Woodward — Air-Ship Propeller Problems. 9 



Meanwhile, my fornmlse are published in the hope that others 

 may find the best designs for the entire mechanism, and the 

 several coefficients of efficiency. 



10. Characteristics of the Ideal Propeller. 



1. The radius must be as large as is practicable. 



2. The blade surfaces nmst be parts of right helicoids {i. e. 

 like the bearing surfaces of a square-threaded screw) . 



3. Every blade must run to the central hub with jull depth. 



4. The ''pitch'' of the screw surface must be determined by 

 the speed of rotation of the shaft and the velocity of the air 

 through the propeller. 



For Example: suppose the air-ship frame be anchored, and 

 that the required thrust, or pull, of the propeller is 100 lbs., 

 and the radius of the propeller be 8 ft. 



Then .4 = 201 



P_ 100 

 A~ 201 



^l 



25'P 80 



-^^ = ---^ = 20 nearly 



That is, the backward current of air passing the propeller must 

 be 20 ft. per second. If T be the revolutions per second and s 

 the pitch of the screw we have Ts = v = 2Q in the case assumed. 



As T is generally known for a motor doing its maximum 

 work we have 



_jj_20 



" rp rn 



If r be 4, we have the pitch = 5 feet, if the ship is anchored. 



If now the air-ship is moving 15 miles per hour, we have 

 2;' =22, so that the air passes the propeller at the rate of 20 + 22 

 ft. per second. Hence the pitch of the helicoidal blades must be 



V ~l~ v' 42 

 s = pitch = —^ — = — = 10^ feet. 



If there are six blades, the depth of each should be 1.75 ft., or 

 21 inches, and each should subtend a circular arc of G0°. 



