than a comparable wooden propeller, which
allowed for faster cruising speeds due to less drag from compressibility.
Thinner blades also improved efficiency at higher speeds. The only drawbacks to
the early metal propeller were their weight and fixed pitch blade angles. The
development of the ground adjustable propeller was a major improvement. The best
propeller of this kind at the time was the dural-blade ground adjustable
propeller. With this adjustable propeller, the pilot could choose whether or not
they wanted to have great takeoff performance or great cruise performance. In
1927, the idea of changing the pitch of a propeller was taken one step further
with the development of the in-flight adjustable propeller. This gearshift
device allowed pilots to change the pitch angle in flight to get the best
performance out of their aircraft during takeoffs and during cruise. One of the
most interesting developments during this period was the introduction of a
propeller that could “feather”. This greatly reduced prop drag and was a
multi-engine pilot’s savior when one of his engines quit. Hamilton Standard,
on their Hydromatic propeller, introduced the “feathering blade”. After
WWII, the Hydromatic propeller was improved by Hamilton Standard to include
features such as reversible pitch, automatic synchronization, and electrical
blade deicing. Many large propeller transports switched to this new system for
its reliability and pilot friendly features. The age of the Turboprop brought a
few changes to the propeller. Four bladed, wide chord, aluminum alloy
propellers, were utilized by most turboprop transports because of their
durability. Engineers designed wide, super-thin, hollow blades to increase the
performance of the aircraft at high speeds. Advanced applications of the
propeller are currently being experimented by Hamilton Standard. The new idea
deals with transport category aircraft and the introduction of the “un-ducted
fan”. This design incorporates the reliability of the turbine engine, with the
efficiency of a prop. Expected savings of 25% in fuel costs drive the ongoing
interest in this application. The design utilizes 8-10 thin but very wide,
closely spaced, swept angle blades to propel an aircraft at speeds approaching
the speed of sound (mach .8). It will be interesting to see how the role of the
propeller develops as time goes on. This report has sparked my interest in
propellers. I have never researched this topic before and feel that I’ve
benefited from writing it. I enjoyed researching the history of the propeller
and it’s contributions to aviation milestones. I’ve taken you, the reader,
from the early experiments of da Vinci, the wooden props of the Wright brothers,
the design of the variable pitch propeller, through the advanced concept of the
“un-ducted” fan. I hope this report was as interesting to read as it was to
write.