274 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
stresses which an aircraft puts upon the pilot—those due to accelera- 
tion or rapid change of motion and those due to high flying in the 
rarefied air of the upper atmosphere. 
In the last hundred years man has increased the speed at which he 
can travel more than tenfold, but there is no reason to suppose he is 
approaching any human limit in speed for, provided that he is pro- 
tected from wind pressure by a closed cockpit and that the motion 
does not change rapidly in direction, there is no more mechanical stress 
on the pilot than if he were sitting on the ground. 
If the human body is moving uniformly there is no force acting on 
it other than that due to gravity, recognized as weight. But when the 
motion changes in either magnitude or direction, large forces come 
into play; for example, while launching an airplane by catapult. 
During this linear acceleration the pilot has the sensation of being 
driven backward against his seat by forces equaling several times his 
own weight. This is seen in the retracting of the skin of his face 
which bares the teeth like a snarling dog. In this case, the accelera- 
tion acts transversely on the body and lasts only a few seconds and in 
this direction the pilot can easily withstand many times the accelera- 
tion of gravity provided his head and shoulders are well supported. 
When a fast-moving airplane changes its direction and turns, air- 
plane and pilot are both subjected to very large forces. The phenome- 
non known as blacking-out came into prominence in the Schneider 
Trophy races; pilots found that in turning at high speed their vision 
became blurred and that for a few seconds in the turn they frequently 
became blind. This is now a common event in aircraft and is well 
understood by fighter pilots. 
When an airplane travels in a curved path in turning or pulling out 
of a dive a large centrifugal force tends to force the airplane and pilot 
away from the center of the circle. The magnitude of this force in- 
creases with the square of the speed and decreases as the radius 
increases. Subjectively, a pilot experiences a great increase in weight 
of all parts of his body as the centrifugal force tries to drive his body 
out through the bottom of the airplane. The magnitude of the acceler- 
ation acting on the pilot is expressed in terms of g, the force due to 
gravity normally acting on the body which causes it to have its normal 
weight. Thus in a turn producing 49 or four times the force of grav- 
ity, if the pilot’s seat were fixed to a spring balance it would register 
four times his normal weight and the pilot and all parts of his body 
become extremely heavy. This is seen in the sagging of the soft part 
of the face which occurs in a tight turn (pl. 1). A turn at 300 miles 
per hour and 1,000 feet radius produces 69, and a pilot in effect weighs 
about half a ton and his blood virtually becomes as heavy as molten 
iron. The blood is normally being pumped to the pilot’s head by his 
