50 



SMITHSONIAN CONTBIBUTIONS TO KNoui.iim.i 



vol 27 



cannol t oil in fact, then, with exactness how to adjust the wings so that CGi - -Cl\ 

 may be 9 cm., and equilibrium was in fad obtained in flighl when (the empiri- 

 cally determined) ( 6?, <!', 3 cm. 



Again, lei it be supposed thai CP, was really overCG, . . . 

 center of gravity below the center of pressure is 4.'! cm.=i?. 



4.5x25 



The distance of the 



Then 



a=sm~ 



12 nearly. 



12.5 x -43 



The doubt as to the actual position of the resultant center of pressure, then, ren- 

 ders the application of the rule uncertain. In practice, we are compelled (un- 

 fortunately) after first calculating the balance, by such rules as the above, and 

 after it has been thus found with approximate correctness, to try a preliminary 

 flight. Having- witnessed the actual conditions of flight, we must then readjust 

 the position of the wings with reference to the center of gravity, arbitrarily, 

 within the range which is necessary. This readjustment should be small. 



Fie;. G. Diagram showing effect of Penaud tail. 



In the preceding discussion it has been assumed that, if there is a flat tail or 

 horizontal rudder, il supports no portion of the weight. This is not an indis- 

 pensable condition but it is very convenient, and we shall assume it. In this 

 case the action of the so-called Penaud rudder becomes easily intelligible. This 

 is a device, already referred to in Chapter II, made by Alphonse Penaud for the 

 automatic regulation of horizontal flight, and it is as beautiful as it is simple. 



Let .1/; (Fig. (i) be a schematic representation of an aerodrome whose sup 

 porting surf ace is Bb, and let it be inclined to the horizon at such an angle a 

 thai its course at a given speed may be horizontal. So far it does not appear 

 that, if the aerodrome be disturbed from this horizontal course, there is any 

 self regulating power which could restore it to its original course; but now let 

 there be added a Hat tail AC set at an angle -a with the wing. This tail serves 

 simply for direction, and not for the support of the aerodrome, which, as already 

 stated, is balanced so that the ('<■ comes under the CP of the wing Bb. 



It will be seen on a simple inspection that the tail under the given conditions 

 is horizontal, and that, presenting its edge to the wind of advance, it offers no 

 resistance to it, so that if the front rises and the angle a increases, the wind will 

 strike on the under side of the tail and thereby lend to raise the rear and depress 



