Failure - Interactions

Fin-Rudder's

Restoring

Yaw-Moment


Due either

to   Beta

or to

Rudder- δR


Free-trailing

or

Fixed Rudder

Rudder

Lock-Over

Solid is  Rudder  FIXED

Dashed is for  Rudder FREE.

"Yaw"  

really  Beta

effects  Rudder Hinge Moments:

Pedal-Force

Reversal.

Yaw

(sideslip)

induced

Roll

Bill Cook    [_Road to the 707_]      reviewed the  investigation of the

B-307 Stratoliner   NX19901    inflight upset,    and  discussed the lesson learnt.

   An Engineer's Lesson

              every  new    engineered safety feature 

                              introduces a new failure-mode.


  Discrepant   Rudder

       New-    another    Failure-mode    

due to      Hydraulic  Boost 


TWA B307 Stratoliner /1Jan41 (date from Rummel pg 56)   cold wx,  T/O from KC  with low ceiling,    during climbing turn,   inflight upset.   Recovered control,  returned & landed KC.    Capt was Harry Cambell,  said he came close to "buying the farm".   The F/E, Larry Beggs, resigned rather than fly again  [this post-mishap  resignation of a mishap-pilot   also  happened in later decades].    Passengers  reported looking up at the roofs of buildings along Independence Avenue.


Airplane grounded,  inspected,  all controls found normal,  lack of any identifiable aircraft faults  suggested cause of upset was pilot error.


Boeing investigated problem,  identified cold soaking  as a factor,   plane had been sitting on ramp for twelve hours with OAT less than zero degrees F.     Hydraulic fluid congealed inside the Rudder Boost System,   resulting in discrepant Rudder displacement.   


Boeing modified the Rudder Boost system  with bleed holes so that warm hydraulic fluid would circulate  inside the Rudder hydraulic cylinder after engine start.


[According to Ed Betts,  _AAHS_, Fall '93,   pg 171;    no specific date listed for mishap.    No corroborating AAR mentioned,  story may be constructed from hearsay,    but the F/E's resignation might be verifiable -- no other information.] 


Ironically,   the Rudder Boost system was an engineering fix,  one of three Boeing modifications to the Stratoliner's empennage -- added after the 18Mar39 upset/breakup  (that killed men of Boeing and KLM,  and Harlan Hull, TWA's Chief Pilot).    The  post-accident modification to the B307  was Boeing's first use of hydraulic boosted rudder.  


This upset mentioned by Ed Betts    might be the first  mishap in which  a hydraulic related anomaly caused discrepant rudder,  and induced   a yaw/roll upset.  Bob Rummel, _Howard Hughes and TWA_, pg 56,  cites this exact date for this cold-soak mishap,  but Rummel mistakenly states that the upset was a pitch-Elevator hard-over.   See AA Flt One/1Mar62.​

Hydraulic-Boost  For Rudder

B307  design was modified  by May 1940  to also  include  Rudder-Boost

Even after George Shairer's   design modifications    (Dorsal Fin, less area of Rudder aft of the hinge),  Boeing  decided to install Hydraulic Rudder Boost:  to ensure positive control of that  Rudder surface (prevent Lock-Over).  Below is an excerpt from page 116, _Aviation_, May 1940.  


[About two decades later,  Boeing  proceeded with a similar design-evolution of the Fin-Rudder on the B707 -- see later web-pages discussing the   design-errs, mishaps, and the evolution of B-707  Fin-Rudder  (size, boost, artificial stability of "Yaw Damper").]

Loss in stability

Added Dorsal

NX19901:

 Reversal in Pedal Force required

  Beware of the transition

           after March 1939

This cognitive battle   between    "yaw angle"    and   Sideslip Angle

   evolved after Mar'1939.               

Careful

Schairer wrote this in 1940 --

    so his concepts of

"at various angles of YAW"

  might have evolved from that

earlier work done aboard NX19901,     into a later exact measure of

Beta-angle           

    on the later tests with the

re-designed  Fin-Rudder,  using his

        Beta Vane   during testing.

 - - Note:  photo of one Flight test configuration - -

asymmetric, #3 and #4 prop's feathered,  yawing moment balanced initially with Left Rudder:    with large Beta, excess Pedal Forces due to excessive Hinge Moment,  pilot soon unable to fetch balancing-Rudder from the Lock-over position.

Schairer, George S.     "Directional Stability  and Vertical Surface Stalling."    

Presented at  IAeS' Aerodynamics  Session, Ninth Annual Meeting, NY;   January 31,  1941.  

Published in IAeS' Journal of Aeronautical Sciences, May'41; pg  270-75;   Institute of Aeronautical Sciences.   Paper describes directional  instability of prototype's Fin-Rudder design and excess Rudder Hinge Moment during flight with large Beta.  Boeing B307, Stratoliner, NX19901, March 18, 1939, Alder, Wa; aircraft accident, Air Safety Board investigation; re-design of Vertical Stabilizer, Rudder, Dorsal-Fin, Yaw, sideslip, Pedal Force, Rudder Lock-over; design revolution.  [George Schairer came to Boeing from Consolidated in 1939 to run aerodynamics -- replacing R. Cram  who had been killed in the Stratoliner crash].