Wooden Propeller 1
Now that we have definitely arrived in the 21st Century, you'd think that the military with their substantial budgets would have only the latest and most fancy technology flying in the sky. They do, and surprisingly, not only does some of it run on propellers - the propellers are made of wood!
To an aeronautical engineer, a propeller is just a wing that happens to be rotating. You can feel what air does to a wing if you recklessly put your hand out of the car window, while traveling at speed. If the tips of your fingers are angled upward, the air pushes your hand upward (and correspondingly, the same for downward). The blades of a propeller cut through the air or water, and because they push the air or water backward, the propeller goes forward. (By the way, in the United Kingdom, they call front-mounted propellers "airscrews", because they "screw" through the air and pull the craft behind them.)
The amount of push, pull or thrust that a propeller generates depends on two factors - first, how much mass of air or water that it's pushing, and second, how rapidly it accelerates that mass of air or water. In general, the more efficient propellers shift a lot of mass, but with relatively small acceleration.
The blades can be set to cut through the air at a shallow angle, or at a steep angle. So in theory, a 41-pitch propeller would move forward through the air 41 inches when the propeller blade had spun one complete revolution. In the same way, a 50-pitch propeller would move forward 50 inches for one spin.
Then it gets a little complicated.
Propellers can be either variable-pitch or fixed-pitch.
A variable-pitch propeller can change the angle at which it cuts into the air. This means that it can be set at one angle for taking off, and then a different angle for high-speed cruising. If an engine fails in flight, variable-pitch propellers can be "feathered". "Feathered" means that the blade is turned to point edge-on to the line of flight, so it produces the least drag, and the crippled plane can glide a long way. But variable-pitch propellers have a lot of complicated machinery in the hub, which makes them more expensive.
Fixed-pitch propellers are cheaper, but they are always a compromise. A target-drone, which is designed to be shot at for training practice, usually comes with a fixed-pitch propeller. The fixed-pitch propeller of a target drone would typically be rated at 90% efficient when it's cruising at 480 kph. But the same propeller is so inefficient that at low speed, it can't get the target drone off the ground. So, to take off, it actually has to be catapulted into the air. A fixed-pitch propeller is much like a car that has only one gear.
Today, there are metal propellers and wooden propellers - and even in the 21st century, 10% of all the aviation market runs on wooden propellers. They're mostly Sensenich propellers.
Sensenich propellers got started on a summer day in 1928, when Martin and Harry Sensenich bolted an engine and a propeller to a farm wagon, and took a crazy high-speed ground run on the dirt roads of Lancaster County, in Pennsylvania. They created such a furor, including stampeding several herds of cattle, that they were banned from running their propeller-powered farm wagon on the roads.
But the boys weren't going to be stopped, and by winter they had harnessed their engine and propeller to an ice sled. They staked down a strong piece of wood in a nearby frozen river, and they tied their propeller-driven ice sled to the wood with a 30-metre rope. They had lots of fun doing 60-metre circles until the rope broke, and they got thrown into the bushes on the frozen riverbank. They were able to pick themselves up, but their propeller was shattered into a thousand pieces. So they used tools that normally make wagon wheels to make propellers instead - and three quarters of a century, later their company is still making propellers.
In that time, Sensenich has made around 450,000 wooden propellers. In the USA, Sensenick supplies almost all of the fixed-pitch wooden propellers that pass Certification by the Federal Aviation Administration. Their factory, which employs 20 people, turns out around 4,000 fixed-pitch wooden propellers each year.
Wooden props are stuck with a fixed pitch, because they are fashioned as a one-piece entity along with the hub. But metal props can have variable-pitch, though believe-it-or-not, wood still has advantages over metal, and that's what I'll talk about next time...
Published 19 June 2003
© 2024 Karl S. Kruszelnicki Pty Ltd
