TECHNICAL NOTE NO.135 - MEASURING AN AIRPLANE'S TRUE SPEED IN FLIGHT TESTING - By W.G. Brown

Sep 2, 2026

This technical note analyzes how to determine an airplane’s true speed during flight testing. It explains why airspeed indicators function mainly as stability cues for routine operations, but require accurate calibration for competitive measurement and wartime research. The note describes calibration error as three components and discusses practical timing methods to infer wind velocity and correct ground-speed observations into true airspeed, including course timing and triangulation approaches.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages15

Reading Time38 min

AccessFree to download

Measuring an Airplane's True Speed in Flight Testing

Purpose and calibration of airspeed indicators

Components of airspeed meter error

Timing methods to infer wind and true speed

N 625213510NALA 3RY C0MMTTCR AE30NAUTICS2g # TECHNICAL NOTESNATIONAL ADVISORY COITTLE FOR AERCNAUTICS. No.135 人200 37Ne # MEASURING AN AIRPLANE¹S TRUE SPEED IN FLIGHT TESTING,By W.G.Brown. April,1933. # NATIONAL ADVISORY COEMITTCE FOR AERONAUTICS. TECHNICA NOTE NO.135, # MEASURING AN AIRPLANEIS TRUE SPEED IN FLIGHT TESTING.By W.。Brown. For the ordinary range of flying where the pilot neverknows the exact velocity and direction of the wind,the airspeedmeter sezves more as a stability indicator than it does as a trueinstrument of navigation.The pilot depends uron it as he de-pends upon the sound of the interplane wires or the stiffnessof the controls to tell in wha.t part of the flying range theairplane is operating.Except possibly in navigating over thesea where the winds are steady and can be allowed for,an errOTof a few miles per hour is of no great consequence. In competitive performance measurement or full scale scien-tific research a more exact knowledge of speed is necessary,Since many of the airplanes in use during the war were equi.ppedwith airspeed meters,of the Pitot or Venturi type it becamecomnmon practice to carefully calibrate these instruments and touse their corrected readings for scientific purposes.Toussaint*in France,and Tizard**in England developed similar methods fordetermining true speed in performance testing which with modifi-cations are still in use today.Briefly,their methods consistin dividing the airspeed meter error into three parts:(1)The *Notes on Methods and Calculations for St atic Tests and F-llflight Performance Tests for New Military Airplanes,by CaptainToussaint,Chief of the service of Tests of the S.T。Ae,1918.**Lecture before the Royal Aeronautical Society,1917,by Tizard. —2— error of the gage in transnitting the pressure difference pro-duced by the tube into speed under standard and ideal condi-tions;(2)the error due to variations of the density from thestandard for which the instrument is calibrated;and(3)theerror due to the particular location of the airspeed meterhead on the airplane. The first error is determined by calibration in the labor-atory.The second is computed from the observa,tions of pres-sure and temperature during tlie test.The third is the resid-ual error'determined by timing the airplane over a known dis-tance,and is by far the most difficult to determine precisely. Unfortunately,since the airplane flies through the airits speed must be observed relative to the air and not relativeto the ground,or if observed relative to the ground a correc-tion must be made for the amount by which the airplane!s speedis reduced or augmented by the wind. To the writer's knoledge five distinct methods shown inFigs.1~5,have been used for timing an airplane over a givendistance and getting a measure of the wind.veloci ty and hencethe true speed of the airplane.Fig.1 shows a straight cou.rselaid out a.long the prevailing vwind direction.The ai.rplane isflown first with and then against the ind and the true air-plane speed is taken as the average of speed in the two dizec-tions.The time over the course may be observed from the ground,the time of passage over each end observer being communicatedto the other end electrically,or the observation may be made —3— from the airplane using a pendulum sight. Ti.g.3 shows a course marked by two parallel lines.*Thepilot holds his airplane on a compass course at right angles tothe end lines and flies back and forth as before. If the tim-ing is done from the ground each end station may consist of twoparallel aires stretched one above the other so as to enablethe observer to sight the airplane at the instant it passesthrough the vertical plane containing the wires。 The averageof the speed in the two directions gives the airplene speedregardless of the wind direction. A third type of speed cauree which has become very popularis triangular in shape(Fig,3),The speed of the airplane canbe determined from the ground speed

FAQs

Why is an airspeed meter not a true navigation instrument in ordinary flight?

In routine flying, the pilot lacks the exact wind velocity and direction, so the airspeed meter mainly serves as a stability indicator rather than a true navigation measurement.

How is airspeed meter error broken down in the note’s method?

The note divides the total error into three parts: gage transmission error under standard conditions, density variation error from calibration standards, and a residual error tied to the airspeed meter head location.

What methods are used to obtain true speed from flight timing?

The note explains multiple approaches to time the airplane over known distances, obtain wind velocity, and apply corrections—such as flying courses with and against the wind, using triangular or parallel-line course patterns, and employing triangulation or camera-based observations.

Sep 2, 2026

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