Woodward — Air-Ship Propeller Problems. 7 



That is to say, a complete mechanism consisting of propel- 

 lers and a 60 horse-power motor, which when anchored can pro- 

 duce a thrust of 650 lbs. — that being the thrust required when 

 a certain air-ship is moving 15 miles per hour — can actually 

 drive that air-ship only 13.3 miles per hour, unless the limit of 

 60 horse-power is exceeded.* 



7. Numerical Results, 



The following table is of value in estimating the power required 

 with propellers of various sizes for pulling or lifting different 

 amounts when the frame is anchored in still air. The propellers 

 are supposed to be ideally perfect in design and construction, 

 and no allowance is made for cross currents and for friction. 



TABLE SHOWING HORSE-POWER WHEN P THE THRUST, PULL 



OR LIFT, AND THE RADIUS OF THE PROPELLER, OR THE 



TOTAL PROPELLER AREA ARE GIVEN. 



P^pull or 

 lift in lbs 



r = radius A =^ total area 

 of equivalent of all propellers i 

 propeller in ft. in sq. ft. 



H = horse- 

 power required 



* Throughout this paper I mean by one "horse-power" 550 foot-lbs. 

 of real "work" per second, I make no use of a so-called "nominal horse- 

 power." 



t In a recent number of "Motor" (London), Mr. Rankin Kennedy says : 

 "It would be a simple matter to prove by calculation that the power 

 required of a propeller to sustain one pound weight in the air is 0.03 B.H.P. 

 In any case, theoretically, 0.03 B.H.P. must be allowed for every pound 

 weight to be lifted." Mr. Kennedy then goes on to say, that it would take 

 only 12 HP. to lift or sustain 400 lbs.! The statement is dangerously loose. 

 It would be true only on condition that the effective area of the propeller be 

 also increased 400 tunes! With the same propeller, it would take 240 horse 

 power to lift his 400 pounds! See Formula [X]. 



