>> > > The pitch of my old wood propeller which I am retiring is given
>> > in
>> > > inches, while my new Warp Drive uses degrees to set pitch. No
>> > > reference
>> > > to inches.
>> > > What is the relationship between the two methods of
>> > measurement?
>> > > Can you direct me to reference material (for the layman)?
>> > > Yvon
There is only one true measure of the pitch of a propeller */as
installed on a particular airplane!/* That is the effective pitch, which
is the amount the airplane goes forward, in inches per revolution, in
horizontal flight. It's an easy formula: TAS, mph, times 1056 divided by
rpm.
At most airspeeds and rpm, */in level flight, /*the effective pitch
of my strange prop is 76". It has to be taken in level flight since the
prop load is different climbing or descending. If your prop's effective
pitch changes much at different speeds and rpm, its twist and/or chord
distribution is not optimum. As you can see from the previously shown
helix-angle tables, most fixed-pitch props have insufficient twist as
compared to the tables, and actually generate reverse thrust in the root
area which, besides considerably reducing the prop's efficiency, blocks
the cooling airflow, requiring larger cooling inlets, thus more drag.
I've heard many stories about how, even though someone has given his
plane more throttle and rpm, the speed didn't change. If the rpm
increased and the speed didn't increase proportionally, then the prop is
on the downward-slope of its efficiency curve. So a 1% rpm increase
*
/should result from/* 1% more speed, not the other way around*. Your
speed increases from increased hp, and your prop now turns faster! Now
if you have a 100 mph TAS plane and you advance the throttle to where
your rpm goes from 3000 to 3030, your speed should increase to 101 mph;
that's pretty hard to see! However, if your rpm increases, say, 5%, from
3000 to 3150, then you should see 105 mph TAS. If not, your prop has
insufficient twist and you are generating reverse thrust inboard which
works against increased thrust outboard.
*I modified the tips of my friend's prop on his Lancair to reduce tip
drag. The speed increased from 215 mph at 2600 rpm to 219 mph at 2650
rpm. To increase speed that much required (219/215)^3, 5.7% more power.
But 2650 rpm/2600 rpm is only 1.9% more rpm, so only 1.9% more power.
And the speed increase was 1.9%. So, 1.9% more speed, 1.9% more rpm! The
speed increase was due to the prop's increased efficiency due to much
less tip drag!
Paul [Lipps]
Thanks Paul! Very very informative.
Paul Lamar ...No rotor no motor.
This is a very simple prop program that incorporates
Paul's idea of using a wide cord near the root.
Any arbitrary blade plan form can be used by changing
the numbers in the DATA statement.
It can be expanded to calculate the bending load
on the blade as well as the HP consumption of the
prop. It uses the segment method common to some prop
programs. To increase or decrease the pitch you would
increase or decrease the lift coefficient number.
It also works at altitude by changing the air density
number. Still a work in progress.
This is the lift normal to the cord so it is
not adjusted yet for blade angle. Yet to be done. It
over estimates the contribution of the root area to thrust.
Hopefully the idea will be apparent.
RPM = 2700 MPH = 200 Lift Coef. = .5 Air density = .0026
Radius Tang Vel Angle Cord Vel Cord Seg Area Seg Thrust
Inches MPH Degrees MPH Inches Sq. feet pounds
3.00 48.2 76.5 206 7.20 0.150 16.5
6.00 96.3 64.3 222 6.60 0.138 17.6
9.00 144.5 54.1 247 6.00 0.125 19.8
12.00 192.7 46.1 278 5.40 0.112 22.6
15.00 240.9 39.7 313 4.80 0.100 25.5
18.00 289.0 34.7 351 4.20 0.087 28.1
21.00 337.2 30.7 392 3.60 0.075 30.0
24.00 385.4 27.4 434 3.00 0.063 30.6
27.00 433.5 24.8 477 2.40 0.050 29.6
30.00 481.7 22.5 522 2.40 0.050 35.4
33.00 529.9 20.7 566 1.80 0.038 31.3
36.00 578.0 19.1 612 1.20 0.025 24.3
Total Thrust = 311
OPEN "pitch200.dat" FOR OUTPUT AS #1
CLS
DIM Cord(25)
Thrust = 0
AirDen = .0026
MPH = 200
CsubL = .5
RPM = 2700
PRINT #1, "RPM ="; RPM;
PRINT #1, " MPH ="; MPH;
PRINT #1, " Lift Coef. ="; CsubL;
PRINT #1, " Air density ="; AirDen
PRINT #1, "Radius Tang Vel Angle Cord Vel Cord Seg Area Seg Thrust"
PRINT #1, "Inches MPH Degrees MPH Inches Sq. feet pounds"
FOR radius = .25 TO 3 STEP .25
RadiusInch = radius * 12
PRINT #1, USING "###.##"; RadiusInch;
TangVelFPS = (6.28 * radius * RPM) / 60
TangVelMPH = (TangVelFPS * 60) / 88
PRINT #1, USING "#######.#"; TangVelMPH;
tan1 = MPH / TangVelMPH
angleRAD = ATN(tan1)
angleDeg = (angleRAD * 57.295)
PRINT #1, USING "#######.#"; angleDeg;
CordVel = (TangVelMPH ^ 2 + MPH ^ 2) ^ .5
PRINT #1, USING "#########"; CordVel;
READ Cord
PRINT #1, USING "########.##"; Cord * 12;
SegArea = .25 * Cord
PRINT #1, USING "########.###"; SegArea;
ThrustInc = SegArea * (CordVel) ^ 2 * AirDen
PRINT #1, USING "#########.#"; ThrustInc
Thrust = Thrust + ThrustInc
NEXT
DATA .6,.55,.5,.45,.4, .35, .3, .25, .2,.2,.15,.1
PRINT #1,
PRINT #1, "Total Thrust =";
PRINT #1, USING "#####"; Thrust
CLOSE
SHELL "more < pitch200.dat"
SYSTEM
Paul Lamar ...No rotor no motor.
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