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9.1 Possible Applications Of Soaring Technology To Drag Reduction In Powered General Aviation Aircraft John H. McMasters and George M. Palmer Purdue Universlty Introduction The term "General Aviation" usually brings to mind the range of powered aircraft encompassing the Piper Cub through executive jet transport aircraft. Depending on one's definitions and biases, however, a case can be made for inclusion of other types of aerodynamically supported vehicles such as the sailplane and their powered derivatives (self-launched sailplanes or motor gliders) and perhaps even the lowly hang glider. While participation in soaring in this country is rather limited and the economic impact of sailplane manufacture is minlsculet the current level of technology in this branch of light aviation |s extraordinary - particularly in the areas of aerodynamic efficiency and utillzatlon of advanced materials and fabrication techniques. The purpose of this brief discussion is to outline the present state-of-the-art in soaring performance and review some of the techniques (particularly in the area of drag re- duction) used to achieve this performance. It can legitimately be objected that the performance requirements of sailplanes and light powered aircraft are quite different and that sailplane manufactures are not bound by the same economic constraints as their counter parts in powered flight. However_ to ignore the aerodynamic lessons learned in sailplane development would be_ in our view, a serious oversight. In view of the fact that sailplane technical literature is infrequently consulted by many aeronautical engineers t particularly those at universitles_ this brief review is considered appropriate.
State -of-the-Art Most modern soaring aircraft are pure sporting devices, the most elegant and advanced of which are optimized for competition - which today implles racing. The classic design problem is one of optimizing an aircraft for two design points: (1) low speed (minimum sink rate) flight in a rectilinear or banked turning attitude to max- imize rate of cl imb and (2) minimum glide angle (or maximum llft-drag ratio) in high speed rectilinear cn_,ise. In racing pedormance, however, absolute maximum L/D
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is less important than maintaining a "low" sink rate (e.g. 2m/sea) at the highest possible speed. * At present two major types of competition sailplanes are in wide spread use: Standard Class, with spans limited to 15m (49.2 ft) with water balast (to increase wing loading in strong lift conditions) and only simple hinged flaps not connected to the ailerons permitted, and Open Class where anything is permitted. Under pressure mainly from European designers t the Standard Class will be divided into two classes for inter- national competition after 1976, with one branch becoming an "unllmited" class keeping only the 15m span limit and the other basically retaining the present Standard Class rules.
The other category of soaring device of interest in this discussion, the "motor gilder", is slowly becoming more popular in Europe and the United States. It is basically a moderate performance sailplane fitted with an engine providing it with a self-launch and out-landing retrieval capability.
Some typical modern sailplanes and motor gliders are shown in Figure 1. Per- formance and geometric data for several typical types are listed in Table 1 . Performance capabillt|es are further clar|f|ed |n Figure 2. Also shown for comparison in Figure 2 are gllde polars for several other types of low speed flying device from (1). There are few standard handbook type references available on sailplanes and soaring technology. Probably the best sources of information are Soaring magazine, Technical Soaring (12) and the publ ications of the Organization Scientifique et Technique Inter- natlonale du Vol-a-Voile (OSTIV) available from the Soaring Society of America (SSA).
Important recent material is available in (5.6).
The basic configuration of the high performance sailplane was well established prior to WWII. Performance increases since that time have been very large, however, due mainly to three factors: 1. A greater appreciation of the importance of the quality of the aerodynamic surfaces and the necessity of seal ing gaps and flow leakage in reducing drag.
2. Advanced airfoil designs with greatly improved (compared with G_ttingen and NACA 4 and 5 digit airfoils) characteristic in the Reynolds number range characteristic of sailplane operation.
* The ideal sailplane glide polar would be as "flat" as possible over the widest possible speed range. Sailplanes, as in the case of most other aircraft types, seldom "cruise" at the speed for L/D max.
3. The introduction of fiberglass construction.
These factors will be discussed in more detail later. Some comparisone based on data from Table 1, for typical sailplanes of good pre-war technical vintage and modern technology are presented in Table 2.
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• " R_5 (m/see) _J \ Figure 2.(continued) Sti!! Air Seo Level Glicle Polors For Several Haturol And Mon-Mode Flying Devices Table 2. Performance Improvement Comparison Weihe ASW-12 % Improv.
(1938) (1964) Min. _ 120 fpm I09 fpm + 9% V at Zmin 35 kt 43 kt -23% L/Dma X 31.5 43.3 +38% V at L/Dma x 41 kt 48 kt +17% V at Z = 6 fps 62.5 kt 88 kt +41% L/D at Z = 6 fps 17.6 24.7 Ka 6 CR Std. Cirrus % Improve.
(1955) (1969) Min. _ 134 fpm 134 fpm 0% V at Zmi n 36 kt 42.5 kt -18% L/Dma x 29 35.2 +21% V at L/Dma x 42 kt 51 kt +21% V at Z _ 2m/s 70 kt 85.5 kt +22% L/D at Z = 2m/s 18 21.9 It should be noted that these gains in aerodynamic efficiency have not been accompanied by serious deterioration in stability, control or safety.
Technical Considerations A number of practical factors make the sailplane design problem difficult, not all of which are directly related to the absence of an engine. For good climb performance (low sink rate) a low wing loading, low weight and excellent aerodynamic efficiency are desired. Further, if climbing is to be done predominantly in thermals, trim drag in moderately steep turns must be low and the speed for minimum sink rate (maximum climb rate) should be low to minimize turn radius (which, for a given bank angle, varies directly with speed squared). On the other hand, for high speed cruise the main concern is aerodynamic efficiency (high L/D). In a first order analysis (i.e. neglecting Reynolds number effects), L/D is independent of weight and thus for a given wing area, a "high" wing loading is desired. The obvious solution of use of variable geometry (e.g. Fowler flaps) to ameliorate the wing loading conflict is limited by several factors (e.g. class rules, economic and/or drag considerations, manufacturing d_fflcultles) some of which will be discussed later. The demands of high aerodynamic efficiency in both climb and cruise require that great care be taken to minimize both parasite and in- duced drag. The latter is "easily" accomplished by use of high aspect ratio wings of near ideal planform. Given presently achievable values of parasite drag coefficient (about 0.010 based on wing area), the optimum compromise aspect ratios for Standard Class (span limited) sailplanes are between 18 and 22. Corresponding values for Open Class machines are between 25 and 30. * The use of high aspect ratio wings of moderate area at typical sailplane speeds, means that the wlng operates in a Reynolds number range well below that of conventional GA aircraft. For example, assuming a machine with an aspect ratio of 22 and wing area of 110 ft 2 , with a useful speed range of 40 to 100 kt, the corresponding Reynolds number range (based on average wing chord) is 1.0 to 2.4 x 10 6 at sea level. If the machine weighed 700 Ibs. loaded, the corresponding lift coefficient range at sea level would be C L = 1.18 to 0.19. The general speed/Rn ranges for several types of low speed flying machines are shown in Figure 3. Sailplane experience indicates that with a little care, GA aircraft designers need not be overly concerned about the adverse influence of lowered Reynolds numbers on wing drag when large reductions in wing chord are contemplated.
Parasite Drag Reduction Post-war advances in sailplane performance began when Rasper (7) demonstrated the startling performance gains achievable by systematlcally cleaning up a machine of initially good aerodynamic layout. The machine used was Dick ,Johnson's one-of-a- kind R,J-5 Open Class sailplane which was of conventional layout and construction (largely wood) employing an NACA 6-series laminar flow airfoil. The results of the successive improvements resulting from careful sealing of gaps and leaks, and reduction of wing waviness and roughness are shown in the now classic Figure 4. The total cleanup resulted * The "optimum" in this case is not really clear, although practice indicates that machines with span greater th_an.n about 22 meters encounter serious flight and ground handling problems.
Required wlng al'ea d_pends on the extent to which variable geometry can be achieved and desired wing loading. Thus overall operational consideration defln_ng span and area Hmlt optimum aspect ratios based an achievement of pure maximum L/D in both Standard and Open Class machines.
4.19 in a 25% reduction in parasite drag at L/Dmax, about 40% of which was achieved by simple seallng and smoothing. A sllghtly more modem discussion of these effects has been presented by Wortmann (8). The impressive results obtained by Wil Schuemann in a general cleanup of an H-301 L|belle are discussed in (9).
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Low-Speed Airfoils The requirements of low drag over a reasonably wide llft coefficient range and the generally low values of operating Reynolds number make airfoil selection for tQ • sailplanes somewhat difficult. Pre-war sailplane designers relied primarily on Gottmgen and INACA 4 and 5 digit airfoil, some of the former type (e.g. G_ 549) being specifically tested for sailplane applications. The advent of the NACA 6-serTes airfoils held promise of substantial drag reduction over at least the C L range of the "laminar bucket" and, provided care was taken in manufacture, there appeared hope of obtaining the "bucket" in practice. A number of very successful designs were thus produced in the late 1940_s and 1950's using various NACA 6x - 4xx and 6x - 6xx airfoils; the moderate camber of these sections representing a reasonable compromise for centering the "bucket" be- tween the high and low speed extremes in required CI..
The theoretical work of R. Eppler and F.X. Wortmann in Germany, beginning in the 1950's, showed that by careful contouring of the airfoil envelope and camber line, the transition point on low-to-moderate speed laminar airfoils could be controlled with some precision. This work lead to a family of Wortmann airfoils (the FX or Franz Xavier series) which have been almost universally adopted in sailplanes designed during the last decade. Wortmann's work is well summarized in his paper in (5) and his air- foils have produced something of a revolution in modern sailplane performance.
Wortmann has shown that by carefully contouring the upper surface of a fairly highly cambered airfoil, the upper end of the laminar flow range can be extended to section CI. values required for low sink rate.
When a highly cambered airfoil is operated at low CL values, however, the airfoil is frequently flying at a negative geometric angle of attack, and thus the lower surface of the airfoil is the one on which transition (and/or separation) is of primary concern in maintaining low profile drag. Thus, by careful contouring of both the upper and lower surfaces, the low drag "bucket" can be significantly extended (in operating C L range) compared to NACA 6-serles sections of similar thickness and minimum drag.
The extent of the bucket can often be further increased by adjusting the camber llne with a small chord (10 - 20%) simple hinged flap at the trailing edge. Examples of the possible improvement are shown in Figure 5. Several typical sailplane and related air- foils are shown in Figure 6, and the general trend in maximum section lift-drag ratio with Reynolds number for several Wortmann and NACA Sections are shown in Figure 7.
While Wortmann's results are impressive the limited data available on the new Liebeck (10) sections appears spectacular. Whether such airfoils, which appear to approach some sort of theoretical limit in single element airfoil lift-drag ratio, can perform in practice when built into a practical wing remains at present an open question.
Wortmann_s investigations of the same type airfoils is reported in his paper in (6).
High Lift Devices In the modern gospel of sailplanes airfoil design according to Dr. Wortmann the wing contour must be absolutely smooth and unbroken as far aft as possible. Thus, leading edge high lift devices, wide chord flaps or ailerons and particularly conventional Fowler flaps of significant chord are out of the question in sailplane design. Thus the designers choice of high lift devices is severely limited. As one example of a way to circumvent this problem, and provide the performance benefits theoretically available from use of area changing flaps *, Wortmann tailored a unique airfoil/flap system speci- fically for the very advanced British "Sigma" sailplane project (see Figure 1 ) .
The FX 67-VC-170/136 section for "Sigma" is fully described in (11) and the combined polar at Rn=3xl06 is shown in Figure 5. The flap of this airfoil is "hidden" inside the basic FX 67-VC-170 airfoil when retracted, thus avoiding flow disruption at high speed.
When extended, it produces a 36% increase in chord. An even more exotic scheme has been proposed and tested by Wortmann (11) which involves deploying a large sheet of sailcloth (e.g. dacron) allowing chord extensions of greater than 50% in the high C L range.
Structures The third component in the post-war revolution in sailplane performance has been the introduction of fiberglass as the main construction material; as pioneered by N_'gele, Eppler, Stender and H_nle in Germany. The use of fiberglass wing skins allows fabrica- tion of relatively wave free surfaces of unexcelled smoothness. A further consequence of the use of relatively low modulus of elasticity fiberglass is that in order to maintain desired levels of torsional and bending stlf_ness, wing skins must be quite thick and correspondingly stronger than required by existing sailplane airworthiness standards. One thus finds fiber- glass sailplanes with load factors approaching those of modern fighter aircraft with little weight penal_ (due to the low speclflc gravity of the fiberglass). Considerable room for further improvements in structures exists by use of advanced composites and materials such as DuPont Kevlar (PRD-49) with nearly three times the modulus of elasticity of ex isting E-gl ass systems.
* Partial span Fowler flap systems have been extensively tested on the South African B J- series of sailplanes with generally poor results.
2.0 Liebeck 1973 (Turb.)
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o o °__ OIJ.V_I 9V_Q-J..=II'I NOIJ.C)3S "llO-,IkllV "XVIN Existing f|berglass systems have several disadvantages, however. Since most high performance sailplanes are produced in limited quantities, most fabrication is done by hand lay-up resulting in high cost and major quality control problems. Inspection for structural integrity remains a major difficulty. Questions also remain about the aging characteristics of existing fiberglass systems, and the problem of ultra-vlolet degradation of the structure carries the commerlcal disadvantage of offering the customer a choice of the basic color scheme of his aircraft. Thus, alternative fabrication schemes to pre- serve the beneficial aerodynamic qualities of fiberglass construction, whilereducing cost, etc. continue to be explored. Notable among these schemes are the use of bonding (e.g. the Liaster "Nugget") (4), the use of "plastic" coatings over conventional alum- inum structure (e.g. Schweizer 1-35) (4) and the extrusion of major structural assemblies as described by Morel li (5).
Concluding Comments The design of modern high performance sailplanes is an enormously challenging task. The absence of an engine means that the designer cannot indulge in the "luxury" of simply fitting the machine with a larger powerplant to obscure deficiencies in aero- dynamic design, weight overruns or to provide the basic design with *'growth potential".
Further, the machine must have very low drag values over a relatively wide llft co- efficient range and these values must be achieved in a relatively low Reynolds number range. It is for these reasons that a careful study of the remarkably successful methods sailplane builders have used to achieve these goals may well repay the designers of powered General Aviation aircraft. It has not been the intention of the authors of this brief discussion to advocate or imply that all sailplane technology is applicable to General Aviation aircraft in general or that the operational and economic constraints faced by the GA designer make incorporation of applicable features feasible. However, even a brief examination of the performance figures achieved by modern soaring machines and a little reflection as the often huge disparity in L/D values between sailplanes and GA aircraft indicates that careful attention to the lessons learned in sailplane design and manufacture hold realistic promise for substantial gains in the aerodynamic efficiency of several GA types. The fuel crisis, whether transient or permanent, may force a re- direction in GA design with greatly increased emphasis on operation "economy", perhaps even at the expense of speed (or productivity) and initial vehicle cost. Modern soaring technology indicates one path along which future development might progress.
Research Needs 1. Performance of Wortmann type sail plane airfoils in wings with roughness, waviness and Reynolds numbers typical of light powered general aviation air- craft need to be further investigated.
2. Simple, economical methods of construction need to be found which lead to improved surface finlsh and manufacturing tolerance to approach the performance of sailplane types for powered general aviation aircraft. Possible examples are: - "Plastic" coatings over conventional structures.
(e.g. Schwelzer 1-35 sailplane) - Metal bonding techniques (e.g. the Laister "Nugget") - Major assembly extrusions from metal and plastic 3. The optimum configuration design implications of the use of sailplane technology in powered general aviation aircraft needs to be investlgated_ Specifically: - optimu m wing geometry engine/thrust producing device location - requirements for high llft devices - wing/body integration (i.e. wing position and body shape) to minimize adverse interference 4. The economic and configuration impllcations, in light of the fuel crisis and sailplane technology, of optimizing the design of a given general aviation air- craft toward maximum "transport economy" even at the possible expense of "productivity'* needs to be evaluated.
References • McMasters, J.H., "An Analytic Survey of Low-Speed Flying Devices - Natural and Man-Made", AIA.A Paper P, Io. 74-1019, Sept.1974.
(Alse contained in Ref. 6 below). An expanded version of this paper will be publ ished in Tech. Soaring, Vol. 3, 1975.
2. Bikle, P., "Polars of Eight- 1971", Soaring, June 1971.
• Zacher, H., "Some Flight Tests on Self-Launching Sailplanes", NASA CR-2315, Nov. 1973. "Flug messung mit 52 Segelflugzeugen", Aero Revue, Oct., Nov., Dec. 1973.
4. Soaring, August 1974 (Journal of the Soaring Society of America, P.O. Box 66071, Los Angeles, Cal ifornia 90066).
5. Nash-Webber, d.L., "Motorless Flight Research - 1972", NASA CR 2315, Nov. 1973.
• _lP Proc. of the Second Inter. S_,mp. on the Tech. and Sci. of Low-Speed and Motorless Flight, MIT, Sept. 1974. (Available from SSA, P.O. Box 66071 Los_ Angeles_ California 90066 for $10.)
a Rasper, A. "Systematic Improvement of the Drag Polar of the Sailplane RJ-5", Soaring, Sept., Oct. 1951.
e Wortmann_ F.X., "Drag Reduction in Sailplanes", Soaring, June and July 1966.
Q Scheumann, W., 1972 Symposium on Competitive Soaring. (Available from SSA).
10.
Liebeck, R. H., "A Class of Airfoils Designed for High Lift in Incompressible Flow", AIAA Paper No. 73-86, ,Jan. 1973.
11.
Wortmann, F. X., "AirfolJs of the Variable Geometry Concept", OSTIV Pub. XI, 1974. (available from SSA).
12.
Technical Soaring, Vol. I and II (4 issues each), available from SSA or Dr. Bernard Paiewonsky, 9309 Burning Tree Rd., Bethesda, Maryland 20034.