“The Black Art”
Since the dawn of aviation there have been many efforts to analyze and design the best propeller. The efforts to work from the design end have suffered, at best.
The problems of designing for prop performance on paper is mainly the result of the large number of recognized and unrecognized variables.
The recognized variables include prop diameter, pitch, number of blades, rpm and twist.
These interact in true aviation applications where engine output is influenced by propeller efficiency. Many efforts to codify propeller design look more like
black art then accurate design. This is because of the many variables. As shown below, the efforts to identify basic factors is confusing enough.
Variables such as V/nD, and Cs make things look way too complex.
There has to be an easier way.
While qualitative assessment has worked for years, it would be better if we could easily test and assign prop efficiency numbers to the current ship and prop.
Now there is an easy set of tests that anyone can do to closely evaluate prop efficiency. With this approach we can get an actual number which can be used to evaluate propeller configurations.
By a trick of geometry we can use easily measured information to determine an amazing amount of data. You do not have to be an aerospace engineer (that used to be Aeronautical Engineer, but rocket flight put an end to that.) to do some real data finding.
Notice in the figures, in each case, the angle between V and dH is identical to the angle between W and D.
That means that D/W = dH/V. We know W is total weight inlbs. We know V is airspeed from the ASI. We can record dH by measuring change in height and converting it to mph.
Then D = W*dH/V. And, by the way, drag horsepower is = DV/375.
For climb, the same holds true except, hidden in the figure is D. So when we get dT it is net thrust and must be added to drag. Hence the climb test and descent tests need to be at the same flight speed.
When you simplify it as much as possible, the final equation becomes:
Prop Eff % = (75*W*((1/sa)+(1/sd)))/H
H = Engine Horsepower,
sa is time to ascend 400 feet,
sd is time to descend 400 feet, and
W is total weight.
Assumed average test altitude is 3000 MSL.