W oodward—Air-Ship Propeller Problems. 9 
Meanwhile, my formule 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 must be parts of right helicoids (7. e. 
like the bearing surfaces of a square-threaded screw). 
3. Every blade must run to the central hub with full 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 ibs., 
and the radius of the propeller be 8 ft. 
Then A=201 
_P _ 100 
a OT 
Ne a WO al 
v= ie sere ve ia nearly. 
That is, the backward current of air passing the propeller must 
be 20 ft. per second. If 7 be the revolutions per second and s 
the pitch of the screw we have 7s=v=20 in the case assumed. 
As T is generally known for a motor doing its maximum 
work we have 
BOO 
a IR i 
If T 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 
v' =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 
s = pitch = —>— = — = 104 feet. 
If there are six blades, the depth of each should be 1.75 ft., or 
21 inches, and each should subtend a circular are of 60°. 
