NO. 3 LANGLE1" MEMOIB ON MECHANICAL FLIGHT 5 1 



the froril again, [f the angle a diminish, so that the front drops, the Wind will 

 strike the upper surface of the tail, and equally restore the angle a to the amount 

 which is requisite to give horizontal flight. If the angle a is not chosen originally 

 with reference to the speed so as to give horizontal flight, the device will still 

 tend to continue the flight in the straight line which the conditions impose, 

 whether that be horizontal or not. 



From this description of its action, it will be seen that the Penaud tail has 

 the disadvantage of giving an undulatory flight, if the tail is made rigid. This 

 objection, however, can be easily overcome by giving to it a certain amount of 

 elasticity. It does not appear that Penaud gave much attention to this feature, 

 but stress is laid upon it in the article "Flight," in the ninth edition of the En- 

 cyclopaedia Britannica, and I have introduced a simple device for securing it. 



The complete success of the device implies a strictly uniform velocity and 

 other conditions which cannot well be fulfilled in practice. Nevertheless, it is as 

 efficient a contrivance for its object as has yet been obtained. 



More elaborate devices have been proposed, and a number of them, depend- 

 ing for their efficiency upon the action of a variety of forces, have been con- 

 structed by the writer, one of which will be described later. This has the ad- 

 vantage that it tends to secure absolutely horizontal flight, but it is much inferior 

 in simplicity to the Penaud tail. 



Apart from considerations about the thrust, the CP is in practice always 

 almost directly over the CG, and this relationship is, according to what has been 

 suggested, obtained by moving the supporting surfaces relatively to the CG, or 

 vice versa, remembering, however, that, as these surfaces have weight, any move- 

 ment of them alters the CG of the whole, so that successive readjustments may 

 be needed. The adjustment is further complicated by another important con- 

 sideration, namely, that those parts which change their weight during- flight (like 

 the water and the fuel) must be kepi very near the CG. As the water and fuel 

 tanks are fixed, it appears, then, that the center of gravity of the whole is prac- 

 tically fixed also, and this consideration makes the adjustment a much more 

 difficult problem than it would be otherwise. 2 



- The following formula; proposed by Mr. Chas. M. Manly show how the center of pressure may 

 be moved any desired distance either forward or backward without in any way affecting the center of 

 gravity, and by merely moving the front and rear wings the same amounts but in opposite direc- 

 tions, the total movement of each wing being in either case five times the amount that it is desire! 

 to move the mean CP ly and the direction of movement of the front wing determining the direction 

 of movement of CP,. 



In Figure 7, CP/ W and CP rv are the centers of pressure of the front and rear wings respectively; 

 the weights of the wings, which are assumed to be equal and concentrated at their centers of figure, 

 are represented by to, W, and a is the distance of the center of pressure in either wing from its center 

 of figure. The original mean center of pressure of the aerodrome is CP„ W is the weight of the 

 aerodrome, supposed to be concentrated at CO,, while m is the distance from CP,,,- to CG,. 



Now, if we have assumed that the rear wing, being of the same size as the front one, has a lifting 

 effect of only 0.66, and on this assumption have calculated the proper relative positions of the front 

 and rear wings to cause the CP, to come directly over the CG,, and upon testing the aerodrome find 



