X-Mozilla-Keys:
While looking for drawings of PT6's in Aircraft Gas Turbine Engine
Technology I ran across this discussion of prop fans.
These are some of the reasons Perry Mick should hang in
there on the development of rotary engine powered ducted fans.
Speeds beyond mach .6 are the goal. To reap the benefits
its going to take a lot more horse power Perry.
Paul Lamar
Quote page 17 Aircraft Gas Turbine Engine
Technology. Third Edition Treager.
" The Propfan Engine
To reduce specific fuel consumption and gain the advan-
tages in the following list, a number of manufacturers,
specifically Pratt & Whitney, General Electric, and Allison,
have designed and built what were essentially ultra-high-
bypass-ratio turbofan engines. At the time of this writing, no
engines of this type have been placed into production, but
increased fuel costs may hasten their development and use.
The propfan characteristics and uses are as follows:
1. The Propfan is expected to be at least 80 percent effi-
cient, that is, able to conven 80 percent of the engine's
horsepower to thrust at Mach 0.8, at an altitude of
35,000 ft. This efficiency is similar to that of a con-
ventional modern propeller but better than that of a
turbofan and should result in at least a 20 to 25 percent
savings in fuel over the turbofan with which it is in
competition.
2. The modern turbofan has a bypass ratio of as high as
5:1 or 6:1, while the propfan is designed to have
bypass ratios of 80:1 or more. While the fan duct or
shroud does improve that unit's efficiency, the
increased drag that results from the large duct tends to
cancel this advantage. Large ducts also present struc-
tural problems. such as ovalization of the duct during
abrupt maneuvers (see chap. 20).
3. The propfan can absorb more horsepower than the
turboprop for a given diameter because of the high
anticipated disc loading of 35 or more. (Disc loading
equals the horsepower divided by the square of the
propeller diameter.) Disc loading for general aviation
aircraft is about 7, while for an airplane like the
Lockheed Electra aircraft, it is about 12.5. High disc
loading is necessary to keep the propeller diameter
within reason.
4.As stated earlier, new propeller designs are no more
efficient than conventional designs, but the conven-
tional propeller begins to generate shock waves when
aircraft speed reaches about Mach 0.6. with a corre-
sponding increase in drag. The curved leading edge
of the newer propellers lowers the effective Mach
number, a reduction proportional to the cosine of the
sweep angle at any point on the blade. For example.
a sweep angle of 300 experiences an effective Mach
number of 0.87 while the blade is traveling at Mach
1. Sweeping the blade to 45 degrees lowers the effective
Mach number to 0.71. the cosine of that angle. Shock
wave formation is further delayed by using low
thickness ratios. (i.e.. the relationship between the
thickness of an airfoil to its chord). The Lockheed
Electra turboprop aircraft propeller has a thickness
ratio of about 2.5 percent at the tip. 8 percent at one-
half the span, and 35 percent at the propeller spinner
junction, while the newer propellers have thickness
ratios of 2, 4, and 20 percent. respectively, at the
same points.
5. By sweeping the inboard section of the newer pro-
peller designs, forward, aerodynamic as well as
centrifugal balancing is enhanced, resulting in the
need for a less powerful and lighter pitch-changing
mechanism.
6. Finally, while the high-bypass-ratio turbofans are
quiet, the new propfans will transmit considerably
more noise to the airframe structure and the surround-
ing environment, and if the propfan is to be driven in
a conventional manner, that is through a reduction
gearbox, very large gearboxes will be required to
transmit the l5,000 horsepower necessary to drive
commercial-sized aircraft see [Fig. 2-47 on the
General Electric Unducted Fan (UDF)]."
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