Lift generated pressures produced by large, heavy aircraft at low altitudes were investigated due to concern over their possible effects on ground objects. Aircraft lift generated pressures were calculated using elementary airfoil theory, and these values were compared with ground level measurements made during an overflight program. The predicted and the measured values were in relatively good agreement. Due to lack of experimental investigations of this phenomenon, opportunity was taken during an overflight program to use a specially instrumented test range to measure the ground pressures produced for a range of aircraft weights and distances

Findley, D. S.

English

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NASA TECHNICAL	 NASA TM X- 72707
MEMORANDUM
	
r i.	 (NASA-TM-%-72707) COMPARISON OF MEASURED
	 N75-25870t i °	 AND CALCULATED AIRCRAFI.LIFT GENERATED
	
n	 PRESSURE'S (NASA) 17 p HC $3.25
	 CSCL OIAP	
,
Unclas
Fes—	
G3/02 26607
a
f	 COMPARISON OF MEASURED AND CALCULATED
jtl	 AIRCRAFT LIFT GENERATED PRESSURES
l
by
,3031 9p^
Donald S. Findley	 ,^	 9
tom ^,j C
l i
	 June 1975
S	 .
This informal documentation medium is used to provide accelerated or
special release of technical information to selected users. The contents
- gay not meet NASA formal editing and publication standards, may be re-
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NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
	
LANGLEY RESEARCH CENTER,, 6`11A,MPTON, VIRGINIA 23665 	 { a
T1	 Report No 2 Cwvernment Accession No 3 Rec-prant's Caui"" No
TM x	 72707
4	 Tate and Subtitle 5 Repo" [)*If
Comparison of Measured and Calculated Aircraft Lift
Gene: ated Pressures 6 hnorr"atig OrganiratHm Coda
26.200
1	 Authorls) o Pe►torming organiretion Report No
Donald S.	 Findley 10 Work Unit No
505-03-12-05
9 Perfor mrng Orpnintion Nana and Address
NASA Langley Research Center
11 Contract or Gram NoHampton, VA	 23665
13. Type of Report anti Period Covered
NASA Teclunical Memorandum
12 Sponsoring App , Name and Adore,•
National Aeronautics and Space Administration
Washington, D.	 C.	 20546
14 Sponsoring Agency Code,
15	 Supple-tantry Notes
This information was plblished in an informal document in 1968.	 No attempt
is made to update this material to reflect the current state of the art.
Proposed ,idespread use cf very large, heavy, aircraft for commercial and mili- I
tary purposes has caused concern about the lift generated pressures that will be
produced at or near ground level due to low altitude operations. Such pressure
may have significant effects: on objects and structures in the immediate vicinity'
of airports.
Aircraft lift generated pressures were calculated using elementary airfoil
theory, and these valu..s were compared with ground level measurements made
during an overflight program. The predicted and the measured values
were in relatively good agreement.
Due to lack of experimental investigations of this phenomenon, opportunity
was taken during an overflight program to use a specially instrumented test
range to measure the ground pressures produced for a range of aircraft weights
and distances.
11. Key Words (Suggrated by Authwlsl l ISTAR category underlined) 	 IN Distribution Statement
Acoustics, flight Lasts, aircraft
noise	 I Unclassified - Unlimited
19 Secwity Ctaurt lot this report)
Unclassified
20 Security Ctanrt. lot this papal
Unclassified
21 No of Pages	 22 Rice
17	 1	 $3•P'.`
The National Technical Inforr cation Service, Springfield, Virginia 22151
Available from
ISTIFINASA Scientific and Technical Information Facility, 10 .0 Box 33, College Pok, MO 20740
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0
COMPARISON OF MEASURED AND CALCULATED
it
AIRCRAFT LIFT GENERATED PRESSURES
By Donald S. Findley
INTRODUCTION
The proposed widespread use of very large, heavy aircraft for commercial
and military purposes has caused concern about the lift generated pressures
that will be produced at or near ground level due to low altitude operation's.
Such pressures may have significant effects on objects and structures in the
immediate vicinity of airports'.
Prandtl and Tietjens in reference 1 have dealt with the problem of the
transfer of the weight (lift) of an aircraft to the surface of the ground.
Expressions are given in reference ;{1 for the pressure increment due to
aircraft lift as a function of distance, where the distance is assumea'to be
large compared to the dimensions of the lifting surface.
Due to the lack of experimental investigations of this phenomena,
opportunity was taken during a recent overflight program to use a specially
instrumented test range to measure the ground pressures produced for a
range of aircraft weights and distances. The purpose of this paper is to
present measurements of the ground pressures and to compare them with
calculations made by the theory of reference 1.
ANALYTICAL STUDIES
The main concepts of the theory of reference 1 and a definition of the
quantities involved are illustrated in figure 1. An aircraft flying at low
speed, and at a distance above the ground large compared to its dimensions,
Is assumed to generate a pressure pattern which bas rotational symmetry with
respect to the location of the airplane. By means of elementary airfoil
theory, the lift generated pressure at any point below an aircraft is predicted
by the relationship
Lh
P
2nR3
where p = the incremental precs,me over ambient pressw`e,'
L = the lift (or weight) of the aircraft
h the altitude of the aircraft above thte grouri`l
R = the slant range distance between the a_craft`and the
point on the ground where the pressure is observed
i
i
n
F	 ^	 L
a
l1
F,or the overhead pgpition of the aircraft, where R = h, it can be seen from
the equation, thaiwY'the incremental pressures, are proportional to the weight of
the aircraft and Jnvercely proportional to the square of the distance between
the aircraft and,4ihe pdint of observation.
Using the above equation, raidulationc were made of the incremental
pressure that would be produced below an aircraft, for various distances from
the point of observation to the aircre:Ct -:,The results of these calculations
are presented as incremental preasurl: per unit weight of the nircraft as a
function of lateral distance from ttO mound track for several altitude^t in
figure 2. It can be seen from thn Vigvro that th%: calculated increment;l
pressures are relatively high under.,tho alrcraf Cand decrease rapidly,%with
lateral distance for low altitudec,"  On the other hand at higher altitudes,
the maximum increments) prussure values arr lower and thoy decrease at a
markedly slower rate as a function of lateral distance. IL follows that the
relatively high incremental prensuran associated with low airplane altitudes
are, confined to the region near the ground track.
At a given point on the _round, the lilt: genur.atod Incremental pressures
will vary as a function of time ac-the aircraft passes overhead. Based on an
assumed speed of 150 lets, the time histories of pressure per unit aircraft
weight associated with two different altitudes are given in figure 3. It can
be seen that pressure increments are predicted for time spans of several
seconds, the shorter time and the higher pressure being associated with the
lower altitude.
To indicate the order of magnitude of increme,ltal pressure predicted for
various combinations of aircraft weight and altitude, the calculated data of
figure 4 are presented. The incremental pressures are those predicted on the
ground track and hence are the maximum values. For instance the maximum
incremental pressure predicted for---a 750,000 pound aircraft- at a distance of
200 feet is about 3 pof.
For comparison with the values of lift generated incremental pressures
bf'figure 4 some data for changes in pressures due to altitude and wind
ivelocity are presented in figures 5 and 6. The nres^ure change, p , as a
funejtion of altitude was taken from rcferrnce 2 and indicates that a value of
3'Paf is associated with an alfiLur,:e chaugo near sea level of about 40 feet.
'The pressure exurtud ar,-a function of wind velocity in predicted from
the ;expression
li
pw 112e2 sin 0
where ; pw is !che impact prossuru, p Is the density of the air, V is the
wind v=lq-i''cy and 0 is the angle of the wind vector relative to the
impingei,.gnt surface ( ,0 is assumed to be 901 for tlse calculations of
figure 6.`' For instance, a pressure value of 3 psf is.associated with a wind
velocity of about 30 kts.)
ORIGINAL Pe,GY.I.RS
OF POOR CtnI.,.jr
ll
hF
{i 11'.-._: 	 _....... 	 .	 .I	 . ..
PUBRIMGNTAL STUDIOS:
Test Conditions
For direct comparison with the calculations, measurements were made of
the lift generated pressure fields of two different aircraft during a recent
overflight program at the NASA Wallops Station, Virginia. The test site,
which is apprl)ximately 30 feet above sea level, is generally flat with some
`wooded areas. The measurement location was in a large open paved area, as
shown in the sketch of figure 'r, with the nearest buildings, trees, and other
large objects at least 100 feet away. Plight tests were made in the second
and third weeks in November 1967, during which time the weather was generally
fair. Strong winds, however, did occur on one day, and the adverse effects ui
wind on the measurements are noted.
The two aircraft used during the tests are illustrated in figure 8(a)
and (b). Airplane A was a large four-engine turbofan-powered military transport
operated by the Air Force. -. ` Irplano B was a large four-engine t!Jrbo,jet-
powered civilian transport operated by the Federal Aviation Administration.'
The scheduled aircraft ground track passed about 140 feet north of the
measurement site_ in a west to east direction. The aircraft were tracked by
radar for all flights. Pertinent information about the operating 9'Onditions
and the positions of the aircraft are given in Table I. 	 j
Instrumentation
A microphone system similar to that presented in figure ) was used. It
has a,useable frequency range of zero Hz to 10 IIHz, and maximum sound pressure
level capability of 140 dB. The ;system consisted of a specially modified
condenser microphone, an PM tuning unit and an amplifier. To monitor and
examine the signals, an oscilloscope and a graphic level recorder were used.
All data signals, an IRIG time code signal, and a voice description of the
tests. were recorded on an FM tape recorder.
i	 RESULT.? AND DISCUSSION
The results of the tests are presented,in figures lU through 12.
Figure 10(a) and (b) contains example , pressure time history data for two
different frequency ranges for a typical flyover of airplane A. Figure 10(a)
relates to the conventional noise frequencies (10 - 10,000 Hz) whereas the datn
of figure 10(b) relate to the lift-generated pressures which are slowly
varying (non-oscillating) in nature and are represented by that part of the
spectrum near zero frequency (for convenience 0 - 5 Hz), It can be seen that
t	 the high-frequency pressure^q_, increase in amplitude gradually to a peak value
which occurs after the fircraft has passed overhead. The lift pressure trace
on the other hand reaches a peak value neaa the time that the aircraft is
closest to the measuring station. It can be seen that the lift pressure trace
Is generally similar in nature to the calculated traces of figure 3.
3 X
S
Measured pressure data obtained from records such as in flrarre 10(b) are
plotted in figure 11 for three different flir)it conditions of tIrplane B.
Ghown also on the figure are calculated curvev for the ooinu flitghL conditions
for direct comparison. It can be seen that the measured and calculated data
are in general agreement. It should be noLed, however, that the calculated
maximum values (corresponding to the shortest slant range) exceed the measured
maximum values in each case. An additional finding is that small negative
pressure values are measured after the aircraft has passed by, whereas the
theory predicts only positive values. The presence of these small negative
pressures has been confirmed even though the meanurement accruraoy may be
degraded because of the response of the meanurement nystiMn to atmospheric
pressure di6turbances which are of the Name order of matthitude. 117rese small
negative pressures may be associated with near-Hold ofPc •cts riot accounted
for by the theory of reference 1.
The data of figure 12 are similar In nature to thOuo of flr;urc 11 but
for airplane A and for somewhat different flight eondibior,s, an listed In
Table I. Airplane A was heavier than airplane D and benco thu mnximurn
incremental pressures were somewhat higher than those of figLi v 11 for roughly
--the namo operating conditions. The spread of the data points is believed due
to the gusty winds of 7 to 14 kts which were observed at Lhe microphone
location. Gusty winds have associated with them pressure variaj^ons which are
superposed on the;,lift pressures, and it in sometimes dlficult to separate
them. Thus, the d&ta of figure 12 are riot believed to be an `accurate as those
of figure 11 because of the adverse weather condi.tibns.
t	
^
CONCLUSIONS
FAircraft lift generated preosures were calculated using elernvntary
airfoil theory, and these values wigre compared, with ground level measurements
made during an overflight program. The predicted and the mcas-jred values were
In relatively good agreement.
As: an indication of the order or unagnitude of the prusaurrs Involved, a
750,000-pound airplane at a diaLance of 200 feet would produce ah incremental
pressure due to lift of about, 3 pnf. 'Phis pressure change wouLtl rour;lily
correspond either to a change in altitude near oea 1 ;evcl of 110 fcut or the
impact pressure of a 30-kt wind.
ItGL'L^^NCLS
1. Prandtl, L.; and Tietjeno, 0. G.: Applied Hydro- turd Aeromechanics,
Dover, New York, 1,934•
2. U.S. Standard Atmosphere. U.G. Comnutticc on Lrtension to the Standard
Atmosphere, if G. J
ORIGINAL PAGE 1S
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LATERAL DISTANCE FROM GRWID TRACK, FT.
Figure 2. - Calculated incremental pressures per unit aircrA
weight as a function of lateral distance from tt
ground track for sltitudes of r0 ft., 100 ft.,
?nn ft., 500 ft., and 1000 ft.

1000
500
	 h-50 FT
100
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AIRCRAFT WQGHT, X 10`' KUNLS
Figure lt. - PREDICTED MAXIMUM INCRFT ENTIAL PRFSGURK; ON THE GRO [ IND TRACE;
FOR DIFFERENT AIRCRAFT WEIGHTS AND ALTITUDES
ALTITUDE, FT
FIGURE 5. - CHANGE IN ATMOSPHF21C PUSS[1RE AS A FUNCTION OF CHANGE
IN ALTITUDE NEAR SEA LEVFL
i
0	 0	 146
F
1
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0	 0	 10	 %0	 sU	 140
WIND VELOCITY, KNOTS
FIGURE '^. — PRESSURE. FKF.RTFIO BY VF. WIND AS A FTNCTION OF VELOCITY.
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Time, sec
Mission 233-B
Figure ll.- Calculated and meabured incremental pressure time hiatoriea
for three different missions of aircraft B.
0.3
0.2
0.1
0 -0
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A
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0.1
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0
Approaching
	
Leaving
-0.1
-3	 -2	 -1	 0	 1	 3
Time. sec
Mission 69-A
Fiyure 12.- Calculated and measured incremental pressure time
histories fortt,ree different missions of aircraft A.


Comparison of measured and calculated aircraft lift generated pressures

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