Fig. 10. Thin Parabolic Aerofoil with Pressure Distribution. Fig. 11. Pressure Distribution of Thick Bird's Wing Type. (Eiffel)
Fig. 12 shows the effect of changing the angle of the bird wing from zero to 8 degrees. The lift per square foot in each case is shown at the upper left hand corner of the diagram while the percentages of the upper and lower surface lifts are included above and below the wing. For these curves I am indebted to E. R. Armstrong, formerly of "Aero and Hydro." As the angle is increased, the suction of the upper surface is much increased (0.541 to 1.370 pounds per square foot), and the pressure at the leading edge increases from depression to a very long thin peak. The maximum under pressure is not much increased by the angle, but its distribution and average pressure are much altered. At 0° and 2° the usual pressure is reduced to a vacuum over the front of the section as shown by the lower curve crossing over the upper side of the wing, and at this point the under surface sucks down and acts against the lift.
Fig. 12. Effect of Incidence Changes on the Pressure Distribution of a Thick Bird's Wing. (After Eiffel)
Distribution of Drag Forces. The drag as well as the lift changes in both direction and magnitude for different points on the wing. In the front and upper portions the drag is "Negative," that is, instead of producing head resistance to motion it really acts with the propelling force. Hence on the upper and front portions the lift is obtained with no expenditure of power, and in fact thrust is given up and added to that of the propeller. The remaining drag elements at the rear, and on the lower surface, of course more than overcome this desirable tendency and give a positive drag for the total wing. The distribution is shown by Fig 13 which gives the lift, drag and resultant forces at a number of different points on two circular arc aerofoils having cambers of 1/13.5 and 1/7 respectively. In this figure, the horizontal drag forces are marked D and d, and the direction of the drag is shown by the arrows. The lift is shown by L and the resultant by R as in the Fig. 7.
As shown, the arrows pointing to the right are the "Negative" drag (d) forces that assist in moving the plane forward, while the drag indicated by arrows (D) pointing to the left are the drag forces that oppose or resist the horizontal motion. With the smaller camber (1/13.5) the drag forces are very much smaller than those with the heavier camber of 1/7, and the negative drifts (d) are correspondingly smaller. All of the drag due to the lower surface, point to the left (D), and hence produce head resistance to flight. The drag to the rear of the center of the upper surface are the same. In front of the upper center we have right hand, or negative drifts (d), that aid the motion. These forward forces obtained by experiment prove the correctness of Lilienthal’s "Forward Tangential" theory advanced many years ago.
Fig. 13. Direction of Drag Over Different Portions of Circular Arc - Aerofoils.
Fig. 14a. Distribution for Wright Wing. (b) M. Farman Wing. (c) Breguet Wing. (d) Bleriot Wing. (e) Bleriot 11-Bis.