Years ago, while visiting an air museum, I overheard a conversation that has stuck with me.
This particular facility employed several friends. I was a regular visitor there, an occasional helper, and a forever fan of the collection.
A man on the ramp was gazing at a P-51 Mustang as he mused aloud within earshot of an employee, “I wonder why this one has four propeller blades but that one (an L-4, the military version of the J-3 Cub) only has two.”
The employee was a good friend and an absolute whiz of an engineer. He knows more about the inner workings of flying machines than I could ever cram into my head. He’s also a remarkably kind and patient man.
“The extra blades are necessary to transfer the horsepower of the engine into the air,” offered my friend. “More power, more blades.”
The visitor thought about that insight for a moment. He scratched his chin, tousled his hair, and ran little circles in the air with his index finger as he considered this newfound information. Then he spoke. “Nah, that can’t be it.”
Propellers are a mystery to most. Even to pilots they are little more than an afterthought. They seem simple enough. They are little more than a fixed form attached to a spinning shaft that produces thrust when run up to enough RPMs.
Even during the pre-flight inspection many pilots put no more thought into the propeller than to assure themselves it is indeed attached to the aircraft.
How it works, why it works, and what can be done to make it work more efficiently rarely comes up in conversation.
Well, let’s address some of those considerations.
My friend with the bright engineering mind was exactly right. The blades of a propeller are in many ways similar to the vertical stabilizer mounted to that same airplane. Surface area matters.
The Lockheed Constellation has a triple tail for the specific reason that a large surface area was needed to provide directional control for that beautiful piece of flying art deco machinery. The vertical could have been a traditional single tail, but it would have been huge. Too large to fit into most hangars of the day. But split that square footage into three parts and the overall height of the tail becomes quite manageable.
The same is true for propellers. The J-3 only needs two relatively short blades because that’s all physics requires to transfer the might of a 65-horsepower engine into the air.
The P-51, on the other hand, can produce as much as 1,500 hp or more. Those same two minuscule blades just aren’t going to get the job done. So the Mustangs propeller sports twice as many, longer, wider, adjustable pitch blades in an effort to get as much of that Merlin power transferred into the air as possible.
It may surprise many to know that the minimum number of blades required for a propeller to produce useable thrust is one. Few of us have ever seen such a contraption, but they do indeed exist.
They were developed by the Everel Propeller Corporation and commercially available in the 1930s. Yet, they never caught on in large numbers. But they’re out there, whirling around on the nose of some small displacement engine and proving the point that one can do it.
On the other end of the spectrum are the eight, eight-bladed, contra-rotating, constant speed propellers of the AN-22, each one measuring in at nearly 20 feet in diameter. Complex? Yes. But how else could one put as much as 15,000 shaft horsepower into the air?
The J-3’s two fixed pitch blades of roughly six feet in length just aren’t going to do it.
The Wright brothers hand carved their two wooden propellers. They did a good job of it. Their work was surprisingly efficient and practical considering the state of the industry and the technology of their day.
Today we benefit from more than a century of experimentation in aeronautical propulsion. The simple two-bladed fixed pitch propeller is still a common sight at airports across the land. But there are new ideas on the horizon that might just change our perception of that all-important fixture pulling or pushing our aircraft forward.
Over the years we’ve seen some peculiar shapes show up. From elliptical and tapered tips, to curved trailing edges, and big squared off paddles. And let’s not forget Q-tips and scimitar blades. There are so many options you could write a book about them. In fact, people have done just that.
The floating gas bag gave rise to the airplane. The airplane led to experimentation and the eventual development of the helicopter. New, mission specific aircraft have been in development since the Montgolfier brothers first sent a collection of livestock skyward in a hot air balloon in the late 1700s. All of it struck the public as fascinating, a bit scary, but largely useless — until they realized it was in fact absolutely life altering.
The pace of change continues. Today, drones are making headway into our daily lives, which has led to even more unique propeller design ideas coming to the fore. Yes, I speak of the toroidal propeller.
A peculiar looking sci-fi inspired device with blades that loop back on themselves to rejoin at the hub. These propellers can be roughly as efficient in terms of lift as a traditional two-bladed design. Their benefits aren’t found in the realm of thrust, however. It is in their ability to produce thrust very, very quietly.
Might aircraft of the future be quiet, efficient, beautiful, and practical? Yes, I believe they will be, even more than they are today. But I will always have a soft spot in my heart for the Constellation and the J-3.
The classics have their place even as we set our sights on the moon and Mars. Efficiency isn’t everything.
