XV-15 litho - NASA's History Office

Figure 28.
Performance tests of 5-ft.
diameter proprotor in the
Army 7- by 10-ft. wind tunnel
at the Ames Research
Center. (Ames Photograph
AC98-0209-4)
24
test, also conducted with Bell
Helicopter as the hardware and technical
support contractor (jointly funded
by the NASA, the Army, and the Air
Force), demonstrated the feasibility of
the airplane-mode rotor stopping and
blade folding, and of the blade deployment
and spin-up process. 18 The
stop/fold tilt rotor, however, had the
additional penalties of the increased
complexity and increased weight of the
stop/fold mechanism, and, with the lack
of a developed convertible engine, it
was put aside as a potentially feasible
concept that would require further
advancements to be an effective
contender.
Another major deficiency revealed by
the XV-3 was the poor propulsive efficiency of the rotor (frequently referred to as a
“proprotor” when used on a tilt rotor aircraft) in the airplane (or cruise) mode as
well as poor performance in hover. The tilt rotor design philosophy that evolved
during this period was that the proprotor should meet stringent performance requirements
in the hover and airplane modes of flight but should not be significantly compromised
to meet helicopter-mode (edgewise flight) design conditions. This meant
that the proprotor blades could be designed with considerable twist, similar to that
of airplane propeller blades, instead of the moderate twist of helicopter rotor blades
(to accommodate the edgewise operation). While the opportunity to use twist more
freely as a design variable could improve performance, the significant differences in
blade loading (both in distribution and level) and in the distribution of air inflow to
the proprotor between the hover- and airplane-mode conditions provided a challenging
problem for the design engineers. Furthermore, the large diameter (low disc
loading) proprotor which allowed the tilt rotor aircraft to hover at helicopter-like
low levels of horsepower, results in a proprotor that is much larger than is required
for maximum efficiency in the airplane mode. A search of prior experimental
reports for applicable airplane mode test results showed that insufficient empirical
data existed at this unusually light airplane-mode loading. NASA Ames and the
Army AMRDL, therefore, sponsored and conducted several analytical and test
activities to investigate both the hover performance level and airplane mode efficiency
achievable with a properly designed proprotor.
In 1968, Boeing Vertol was awarded a contract by Ames to investigate the
effect of blade twist on the performance of model-scale proprotors. Under
18 Anon., “Large Scale Wind Tunnel Investigation of a Folding Tilt Rotor,” NASA CR 114464,
Bell Helicopter Co., May 1972.