Wright Flyer Propeller Investigation



Wright Flyer Propeller InvestigationW B Garner, July 2017IntroductionIn late June of 2017 I received an Email from Robert (Bob) Kokuc of Amherst N.H. He said that he had been working for about 2 years on reverse engineering the Wright brothers 1903 Wright Flyer. He was currently working on their propeller design. He had found my white paper about model airplane propellers on the internet and contacted me with a number of questions.One of the questions, and the subject of this paper, was related to the position of what the Wright’s called the Center of Pressure along the blade radius and which I call the Center of Thrust. This is the location along the blade radius where half of the thrust is generated on the hub side and the rest on the tip side. My paper indicated that this point occurs at about 75% of the blade length for typical model propellers while the Wright propeller COT occurs at 5/6 or 81% of the blade length. The question, then, was this a result of random actions or was it deliberately selected based on the Wright’s knowledge and experiments. Bob supplied drawings of the propeller blade showing dimensions, pitch angles and blade cross sections. The blade is narrow at the base, widening at a constant rate to the tip where it is maximum. Model propellers generally are wider near the base and taper toward the tip. Incremental thrust at some radius is proportional to the incremental area, the velocity squared and approximately to the square of the local radius. Hence it follows that the Wright propeller COT is located nearer the tip since the area is greater out there compared to a typical model propeller.The question, then, is it possible to confirm that the COT is actually at 81% by analysis?Information AvailableThe drawings provide the required dimensions and pitch parameters. Table 1 summarizes the geometric information. The blade length is 4.25 feet with a rounded tip. The airfoil cross sections appear to be thin arcs of nearly constant thickness. The lift and drag characteristics are unknown.Table 1 Geometric InformationStation, feetChord, inchesPitch Angle, degreesPitch, feet14.0517.525 3/163910.236 3/83111.347 3/162511.7The propeller was designed to operate at airspeed of 25 mph and an rpm of 330. The glide angle is stated to be 6 degrees where glide angle is defined as Arctan (D/L) where D is drag and L is lift. This angle seems to be the angle between the direction of airfoil lift and the resultant force taking into account the drag component.AnalysisThe Simple Blade Element Theory described in F. Weick’s book, Chapter 4 was used for the analysis with the specific steps taken from his Table 1 of that chapter. Since the required data was incomplete some assumptions were made to do the analysis.A formula was generated for the pitch angle, beta, as a function of station radius using the data in Table 1.Beta = 1.5*(r/R)^2 -16.1*r/R +65.5degreesThe blade was divided into 13 increments each of 0.25 feet length and starting at r = 1 foot. The chord width was calculated assuming the width increased uniformly along the full length of the blade from 4 inches at 1 foot to 7 3/16 inches at 4 feet.The coefficient of lift, Cl, as a function of Angle of Attack, AOA, polar characteristics are unknown so Clark Y values were assumed. An aspect ratio of 11.7 was assumed and the Cl slope adjusted accordingly. The value of gamma in the theory was assumed to be 6 degrees from the Wright data. The Wright notes say that this angle was approximately constant for AOA from 3 to 10 degrees. Therefore it was assumed constant everywhere in this analysis. In any case variations have negligible effect on the Center of Thrust.ResultsFigure 1 is a plot of the results; the normalized thrust as a function of r/R. The Center of Thrust occurs at r/R = 0.82, essentially the same as reported by the Wrights. Also note that the center 50% of the propeller produces virtually no thrust.Bob indicted that this result convinced him that the Wrights deliberately designed the propeller around this point, supported also by data from other tests on a smaller propeller and tests on airfoils. Figure 1 Wright Brothers 1903 Cumulative Normalized Thrust as a Function of Location along the parison to Model Airplane PropellersA similar analysis was conducted for two model airplane propellers from different manufacturers. Figure 2 is a photo of the Wright 1903 propeller, Figure 3 is a photo of an APCE model airplane propeller and Figure 4 is a photo of a Master Airscrew model airplane propeller.Figure 2: the Wright Brothers 1903 PropellerFigure 3: APCE Model Airplane PropellerFigure 4: Master Airscrew Electric Model Airplane PropellerFigure 5 is a plot comparing the cumulative normalized thrusts of the three propellers as a function of blade position. The Wright center of thrust occurs at about 82% of the blade length while the model airplane propeller’s centers are about 70% of the blade length. The effect of placing most of the area near the tip is apparent.Figure 5 COT ComparisonChanging operating conditions such as airspeed or rpm had only minor effects on the COT for all three propellers.Figure 6 compares the chord distributions of the model propellers to that of the Wright one.Figure 6 Chord Distribution ComparisonsThere is a distinct difference relative to the Wright design, explaining why the COTs are so different.ReferencesRobert Kokuc, email rekokuc@Fred E. Weick, “Aircraft Propeller Design”, McGraw-Hill Book Company, Inc., New York & London, 1930, Chapter 4.rcaeronotes.my website ................
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