NASA Technical Memorandum 88947 

AIAA-87-0530 

f 

Structureborne Noise Control in 

Advanced Turboprop Aircraft 

[NASA-TB-88947) STBUCPUREECSNE NOISE Ne7-165E7 

COYTEiClL IN ADVANCED TDEBCPXIGE AlIiCRAFT 

(hASA) 24 p CSCL 20A 

Unclas 

G3/71 43313 

Irvin J. Loeffler 

Lewis Research Center 

Cleveland, Ohio 

Prepared for the 

25th Aerospace Sciences Meeting 

sponsored by the American Institute of Aeronautics and Astronautics 

Reno, Nevada, January 12-15, 1987 

A 
STRUCTUREBORNE NOISE CONTROL IN ADVANCED TURBOPROP AIRCRAFT 

by 

Irvin $1. Loeffler 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135 

Abstract 

Structureborne noise is discussed as a contributor to propeller aircraft interior noise 

levels that are nonresponsive to the application 

of generous amounts of cabin sidewall acoustic 

treatment. High structureborne noise levels may 

jeopardize passenger acceptance of the fuelefficient high-speed propeller transport aircraft 

designed for cruise at Mach 0.65 to 0.85. 

single-rotation tractor and counter-rotation tractor and pusher propulsion systems will consume 15 

to 30 percent less fuel than advanced turbofan 

systems. 

These 

Structureborne noise detection methodologies 

and the importance of development of a structureborne noise sensor are discussed. A structureborne 

noise generation mechanism is described in which 

the periodic components of propeller swirl produce 

periodic torques and forces on downstream wings 

and airfoils that are propagated to the cabin 

interior as noise. Three concepts for controlling 

structureborne noise are presented: (1) a stator 

row swirl remover, (2) selection of a proper combination of blade numbers in the rotor/stator system of a single-rotation propeller, and the 

rotor/rotor system of a counter-rotation propeller, and (3) a tuned mechanical absorber. 

Introduction 

Propeller-driven aircraft are characterized 

by noise and vibration levels which in many cases 

adversely affect the comfort of passenqers and 

crew. Generous amounts of acoustic treatment 

applied to the fuselaqe sidewalls have often been 

ineffective in reducing the interior noise to 

desired levels, indicating a probable noise floor 

due to an alternate propaqation path. As indicated in Fig. 1, externally generated noise from 

the propeller and engine propagates through the 

air and excites the cabin sidewalls, which then 

radiate noise to the cabin interior. However, 

vibrations from the propeller, enqine, gearbox, 

and propeller wake propagate throuqh the wing 

structure and the consequent excitation of the 

cabin sidewalls causes them to radiate noise to 

tne cabin interior. Thi5 "btr uituieborne noise" 

(SBN) appears to be a prime contributor to interior noise levels that are nonresponsive to the 

conventional means of suppressinq airborne noise 

(ABN) . 

coniervation has made the advanced turboprop, a 

hiqh-speed propeller also known as a propfan, a 

strong candidate as a propulsor f r transport aircraft in the 1990's and beyond.l,? Both sinqlerotation (SR) and counter-rotation (CR) hiqh speed 

propeller propulsion systems are beinq developed 

to cruise at flight speeds of Mach 0.65 to 0.85. 

These systems are expected not only to match the 

flight speeds of the 1980 vintage turbofan fleet, 

The current world-wide interest in energy 

This paper is declnred a work of the U.S. Government and is 

nut subject to copyright proteclion in the United States. 

but to operate on 50 to 60 percent less fuel. 

They will consume 15 to 30 percent less fuel than 

advanced turbofan enqines. 

A model of an SR tractor propfan is shown in 

Fig. 2. A winq-mounted installation of this propeller is pictured in Fiq. 3. The General Electric 

UDF (unducted fan) engine is a unique qearless CR 

pusher hiqh-speed propeller system desiqned for aft 

mountinq, as shown in Fig. 4. A geared CR tractor 

propeller system suitable for wing-mountinq is 

shown installed on a four-engine military transport in the illustration of Fig. 5. All three of 

these installations present a strong potential for 

unacceptably high SBN levels in the aircraft 

cabins. 

Although high-speed propellers are designed 

for lowered noise with their thin highly-swept 

blades, the supersonic tip speed and the hiqh blade 

loadinq of these propellers produce noise levels 

some 25 dB or more higher than those of conventional propellers. These high noise levels may 

require fuselaqe sidewall transmission loss values 

of 50 or 60 dB at a propeller blade passaqe frequency (BPF) in the ranqe of 150 to 300 Hz, with 

somewhat less severe requirements at hiqher harmonics of the BPF. If the conditions that produce 

higher propeller ABN levels also result ultimately 

in higher SBN levels, such levels on an advanced 

turboprop aircraft may well be as much as 30 dB 

above the already unacceptably hiqh SBN levels of 

the conventional propeller aircraft. Furthermore, 

SBN technology is i n i t s infancy, and the qeneration and propaqation mechanisms are currently the 

subject of theory, conjecture, and experiment. 

While SBN appears to be a major threat to 

passenger acceptance of the advanced turboprop 

commercial transport aircraft, its very existence 

is based 1argel.y on inference from previous unsuccessful attempts to produce a quiet interior in a 

propeller-driven aircraft. 

author's knowledge, SBN has never been unequivocally isolated and measured in an aircraft in 

flight. This is due to the difficulties in separating the causes of excitation of the interior 

vibrating surfaces--how much was induced by external !,BY impjnqement and how much hy structureborne 

transmission. 

To the best of the? 

In the early 1980's, while responsible, in 

cooperation with the Aircraft Noise Reduction 

Division of the Lanqley Research Center, for planning the interior noise acoustic research program 

for the Lewis-manaqed Advanced Turboprop Project, 

the author espoused a number of concepts involving 

SBN research, as qiven below: 

(1) Laboratory model tests and qround based 

tests on propeller aircraft are very valuable even 

though neither closely simulates inflight SBN 

conditions. 

Cop)right 1981 American Institute of Aeronautics and 

Arlronnvlirs. lor. Yo copbripht i> asurlcd in the Uniled Slatei 

under Title 17. U.S. Code. The US. Government hnsn 

ro,dty-free licenu to exerciv all riyhts under the c0p)ripht 

claimed herein for Govcromentd purposes. All other rights mrr 

rescrvcd by the cep)ripht owner. 
(2) The wake of an SR propeller impinging on 

a downstream wing, pylon, or tail surface produces 

a periodic torque load that can propagate to the 

cabin interior as SEN. 

(3) A stator row downstream of an SR propel- 

(4) The selection of an appropriate combinaler offers a means of reducing SBN. 

tion of blades in an SR rotorlstator system can 

reduce SBN transmitted to an aircraft interior. 

This concept has been expanded in this paper to 

also apply to CR propeller systems. 

near the wing roots should be investigated as SBN 

sensors. 

(5) Strain gauges mounted to the wing spars 

This paper will serve to document these concepts and to cite some of the current pertinent 

SBN research activity. 

Branch of the Langley Research Center is currently 

managing an extensive in-house cnd contract 

research program investigating these and other 

concepts related to SEN in propeller-driven aircraft. 

measurement, the feasibility of various experimental and analytical approaches in SBN research, a 

mechanism for SBN generation as a result of propeller swirl impingement on downstream airfoils, 

and possible methodologies for reducinq SBN in 

propeller-driven aircraft will be discussed in 

subsequent sections of this paper. 

The Structural Acoustics 

The unique problems associated with SBN 

Structureborne Noise Detection 

Separation of ABN and SBN Signals 

In general, propeller-generated SBN cannot be 

directly measured in the cabin of an aircraft in 

flight. As indicated in Fig. 6 the cabin interior 

noise level consists of both ABN and SEN. Because 

of the very high correlation between propeller ABN 

and SBN (the spectrum of each consists primarily 

of the propeller blade passage tone and its harmonics) a microphone in the cabin measures the 

combined acoustic levels at each propeller tone 

frequency, and the SBN contribution is indiscernable from the ABN contribution. Direct measurement 

of SBN requires some means of separating SBN from 

the ABN signal. 

Acoustic Barrier Technique 

reducing the ABN to a level well below that of the 

SBN level, as with a barrier technique as shown in 

Fig. 7. 

technique are: 

One way to directly measure the SBN is by 

Two examples of application of the barrier 

(1) A wall to block the airborne noise path 

(2) An enclosure to completely seal off the 

receiver (cabin exterior) from the ABN 

source 

An approach, developed by William Mayes of the 

Langley Research Center and the author in an SBN 

ground experiment conducted at Langley on a 

DeHavilland Twin Otter aircraftY3 employed the 

second of the two barrier techniques. 

noise from the propeller was measured in the 

unaltered (baseline) state, and again with one and 

Interior 

then two layers of acoustic material wrapped 

around the fuselage. 

the principle that known increases in sidewall 

acoustic attenuation produce a predictable change 

in the ABN level without a significant effect on 

SBN. Thus, if point (1) in Fiq. 8 is a baseline 

condition, increasing attenuation results in 

points (2) to (4). Without SBN points (3) and (4) 

would fall on the airborne noise curve at (3') and 

(4'1, respectively. As the ABN level is lowered 

below the SBN level, the curve becomes asymptotic 

to the horizontal line representinq the SBN level. 

The line is horizontal because SBN is invariant 

with ABN barrier treatment level. One problem 

with this approach is that the acoustic material 

wrapped around the fuselaqe may influence SBN 

propaqation in addition to blocking ABN. 

While it is possible, in principle, to measure SBN by the brute force method described above, 

in a given situation it must be physically possible and feasible to suppress the ABN to a level 

below that of the SEN, and to accurately predict 

or measure the various attenuation levels. Attenuation is most readily predictable in the mass-law 

frequency ranqe, where for single wall treatment 

the transmission loss at a qiven frequency varies 

with the logarithm of the treatment weiqht per unit 

surface area. 

SBN Vibration Path Interruption 

This approach is based on 

It is also theoretically possible to determine 

SBN by eliminatinq the SBN component from the measured interior noise. The level with SBN removed is 

subtracted antiloqarithmically from the combined 

ABN + SBN level. The remainder is the SBN level. 

This method, however, can be used to measure 

structureborne noise only when the structure borne 

noise level is approximately equal to or hiqher 

than the airborne noise. For current vintaqe 

propeller-driven airplanes this is qenerally not 

the case. This approach could best be used in 

conjunction with the testinq of ABN acoustic treatment to ensure that the measured noise levels are 

uncontaminated by the presence of SBN in the total 

noise level. The method has, perhaps, one other 

application. It can be used to demonstrate that 

the structureborne noise component is not dominant 

in those cases where noise measurements with and 

without the structureborne path broken are substantially equal. 

method that have been suqqested are (1) disconnectinq the winq or pylon supporting an operating 

enqine (providing the tail empennaqe is outside 

the propeller slipstream), (2) dismounting and 

separately supportinq the enqinelpropeller system, 

and (3) locating an equivalent enqinelpropeller 

system (perhaps another airplane) near the test 

aircraft fuselage. Methods (2) and (3) can also 

test propeller wake effects on a winq or pylon by 

usinq forward and reverse pitoch or by shiftinq the 

enqinelpropeller axis by 180 . 

path hy method (1) would be by detachinq the fuselaqe from the winqs of a North American Rockwell 

OV-10 Bronco, shown in Fig. 9, by removinq the four 

attachment bolts leavinq the powerplants, winqs, 

and tail empennaqe undisturbed. The lower Dart of 

the figure shows the measured winq and vibration 

mode at 201.5 Hz, which will be discussed in a 

later section of this paper. 

Examples of the use of this 

An example of breaking the SBN propagation 

2 
Inflight Measurement of SBN 

greater difficulties. ABN suppression methods 

such as the barrier techniques are impractical due 

to the imposition of significant added weight to 

the aircraft, unacceptable drag, the inability of 

the structure to withstand flight aerodynamic 

forces, and the effect of the barrier on aircraft 

flight control and safety. Thus, practical considerations require that barrier experiments be 

conducted as ground tests. It may be argued that 

the stresses imposed in an aircraft resting on its 

landing gear under ground static conditions are 

quite different from those imposed in the same 

aircraft in wing-supported flight conditions. 

Nevertheless, it may likewise be argued that if 

the physics of SBN generation, propagation, and 

control can be demonstrated and understood on an 

aircraft on the ground, it is likely that the 

principles can also be applied to an aircraft in 

flight. Consequently, it appears that ground 

tests provide a valuable and necessary means for 

studying SBN in full scale aircraft. Furthermore, 

it is also recognized that the source noise provided by an operating propeller in static conditions on the ground differs greatly from that of 

the same propeller in flight. What is needed is a 

sensor or measurement technique that is capable of 

measuring SBN directly, but which is insensitive 

to ABN. Such a device would provide a capability 

of evaluating the effectiveness of proposed SBN 

control devices on an aircraft in flight. The 

development of such a sensor technique may be the 

real key to success in the development of SBN control technology for propeller transport aircraft 

in flight. 

Measurement of SBN in flight presents even 

Structureborne Noise Generation 

SBN Generation Mechanisms 

Any mechanism by which time-varying forces 

can be transmitted by the enginelpropeller system 

to the aircraft structure is a possible contributor to SEN. Mechanical imbalance of the enginel 

propeller rotating system is one obvious source of 

vibration transmission to the engine mounting system Another is nonuniform or off-axis inflow to 

the propeller that produces a variation of lift 

and drag forces on each blade during one rotation 

of the propeller. In addition blade-to-blade 

variations in lift and drag can yield one-per-rev 

harmonic variations in propeller torque, thrust, 

and lateral forces. All of these forces are 

transmitted through the engine and propeller 

mountings, and the tuning of these mountings to 

absorb the attefidant vibrations cc.!d be one step 

in reducing these vibrations. 

used by Unruh4 to reduce SBN by 10 dB in a 

single-engine light aircraft. 

rior noise and vibration of 2 small propellerdriven business aircraft was greatly reduced when 

the aircraft was converted from a conventionai 

tail empennage to a T-tail where the horizontal 

stabilizer and elevator system was relocated outside the propeller slipstream, an early indication 

of SBN due to the action of propeller wake. 

This method was 

In one instance the unacceptably high inteIn their str ctureborne noise experiment Metcalf and Mayes Y concluded that the 

structureborne noise measured in a ground test of 

a Dehavilland Twin Otter was caused by propeller 

blade wake and tip vortex interaction with the 

winq. The barrier method discussed in the previous section was used to isolate the SBN by two 

wrappings of the fuselage in acoustic material. 

The SBN level was determined to be 10 or 11 below 

the ABN level. Application of a wrap on the wing 

in the propeller wake region reduced the SBN level 

by 6 or 7 dB, and qave confidence that SBN was, 

indeed, isolated. A mechanism will now be described by which the propeller wake and tip vortex 

produce noise and vibration in the cabin interior 

of winq-mounted propeller-driven airplanes such as 

the Twin Otter. 

Propeller Wake SEN Generation 

An SBN qeneration mechanism unique to 

propeller-driven aircraft is provided by the 

reaction of a downstream airfoil to an upstream 

propeller wake. For aircraft driven by singlerotation wing mounted tractor propellers, a winq 

or tail surface located downstream of a propeller 

receives a periodic torque loading at the propeller blade passage frequency and its harmonic due 

to the unsteady flow produced by the individual 

propeller blades. This periodic torque produces a 

"wiggle," a small periodic flexing of the airfoil 

which is attached to the fuselage structure. The 

resultant flexing of the fuselage results in the 

radiation of sound to the cabin interior for periodic torques in the audible frequency ranqe. If 

the center of the propeller slipstream passes 

above or below the airfoil, or if the airfoil is 

swept, there will also be net periodic upward and 

downward forces on the airfoil. If present, these 

forces also contribute to a "wiqqle" in the downstream airfoil. 

Most of the flow behind a propeller typically 

follows a helical or swirlinq path, as shown in 

Fig. 10. However, the detailed flow pattern 

throughout the propeller disk is quite complex, as 

shown in the calculated SR-3 propeller wake secondary flow plot of Fig. 11 taken from Ref. 5. 

Considerable variation in maqnitude and direction 

is evident. The tanqential component of the flow 

is clockwise (CW) at some points and counterclockwise (CCW) at others. While the spanwise flow is 

mostly toward the tip, in the tip vortex reqion 

the flow is toward the hub on the blade suction 

side of the wake. 

gential direction (circumferential) is, of course, 

cyclical; the pattern repeats from blade to blade. 

In Fig. 11 the blade wake locations are evidenced 

by the propeller tip vortex flow as well as the 

pronounced chanqe in spanwise flow on opposite 

sides of the blade. In the midspan and hub region 

where the solidity is high, the flow is characteristic of cascade or channel flow, while in the tip 

reqion the flow is more representative of isolated 

airfoil flow, althouqh this is also due to the 

influence of the tip vortex. 

The nonuniformity in the tanIn tkc prcpeller wake p!ct cf Fiq. 11, it may 

be assumed that the center of the vortex is located 

in the vicinity of the propeller blade tip. 

the region lyinq well inside the blade tip reqion 

the flow undergoes only small changes in maqnitude 

and direction; in the blade tip reqion the changes 

are rather severe. Between the blade wakes the 

flow near the tip has only a small swirl Component. 

In 

3 
It may be useful to consider the swirl flow 

as having two tangential components; a constant 

nontime-variant component analogous to a DC current 

flow, and an oscillating or periodic time-variant 

component analogous to an AC current flow, as 

illustrated in Fig. 12. 

Physical evidence of significant periodic 

tangential forces introduced in the propeller wake 

at blade tip distance from the propeller shaft has 

been obtained. 

measured these oscillating (AC) forces in terms of 

the pressures imposed on sensors mounted on opposite sides of a nonlifting airfoil which was traversed radially in a plane one-propeller diameter 

downstream of the propeller plane, as shown in 

Fig. 13. R.M.S. dynamic pressure traces for the 

advancing propeller vane side are given in Fig. 14 

as a function of radial distance from the propeller axis for wind tunnel flow Mach numbers of 0.6 

and 0.8. 

periodic pressures are associated with the propeller tip vortex region. 

For a propeller mounted with its axis of 

rotation on or near the wing chord line, one 

effect of the swirl flow in Fig. 11 is to impose a 

steady or DC twisting force or torque on the downstream wing, tending to rotate it in the vertical 

plane in the same rotational direction as the propeller. In addition, there is an unsteady or AC 

torque tending to cause an oscillating rotation of 

the wing around the propeller axis, as illustrated 

in Fig. 15. Figure 15 illustrates the motion of 

the wing associated with the first order mode of 

flexing or forced vibration of the wing. If this 

torque were constant with time, i.e., with the DC 

component only, it would pose no problem with 

respect to cabin interior noise and vibration. 

However, as will be discussed in detail later, the 

pressure at any point in the propeller wake, such 

as a point on the wing surface, varies with time 

during a cycle. The period of the cycle, T, is 

the time required for the pattern of Fig. 11 to 

repeat. 

ler rotational speed, each blade wake passes any 

given point at the propeller blade passage frequency, or BPF Hence, 

Miller, Dittmar, and Jeracki6 

It is quite evident that the largest 

Since the pattern rotates at the propel1 T=m 

and 

BPF = BN 

where B is the number of propeller blades and 

N is the propeller rotational speed in revolutions per second. 

it is also likely that torsional modes (spanwise 

twisting) will also be excited. 

An example of this kind of motion is shown in 

Fig. 9 of the vibration mode of the NASA Lewis 

OV-10 North American Rockewll Bronco aircraft as 

measured by Langle~.~ Here the right wing is 

excited by a white noise signal from a shaker 

applied to the right wing. The nodal line at midspan is at the enginelpropeller location. The 

mode shown is for 201.5 Hz, approximately twice 

the BPF of the propeller at cruise speed. Many 

other modes were also measured, many of them 

In addition to the bending modes of the winq 

involving torsional vibration, or a twisting motion of the winq about a spar or a leading or 

trailing edge. With reference to Fiq. 15 it would 

seem that strain qauqes mounted near the top and 

bottom of the winq main spars near the winq root 

should be a preferred choice for a sensor to 

detect the flexural or bendinq wave in the wing as 

shown. The fuselaqe structure must react directly 

to the forces imposed upon it at the wing attachment point. Consequently, the details of winq 

deformation or motion away from the wing root are 

probably of little significance. Therefore the 

dynamic strain qauge near the root senses the 

periodic strain, probably a good measure of the 

vibration level reaching the fuselage. Additional 

combinations of gauges mounted fore and aft can be 

used to sense the torsional strain at the wing 

root. 

The main objective of the Langley OV-10 

research effort was to determine the wing and 

fuselage dynamic response of a full scale aircraft 

to a variety of excitation modes. The winq was 

excited by impact hammer, by single and by multiple phased shakers, and by the propeller itself. 

Motion of the wing and fuselage was detected by 

accelerometers and strain gauqes. 

objectives was to determine appropriate locations 

for SBN sensors. From Fiq. 9 it is quite evident 

that a nodal point on the winq would not be a qood 

location for an accelerometer intended for sensing 

winq vibration response at a frequency of 201.5 Hz. 

The development of an SBN sensor or SBN 

detection methodology is one of the objectives of 

the laboratory model SBN experimental research 

program conducted by Unruh8 of the Southwest 

Research Institute, currently in progress. This 

work is being done under contract to Langley. 

Unruh's test uses a 102 cm ($0 in.) diameter fuselage model enclosed in a 165 cm (65 in.) diameter 

concrete pipe to provide ABN shieldinq. A propeller, an impact hammer, a speaker, and possibly 

phased shakers will be used to excite the model 

wing attached to the fuselage and protrudinq 

through a sealed opening in the ABN shield. 

Ground-based transfer function measurements relating input forces to wing root responses by accelerometers and strain gauges and to fuselage 

interior noise levels will be used to develop a 

method for measuring SBN inflight. 

One of the 

Propeller Periodic Torque Load on Downstream 

Airfoil 

The experimental determination of the torque 

loading on the wing at a qiven instant would 

require a distribution of th instantaneous pressures with time for a suitable qrid covering the 

winq surfaces exposed to the propeller wake. 

Then, for each selected time, the inteqrated product of the pressure, the grid area, and the distance from the propeller axis can be algebraically 

summed to yield a net torque actinq on the winq. 

From these values the time history of the net 

torque for a complete period can be determined. 

Since an experimental study involves difficulties in providing a sufficient number of sensors to obtain even a modest resolution of the 

unsteady pressure distribution on the winq behind 

a propeller and also the need for extremely accurate phase information, an analytical study was 

included in the Langley SBN program. The analysis 

4 
by Weir9 readily accommodates the parametric 

investigation of such variables as propeller 

location with respect to the winq, propeller 

thrust, propeller blade number, wing sweep and 

taper, and advance ratio. Test caSes coiiiputer for 

the Twin Otter, the OV-10 Bronco, and the NASA PTA 

aircraft showed significant differences in the 

temporal variation of the spanwise lift distribution and the wing root moment. 

system is available for analysis of winq 

structural response. 

An effort to completely model the SBN generation and propagation process is currently underway 

by the Cambridge Acoustical Associates. This 

Small Business Innovative Research contract work 

is managed by the Langley Research Center. 

obtain experimental data to guide the effort, 

input and transfer impedances of the wing and 

fuselage structures of the NASA-Langley Beechcraft 

Baron 58P laboratory test airframe were measured 

by Cole and Martini.l0 Engines and fuel were 

not present for the tests. 

transfer functions of the fuselage both with and 

without attached wings, structural input impedance 

measurements, and structural-acoustic crosscorrelations were obtained. The transfer functions between cabin pressure and excitation force 

applied to the structure indicated substantial 

variability with frequency, with no tendency to 

become asymptotic with higher frequency. 

more, the dynamics of the aircraft structuralacoustics systems provides close couplinq of the 

acoustic space in the cabin to many degrees of 

freedom of the structure. These findings indicate 

a probable difficulty in "fine tuning'' a structure 

to minimize cabin response at propeller tone 

frequencies. 

The analysis 

To 

Structural-acoustic 

FurtherSBN Propagation in Wing or Pylon 

From an acoustics viewpoint, the elastic wave 

of most interest in a structure is the bending or 

flexural wave a combination of compressional and 

shear waves.l* Because of the large transverse 

displacements associated with these waves, they 

readily couple to compressional waves in surrounding fluids. 

excited in a structure such as a wing or fuselage 

by compression waves in a surrounding medium, and 

the structure is likewise capable of exciting compression waves in a surrounding medium. However, 

Langley tests with strain gauges on the main spars 

of the OV-10 indicated that a pure compression 

wave in the main wing spar may also contribute to 

the propagation of SBN through a wing to the fuselage. While the velocity of a compressional wave 

in a bar or plate is invariact with frequency, the 

velocity of the bending or flexural wave varies 

with the square root of the frequency. 

propagation process is dispersive, and the wave 

form is not preserved. Considerable research is 

indicated to properly define the precise nature of 

the propagation of SBN in aircraft structures. 

Thus, bending waves are easily 

Thus, the 

rotor results in periodic torque and thrust which 

would ordinarily be carried by the propulsion system mounting and ultimately by the main structure 

of the aircraft. Thus, the CR propeller may also 

function as an SBN generator. 

GE/NASA UDF enqine with the two counter-rotatinq 

blade rows carried by free turbines, strong periodic reaction torques should not reach the enqine 

mount. A periodic thrust, however, may provide a 

source of SBN qeneration. 

However, for the 

SBN Control Methodology 

SBN Control Concepts 

propeller-driven aircraft appear to have some 

potential. Among these are: 

A variety of SBN control concepts for 

(1) Stator row swirl remover 

(2) Blade number combination selection 

(3) Tuned mechanical absorber 

(4) Tuned wing/fuselage (or pylonlfuselaqe) 

(5) Structural damping 

(6) Compliant airfoil surfaces 

attachments 

Only the first three will be discussed in this 

paper. 

Stator Row Swirl Remover 

The first proposed device consists of a 

number of radially oriented turninq vanes somewhat 

like a propeller that remains stationary instead 

of rotating. These turninq vanes or stators are 

mounted behind the propeller and are designed with 

blade element shape and camber to efficiently turn 

the incoming flow from the propeller to the axial 

direction upon leaving the turning vane as shown 

in Fig. 16. The vanes acting together thus remove 

th4 swirl from the propeller exit flow. 

As mentioned earlier, if the swirl flow behind 

the propeller were to have constant magnitude and 

direction, it would present no problem with respect 

to structureborne noise. 

radial distance from the propeller axis, the flow 

has a constant or DC component and a varyinq or AC 

component. In fact, it is not necessary to remove 

the constant or DC swirl, the component imposinq a 

constant torque force on the wings, in order to 

eliminate structure-borne noise due to swirl 

interaction with the wings. A stator row system 

designed to function solely as a flow straiqhteninq 

device, such as the flow straightening honeycomb 

screens in a wind tunnel, would be quite adequate. 

However, such a stator system provides a very 

attractive means to also remove the 3C swfrl ccmponent, and thereby increase the propeller propulsive efficiency. 

To be effective in eliminating the AC swirl 

component from the propeller exit flow the downstream stator row must have a reasonable hiah 

However, at any given 

SBN Generation by Counter-Rct:ticn Propellers solidity from hub to tip. Optimum values for a 

given design, of course, must be determined by 

analysis and experiment. The requirement for flow 

row operating in the circumferentially-varying straightening (removing the AC component swirl ) 

swirl from the upstream blade row experiences a is, indeed, a function of the structureborne noise 

cyclic or periodic variation in torque and thrust reduction required in terms of the baseline strucloading determined by the passage of each down- tureborne noise level, the desired final interior 

stream blade through the wake of each upstream noise level, and the benefit provided by other 

blade. The periodic loading of the downstream means of structureborne noise reduction. 

For counter-rotation propellers the aft blade 

5 
Performance and Acoustic Benefits of Stators 

for SR Prooellers. The use of stators to efticiently recover the swirl energy from the propeller 

exit flow provides highly significant additional 

benefits. For high speed, high performance aircraft such as currently being considered in the 

advanced turboprop aircraft concept, the propeller 

swirl results in lower propeller efficiency due to 

kinetic energy lost in the swirl flow, complicated 

nacelle and wing contouring requirements to minimize drag, complicated canted engine inlet designs 

due to off axis flow entering the inlet, and low 

speed stability and control problems due to nonsymnetrical wing contours aft of the engines. 

In a parametric design study of counterrotation propellers for advanced turboprop aircraft,l3 the propulsive efficiency of a system 

with seven stators located downstream of a tenblade singlerotation propeller was calculated. 

The propeller efficiency went from 80 to 87 percent. 

the efficiency calculated for the recommended 

counter-rotation design of two counter-rotating 

propellers with five propeller blades each. 

tureborne noise device are a: follows: 

This value was only 1.1 percent point below 

Some advantages of the proposed stator struc- 

(1) Reduces structureborne noise produced by 

the oscillating aerodynamic loadinq of a 

propeller slipstream on a wing or pylon on 

which it is mounted. 

(2) Increases propulsion system efficiency by 

recovering propeller swirl energy otherwise 

lost. 

For a wing-mounted advanced turboprop propulsion systems designed for cruise at altitudes of 

25 000 to 35 000 ft at flight speeds of Mach 0.6 

to 0.8, the following additional benefits can be 

realized: 

(1) Eliminates or reduces the amount of wing 

(or pylon) leading edge and nacelle 

contouring otherwise required to achieve a 

low drag propulsion system installation 

(2) Eliminates need for complicated canted 

core engine inlet design by eliminatinq 

off-axis flow into the inlet for inlet 

located aft of the stator 

(3) Reduces low speed stability and control 

problems by eliminating nonsymmetrical flow 

over the opposite wings for same rotation 

direction enqines on a twin or four-enqine 

aircraft 

(4) Allows the single-rotation advanced 

propeller (prop-fan) system to approach the 

propeller efficiency of an equivalent 

technology counter-rotation propeller 

system 

Swirl from CR Propellers 

Since a counter-rotation (CR) propeller system 

is designed to remove propeller swirl, such a system may seem ideally suited for a wing-mounted 

tractor propeller configuration with respect to 

structureborne noise. However, while the CR system 

can readily remove the DC swirl component, the flow 

downstream of the second propeller would exhibit a 

relatively high level of AC swirl component, particularly if the number of blades in each row is 

small, e.g., on the order of one-half the number 

of blades in an equivalent single rotation propeller design. The effectiveness of swirl removal, 

of either the DC or AC components, is determined 

primarily by the solidity of the downstream blade 

row, be it counter-rotating or fixed. 

a single propeller blade or stator vane would 

process only a small portion of the total flow. A 

sufficient number of rotor blades or stator vanes 

with appropriate chord length, i.e., sufficient 

solidity, is required to substantially reduce the 

AC swirl component. 

For example, 

Blade Number Combination Selection 

SR Propellers. An important parameter to the 

reduction and phasing of the AC swirl reaction 

torque is the number of stator vanes relative to 

the number of rotor blades. If the number of propeller blades is qiven by B, each downstream stator 

vane "sees" a peak pressure each time a propeller 

blade passes by, or B torque pulses per revolution. 

At a propeller rotational speed of N rps, the stator row experiences a periodic torque at a frequency of BN Hz. If the number of stator vanes is 

also B, all peak torque pulses occur simultaneously 

with a magnitude of B times that of a sinqle pulse 

at the blade passage frequency of BN Hz, the blade 

passaqe frequency. However, if the number of stator vanes is some other number V rather than B, 

where B and V contain no common factors, then the 

periodic torque on the stator consists of BV small 

pulses per revolution with a frequency of BVN Hz, 

or V times the blade passage frequency. 

Fig. 17(a), if eight equally spaced stators are 

placed behind the 8-blade SR-3 propeller, the eight 

stators will each experience the reaction torque 

from the nearest propeller blade wake simultaneously. 

are each 45 apart, when propeller blade no. 1 

passes stator vane A, (see Fig. 17(a), a simultaneous passage occurs for blade no. 2 and vane B, 

and for the remaininq combinations of no. 3 and C, 

no. 4 and D, etc. Thus, there will be a torque 

pulse consisting of the combined (added) torques 

for all eight stators for each passage of blade 

no. 1 with one of the stators. The correspondinq 

reaction torque on the nacelle is represented in 

Fiq. 17(b). 

However, the nacelle torque reaction is very 

much different if nine equally spaced stators are 

used instead of 8, as shown schematically in 

Fiq. 18(a). 

only 40 

before, 45 apart. Thus, at the moment the propeller wake of blade no. 1 Is passing stator A, 

blade w$ke no. 2 is still 5 away from stator B. 

After 5 more of propeller rotation, or 5/360 or 

1/72 of a revolution, blade wake no. 2 will pass 

stator B. 

5' or 1/72 or a revolution, blade wake no. 3 will 

pass stator C, no. 4 stator D, etc. 

complete revolution, or 72 5O increments later, 

blade wake no. 8 will have passed stator I, and 72 

small torque pulses will have been imposed on the 

nacelle as shown in Fig. 18(b). As discussed previously, with 8 stator vanes, there were only 8 

larqe torque pulses per revolution, as shown in 

For example, as indicated in the schematic of 

Siye the rotor blades and stator vanes 

In this case the stator vanes are 

apart while the rotor blades are, as 

At subsequent increments of rotation of 

After one 

6 
Fig. 17(b). 

tor of Fig. 17(b) are also shown by the dashed 

lines in Fig. 18(b) for comparison. For the general case, the number of torque pulses P per propeller revolution is given by the relation 

The large pulses from the 8-vane staP = RS 

where R is number of rotor blades, and S is 

the number of stator vanes, and where R and S 

have no common factor. 

tion of S for a given value of R, the strength 

of the torque reaction pulses on the nacelle supporting the stator row may be reduced by the factor 

S. Furthermore, the frequency will be multiplied 

by the factor S. 

A process that would cause a shift to higher 

frequencies would be quite advantageous for ABN. 

Noise reduction as a function of frequency is 

plotted in Fig. 19 for a representative advance 

turboprop aircraft double-wall acoustic design. 

The data are from Fig. 89 of Ref. 12. In the 

experiment by Lockheed-California a 9.0 m, 

(29.5 ft) long section of a Swearinqen Metro I1 

aircraft was used as a 0.43 scale model of an 

advanced turboprop fuselage. 

placed in an anechoic chamber and the sidewalls 

were excited by simulated propeller noise and by 

white noise. The 3.5 m (11.5 ft) long test section was fitted with a variety of experimental 

high suppression wall treatments designs. 

shown in the figure, increasing the BPF from 300 

to 1200 Hz, an increase by a factor of only 4, 

increases the noise reduction by about 30 dB. 

Thus, the test double-wall reradiated ABN to the 

cabin interior less readily at 1200 Hz than at 

300 Hz. If the reradiation of SEN occurs in a 

like manner, the shifting of the reaction torque 

pulse frequency to a higher frequency would be 

very attractive. 

peller exit flow will have a small oscillating 

swirl flow. Referring again to Fig. 11, it is 

clear that as the 8-blade propeller turns the 

swirl pattern also rotates. Hence, a stator will 

be impinged by the vortex flow from one blade at 

one instant followed by the flow intermediately 

between adjacent blades and at 1/8 of a revolution 

later, be impinged again by the vortex flow from 

the following propeller bladc. Consequently, the 

flow entering the stator will exhibit a range of 

incidence angles, and the turninq of the flow will 

not be exactly to axial for all circumferential 

locations. The dicrepency in the tangential flow 

component, a variation in the stator deviation 

angle with a period corresponding to the tiqe for 

1/8 revolution, for each spanwise location, will 

be quite small relative to the variation in the 

incidence angle for the same spanwise location. 

Thus, while the time averaged net swirl, the DC 

swirl component, will be zero or nearly zero at 

any spanwise location, the instantaneous values 

will vary from very slightly underturned to very 

slightly overturned during a period. As a consequence, there will be a small oscillating torque 

on the wing even with the stator providing zero 

net swirl upstream of the wing. 

Thus, by the proper selecThe test article was 

As 

Even with properly designed stators the proThe chordwise flow along a given stator surface will also vary with time. The secondary flow 

(flow perpendicular to the propeller axis) pattern 

on Fig. 11 will rotate at the propeller rotational 

speed. Consequently, if at a qiven instant the 

stator leading edge is coincident with a vortex 

center of Fig. 11, at a moment later the blade 

leading edqe will be coincident with a different 

portion of the flow in the CCW direction, and will 

ultimately be coincident with the vortex of the 

following blade. The stator will have processed 

flow for a complete cycle, i.e., from one blade to 

the following one. Since this is true for all 

stators as well as for all angular locations 

between stators, the flow downstream of the stator 

will exhibit the range of variation from blade to 

blade during the interval of a period and at an 

axial distance or wavelength of llBPF x V,, where 

V is the velocity of the propeller slipstream 

fjow relative to the propellerlwing system. 

there will remain, even in the case where no net 

average swirl remains, instantaneous CW and CCW 

swirl components which, though cancelling each 

other, nevertheless, introduce an oscillating 

pressure force on the downstream winq surface with 

a much reduced magnitude relative to the original 

unsteady swirl flow, and at the same BPF frequency 

as the original swirl. The further reduction of 

this residual oscillating swirl, if large enough 

to jeopardize the attainment of a desired interior 

noise or vibration level, can be accomplished by 

application of a second structureborne noise control device, the tuned mechanical absorber, to be 

discussed later. 

In a counter rotating propeller the selection of the number of forward and aft 

rotor blades also has an important effect on 

vibration transmitted to the enginelpropeller system mounting, and ultimately to the fuselage 

itself. 

single rotation propellers can be used to reduce 

the vibrations transmitted by a counter-rotation 

propeller to the mounting pylon and subsequently 

to an aircraft cabin. For example, consider the 

cyclic torque forces on the aft rotor due to the 

cutting of the wakes of the upstream rotor. 

there are F blades in the forward rotor and F 

blades in the aft rotor, there will be 2F torque 

pulses per revolution on the aft rotor with the 

rotors turning at the same rate but in opposite 

directions. The periodic torque load frequency on 

the aft rotor will be 2FN Hz. However, if the 

number of blades in the aft row is A instead of F, 

and A and F contain no common factors, there will 

be 2FA pulses per revolution of the aft rotor with 

a strength of 1/A the magnitude of the pulses of 

the previous arrangement. 

vibrations will occur at a higher frequency of 

2FAN Hz and may be more highly attenuated by the 

aircraft structure. 

Thus, 

CR Propellers. 

An approach similar to that used for 

If 

The reduced strength 

Tuned Mechanical Absorber 

The tuned mechanical absorber is particularly 

suited for reducing vibratory motion in nonplatelike structures with low surface strains, and for 

which the vibration frequency spectrum of the 

response has a single resonance, a number of . widely-jeparated .- 

separated groups of modes. 

effective in reducing the magnitude of a flexural 

wave in a wing or pylon supporting an engine/ 

propeller system. 

is rigidly fastened to the vibrating member at a 

point of hiqh amplitude response, such as an 

re~oxancel~ 31" B ns~'mhnr of widely 

It should be 

In this approach a tuned mechanical absorber 

7 
anti-node. 

absorber for a single de ree of freedom system is 

illustrated in Fig. 20(a7. A schematic of a typical physical arrangement of the system is shown in 

Fig. 20(b). In this case a viscoelastically-damped 

rod supports a mass to form a spring-mass oscillator system. Figure 21, taken from Ref. 14, 

indicates that the vibrational energy removed from 

the system is very sensitive to frequency for 

lightly damped systems (low values of the damping 

loss factor, no). The sharply tuned and lightly 

damped system provides the 1 owest damping system 

weight for a single frequency response in the 

vibrating member. Since advanced turboprop propellers are intended to cruise at constant propeller speed (and constant blade passage frequency) 

the tuned absorber may be an attractive device for 

reducing vibration levels in a wing, pylon, or 

tail surface responding to the vibratory loads 

imposed by a propeller. 

ers can be finely tuned to the forcing frequencies, 

in this case, the BPF and possibly some higher 

harmonics . The effectiveness of the tuned mechanical 

absorber has already been demonstrated in reducing 

vibration in an aircraft cabin from the vibration 

set up in the mounting of an out-of-balance engine 

system. Some of the early production models of the 

Douglas DC-9 experienced a difficulty with interior noise in the last two or three window seat 

positions. 

about 10 dB higher than the forward cabin noise 

and was dominated by two tones, one at 120 Hz and 

the other at 180 Hz. In the cruise regime, the 

rotor out-of-balance vibration frequencies of 

interest are approximately at 120 Hz for the low 

speed rotor, and 180 Hz for the high speed rotor. 

Higher frequencies, harmonics, or vibration due to 

blade passage did not cause significant sound 

levels. However, the engine vibration levels 

were, in general, within specification limits. 

The low speed rotor includes two fan stages, four 

compressor stages, and three turbine stages; the 

high speed rotor includes seven compressor stages 

and a single stage turbine. 

analysis in conjunction with experiments by Douglas 

determined that an upward and downward motion of 

the engines caused an oscillzting torque in the 

engine support structure. 

the engine mounting and pylon structure caused a 

structural response in the rear bulkhead to which 

the engine mounts were attached. Computer analysis of the modal patterns for fuselage shell 

vibration for a number of frequencies of interest 

were calculated. The amplitude of vibration was 

on the order of only 0.001 in., as determined by 

the analysis. 

Douglas engineers resolved the problem by 

fabricating tuned vibration absorbers, designed 

for 120 and 180 Hz resonance frequency, and 

attaching these to the engine supports. These 

devices were designed for a noise reduction of 

5 dB for the 120 Hz tone and 12 to 14 dB for the 

more dominant 180 Hz tone. The most probably 

level of the loudest seat on an airplane was 

reduced from 102 dB to 97 dB, while the upper 

limit was reduced from 112 to 102 dB. The "fix" 

was successfully applied to several other noisy 

DC-98 aircraft and shortly thereafter was included 

on production DC-9's. 

The ideal simple tuned mechanical 

Tuned mechanical absorbAt these locations the cabin noise ran 

A forced vibration 

The forced vibration in 

Summary and Conclusions 

Structureborne noise in propeller-driven 

aircraft has until recently received little attention from engineers and scientists. Now, however, 

as the advanced turboprop propeller offers a significant fuel saving over the currently popular 

turbofan engines at comparable flight speeds, considerable research is needed to ensure that structureborne noise will not hamper the utilization of 

this very attractive propulsion system. 

rotating system and unbalanced aerodynamic forces 

on the propeller, the unsteady components of propeller swirl produce periodic torques on downstream 

airfoil surfaces that propagate to the aircraft 

cabin as noise. 

tureborne noise for a sinqle-rotation tractor propeller-driven aircraft is a specially desiqned 

stator row mounted downstream of the propeller. 

By proper selection of the number of stator vanes, 

the strength of the torque reaction forces transmitted to the fuselage can be greatly reduced and 

the vibration frequency can be shifted to a much 

higher frequency where attenuation by the aircraft 

structure may be more effective. Selection of the 

appropriate blade number combination may also 

reduce SBN in a CR propeller system. 

absorber, a damped mass and spring system, is 

rigidly fastened to the vibrating wing or engine 

mounting pylon to reduce the vibration levels 

transmitted to the fuselage and thereby reduce the 

SBN in the cabin interior. 

Application of the structureborne noise 

theory discussed in this paper and the proposed 

control methods, it is believed, can lead to 

greatly reduced structureborne noise in propellerdriven aircraft. In any event, the experimental 

and analytical investigation of the concepts discussed here should at least serve to advance current understanding of the structureborne noise 

phenomenon. 

In addition to mechanical imbalance of the 

One approach to reducing strucIn another approach a tuned mechanical 

References 

1. Duqan, J.F., Miller, B.A., Graber, E.J., and 

Sagerser, D.A., "The NASA High-speed Turboprop 

Program," SAE Paper 801120, Oct. 1980. 

Savings Potential of the NASA Advanced Turboprop Program," NASA TM-83736, 1984. 

3. Metcalf, V.L. and Mayes, W.H., "Structureborne 

Contribution to Interior Noise of Propeller 

Aircraft," SAE Paper 830735, Apr. 1983. 

4. Unruh, J.E., and Scheidt, D.C., "Design and 

Test of Aircraft Engine Isolators for Reduced 

Interior Noise," SRI-06-4860, Southwest 

Research Institute, San Antonio, TX, Dec. 1982. 

5. Bober, L.J., Chaussee, D.S., and Kutler, P., 

"Prediction of High Speed Propeller Flow 

Fields, Using a Three-Dimensional Euler Analysis," AIAA Paper 83-0188, Jan. 1983. 

2. Whitlow, J.B., Jr., and Sievers, G.K., "Fuel 

8 
6. Miller, B.A., Dittmar, J.H. and Jeracki, R.J., 

"The Propeller Tip Vortex - A Possible Contributor to Aircraft Cabin Noise," Journal of 

Aircraft, Vol. 19, No. 1, Jan. 1982, pp. 84-86. 

tureborne Noise in Aircraft:Modal Tests," NASA 

7. Clevenson, S.A., and Metcalf, V.L., "StrucTM-87739 , 1986. 

8. Unruh, J.F., llPropeller-Induced StructureBorne Noise: Laboratory-Based Test Apparatus," AIAA Paper 86-1938, July 1986. 

9. Weir, D.S., "Wing Loads Induced by a Propeller 

Wake," AIAA Paper 86-1967, July 1986. 

10. Cole, J.E., and Martini, K., "Experimental 

Test Results on the Beechcraft Baron 58P, 

Phases I and 11," Cambridge Acoustical Associates Report U-1415-349, Sept. 1986. 

11. Beranek, L. L., Ed., Noise Reduction, McGrawHill, New York, 1960. 

12. Prydz, R.A., Revel1 , J.D., Hayward, J.L., agd 

Balena, F.J., "Evaluation of Advanced Fuselage 

Design Concepts for Interior Noise Control on 

High Speed Propeller-Dri ven Aircraft." 

LR-30192, Lockheed-California Co., Burbank, 

CA, Sept. 1982. (NASA CR-165960) 

13. Strack, W.C., Knip, G., Weisbrich, A.L., 

Godston, J., and Bradley, E., "Technology and 

Benefits of Aircraft Counter-Rotation Propellers." NASA TM-82983, 1982. 

14. Rogers, L., Ed., Conference in Aerospace Pol - meric Viscoelastic Damping Technology for th: 

1980'~~ AFFOL-TM-FBA-78-78, July 1978. 

15. Van Dyke, J.D. Jr., Schendel, J.W., Gunderson, 

C.O., and Ballard, M.R., "Cabin Noise Reduction in the DC-9," AIAA Paper 67-401, June 

1967. 

9 
QRtGrnAL PAGE ls 

OF POOR QUALITY 

r STRUCTUREBORNE 

1 NO I SENIBRATI ONS 

PROPELLER WAKE 

-'. ''. 

4- AIRBORNE NOISE 

PROPELLER 

ENGINE 

FIGURE 1 .- CABIN ENVIRONMENT-AIRBORNE NOISE AND STRUCTUREBORNE NOISE. 

FIGURE 2. - SINGLE-ROTATION ADVANCED TURBOPROP PROPULSION SYSTEM. 
ORlGMAL PAGE Is 

OF POOR QlJAun 

FIGURE 3. - WING-MOUNTED SINGLE ROTATION HIGH SPEED PROPELLER INSTALLATION. 
ORIGrnIAL PAGE Is 

OF POOR OUALITY 

FIGURE 4. - G.E./NASA UDF AFT-MOUNTED PUSHER ENGINE INSTALLATION. 
@R‘aGIMAL PAGE 1s 

OF POOR QUAW 

C-86-0139 

FIGURE 5. - WING-MOUNTED CR TRACTOR PROPELLER SYSTEM INSTALLATION. 

rAIRBORNE NOISE RADIATED 

/ TO CABIN INTERIOR 

STRUCTUREBORNE 

NOISE RADIATED 

TO CABIN INTERIOR1 I 

STRUCTUREBORNE i;,/‘ 

NOISE VIBRATIONS-” 

FIGURE 6.- PROPAGATION OF AIRBORNE NOISE AND STRUCTUREBORNE NOISE 

TO THE CABIN INTERIOR. 
STRUCTUREBORNE NOISE 

RADIATED TO 

CABIN INTERIOR 3 

\\ 

!'\ -rBARRIER BLOCKS 

STRUCTUREBORNE 

NOISE PENETRATES 

BARR I ER 

FIGURE 7.- STRUCTUREBORNE NOISE ISOLATED BY EANS OF AIRBORNE 

NOISE BARRIER IN GROUND TEST. 

AIRBORNE 

NOISE BARRIER 

\ 

7\ I 

NOISE + STRUCTUREr AIRBORNE NOISE 

STRUCTUREBORNE NOISE 

4' 

BARRIER ATTENUATION, dB 

FIGURE 8.- VARIATION OF CABIN NOISE LEVEL WITH VARYING 

BARRIER TREATENT . 
FIGURE 9.- WING AND FUSELAGE VIBRATION MOM AT 201.5 HZ WITH WHITE 

NOISE SHAKER EXCITATION ON RIGHT WING. 

rSWIRL FLOW IMP I NGING 3)- ,,,/-WING 

e (( 

/ 

\ \ LENGINE NACELLE 

\ 

PROPELLER 

FIGURE 10.- SWIRL FLOW FROM PROPELLER IRINGING ON 

DOWNSTREAM WING SURFACES, 
CS-Q3-0 199 

FIGURE 11. - PROPELLER WAKE SECONDARY FLOW. 

r PERIODIC COMPONENT 

/ 

TIE - 

FIGURE 12.- PERIODIC AND STEADY COMPONENTS OF PROPELLER 

SWIRL VELOCITY. 
FLUSH MICROPHONES ON 

WIND TUNNEL CEILING 

A/ if 

'-FLUSH MICROPHONES ON 1 

EACH SIDE OF RADIALLY y/ TRAVERSING VANE 

(SIDE OF VANE VISIBLE IN SKETCH 

TERNED "ADVANCING PROPELLER SIM 

OF VANE") 

FIGURE 13.- INSTALLATION OF PROPELLER, RADIALLY TRAVERSING VANE, 

AND MICROPHONES IN WIND TUNNEL. 

V 

J 

0.6 3.06 

---- 0.8 3.06 

1.84 0.86 

---- 0.8 3.06 1.66 1.15 

\*LE, 

1.1 

0 

.7 .8 .9 1 .o 

FRACTION OF PROPELLER RADIUS, r/R 

FIGURE 14.- RADIAL VARIATION OF VANE SURFACE R.M.S. 

DYNAMIC PRESSURE MEASURED IN PROPELLER WAKE. ADVANCING PROPELLER SIDE OF VANE. 
FUSELAGE SHELL 

VIBRATION RODE 

/ 

/ 

7 \ 

-L STRAIN GAGES ON 

WING SPAR 

i 

WING VIBRATION MODE 

FIGURE 15.- WING AND FUSELAGE VIBRATION MODES DUE TO PROPELLER 

WAKE PERIODIC TORQUE LOAD. 

,,- NONROTATING 

1 I TURNING VANES 

// I r SWIRL FLOW TURNED // TO AXIAL BY TURNING /I 

,I I 

FIGURE 16.- SWIRL FLOW TiRtiED TO AXiAL BY NONROiAiiNG TURNING 

VANES, 
2700 

goo 

O0 30' 60' 9oo 

WAKE POSITION OF BLADE #I 

FIGURE 17.- REACTION TORQUE ON NACELLE FOR 8-BLADE 

PROPELLER WITH 8-VANE STATOR. 

8-BLADE PROPELLER 

ROTATING CLOCKWISE 

I n kr8-VANE 

I I STATOR 

I I REFERENCE 

O0 30' 60' goo 

WAKE POSITION OF BLADE #I 

FIGURE 18.- REACTION TORQUE ON NACELLE FOR 8-BLADE 

PROPELLER WITH 9-VANE STATOR. 
LENIFLOPE OF INTERIOR 

MI CROPHONE AVERAGES 

30 - 
r BASE 

RESPONSE 

MATER I AL I I 

(B) TYPICAL PHYSICAL ARRANGEMENT. 

FIGURE 20.- TUNED MECHANICAL ABSORBER. 
I I I 

.5 1.0 1.5 2.0 2.5 3.0 

FREQUENCY RATIO 

FIGURE 21 .- VIBRATIONAL ENERGY DISSIPATION OF TUNED 

MECHANICAL ABSORBER, 
2. Government Accession NO. NASA TU-88947 

AI AA-87-0530 

1. Report No. 

4. Title and Subtitle 

Structureborne Noise Control in Advanced Turboprop 

Aircraft 

7. Author@) 

Irvin 3. Loeffler 

9. Performing Organization Name and Address 

111. Contract or Grant No, 

National Aeronautics and %ace Administration 

3. Recipient's Catalog No. 

5. Report Date 

6. Performing organization Code 

535-03 

8. Performing Organization Report No. 

E-3362 

10. Work Unit No. 

Lewis Research Center ' 

Cleveland, Ohio 44135 

National Aeronautics and Space Administration 

Washington, D.C. 20546 

13. Type of Report and Period Covered 

14. Sponsoring Agency Code 

2. Sponsoring Agency Name and Address I Technical Memorandum 

9. Security Classif. (of this report) 

Unc 1 as s i f i ed 

20. Security Ciassif. (of this page) 21. No. of pages 22. Price' 

Unclassified 

6. Abstract 

Structureborne noise is discussed as a contributor to propeller aircraft interior noise levels that are nonresponsive to the application of generous amount of 

cabin sidewall acoustic treatment. High structureborne noise levels may jeopardize passenger acceptance of the fuel-efficient high-speed propeller transport 

aircraft designed for cruise at Mach 0.65 to 0.85. These single-rotation tractor 

and counter-rotation tractor and pusher propulsion systems will consume 15 to 

30 percent less fuel than advanced turbofan systems. Structureborne noise 

detection methodologies and the importance of development of a structureborne 

noise sensor are discussed. A structureborne noise generation mechanism is described in which the periodic components of propeller swirl produce periodic 

torques and forces on downstream wings and airfoils that are propagated to the 

cabin interior as noise. Three concepts for controlling structureborne noise 

are presented: (1) a stator row swirl remover, (2) selection of a proper combination of blade numbers in the rotor/stator system of a single-rotation propeller, and the rotor/rotor system of a counter-rotation propeller, and (3) a tuned 

mechanical absorber. 

7. Key Words (Suggested by Authofls)) 

Structureborne noise; Propeller noise; 

Advanced turboprop noise; 

Interior noise 

18. Distribution Statement 

Unclassified - unlimited 

STAR Category 71 

'For sale by the National Technical Information Service, Springfield, Virginia 221 61 

~ ~~___ ~