XV-15 litho - NASA's History Office

component occurred during the STO tests. This component, called the “yoke,”
to which the blades are attached, was manufactured of titanium because it
afforded valuable weight saving over steel while still providing the required
fatigue life.
On October 1, 1982, while performing STO operations at the Crows Landing
NALF, at the XV-15’s maximum takeoff gross weight, a telephone call was
received by the Ames test director Shorty Schroers, from engineers at the Bell
facility in Texas. They informed Schroers that they had just discovered that
the strength of titanium material used for the rotor yokes was significantly
lower than that used in their design. The flight crew was informed about this
new and somewhat disturbing development while in flight. They landed the
XV-15 safely and removed the weights added for the STO tests. After further
consultation with Bell engineers, it was decided to “gingerly” fly the aircraft
back to Ames taking special care to keep the hub yoke oscillatory loads at a
low level.
The full story regarding the titanium fatigue strength anomaly emerged later.
While performing design work for another project, a Bell engineer came across a
published fatigue strength allowable load level for titanium that was lower than
that used for the design of the XV-15 yokes. Although the titanium identified by
the Bell engineer and the titanium used for the proprotor were the same, a difference
existed in their fatigue strength because of heat treatment (a process by
which the strength and other properties of metals are altered by exposure to specific
thermal conditions). As luck would have it, the heat treatment for the
titanium used for the XV-15 yokes was the one which resulted in the lower
fatigue strength. This meant that aircraft N703NA had been operating at significantly
higher loads than the lower strength titanium could bear for the duration
of the flight program.
Operation of both XV-15 aircraft was continued but with the installation of a
new set of titanium yokes and with the allowable loads reduced until a better
solution was found. The solution was replacement of the titanium yokes with
steel yokes of the same design. Steel yokes were installed on aircraft N703NA in
July of 1985 and have been used continuously since then without incident.
Flow Visualization Studies
In the early 1980s, a number of tilt rotor technical issues remained unexplained.
One of these was that acoustic measurements in the hover mode of flight
revealed that noise, rather than being at about an equal intensity around the aircraft,
was greater behind the aircraft than at an equal distance along its sides.
Another issue was that, although the magnitude of the download was now
accepted as being greater than initially estimated (based on recent performance
investigations), verification of the reason for this was needed. In an attempt to
answer these questions and to better understand the airflow around the tilt rotor
71