Dayton, June 18, 1903
Your letters of June 7th and 14th are received, together with the various clippings, &c., which you mention. We thank you for your kindness in sending them. The only point of importance in the Maxim articles is the statement that the speed in his final trial was about 37 miles, whereas I have seen it stated elsewhere that it was 42 miles. The lower number agrees better with our calculations of what it should be possible to attain with the screws and engines he used, and seems to confirm the correctness of our theory.
The papers on screws, by various writers, do not seem to me of very much value. The chapter in the French book of Andre which is devoted to screws, seems about as good as anything, but the final conclusion is that very little is known of the action of screws in motion forward. The action of screws not moving for ward presents a very different case, and experiments based on such conditions are not applicable to the conditions met in practical flying. Many of the writers seem to think that there is some connection between the thrust per horsepower standing still and the real efficiency of the screw. This really has nothing to do with the efficiency except in flying machines built on the vertical screw system. In other machines the thrust must be adapted to the speed at which the machine flies. Some of the writers see that running forward introduces new conditions but they do not seem to have any very definite ideas as to the amount and nature Of the differences. We think we have a method of figuring a screw in action but of course it is all mere theory as yet.1 We will know more about its correctness when we have had a chance to try it.
We note that the memorandum on the Santos Dumont engine places the power at 16 to 18 horsepower. In the Andre' book the size of the piston is given at about 3 1/4 inches and the stroke 4 inches so it is only 111/166 as large as our engine. Our engine develops at the brake 15.6 horsepower and we are convinced that this is very close to what we will be able to reach as a maximum. It furnishes in foot pounds at the brake energy equivalent to 23 percent of the heat units contained in the fuel, so its efficiency is some twenty percent higher than the usual efficiency of gasoline engines which ranges from 14 to 18 percent of the total power contained in the fuel. The Dumont motor may develop more than 10 horsepower but it is with me a case of seeing before believing. The screw of the airship was 4 meters in diameter and had a pitch of 4 meters, and the engine at 1,200 revolutions (which is the speed of maximum power of motors of this size) drove it 200 turns per minute and gave a thrust of 154 to 165 lbs. Now if you multiply the thrust by the pitch divided by the circumference of the screw, and multiply this by the travel per minute of the center of pressure (which is about 5/6 of the circumference of the screw), you will find that the result is about 10 horsepower. Or if you figure the resistance of his balloon at 19 miles an hour it is found to be equivalent to less than 3 horsepower. If we add another horsepower to overcome the resistance of the car, and multiply the result by two to make allowance for an efficiency of only 50 percent of the screw, the total power required to make this speed is only about 8 actual horsepower. The power of the Santos Dumont engine as given in the descriptions is evidently the nominal and not the actual power. All French motors are recognized by American engine builders as being rated at nearly double their actual power. That is one reason why we thought it best to build our own engine. Then we could depend on the weight and power.
That portrait business is a real nuisance, however, if we can get pictures pretty enough to do us justice (?), we will try to send you some within a few days.
I have finished the manuscript of my address and find that it contains about 5,500 words. In delivery I may not follow the manuscript very closely in some places but the matter will probably be about the same in amount.
With kind regards, [&c.]
1 The Wrights' method of calculating propeller performance was mere theory only in the sense that any theory remains a theory until proved by actual experiment. The propellers had already been completed. See above n. 1, Octave Chanute to Wilbur Wright, June 7, 1903.
Orville Wright in later life (Orville Wright to F. R. Cordley, Nov. 13, 1923) announced his intentions of publishing the theory, evolved by himself and Wilbur, on which the design of the Wright propellers was based. Parts of four notebooks were devoted to propellers, but the theory itself was never set forth in so many words. The contents of these notebooks and what can be deciphered of the theory are discussed in detail in Appendix III, The Wright Propellers.
Octave Chanute to Wilbur Wright, June 30, 1903