NACA mach number warning device for use in flight

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Material Information

Title:
NACA mach number warning device for use in flight
Alternate Title:
NACA wartime reports
Physical Description:
8, 5 p. : ill. ; 28 cm.
Language:
English
Creator:
Goodman, Jerry
Langley Aeronautical Laboratory
United States -- National Advisory Committee for Aeronautics
Publisher:
Langley Memorial Aeronautical Laboratory
Place of Publication:
Langley Field, VA
Publication Date:

Subjects

Subjects / Keywords:
Mach number   ( lcsh )
Aerodynamics   ( lcsh )
Genre:
federal government publication   ( marcgt )
bibliography   ( marcgt )
technical report   ( marcgt )
non-fiction   ( marcgt )

Notes

Summary:
Summary: 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 when a predetermined limiting Mach number is approached. The operation of the instrument is based on the relation between Mach number and the ratio of impact pressure to total pressure. These pressures are obtained from the pitot-static installation on the airplane. The accuracy of the device, exclusive of errors due to the pitot-static installation, is ±1 percent.
Bibliography:
Includes bibliographic references (p. 8).
Statement of Responsibility:
by Jerry Goodman.
General Note:
"Report no. L-203."
General Note:
"Originally issued July 1944 as Advance Restricted Report L4G31."
General Note:
"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 war effort. They were previously held under a security status but are now unclassified. Some of these reports were not technically edited. All have been reproduced without change in order to expedite general distribution."

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 003804356
oclc - 123558043
System ID:
AA00009402:00001


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Full Text
L- 03


ACR No. L4ACG


NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS






WA1RTIMEll RE PORT
ORIGINALLY ISSUED
July 1944 as
Advance Confidential Report L4G31

NACA MACH NUMBER WARNING DEVICE
FOR USE IN FLIGHT
By Jerry Goodman


Langley Memorial Aeronautical LI
Langley Field, Va.












WASHINGTON

WASHINGTON


laboratory


j. 4. ; ..
-w...


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 war 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


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Digitized by Ihe Inlinel Archive
in 2011 with lunding from
University ol Florida, George A. Smathers Libraries will support from LYRASIS and the Sloan Foundation


hlip: www.archive.org details nacamachnumberwaOIOlang







NACA ACR to. L5G31

NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS


ADVICE COIFID 'NZTIAL REPORT


NACA MACH TUMBER '."!ARTTING DEVICE

FOR USE IN FLIGHT

Ey Jerry Goodman

SUMTTAAP

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 when a predeter-
mined limiting Mach number is approached. The operation
of the instrument is based on the relation between riach
number and the ratio of impact pressure to total pressure.
These pressures are obtained fro: the pitot-static
installation on the airplane. The accuracy of the device,
exclusive of errors due to the pitot-static installation,
is 1 percent.


INTRODUCTION

The maximum safe speed of present-day high-speed
airplanes is limited by adverse co:mpressibility effects.
The characteristic results of' exceeding the safe limit
are large changes in trim, stability, and control forces,
which are usually dangerous and are so:.ietimres accompanied
by severe buffeting. The inordinate changes in pressure
distribution around the airplane nmay cause structural
damage to the v:ings and tail. This condition is usually
encountered during dives. These effects cccur at a dif-
ferent airspeed for every altitude but, cince compressi-
bility effects are a function of only the yach number,
adverse effects always occur at essentially the same value
of the ?,Iach number, regardless of altitude, for any one
airplane.

The usual method by which 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








NACA ACR No. L4G51


to correlate these readings quickly. The NACA ?'ach
number warning device was developed to 'iarn the pilot
directly when the limiting condition is approached.

PTPI.TCTPIE OF OPEATION

The operation of tihe TNA'. P'ah number warning device
is based on the fact that P'ach ruTiber is a function of
the ratio of impact nrecsure to total pressure as shown
by the relation-1

E =k 11 1 (1)
2 1 \ H

where

M a:ach i-umber

H free-stream total pressure, pounds per square foot

p free-stream static pressz;re, pounds per square foot

y ratio of specific heats (1. for air)

The pitot-szatic tube furnisies. the pressures H
and p. The warning device utilizes these pressures to
close a contact when (F p)/4h reaches a predetermined
value. This value is denoted as the operating point of
the Instrunment.

A diagram of the instrument is shown in figure 1.
The inner bellows is evacuated and responds to total
pressure H. The csse is subjected to static pressure p
and the outer bellows is therefore actuated by impact
pressure H p. The effective areas of the bellows are so
proportioned that, at the operatinC point of the device,
the force exerted by the outer bellows is equal and oppo-
site to the force exerted by the inner bellows. Further
increase of H p causes the outer bellows to lift the
inner bellows off its stop and thus close the contact.

The bellows-area ratio in terms of (H p)/H at
the operating point of the instrument may be evaluated
from the following relations:

(H p)Al = (H O)A2
A2 H p
H
1: H
0.C IDI ,T IAL


C 01 IDFT IAL





-rACA ACR No. L.G531


and, from equation (1),
-1
H =- 1-. (0.212 + 1) 2 (2)
H
where

Al area of bellows subjected to impact pressure H p,
square feet

A2 area of bellows subjected to total pressure -H,
square feet

A plot of (H p)/H aZainst Mach number, suffi-
ciently accurate for field use, is given in figure 2,

GErNEPAL DES.CRI OPTION

A basis of design was provided by the NACA Mach
number indicator (reference 1). A diagram of a cross
section of the warning device is shown in figure 1.
Photographs of the device are given as figures 3 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 screw and prevents
sideshake of the bellows. A fixed contact on a flat
spring is mounted directly above the movable contact.
Stops prevent overtravel of the bellows when the operating
Mach number is exceeded. Adjustments are provided for
the internal and external stop screws, 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 21 pounds.
2

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 Prach 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
desiLi. Critical Mach number is defined as the flight


CON 'IDE.ITTAL


COPPI TD TIAL







NACA ACR No. L4G31


,P.ach number at which the velocity of air flow at some
point on the airplanee reaches the speed of sound.

The Mach number at which 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, clearance between this
limiting V'ach number and the Macn number at which the
device operates should be provided. The errors introduced
by the pitot-static installation of the parLicular air-
plane should be taken into account by determining the
relation between the actual limiting MacI number and the
indicated limiting .Iach number for the installation and
then basing the operating point of the instrument on the
i;diicated limiting 1ach nunber. A discussion of pitot-
static installation errors is given in reference 2.

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 warninur signal can be easily
determined. The instrument lag is difficult to oredict,
however, since it is ~cslble for the instrument either
to ledd or laC the operating I.:acch number; for example,
in a dive the static pressure may lag more than the total
pressure because of the lar-rer static-side volume and the
instrument will therefo.'e irsue its signal before the
limiting Mach number is actually reached. The clearances
needed I .us therefore be determine-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 FEATURES

The bellows 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
quite 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 bellows spring forces and the operating point depends
on the ratio of bellows areas. Hysteresis, drift, and
aftereffects of the bellows are nerlir-ible because of the
small reflections and low stresses involved. Both bellows
are of similar metal and have received the same heat
treatments; hence, both should respond similarly to
temperature and aring effects and keep the operating point
unchanged.
C O(T F IDETT TIL


C ON FIDENT IAL






:.ICA ,CR No. L4,L51 CONF'TD-NTTTAL 5

Sliding line contact is present between the large
silver contacts to remove- oxide film or dirt on the
contacts. The current-carrying capacity is sufficiently
high that a warning indicator can be actuated without
the use of intermediate relays. :elding of the contacts
may result in a slightly lower value of operating point
on breaking contact than on making contact; however this
shift in operating point should be negligible.

LABORATORY TESTS

Laboratory tests of the prototype of the ITACA I.ach
number warning device with an operating point at M = 0.65
disclosed the following characteristics:

(1) Altitude effect: 1-p3rcent shift in operating
point betv'een sea level and 50,C0O feet. This error
mi,'-ht e reduced by a more careful adjustment of the
bellows. A residual error of 0.1 percent is inherent in
the mechanism because of the bcllo-ws spring constants.

(2) Temrperature effect: A teirrerature change
of 100 F (from 900 F to -100 ) s'-ifted the operating
point less than 1,4I percent. This shift is probably due
to stiffening of the bellows.

(5) Contact rating: Io 'dverse effects were noted
at voltages of 12 to 24 volts and currents up to
0.5 a-apere. For service use, the contacts are conserva-
tively rated at 24 volts and 1/. a.-pere, or 110 volts
and 0.03 ampere.

(') Acceleration effects: k-.ccelerations along the
bellow? n:-is shifted the ope;'atin- point 1 percent per g.
Acctle'1ions perpendicular to trh bellows axis caused a
-percent shiit per 12g. (See installation recommendations.)
4 L
(5) Hysteresis: Repeate=d tests in which the
operating: point was approached front both above and below
indicated that the instrument will always issue its
signal within 1/1 percent of the operating point.
(6) Vibration effects: The operation of the
instrument was checked while it was being vibrated at
0.04-in-ich double amplitude and frequencies of 10 to
60 c:.cles per second. The oo'crating point was found to
shift 1/I percent. After 2 hours of vibration under the
same conditions, the operating point was again checked
and was found to have shifted I/' percent.
SC CTID FT ML






NACA AC3 No. L4G31


(7) Lag: The lag of the instrument is defined as
the time elapsing between the ettai~mient of the operating
pre-sure ratio at the pitot-static tube and the issuance
of the warning signal. The warning device was connected. -
to 50 feet of tubing 3/16 inch in diameter. A rate of
proeaure rise equivalent to that occurring when the airplane
experiences a longitudinal acceleration of 2g at 500 miles
per hour was applied. The lag vwa less than 0.05 second.
Si-mlar tests simulating dives gave the same result.

FLIGHT TESTS

The _rACA rach numr.':er warning device used in the
laboratory tests (operating ocir.t at Y = 0.65) was
installed in the left-w .r.g giu- bav of a P-h7D airplane
(fig. 1). The instrument was bolted firmly to a gun-
mount bracket and was connected t. a warning light on
the instru-mnt panel. Alti-notr and airspeed indica-
tions were noted by the pilot during shallow dives from
25,0CO feet. An indicated airFpeed of 500 miles per hour
was maintained. Correlation ',.th the altiimeter and
aircpecd indications showed that the warring device made
contact at = 0.64 a.nd broke contact at N = 0.62.

RA.JGES ATD I-CJU3IUS TS

The- operating point of tl..e warning device has been
shown to defend on the ratio of belloC w areas. A suf-
ficiently large selection of beliows sizes is commer-
cially available that a warning device can be designed to
give any desired operating point in the existing range
of limiting -Mach number v:ith..n 2 percent. The effective
areas are controlled to aboat 2 percent during manufacture
so that bellows can be individually matched to obtain any
dcsire.l operating point if greater accuracyy is warranted.

Further adjustment of the operating point within a
1-percent range can be obtained by permitting a slight
amount of residual air in the evacuated bellows. The
pressure in this bellows must not exceed 7 millimeters
of mercury to avoid temperature land 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 "hen evacuated, and the .eight of the inner
bellows must be adjusted so that the outer bellows is not
deflected when the heads are f'?st.=ned beether. Errors
Ci: 1,T TT : "TT TAT,


CN TJTD7.TIAL









FACA ACR No. I)4G51


in these settings are manifested as a change in operating
point with altitude and can be detected by suitable
ground tests in an altitude chamber.


RECOMMENDATIONS


The NACA 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 pitot-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 bellows 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, la.


CON FIDENT L4L


CONIFTDS TTI.L








NACA ACR No. L4G31


REFERSilTCLS


1. Smith, Norn-an F.: UNCA iMach IJumber Indicator for
7se in Hihi-Speed Tunnels. TACA ACR No. 5G31,
19h3.

2. Thompson, F. L., and Zalovcll, John A.: Airspeed
'eaeurements in Flight, at High Speeds. NACA ARR,
Oct. 1942.


CON 'FIrT TIAL


COJFIDT'.iTIAL







NACA ACR No. L4G31


S461c conn/ect.on -l


NATIONAL ADVISORY
COMMITTEE 10o AERONAUTICS

..ure i.- Cross-sect,onol vew of NACA Mach number
worninn devIce.


Fig. 1








NACA ACR No. L4G31


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NACA ACR No. L4G31





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Fig. 3


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NACA ACR No. L4G31


Figure 4.- NACA Mach number warning device.


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Figure 5.- Evacuated inner bellows of N\CA Mach number warn-
ing device mounted on base.


Figs. 4,5






NACA ACR No. L4G31


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.


Figs. 6,7




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