JUL 6 mi 



SYMPOSIUM ON AERONAUTICS. 



{Read April 14, 1917.) 

 I 



DYNAMICAL ASPECTS 

 Bv ARTHUR GORDON WEBSTER. 



In opening this symposium I can undertake to do no more 

 than explain, in a most elementary way, the dynamical principles 

 upon which artificial flight depends. It is difficult to do this with- 

 out the use of dift"erential equations, which would be out of place 

 in a popular discussion, so that my treatment must confine itself 

 to the merest outline. We must distinguish at the outset between 

 aeronautics properly so-called, in which we have to do wnth airships, 

 that is apparatus possessing natural sustentation through the buoy- 

 ancy of the air displaced, which is at least as heavy as the airship, 

 and aviation, which is the operation of apparatus that has no nat- 

 ural sustentation or buoyancy, being heavier than the displaced air, 

 and. like a bird, possessing sustentation only when in motion. Un- 

 fortunately we have no generic term for the latter apparatus, cor- 

 responding to the recently coined French word " avion," and we are 

 obliged to make use of the word aeroplane, although the term plane 

 is not always accurate. \\'hile the principle of Archimedes, namely 

 that a body is buoyed up with a force equal to the weight of the 

 displaced fluid, this force acting at a point coincident with the center 

 of mass of the fluid displaced, is sufficient for the study of the equi- 

 librium of the airship, totally different principles are involved in 

 connection with the aeroplane. 



The first principle that we shall use is that of relative motion 

 of the aeroplane and the air. It will be admitted that the forces 

 involved are the same whether, as in the case of the kite, the ob- 

 ject is at rest and the air in motion, or as in the case of the aero- 



PROC. AMER. PHIL. SOC, VOL. LVL L, JCXE 15, IQI?. 



