Letters Between George A. Spratt and the Wright Brothers
(Thanks to the diligent efforts of Mr. Donald C Paulson, who transcribed the following from copies of the originals, we are able to offer the contents of thirty-eight letters between the Wright Brothers and George A. Spratt:)
Sept. 21, 1901
Mr. G.A. Spratt
Coatesville, Pa.
Yours of recent date not yet recd. Has it been written? Perhaps you were waiting to hear from us though. Well after you left camp, conditions which you know were none too pleasant in some respects became even worse, as they were no longer relieved by your funny stories and pleasant company, so four days after you left we also broke camp and returned home. We went up to Eliz. City on a sprit sailboat with Dan Tate & Ed. Had a fine trip of only about five or six hours. When we came to pack up I made the unpleasant discovery that one of my blankets that had lived with me for years in terms of closest intimacy even sharing my bed, had abandoned me for another and had even departed without a word of warning or farewell. Although I regretted to part with it yet I felt happy in the thought that its morals were safe as it was in the company of one who made "character building" rather than hard labor the great aim in life! Mr. Huffaker left Sunday. He looked rather sheepish on departure, which I attributed at the time to the fact that he was still wearing the same shirt he put on the week after his arrival in camp. Well, some things are rather more amusing to think over, than to endure at the time. I enclose a few prints. That of the Huffaker machine you will please not show promiscuously. I took it as a joke on Huffaker but afterward it struck me that the joke was rather on Mr. Chanute, as the whole loss was his. If you ever feel that you have not got much to show for your work and money expended, get out this picture and you will feel encouraged.
We have been quite busy since returning home putting in some new machinery and finishing a new gas engine on which we have been working for some time.
By Mr. Chanute's invitation I was up to Chicago a few days ago to address the Western Society of Engineers on our experiments. His faith in Lilienthals tables is beginning to waver though it dies hard.
Hope to receive a long letter from you soon.
Yours truly,
Wilbur Wright
Oct. 19, 1901
Mr. G.A. Spratt,
Coatesville Pa.
Dear Sir:
Your interesting letter received several weeks ago and read with much pleasure. Am glad that you know where to go to look for your glasses when you need them. Hope you will take a diving bell along when you go after them.
Since returning from Kitty Hawk we have made a number of experiments to determine just how much the Lilienthal tables are off, and find that it is not so bad as we had thought. At 5 degrees they are about 50% too high, that is Lilienthal makes the lift at that angle 64 percent of the total pressure at 90 degrees, whereas we find it only about 40 percent. But Lilienthal also makes the pressure at 90 degrees about fifty percent too high so that altogether estimates made according to his formula are about double the correct figure. At least so it looks to us.
Last month I went up to Chicago and talked to the Western Society of Engineers. My address will be printed with illustrations, in December number of their transactions. Visited Mr. Chanute at his home and found his private office at his residence filled with several hundred kites and models of flying machines and nearly a dozen stuffed buzzards, pelicans, frigates, gulls, hawks, etc. etc. It goes without saying that I enjoyed the visit very much.
Have had no word from Chucky City. But neither has he had any word from us, so it is an even thing. I suppose that you have written to him as you were heard inquiring his address on one occasion (?!) I have never said anything to Mr. Chanute in regard to certain matters connected with Mr. H.-, because I thought that their relations were about ended and it was not necessary. Any how it would come from some one else better than from me.
How are you coming on with your experiments? Have you your machine done? In our recent experiments we found it a good thing to straighten up the wind direction by mounting our instrument in a long trough or square tube, with a window just over the instrument and dividing of the current before it struck the surface to be tested, by means of numerous vertical and horizontal partitions like this.
(drawing)
We made these partitions of sheet iron so as not to obstruct the flow of air. By this means we got a current of almost perfectly uniform direction. Shall hope to have a long letter from you soon as your letters are always read with pleasure.
Yours truly,
Wilbur Wright
Dec. 15, 1901
Mr. G.A. Spratt
Coatesville, Pa.
Dear Sir:
We were pleased to receive your letter and the photograph of your new testing machine. It seems quite ingeniously designed and I think should give good result. As you say the greatest trouble will possibly be with the changeableness of the wind. If I understand you properly, the machine is intended for locating the centre of pressure at any angle (or rather locating the angle of any centre of pressure) and for finding the direction of the resultant pressure as measured in degrees from the wind direction so that the ratio of lift to drift is easily obtained, the lift being the co-tangent and the drift being the tangent of the angle at which the arm stands. Does the machine also measure the lift, in terms of percent of the pressure at 90 degrees so that you can make tables like that of Lilienthal?
I think I told you in my last that we had been experimenting with a "lift" measuring machine. We have carried our experiments further and have made a measurement of the lifts of about 30 surfaces at angles 0 - 2 1/2 - 5 - 71/2 - 10 - 12 1/2 - 15 - 17 1/2 - 20 - 25 - 30 - 35 - 40 & 45 (all degrees). The results have rather surprised us as we find at angles of 7 to 15 with some surfaces a greater lift than Lilienthal gives in his table. Our #7 surface which a rectangle 1:6 with a depth of curve of 1/2 the chord has a "lift" of one hundred and nineteen percent at 17 1/2 degrees.
Lilienthal only claims about 80 percent. But at 3 degrees our measurement is way below him. I will try to send you a blue print showing the lifts of some of the surfaces we have tested. Some surfaces which lift big at very small angles are no good at large angles and vice versa. We have not attempted to trace the travel of the centre of pressure except that by holding some of the surfaces between the tips of our fingers we were able to roughly determine which ones tended to reverse and which did not. It seems that surfaces with rather flat upper sides and thickened front edges lift more at small angles than plain curves and have little reversal of the travel of the centre of pressure. Thickening the front edge does not seem to add near as much to the drift as I expected though it adds some.
(diagram, page 5)
We have found less drift with surfaces 1/20 deep than with curves 1/12 deep. What is your experience ?
Jan 1, 1902
This being January 1st I am beginning a new sheet. The letter was laid aside till the sun should shine long enough to make blue prints, but I had to wait two weeks on it. I also got a photograph of the machine, but it is a poor one. The wind is supposed to be blowing straight from you toward the machine.
In using this machine no figuring at all is required in making a table unless the surface to be measured is of different area from the resistance planes RRRR. We simply set the surface at the desired angle of incidence, then get the reading on the dial of the pointer P; and from a table of sines and cosines get the sine of the angle indicated by P. It is the desired number for our table of "Lifts". We can make a complete chart of the lifts of a surface from 0 to 45 degrees in about an hour. On the backs of the blue prints are shown the shapes and curvatures of the surfaces whose lifts are shown on the charts.
We stopped experimenting about two weeks ago and shall probably not be able to resume them till next Fall as our busy season is about here.
With kind regards
Yours truly,
Wilbur Wright
Dayton, Jan. 23, 1902
Dear Mr. Spratt:
Your interesting letter received. I see from your remark about the "blues" that you still retain the habit of letting the opinions or doings of others influence you too much. We thought we had partly cured you of this at Kitty Hawk. It is well for a man to be able to see the merits of others and the weaknesses of himself, but if carried too far it is as bad, or even worse, than ” seeing only his own merits, and others' weaknesses. In the present case there was no occasion for your "blueness" except in your own imagination. Such is usually the case.
You seem to be having trouble to obtain satisfactory surfaces. Possibly our experience will be of assistance in deciding on proper material. But first you ought to decide on the length, breadth and thickness of your surface. It ought to be as near as possible proportionate in every dimension to the surfaces to be used in large machines. Now the experiments of Lilienthal, Langley, Dines, Maxim, Wenham, and others, as well as our own recent experiments show that the efficiency at small angles is greatest in surfaces having the latitude much greater than the fore and aft dimension. And this agrees with what we find in birds, as the tip to tip measurement is never less than five times the longitude of wing from front to rear, and in the sea birds which live on the wing the tip to tip spread is sometimes twenty times the fore and aft dimension. If you would copy nature your surfaces should have a lateral breadth not less than six times the length fore and aft, nor more than twenty times.
As the tip to tip dimension is determined by the size of your testing machine your surfaces should be twelve inches from tip to tip and from one and a half to two inches the other way. Next thickness is to be considered. Our large machines varied from sixty inches in fore and aft length, to eighty -four inches; while the thickness in the front spar was 1 1/2 inches. The thickness was therefore about 1/40th of the length. As the length of your surface I have assumed to be about 1/2" to 2", the thickness ought to be about 1/20" or 1/30". Now I think surfaces of such dimensions can be made only of steel. That was our conclusion at least, and we made our surfaces of that material with quite satisfactory results. We used surfaces 1" long and the steel was twenty gauge or about 1/32 of an inch. I think that 20 gauge steel would be thick enough for surfaces 2" x 12". This thickness of metal can be easily cut with a good pair of tin shears and you can easily shape the curvature to what you want with a hammer. If you wish to make surfaces with thickened front edges you can add an extra piece of tin bent to the shape you wish the outside to be.
(See drawing, pg. 8)
With a pair of tin shears, a hammer, a file and a soldering iron you can get almost any shape you want. I am sending you four of the surfaces we used in our experiments. When you have examined them all you wish, I would like to have them returned to me as I wish to preserve them.
We obtained our wind current by using a fan driven by our gas engine. The velocity is probably twenty five miles per hour. The pointer fluctuates on the dial about a quarter of a degree while small angles are being measured and somewhat more at larger angles. By gradually applying friction the pointer is brought to rest at the correct point. It gives the same result each time within a fraction of a degree. The possible error in results owing to this trembling is, I think, not over one or two per cent. It gives us little trouble.
Mr. Chanute told me some time ago that he was thinking of recommending you to the Smithsonian for an appropriation and I told him that I thought your machine was O.K. in its principles and method of operation. I hope the application will be successful and that you will have opportunity to carry through a more complete series of investigations than any one has yet made.
We have done nothing in this line for some time and probably will have no more time for it till next Fall. We shall be glad to hear from you frequently, and will gladly give you any information you wish provided we have got it.
Yours truly,
Wilbur Wright
P.S. I learn that you have had, or are to have, a letter from Mr. Huffaker. Hope you will write him a long letter and tell him all about things. When I came to look at our surfaces again I found them rusting, so I put a coat of enamel on them and put them in the oven to bake but forgot about the solder and let them get too hot. It does not improve their appearance any, but as it opened up the joints, it discloses the method of construction very clearly. Hope you will never have such bad luck.
WW
Dayton, February 17, 1902
Dear Mr. Spratt,
Your letter and the surfaces received. Sorry you do not find sheet steel applicable to your needs. I recommended it because it is so stiff, so easily formed into any desired shape, and so cheap. In fifteen minutes any curvature desired can be obtained with no other tools than a hammer and anvil, either a plain curve, or spoon shaped, or bowl shaped. Carving surfaces out of wood is all right, but it is awfully tedious work.
Your choice of 20 ft. x 5 ft. for your large machine is a wise one I think. Our 1901 machine had too great length fore and aft. It was 7 feet, you remember. Your reasoning in regard to two 6" x 12" being equal to one 6" x 24" in lift is hardly sound. The two halves of a broken pitcher are equal in weight to the unbroken pitcher but they will not hold as much water. Your two surfaces will equal the larger one in area but it does not necessarily follow that they will be equal in any other respect. As a matter of fact the conditions are materially different owing to the fact that in the 6" x 24" surface there are only two ends (tips) for the wind to escape around, while in the two 6" x 12" surfaces there are four such ends. Thus there is less loss in the 6" x 24" surface, and its lift is relatively greater. It is probable, indeed almost certain, that a small surface having its length and breadth in the ratio of 5:20 would give results very similar to the full size surface.
I am sorry that Mr. Langley has not adopted Mr. Chanute's suggestion to help you in your experiments with an appropriation. I sometimes think that Mr. L. is a little bit jealous of Mr. Chanute and takes pleasure in slighting him in such a way as this. However it may be that he is honest in his refusal. If he has had many like Mr. Huffaker in his employ I would not blame him for not having much confidence in outside work. How much do you think Mr. H. would do if given a small appropriation for work to be done at home?
You are quite right in saying that a gas engine and tools are an advantage in experimenting. It is possible that in the long run you could do more experimenting in a given time by spending part of your time earning money by running a bicycle repair shop, or something of the kind, and incidentally fitting up with an engine which would come in handy in aeronautical work. In the present stage of the game aeronautical experimenting alone is not a very sure way of earning bread and butter. Except for the time waiting for suitable weather I would prefer natural wind to any fan arrangement.
With best wishes
Yours truly,
Wilbur Wright
Kitty Hawk, N.C. Sept 7, 1902
Dear friend Spratt,
We are back at the old camp at Kill Devil Hills. Except that mosquitoes are lacking things seem very natural. We have put posts under our building and erected an addition 16' x 16'at one end thus making the building 16' x 40'.
Your friends Dan Tate, Bill Tate and Captain Midgett have inquired particularly whether you were coming down and were much pleased that there was a prospect that you would.
Capt. Midgett, of the Lou Willis sails from Elizabeth City for Kitty Hawk every Tuesday night or Wednesday morning. When you come, you should arrange to reach Eliz. City not later than Tues. eve. and immediately inquire for the schooner Lou Willis at the wharves. It usually sails at daylight Wed. morning. The hotel at Nag's Head is closed so no steamer runs there now.
Hoping to see you soon.
Yours truly,
Wilbur Wright
We expect to be here till about Oct. 25th
Kitty Hawk, Sept. 16, 1902
Dear Spratt:
Yours of 9th rec'd. We learned with much regret that there is a possibility that you may not get down here this year, as we had looked forward to your visit with pleasure. Everything is so much more favorable this year than last that it would be a pity to have your ideas of camp life here based on your experience of one year ago. First, we have not seen a dozen mosquitoes in the two weeks and a half we have been here. I have not seen a half dozen myself, we fitted up our living arrangements much more comfortably than last year. Our kitchen is immeasurably improved, and then we have made beds on the second floor and now sleep aloft. It is an improvement on cots. We have put battens on the cracks of the whole building including the addition so it is much tighter and water proof than before as well as more sand proof. Our new well goes down six or eight feet below low water mark on the ocean, (about ten feet deeper than last year) and we now have good water. We also have a bicycle which runs much better on the sand than we hoped, so that it only takes about an hour to make the round trip to Kitty Hawk instead of three hours as before. There are other improvements too numerous to mention, and no Huffaker and no mosquitoes, so we are having a splendid time. The main thing though is the new machine. We have the two surfaces completed and the uprights in place, but the rudder is not yet quite done. It is 32' x 5' spreading an area of 305 sq. ft. altogether. The curvature is about 1 in 25. We had it out making some tests of its efficiency today and are very much pleased with the results of our measurements. The indications are that it will glide on an angle of about 7 to 7 1/2 degrees instead of 9 1/2 to 10 degrees as last year. The drift is only about 1/8 of the weight. In a test for "soaring" as a kite the cords stood vertical or a little to the front on a hill having a slope of only 7 1/2 degrees. (Diagram, p. 24) This is an immense improvement over our last years machine which would soar only when the slope was 15 to 20 degrees as you will remember.
Mr. Chanute is sending down two machines. One built for him by Mr. Herring, and one built by Mr. Lampson (of kite fame). He is expecting to come down himself about Oct. 1st. Mr. Herring will come down to manipulate the Chanute machines.
At present Orville and I are alone in camp. We made arrangements before coming down, to have Dan Tate with us as soon as we were ready to begin experimenting. This is all the force we absolutely need, as we will do little measuring and photographing till later when we have more men. We do not absolutely need a fourth man, yet he would not be an incumbrance by any means, especially if he was as good a companion as I know you to be. If you are really intending to build a large machine you ought if possible by all means to see the three machines at Kitty Hawk this year.
If you find it possible to come down you will need warm clothing and plenty of bed clothes as it may get cool before you get away again.
Yours truly,
Wilbur Wright
Dayton Nov. 12, 1902
Dear Mr. Spratt:
Your letter received and read by Orville and myself with a great deal of pleasure or rather amusement, for I would be sorry to take pleasure in such misery as you experienced in visiting your brother doctor at Kitty Hawk. I believe, though, that a man who would make such a fuss over a few bugs would slap at mosquitoes before they bite! I am interested in your bright idea and what will come out of it. I was not quite certain when you first spoke of it that it would cover all the points involved in fixing the lift of a surface, but you seemed to have the germ of an idea well worth working out, and one which would probably lead to some valuable results. I think so yet. It will probably open a field which will give you something to think about for some time. If any information in my possession will be of assistance to you I will be very glad to furnish it.
I am sending you the copies of our tables of "lifts" or pressures at right angles to the wind, which I promised you. Descriptions of surfaces are on the back. The tests were made in an artificial blast having a speed of about 30 to 35 miles per hour. I will also send you a print of the resolutions of forces and names of forces as we use them as soon as I get a chance to prepare a good blue print.
The three days after you left camp were the best practice days we had during our whole stay. We made about one hundred and fifty glides one day and over a hundred the next. The wind varied from 10 to 16 2/3 meters per sec. so it was no work to get up the hill and no run was required to start. We increased our record for distance to 622 1/2 feet, and for angle to five degrees even for a glide of 156 ft. on two occasions. We, of course, started faster than we landed in these 5 degree glides. The machines real minimum angle of descent is 6 1/2 degrees.
We broke camp at daylight Tuesday Oct 28th and walked to Kitty Hawk facing a cold drizzling storm blowing over 30 miles per hour. A little before noon the wind died down some and the Lou Willis started but at dark had not quite reached North River so the captain ran back and anchored at Powell's Point for the night. The next day the wind rose again and Capt. Midgett would not sail, so we transferred to the "Ray" at noon, and reached Eliz. City at 8:45 P.M. after a very rough trip. We averaged almost one mile per hour for the 36 miles to Eliz. City from Kitty Hawk.
Regarding the ten dollars you enclosed, I will say that we refused to accept any pay from either Mr. Chanute, Mr. Herring or my brother Lorin, for camp expenses so we see no reason to make an exception in your case. Moreover we feel that your help was worth more than your board, so you owe us nothing anyhow. But as I do not wish that money to be the first thing you see when you open this letter, I will send it later. We owe you, not you us.
With best regards,
Yours,
Wilbur Wright
Dayton Dec. 29, 1902
Dear Mr. Spratt:
I received your letter of Dec. 21st a few days ago with great pleasure. I had expected to end the second chapter of my last letter long before this, but have had so many other things on my mind that I have neglected longer than I intended. You will find enclosed the table of "lifts" or rectangular pressures that is pressures at right angles to the wind direction. Also a sheet showing the dimensions and curvatures of each surface. I have not had an opportunity to make any prints of photos since I wrote you last, but will make some of these days and send you some.
We have recently done a little experimenting with screws and are trying to get a clear understanding of just how they work and why. It is a very perplexing problem indeed.
I hope you will hang on to this problem you are on till you get everything clear. There is evidently something there worth knowing, though it may turn out different from what you originally expected.
We are thinking of building a machine next year with 500 sq. ft. surface, about 40 ft x 6 ft 6 inches. This will give us opportunity to work out problems connected with the management of large machines both in the air, and on the ground, such as starting etc. etc. If all goes well the next step will be to apply a motor.
I saw in the papers a short time ago that the Atlantic Coast had been visited by a very severe storm. Wonder whether our shanty is still on its pins or whether it is down in a hole again. I have no word from Kitty Hawk since we left.
With best wishes for a Happy New Year.
Yours truly,
Wilbur Wright
Dayton, February 28, 1903
Mr. G.A. Spratt,
Dear Doctor:
Your letter received and read by Orville and myself with a great deal of interest. Your idea that the effect of curved winds on straight surfaces is equivalent to straight winds on curved surfaces, seems to me to be sound, and so far as I am aware it is entirely new. The only difference I can see in the two cases is that due to the fact that when the wind or path is curved, there is a slight difference in the wind velocity on different parts of the surface due to the fact that some parts are farther than others from the center of gyration. I think you are possibly right in thinking that these curved winds may explain some things which have been puzzles heretofore. I had myself often thought that a wind gust had more power than the increase in velocity alone could account for, and that when the gust died out the support fell off too rapidly. Now your theory may explain this for if the increasing wind has a slight upward curve.
(diagram)
as is undoubtedly the case, and the sinking wind a downward curve
(diagram)
then the first would be equivalent to a surface with greater curvature, while the latter might be equivalent to a flat surface with greatly reduced lifting power. Your chart of falling weights, I do not fully understand as I am not sure which end is which. I am returning it, and request that you draw a line representing the tracing made by a fixed point during the same time. You see I am not sure whether the center on which the plate swings is above, or diagonal or to one side of the chart you send, and I do not just understand which way gravity is pulling whether toward the short or the long side of the sheet. Until I get these points clear in my head I can not get the real bearing of the experiment. I suggest that you try a very much lighter surface and weight sometime. A surface 6" x 6' and total weight of over 900 grams would be equivalent to 1/2 lb per sq ft. and would require a falling speed of 18 ft per second to obtain a sustaining reaction, a speed which it would probably not acquire in less than a second. As your chart probably covers less than a quarter of a second you see the advantage of a lighter surface and slower falling speed. You say "the surface that deflects the greatest amount of air through the greatest angle at any given speed and with the least checking or retarding of currents is superior to its fellows". This is true I think as regards the amount of pressure. But in flight the direction of the pressure is also to be considered for which reason it is better to use surfaces at rather small angles although they then deflect less air and with less change of direction. Otherwise your rule seems good. However, if I understand you, you seem to draw the deduction that only spar length influences the amount of air deflected, and that chord length is of no value. I can not agree to this. It seems to me that the amount of air acted upon is in direct proportion to the chord length other conditions remaining the same. In the drawing, ab has 1/2 the chord length of a'b'. It acts upon a layer of air having a depth of ae which is just half of a'c' which a'b' acts upon. If the wind velocity is equal in each case, a'c' will act upon twice as much air and deflect it at the same angle. The quickness of deflection is also equal, for while e'b' requires twice as much time for the passage of the wind at a given speed as cb the distance the air is deflected is also double, and since the air is deflected twice as far in twice the time, it is deflected once as far in once the time, i.e. the speed of deflection is equal. Therefore since a'b' moves twice the quantity of air and deflects it at the same angle and speed it does twice the work. Its power is in direct proportion to the chord length. Do not misunderstand one however. I do not mean that a surface 2 x 2 is equal to two surfaces 1 x 2. I mean that a surface 2 x 2 is equal to four surfaces 1 x 1. The other is true however if no air is allowed to escape off the ends; but as in long surfaces the escape off the ends becomes excessive it is best in practice to make the ratio of spar to chord at least 6 to 1. Our 1902 machine was 32 x 5 ft. The 1903 is to be 40 x 6.5 which is practically the same thing. We recently built a four cylinder gasoline engine with 4" piston and 4" stroke, to see how powerful it would be, and what it would weigh. At 670 revolutions per min. it developed 8 1/4 horse power, brake test. By speeding it up to 1000 rev. we will easily get eleven horse power and possibly a little more at still higher speed, though the increase is not in exact proportion to the increase in number of revolutions. The weight including the 30 pound fly wheel is 140 lbs. I return the X. The fact that I return it does not mean that I have not noticed and felt the spirit which prompted you to send it. You did not owe it.
With kind regards,
Wilbur Wright
Dayton, Ohio March 28, 1902
Dear Doctor;
Yours 8th rec'd some time ago. I am not certain that I exactly understand you on some points, and while I can see that we do not entirely agree, it is difficult to tell just how far we disagree. It is my belief (1) that the lifts and the drifts of similar surfaces (I use the word similar in a geometric sense, meaning alike in all respects except size) are each exactly in proportion to the areas, and that their ratio to each other remains constant regardless of the area. If a given surface has, at a given angle and speed, a lift of 6 lbs. (2) But for end slip, surfaces having the same depth and shape of curvature would have lifts and drifts in proportion to their areas regardless of the ratio of length to breadth etc. A plane 1 x 4 will have exactly the same lift and exactly the same drift as a plane 4 x 1 if the end slip is entirely prevented in both cases. The only reason why spar length is more valuable than chord length is because the latter increases the length of the ends and therefore increases the loss from end slip. (3) If end slip did not exist the amount of air acted upon would always be in proportion to the chord length because the air would be disturbed to a depth proportional to the chord length,
(drawing page 33)
this is true regardless of angle, and regardless of whether the machine "falls" or not. As the pressures are in direct proportion to the weight of air acted upon, the pressures are also proportionate to the chord length. (4) I can not see that the "time used in deflecting" is a factor in this case, unless you mean by this expression wind velocity. Of course velocity counts; but if the weight of air and the distance it is thrown down in a second remains constant, it seem to me immaterial whether each particle of air acts for a hundredth of a second or whether it acts for a tenth of a second. It is simply a question of "gear" and does not affect the final result. The relative value of spar and chord lengths depends entirely on the angle of incidence. Thus at 0 to 20 degrees a surface 1x4 is much better than a similar surface 4x1; but from 30 to 90 degrees the 4x1 has the greatest lift. This is because at small angles the wind slips off the ends chiefly, while at big angles the slip is mostly off the front edge.
(drawing page 33)
Thus a spar length is disadvantage at big angles. Possibly all this that I am writing to you is just what you have been trying to write me and that we do not understand each other readily because we look at things in different directions and it takes time to understand we are looking at the same thing.
Yours truly
Wilbur Wright
Dayton, April 12, 1903
Dear Mr. Spratt,”
You are quite right in saying that my creed is in part mere theory, yet so long as it leads me aright it is as good as proved facts. When I find it leading me astray, I will drop it. It may need something more tacked on, if so the tack and hammer are ready.
I only recall two laws regarding the elasticity of the air just now. First, pressure is communicated at the rate of 1100 ft. per second, which is a rate so much beyond that at which we are able to strike it in flight that the air may be said to communicate pressure instantly. Second, the pressure or reaction produced by striking the air varies as the square of the speed of striking. Thus double the speed gives four times the reaction, and one hundred times the speed gives ten thousand times the reaction. So far as my knowledge goes there is no ground for believing that the streams of air follow different paths at different speeds, but rather that they follow the same path at all speeds. In our testing machine experiments the wind velocity made no change in the readings. If the increase of speed had caused a change in the path of the air streams it is almost certain that the changes on the inclined curves would have been different from the changes on the perpendicular resistance surfaces and the indicator would have given a different reading. If this assumption regarding paths is true I see no theoretical reason for expecting the reactions to follow any other law than that of the square of the velocity. However I am open to conviction, as I know that an ounce of fact outweighs a pound of theory. We must be sure though that we apply the facts right, or rather that we do not misapprehend what the facts really are, and we must adapt them to the other mans method of looking at things. You say it makes a difference whether surfaces it the air quickly or not, now I would state the same thing in another way, viz the reaction varies as the square of the speed. Your falling plane at first struck the air at a speed equal to that of the plane alone, but after the air had been set in motion the conditions were different and it struck the air with a velocity equal to the difference in their actual speeds so the resistance was less for a time than at first. (written addition to the side) This explanation is not quite correct. It is the formation of a descending column above the surface which rushes down and fills the vaccuum on the upper side that reduces the pressure. The conditions in front remain practically the same or rather obeyed the law V squared all the time. As I see it the remedy is not to try to strike the air quicker, but rather to get a hold upon some air not already set in motion. The loss comes not from the air giving way under you, but from grabbing hold of air for your next stroke that is in downward motion before you attack it. It is just like rowing a boat up stream, you lose both the slip of the oars and the flow of the current. These losses are, however entirely different things from each other. In flying, the machine continually moves forward on to fresh air and so avoids loss from downward currents created by itself.
I still fail to understand why you say that a surface with a great spar length strikes the air quicker than one nearer square. It looks to me as if twenty miles an hour is twenty miles an hour regardless of the shape of your surface.
Mr. Chanute left America about the first of January for a trip through Egypt, Italy, France, Germany and England. He was expecting to reach home about the first of May.
As ever your friend,
Wilbur Wright
Dayton April 20th 1903
Dear Doctor;
Yours of 15th rec'd. I see that you are back at your old trick of giving up before you are half beaten in an argument. I felt pretty certain of my own ground but was anticipating the pleasure of good scrap before the matter was settled. Discussion brings out new ways of looking at things and helps to round off the corners. You make a great mistake in envying me any of my qualities. Very often what you take for some special quality of mind is merely facility arising from constant practice and you could do well or better with like practice. It is a characteristic of all our family to be able to see the weak points of any thing, but this is not always a desirable quality as it makes us too conservative for successful business men, and limits our friendships to a very limited circle. You envy me, but I envy you the possession of some qualities that I would give a great deal to possess in equal degree. Regarding the matters on which you have asked my advice I must confess that I am at a loss just what to say. If you ask me to give advice of the best method of turning hand springs or some other trivial matter I would gladly give you bushels of it, but regarding a matter which might affect the whole course of a mans life, I almost fear to give any, lest injury might result from it, instead of good as intended. I suffer the consequences of my own mistakes with some composure, but I would hate awfully to see some other person suffering from an error of judgement of mine. Nevertheless, I have a great desire to see you succeed and if you feel free to communicate the matters you have in mind, I will promise to do what I can to help you either with advice or with assistance in obtaining help to carry forward the work, provided of course that the matters communicated are in my judgement meritorious.
Regarding the matter of the advisability of immediate publication, and of the policy of communicating your ideas to any other person interested in the solution of the flying problem, you alone are in position to give judgement. So much depends on the nature of the discoveries you have made, and the completeness with which they are worked out. If the ideas are complete and merely demonstration is needed, you would probably be safe in communicating with any honorable person; if you have merely the germ of an idea which might develop in unexpected ways, more would depend on the character and inclination of the person chosen as your confidante, and such would depend on whether or not it was possible to proceed absolutely alone. Stripped of superfluous words my advice would be “Do what you yourself think is best”. Orville and I were very sorry to hear that your health is not good, and hope that you may soon be better. You should by all means avoid laying awake at nights studying out problems, as you did sometimes at Kitty Hawk. We hope that other maters will right themselves in time.
I have a letter from Mr. Chanute who had reached Paris, stating that he expected to reach Chicago about May 7th.
Truly your friend”
Wilbur Wright
Dayton April 27, 1903
Dear Friend Spratt;
Yours of 24th with manuscript rec'd. It was not my intention to advocate dishonesty in argument nor a bad spirit in a controversy. No truth is without some mixture of error, and no error so false but that it possesses some elements of truth. If a man is in too big a hurry to give up an error he is liable to give up some truth with it, and in accepting the arguments of the other man he is sure to get some error with it. Honest argument is merely a process of mutually picking the beams and motes out of each others eyes so both can see clearly. Men become wise just as they become rich, more by what they save than by what they receive. After I get hold of a truth I hate to lose it again, and I like to sift all the truth out before I give up to an error.
There are many points in your letter I would like to discuss but will scarcely have room in this letter so I will proceed to the paper. I have read it over several times carefully but have not fully grasped all of your ideas yet. It seems to me that it would be of advantage to make the paper a little fuller so that the reader will not have to supply anything himself in order to grasp the points clearly and fully. My judgement would be that in a paper of this kind where so many entirely new laws and principles are enunciated you ought to give sufficient explanations to make it impossible for any one to fail to understand and impossible to doubt the truth of the laws stated. As one illustration of my idea of what such a paper should be we might take the subject of the superiority of curved surfaces over planes. The idea that curved surfaces were better than planes was hundreds of years told before Lilienthal first explained why they were better and how much better and proved it, so that what before was mere conjecture and without any particular value to the world, at once became a solid part of the worlds' knowledge and Lilienthal is justly entitled to the credit of being the inventor or discoverer of curved surfaces. Likewise Darwin merely offered the proof and explanation of the origin of species, the idea was old. The point I make is that you should not merely announce your ideas, you should also accompany the announcement with such explanations, arguments and proofs as will leave no place for a Lilienthal or a Darwin. In reading over this paper I have tried to place myself in the position of a stranger reading a paper on a subject largely new to him, and have noted some points that raise questions or doubts and seem to need further elucidation.
Page 5 (x1) Make a drawing showing second position and explain a little more fully what is meant by "each will rotate on their pivots the same number of degrees." Also show the second position of the machine with the surfaces transposed.
Page 5 (x2) Explain fully just how the centrifugal forces are to be equalized in making the experiments, and also how the inertia of the surface moved in the curved path is corrected.
Page 6 (x3) Explain more fully why the curved wind against a plane is exactly equivalent to a curved surface against a straight wind. Since all matter tends to move in straight lines the question may be raised whether the curved wind after striking the plane will rebound in a straight line or in a curved line. It must rebound in a curved line to make the conditions similar.
Page8 (x4) The words "any longitudinal section or number of sections give the same response" are somewhat ambiguous. In what position are these sections mounted? Explain more fully "why" and "how" it demonstrates that "the forces of rotation etc are focussed at the center." Explain fully why "we may conclude therefore etc". In a paper of this kind it is important to meet any question that can possibly be raised. The statements made in this part of your paper are very important and should be made more prominent by fuller discussion.
Page 8 (x5) Are you sure of the truth of this statement? Of course I have not had opportunity to try it as yet, but it is a most remarkable fact if true. Does not the length of the string make any difference? It should be explained more in detail how it shows that the forces are focussed at the center. Very few of your readers will do any thinking for themselves. They will follow you, but will not take a step beyond where you stop.
Page 8 (x6) I do not clearly understand just what you mean by the words "the lift is shown". Do they mean something in regard to the dials shown in the drawing?
Page 9 (x7) This statement is very important. You should therefore give your process of reasoning, and the exact data of some of the experiments showing its truth, so that conviction may be carried to the mind of the reader.
Page 9 (x8) I am not certain that I understand this paragraph at all. Can you not make it a little clearer? You must understand that many of your readers will be unaccustomed to your methods of thought and expression and will fail to grasp points that are clear and obvious to you.
Page 9 (x9) Explain more fully what is meant by "fixed quality".
Page 9 (x10) Is the word "always" to be limited in its application in any way?”
Page 10 The statements regarding drift are very important. You should therefore forestall possible denial of their correctness by giving a pretty full statement of the reasons, and the data of the experiments on which your conclusions are based.
(continuation of above letter)
Dayton, Apr. 28, 1903
I was expecting to get this letter finished last night but did not, so I add a few words tonight. Orville and I expect to go to Kitty Hawk again this summer or Fall. We have designed a 500 sq. ft machine and propelling mechanism and have a large part of both already constructed, but much yet remains to be done before we are ready for our trip. We would be very loath to take up any thing new that would prevent us from carrying through the experiment we are already started upon. We would be very glad to give you such assistance as we are able to give, provided it did not take too much of our time, or interrupt our own work. Have you tested all the important features that you intend to incorporate in your large machine, by means of testing apparatus or small models? Whenever any feature lends itself to this mode of testing it is economy to try it before going to the trouble and expense of building a machine on a large scale. At all events we will expect to see you in Kitty Hawk when we go into camp this year.
Yours truly,
Wilbur Wright
Dayton, May 6, 1903
Dear Doctor.
I have yours of 4th May. Your statement regarding the effect of winds on a cylinder or longitudinal sections of same when it is pivoted on an immoveable point at its center, I entirely assent to (although I have not tried it) because it seems reasonable. However I think my own way of stating the case would be a little different from yours, in view of my different point of view, or way of looking at it. Since it is impossible to apply force to a frictionless surface in any direction except vertically to the surface it follows that in a cylinder or it's segments no force can be applied except toward or from the center. In your sentence "the forces of rotation are focused at the center", are not the words "of rotation" entirely superfluous? However this is simply a difference in definition; the idea remains the same.
What connection all this has with practical flight is a complete mystery to me. You must think me very blind but for the life of me I can not see how the principle is to be utilized in view of the seeming fact that the only way to get an immoveable point at the center of the curvature of the surfaces would seem to be by making the curvature equal to that of the surface of the earth and hanging the weight at the earths' center. But in that case the weight would merely turn round and round and would never get anywhere no matter how far the surface traveled. So you see I am quite a good distance from stumbling on to your application of the principle which you feared would be so obvious.
You will find my opinion of the relative lifts of arcs and hyperbolas by comparing our #9 surface with our #12 surface, at 2 1/2, 5, 7 1/2, 10, 12 1/2 degrees. It depends on the angle.
(drawing)
The experiment of which you sketch is about as annexed still seems to me most remarkable as it would, if true in the sense I have understood it, conflict with what seems reasonable to me. If the string be of infinite length it would of course be practically parallel with the course of the wind, and in order for it to point to the center of curvature of the surface, the surface if shallow would have to present its convex side almost at right angles to the wind as in fig. 2
(drawing)
It may be that that is just what it will do but at first sight it seems almost preposterous. However having stopped a moment at this point to consider the case further I am not so sure that the string will be parallel to the wind for the pressure on the concave side will be principally on the front edge and this will tend to turn the surface at so strong a negative angle that the pressure on the convex side will much exceed those on the concave side and the entire surface will therefore be forced toward the concave side and draw the cord in that direction, as compared with the wind. The more I study the situation the more reasonable your statement seems, and I get back to the point that all pressures upon a surface must be normal to the surface at the point of application, consequently the cord should be normal to the surface and should therefore form a continuation of a diameter of the cylinder. I think I shall try the experiment tomorrow if I can find time.
The reasoning of Lilienthal on the subject of why curves are better than planes as I understand his drawings (I have not read the text as it is in German) is about as follows.
(drawing)
The wind each molecule hits the plane once and bounces off it at a big angle and would never touch it again but in bouncing off it gets tangled up with, and gets in the road of other particles of air until they waste their force on each other and dam up in front of the surface and forma sort of wedge of air which splits the on coming current and prevents it from exerting its power as it should, since part of the air is thrown upward and part downward and none of it reaches the surface at full speed. But in the curve the air hits the front edge and bounces off at a slight angle till it hits again and again like the sides of a polygon in a circumscribed circle. As the change is so gradual there is little tendency for the air to be churned up or form a wedge and the full power is therefore exerted. His explanation of the cause of negative tangential in curved surfaces is that since all pressures are normal to the surface and since the pressures toward the front are stronger than toward the rear therefore the average or resultant pressure "R" must be inclined in front of "N" a line normal to the chord. These may not be his exact methods of explanation but I think they are substantially so.
Yours truly,
Wilbur Wright
Dayton, May 6th
Fig. I Straight wind striking curved surface and bouncing off in straight lines time after time.
Fig. II Curved wind striking straight surface and bouncing off in curved lines time after time.
Yours truly,
Wilbur Wright
Dayton, May 15, 1903
Dear Doctor:
Yours of 9th rec'd, I have been very busy and hove not had a chance to try the experiment of the floating segment of a cylinder, but on further study of the situation I am quite certain that it will give the result you obtained. I really do not see now how it could be otherwise though it struck me as very strange at first announcement. Regarding the question of whether forces of rotation do or do not exist, it may be that it is rather a matter of definition than anything else that causes our lack of agreement. I therefore suggest that in your paper you define as clearly as possible just what you do mean and what you do not mean by this term. I think your use of the term "focussed" is very good, and really better than "equalized".
In your letter you say "If such a surface (=1/4 cylinder) be
(diagram)
used with an arrangement to pivot the string at intervals along the diameter" etc. I really do not understand how the string can be pivoted as described. I can understand that if the section of cylinder be mounted on a cork say as large in diameter as the circumference of the cylinder and pivoted at the point C, the surface will stand still because the pressure zc and cy are at right angles to the arc af and therefore have no component tangent to the arc.
(diagram)
But if the surface be pivoted at B the force cy will create a turning force tangent to the smaller circle whose center is B and zc will create a turning force tangent to the larger circle and in the same direction. Therefore the surface will duck forward. But if the surface be pivoted at B' the forces will act on the opposite side of the circles and the surface will move backward toward f.
(diagram, Fig. 2)
In the second figure where all the pressures are from the center outward the surface will move forward (if pivoted at B) whenever cy is stronger than cz and will move backward if cz is stronger than cy. If pivoted at B' the motions will be in the opposite directions. This is my idea of what you probably mean by the experiment of pivoting the string at intervals along the diameter oc. Is this correct? I mean, is it the same in theory?
I am unable as yet to understand the application of this to the practical case you mention i.e. the death of Lilienthal. The conditions seem so very different. In your experiment c is a motionless point, while in Lilienthal's case gravity was a strictly vertical force which constantly shifted its position horizontally. So AXW far as I can now see the case shown in Fig. 1 would result in a duck forward no matter whether Lilienthal were at B or C or B'. It would apparently depend on whether the center of pressure of the surface was in front or behind a vertical line drawn through the center of gravity. Since gravity is a strictly vertical force, the center of gravity would always seek a point directly under the center of pressure or vice versa the c. of p. would seek a point exactly above the c. of g. no matter how far the weight hung below or above the center of the arc.
You have confused our lift testing machine with the tangential measuring machine. The one measures the amount of pressure compared with a uniform standard (a sq. plane of equal area in equal wind). The other machine shows only the direction of this pressure. The #2.5 surface at 0 degrees lifts more than #7 because it has equal upward pressure on the underside, and less downward pressure at the front of the upper side. (diagram, p. 53)
If you will chart a few of our surface tests, say #7 and #10 and #20 and then when you find the lines on the chart taking unexpected courses, make a drawing of the surface and draw a line showing the direction of the wind at the angle of incidence you are dubious about, you will in every case I think find from your study of the way the wind strikes, especially at the front, a full explanation of why the lines on the chart take such peculiar directions. A surface with a thickened edge presents its upper side at a greater angle of incidence than the lower. Its effect is therefore equal to that of the average angle approximately.
Lilienthals explanation of the manner in which the air acts upon various surfaces has been visually proved by Prof. Marey (sp?) whose photographs of smoke streams you saw at Kitty Hawk two years ago. Do you not remember the photos Mr.Chanute had just received and showed to us? Some of them were published in the Scientific American in Feb or March 1902. Obermayer a German by a different process also obtained photographs of the course of the air streams, and Von Lossel by holding candles in front of surfaces was able to trace the direction of the streams by the direction the blaze was turned.
Your demonstration of how a curved wind striking the flat surface is equivalent to a straight wind striking the convex side of a curve is all right because in this case all the streams can bounce off in straight lines after hitting once. The case I was cautioning you to guard was that where the wind strikes the concave side and bounces time after time before it gets away from the surface for good. The similarity of the curved wind on straight surface is less obvious in this case.
I think you will do well to keep hatching at your egg till you see what does come out of it. Even if you fail to get the chicken you expect, it may be all right anyhow. Turkey is just as good as chicken. Your experiments are very original and will attract attention if you are careful to bring them within the comprehension of the ordinary student.
As ever your friend,
Wilbur Wright
Dayton May 24, 1903
Dear Doctor
I see that in some respects we are in agreement and we do not agree at all. We agree that a segment of a cylinder will not move if pivoted at center of cylinder upon a fixed axle. But when we come to consider flying conditions you see similarities where I can see none. I tried the experiment you suggested but as I used a surface of the lightest possible material instead of tin, and used a heavy chunk of metal for the weight in order to reduce its resistance below that of a nail placed transversely, the results were quite different from those you described. I think that in your experiment the center of gravity and the center of pressure were both too near the center of the entire structure.
(drawing)
I suggest the following method of studying the problem.
(drawing)
Cut a piece of paste board or time to the shape of a quarter circle and pin it against a wall by driving a nail at the center c. Drive other nails at P P etc and connect the nails to the perimeter of the circle at the points a a by rubber bands. Now it makes no difference whether the rubbers are stretched equally tight or not, the arc a a will not rotate. Neither will it rotate if all the rubbers are cut but one, no matter which one is left. This is the case of your cylinder pivoting at the center. If we keep clearly in mind that there can be no action without a reaction, and that they must be exactly equal, and exactly in opposite directions it is clear that while the cylinder segment is exactly in equalibrium in all directions, the center c is under unequal strains and will tend to be moved to right or left according as the right or left rubbers are drawn tightest. As the point c is capable of greater resistance than the force tending to move it the whole structure remains motionless regardless of differences in strains and regardless of how many or what rubbers are cut.
But if a weight is substituted for the pin at c the arc a will immediately change its inclination unless all the rubbers pull equally. If all but the center rubber be cut the arc will still be motionless, but if all but one of the others, be cut, the arc will tilt up till the weight, comes in a vertical line with the point of suspension. The dotted lines show the position at the moment the rubbers are cut; the solid lines the new position it takes. If two rubbers are left uncut, but one with greater strain than the other the structure will take a position with the resultant vertical and the weight will fall in line with it. If the weight had been hung at a point a little above or below the center c the tilting would be in the same direction. The principle involved is this that since the only reaction in the weight which pulls vertically downward, therefore the only action that can be exerted upon the surface must be vertically upward and the lines of the two forces must coincide. If the pressure on the surface is at first in some other direction the surface must rotate or tilt till the resultant pressure becomes vertical, and even if vertical it must also be in line with the weight. Thus a surface having a vertical pressure at the point y will not be in equalibrium unless the weight z is moved to z' thus bringing g into line with y and x.
(drawing)
The weight must move or the surface must tilt. The distance from the center does not prevent the tilting, but slightly afects its force. The tilting force increases with distance from surface. If the weight had been at y there would be no tilting if the force xy had been applied. Applying these results of this system of experiment to your floating cylinder, the conclusion is that while there is no force tending to rotate the cylinder around the center, there is a force tending to rotate the center around the cylinder segment and in practical flight the effects are similar because a weight pulling vertically gives no horizontal reactions to prevent it from starting to rotate around the wings. It is the experience of both birds and men that a center of gravity about on a line with the surfaces gives the best results. The buzzard hangs below and pays the penalty of its laziness by its unsteady flight as compared with hawks. As I understand the Lilienthal catastrophe he was floating along when a wind gust with a upward trend struck him and caused the center of pressure to move slightly forward (arched surface with larger ang. of incidence) this caused the machine to turn up in front and loose headway the reduced speed made the angle of incidence still greater and moved the c of pressure still further forward. He moved forward to the limit to turn the machine down in front. But the gust suddenly dying out with a downward trend the center of pressure suddenly moved way back far beyond the point to which he was able to move his body backward, so the surface tilted forward with a "bang" and probably broke the stop which prevented the tail from falling below a certain point. (It was free to fold upward you remember). In this position the speed increased rapidly, and with the smaller and smaller angle of incidence the c of pressure moved farther and farther back and turned the surfaces over more and more till it turned entirely over and struck the ground with frightfull velocity partly upside down.
(drawing)
Yours truly,
Wilbur Wright
Dayton June 7, 1903
Dear Mr. Spratt:
While I am aware that the shock of receiving a letter from me is apt to bring on a fit, yet I assure you that my writing is with no "design" on your life, which you mention in your letter to Wilbur. We both take great interest in your letters, and my not writing to you is not from a lack of interest in what you are doing, but rather from a lack of ability as a letter writer. Will seems to enjoy writing, so I leave all the literary part of our work to him. But I see that he has failed to make you understand exactly what our ideas are on some of the points that have been under discussion.
In the first place the term "force of rotation" has been used so loosely that I think a great deal of the misunderstanding has been due to it. I think in his letters that Will has used the term "force of rotation" sometimes where the term "force of revolution" would have been better. To avoid any misunderstanding on this point, let us use "force of rotation" as meaning a force which tends to turn a surface on some point within the surface itself (as at the point a for instance) and the "force of revolution" as a force which tends to turn the surface about a point outside the surface itself, as at b.
(drawing)
We can not agree with you on some of the points under discussion, if our understanding of what you mean is correct. We seem to be agreed on the point that there would be no tendency for a surface to revolve about the point (or line) in which all the forces acting on the surface focus, however much these forces may vary or differ in intensity. In figure II. there would be no tendency for the surface to revolve about the point b, although the force bc be twice that of bd. We will let be represent the direction of gravity, which never changes. Now, if all the forces acting on the surface be resolved into one force acting in the direction bg, there would be no tendency for the surface to either rotate on any point within itself, as at a, nor to revolve about the point b, at which all the forces focus, when the force bg happens to exactly coincide with the line be (gravity); but if the result of all the forces acting on the surface should be in any other direction than that of be (as bh or bi) then the surface would have to rotate on some point within itself until the line bh (or bi) again coincides with the line be in direction,