hCl\L'M ACE No. Li+GU NATIONAL ADVISORy COMMITTEE FOR AERONAUTICS WARTIME REPORT ORIGINALLY ISSUED July l^kk as Advance Confidential Report L14-G31 NACA MACH NUMBEE WAEKIKG DEVICE FOR USE IN FLIGHT By Jerry Goodiaan Langley Memorial Aeronautical Laboratory Langley Field, Va. NACA WASHINGTON NACA WARTIME REPORTS are reprints of papers originally issued to provide rapid distribution of advance research results to an authorized group requiring them for the wax effort. They were pre- viously held under a security status but are now unclassified. Some of these reports were not tech- nically edited. All have been reproduced without change in order to expedite general distribution. L - 203 \ Digitized by tlie Internet Arcliive in 2011 witli funding from University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation http://www.archive.org/details/nacamachnumberwaOOIang Ij} fy^oij iKo^f^fi- NAG A ACR Fo. IJ4.G3I NATIONAL ADVISORY COJ^flTTEE FOR AERONAUTICS ADVANCE GONPID'.ilNTIAL REPORT NACA MACH NUMBER V'/ARNING DEVICE FOR USE IN FLIGHT By Jerry GoodiTian SUmiARY An instrument for warning the pilot of the approach of the airplane to critical speed conditions has been developed at the Langley Memorial Aeronautical Laboratory. The device closes a contact that completes the electrical circuit of a suitable warning indicator v/hen a predeter- mined limiting Mach number is approached. The operation of the instruiaent is based on the relation between Mach number and the ratio of impact pressure to total pressure. These pressirres are obtained from the pit ot- static installation on the airplane. The accui'acy of the device, exclusive of errors due to the pltot-static installation, is ±1 percent. IJWRODUCTION The iraximi-Tm safe speed of present-day high-speed airplanes is limited by adverse corapressibility effects. The characteristic results of exceeding the safe limit are large changes in trim, stability, and control forces, which are usually dangerous and are soraetimes accompanied by severe buffeting. The inordinate changes in pressure distribution around the airplane may cause structural damage to the wings and tall. This condition is usually encountered during dives. These effects' occur at a dif- ferent airspeed for every altitude but, since compressi- bility effects are a function of only the Kach number, adverse effects alv/ays occur at essentially the same value of the Mach number, regardless of altitude, for any one airplane . The usual method by v^hich the pilot determines the proximity of the airplane to the limiting speed involves reading an altimeter, an airspeed indicator, and a placarded table of predetermined limiting speeds and altitudes for the particular airplane. In high-speed flight or dives, it is obviously difficult for the pilot confid"::ntial haca acr no. lI^gji to correlate these readings quickly. The NAC/l Mach number wcrninr; device was developad to warn the pilot directly when the limiting condition is approached. PRIHCIPIE OF OPiRATJON The operation of tlie NAOA T.'ach rumher wai-ning device is based on the fact that Mach number is a function of the ratio of impact pressure to total pressure as shown by the relation yf Y - 1 • 1 (1 \ TT - O H - 1 (1) vjhere M Ivlach jiumber H free-stream total pressure, pounds per squ.are foot p free-stream static pressijire, pounds per square foot Y ratio of specific heats (1.'; for air) The pitot-static tube furnishes the pressures H and p. The warning device utilizes these pressures to close a contact when (R - p)/^ reaches a predetermined value. This value is denoted as the operating point of the instrximent. A 6 iagram of The inner be Hows pres sure H. The and the oute r bel pressure H - p. prop orti oned that the force exerted site to the force incr ease of H - inner be Hows off the instrument is sh is evacuated and res case is subjected to lows is therefore act The effective areas , at the operating po by the outer bellows exer-ted by the inner p causes the outer b its stop and thus cl ov.'n in figure 1. ponds to total static pressure p uated by impact of the bellows are so int of the device, is equal and oppo- ci e 1 1 ow s . Pur the r ellovv's to lift the ose the contact. The bellows-area ratio in terms of (H - p)/^ at the operating point of the instrument may be evaluated from the following relations: (H - p)A2 = (H - 0)A2 ^ = H - P A-, H COIIFIDTJTIAL N^^ICA ACR No. li+GJl COFFIDENTIAL and, from equation (1), .1 ^-^ = 1 - (o.2m2 h- i)2 (2) H where A-j_ area of bellows subjected to impact pressure H - p, square feet Ag area of bellows subjected to total pressure H, square feet A plot of (H - p)/R against Mach number, suffi- ciently accurate for field use, is given in figure 2, GENERAL DESCRIPTION A basis of design was provided by the NACA Mach number indicator (reference 1). A dia--Tram of a cross section of the warning device is shown in figure 1. Photographs of the device are given as figures 5 to 6. The inner bellows is evacuated and is prevented from collapsing by an internal stop. The heads of the bellows are fastened together and sealed. An insulated silver contact is attached to the fastening screw, A flat spring fits the collar of the fastening screv; and prevents side shake of the bellows. A fixed contact on a flat spring is mounted directly above the movable contact. Stops prevent overtravel of the bello'ws vvhen the operating Mach number is exceeded. Adjustments are provided for the internal and e>iternal stop screv;s, inner-bellows height, and contact gap. Both contacts are brought out of the airtight case through insulated and sealed terminals, The device is constructed entirely of brass and weighs about 2— pounds. DESIGN CONSIDERATIONS The Mach number at which adverse effects are encoun- tered is essentially constant for a particular airplane design, varying only with flight lift coefficient. This limiting Mach number is usually from 0.05 (about 55 mph ) to about 0.10 (about 70 mph) greater than the critical Mach number of the airplane, depending upon the particular design. Critical Mach number is defined as the flight C0N''-^^IDE"'\TTIAL ^ CONFI.DENTIAL NACA ACR No. ll|.G51 Mach nioriber at which the velocity of all' flow at some point on the airplane reaches the speed of sound. Tlie Mach number at vhich a warning should be issued .. can be determined in high-speed wind-tunnel tests or in flight tests. In order to allow for instrument lag/ air- plane acceleration, and pilot lag, cleai-ance between this limiting Fach number and the Mach number at which the device operates should be provided. The errors introduced by the pitot-static installation of the particular air- plane should be taken into account by determining the relation between the actual limiting Mach nijunber and the indicated limiting ?>''ach number for the installation and then basing the operating point of the instrurr.ent on the indicated limiting ?''ach nuinber. A discussion of pitot- static installation errors is given in reference 2. cr- The actual clearance allowed depends on the maximum acceleration of the airplane near the limiting Mach num- ber and the sum of instrument and pilot lag. The reaction time of the pilot to the warning signal can be easily determined. The instrument lag is difficult to predict, however, since it is possible for the Instrw.ient either to lead or lag the operating Kach number; for example, in a dive the static pressure may lag more than the total pressm-^e because of the larger static-side volume and the instrument v*fill therefoi'e issue its signal before the limiting Mach number is actually reached. The clearances needed must therefore be determiiic-d by experience. In order to avoid handicapping the performance of the air- plane, it is important that the warning device should not issue the signal too soon. DESIGN PZ AT IRES The bellov/s areas in the warning device are large so that large forces are available to actuate and assure positive operation of the contacts. The bellows are qi:ite flexible and the required movement is small with the result that the bellows spring forces are negligible. The instrument is therefore practically independent of the belD.cv/s spring forces and the operating point depends on the ratio of bellov;s areas. Hysteresis, drift, and aftereffects of the bellows are negligible because of the small deflections and lovif stresses involved. Both bellows are ot" similar metal and have received the same heat treatments; hence, both should respond similarly to tem.perat\ire and aging effects and keep the operating point unchanged . CONFIDENT LAL ]^^c^ ACR No. rJ+Gji conftdeftial 5 Sliding line contact is present "betv/een the large silver contacts to remove oxide film or dirt on the contacts. The current -carryin£^ capacity is. sufficiently high that a warning indicator can be actuated without the use of intermediate relays. Velding of the contacts may result in a slightly lower valiie of operating point on "breaking contact than on making contact; hovjever this shift in operating point should "be negligible. LABORATORY TESTS La"'ooratory tests of the prototype of the NACA Llach number warning device with an operating point at M = O.65 disclosed the following characteristics: (1) Altitude effect: ±l-p3rcent shift in operating point between sea level and 50,000 feet. This error might be reduced by s more careful adjustment of the bellows. A residual error of 0.1 percent is inherent in the mechanism because of the bellov.7s spring constants. (2) Temperature effect: A temperature change of 100^ F (from 90° F to -10° F) shifted the operating point less than l/lj. percent. This shift is probably due to stiffening of the bellows. (5) Contact rating: No adverse effects were noted at voltages of 12 to 2I4. volts and currents up to 0.3 ampere. For service use, the contacts are conserva- tively rated at 2i± volts and I/8 ampere, or 110 volts and 0.05 am.pere. (.a) Acceleration effects: Accelerations along the bellovjs axis shifted the operating point 1 percent per g. Accelerations per loendicular to the bellows axis caused a 1 • — percent shift per 12g . (See Installation recommendations.) k (5) HysteresivS: Repeated tests in v/hich the operating point was approached from both above and below indicated that the instrument will always issue its signal within l/Lj. percent of the operating point. (6) Vibration effects: The operation of the instrument was checked while it v;as being vibrated at 0.0[{.-inch double amplitude and frequencies of 10 to 60 cycles per second. The operating point v;as found to shift l/li percent. After 2 hours of vibration under the same conditions, the operating point viras again checked and was found to have shifted l/ij.. percent. CONFID'illvTiAL 6 CONPIDEWTIAL NACA AC'R No. 1J4.G5I (7) Lag: The la2' of the instru-tient is defined as the ti'Tie elapsing between the attainnient of the operating pressure ratio at the pitot-static tube and the issuance. of the warning signal. The warning device v;as connected •. -s to 50 feet of tubing 3/1^ inch in diarrieter . A rate of pressure rise equivalent to that occurring when the airplane exroerienees a longitudinal acceleration of 2g at ^00 miles per hoior was applied. The lag was less than O.O5 second. Similar tests simulating dives gave the same result. FLIGHT TESTS The FACA '"^ach number warning device used in the laboratory tests f operating Doint at }". = O.63) was installed in the left-vn'.ng gun bay of a P-UYD airplane (fig. 1). The instrument was bolted firmly to a gun- mount bracket and was connected to a warning light on the instrument panel. Altimeter and airspeed indica- tions were noted by the pilot during shallow dives from 2S,0C0 feet. An indicated airspeed of JOO miles per hour vms maintained. Coi-relation with the altimeter and airspeed indications showed that the warning device made contact at M = O.olj. and broke contact at I'T = 0.62. RANGES Aim ADJUSTivEKTS The operating point of the warning device has been shown to depend on the ratio of bellows areas, A suf- ficiently large selection of bellows sizes is commer- cially available that a warning device can be designed to give an3'" desired operating point in the existing range of limiting I.Tach n-umber within ±2 percent. The effective areas are controlled to aboat 2 percent dioring manufactiore so that bellows can be individually matched to obtain any desired operating point if greater acc-uracy is v;arranted. Further adjustm.ent of the operating point within a 1-percent range can be obtained by permitting a slight amount of residual air in the evaciiated bellows. The pressure in this bellows must not exceed 7 millimeters of merctiry to avoid tem.perature and altitude errors. An additional 1-percent shift in the operating point can be obtained by adjusting the contact gap. In order that the device be free from altitude effect, both bellows must be correctly adjusted. The internal stop must prevent any deflection of the inner bellows when evacuated, and the height of the inner bellows must be adjusted so that the outer bellov/s is not deflected when the heads are fastened together. Errors CONFIDENTIAL MAC A ACH No. L)iG-31 COM?IDS:JTlAL ''' in these settings are manifested as a change in operating point vi/ith altitude and can be detected by suitable ground tests in an altitude chamber, RECOMMENDATIONS The NAGA Mach number warning device as developed has been tested and found to be sufficiently accurate for the use described herein. The applications of this .device are numerous, as it can be employed to operate any electrical apparatus through the use of suitable relays. It is recommended that the device be mounted as close as possible to the pltot-static head in order to reduce lag. This location of the instrument is possible in most installations because the device is small and requires only two electrical leads to a warning device on the instrument panel or to the equipment being operated. The instrument should be installed in a horizontal position with the bellov/s axis parallel to the lateral axis of the airplane in order to minimize acceleration effects. If vibration is excessive, the use of vibration- absorbing mounts may be advisable. It is possible, at a sacrifice in accuracy, to decrease the size of the instrument by replacing the bellows with smaller bellows or diaphragms of proper design. The weight can be decreased by using dural in place of brass wherever possible. The static-pressure and total-pressure volumes may be altered by the use of suitable blocks in order to decrease and equalize lags. Langley Memorial Aeronautical Laboratory National Advisory Committee for Aeronautics Langley Field, Va . CONFIDENTIAL 8 CONFIDT.viTIAL NACA ACR No. i4g31 REPERTi]NCE3 1. S:nlth, i'^oriT'.an F. : NACA Mach N-uiribcr Indicator for Use in Hlq;li-Speed Tunnels. NACA ACR No. 3G51, 2. Thompson, F. L., and Zalovci^c, John A.: Airspeed Measurements in Flight at High Speeds. NACA ARR, Oct. 19l;2. CONFIDENTIAL NACA ACR No. L4G31 Fig. 1 Sif^/-/ c conn e c //on Coi/e/- Interni}/ stop Spr/nci m oun/-pn/ cont^r.i- Coryfact F-^steninci screw SJdes/MAe restrc7/nin6 s prin6 Seal -off tube E/ectr/Cd/ fi/u(i Pi tot connecf/on NATIONAL ADVISOPY COMMITTEE FOt AERONAUTICS Future I.- Cross- sectional view of NACA Mach number warninc^ device. \ NACA ACR No. L4G31 Fig. 2 1 1 i. ' '" T " — I — i _;i- ife 1 i ■ .__..__. |.,.„,^.. ; - - '-'r- ...._ .■i_.,. -:-i ■ --:- . ,,,,.. ' T" I -' ii -- t " y 1 -..1. ) ■ 1 ^ 1 1 i^-^ , ii'.', k ,.,.„ -^ -r ^- .:_ i t. j ■-i-- — - --- U:i '\ - - ....L... .. !.- 1 -T- ^ ^-■^ "■ r*; \ 1 -- ,,., rj 4- — i--.- i ": ... "- rr \ 7a:" ■ ^ r'i: \ -- \ . -■-- \ ., -!-.. It v'' \ i Ml 1 >t c. \ - i a M : : ^ 51'""" -N ■■-;- :: \ j I : 1 ; tUi [r— rj--- . ! 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A L b^ ^ (U •d bo C! •H c! a Li 0) 3 G o cd < < 2; 0) •H XI S 0) (0 CO cd CO I to (U bo NACA ACR No. L4G31 Figs. 4,5 Figure 4.- NACA Mach number warning device Figure 5.- Evacuated inner bellows of N VGA Mach number warn- ing device mounted on base. NACA ACR No. L4G31 Figs. 6,7 Figure 6.- Contact and terminals of NACA Mach number warning device assembled in case; outer bellows and fastening screw mounted. Figure 7.- NACA Mach number warning device installed in left-wing gun bay of P-47D airplane. \ UNIVERSITY OF FLORIDA 06 465 UNIVERSIT/ OF FLORIDA DOCUMENTS DEPARTMENT °i0MARSTON SCIENCE LIBRARY P.O. BOX 117011 ,^,,MQA GA1NESV1LLE.FL 32611-7011 USA \