The Northrop Turbodyne (XT37)
Strength Requirements for Gas Turbine Power Plants
in Navy Patrol Bomber Airplanes
by CDR William H. Miller, Head of BuAer Structures Section
4 Jun 1943

1. Strength shall be provided in the power plant unit including means of attachment to its supporting structure, for the critical combinations of power and accelerations expected to be encountered in catapulting, landing, and in flight when installed in Class VPB land-based airplanes. These requirements are applicable to those installations in which the power plant center of gravity lies not more than five (5) feet aft of the airplane center of gravity. The design limit loads (see paragraph 2) shall be not less critical than those resulting from the following airplane accelerations and velocities:
  1. linear catapulting and landing accelerations as follows:
    1. 1.5 g acting forward.
    2. 4 g acting upward.
    3. 4 g acting upward in combination with 1.33 g acting aft.
    4. 4 g acting upward in combination with 1 g acting sidewise.
  2. Flight conditions with the power plant operating at the military  rated engine rpm as defined in the applicable issue of Army-Navy Specification AN 9502.
    1. Linear acceleration of 4 g upward combined with a positive pitching velocity of 1.5 radians per second.
      1. Power plant at thrust and torque corresponding to 175 mph
      2. Power plant at zero thrust and torque due to instantaneous power shutoff
    2. Linear acceleration of 2 g downward combined with a negative pitching velocity of 1.5 radians per second.
      1. Power plant at thrust and torque corresponding to 175 mph
      2. Power plant at zero thrust and torque due to instantaneous power shutoff

A positive pitching velocity is defined as an angular velocity tending to raise the airplane's nose. For a single propeller the gyroscopic moment resulting from the specified pitching velocity shall be based on a propeller moment of inertia to be specified by BuAer. For contra-rotating propellers the opposed gyroscopic moments applied to the shaft shall be based on propeller moments of inertia to be specified by BuAer

2. Factors of Safety. The power plant unit including means of attachment to its supporting structure shall withstand without permanent set design yield loads equal to 1.15 time the design limit loads and shall withstand without failure design ultimate loads equal to 1.5 times the design load limits.

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