1、NASA COTRACTOR . NASA CR /760REPORT 7V./3L, 7-0 7-/-/ # k_ CHH 74-28(NASA-CR-114760) NOISE LEVELS OF N74-20663,OPERATIONAL HELICOPTERS OF THE OH-6 TYPEDESIGNED TO MEET THE LOB MISSION (HughesHelicopters, Culver City, Calif.) (/0 p UnclasHC $6.50 CSCL 01C G3/02 35587 )NOISE LEVELS OF OPERATIONAL HELI
2、COPTERSOF THE OH-6 TYPE DESIGNEDTO MEET THE LOH MISSIONBy: R. A. WagnerPrepared Under Contract No. NAS 2-7254BY: HUGHES HELICOPTERSDivision of Summa CorporationCulver City, CaliforniaFor: Ames Directorate, U.S. Army Air Mobility R 365 for Muffled2 = Main rotor speed (rad/sec)10Provided by IHSNot for
3、 ResaleNo reproduction or networking permitted without license from IHS-,-,-8. Fuel systemWFS = 10 + 0.063 x (lbs of fuel)9. Muffler weight = 48 poundsPERFORMANCE OPTIMIZED DESIGNThe OH-6A with the Allison C-18 gas turbine had strict and enforced require-ments for light weight and high performance i
4、n its basic design. Literally,ounces were considered in comparing alternate configurations and param-eters. As a result, the OH-6A was an excellent baseline from which tospring when investigating performance optimized designs with the C-20engine.The helicopters studied for the operational OH-6A type
5、 optimized for per-formance using the Allison C-20 engine had the following range of parameters:Main rotor radius varied from 13. 16 (OH-6A) to 15. 0 feetMain rotor tip speed varied from 600 to 750 feet per secondTail rotor tip speed varied from 623 to 779 feet per secondThese machines were required
6、 to meet the mission requirements previouslygiven on page 3.Initially, it had been planned to investigate main rotor tip speed as low as550 feet per second for the performance optimized designs. It was alsodecided to keep the blade aspect ratio the same as that of the OH-6A to main-tain as much simi
7、larity to OH-6A as possible. With these constraints, the550 feet per second tip speed resulted in too high value of CT when using thefull power available at the design hover condition of 6240 feet and 95F.This would result in excessive extrapolation of the OH-6A test data which isvery sparse at thes
8、e high values of CT but it is likely that the implied near-ness to stall would result in a hover performance reduction. Further, thevehicle could not reach the required VNE because of retreating blade stallunless a substantial increase in rotor solidity was utilized. (Actually, thevalues of tip spee
9、d selected are greater than 600 feet per second, hence, theomission of the 550 values is academic.)11Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-All of the helicopters of Table I, meet the OH-6A performance requirementsexcept cases 1 and 7. These
10、 cases are for 13. 16-foot rotor radius, at 660and 600 feet per second tip speeds, respectively. These machines have aVNE of 108 and 114. 9 knots, respectively. Only at a higher tip speed can arotor of this radius and solidity absorb the power of the C-20 withoutencountering retreating blade tip sta
11、ll which in turn causes roughness whichlimits VNE. In all other respects, all these machines meet or exceed thestated requirements. (Case 7 also fails to meet the autorotation criteria.)(See Figure 6.)The pertinent data for the designs studied are given in Table I. Data forcases 1 through 9 are plot
12、ted on Figure 1, Payload versus Tip Speed. It canbeen seen that, as expected, payload increases with rotor radius, and withreduced rotor speed (within the ranges studied). Although the rotors shownfor cases 1 through 6 are 4-bladed, and those of cases 7 through 9 are5-bladed, the significant differe
13、nce is not the number of blades, but ratherthe change in solidity necessitated by the lower tip speed of cases 7 through9.Figure 2, Payload/Empty Weight Ratio Versus Tip Speed shows that the 14. 0foot rotor at 615 feet per second tip speed has the maximum value of the800BEST PAYLOAD/EMPTYRATIO DESIG
14、N POINTPAYLOAD = 650 LBco 700-JCCASE 3. 0 MAINo IBLADE- 1 RAD.=-I BEST PAYLOAD/EMPTY0.476 RATIO DESIGN POINTSMAIN BLADE RAD. = 14.0 FT143VMAX (max cont) (kt) 141 143VMAX (T. O. Power) (kt) 145 150As can be seen from Table I, all of the tail rotors studied had 26-27 horse-power required. From Table I
15、I, it can be seen that all tail rotor weightswere within a range of +1. 9 pounds. Neither of these terms vary enough tobe significant over the range of values studied.14Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-DISCUSSION OF NOISE FORMULAS FOR
16、DESIGN PURPOSESThe presentation of data in Reference 1 is satisfactory for showing thevariation of noise levels when a single parameter is varied. For example,Table I of Reference 1 displays the component noise source, along with thepartial derivative or explanation. This presentation is useful when
17、 evaluat-ing the effect of a change in a single parameter, e.g., tip speed, in an exist-ing vehicle.Several parameters a designer has at his disposal when designing a helicopterare important in defining the noise level. These parameters are:1. Tip speed of main and tail rotors2. Thrust of the main a
18、nd tail rotors3. Power required of the main and tail rotors4. Engine power and rpmInitially, it had been planned to express the sound pressure level ofthe rotors asSPL = K1 + K2 (VT) + K3 (H. P. ) + K4 (T)In conducting the tests, when, for example VT is held constant, horsepowerand thrust are both c
19、hanging. When comparing constant thrust points, tipspeed and horsepower are both varying0 As a result, determination of thevalues of the constants is ambiguous, and different values can be derivedwithout a clear way of deciding which set is the best to describe the actualdata.On the basis of the tes
20、ts reported in Reference 1, the noise levels of themain and tail rotors, and the engine were separately displayed. It has beena relatively simple matter to express these separate noise levels in follow-ing form (for the main and tail rotors):OASPL = K1 + K (VT) + K3 HP x T15Provided by IHSNot for Re
21、saleNo reproduction or networking permitted without license from IHS-,-,-2800.-JCSE 3I-S -r BLADE RAD. =i _ _15.0 FTS2700U0 BEST PAYLOAD/EMPTYn RATIO DESIGN POINT0zo 260013.162500550 600 650 700 750 800MAIN ROTOR TIP SPEED - FT/SECFigure 3. Mission Gross Weight Versus Tip Speed Standard Designsand f
22、or the engine:OASPL = K1 + K2 (HP)+ K3 (%NZ)where the Ks are constants to be determined by the test dataVT = tip speed (feet per second)HP = horsepowerT = thrust (pounds)S2cN = engine rpm expressed as a percentage of normal rated speed.Intuitively, noise will increase as both horsepower and thrust i
23、ncrease, andsince thrust will generally increase as horsepower increases, the squareroot of the product of horsepower and thrust seems a not unreasonable16Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-parameter against which to plot noise level. Us
24、ing the square root retainsthe semblance of “linearity“ of parameters. Although this “artificial“ quan-tity is not correct if the thrust goes to zero (as it can in some of the tailrotor tests), we are really not interested in this extreme case, since in ourapplication, both main rotor and tail rotor
25、 thrusts are confined to a compara-tively narrow range of hovering values.Of course, the data as presented in Reference 1 and this report can be usedto express noise levels in terms of torque, or thrust, or any other quantitythat may be available to the designer at a given time.This presentation is
26、valid only for an OH-6 type vehicle, although the varia-tion in noise levels with variation in parameter could be used as the basis forchecking theory, or making corrections to theory.No attempt was made in this program to relate the noise levels to any theory,or method of calculating noise levels.
27、The effort here is to present the testdata in a convenient form for a designer to estimate the noise levels of anOH-6 type helicopter when tip speeds, thrust and power are known.The derived formulas follow:Standard Helicopter - Square tipped, 4-blade main rotor2-blade tail rotorMain rotor onlyOASPL
28、= 45. 9 + 0. 051 VT + 0. 0077 HPx T(Test runs 148-161, Table II, page 29)Tail rotor onlyOASPL = 65. 0 + 0. 024 VT + 0. 094 HP xT(Test runs 162-178, Table II, page 31)Engine onlyOASPL = 68. 9 + 0. 051 (HP) + 0. 036 (N2 )(Test runs 200-204 Table II, page 33)17Provided by IHSNot for ResaleNo reproducti
29、on or networking permitted without license from IHS-,-,-Quieted Helicopter - Tapered tip 5-blade main rotor4-blade tail rotormuffled engine exhaustMain rotor onlyOASPL = 38. 9 + 0. 054 VT + 0.0086 HP x T(Test runs 90-103, Table III, page 55 and test runs 242-255Table III, page 57, averaged)Tail roto
30、r onlyOASPL = 60. 5 + 0. 021 VT + 0. 081 HP xT(Test runs 65-77, Table III, page 57)Engine onlyOASPL = 57. 9 + 0. 033 (HP) + 0. 118 (%NZ)Test runs 20-34, Table III, page 59)(All test runs cited above are from Reference 1. )(All the above OASPLs are linear dB referenced to 0. 0002 dyne per squarecenti
31、meter)(Note: In the data for the quiet tail rotor only, runs 65 to 77, there are novalues of power required. To obviate this omission, the relationship betweenthrust and power was determined from other “quiet“ tests where both thrustand power values were given. The appropriate values of power were t
32、henused in the determination of the constant terms.)Although the noise levels of the engine gear box and main and tail rotor trans-missions are not separately determined nor displayed, these componentswere present and their influence was present when the tests were run. Thus,we can synthesize the no
33、ise level of any OH-6 type helicopter by estimatingthe noise levels of the main and tail rotor, and the engine separately, andcombining as shown in Figure 3. 7, page 59 of Reference 4.This has been done for the 9 “unquieted“ designs and the data is shown inTable IV. This data shows that, as expected
34、, when designs are made usingessentially constant power, the noise level is a function only of the tip speed.(The estimated OASPLs for designs of the same tip speed vary only 0. 1 dBwith the smaller rotor always the less noisy.)18Provided by IHSNot for ResaleNo reproduction or networking permitted w
35、ithout license from IHS-,-,-TABLE IVCOMPUTED OASPLS FOR “STANDARD“ HELICOPTERSHOVERING AT MISSION GROSS WEIGHTMain Rotor Tail Rotor EngineEstimatedHelicopterCase VT HP Thrust OASPL VT HP Thrust OASPL HP %NZ OASPL OASPL1 660 192 2534 84.9 692 24 139 87. 1 221 100 83. 8 90.22 660 192 2625 85. 0 692 24
36、 140 87. 1 220 100 83.7 90. 33 660 192 2737 85.1 692 24 141 87.1 220 100 83.7 90.44 750 192 2505 89.5 779 24 122 88.8 220 100 83.7 92. 85 750 19Z 2590 89.6 779 24 122 88.8 220 100 83.7 92. 96 750 192 2706 89.8 779 24 122 88.8 220 100 83.7 93. O07 600 192 2594 81.9 623 24 152 85.7 220 100 83.7 88. 98
37、 600 195 2713 82. 1 623 24 156 85.8 224 100 83.9 89.09 600 196 2815 82.2 623 24 159 85.8 226 100 84.0 89. 10Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Figure 4 shows OASPL versus tip speed for the 14. 0 foot rotor radiushelicopter selected for t
38、he best payload/empty ratio.QUIETED DESIGNSFigure 2 illustrates the small change in payload/empty weight ratio occa-sioned by rotor radius change from 13. 16 feet to 15 feet. This same smalleffect will be present in a “quieted“ version since a major part of the quiet-ing comes from the tail rotor.Th
39、erefore, the same radius rotor as used in the best payload/empty ratiovehicle is used in the quieted version.Basic vehicles studied are as shown in Tables I and II. Case 10 has a5-bladed main rotor with tip speed of 615 feet per second (the same as thebest payload/empty ratio design) but has tapered
40、 tips, and has a 4-bladedtail rotor with tip speed of 450 feet per second, and the engine is muffled.Case 11 has the main rotor tip speed slowed down to 550 feet per second,increased blade chord, a 4-bladed tail rotor at 450 feet per second tip speed(slightly larger in blade radius and chord compare
41、d with Case 10 to achievehigher thrust necessitated by the lower tip speed main rotor), and the engineis muffled,110_OH = 6A MISSION PAYLOAD300 I I70 75 80 85 90 95 100OASPL - dBFigure 5. Payload Versus OASPL for Hover at Mission Gross Weight.Mission Gross Weight Based on Hover Ceiling of 6240 FeetI
42、GE at Mil Power23Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-RECOMMENDATIONSHughes Helicopter quiet helicopter program has demonstrated a “quiet vehicle“,but one not totally suited to operational use because the quieting involved rotorspeeds at w
43、hich emergency autorotation was not satisfactory.This study has identified the range of payload penalties in terms of pounds perdB (OASPL linear scale) if the vehicle is quieted and is operationally suitable.Not demonstrated is the tactical significance of “quiet“. It would appear thatthis demonstra
44、tion is not particularly amenable to analytic determination.Even if PNdB had been used instead of linear OASPL as the reference, theaspects of directional effects, the effect of forward flight, the effects ofmodulation due to inter-rotor action (none of which were measured in thisprogram), and the i
45、nfluence of ambient noise levels and ground cover are suf-ficiently complex as to render analytic conclusions difficult to arrive at, andperhaps more difficult to believe in.Therefore, it is recommended that several OH-6A helicopters of an agreedupon quiet be procured, and appropriate field testing
46、be conducted to assessthe value of quiet in tactical situations. These machines could have, say,two levels of engine quieting and tail and main rotor quieting to permit assess-ment of the several ranges of frequencies represented by the three noiseproducing components.It is recommended that developm
47、ent work be pursued leading to reducedengine noise. This work should be done both as part of the basic enginedesign, and as an add-on muffler. The object should be to reduce noise withthe least penalties to power available.CONCLUSIONS1. A range of penalties of 6 to 30 pounds of payload per dB of OAS
48、PL isshown for an OH-6A type helicopter designed to meet the LOH mission.The efficiency of the muffler and the amount of quieting required deter-mine where in the above range the actual penalty falls. Payload lossassociated with weight of a muffler, and increased fuel requiredbecause of the effect o
49、f the muffler on engine fuel consumption aresmall compared to the loss of payload occasioned by reduced enginepower available.Substantial reductions in noise can be accomplished by designing thetail rotor for reduced tip speed and muffling the engine without altering24Provided by IHSNot for ResaleNo reproduction or networki
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