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2.2 General Overview of Drag S .A. Anderson Ames Research Center The importance of reducing drag for general aviation aircraft is increasingly evident for the reasons noted in Figure 1. This includes rising fuel costs and the demand for improved performance to meet foreign competition. Equally important is the impact of more stringent noise and pollution standards because these factors indirectly affect aerodynamic performance. Although the general principles of how to achieve drag reduction are known to aircraft designers, applications to general aviation aircraft are a s|gniflcant challenge because this aircraft category is particularly sensitive to costs, maintenance, marketing, safety utility, and even stability and control.
How much do we really know about potential drag reductions for a typical high-performance business aircraft? A casual inspection of a current twin shows an abundance of brazier head rivets on all parts of the aircraft, several large external antennas, a lack of wing-fuselage filleting, lapped skin joints, many air inlets at obviously undesirable aerodynamic locations, and a single large-diameter exhaust pipe protruding at close to 90 ° to the airstream. On one twin turboprop aircraft, seven separate NACA flush inlets were located on each engine nacelle, some obviously of questionable value for pressure recovery. Although it is recognized that little systematic research on drag for current aircraft configurations has been conducted recently, many of the results of early NACA research can be usefully applied to current aircraft. Obviously, there is little similarity between the blunt, radial-englne transport aircraft of the late 1930!s, for which most of the early research was conducted, to today's sleek business jet, so few would question the need for addltlonal research.
As noted in the program for this workshop, it is timely to identify the state-of- the-art on aerodynamic drag reduction and develop a program plan for achieving meaningful results. There are, of course, many elements making up the total drag of an aircraft, including fuselage, wing, nacelles, trim, interference, tail, and cooling drag. The various topics to be covered in the next three days are shown on Figure 2.
Note that although cooling drag can be a large percentage of total drag (as high as 25%), it has previously been covered in a NASA/University/Industry workshop and will not be considered explicitly at this workshop. As noted in Figure 3, the purpose of this paper is to review the relative drag contributions of these various elements,
pausingbriefly to note what is known about each from past research, thereby
identlfying gaps in the knowledge for further consideration by speakers who will follow with detailed discussions.
Basic Sources of Drag ]t is important to identify the basic sources of drag in order to gain a better understanding of how improvements in performance can be made. Shown on Figure 4 are the following: (1) skin friction due to the air molecules rubbing the surface, the magnltude being a function of the flow conditions (laminar or turbulent) and the amount of wetted area; (2) induced flow or vortex flow primarily a function of wing aspect ratio; and (3) pressure effects associated with the profile or form of various parts of the aircraft.
Shown on Figure 5 is the variatlon of flat plate drag coefficient based on wetted area with Reynolds number for fully turbulent and laminar flow conditions.
Note that at large Re numbers typlcal of flight cruise condffions, the drag associated wlth turbulent flow is ten times higher than for laminar flow. In another example of the effect of flow conditions, Figure 6 compares the equivalent drag of a laminar flow airfoil and a circular wire. If nothing else, this is an incentive to avoid using exposed landing wires.
Drag Prediction Techniques Moving along to the flrst topic of our workshop, the various drag prediction techniques in use today are noted in Figure 7. The empirical approach takes advantage of semi-analytical methods in which wind-tunnel and flight-test results of similar type aircraft are factored in to establish a data base. Wind t_nnel measurements of drag for a new design are usually made, particularly for high-performance aircraft.
Extrapolation of small-scale (low Re no.) data to flight conditions can be difficult when including power effects and the accuracy of how well the small-scale model represents the actual aircraft. Finally, theoretical estimates, although used extenslvely in the past, have become more popular because of the availability of large capacity digital computers. Solutions of 3-dlmenslonal viscous flow effects appear to remain a challenge even with very large (and expensive) digital computers such as the ]LL]AC IV based at the NASA Ames Research Center.
Relative Contributions to Drag An example of results from drag prediction methods developed at NASA-Ames is shown in Figure 8. The aerodynamics subroutine calculates a series of factors which are used to establish drag values. Form factors are used for each component to represent drag increases above that of a flat plate to account for 3-D effects, interference, roughness, and excrescences. These calculations were made for the Learjet, Citation, Cessna 340, Piper Arrow, and Cessna 1.50. Note first, not unexpectedly, that the wing and fuselage are responsible for the largest source of drag.
Of interest in the last column is the amount to be added to match flight values of drag. This item varies greatly, gblng from less than 2 percent for the Lear]et to 37 percent for the Cessna 150. Improvements are needed to more accurately account for such factors as 3-D effects, cooling drag, landing gear, slipstream drag, etc.
Factors Influencing Fuselage Drag In the next item of our workshop agenda, Figure 9 gives several factors which affect fuselage drag. The surface conditions are very important because of the large wetted area. Windshield shape can significantly affect total drag at the higher Math numbers. Fuselage shape in terms of fineness ratio, nose shape and rear-end shape must be carefully considered. Shown in Figure 10 is the effect of afterbocly contraction ratio on drag. The contraction ratio must be greater than 2.0 to avoid a drag increase.
A similar consideration must be given in the vertical plane.
Factors Influencing Wing Drag Figure 11 lists several factors which are considered in selecting a wing for a new aircraft design. A large background of data is available from NACA research on airfoil sections and newer types such as the GAW-1 airfoil to challenge the designer in selecting the correct airfoil section for his aircraft. The NASA has underway a program on airfoil development aimed primarily at optimizing airfoils for specific operating conditions. Thickness ratio effects are generally well-documented. Planform and aspect ratio effects are also important as influenced by structural considerations.
Wing-tlp effects on induced drag will be covered specifically in a Langley Research Center paper describing the trade-offs on using "Winglets."
Another reminder of the importance of surface conditions and thickness ratio on drag is given in Figure 12. These NACA data tend to exaggerate the effect of roughness because the lower curve represents a mirror-finish surface condition. The
lrend today for the higher performance .jets is to use thicker airfoil sections for
structural and fuel volume considerations.
Factors ]nfluenclng Trim Drag Of the various factors shown in Figure 13 which affect trim drag, tall location and static stability have recently been glven increased affention. A tail location out of the slipstream ("T" tail designs) offer some drag decrease, and canard horizontal tall locations have appeared on experimental alrcraft. |n consideration of the small percentage of the tall surfaces to total drag indicated previously, one must be careful not to compromise stabillty and control in looking for performance imporvements. In this connection the control configured-vehicle (CCV) and relaxed static stability have recelved attention recently. An illustration of the effect of reducing static margin on the horizontal tail area required is shown in Figure 14. These curves indicate the variation of tail size with static margin to trim out the wing-fuselage pitching moment and the tail area needed for maneuverlng. To achieve the minimum tall area and therefore the least amount of drag, the static margin must be slightly aft of the neutral polnt (d_e/dCI" = 0) but ahead of the maneuver point (dFe/dAz = 0). Obviously, some for of stability augmentation must be provided to meet the FAR if minimum tall size is desired. At this point one would logically question the merits of reducing static margin for most General Avlatlon aircraft.
Considerations for Drag of Complete Aircraft In the final analysis, drag of the complete configuration is the most difficult to rationalize. As noted in Figure 15, cost is a factor that must be considered in each aspect of aerodynamic drag reduction. Cost aspects will be dlscussed in a paper later in the workshop. In this regard use of composltes may offer promise in that extremely smooth surfaces with attendant low drag can be achieved without high-cost manufacturing techniques. The second point, aerodynamic drag of the complete configuration, must take into account items such as wing nacelle and tall location, fuselage camber, wing and nacelle incidence, wing loading, cruise lift coefficient, etc. This area will be covered also on the last day of the workshop. The next item, propulsion system integration, is an important area, particularly for higher performance aircraft. Nacelle size and location can significantly affect high subsonic Mach number performance, as will be discussed by NASA Lewis Research Center. Fabrication details, the next item, must be considered in the light of cost and aircraft appearance. A smooth-looking aircraft not only has the potential for higher performance, but also sales appeal. Finally, an important point to know is the relative magnitude of the various sources of drag because of the many trade-offs in aerodynamic drag reduc- tion. This leads to the next point of discussion.
In Figure 16 the relative drag values are compared for a high performance aircraft. Leading the llst is the friction drag, with induced drag a close second.
Cross flow or 3-D effects can cause drag problems and are unfortunately the most difficult to predict. Induced drag primarily a function of wing aspect ratio can be reduced by wing-tip modifications, as will be covered by Langley Research Center.
Historical Survey of Drag Figure 17 presents the variation of drag based on wetted area as a function of tlme. Starting with the Wright Brother's design as the highest drag vehicle--not too surprising if you recall how large a drag penalty wires can create. The lowest drag values correspond to fighter aircraft such as the Douglas A-4 and LTV F-8.
There is no question that improvements have been made with time, but how well are we doing in realizing the goals of drag previously noted. Shown in Figure 18 is a comparison of flight drag data with flat plate skin fraction curves for turbulent and laminar flow conditions. The data which are for typical general aviation aircraft fall short of even achieving the turbulent flow drag values. The lowest drag value quoted is for the black buzzard (coragyps atratus) which in some 150 milllon years of evolution has no doubt managed to achleve reasonably good flow conditions without having to contend with cooling drag and propeller slipstream effects. There are |nd|cat|ons that these idealized goals can be approached by aircraft wlth good surface finishes, such as the point for the Lear]et at 30 million Re.
Concluding Remarks In conclusion, three main points should be kept in mind during the next three days (see Figure 19). We need to more accurately clarify the sources of drag for general aviatlon-type aircraft so that new designs can benefit from more accurate prediction techniques. Next, by knowing more about the sources of drag it will be possible to bring out the greatest potential for drag reduction. Finally, we must use our expertise to identify gaps in knowledge and point out areas which should receive high priority R and D efforts.
In closing, | would like to mention my personal drag reduction program carried out on a Vultee BY-13 trainer aircraft. By using NACA drag information, the high speed of this aircraft was improved from 160 to 210 mph, representing a change |n equivalent flatplate area from 16.1 to 7.2 square feet.
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3. PAPERS OF SESSION I - STATUS OF DRAG PREDICTION METHODS 3.1 Overview of Drag Prediction Methods D. Ruhmel, Cessna Aircraft Company 3.2 Prospects and Time Tables for Analytical Estimation of the Drag of Complete Aircraft Configurations F. O. Smetana, North Carolina State University 3.3 Summary of Drag Cleanup Tes_ in the NASA Langley Full- Scale Tunnel M. O. McKinney, NASA Langley Research Center 3.4 Simpllfied Theoretical Methods for Aerodynamic Design ,J, Tulinius, NASA Langley Research Center 3.5 Drag Reductlon/Back to Basics O. W. Nicks, NASA Langley Research Center 3.1 Overview of Drag Prediction Methods D. Ruhmel Cessna Aircraft Company This paper was not submitted for inclusion in these proceedings.