TITLE: Estimating Aircraft Performance AUTHOR: Isadore Herman
A collection ol articles on Ihe historical, operational, doctrinal, and theoretical aspects ol inleflkjcnec.
All sisiemcnts of fact, opinion or analysis expressed in Studies in Intelligence arc those of
ihe authors They do not necessarily reflect official positions or views of the Central intelligence Agency or any other US Oovemment entity, past or present Nothing in the contents should be consinied as asserting or implying US Government endorsement of an article's factual statements and interpretations.
Ramified process of determining the characteristicsew model displayedoviet air show.
ESTIMATING AIRCRAFT PERFORMANCE
the Soviet Union unveils an airplane of new design, as it did in some numbers at its air show last July,. Air Force has an immediate requirement for an estimatehe machine's performance characteristics in order to assess its place and contribution in the complex of Soviet air power. Such an estimate can be made with good reliabilityew photographs of the plane have been taken from the ground. The task begins with the photogrammetrtst and the photo
Drawings to Scale
The firstit Isimpleto transmute the photographshree- or six-view drawing properly dimensioned. It Is the photogrammetrist who makes thefor these drawings. He begins by determining the true shape of the aircraft and the proportion its dimensions bear to each other. Absolute values, the scale of the drawing, can comereliminary step Is to get correctionfor any distortion in the photography due to the camera itself. These should be readily available; all attach* cameras are checked and calibrated before being sent out to the field. The proportional drawing then becomes an optics problem to be solved by descriptive geometry and spherical trigonometry.
ectangular block is photographed from an angle, the lengths of the three sides on the Image do not bear their true proportions to one another and the angles are not right angles. Knowing that the three sides are actually at right angles, however, we can calculate what attitudes the block could have been in to produce this Image and what theproportion of the sides to one another would be at various look angles. If we liad several photographs of the
block from different angles, we could plot each of these look anglesunction of the apparent proportions of the sides In each. The intersection of these lines, since they all refer to the same block, would be the point which defined the true proportion ol the sides to one another. (See
An airplane has some of the geometric regularities of ablock and one of the methods used to find ItsIs similar tone^rawn^between thejwo wing tips of any plane must be'^erpendlcular to the center line of the fuselage and the wing tips must be equidistant from this center line. The tail must be perpendicular In the third dimension By measuring the apparent length, wing span, angle between tbe line connecting wing tips and the center line, and tall height, the photogrammetrlst cantheir true proportions as though theylock. Then, using this true ratio of length to span and height to span, he can work the equation backwards for any one photo-
graph and calculate what the roll, pitch, and yaw of thehad been with respect to the camera plate. (See
This data Is furnished to the photo Interpreter, whothe aspect of the photographic image and produces the required three-view proportional drawing. The photohere really wears two heads. He must usehoto interpreter to And and reproduce visibleof the airplane; but he must also use his uigenulty as an illustrator to fill in the areas that are not seen so that they will be properly portrayed. In reconstructing theseareas, there Is an important Interplay between the photo interpreter and subject analysts expert In aircraft com-ponenta
The next problem Is that of scaling the drawing, ofthe absolute dimensions of the aircraft. If we know the exact range from which the photograph waslikely If the plane was not Incan calculate the scale directly
as the quotient of the camera's focal length by the rangethe absence ol this Information we must rely eitheraircraft or other objects also in the picture orrecognlied from earlierthings asblisters, radar domes, andthatstill the same site. Analysts may have documentaryclues to the size of external components, Thethus completed by
personnel of the Foreign Technology Division of the Air Force Systems Command, which has central responsibility forthe performance characteristics of the aircraft. Many units of the FTD are Involved In the performanceAircraft Directorate, the Propulsion Directorate, the Engineering Analysis Directorate, the Hectronieaand the Weapons and Industry Directorate. Theyspecialists In propulsion, preliminary design structures, aerodynamics, performance, weights, armament, andThese are all representedask force assembled for the estimating project. The Aircraft Directorate. Inmonitors the progress of the analysis. Allunits are now given copies of the drawing.
The Propulsion Directorate has the task of estimating the power available to the aircraft and the performance of Its Jet engine. They have from the drawing the exhaust portand an Inlet configuration and size. First they try to correlate these with some engine known to be available, but more often than not this is not possible. Then they take whatever background InformaUon there Is, make someand perform several analyses of alternativefor the engine cycle to arrive at an Initial estimate. Thishrust-velocity curve for sea level and one for some altitude such0 feet (See
The weight analyst meanwhile la catenating the take-off gross weight of the airplane and breaking It down Into fuel, structure. Unding gear, tail, wings, etc. The method Is es-
'Bee Kenneth K. Oofrone'i TnteDlftnee Photomphy- in studies VtplB.
VELOCITY FMva* 3
scnlially theaa that used In Industry for preliminary design, approximating the component weights that have been empirically determined to correspond to such-and-suchvolumes, velocities, etc. For example, the weightingunction of Its dimensions, Its structural material and design, the speed regime for which It is Intended, and the weight of the airplane. The trick, supposing that we can get values for these factors from our photographs, ls to formulate the precise relationship amongeight engineers have devised complex formulae which vary with theone for an aircraft built by Douglas, for example,ifferent oneoeing airplane. It Is our aim to find the formula that applies ln the USSR and ultimately its variations for individual design bureaus ln the USSR In this we stillong way to go.
ora ipeclflc Ulustrabori of this and soma of the other methods usedarrow spplkaUoa of performance analysts, seeeorge's "The CakulaUoa of Soviet HelicopterIn SftidMr in.f.
The structures specialist, working from the Ihrce-view drawing and any supporting Information on such things as rivet lines, determines the structural layout of the airplane. This serves two purposes: It helps production analysishow the aircraft was built up and itheck by limited stress analysis on whether the structural limits of the airplane are exceeded by the performance estimated. No complete stress analysis is run.
the equipment, fuel,n the skeleton of the three-view drawing In functionally correct arrangement andaccommodation for tlie volume of fuel estimated by the weight analyst The layout to also used In deriving the -weight distribution and balance of the plane.
Armament, electronic, and equipment specialists use the dimensional data of the drawings along with featuresin the photographs to reconstruct the armament,and other component systems used In the plane These are not necessarily of Importance In determining the performance of the airplane Itself, but they are later used by weapons systems analysts when they evaluate its operational effectiveness.
The aerodynamics specialists determine the drag and lift factors affecting the airplane's performance. Dragfor supersonic flow is complex, usually including skin friction drag, compressibility drag, ware drag. Interference drag, and drag due to lift. Skin friction dragunction of the area of the aircraft exposed to the air stream (the "wetted" area. In aerodynamicompressibility drag Is encountered when speed becomes sufficient tothe air around the forward surfaces; Itharp Increase In total drag In the transonic region. Wave dragesult of pressure distributions unique in supersonic flow. Interference drag Is caused by tha proximity of oneof the airplane to another; for example, an airplane with external tanks, because of the Influence of the pressurefrom the fuselage and wings on the tanks and vice versa,otal drag greater than the sum of that for the clean airplane and that for the tanks In Isolation Drag due to lift In supersonic Sow Is similar to that In subsonic flow, but with an additional component In supersonic flow the
center of pressure Is located halfway back along the wings (aboutercent of wing chord, ln technical language) rather than at the forwardercent chord) as In subsonic, and there mustrimming of the aircraft to compensate for this shift in center of pressure. The trim drag thus Induced is the additional supersxmic component of the drag due to lift.
The foregoing types of drag are only those arising In the tfcternall^awts--along with the engine performance problem Called spillage or additive drag. It results from pressure differences around and Just Inside the Up of the engine air Intake. It ls ofmagnitude to require inclusion In estimates onaircraft.
The method of drag estimation used In FTD was chosen from among those used by several aircraft companies after determining which of them was most closely substantiated by wind tunnel and flight testa But knowledge of high-speed aerodynamics Is undergoing continual change as flight speeds go up. and methods of performance estimation are advancing accordingly. These advances are kept under constant study and FTD methods are revised and supplemented to keep them up to date.
In estimating lift, we are handicapped by the fact that exact wing profiles cannot usually be established fromBut measurements of thickness, aspect ratio, area dimensions,nable us toypical airfoilthat of the airplane. Data obtained from the National Aeronautics and Space Administration on similar airfoils can then be used to construct lift coefficients.
Now having data on weight, balance, stress limits, lift, and drag, we check the power required to fly the airplaneegime of flight speeds against the Initial estimate of engine performance prepared by the Propulsion Directorate. Ituestion of deciding whether our reconstructed airplane and engine are compatible in combination or whether we should rcstudy the engine or the aerodynamics. There are several choices that can be made both In engine parameters and in type of engine. For example, if the tailpipe Is large. It could
igh-thrust engine with relatively high specific fuelor It couldy-pass engine with much less thrust but lower specific fuel consumption. Decisions on such points as these are now made by the Aircraft Directoratemonitors on the basis of all intelligence availablethe aircraft or the requirements It was designed to satisfy.
OncelUuLS been decided thatlon rrtaKcTOnse/'the'pro^
thrust and fuel flow curvesunction of velocity atrange of altitudes, and the aerodynamics specialist computes drag and lift coefficientsunction of velocity at theseThese two sets of date, together with that on weight, are then turned over to the mission performance speeUlist* in the Engineering Analysis Directorate.
The mission on which the plane's performance Is to beIs divided into take-ofT run, climb to cruising altitude, cruise to combat point, combat, and finally cruise home and landing. Best climb performanceet aircraft Is denned as that ln which It reaches Its desired cruising altitude in the minimum of time. In order to determine thisarticular airplane It Is necessary to find the forward speed that yields the highest rate of climb at each of the whole range ofIn composite the speed profile necessary for reaching the cruise altitude In the shortest period of time. In moat flight-testing activities, this Is achieved by what arecalled "saw-tooth climbn which the airplane Is required to fly through an altitude span at variousand the speed at which the maximum rate of climb Is achieved Is then established as best for that altitude and weight.
We do essentially the same thing by calculations,the thrust available with the thrust required for thealtitudes and weight condition* during the climb. When rate of climb is plottedunction of velocityiven altitude and weight, the top of the curve represents the speed for best climb and the point at which the curve crosses the axis Is the maximum speed for that altitude. (Seeo these results there must be applied an accelerationto account for velocity changes with altitude; this Is taken care of In the computation.
The power settings, altitudes, and speeds for cruise are the chief factors In determining the maximum radius or range for the airplane. The rules governing best performancethe cruise portion of the mission are Important because the majority of the time In flight, at leastomber. Is spent In cruise and the largest amount of fuel Is used. In accordance with standard militaryonstant potential rate of climb Is maintained during the cruise for the given weight condition, the variables being altitude and speed. In designing an optimum mission performance, we pick arate of climb that will yield the martmnm in nautical miles per pound of fuel. This is not necessarily at thealtitude, as one might conclude at first glance from the fact that Jet engines normally operate most efficiently with respect to fuel consumption at the highest altitudes.
The type of combat and the power setting used therein are important delerroinants of the amount of fuel consumedthe combat portion of the mission. As throughout themission, the weight of the airplane is Important, and we must take into consideration the amount of fuel burned at any point. The weight of the bomb or ammunition also needs to be considered.
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reat deal ofIn standard requirement* for fuel reserves on landing. Normal military specifications callminut< flying time reserve, butercent of the Initial fueL If you take offound load thU means landing0 pounds of fueleserve seems to us excessivestimating the radiusomber. aUowmlniJt* res*rve endurance, but do not en3uTanw,"cond^
operating is determined accordingly. For the BISON this meant two engines operating and two dead; when two engines were operated at high power, the specific fuel consumption was lowest and less fuel was required forminutecmp^itaiion
As must by now be evident, therereat deal ofrequired inerformance estimate. To be more precise,ngineer man-hours used to beon the performance estimate for one airplane With the aid of automatic computers, however. It ls now possible to obtain in less than an hour an amount of data that had previously takenan-hours There are stillrours of engineering time required, but further research Indicates that we may be able to reduce tbls residue
Roughly similar to this process of aircraft evaluation Isevaluation; but evenruise rrussile. the mission pro-tm, the type of power plant, and the aerodynamics are slightly difTerent. They are different again In the ballistic missile, where, however, automatic computers are particularly useful in performing the tedious Integrations necessary In calculating the trajectory.Original document.