By Bernie Willis
When I was a kid, I thought arm wrestling proved strength and prowess. Timing
and strength were critical to winning. Now its not about me but my airplane. I’ve
discovered that nothing has really changed. The power of your propeller driven
airplane is generated by timing and strength, or in mechanical terms RPM’s and
torque. Let’s explore together.
Propellers are a marvel of engineering or alchemy. As an airscrew pulling its way
through jello makes sense, that one revolution of a 50 in. pitch propeller would
move forward 50 in. It would be like a wood screw turning into soft wood. Each
rotation of the screw driver would pull the screw further into the material.
However, I think of alchemy because with air there are complicating issues. For
instance, air density is variable so there is slippage in the screwing process. Cold
dry air is much more dense than hot moist air. How much will this affect your
propeller? It all depends, so for now just know it makes a difference.
The propeller is like a wing. It has an airfoil, a span and cord. It spins like an
aircraft in a spin but much faster. Here there is a challenge. To compensate for
the increase in speed, foot per second if you will, the pitch or angle of attack
decreases toward the tip. This twist change or washout, certainly helps but it
can’t eliminate the elephant in the closet.
That elephant is the speed of sound. If you’re a fighter pilot you don’t hear it
because you’ve left it behind. If you lived near Burbank CA in the 1950’s you
heard it daily. The sonic boom comes from the air not conforming around the
wing or propeller at high speed. It buffets against the wing. Chuck Yeager
demonstrated the “barrier” could be broken but to this day the “boom” is still
annoying. Late sleepers at Campbell Lake know the sound of prop tips flagging
the air instead of making thrust. I was confronted with this problem at the
Aviation Museum this summer. How could a 90 horse power engine get a 2000
pound airplane airborne? How could such a craft work the sand bars in early
Alaska? Today a 90 horse power cub is no rocket ship. Everyone wants at least
150-160 hp and some 180 or more.
James Watt, the steam engine builder, gets credit for determining horsepower. A
story says that a potential customer wanted an engine that could equal the power
of his best horse. Watt figured that a horse could turn a 12 ft. diameter mill 144
times an hour pulling 180 pounds. Nice horse! Or as figured today lift 550 pounds
one foot in one second. Nice horse! But lifting weights or turning a mill won’t
produce the thrust we need to fly. So, what will? Do you think more horse power.
While this is sort of true, more power also increases speed which out at the
propeller tip can be useless. Take the 86” C-185 prop turning at 2850 RPM. The
circumference of the tip travel is 270” or 769,614 in./min. or 64135 ft./min. or
728 mph. The speed of sound at 59 degrees Fahrenheit, standard day, is 761
mph. No wonder within 33 mph of the speed of sound we hear buffeting,
screaming if you will, noise that isn’t producing thrust.
At the Alaska Aviation Museum we have an assortment of old aircraft engines.
One of them is an OX6 the same as the OX5 but with dual ignition. This engine
despite its low horse power produced torque approaching that of the 180 hp
Lycoming because it produced the power at only 1400 RPM. It was like a diesel
truck churning its way up a hill with a heavy load. It turned that power into thrust
with a long and very course prop. There was no buffeting at the tips, just power
producing thrust. Yes, it was heavy, water cooled, with exposed rocker arms and
a TBO of maybe 50 hrs. You don’t want one for your C-185 but it would beat your
Super Cub in an arm wrestling match with 337 ft. pounds of torque.