A take-off propeller, for example,
Propellers For The 1192-cc Volkswagen Engine by Stanley A. Hall (EAA 10883) 1530 Belleville Way Sunnyvale, California Project Engineer, Airborne Systems, Lockheed Missiles & Space Co.
HE GROWING POPULARITY of T the Volkswagen engine as a powerplant for homebuilt aircraft suggests
that some discussion on the matter of propellers for this remarkable little
engine might be useful. This article addresses the selection of direct-drive, fixed-pitch propellers for the 1192-cc. VW engine rated at 40 hp at 3900 rpm. Data on the engine comes from Huggins in Reference 1. In order to set the stage, some preliminary remarks are in order concerning the types of propellers considered and with the test data upon which the technical aspects of this article are based. In the first case, the designer of aircraft delivering both higher horsepower and higher airspeeds than those capable of being delivered by the 40-hp VW engine has several operating regimes with which to trade off. It is necessary to effect these trade-offs because a fixed-pitch propeller will operate at maximum efficiency only under one set of conditions. He has to decide on the conditions he wants to satisfy and proceed on that basis. 10
JULY 1970
although exhibiting good take-off properties, delivers relatively poor performance at top speed; good takeoff—low maximum speed! A propeller designed to operate at top efficiency at top speed will, on the other hand,
suffer at the take-off end; good maximum speed- poor take-off! In addition to the take-off and maximum-speed classifications, propellers are also identified in the "climb," "cruise," "economy cruise," and "maximum cruise" categories. There are undoubtedly others as well, depending on the objectives being sought. Insofar as concerns the 1192-cc. VW engine, the choice of propeller
classifications would seem to reduce to two- the climb propeller and the cruise propeller. Analysis shows that the climb propeller will perform very nearly the same as the take-off propeller in the low speed regime represented by aircraft powered by 40 hp. For this
reason they are combined in this article under the "climb" heading. Similarly, all the cruise propellers are combined under the "cruise" heading. This is because the 40-hp VW engines, as used in most homebuilt aircraft of the day, have so little reserve power that they are commonly operated at nearly full throttle all the time, even under "cruise" conditions. Separate classifications would, on this basis, appear superfluous. On the matter of the test data upon which this article is based it should be pointed out that one of the obstacles to accurate treatment of the general problem of VW propellers is the shortage of controlled test data suitable for the purpose. The mainstay of this article, NACA Report No. 640 (Ref. 2), for example, deals only with propeller blade angles of 15 degrees and greater. Due to their high rotational speeds VW propellers often require blade angles down to ten degrees or less. K.D. Wood (Ref. 3) shows test data for angles down to eight degrees. However, the propellers he used in his testing and the conditions under which he tested them did not yield data altogether useful to the VW application. All the investigator can do in these cases is to extrapolate from what data are available. This process calls for curve fairing, eyeball adjustment of data, etc., and as such is not always rigorously dependable. Although appropriate adjustments
have been made here to describe the VW situation, the reader should recognize it for what it is—a pencil and paper extension of test data. With that out of the way we may now proceed. In order to get the most
out of an engine-propeller-airplane combination one needs to "match" the propeller diameter and pitch, not only to the horsepower of the engine and its rpm, but to the airspeed of the airplane. What the designer needs to know is, what combination of propeller diameter and pitch will convert the power available in the engine to the maximum possible amount of thrust at the desired airspeed? Fig. 1 is intended to answer that question. It plots aircraft design airspeed against propeller diameter and pitch for both the climb and cruise
conditions. The data in the figure derive from tests undertaken by NACA on the two-blade 5868-9 propeller, and from K. D. Wood whose propellers are identified as "lightplane" propellers. The NACA propeller is of metal construction, uses the Clark Y airfoil at the 75-percent radius reference point and has, according to Wood, an "activity factor" of 77.5 per blade. K. D. Wood's propellers are constructed of wood and have an activity factor of 93 per blade. The term "activity factor" relates to the distribution of area along the radius of the blade and has meaning only to engineers. It has no particular significance in this article. Propellers most commonly used on VW-powered airplanes appear to have about the same airfoil and roughly the same activity factor as the NACA propeller. Although they are made of wood and do not have exactly the same blade planform as the NACA propeller it is likely that the data shown in Fig. 1 will provide reliable data for most 1192-cc. 40-hp at 3900-rpm VW aircraft applications. Turning first to the "cruise" case, Fig. 1 is intended to be used in the following manner. Enter the graph at the maximum level-flight airspeed the aircraft is expected to reach on 40 hp and read the corresponding propeller diameter and pitch. This particular combination of diameter and pitch will "match" the highest propeller efficiency with the airspeed selected. The question naturally arises as to exactly what "expected" design airspeed to use. The only way by which this can be determined with any accuracy at all (and even this is debatable) is to run a complete drag analysis of the airplane, including a special analysis of the drag properties of the engine cowling. These can both be formidable tasks. A more practical way for the homebuilder to choose the proper design speed is to estimate it on the basis of the top speeds reached by airplanes already flying on 40 hp. The error involved is not likely to be significant.
I
I
!
more thrust at low speeds than at high
I
ones. Selection of a climb propeller for the 3900 rpm VW engine turns out to
Climb Prop Dia.
be based almost entirely on tip speed; r^——Cruise Proo Dia.
Volksplane «, Brown's J o d e l
70
80
90
100
110
120
130
Design Airspeed, mph F i g u r e I - Propeller Diameter and Pitch versus Design Airspeed
Parenthetically, the possibility for error would be reduced if builders would only report the performance of their aircraft accurately. Some claims tempt credibility, seeming to represent enthusiasm rather than accuracy. Four 1192-cc., 40-hp VW-powered aircraft for which apparently reliable maximum-speed data are available to
the author are the Volksplane (Ref. 4), the "specification" Jodel D-9 (Ref. 5),
Capt. W. E. Brown's Jodel DF-9G (Ref. 6), and the "specification" Sportavia RF4d (Ref. 7). The RF4d is the powered glider-like aircraft that Mira Slovak recently flew across the Atlantic and crashed at Santa Paula, California on the final leg of his trip. The maximum speed claimed for each of these aircraft is shown in Fig. 1 as a vertical line. These lines intersect other, horizontal lines to show the corresponding propeller diameter and pitch for which the propeller efficiency is theoretically maximum. These propellers will absorb 40 hp at 3900 rpm at the speeds for which they were selected, and convert the power available at those speeds into the maximum amount of thrust possible. Note that the minimum design airspeed in the graph is 70 mph. At 3900 rpm the tip speed of the propeller chosen to yield the maximum efficiency at 70 mph is right on the edge
of the critical velocity for a rapid
decay in efficiency. A design speed lower that 70 mph would require a larger-diameter, lower-pitch propeller. At 3900 rpm the propeller tip speed would approach the velocity of sound with the result that the propeller efficiency would take a significant dip. K. D. Wood (Ref. 3) and Diehl (Ref. 8) state that the efficiency can be expected to drop from six to ten
percent for each ten-percent increase in tip speed over the critical velocity,
with losses starting to appear between
950 and 1050 feet per second, depend-
3900 rpm is a lot of rpm. With the maximum rpm for any speed set at 3900 and the critical tip speed set at 950 ft./sec. at sea level, the diameter of the propeller computes to 4.61 ft., or 55.32 in, Essentially then, regardless of airspeed, a propeller turning at 3900 rpm cannot have a diameter greater than 4.61 ft. if the tip speed is to be held to 950 ft./sec. The effect of airspeed on the design of the propeller is to change only the pitch; the higher the airspeed, the higher the pitch. Fig. 2 plots thrust versus airspeed for a number of propellers. Each graph in the figure compares a climb propeller and a cruise propeller, both design-
ed to absorb 40 hp at 3900 rpm at the design airspeed shown.
The curves reveal an interesting feature, one which is quite unexpected when considering faster, higher-horsepower aircraft. They show that although the climb propeller delivers
higher thrust under static conditions
than its cruise counterpart, the advantage quickly disappears in favor of the ratio. cruise propeller. In the case of faster, As Fig. 1 shows, the higher the more-powerful aircraft one would exdesign airspeed the lower the propeller pect this advantage to develop at diameter and the higher the pitch considerably higher airspeeds. Whererequired to absorb the same power. as, as the graphs show, the thrust of Two advantages accrue from the clean the climb propeller begins to decay as aerodynamic design implicit in higher soon as the aircraft starts to move, the airspeeds. The most obvious one is thrust of the cruise propeller, although that clean aircraft have less drag and starting out at a lower value, actually
ing on the blade thickness-to-chord
would, naturally, be expected to fly faster for the same power. A less obvious but equally important advantage is that up to a point the propeller efficiency increases with aircraft
speed.
The efficiency of the RF4d propeller called for in the graph is about 0.82, which means that at maximum speed 82 percent of the available engine power is being converted to thrust. The slower Volksplane, on the other hand, has a propeller efficiency on the order of 0.75. The two aircraft
develops a higher thrust for awhile, then decays at about the same rate as the climb propeller. It is significant in the case of the 1192-cc. VW engine that the cruise propeller excels the climb propeller, even in take-off. One likely explanation for this is that although both propellers are operating at a high angle of attack during the take-off roll, and the cruise propeller is in or at least on the edge of
stalling over most of its radius, the
latter quickly moves out of the stall region once a little air gets through it are designed to different objectives and and starts to "fly." And once it starts should not, therefore, be compared to fly, it flies better than the climb except for illustrative purposes. Unless propeller. This is why a good pilot of the designer of a new VW-powered any fixed-pitch propeller airplane, in aircraft builds his own propeller he seeking quick take-off, will ease the will eventually be faced with the probthrottle in gradually during the takelem of choosing the right propeller off roll, letting the propeller "bite" from among those currently being into the air rather than stalling built by someone else. He will not through it by the pilot standing on the likely find exactly the right one. Howbrakes with the throttle wide open and ever, the data shown in Fig. 1 should then letting go. assist him in narrowing down the Pilots and builders alike are interchoice. ested in static rpm, the next subject of Turning now to the case of "climb" this article. Pilots are interested in propellers, one observes from Fig. 1 static rpm for purposes of pre-flight that they tend to be larger in diameter checking of the magnetos and for full and flatter in pitch than cruise propelpower output, the builder for deterlers. They are designed to develop (Continued on next page) SPORT AVIATION
11
200 150
100
20
30
40
50
oO
70
80
90
100
F i g u r e 2 - T h r u » t v e r » q » Airspeed (or Several Propeller*
(Fall Throttle)
• Static rpm - Static thrust, l b s .
HP absorbed
• HP absorbed, c r u i s e props HP absorbed, C l i m b props 40
40001 3500 3000-
2.500Static rpm, cruise props .20
10001500 --150 1000 - 100
15
Static thrust, c l i m b props Static thrust, c r u i s e props 1ZO
A i r s p e e d (mph) f o r W h i c h P r o p e l l e r is D e s i g n e d
Figure 3 - Full Throttle Static Propeller Performance (Sea Level, Standard Day)
12
JULY 1970
130
mining if he has the right propeller and/or a good engine in the first place. Fig. 3 shows what the various propellers considered in this article ought to turn up on the ground in zero wind conditions, at sea level. If the engine is known to be capable of developing its full power, if the propeller exhibits reasonably good workmanship, if it has a reasonably good airfoil, if its twist distribution and activity factor are proper, and if the run-ups are made anywhere near sea level standard conditions, the static rpm ought to be close to the values shown in the figure. On the matter of one building his own propeller, the experts advise against it. This is a job better left to the professionals. However, for the EAA'er who is unusually talented and knowledgeable in woodworking there is no solid justification for not tackling the job—provided he has the advice and guidance of someone thoroughly experienced in the art. All rotating machinery is potentially hazardous, especially propellers. Consider this: A four-foot diameter propeller turning at 3900 rpm is trying to pull itself apart at the hub with a force of over 30,000 lbs. And this considers only the centrifugal forces. Bending and twisting stresses must also be added. And consider balance. If the propeller is not properly balanced the potential hazard is even more pronounced. Some' VW engines are simply bolted to a 3/4-inch thick plywood bulkhead. It is easy to see how only a small amount of imbalance, being aggravated by a 3900 rpm buzzsaw, can take an engine right off the airplane. Obviously, most careful control of materials, workmanship, and balance need be exercised. A professional propeller builder knows better than anyone else just how to do this. If, in spite of the admonishment cited above, the homebuilder still wants to build his own propeller, he can turn to Table 1. Table 1 shows the blade planform of the NACA 5868-9 propeller, modified to accommodate wooden construction. The table shows the blade width at each station in percent of the total propeller radius. The blade thickness of the NACA propeller at the 75-percent radius reference point is about nine percent of the blade width. From the structural standpoint this is considered somewhat thin for a wooden propeller. K. D. Wood shows an 11.5-percent thickness at the 75-percent radius point for the "lightplane" propellers cited in his work, an increase of roughly 25 percent. The values in the thickness (t max) column in Table 1 include this 25-percent increase, which 'is faired from the
original NACA thickness at 30-percent
radius point. The Clark Y airfoil is, obviously, modified to this extent for wooden propellers, although it is still technically a Clark Y airfoil. As the sketch in Table 1 shows, the blade is faired into the hub from the narrower NACA planform, starting at about the
20-30 percent radius point. It is important that the blade not be necked
down too far at its narrowest point because of the high stresses in this area. During actual construction the builder may find that the maximum thickness near the root will vary somewhat from the values shown in the table. The amount it varies depends on the maximum thickness of the hub which, in turn, is based on the blade angle being used. A large angle, for example, will require a thicker hub than a smaller angle and the thickness variation near the hub will be greater. The thickness distribution shown in the table is for a propeller having a blade angle of 15 degrees at the 75-percent radius reference point (comparable to the 95-mph VW cruise propeller). Fairing this blade into a VW hub from the 20-30 percent radius point will yield thicknesses at those points which are close to the stated values. For propellers having other blade angles the shape should be faired smoothly into the hub with no particular regard to the thickness inboard of the 20-30 percent point. Outboard of the 20-30 percent point, however, the builder should closely regard the stated thicknesses in order to achieve the desired propeller efficiency. As to twist distribution, Table 2 shows the variation of blade angle along the radius for cruise propellers. The twist changes in such manner that the pitch of the propeller at each blade station is approximately the same. Pitch, of course, is the distance each blade element moves forward for each revolution of the propeller, assuming no "slippage" through the air. Since the tip moves a greater circumferential distance than a point nearer the center, the blade angle has to be less at the tip than closer in so as to move the same distance forward per turn. In conclusion, it is hoped that this article has afforded the reader at least a degree of practical insight into propellers in general and the 40-hp, 3900-rpm VW propeller in particular. There is as much room for experimentation and improvement in propellers as there is for the aircraft they drive, and propeller manufacturers still engage in considerable research and development in quest of improvement. From the technical standpoint propeller design is not quite a black art, but one is often given to ponder the
Table 1 - Blade Planform and Thickness Distribution
NACA
Blade Cross Section pet. R-»20
30
40
50
60
70
80
90 95
Blade Planform
Pet. R
b, in pet. R
'max' 1 "!*'-
20 30 40 50 60
7.6 10.6 13.8 15.2 14.8
Faired 36.0 17.9 13.8 12.4
b
Pet. R
b. in pet.
70 30 90 95
13.2 11.2 9.0 7. 5
Table 2 - Cruise Propeller Twist (fi)
11.8 11.2 10. 7 10.5
Distribution
A i r s p e e d (mph) for W h i c h Propeller is Desiened
^^ rotation
70
80
90
100
110
120
130
Pet. R
ft
£
£.
£
E
£
£
20 30 40 50
31.2 25. 7 21.1 16.8 13.6
35.8 30.0 25.3 20.9 17.6 14.8 13.0 11.8 11.4
37.8 32.0 27.0 22.8 19.4 16.7 14. 7 13.6 13.2
39.6 33.6 29.6 24. 3 21.0 18.1 16.2
40.9 34.9 29.9 25.6 22.1 19.3 17.4 16.2 15.7
42. 5 36. 5 31.4 27.0 23. 7 20.8 18.8 17.6 17.2
60 70 80 90
95
10.9 9.0 7.9 7.4
33.6 27.9 23.3 19.0 15.6 13.0 U. 1
9.9 9.5
15.0 14.6
i*
point. Until the advent of V/STOL
aircraft and helicopters the last good
work in propeller theory came out in the late 1930's and early 1940's. The helicopter and V/STOL people, of course, look at propellers from a different point of view than do we. When one gets right to it, much dependence must still be placed by we homebuilders upon the expertise developed over the years by the Hegys, the
References: 1. R. G. Huggins. Information packet on conversion of the Volkswagen engine for aircraft use. 2. Hartman, E. P. and Bierman, David. "The Aerodynamic Characteristics of FullScale Propellers Having 2,3 and 4 Blades of Clark Y and R.A.F. 6 Airfoil Sections." NACA TR No. 640. 1938
3. Wood, K.D. Aerospace Vehicle Design, Vol. 1, "Aircraft Design." 1963
Fahlins, the Troyers, and the Andersons. These men know their business and, should they be willing, ought to be consulted whenever the opportun-
4. Evans, W. S. "Der Volksplane." Sport Aviation. May, 1969
The assistance and counsel of J. H McVernon, Aerodynamicist at Airborne Systems of Lockheed Missiles & Space Co., and Ole Fahlin of Fahlin propellers in the preparation of this article are gratefully acknowledged.
7. Aero Sport Aircraft Imports. "The Fournier RF4d." Advertising brochure. 8. piehl, Walter S. Engineering Aerodynamics, Revised Edition, Sixth Printing.
ity presents itself.
5. Jane's All the World's Aircraft. 1958 6. Brown, Capt. William E. "Flight Report
on the Jodel D-9." Sport Aviation. July, 1966
April, 1943
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