Sunday, July 8, 2018

WRight Perspective - Article 3 of Four


        The  WRight  Perspective – Article  Three
By Joe Bullmer



 The two illustrations including captions ( above) are from the publication The Wright Flyer, an Engineering Perspective

   This is the third article in a series discussing the Smithsonian compilation document The Wright Flyer, An Engineering Perspective, cover pictured below.*


    [The two previous] articles have addressed the section discussing the Wrights as aeronautical engineers and the section on aerodynamics, stability, and control.  This article discusses the third section, titled Longitudinal Dynamics of the Wright Brothers’ Early Flyers. 

  The purpose of these articles is to address differences that have been pointed out concerning information presented in this author's book The WRight Story and that presented in the Perspective.  The purpose of The WRight Story is to record an accurate description of the Wright brothers’ work.  Original documents supporting the following comments are referenced in that book.



 
     At the start of this section, page 45 of the Perspective, the author [Frederick J. Hooven] presents an entertaining description of his close relationship with Orville Wright from 1925 until Orville’s death in 1948.  Unfortunately, throughout these years Orville apparently allowed the author to believe that Otto Lilienthal's lift data were found to be wrong.  On page 48 the author states that at “The end of the 1901 season…..having found Lilienthal’s data to be mistaken….. they realized that they would have to develop their own aerodynamic information…”  This contradicted a 1902 letter Wilbur wrote to Octave Chanute saying that Lilienthal’s data “were as accurate as is possible."

Otto Lilienthal (1848 - 1896) and one of his magnificent gliders
   
   Even more amazing in view of his lengthy and close relationship with Orville, the Perspective author was apparently totally unaware of the instability of early Wright aircraft until he read Charles Gibbs-Smiths 1966 book The Invention of the Aeroplane. 



    On page 46 of the Perspective the author claims that book “made the first mention I had ever seen of the longitudinal instability of the Wright machines.”  So evidently Orville never mentioned it.  In fact, the author claims to have not believed it until he commenced two-dimensional computer simulations in 1978.

  On page 47, Edward Huffaker's note from July 29, 1901 concerning the Wrights’ glider is quoted stating that “The equilibrium is not satisfactory and the Wrights think of making radical changes, placing the rudder in the rear, or rebuilding the machine”.  Interestingly, the surviving notes of the Wrights as presented in McFarland's compilation** mention nothing about considering an aft elevator after mid-October of 1900. 

 
     
   Page 49 notes that the Wrights tried to reduce the pitch instabilities of their vehicles by altering center of gravity (C.G.) locations.  They first moved the C.G. of their 1904 machine from the 29% chord point farther aft to the 32% point.  Finding this worsened the pitching problem, they reversed the C.G. location to the 23% point by adding 70 pounds of iron to the canard supports.  Obviously, although they knew the location of the C.G. was important, the Wrights didn’t yet understand the control implications of the C.G. location relative to the center of lift.  Actually, the Wrights did not develop a way of determining the locations of the centers of lift of their vehicles.


    
    Page 50 begins with the statement that by 1905 the Wrights had eliminated the pitching problem with their aircraft.  “From August 24th [1905] onward…..there was no longer the tendency to undulate [in pitch] and there were far fewer crashes”.  This contradicts movies taken in 1909 from their aircraft in flight clearly showing constant rapid movements of the elevators to cope with pitch instability.  


[A 1909 inflight movie of a Wright plane shown above. It doesn't take a rocket scientist
 to note the pitch instability. Ed.]


Image result for glenn curtiss
Glenn Hammond Curtiss (1878 - 1930), the pioneer who got it right..
 
  This statement also conflicts with another on page 51 that implies that, if they’d had more time, the Wrights might have put their elevators in the back.  The interesting observation is made that the Wrights kept their unstable canard design after 1905 because “They had quite enough to do to build more machines and prepare for public flying without trying to develop a radically different machine for 1908”.  Obviously one of their real fears was that they would fall far behind others that were building flying machines, especially Glenn Curtiss  in the U.S. and numerous others in Europe.  Another may have been that the canard being an obvious feature, abandoning it would have made their patent much
less enforceable. Either of these outcomes would have jeopardized their chances of cashing in on their work.

    On the next page appears the popular statement that “the Wrights conceived the airplane from the very first as a craft that, like the bicycle, depended upon its rider to maintain its equilibrium.” As discussed in the previous article in this series, this is not true. In a legal deposition written in 1920, Orville wrote that they originally thought they would have a stable machine because the data they had on hand showed the center of lift to move exactly opposite of the way it actually did with changes in angle of attack.

   On page 52 it is also mentioned that the wind at Kitty Hawk when the Wrights flew in 1903 averaged about 20 mph.  This is curious since other sources, including Orville Wright and the National Weather Service, claimed it to be 25 to 27 mph.

    On that same page it is pointed out that the Wrights suffered almost two dozen destructive crashes in 1904 and 1905 learning how to turn their aircraft.  This clearly contradicts the first section of the Perspective in which the statement is made that the 1902 glider (and by inference the 1903 Flyer) was capable of making “smooth banked turns.”

   From page 51 to 58 some of the theory and results of two dimensional computer flight simulations of the 1903, ‘04, and ‘05 Wright Flyers are presented.  These simulations do not attempt to represent any actual flights by the Wright aircraft.  They were done to determine if the vehicles actually were longitudinally unstable.
 
   Since the simulated vehicles were indeed longitudinally highly unstable, a simulation of a pilot’s control actions had to be included to avoid immediate crashes.  Pages 53 and 54 reveal that the average pilot reaction times used for pitch control in these simulations were 0.04 seconds for the ’03 and ’04 aircraft and 0.05 to 0.07 for the ’05 and ’07 aircraft.  Any more and they would crash.  One wonders how realistic a pilot reaction time of 1/25th of a second is, particularly since there is no requirement for lateral control in these two dimensional simulations.  Also, allowing slower pilot reactions for the more stable ’05 and subsequent aircraft indicates that reaction times were set by the requirements of the vehicles rather than the improving capabilities of the pilots.

 
    Page 70 points out that wind tunnel tests of accurate models of the 1903 Flyer showed the drag of the complete vehicle to be higher than the Wrights’ composite estimate.  But it then says that horsepower of the engine was correspondingly higher than claimed because of ingesting cooler air near the sea and better cooling with the cooler air.  It is further claimed that, due to proximity to the ground, induced drag would be lower than measured in the tunnels.  And finally, with no explanation it is stated that propeller efficiency was better than the Wrights claimed.

   The validity of the simulation results could be questioned because of the assumptions just described regarding power, drag, and the propeller.  But perhaps the most damaging assumption affecting the usefulness of the results is the two dimensional nature of the simulation.  The title of the article does state that it is limited to longitudinal dynamics, but how useful is that if the pilot doesn’t have to concern himself with roll control of a vehicle that was also laterally unstable because of its anhedral?  In such a vehicle a substantial percentage of the pilot’s attention must be devoted to roll control, certainly at least 20 percent.  This could well distract the pilot from pitch control for a second or more, allowing the aircraft to crash from pitch instability.  In fact, in a September 20, 1902, entry in his diary, Orville admitted that he had crashed for exactly that reason

  The final five pages of the section is a line listing of the simulation program. 

   The next and final article in this series will discuss the sections of the Perspective concerning propulsion and structures. 

   *The document is available online in various formats here. Because we frequently refer to specific sections and page numbers, we recommend downloading the PDF version, available here.

  **Marvin McFarland's complete compilation is online. We have linked you to Volume I of the two volumes.





Joe Bullmer, above, has a Master's degree plus advanced studies in Aeronautical Engineering. His first contribution to the"Truth in Aviation History" series of articles is "Joe Bullmer Rebuttal to Tom Crouch in the"Huffington Post." about the claimed fourth flight picture of the Wrights in 1903.     


All of the pictures and most of the links in this essay were selected and added by the founding editor of "Truth in Aviation History."
 
       
    

Thursday, May 31, 2018

WRight Perspective - Article Two of Four




The  WRight  Perspective – Article  Two
by Joe Bullmer 
 
The Wright Brothers, left to right, Wilbur and Orville, at their home in Dayton, Ohio.



     As pointed out in the introduction to the first article in this series, the impressively technical Smithsonian publication The Wright Flyer, An Engineering Perspective,  (hereafter referred to as the Perspective), [full text found here] has influenced the beliefs of many interested in early Wright aircraft.  But the Perspective compilation includes many non-technical and some technical statements and opinions that contradict information in the Wrights’ records and in this author’s book The WRight Story. 
      The purpose of this series of articles is to present this author’s positions on these differences. 
 Image result for wright story bullmer
    In addition to other material, over twelve hundred pages of the surviving records of the Wrights’ work were studied to create The WRight Story.  Consequently it corrects dozens of common misconceptions concerning their work.  These errors have been repeated for many decades and still are by most whose reputations have been built in part on the traditional Wright brothers’ story. The WRight Story contains hundreds of references to the Wrights’ own words and records, and detailed proofs of all points made in these discussions. 
 
     The author of The WRight Story and this series of articles has a Masters Degree in Aeronautical Engineering along with additional post graduate studies from the University of Michigan, and has worked in the field of aircraft design and performance for decades.  Consequently this author is knowledgeable of the technical detail presented in the subject Smithsonian publication.

     This discussion addresses the second paper in the Perspective titled Aerodynamics, Stability, and Control of the 1903 Wright Flyer.  Although that section is fairly technical, effort has been made to make this discussion much less so.  The length of the subject Perspective article, 20 pages of double column print, has dictated the length of this discussion.  The subject article was originally an American Institute of Aeronautics & Astronomics (AIAA) Wright Flyer Project paper numbered WF 84/09-1.

     The first page of the article, page 19 of the Perspective, includes the statement that by the end of 1903 “the Wrights had in hand all of the fundamental understanding and knowledge they needed to show the world how to fly”.  This is a major overstatement since, as clearly shown in their writings and patent, they did not understand how cambered wings lifted their vehicle off of the ground, nor did they yet know how to control the vehicle’s direction of flight.

Edward Huffaker, important aviation


 pioneer, who, together with Dr. George Spratt,
 provided early research and essential advice to the Wright brothers





      

     The next paragraph states that the Wrights “conducted the necessary tests….. to learn just what they required to succeed” giving the impression that they determined what all of these necessary tests would be.  But it should be noted that they were talked into a most critical test and assisted in carrying it out by visitors to their Kitty Hawk camp, namely George Spratt and Edward Huffaker.


      This test proved that the movements of the center of lift of their wings were opposite to what the Wrights had thought and exactly as Spratt and Huffaker had said, thus explaining their vehicles’ instabilities.  Nonetheless, they still did not change their aircraft’s configuration.  Also, both  Octave Chanute and Dr. Spratt 
Image result for octave chanute
Octave Chanute, one of the first aviation pioneers. He was the author  of "Progress in Flying Machines," an early reference for all of the aviation pioneers, including the Wrights.

familiarized them with wind tunnels, showing them photos of tunnels and the scheme for the lift-vs-drag balance that the Wrights were to use.

  On the next page it is stated that their wind tunnel data “served them well for a decade”.  This may be true, but unfortunately the data served no one else.  Contrary to common practice within the fledgling aviation fraternity, the Wrights never published their data, according to Marvin McFarland and others.*



Otto-lilienthal.jpg
Otto Lilienthal, glider pilot and great aviation pioneer.

     Page 20 of the Engineering Perspective also claims that “Otto Lilienthal   had used a whirling arm apparatus to measure the lift and drag for various airfoils” clearly implying that was a likely source of errors.  This is irrelevant.  The data the Wrights used was developed by Otto in a natural straight steady wind.  A whirling arm had nothing to do with it.  This is easily proven by comparing the Lilienthal data that the Wrights used to the plates at the end of Lilienthal's book, "Birdflight as the Basis of Aviation."

1184222


     The next paragraph states that “the difficulty [in generating lift] lay with "Smeaton's coefficient"  This is also not true since Smeaton’s coefficient primarily affects wing area and the very successful 1902 glider had essentially the same wing area as the poorly performing 1901 machine.  The reason the 1901 machine performed so badly was that both the 1901 and 1900 machines had extremely poor aspect ratios and camber shapes while the well performing 1902 vehicle did not.  These were the Wrights’ own mistakes as they discovered with their wind tunnel and as they admitted in a November 24, 1901, letter to Octave Chanute. 

     Then on the next page the authors go on to say that “With a clever combination of their wind tunnel data and a few tests with a wing from their 1901 gliders (sic) they concluded that the correct value [of Smeaton’s coefficient] was 0.0033.”  In actuality, on October 6, 1901, over a month before they built their wind tunnel, Wilbur told Chanute that "I see no good reason for using a greater [Smeaton's] coefficient than [Langley's value of] 0.0033." They had previously tried to calculate the coefficient with bicycle tests but concluded that those tests were not sufficiently accurate.  So, since it seemed consistent with their glider data, they adopted Langley’s value.


     On page 22 it is claimed that the great German glider pioneer Otto Lilienthal died because of  “a vertical gust, or by…. raising the nose too far.”  Although the Wrights had no information to dispute this, it seems extremely doubtful that, with nearly 2,000 glides under his belt, Lilienthal could have made such a novice mistake.  Other more likely explanations made at the time by those familiar with his equipment include that he was of necessity practicing with a poorly maintained glider that broke a tail support, or that he was testing a new pitch control that failed.

     Also on this page it is stated that “whether their aircraft were stable or unstable was an accidental matter” and so “the question of the Wrights’ intentions to design an unstable airplane is meaningless” as if they never addressed the problem, didn’t care, or their intentions can not be determined.  In fact they stated quite clearly that they did care and intended to design a stable vehicle, but the movement of the center of lift with changes in angle of attack was in the opposite direction from what they thought it would be. 
                                             
     During his speech to the Western Society of Engineers in 1901Wilbur stated, "Our peculiar plan of control by forward surfaces instead of tails was based on the assumption that the center of pressure would continue to move farther and farther forward as the angle [of attack] became less”.  In his 1920 legal deposition Orville recalled their perplexity over the situation thus: “Our elevator was placed in front of the [wing] surfaces with the idea of producing inherent stability fore and aft, which it should have done had the travel of the center of pressure been forward [with decreasing angle of attack] as we had been led to believe.  We found, however, that these machines were anything but inherently stable fore and aft.”  Thus their original intentions are quite clear.  They did address the problem and wanted a stable aircraft, but their basic understanding of aerodynamics was incorrect.  Perpetuating the result of this mistake turned out to be extremely detrimental to the acceptance of their planes.  They finally abandoned canard elevators in 1910.

     Figure 3 on page 22 is incomplete, and Figure 4 on page 23 is incorrect and confusing.  Figure 3 shows aircraft axes and positive rotational moments, but as a setup for Figure 4 the directions of positive angular displacements should also have been indicated.  

Figure 3:


Figure 4:



     Figure 4 contains three plots intended to show how stabilizing moments should vary for angular displacements along each of the aircraft’s three axes of rotation.  To properly envision this problem one need first realize that the axis, angle, and moment directions are all supposed to follow what are called “right hand rules”.  That means that if you take your right hand palm down and extend the thumb to the side, the first finger straight out, and the second finger straight down, then the axes of the airplane are labeled in that order.  The thumb is the positive x or roll axis through the nose, the index or first finger is the positive y or pitch axis out the right wing, and the second finger is the yaw axis z positive straight down.
     Then if you stick your right thumb out again and curl your fingers, as you point your thumb in the positive direction of an axis, your fingers indicate the positive direction of aircraft rotation about that axis, and the positive direction of a moment, and moment coefficient, about that axis.  For example if your thumb is pointing out the nose of the aircraft your curled fingers show the direction of positive roll (right wing down) and positive rolling moment coefficient.  These are standard academic conventions for the mathematics used to describe aircraft maneuvering. 

     A little thought reveals that, using the conventions just described, a given positive rotation of an aircraft should generate a negative torque or moment in the opposite direction to bring a stable airplane back to its original position, and visa-versa.  That’s really all one needs to know.  Standard graphic presentations show positive displacement angles to the right versus the moment coefficients generated plotted positive upward.  Thus lines of positive stability about any axis should always have a negative slope, a positive angular displacement generating a moment in the opposite direction.  The more negative the slopes, the stronger the stabilizing influences.  Unfortunately, in the Perspective, Figure 4a is unnecessarily complicated, Figure 4b shows the wrong axes and slope, and Figure 4c is mislabeled.

     The non-technical reader may not care about this tutorial on aircraft stability diagrams, and I would agree that it is unnecessary to a reasonable understanding of how the Wright Flyer flew.  But it is instructive to see how something so seemingly impressive as Figure 4 can in fact be so confused.

Figure 5A:


Figure 5B Canard Configuration:


     Figure 5A on page 24 presents four airplane stability sketches.  The canard configuration shown at the top right, and shown here as Figure 5B, is claimed to be stable because the heavy positive load on the canard would cause it to stall before the wing does as the airplane pitches up, thus lowering the nose.  At the limit this is indeed a stable reaction.  But stability in flight is more concerned with the effects of small perturbations from nominal.  In that case, as the nose of this supposedly stable canard configuration is pitched up slightly, the canard’s increased positive angle of attack would cause more pitch up.  Although the main wing would develop slightly more lift, it would do so closer to the center of gravity tending to negate its stabilizing effect.  Thus, unless the canard is very small, these effects would result in a further pitch up of the aircraft. This unstable pitch reaction contradicts the labeling.

     The Wrights eventually reduced this problem somewhat by moving the center of gravity well forward putting a heavy positive load on the canard.  (Interestingly this altered their canard configuration toward the tandem wing configuration used by Langley.)

     Pages 25 through 34 display a Vortex Lattice analysis and the results of two wind tunnel investigations.  Although somewhat impressive, neither contains any surprises or unexpected results, and thus they add little toward explaining the overall flight characteristics of the 1903 Flyer.  The authors say as much in their summary at the end of the article.

     However these pages do contain some noteworthy comments.  On page 31 it is claimed that “from the beginning of their work, the Wrights chose not to use dihedral." In support of this a February, 1902, letter to Chanute is cited.  However, 1902 is a long way from the beginning.  In fact their first glider in 1900 began tests with dihedral in its wings, but it was later taken out leaving the wings perfectly straight.  The 1901 machine also started with straight wings, but by the end of testing they had decided to arch their wings thereby using  anhedral. All subsequent vehicles into 1905 used anhedral.

     The article also claims that “the Wrights’ gliders had anhedral….to allow more effective use of the warp control”.  It certainly made the use of a roll control more necessary, but other than making the vehicle unstable, anhedral added nothing to control effectiveness.  They actually used anhedral to keep winds from blowing their vehicles back into the hillside while traversing the hill.  Although dihedral would cause the wind rushing up the hillside to tilt a traversing aircraft into the hill, anhedral caused the vehicle to tilt away from the hill. 
          
     On page 31 the effect of dihedral is explained as wind blowing on the tops or bottoms of wings.  This is a common explanation.  However a document of the technical stature of the Perspective should point out that dihedral actually makes wings roll away from cross winds by increasing the effective angle of attack of the upwind wing and decreasing it on the down wind wing.

     There is a discussion on page 32 describing the problems the Wrights had in 1904 and 1905 trying to learn how to configure and operate their vehicles to make turns.  Notably, this directly contradicts the comment in the previous section of the Perspective that claims the 1902 glider was capable of making “smooth banked turns”. 

     Page 34 states that the “combination of warp and rudder deflection will produce….a more coordinated turn”.  That is not why the Wrights connected them.  They clearly stated more than once that when the aircraft inadvertently banked and they tried to right it, warping would cause it to roll, yaw and spin into the lower downward warped wing having increased drag.  The original rudder was fixed straight ahead to keep the vehicle on course but this didn’t work.  The rudder was then made moveable to counteract the yaw due to warping, but was only given enough deflection to keep the aircraft going straight when warping was used to correct an inadvertent roll, not enough to yaw it into, or out of, a coordinated turn.  That is why the two controls had to be disconnected in 1905, to enable performing both simple roll corrections and coordinated turns.

     Pages 35 to 41 are devoted to an extensive Root Locus analysis of the closed loop response of the aircraft to control.  Much of the results of this analysis are dependent upon assumptions concerning the responses of the pilot which are represented as Kp in the diagrams.  This analysis adds, little since it concludes that the vehicle can barely be controlled by a skillful and well-practiced pilot, something we already knew.  However it does lead to an interesting and descriptive quantitative conclusion on page 35 stating that "stabilizing the 1903 Flyer is roughly equivalent to balancing a yardstick vertically on one's finger!"

     On page 41 the argument is made that “far from abandoning warp/rudder interconnection” both wing warping and rudder movements were controlled by different movements of the same stick on later models.  This is a specious argument because the original mandatory non-variable interconnection was the special feature of the Wright brothers’ control scheme that was patented.  As previously mentioned, this feature had to be abandoned in 1905, a year before the patent for it was issued, in order to make turns.  Although this feature had been abandoned, it was vigorously defended by the Wrights in subsequent patent infringement battles.

     The next article in this series will discuss the longitudinal dynamics of early Wright Flyers. 

* Marvin MacFarland's "The Papers of Orville and Wilbur Wright" are fully digitized  at hathiway trust.org. The link is to Volume I of two volumes.

 _________________________________________________________



Joe Bullmer, above, has a Master's degree plus advanced studies in Aeronautical Engineering. His first contribution to the"Truth in Aviation History"series of articles is "Joe Bullmer Rebuttal to Tom Crouch in the"Huffington Post."       

All of the pictures and most of the links in this essay were selected and added by the founding editor of "Truth in Aviation History."__