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8.2(][) Drag of the Complete Configuration
Aerodynamic Conslderat ions Jan Roskam University of Kansas Introduct ion The purpose of this part of the paper is to focus on a number of drag items and relate them to the performance of the complete configuration.
First, the effect of fuselage camber, wing and nacelle incidence are discussed from a viewpoint of design decision making.
Second, the effect of overall cruise drag on the design gross and empty weight of the airplane is discussed. Examples show that cruise drag can have a very important influence on total airplane weight.
Third, the effects of usable cruise lift-to-drag ratio and wing-loading are shown to be important.
Finally several research needs relating to design of the complete configuration are reviewed.
Effect of Fuselage Camber, Wing and Nacelle Incidence in putting together a new airplane1 a number of fundamental geometric choices must be made. Typical examples of such choices are: - extent of fuselage camber; - wing incidence on fuselage; and - nacelle incidence and position relative to the wing.
In determ;ning the extent of wind tunnel testing required to "optimize" the configuration, the aerodynamiclst is confronted with a large number of variables. For example, if it is assumed, that two camber shapes, two wing-fuselage incidence angles and two wing-nacelle incidence angles are to be investigated, this alone leads to eight combinations to be tested. Under the economic constraints of the general aviation industry it is usually not feasible to do this much testing.
Major aircraft manufacturers, on fighter, bomber and even on some tTans- port programs, obtain significant inputs from NASA in terms of systematic wind tunnel configuration testing.
How does the general aviation designer choose the best configuration ? Well, very often he ends up guessin_ or, the shaping decision (for lack of definitive aerodynamic input) is made for him by engineers or managers outside of aerodynamics.
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Inputs such as tooling costs and marketing opinions outweigh the aerodynamlcist in the decislon making process, primarily because the aerodynamlclst does not have convlnclng arguments one way or the other.
To illustrate these points and to point once more to the need for systematic tunnel testing of general aviation research models, the following examples are given.
Note from Figure 1 that three different vertical nacelle installations are being used for turbopropeller airplanes. Note also, that all three use rather differing aft fairing shapes. The question arises: can they all be right?
Observe from Figure 2 that one manufacturer employs two quite different piston engine nacelle configurations. Figure 3 illustrates two more and again different nacelle shapes. The questions arises again: can they all be rlght?
Possible pay-offs of such research are illustrated in Figure 4 taken from Reference 1 (1942). Figure 4 shows a range of wing-body-nacelle drag coefficients of .1250 to .1050, (.0078 to .0066 based on wing areal) dependlng on vertical nacelle location alone. In other words, there are 12 drag counts to be galned by selecting the vertical nacelle location.
It would seem that the industry could derive significant beneflts from a series of systematic wind tunnel test to determine the best (lowest drag) shape of such wing-nacelle installations. Such research should also account for the effect of thrustline location and orientation, as well as for the possible beneficial effect of forward propeller shaft extensions, such as used on the hlavajo.
Drag Effect on Airplane Weight and Airplane Market Price Aerodynamic drag is not generally thought of in general aviation airplane design as an important factor affecting airplane weight. The reason may be the fact that usually new airplane "designs" consist of adaptations of components which are already in production, to a new alrplane. The term "tinker toying", although not a kind description probably applies to much of general aviation airplane design.
However, every now and then a truly new design evolves and then the effect of drag on weight can be important as will be illustrated in the following simplified analysls.
Assume that total airplane weight is broken down as follows:
(1)
W = WpL + W F + W E whe re: WpL - payload weight W F = fuel weight (including reserves) W E = empty weight Fuel weight and empty weight are assumed to be broken down as follows: W F = A + TxSFC x _ and: (2) R (3) W E = B" +'CT + D'W F whe re: A = weight of reserve fuel T = cruise thrust SFC = cruise fuel consumption Ibs/lbs/hr V = cruise speed R = cruise range B = empty weight without power plant and fuel system = weight of power plant per Ibs of cruise thrust D = weight of fuel system per Ibs of cruise fuel In cruise flight: T = W and L = D
(4)
I ift drag so that
T = W(D) (5)
Substituting equations (2) through (5) into equation (1) yields: Upon solving for W it is found that: VU_,t. +-A, t !_ .,,. "_ W _ ,,, or By introducing: it is possibleto rewrite equation (7) as: _kJ_ o_ - b//%.j_ D To determine the effect of drag on airplane weight, the differential can be found from equation (10) as: bk_J --4 b 0 0
= CV=-
Table 1 presents data from which ;>_/'J_ can be calculated for a typical general aviation piston engine driven twin.
So using equation (11): "_ Ud
= OI- z L 32- _ - Itq
This means that per unit L/D, the airplane gross weight can be lowered by about 120 Ibs, a significant saving when compared to ff_e empty weight, W E- Figures 5 and 6 illustrate similar results obtained in Reference 2 on small two-place turbofan (1200 Ibs max thrust) airplanes.
Table 1 and Figure 5 and 6 all show the importance of designing to the maximum possible cruise lift-to-drag ratio, if the lowest possible airplane weight is to be achieved.
It should be noted, that lower empty weight, achieved by better aerodynamic design has a very significant effect on the marketing price of an airplane. Table 2 shows typical market prices related to gross and empty weights for general aviation twins.
For the example twin of Table 1 the typical market price per pound of empty weight would be about 34 $/Ibs. Attaining a 120 Ibs saving would cut the market price by $4,080, a rather significant competitive advantage l Table 1.
Data for Calculation of_/_(,LIzI)_ for a Typical Twin.
Engines 2 x 300 hp. at 450 Ibs each WE = 3700 Ibs WF = lO00 Ibs WpL = 1600 Ibs SFChp = .45 Ibs/hp/hrs W = 6300 Ibs Assume propeller and engine weight =llO0 Ibs Assume fuel system weight = lO0 Ibs = 5,800 Ibs Assuming a cruise L/D = II and Wave cruise T cruise = 527 Ibs = 303 Assume Vcruise = 216 mph, then HPcruise Fuel flow in cruise then is 136 Ibs/hr.
This yields a range of (1000-200 (reserves)) 216 = 1270 miles The value of SFC is 5-,27= .26 lbs/Ibs/hr I So, A = 200 Ibs B = 3700-1200 = 2500 Ibs - IlO0 - 1O0 C - 527-" = 2.1 D = 1,000-200 = .13 From equations (8) and (9): a = 1600 + 200 (l + .13) + 2500 = 4326 Ibs b ,26 x T_ (l + .13) + 2.1 = .05 + 2.1 = 2.15 Lift-to-Drag Ratio and Wing Loading Effects Revisited Light airplanes, such as the Cessna 172 typically cruise at lift coefficients in the range of.
C. _ .3 -Eo ._-- Figure 7 shows that the corresponding L/D value varies from 10.0 to 13.2. This compares with a maximum L/D value of 13.8 indicating that significant improvements must be attainable by increasing wing loading. Increasing wing loading not only will bring the cruise C L closer to I/D/max on the polar but it will also shift the polar to Table 2. Typical General Aviation Light Twin Airframe Prices, (1975 Flying Annual Data) GrossWeight Price Price Price Empty Weight T w (Ibs) Type wE (Ibs] $/lbs
i'
$/Ibs 13.7 Cessna Skymaster 4,630 2,710 63,300 23.4 14.0 4,570 2,770 63,995 23.1 Piper Seneca Piper Aztec E 5,200 3,042 88,200 17.0 29.0 17.5 Beech Baron B55 5,100 3,155 89,000 28.2 Cessna 310 3,251 89,950 16.4 5,500 27.7 _J Averages 5,000 2,986 78,889 15.7 26.3 Rockwell Shrike 19.0 27.8 128,150 4,608 Commander 6,750 22.0 37.0 138,500 3,741 Cessna 402 B 6,300 21.4 13g,100 3,930 35.4 6,500 Piper Navajo B 17 4,950 4,042 Cessna 414 27.6 6,350 43.3 219,450 4,265 32.4 51.5 Beech Duke 6,775 Averages 6,535 ! 4,117 160,030 24.5 i 39.0 the left in the higher C L range (note that CDo actually will increase because it is based on a smaller wing areal). This fact has been previously demonstrated also in such papers as References 3, 4, and 5.
Figure 8 illustrates some typical results. Cutting wing area in the chordwlse direction by 30 percent results in a 10 percent reduction in thrust required and therefore _n fuel flow. F_gure 9, shows the relative aerodynamic "cleanness" of 1975 general aviation single engine airplanes compared to what is felt feasible in the future. To achieve this however, will require the introduction of new designs and new manufacturing technology.
Research Needs It appears that research into the follow ing areas would have significant potential for paying off in imporved general aviation airplanes: - Nacelle shape and nacelle location on wings (for horizontally opposed piston engines and for turbo propeller installation); - Improved methods for predicting the effect of drag on weight (Adaptation of NASA/Ames GASP?); and - Expansion and specialization of GASP to single engine and twin engine propeller driven airplanes with detailed accounting for weight, stability and control and propulsion interference factors.
Refe re rices • Becker, G.V. and Leonard, L.H.; HighSpeed Tests of a Nbdel Twin-Engine Low-Wing Transport Airplane; NACA TR 750, 1942.
2.
Heldenbrand, R.W.; Merrill, G.L. and Burnett, G.A.; Study of Small Civil Turbofan Engines Applicable to Military Trainer Airplanes: Final Report; NASA CR 137575; Garrett Airesearch 74-210987-A; April 1975.
e Roskam, J.; Opportunities for Progress in General Aviation Technology; AIAA Paper 75-292, Presented at the A]AA 1 lth Annual Meeting and Technical Display, Washington, D.C. February 24-26, 1975.
a Roskam, J.; New Airfoils and Higher Wing Loadings: A New Look at General Aviation Airplane Design; Paper presented at the Technological University of Delft, The Netherlands, May 20, 1974.
e Roskam, J. and Kohlman, D.L.; An Assessment of Performancy, Stability and Control Improvements for General Aviation Aircraft; SAE Paper 700240 pre- sented at the SAE Business Aircraft Meeting, Wichita, Kansas, Apral 1970.
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Mitsubish| MU-2G twin-turboprop utility transport )A[i , Beechcralt iS lirliner sevtntenn-soat lilhl transport Piper PA-31T Cheyenne six/night-;eat liirht transport aircraft (PQ_ol Prs_) Figure 1. Examples of General Aviation Turbopropeller Installations of the Beechcraft Baron 58 four/six-seat cabin monoplane "/kree-vlew drawing of the Bee©heruft Duke B$0 4/8-sent preNnrited transport (two 380 hp Ly¢omin| TIO-S41-El¢4 en=ines) Figure 2. Different Piston Engine Nacelle Shapes Used by One Manufacturer 34.5
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i Cessna Model 414 pressurised light transport Three-view drawing of the Piper PA-31P Pressurised Navajo (two 425 hp Lycoming TIGO-S41-E1A engines) Figure 3. Further Examples of Piston Engine Nacelle Shapes . . ".: tE • /.+# • ;_: ::,: u: +A,=,-.--,,oo.,_ _.
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Typical Range of Wetted Areas and Equivalent Parasite
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Drag for General Aviation Airplanes