The design of propellers made of Micarta is discussed. The advantages of the material are noted, especially as compared with wood. The design changes necessitated by the use of Micarta are discussed with reference to the hub boss, the narrowing of the blade tips, the thinning of the blades, the angles of the leading and trailing edges, and the adjustment of the pitch. Results of flight tests show that the Micarta propeller gave a top speed of 2 miles per hour more than the wooden propeller while turning about 120 r.p.m slower, with about the same rate of climb. At top speed, the Micarta propeller shows an improvement of about 7 percent in fuel economy, although the plane is flying 2 miles per hour faster

Caldwell, F W

Clay, N S

English

NASA Technical Reports Server

.... 
FILE COpy 
NO.I-W c 
TEC HN ICAL NOTE S 
c 
NATIONAL ADVI SORY COMi.U TTEE FOR AE RONAUTI CS 
No . 200 
MI CARTA PROPELLERS - III 
GE NERAL DE SCRIPTION OF THE DESI GN 
By F. W. Ca l dw e l l and N. S Clay 
Wash ingt on 
AU b'U.st , 1924 
Reproduced by 
NATIONAL TECHNICAL 
INFORMATION SERVICE 
__ SPringfield, Va. 22151 
, 
11 
https://ntrs.nasa.gov/search.jsp?R=19930080993 2020-03-17T04:11:59+00:00Z
NATIONAL ADVISORY COM.MITTEE FOR AERONAUTICS . 
TECHNIOAL NOTE NO. 200. 
MIOARTA PROPELLERS - III. 
GENERAL DESCRIPT ION OF THE DESIGN . 
By F. W. Oaldwell* and N. S. Clay.** 
I n working out a desi gn of prop el lers to be made of mi carta 
it is nece s sary to depart s omewhat from the conventional types 
of propeller 6esi gn so a s to make the most advanta geous use of 
the properties which are peculiax to the mat erial. I t would b e 
possible to take the design of an ordinary wooden p rope l ler a nd 
~ build it in micarta . Th i s would give a very heavy propeller, 
.. 
h owev e r, W'~1 ich, of course, would be a very serious fau lt in a 
propeller to b e used i n aircraft. The resulting p ropeller would 
also be no more efficient than the corresponding wooden propeller . 
The heav iest part of the wood en propeller is t h e hub boss . 
It is ne ces sa ry to make this large on ac count of the high crush-
i ng s tress i mp os ed by the drive a nd a lso on a cc ount of the ten-
d ency to split out through the hub and. bolt hole s . 
I n micarta we are dealing with a materia l of enormou sly 
greater cl"'ushing st rength than wood . At the same time, the grain 
is in only one plane in the ca s e of micarta so that we may make 
the p l ane of the grain perpend icular to t h e axi s of rotation and 
avoid any danger of splitting out through the hub bos s . Of 
*Aeronautical Eng i neer, Air ervice, War Department. 
* * Material and Proces s Eng i nee ri ng Department, West inghouse 
Elect ric and J1anufacturing Company . • 
N.A . C. A. Techn ical Notc No. 200 2 
course, i n the case of the wooden p ropel l er the splitting may 
occur in any p l ane paral lel to a radial line through the main 
p ortion of the blade . 
The us e of m ica rta thus p er.~l its a much small er hub b os s than 
the corresponding vlfOoden p rop cl l cr . Th i s in itself makes f or a 
considerable saving of wei ght. 
At the same time, freedom f rom the tendency to split ma kes 
it possible to u s e t h i nner secttons with mi ca r ta b lades than is 
p oss ibl e with mos t wooden b l ades , and as a re sult there is an 
i mprovement in efficiency. 
I t i s a l s o p os s i b l e to us e s omewhat narrower tips with mi-
ca rta p ropell e r s than with wooden ones, and thi s appea r s to b e 
an advantage from the s tandpotnt of efficiency i n h i gh speed 
p ropell ers . I f the wooden p ropellers are made ve r y narrow at 
the tips , they b ecome too f ra gile to stand the ab rasion of small 
st ones, gr a s s , et c . On a c count of the greater tou ghness of th e 
ma terial, and it s f r eedom f rom any tendency to split, t h e micarta 
b lade s may be ~nad e quite th in at the tips wi .t hout i mpairing t hen 
fr om the serv ic e standpoint. 
I n the p rac t i cal des i gn of a ircraf t p ropellers one of the 
mos t dest ructive f or ces encountered i s a form of t orsional vib ra-
tion usually descr i bed as f lutter. Th i s form of stress is of ten 
pa r t i cularl y seve r e b e caus e, when once started, it i s apt to in-
_ c rea s e i n severity due to the absenc e of damp i ng. 
From Figure 11 it i s obv i ous that the effect of t h e air pre&-
-. A. a.A . Technical Note No . 200 3 
sure on the b l ade will be to caus e it to twist in such a way as 
to in creas e it s angle. As soon as the angle is increased the 
actual amount of a ir pressure is increased, due to the h i ghe r 
angle and at the same time the posit ion of the line of resultant 
air pressure moves toward the leading ed ge; ( shift of center of 
pressure vvi t h increase of angle) so that the twisting moment is 
con siderabl y increased by a relatively small change in angle . 
The only force tending to restore the "b lad e im its initial angle 
is the elasticity of the material . Afte r this vib ration gets un-
der way, the ener gy of the oscillation continues to increa se 
unti l it s rate of increas e is either balanced by the internal 
work of f riction of the materia l or it continues to increase until 
failure of the part takes place. 
The mos t obviou s ;aethod of p rovid i ng a damp i ng means for this 
form of oscillation would b e to so design the blade that t l1G ai r 
pressure will cause the angles t o dec rease rathe r than increa s e . 
The me thod used in wooden p rope1 lers to approach this cond i tion 
i s to des i gn the propellers with a sharply curved leading edge 
and n earl y st raight trailing eo_ge e This method was appl ied in 
the case of the micarta propellers fo r the Liberty engi ne a nd f or 
the 300 HP. Wright engine (Figure 12) . 
Anothe r nethod of mak i ng the angl es decrease under the i nf l u-
enc e of the air p ressure i s to ma ke the leading edge st if fe r than 
t h e rema i nder of the blad e, so that the virtual center of gravity 
i n torsion i s moved neare r the leading edge. 
N. A . C. A. Tcchnica l i~ ote No . 20Q 4 
_1.1-:. inspection of Figurc 13 will show that the effect of mov-
i ng t he v irtual ccnte r of gravity of the section i n t ors ion f ar 
en ou gh f orward i s to cause the blade angl e to decrease under the 
ef fec t of the air loading. Thi s will p roduce a damping cffect so 
tha t the to r s ional oscillations a re never apt to become e xcess i ve . 
I n all of the micarta bl ades an attempt has b een made to reach 
this condition by molding in small p iano wires or other rcinforc-
i ng material s ome:.vhat as shown i n Fi gure 13. As will be shown 
la t e r on i n thi s article, this lTIc t hod i s preferab le from the de-
sign standpoint to t he use of an excess ively curv ed l ead i ng edge 
( F i oure 14) , because the great curva ture of the l eading edge usu-
ally i n troduces centr i fuga l bend i ng moments which may b e qu i tc 
severe . 
These cent ri fugal b ending inoment s ar e part i cularly obj ect ion-
able i n reversible propellers s i nce the b ending Il1 0ment s due to 
the ai r f orce s are usually added to thos e du e to centrifu ga l 
force wh en the propeller i s set in the position of rever s e thrus t . 
For this reason, the b l ades fo r the reve rsible and adjustable p ro-
pelle r s were made symmetri cal, the centers of gravity of the var-
ious c ross sec tions b ein g spac ed in a radial line perpendicular 
to the axis of rotation. Thi s arrangement elimi nates the bendi ng 
moments due to centrifugal fo rce f rom the sta tic st re ss analysis 
s i nce all of these moments 'will be zero. How the centrifugal 
fo rc es effec t the stress when the p ropeller i s subj ected to de-
flcction will be d iscussed later . 
N.A. O.A. Techn ical Not e No . 200 5 
Oons ide r ab le s tudy has been given in all count ries to the 
subj ect of the adjustable pitch p ropeller as a means of i mprov-
in g the per fo rmance of a irplancG . With the conve nt iona l type of 
gasoline cng i n e p ower p l ant , the propelle r mus t be des i gn ed to 
absorb the pO'.'Ver of the eng ine at the t op speed of the airpl ane 
wh en the eng ine is turning at its maximum safe speed . When the 
airplane is s t and in g on the ground the eng i ne \I'Till be able to 
tu rn th i s propeller at onl y about 6510 of the maximum safe eng ine 
speed while during the climb it \lv ill be able to turn t he p ropeller 
a t only ab out 90% of the maximum saf e. engi ne speed. Thu s at the 
t i me of take-o ff , the eng i ne i s only able to develop ab out 8 5% of 
i ts maximum power, while du r ing the Climb it i s able to develop 
onl y about 9 0% of t h e maximum p ower . By the use of adjustable 
p it ch p ropellers, it is p os s i b le to slightly reduce the angles of 
the b l ades durin g t he climb so that t h e eng ine may tur n at its 
maximum safe sp eed at all times . Since the ad j ustab le propelle r 
i nvo l v e s the chang i n g of the b lad e angl es f rom the p ilo t ! s seat 
wh il e the airp l ane i s in mot ion , ther e i s very little add itional 
c omp licat ion i n makin g the change i n angle great enough so that 
the propeller b ecomes a pushe r i nstead of a t ractor. Thi s type 
of propelle r is kn own as a reversib le propelle r. It is, of course, 
n ot used as a °.1 eans of actual l y backing the a ircraf t except in ' the 
case of an a ir sh ip . I n the ca s e of a n airp l ane, the rev ersed p ro-
p eller i s used a s a ve r y p owe r f ul b rake to br ing the p lane qu i ckly 
to rest when landing i n a c onf ined spac e, or on very rough ground , 
• 
N.A. C. A. Technical late No. 200 6 
etc . Thi s reversing f eature is of no particular value f or landing 
on good, smoot h f ields, such a s the usual airports. As a means 
of extending the f ield of the airpl ane , however; by permitt i ng 
la nd ing i n mnall spac es and a bett er take- of f from these places , 
the adjustab13 and revers i ble p ropeller offers a v ery prom i sing 
fiel d. 
The reve r s i ble and ad j ustable p rop el ler with micarta blades 
shown i n Fi gure 1 5, is one of t he most pract i ca l devi ces yet 
work ed out for this pur pose. I t is quite strong in all details, 
we i ghs very little more than the fixed pitch propeller, and op er-
a t es so eas ily that the pitch may be adjusted with two fingers on 
t h e cont rol leve r when the eng ine is running. 
The b lades of thes e p ropellers are made interchangeable as 
t o p itch and balanc e so that any b lade may be replaced by simply 
scr ewing one b lade out and screwing in a new one . 
Fi gure 16 shows a f ront view of an airplan e equ i pp ed with 
one of these adju stable and reversible p ropellers wit h micarta 
b lades. Figure 17 shows a side view of the same airplane and 
p r opeller. Th e counterwei ghts shown at the front of the p r~el­
l er are designed so that they exactly balanc e the centrifugal 
t wi st i ng moments at all angles and sp eeds , thereb y grea t ly reduC}-
i ng the eff ort requ ir -;d to change the p i tch while the eng i ne is 
runni ng. 
The ferrules into which the blade s sc re in F i gu re 15 are 
provided internally with a bal l thrust bearing which takes up t h e 
N.A . C. A. Technical Note ~o. 200 7 
centrifugal force a nd two radial ball b eari ngs to take care of 
the thrust and t orque bending i'n oments. 
The ferrule i s a l s o p rovided with a series of 36 equally 
spaced holes a t the outer end through which a cap screw passes 
i nto the blad ~~3 to secure them from unscrewing. The cap screw 
hole in the blade is drilled by means of a jig in a fixed rela-
tion to the b lade angles. 
At the othe r end of t h e ferrul e there are 12 equally spaced 
holes, 2 of wh ich ma y b e ma d e to match with 3 of 9 equally spaced 
holes i n the outer control ring, which is made integral with the 
counterweight . This patented di ff erential arrangem ent g ives 36 
relative positions between the ferrule and control ring, corre-
sponding to the 36 holes in the outer end of the ferrule. It is 
t hus poss ib le to keep the control ring always in exactly the 
proper relation to the blade angles . 
Th e method of adj usting the pitch is more or less obvious 
from the photo graphs and need not be described in detail. 
At the cockpit there is a patented devic e to insure throt t ling 
whi l e the propeller is pass i ng through the neutral posit ion, thus 
prevent i ng the engi ne f rom ra c ing with the propeller in neutral 
pit ch . 
In order to test the strength of the attac hment to the fer-
ru le, a series of tests on the shearing strength of threaded can-
vas micarta sp ec i mens was mad e by screwing the threaded specimen 
in a cylindrical metal s leeve and applying pressure in a testi ng 
& 
N. A.O . A. Technical Note No . 200 8 
mach i n e unt il fa ilure occurred by shearing of the threads. The 
values are tabulated in tab le. 
All of the specim ens show a shearing strength in excess of 
4000 l b . per sq . in ,. of shearing area. Contrary to expectations, 
t h e finer thrE.ads show som e1Nha t higher units trength in sh ear 
than the COarser ones. All of the sp ecimens were threaded with 
the but tress form of thread designed to carry load in one direc-
tion only. Th ese tests indica te that the actual attachnent of 
the b lade has a shea ring strength of about 180,000 lb., which i s 
s omewhat i n excess of the tensile st rength of the b lade at the 
shank. The nonnal load of this part in operation is about 20,000 
l b . 
Th es e detachable blades are also made up to i nstall i n fixed 
p it ch p rop ellers. Figure 18 shows a propeller arranged to pro-
vide detachable and interchangeable blades for the 180 HP. Wright 
engine, wh ile Figure 19 shows a three-blade propeller made up f or 
the 300 HP . Wri gh t engi ne . 
Many whi rl ing test s have been run on a ll diff erent typ es of 
micarta p ropel lers. One of the most interestin~ tests was a run 
of 300 hours on the Liberty propeller , most of which was carried 
o~t at an i nput of 500 r~ . This test was run b efore any of the 
propellers were released f or service. A later series of tests 
'\fas run by te s ting f i ve propellers a t 1000 HP. input until destruc-
tion occurred . This s eries of tests resulted in adopting the type 
of hub drive shown i n Figure 20 . 
One of t h e Lib erty micarta propeller s equipped with this 
N.A.C . A. Technical Note No. 200 9 
type of hub d rive withstood a test of 25 hours at 1000 HP . in-
put b efore fa ilure occurred . 
A number of interesting f li ght tests have b een made with the 
Libe rty micarta propeller (Fig. 21) . . Fi gure 22 shows a compara-
tive test of the Lib erty micarta p ropeller and a wooden prop eller 
for the same eng ine and airplane . 
The curv es show that t he micarta propeller ga ve a top speed 
o f 2 miles pe r hour more than t he wooden p ropeller while turning 
ab out 120 R.P.M. slower. I n sp ite of the lower R.P . M. of the mi-
carta propell e r, the rate of climb for the two is about the same. 
At the top speed the micarta propeller shows an imp rov ement of 
7% i n fuel economy , although the airplane is flyin g 2 miles per 
hour faste r . 
I n the ab ove, an effort has been made to describe the mi-
carta propelle r de s i gns in a s emi te chn ical way. I n the n ext ar-
t i c le, an effort will b e ma d. e to ES ive some data on the designs 
f rom the s tandpoin t of aerodynaLlics and st ress es. 
l 
! 
N.A.C.A. Technical Note No. 200 10 
Table 
Shea ring Strength of Mi carta Threads 
Dwg . ! T11reads Diameter No . Hicarta Length No. per in. outside 
~ 
2 Wood 9 6 2 3 
4 Canvas 11 8 2 3 
5 Canvas 10 4 2 3 
10 Oanvas 3 2 2 4 
11 Canvas 7 6 I 2 4 
12 Oanvas 6 4 I 2 4 
Shearing Strength of Micarta Threads (Cont.) 
Root Thread Yield Yield Maximlun Ul t. 
No. diameter area l b . sq. in. load Str. 
a t root lb. per 
sa. i n . 
2 2.74 15.06 46000 3054 85000 5644 
4 2 . 80 15.39 57000 3704 92000 5978 
5 2 .62 14.40 50000 3472 78700 5465 
10 3.25 17. 87 550'00 3078 100000 5596** 
11 3.74 , 20 . 56 
I 
58500 2845 100000 4864*** 
1 2 3 . 62 19 .90 37000 1859 100000 5025*** * 
** 100000 l b . for 28 hours. 
*** 100000 II II 1 5 mi nutes . 
* * * * 100000 II II 4 II 
N.A. C.A . Techni cal lOt9 . No .200 Fi gs . ll, 13 
Lin8 of re sul tant 
air pres sure on 
section 
Virtual C.G . of 
s8ct i o~ i n tors i on . 
Fi g . ll 
Fi g . 13 
Reinf orc i ng 
ma.ter i a. l . 
C . G . of 
s ect i on i n tor si on 
Line of r e s ultant 
a ir pre ssur e on 
s ection . 
N. A. C .A . Te chnic a l No t e No.200 
. 9R 
. 8R 
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. 4R 
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Fi g .12 
Fi g .l2. Plan f o r m 
of Mi carta 
Liberty a nd 300 HP. 
Nri gnt propellers . 
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Rate of climb , ft ./min. 
Fi g . 22. Comparative propeller t est on DE-4. 
".A.C.A . "'e;:1:r.i c 1 Not~ ·k. ~;lU Fi go.l't , 15 ~ 16 
Fi~ . 14 
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Micarta Propellers III : General Description of the Design

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