Viewing page 15 of 23

This transcription has been completed. Contact us with corrections.

April 1913            Fly Magazine                11

An analysis of the diving rotation
Would disappear with tandem surfaces
          by Albert Adams Merrill, Massachusetts Institute of Technology

Several months ago there was an accident to a monoplane which resulted in the death of a French aviator.  A court of inquiry was held and the testimony of two of the witnesses was to the effect that the wings of the monoplane collapsed downward.
This testimony was given by eye-witnesses whose opinions were worthy of respect, and it was the first testimony to call attention of engineers to the danger of the diving rotation, which danger, previous to this time, had been overlooked.  it is the diving rotation which causes the air to strike on top of the surface.  This tends to act as a brake on the machine, and the top pressure per square foot becomes so great that unless the machine has very strong top guy wires the wings will break.
As the result of the investigation into the causes of this accident Bleriot called in his outstanding machines and rewired them, increasing the strength of his top guy wires and thus increasing the factor of safety against downward pressure.
It is the object of this article to analyze the conditions which produce a diving rotation with a view of finding out how the danger produced by this phenomenon can be reduced to a minimum.
For the sake of clearness it is necessary to define accurately certain terms.
1. Pressure Angle.  This is the angle between the chord of a surface and the path of the machine.
2. Angle of Incidence.  This is the angle between the chord of the surface and the horizon.
  It is essential to understand thoroughly this difference and to keep it constantly in mind, otherwise it will be impossible to understand the nature of many of the phenomena which occur in flight.  It is the pressure angle which determines the magnitude, direction, line of action and point of application (center of pressure) of the resultant pressure on any surface.  The angle of incidence has nothing whatever to do with the resultant pressure on a surface, it determines only the magnitude of the H and V components of that pressure.
In Eiffel's work the angles mentioned are angles of incidence, but since the air current is horizontal they are also pressure angles, so that when you read that the X and Y components of at a certain angle have a certain value, it means that these values are constant for that surface at that pressure angle (we are not considering changes in V).
Eiffel shows that in a monoplane when the pressure angle is decreased from about 15 degrees the center of pressure rushes back and produces a couple which tends to cause a diving rotation.  This movement of the c.p. is entirely independent of the angle of incidence.  If the pressure angle was never changed the position of the c.p. relative to the front edge would not change regardless of any change in the angle of incidence. 
At the end of every flight it is necessary of course to come down to a landing, and this involves decreasing the angle of incidence.  This being so the question is, Under what conditions will this decrease in the angle of incidence produce a diving rotation?

                        Fig. 1 (diagram)
Let Fig. 1 represent the force diagram of a machine flying horizontally at uniform speed on the path marked by the long arrow.  C.P. = center of pressure upward; C.T. = center of thrust forward; C.G. = center of gravity downward; C.T.R. = center of total resistance backward.  This represents a balanced system at the pressure angle (symbol).
Let B = angle of incidence.  Then (symbol) = B when flying horizontally (no wind).  The aviator wants to land and he pushed his lever forward a very small distance therefore both (symbol) and B will decrease.  Let(symbol) represent the ratio of decrease.  Then (symbol) will become (symbol) -(symbol) (symbol) and B will become B-(symbol) B.
Now as the pressure angle has decreased the center of pressure will move backward.  It we let C.P. represent the distance from the front edge to the center of pressure, then C.P. will become C.P. +(symbol) C.P. and the tendency of this alone will be to cause a diving rotation.  But note the fact that when (symbol) becomes (symbol)-(symbol)(symbol) the lift decreases, the weight is unsupported and the machine no longer maintains a horizontal course.  if H represents the original height the new height will be H-(symbol) H.  In fig. II (symbol) O represents the first position ((symbol)-(symbol)(symbol)) O, represents the position when the elevator is pushed forward and the new