Created: 9/1/1959

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A collection of articles on the historical, operational, doctrinal, and theoretical aspects ot Intelligence.

All statements of fact, opinion or analysis expressed in Studies in Intelligence are those of

the authors They do not necessarily reflect official positions or views of the Central Intelligence Agency or any other US Government entity, past or present Nothing in the contents should be construed as asserting or implying US Government endorsement of an article's factual statements and interpretations


et of mathematical curves and formulas can be used to convert data derived fromthe stillew whirlybird tofor its performance in action.


The chariness ol the Soviets in disclosing facts about their military establishment and the technical characteristics of their equipment extends even to items not used primarily lor rnilitary purposes. Despite stringent security, however, they are not able to continueew item once It Is in series production and has been issued In quantity to field units. Recognising this fact, they finally relax to the extent of demonstrating new equipment they have in service at such public affairs as the May Day Parade, attended by all foreign military attaches stationed in Moscow. Orictureew Item may appearoviet military journal over some such caption as "Another Great Proletarian Achievement" or "The Highest Performance in the World."

. technical intelligence analyst thus finds before him one or more photographs of some new item of equipment alongerse Soviet description of it implying that it haspassed user tests and may actually be In production. This Is of course not enough. Its performance andmust be determined as accurately as possible If its influence on Soviet military capabilities is to be properly gauged. The analyst can prod the field collector withand wait for more information to come in. On the basis of his appreciation of the Soviet state-of-the-art in the new item's field, he can meanwhile make some guess as to what its performance should be. But on many important items he can do much more, and does. By assembling all the available Information, obtaining dimensions from an accurate scaling of the photographs, and making certain assumptions

Helicopter Performance

if necessary, he proceeds systematically to calculate theperformance of the new piece of Soviet nrUitary This article shows by way of example how the

clplea of mathcroatics and engineering can be applied to

tunal* tbe performanceew Soviet helicopter.

Required Data

The helicopterery complex machine,yriad of moving parts, black boxes, and structural members. Since, however, the principles of helicopter engineering are well understood and the laws of nature apply as Inexorably in the USSR as in other parts of the world, it Is possible,to some extent. developmental experience,umber of significant conclusionsoviet hell-copter from Its outward appearance. The first step is toaccurate dimensions by scaling one or more goodSome of the more Important dimensions to beare the aircraft's total length, Its landing gearthe diameter of its rotor or rotors, and its rotor blade root chord and tip chord length (the width of the blades at their Inner and outerrom these dimensions can beumber of values which will be needed inarea of mdividual rotor blades, the area of the rotor disc (the whole circle swept by tbehe rotor solidity ratio (total blade area divided by discnd the cross-section areas of various parts of the aircraft. The outward appearance of the helicopter should also help to establish whether it Is poweredas turbine or aengine and will show whether it has single, twin,or tandem rotors.

All information about the aircraft obtained fromovert and covert, Is now assembled and recordedform. Two Important additional specificationsengine horsepower (rating for normal continuousand for take-off) and tbe linear speed of rotor bladeif reliable information on these is not available they

usually be estimated: tbe rotor dux area will usually give

indication of the engine horsepowerelicopter of given

size, and the speed of sound constitutes for tbe rotor tip spaed y

an upper limit which cannot be approached (even

COIiriUliwM 1

Helicopter Performance

Sight) without undesirable air compression and separation effects. The type of rotor and Its 'blade and disc area will -' 'also* show trie gross weight of the aircraft. eights and Payload

Having assembled the above information, obtalnsble with some interpolationood photograph, the analyst can now calculate probable performance values. His firstIn my Judgment, should be the weight of the helicopter empty. This he determines by aggregating the weights of Its various sections and component parts, specifically the rotor blades, rotor hub assembly, body group, landing gear, engine section, power plant, power plant accessories, rotor mast, transmission drive shaft, transmission, starting system,system, lubrication system, fuel system. Instruments, flight control equipment, electrical system, furnishings, and communication equipment. Established mathematicalfor the weight of each of these components in terms of the specifications determined above have been shown byanalysis to yield sufficiently accurate results. For example, the weight of the main transmissioningle overhead rotor poweredeciprocating enginehere HP* is the take-off horsepower rating of thos the rotor disc radius, and VT is the rotor tip speed. Similar expressions have been established for each of the other sections, and the sum of these is the weight of the aircraft empty.

This net weight may now be subtracted from the previously determined gross weight toigure for the useful load, comprising the load of fuel, the weight of the crew, and the payload. The fuel weight can be calculated from the range of the helicopter, or if this is unknown It can be assumed atautical miles, the average range of most modern helicopters. The number of crew members, usually one to three, can be estimated from the size of the aircraft, and each can be taken to weigh with his personalounds. The useful load less the weight of fuel and crew Is the payload. and we have thus obtained our.first Important performance value.


Cetlmpi. Speed, and

In order to establish the hover ceiling for the heeilitudr It can maintain without forward flight, itto plot two curves, power required against

and power available against altitude, the altitude at which these curves Intersect is the hover celling. The powerdiminishes with altitude, the gradient of the curveon the type of engine In the aircraft. Plotting data can be obtained from any standard propulsion handbook. The power required, on the other hand, increases with altitude. The same factors apply to propulsion forward, and similar curves can be used to obtain the maximum and normalspeeds at any Elvtn altitude.

The graphs developed for obtaining the hover celling and forward speed can also be used for calculating the vertical and maximum rates of climb. The maximum Is attained inmotion because the power required for forward flight is less than that required for hover. The rate of climbunction of the surplus power available under given operating conditions, and therate of climb can be expressed


efficiency, ahp is powers gross weight, and Bhpm, is the minimum power required for forward flight under any conditions. The rate of climb thus calculated can be used further to establish absolute and service ceilings for the craft The absolute celling is reached when the maximum rate of further climb is zero, and the service celling is defined as the point where rate of climb dropseet per minute. The altitude* at which the available and required levels of power

satisfy the0 are therefore the absolute and service ceilings respectively. Range and Endurance

Thereumber of performance values which

on fuel consumption rate. These Include range (longest one- .. . ayadius (round trip withndurance (time

In theruising speed for maximum range, and...

speed for maximum endurance. These values can be obtained


Helicopter Performance

frora the performance curves already detertnined plus the 8FC/UHP curve (specific fuel consumption ru brake horse-powcr developed) of the engine.FC/BHP curve for this particular engine Is not usuallyurve typical for its power and type (reciprocating or gas turbine) can be obtainedropulsion data handbook. This assumption Is not likely to lead to any serious error.

Tbe cruise fuel rate in pounds of fuel per pound of gross weight per hour an now expressedurve plotted against forward speed. The minimum value ol willwith the velocity (and corresponding power setting) for maximum endurance;angent to the curve from the point of origin will Indicate the velocity and fuel rate forrange. If the amount of fuel carried by the aircraft is known or can be determinedrom the size of tbe fuelhe range and radius can be calculated from these results. Conversely, however, the range of the hebcoptcr can frequently be assumed toautical miles and the amount of fuel it must carry can then be determined by reverse The radiuselicopter is usually less than hah" the range because of fuel consumption in the second warm-up and take-off for the return trip.

Thereumber of other performance values which are of considerable Importance In estimating the effectivenesselicopter in service. Some of these, such as lifeof component parts and time required for overhaul, can not be determined by analytical methods, but only by testing the aircraft under field operating conditions. Others, such as stability and control values, can be found by calculation but in my opinion do not warrant the effort required. The fact that the Soviets have decided toivenmodel is sufficient indication that it responds to Its control Instruments with reasonable promptness and that it does not suffer from serious aerodynamic instability. Lengthy computations to arrive at these conclusions are hardly necessary.

The principal calculations made In estimating Soviet hell-copter performance are therefore those outlined above In very


abbreviated summary. The summary outline- will have beenope, to show the reader how, with relatively little to go on. It Is possible to arrive at significant conclusionsew Soviet model. The performance values thusare of course mere approximations, which shouldbe used only In the absence of more reliable data. As soon as overt or covert collection media can furnish dependable Information, the calculated values should be discarded In favor of more accurate figures based on observation or actual tests

of the aircraft.

Original document.

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