8/19/2019 Percent Power
1/2
How To Determine The Part-Throttle RPM of a Fixed PitchPropeller
At a Given Horsepower
by Stan Hall
Technical Advisor, Engineering, EAA Chapter 62 To get the
most life from an aircraft engine while at the same time getting
acceptable airplane performance the engine manufacturers commo
ecommend their engines not be operated continuously over 75% of
their full throttle rpm. We have thus come to accept this rpm as a
standacruising" rpm.
The problem is, how does the pilot of an airplane equipped with
a fixed pitch prop know when he is pulling 75% since the only
power-relatednstrument he has is the tachometer and the tach
doesn't tell him directly?.
So, the question arises, to throttle back to 75% power should he
come back 25% of the distance the cockpit control provides? No,
because tnly point on the throttle he knows at least roughly what
the power is, is full throttle. Or, should he throttle back to 75%
rpm as shown by the
ach? No again because the relationship between prop rpm and
engine rpm is far from linear. It is, in fact, exponential. The
power absorbed xed pitch propeller is proportional to the cube of
its rpm. Thus, if he reduces the rpm by a quarter he can expect a
power reduction of almoshird - and an IAS far less than he had
anticipated.
But the tach can indicate the power - you can mark it. A way to
determine where to put the tach to yield a 25% power reduction is
to refer to traph shown here. Enter the full throttle (F.T.) level
flight prop rpm on the left and draw a horizontal line to the right
to intersect with the line
abelled 75% F.T. hp then down. Read the part-throttle rpm at the
bottom. An example using the Super Cub PA-18-150 is shown. At 2700
fulhrottle rpm the graph shows the rpm at 75% F.T. power to be 2451
rpm. The Super Cub's owner's handbook shows 2450 rpm.
The graph is based on the use of the so-called "Propeller Load"
equation which is.
HP2 = HP1 (RPM2 /RPM1 )3 (eq. 1)
where HP2 = part-throttle hp, HP1 = full throttle hp,
RPM2= part-throttle rpm, RPM1 = full throttle rpm
Since we're looking for RPM2 we rewrite the equation
to:
RPM2 = (RPM13(HP2/HP1))
1/3 (eq. 2)
Something interesting happens here. Note that HP2 / HP
1 is a ratio of two horsepowers, not the horsepowers
themselves. Since for the 75%
ower case the ratio is .75 we rewrite the equation again and
get:
RPM2 = (RPM
13 x .75)1/3 (eq. 3)
This is the equation upon which the graph is derived. What this
new equation says is, the only things you need know to determine
the Part -hrottle rpm are the full throttle rpm and whatever
percent of full throttle power you choose. The actual horsepower
isn't directly involved exce
8/19/2019 Percent Power
2/2
s reflected in the rpm at which it is driving the propeller.
f you don't know how fast your propeller turns at full throttle
in flight, find out by hopping into your vehicle for a short, full
throttle test on a nicalm "standard" day, level and at the altitude
at which you usually like to fly. You will likely get a more
realistic number this way than by read
n the owner's manual whnt the manufacturer claims for his brand
new engine - while running in a test cell at close to sea level,
with no ram and no engine installation losses such as you most
certainly have in your aircraft.
Back to the graph. If you don't like graphs and/or don't want to
go to the trouble of calculating the cruising rpm from the
equations 1 show,ollowing is a table which lists a selection of
power percentages and multipliers which go with them. Pick out a
percentage you like and multiour full throttle rpm by the
multiplier shown. The result will be the corresponding rpm.
Stick a little piece of tape on your tachometer at that rpm, and
"cruise" at that power setting. In fact, stick several pieces of
tape there, each ane of your chosen power settings. Now you can
fly, knowing whenever the tach needle is on one of the marks, what
percentage power you'ulling.
f you have the very good fortune to know what your full throttle
horsepower is, instead of identifying the tach marks only by power
percentagnnotate the actual horsepower too. Since you know the full
throttle rpm you can figure the horsepower at any other rpm by
using equation l,
ropeller load equation. However, if you're not at ease with
fractional exponents you may need a calculator that is.
You can have all kinds of fun putting this information to work
by, for example, determining fuel consumption versus power, range
versus powtc. Who knows where all this fun can lead?
A final comment about the graph. While the maximum rpm shown on
the graph is 4000, engines such as 2-strokers frequently turn at
over 6pm. This is too fast for an efficient propeller and so these
2-strokers are often equipped with a propeller speed reduction unit
(PSRU). Since raph deals only with prop rpm and not engine rpm all
one has to do in such cases is to divide the full throttle engine
rpm by the speed reduc
actor to get back on the graph. For example, the graph will work
even if your 2 to 1 PSRU is bolted to some wild, screaming fire
breather whurns at 8000 rpm. Even so, a prop turning at 4000 rpm is
likely to show poor efficiency. Better to use a higher speed
reduction factor. But thstory for another time.
©2000-2002 Stan HallQuestions and Feedback can be sent to the
ebmaster
Power Percentage Multiplier 75 O.91
70 O.89
65 O.87
60 O.84
55 O.82
50 O.79
