A down-to-earth assessment of the Wright Brothers' competence in air-screw design
by Paul Jackson
Editor-in-Chief, Ret, Jane’s All the World’s Aircraft
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| Fig 1. William Shakespeare was “a man more sinned against than sinning”
(King Lear Act 3, Scene 2). Over-hyped by fawning supporters he, sadly,
has to be taken down a peg in the cold light of digitalized history |
Wherever their skills permit it, traditional aviation historians employ Shakespeare-like prose to describe, with all the eloquence at their disposal, the inventions and achievements of the Wright Brothers. That is most apt, for the Bard of Avon was as fecund in the realm of the written word as were Orville and Wilbur in developing all aspects of the airplane.
Shakespeare has been credited with coining 3,200 words in the English tongue, plus at least 150 more expressions in daily use. Truly, he was a prolific and popular playwright.
Prolific and popular. Perhaps the shortest English sentence to be an enigmatic oxymoron. Why? Consider baffled Elizabethan audiences leaving The Globe theatre, grumbling that idiot Shakespeare had made it impossible for them to follow the plot because of all the invented words and phrases of obscure meaning littering the script. Why didn’t the fool write in English?
Of course, he did. What has happened is that modern researchers tasked with discovering the origin of any English word have traced it back through literature until they found it in Shakespeare — whereupon they declared that, because the man was genius, he must have invented it. Therefore, there is no need to look any further back.
The Internet has much to answer for in its debasement of scholarship, but it is highly efficient in exposing lazy scholars. The gradual digitalization of pre-Shakespeare literature in recent years has made it possible — in a search taking a few thousandths of a second — to pinpoint a given word in writings from before the Bard’s birth.
Thus, those 3,200 words credited to Shakespeare have shrunk, within the past few years, to 1,700, and the tally is diminishing with every passing month.
Similar exaggerations of the Wright Brothers’ prowess are also being exposed by freshly digitalized documents, but the process is being fiercely resisted by aviation historians who hope to emulate another significant figure in English history: King Canute, resister of the incoming tide.
Whereas it is not an offense to rob Shakespeare of the credit for a word it is now known he borrowed, the home of what purports to be the original Wright Flyer, the Smithsonian Institution in Washington, DC, has a serious legal problem with documents which prove the wrights did not do, or invent, something they claimed they did.
The covenant of November 1948 with the Wright family forbids the Smithsonian from publishing anything which disproves the Wright 1903 Flyer was, “The World's First Power-Driven Heavier-than-Air Machine in Which Man Made Free, Controlled, and Sustained Flight” and that, “By Original Scientific Research the Wright Brothers Discovered the Principles of Human Flight” and “Taught Man to Fly”.
Refusal to acknowledge the relevant pre-Wright documents was not the result of laziness, as was the case with Shakespeare’s writings, but of a legal imperative: Contradict any of these assertions and the Flyer gets removed from the building.
The Old Guard’s problem is that now, any literate child, working at a computer in their bedroom, can download, from the Internet, date-marked documents published by learned bodies which shatter claims of Wright primacy in more than one field.
A prop for the Wright legend of brilliance
The Hartzell company is based in the Wrights’ home state of Ohio and is one of the world’s leading manufacturers of propellers. Surely, it — of all authorities — can be relied upon to present an accurate, unbiased assessment of the chronology of airplane propeller development. Let’s see what its website (http://hartzellprop.com/wright-brothers-propellers) has to say on the matter:
“In the late 1800s, several flying machines emerged from early pioneers who based their propellers on screw-shaped design. But it was the Wright brothers who were the first to acknowledge that an aircraft propeller should be shaped more like a wing than a screw.
“The two brothers reasoned that propellers could act like rotating wings spinning through the air. The idea was that the rotating propeller blades would act as “airfoils” (wing shapes) that produce a pressure differential, displacing air backward to produce forward thrust.
“Using data from their wind tunnel experiments, the Wrights created an efficient propeller design modeled after one of their wing shapes. They then set out to invent a propulsion system that utilized a small engine and two large, slow-turning propellers.”
There; as plain as day: The Wright Brothers designed the world’s first efficient aerial propeller. They did so in their wind tunnel, which was put into service (according to the Smithsonian) in October 1901. The modern propeller was invented in Dayton, Ohio, in 1902 or 1903. The first such propeller was tested, at sub-scale, on a motorised rig at Dayton on December 15, 1902.
Don’t bother with Wikipedia’s history of the propeller; it is exactly the same: “...the Wright Brothers realized that a propeller is essentially the same as a
wing, and were able to use data from their earlier wind tunnel experiments on wings, introducing a twist along the length of the blades.”
Surely, an impossible fact to miss
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| Fig 2. Sidney Hollands described the attributes of an efficient, modern
propeller two decades before the Wrights’ failure to build such a device
(via Royal Aeronautical Society) |
But let’s use the wonders of modern digitalization of historic records to go back in time two decades before “the Wrights invented the propeller” in the wind tunnel at the rear of their cycle shop. Back to London, England, in June 1885, where the Aeronautical Society of Great Britain is holding an exhibition of the latest in aviation science in the Crystal Palace (which was originally built to house the 1851 Great Exhibition).
We know that a certain Englishman, Sidney Herbert Hollands, exhibited an airplane propeller of revolutionary design at that event. This was recorded in the Society’s archives by Baden Baden-Powell (sic), editor of the house magazine, Aeronautical Journal (and brother of Robert, later founder of the Boy Scouts) but, by an oversight, full details were not bound into the Society’s journal of proceedings.
But all is not lost, for a copy of the report crossed the Atlantic and landed on the desk of the doyen of US aviation pioneers, Octave Chanute. In February 1893, Part IX of Chanute’s book Progress in Flying Machines (downloadable at http://invention.psychology.msstate.edu/i/Chanute/library/Prog_Aero_Feb1893.html) had this to say:
Hollands however, made some experiments on the best form of lifting screw-blades, and stated that he had found it advantageous to make the fan blade concave on the driving or lifting side, and that the angle of maximum efficiency was 15° with the plane of motion at the tip and 30° at the root.
The form which he found most efficient was two-bladed; with the blades narrowest at the tips, slightly concave on the lifting side, the tip slightly drooping, each blade being approximately the shape of an elongated shallow spoon or scoop, and with a pitch equal to about two-thirds of the fan's diameter, giving a mean angle of blade of 22° 30' with the plane of motion.
These blades were of thin sheet steel, and their forms will be noted as confirming what has already been stated as to the advantages of the bird-like form of wing. M. Hollands said further:
I find another advantage accrues also from the use of these very thin, sharp edged hollow blades — viz, that there is no appreciable resistance to rotation that does not contribute to lifting effect.
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| Fig 3. A selection of typical, modern propellers, built for light
aircraft by the Hercules company in the UK, home of its designer,
Hollands |
Let’s just recap. A cambered blade; wide at the hub and narrowing towards the tip; the angle of leading-edge incidence progressively reducing towards the tip. Doesn’t that describe a modern, efficient propeller? Chanute was a mentor to the Wrights and they were actively seeking out literature to assist their quest for flight. Does anyone, seriously, suggest that Hollands’ researches did not come to their attention, by active or passive means?
And Hollands’ ideas yet again crossed the Atlantic, but in first-hand form, when the Scientific American published his article, ‘Wind Motors, Ancient and Modern’ in its Supplement of September 1, 1894.
Wright Brothers: innocent bystanders
At this point, regular readers of this blog will be expecting so see a scathing denouncement of the Wrights for stealing Hollands’ ideas — in the manner they appropriated many others. Nothing could be farther from the writer’s mind. Together with the rest of the aeronautical experimenters, the Wrights foolishly ignored Hollands’ careful researches and pressed on with their own, inefficient designs.
No particular criticism can be made of the Wrights for not realising the significance of Hollands’ writings, because all the others failed to take them up, as well. But the Wrights have been elevated by their own boastfulness and the sycophancy of others into a category head and shoulders above “all the others.” Now it appears they were not as smart as they, or their biographers, made out.
To give him his due, Hollands kept trying. In another magazine, he co-authored with G Lacey Hillier a series of aviation articles, saying in Part III
“With further reference to aerial propellers, one of the present writers has found by comparative experiment that a very advantageous feature of design is to make the blades concave on the driving side (and, therefore, convex on the other side), which form really amounts to extending the lifting form of the aerocurve to propeller blades, which latter have been hitherto made flat.” [an ‘aerocurve’ being a lifting surface incorporating camber]
And then, in Part IV, referring to a series of comparative experiments with different propeller designs and numbers of blades:
“4. That it is very advantageous to make the blades with a certain degree of concavity on the driving side* (or ‘conchoidal’), and therefore convex on the other or advancing side.
5. That there is a distinct advantage in making the blades narrow at the tip, and broadest near the root, which form is quite contrary to previous aerial practice in aerial fans.”
[*The term “driving side” might be a little confusing to some. The reason Hollands and almost all others of his time refer to the concave back of the blades thus, is that they thought a cambered surface created lift (or thrust) by building up a pressure on the bottom, or concaved side. They were largely unaware of the dominance of a lower pressure on the convex side, sucking the aircraft forward. In spite of Phillips and Lilienthal publishing fairly correct explanations, the Wrights maintained their mistaken belief until well after they developed their airplanes.]
Part IV includes comparative diagrams of the Hollands pointed-tip propeller and the alternative design with its narrow root and wide tip. For two propellers, each of 10 feet in diameter, the Hollands has its centre of mass at 4 ft 2½ in diameter and centre of pressure at 4 ft 7½ in; for the wide-tipped propeller, the numbers are 6 ft 6½ in and 6 ft 4 in, respectively.
Comments Hollands:
“This reduction of the radius of the centre of mass not only reduces the centrifugal pull of the blades, but the centre of pressure being correspondingly reduced, the bending moment of the arms, and consequently the pull on the back stays becomes much less. Both these conditions conduce to lightness of construction.
“Because of the structural advantages, the reduction of radii of the two centres reduces the necessary torque, or turning effort. After all, it is only logical to make the blades narrow towards the region of the highest velocity, i.e., at the periphery, and, of course, a regularly increasing area towards the root where the velocity of rotation is least."
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Fig 4. Hollands’ 1901 comparison (simplified and further annotated)
of two blade shapes for a 10-foot propeller: his own (left) and the
broad-tip variety employed (although in what was then the future) by the
Wrights. On the left, the bending arm of the center of pressure (P) —
which is trying to snap off the blade — is near to the prop’s axis. On
the right, the leverage of P is enhanced and the blade is more highly
stressed. And this was before the Wrights added even more mass to the
tips of their later props, worsening the situation. Adjacent to P is M,
the center of mass; the greater the distance of M from the rotational
axis, the more horsepower is absorbed just in turning the weight of the
prop. So, the narrow-tip prop scores on both counts |
Apologies, good reader; you have not been provided with references for these two articles. They appeared in January and April 1903 in the UK quarterly magazine, Flying. The Wrights probably saw at least the January 1903 edition, as it also contained a four-page, second-part report of Wilbur’s talk to the Western Society of Engineers.
The articles were published as the Wrights were putting the finishing touches to their patent application (submitted March 23, 1903) and starting manufacture of the Flyer. So, as a further recap: cambered blades with narrowing at the tips and progressively reduced twist, outboard. Do the opposite, and more horsepower is needed to turn the prop; and the blades themselves are subjected to a greater force trying to snap them off backwards.
And, in the light of these vital and timely revelations, what did the wizard Wrights do? This:
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| Fig 5. The Flyer’s propellers at Kitty Hawk in December 1903. Twist,
yes; camber, a little; narrowed tips, no way. An improvement on some
others’ designs, granted; but a pitiful effort compared with what
“aeronautical geniuses” should have been achieving at that time |
America is not entirely without honors in the matter, though, for on February 10, 1901, Augustus Herring had written to Chanute reporting encouraging results of “experiments with curved surfaces and screw propellers with straight & with curved blades.” The following year, Chanute brought Herring to the Wrights’ gliding camp at Kitty Hawk, but it is unclear whether the latter had any influence on the Flyer’s prop.
Historians dazzled by the Wrights’ self-publicity feel obliged to give Herring a poor write-up and stress the later animosity between the two. But as Herring’s letter shows, he was closer to the optimum propeller formula in 1901 than were the Wrights at the time.
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| Fig 6. Herring tells Chanute he has been experimenting with both
cambered and what, today, would be called ‘scimitar’ blades. The date
was February 1901 — nine months before the Wrights even began
experiments in their wind tunnel |
And also was Hollands closer to the optimum propeller formula than he (or any others) realized. Nobody seemed to be taking into account another spin-off resulting from the propeller-is-a-wing-on-its-side concept. The ideal lift distribution across a wing from an induced drag standpoint is for lift to taper off near the tips — a concept which the Spitfire employs par excellence. This reduces the amount of swirling flow from the bottom of a wing around to the top, which destroys most of the lift at the tips; thus the proliferation of “winglets” on modern airliners.
The aerodynamic advantage of tapered tips is every bit as important as the mechanical bending moment advantage that Hollands did describe for that shape.
Cranks
Two basic shapes of Wright blade are discernible in the years after 1903. First came the rounded tip; then, from 1908 onward, the ‘cranked’, or bent tip. The waters are muddied slightly because the first disclosure of the Flyer, in France during August 1908, was with 1903-style propellers as a consequence of the others being damaged in transit. Further confusion arises from the fact that the props look bent only from certain angles. Whatever; there is an obvious kink in their trailing edges and they are appreciably wider at the tips.
The reader should also note that the later Wright propeller design was less close to Hollands’ ideal shape than its predecessor. It had even greater mass (and area), even farther from the hub, meaning it soaked-up even more horsepower for no good reason, and tried even harder to snap off its blades through increased backwards forces at the tip. Furthermore, the aerodynamic tip losses previously described — but not then understood, or taken into account — will have been multiplied (the wider the tip, the greater the loss.)
Yes, folks; the more the Wrights refined the design of their propeller, the more inefficient and dangerous it became.
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| Fig 7. The 1905 Flyer modified with two seats and ‘cranked’ blades
during trials at Kitty Hawk in May 1908. Note that the aircraft still
needs a downhill run to take off |
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| Fig 8. Propellers of the Flyer which killed Lt Selfridge on September
17, 1908, when (according to the Wrights) one blade disintegrated in
flight. Blade incidence to the direction of travel almost varies from 0°
to 90°, and strain on the hub must have been immense |
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| Fig 9. Wilbur flies sister, Katharine on February 15, 1909. The propeller tips seem to be of a compromise design |
If it’s any good: steal it
Propellers are all about converting engine horsepower into forward thrust. Hartzell says the Wrights obtained over 66% efficiency (taking Orville’s written assertion as being true); some aviation historians report that this was later improved (with the cranked design of 1908) to 81½%. That may be compared with the near-ideal 90% of modern shapes.
How overloading the tip area and, consequently, increasing aerodynamic losses can make a propeller more efficient is a mystery which Wright-worshipers prefer not to explain. One conjectural explanation might be that the Brothers became vaguely aware of tip losses and thought the answer was to build a wider barrier to airflow overspill in that location. We now know the opposite to be the true state of affairs. In an analogy: faced with getting a Jeep across quicksand, the Wrights were loading it with concrete blocks to give the tires better grip.
And as Joe Bullmer highlights in
The WRight Story, the Brothers’ airplane patent contains irrefutable, written evidence of a fundamental misconception in aerodynamics: their belief that 100% of lift (and of propeller thrust) generated by a cambered airfoil comes from the lower (propeller’s back) surface. In the real world, some 67% of lift (thrust) derives from suction on the top (prop’s front) surface — hence the tip losses because the top surface is trying to “steal” air from the underside.
In proceedings of the (UK) Society of Engineers in 1908, it was claimed that Hollands’ propeller achieved 26 lb of thrust per horsepower inputted, whereas the Wrights’ best figure was a mere 16 lb. The arguments can go back and forth ad infinitum, involving complex formulae and even more esoteric methods and calculation and comparison. (It helps greatly to be in ignorance of tip loss.) Let us, therefore, base our analysis on a simple human trait which infallibly gives a correct answer in such matters: pure greed.
If the Wright propeller had been the best available, it would have been widely copied. Had it been patented (and, interestingly, it was one of the few aeronautical things the Wrights didn’t attempt to protect), it would have been either built under license or, just pirated. Imitation is the sincerest form of flattery, yet nobody flattered the Wright propeller. This might have been because the rest of the world’s aspiring aviators were so staggered by the Brothers’ brilliance in propeller design that they felt unworthy of copying it; or because they investigated it and found it inadequate. Human nature suggests the latter is the right answer.
[*Let it be explained, here, that some propellers break Hollands’ rules by having constant-chord blades with square(ish) tips. Reasons for this did not concern the early aviators. It will be noted that such props are attached to high-power, or high-revving [eg, microlight] engines. Moreover, they are usually metal or composites in structure. Typical of the square-tipped type are those fitted to early versions of Lockheed Hercules — but each engine is rated at 4,200 hp, compared with the Flyer’s 12 hp. Current Hercules versions have abandoned the square tips for a more tapered design.]
In regular letters to the editor of Flight for most of 1909, Hollands was still stressing the superiority of his design and challenging Frederick Handley Page, and others, to better it. Had he but known, he was pushing at a door that was beginning to open — except in the US, where the Wright Flying School continued to employ the cranked-tipped monstrosity of a propeller that had (according to Orville*) shattered and killed Lt Selfridge, US Army, during the 1908 military trials. The Model B was no different, and even a decade after Kitty Hawk, Wright airplanes were still flying with broad, cranked-tip propellers.
[*The Wrights were their own air accident investigators for this crash and, if they are right, the unnecessarily large strain imposed on the prop, as predicted by Hollands, would be an obvious suspect for the cause of the disintegration. However — as it is planned to discuss in a future blog — later, less partisan analysis also points fingers at engine malfunction; overloading of the airplane; pilot’s unfamiliarity with the new propeller design; and structural failure, allowing the propeller to come into contact with a rigging wire.]
Around the same time, 1908, Hollands was working on a metal propeller, for which 85% efficiency was claimed. If confirmed in practice, this patented design scores only a few percent short of the figure for a typical propeller 100 years later. In fact, Hollands held several patents, including one for a reversible propeller with hydraulic actuation. This was applied for in 1898, some considerable time before Hamilton Standard of the US won the 1933 Collier Trophy for the world’s “first” hydraulic propeller.
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| Fig 10. Barnstorming aviatrice Katherine Stinson was the fourth woman in the U.S. to earn a pilot's license, on July 24, 1912, in a Wright Model B. The massive propeller tips are obvious |
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| Fig 11. By 1917, Katherine had learned the error of her ways and switched to a Curtiss JN4 with a ‘Hollands’ propeller |
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| Fig 12. In 1916, the Wright company, then on its last legs, finally
discovered the (W)right stuff and fitted its otherwise unremarkable Type
L with a cambered, scimitar-shape propeller, having pointed tips. If
Hollands and Herring received letters of grateful thanks from Orville
for their far-sighted observations of a decade-and-a-half earlier, they
never mentioned them |
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| Fig 13. Sopwith’s Camel — one of the more famous participants in the air
battles of the First World War. If a camel is, “a horse designed by a
committee,” then a Camel with a Wright propeller would have been, “an
Allied fighter designed by the Kaiser,” and the Red Baron would have
enjoyed a walkover |
Postscript
To obviate any attempt to misinterpret the purpose of the above study, let it be understood that it is not the writer’s purpose to advance any claim that Hollands “invented the propeller.” There were other, relevant contributions from experimenters such as Lanchester, Drzweiki, and Prandtl. Hollands, perhaps, didn’t do it all by himself — but all the essential data are concentrated in his writings and the key fact is that they all pre-date, by a large margin, manufacture by the Wrights of a much inferior product.
Indeed, Orville’s account of propeller development, in How We Invented the Airplane, is bizarre. The Brothers borrowed books on ship propellers from the Dayton Public Library and found the data could not be applied to aerial propulsion — which is fair comment; air is compressible and water is not, thus an entirely different approach is essential. So, as a next step, they “began the study of the screw propeller from an entirely theoretical standpoint.” Entirely omitted is any expression of the faintest curiosity about what other aviation researchers were doing in the line of propulsion.
Yet, we know for certain the Wrights were cognizant of Hollands’ work because their authorized biographer, Fred C Kelly says, “they did not begin serious reading until 1899. Among the books they read was Octave Chanute’s Progress in Flying Machines...” which, of course, contained Hollands’ detailed formula for an efficient prop. Orville could have told Kelly that they found the air-propeller literature as unhelpful as that for water. They’d have been wrong, but at least admitted they had pursued a blatantly obvious avenue of research.
That said, the Wrights’ reputation for propeller prowess seems to have been thrust upon them by sycophantic historians, including those at Hartzell and Wikipedia. As far as is known, they (certainly Wilbur) never asserted that they invented the cambered, twisted propeller. True, they said things like, “we discovered” or “we reasoned” without claiming they were the first to do so; that was left to a later generation of unknowledgeable, lazy historians. The same false credit was awarded to them for wing-warping flight control. In truth, they came up with a poor propeller design; then modified it to be worse.
Those whose view of the Wrights’ aeronautical brilliance remain stubbornly undimmed, in spite of all the above, might wish to indulge themselves in the ultimate Wright experience. At Dayton-Wright Brothers Airport, Wright "B" Flyer Inc (www.wright-b-flyer.org) offers pleasure flights in a replica Model B, tail number N3786B. Nobody having persuaded any of the innumerable 1903 Flyer replicas to fly in anything remotely resembling a safe fashion, this is the nearest anyone will come to the original Wright stuff. Be assured, it’s perfectly safe; this one has narrow-tip Hollands propellers, built by Sensenich, instead of the over-stressed, super-wide originals. (Something approaching the latter can be seen on an accompanying Model B replica, N2283D, that managed 2½ hours of flight for a film before suffering an accident and permanent grounding. A third, new Model B, due to fly in 2020, appears to have pointed-tip, carbon fiber props. So: flyable Wright replicas without replica Wright propellers; there’s a message in there, somewhere.
Why the Wrights did not check-out Hollands’ more advanced thinking (and he receives several additional mentions in the Chanute volume on account of his parallel work with airplane engines) must remain a matter of conjecture, although one might suspect a certain degree of arrogance or pig-hardheadedness. Two lessons emerge:
- For all the remarkable capabilities ascribed to them, the Wrights failed to apply the basic mechanical knowledge demonstrated by Hollands two decades earlier.
- Not only did the Wrights not arrive at the same conclusion as Hollands, they did not have the wit to copy a good propeller design when it was handed to them on a plate.
And, in view of the earlier revelations of this blog, one question cries out for an answer: How might the Flyer have performed at Kitty Hawk in 1903 with a pair of decent, Hollands propellers?
---by
Paul Jackson FRAeS, former Editor-in-Chief, Jane’s All the World’s Aircraft (1995-2019)
