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MR AU£.. 1941
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NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS ORIGINALLY ISSUED August 1941 as MeIllOrandum Rep ort WI~TUNNEL TBSTS OF FOUR CURl'ISS PROPELI.ERS · EMBODYING Dn'FERENT BLADE SECTIONS By W. H. Gray Langley Memorial Aeronautical Laboratory Lan.gley Field, Va.
WASHINGTON NACA WARTIME REPORTS are reprints of papers originally issued to provide rapid distribution of advance r e sear ch results to an autho rized group requiring them for the war effort. They were pre- viously he ld under a se c urity status but are now unclassified. Some of these reports were not tech- nically edited. All have been repr oduced without change in order to expedite general distribution.
~ I MEMORANDUM RE PORT for Army Air Corps WIND-TUNNEL TESTS OF FOUR CURTISS PROPELLERS EMBODYING DH'FERENT BLADE SECTIONS By W. H. GRAY SUMMARY Tests of four la - foot pr o pe llers were made in the propeller-research tun nel for the Army Air Corps to c heck flight and stati c t hrust test results ma d e on several pro- pellers embodying Clark Y and modified NACA l6-series sec- tions. These propellers were identical as to diameter and acitvity factor and very closely ide ntical in thickness The blades embodied sections ratio and pitch distribution .
with both sing l e - and double -c amb e red Clark Y, modified NACA l6-series, and a combination of Clark Y and modified NACA -16 t from 20 airfoils. T d f bl d 1 eSjS c ove r e a range 0 a e ang es to 70 , and were all made at tip speeds below 280 feet per second.
Although these tests were not conclusI v e i n themselves, owing to the conditions under which they were m ade , the results seem to check th e flight and s tatic tests as closely as wou ld be expected .
- 2 - INTRODUCTION Last ye ar (19 4 0) the Curtiss Prope - ller Comp a ny built and test e d three propellers which differed only in the sections employed. The purpose of these tests was to determine the relative merits of the Clark Y and a modification of NACA 16- series secti on for th e various conditions of flight. The tests consisted of static tests conduct e d_ at W ri g ht Field and flight tests .conduct ed on a Hawk 75 airplane at Caldwell, New Jersey. The re sul t s fr om these tests, listed in Curtiss Report . Number C-1123, (reference 1) indicat e d that the . Clark Y
section wa s 10 to 15 p ercent superi0r to the modified NACA
l6-s sr i 0s f or t he ta ke - off c ondition; but t h at the modifi e d NA CA ].6 - ser ies propell e r was from 0 to ·3 pe rcent more effi- cient at high speed.
Inasmuch as the modified NACA 16-seri e s propelle~s dis- played both bad and good charact e ri s tics, the Curtiss Com p any could not deci de to use th i s s e ction for production. The Army Air Corps was also inter e sted in this matter, so an ar- rangement was made for the NAC A to test these same propellers in the propeller-research tunn e l as well as on the static whirl ri g . Since the tunnel tests could be made only at low tip speeds, the object would be to furnish more complete in- formation regarding the stalling properties of the sections, and perhaps a measure of the relative drags. The static - 3 - tests would provide information relative to compressibility effects .
The NACA l6-series propellers did not incorporate true sections, having been modified by the Curtiss Com pariy a~ the leading and trailing edges in order to improve the manufacturing and serviceability qualities . As there was some question as to th e effect of these modifications on the aerodynamic qualities, it was decided to test these pro- pellers as they were , and then rework the leading edges into conformity with true NACA 16 profiles , after which additional tests would be made .
This report covers the tunnel tests ' of the three pro- pellers which had been fli gh t te sted by the Curtiss Company, and an additional propell er Which incorpo rat ed modified o 0 Clark Y sections. A blade - angle range from 20 to 70 was investigated .
A s epa r a t e rep or t covers the static whirl rig t ests .
APPARATUS AND METHODS Propellers. - The diameter of al l four propellers tested vari ed onl y s lightly from ten i'eet . The dip in the 101332 geometric pitch curves , as shown i~ figure 1, blade-form data, is due t o th e blade angle of the section in transition from Clark Y at the . root to modified NACA-16 at the tip.
The assu m ed chord line rather than the ' zero lift line has
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been used as a basis of computation for the geometri c pitch.
Propeller 101330 consists of a modified NACA -16 blade section throu~lout. The airfoil fs based on data from ref- erence 2, using the basic 16-000 airfoil.
Propeller 101332 has a modified NACA 16-series tip and a Clark Y root. The b l ade sections fair f rom one airfoil to the other between the 30 inch and 42 inch ' stations.
Propell e r 101336 is a Clark Y section cambere d on both upper and lower surfaces. , Propeller 89306-22S has a true Clark Y section throughout.
Following tests of tle 101332 propeller described above, the three blades were shipped to the factory and the leading edge radius was decreased to more nearly conform to the NACA l6-s eries . Subsequent me asurements indicated that the sec- tions were not yet accurately formed , and must be consid6red to be modified NACA 16 sections .
I All of the above propellers were t e sted in a 28-inch diameter spinner on the st r eamline nacelle shown in figure 5.
Driving mechanism . - The propellers were driven by two 25-horsepower electric motors arranged in tandem. (S ee figure 6. ) The set -up was originally designed for t ests of propellers in dual rotation , and for that reason the front motor was directly connected to the front propeller while the rear motor drove the rear propeller through chains and a
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countershaft . For these tests the propeller shafts were locked together .
Measurements . - The net thrust or drag of the propeller- body combination was measured on a thrust balance locuted on the floor of the test chamber .
T he torque of each motor was measured wi th a spring- Selsyn dynamomete r. The motors rested on bearings con - centric with the shaft axiS , and were restrained from ro- tatin g by springs attache d to the fixed fram e . The amount of deflection of the motor t'rtame~ was measured by Selsyn generator units a nd tr an smitted to inc.:.1catin g units on the rloor .
Measurement of rotat i onal speed was made with a con- dens er tachom e t e r developed by the NACA . Frequent checks on the accuracy of t he instrument were ma de by means of a tunin g fork and oscillograph .
'1'he tunnel spe e d rang ed from 0 t o about 110 miles per hour , and the maximum p ropeller speed was not over 520 rpm , or about 287 feet per second rot at ional tip sp e ed .
RESULTS AN D DISCUSSION The measured v a lues hav e b een reduced to the usual c o- efficients or thrust , power , and pr opulsiv e effiCiency .
- 6 - __ effective thrust CT
p n D4
Cp = engine power
p n3 D5
'T\ = CT V
Cp nD where the effective thrust is the measured thrust of the propeller-body combination plus the drag of the body alone.
liD" is the propeller diameter in feet and "n" is the pro - peller rotational speed in revolutions per second.
The results are given in the following figures: LIST OF FIGtmES Figure Characteristic curves for 101330 (modified NACA
7 to 9
16 thoughout) 10 to 12 Characteristic curves for 89306 -22S (Clark Y throughou t ) Characteristic curves for 101332 (mo dified NACA
13 to 15
16 tip, Clark Y root) 16 to 18 Characteristic curves for 101336 (doubl e -camber ed Clark y) NACA 16 and Clark Y propeller characteristic com - parisons - 7 - 20 Characteristic comparisons for three propellers 21 Efficiency ~nvelope comparison~ 22 Efficiency comparisons for constant Cp :: 0 . 2
23 Efficiency comparisons for constant Cp :: 0.4
24 Thrust ratio comparisons The charact e ristic curves (see figures 19 and 20) have been adjusted to the equivalent angle of the Clark Y blades, This is necessitated by for three represen t a t ive angles .
the difference in - the effective blade a ngles based on the assumed chord lines.
Figure 19 is a comparison b e tween the modified NACA 16 and Clark Y propellers only and indicates a higher thrust, power , and con seque ntly efficiency, in the range of take-off ( 0 .3(V(nD ) max . eff . ) for the Clark Y. The same fi gure shows the delayed stalling characteristic of the Clark Y as compared with the early st a ll of the modified NACA 16 .
In figure 20 th e order of delayed st a lling for the four propellers is indic ated as follows: modified NACA 16 - series, combination modified NACA - 16 and Clark Y, double cambered Cl a rk Y, and Clark Y.
Comparison of efficiency envelope curves for the high- sp eed condition . (se e fi gu r e 21) shows v er y little choice betwe e n pr opellers . Th e data indicates that about 2 per- cent higher e f f i cie ncy was r ea liz ed for the modified NAC A - 8 -
16 propeller at ' values of V/ nD below 3 . 0 ; but at higher values
of V/nD the Clark Y propeller was slightly superior . The crossover of the envelope c o uld be accounted for by the fac t that at the higher values ofV/nD the mod i fied NACA 16 pro - file was operating outside the range of lift coefficients for w hich it was designed . This is even more apparent in figures 22 and 23 , which provide comparisons at cons ' tant power .
Low - speed comparisons of thrust in the range of take - off and ' climb , (see figure 24 ) indicate that the double - cambered C lark Y gives the best approach to the available thrust of the Clark Y.
The tests of the propeller which had been returned to the factory fpr minor reworking showed no appre c iable aerody - namic difference, within the' experImental accuracy , from those conducted with the original specimen .
CONCLUDING REMARKS As these tests were made in a low speed , turbulent air - s t ream, the result 's may not be indicative of the relative merits of the different pr6pellers for high - speed operating conditions . Obviously, the delayed compr e ssibility charac- teristics of thR NACA 16 - series sections would not be expected to be made evident by ' the ' se tests. ' The ' tunnel turbulence may have limited the extent of laminar flow ' disproportionately for the various sections . Increased Mach number operating conditions may have an equalizini effect on the stalling characteristics; although these t e sts seem to check those reported in ~efe~erices I and 3.
If it is accepted that " the use of NACA 16-s~r~es sec - tions does result. in, in.ferior tak e -off qual:ities it should be pointed out that the lower drag qualities at high speeds permit " the use of " larger "blade areas, , or 0pel;'atton at higher tip speeds, or the " employm ent of sections " designed for higher values of C , all of which wil l increase the take-off L thrust.
This indicates t hat all future propellers built for experimentally checking the relative merits of the Clark Y and NACA I G -seri es s ec tions should not be designed geomet ri- cally similar except for section, but should be designed for equal take-off thrust, so that they can be compared on a basis of high speed performance alone .
Langley Memorial Aeronautical Laboratory, National Advisory Committee for Aeronautics, Langley Field, Va., August 2 1, 1941.
- 10 - , REFERENCES 1. Anon.: Comparison of Flight Test on Propellers Using Clark-Y, N.A.C.A.-l6 , and Combination Clark-Y and N.A.C.A.-16 Profiles. Rep. No. 0-1123, Curtiss Propeller Div., Curtiss-Wright Corp.; Nov. 11, 19~.O.
2. Stack, John: ' Test 's of Airfoils Designed to Delay the " qompressibil:ity Burble. NACA TN No. 976, Dec. , 1944.
(Reprint' of ACR, June 1939.)
3'. Corson, Blake w., - , Jr. " and Mastrocola, Nicholas: ' Static
Characteristics of Curtiss propellers Having Diffe~ent
Blade Sections. " NACA MR, Aug. 27, 1941. · "
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