For those of us who are air-minded, I think we could all agree that lift, and the devices that create this still mysterious force, are truly remarkable. Without the modern development that led to aircraft and the lift they create, today’s aviators would be like the many generations who came before-- dreaming of being in the air, but left standing on the ground. Thankfully, though, we are surrounded by innumerable inventions which harness the power of moving air. From airplanes and helicopters to windmills, compressors, vacuums, and desk fans, we humans are quite well equipped.
Hidden at the core of each of these devices is a curious little shape known as an airfoil. These little 2-D profiles are rarely seen as they are the inner cross-section figures of the wings, propellers, and blades we rely upon to power our planes and and invigorate our vacuums. You may wonder what is so special about these streamlined teardrop-like shapes. Some think these shapes are integral to the existence of lift. However, as one of my college professors said, “Even a barn door will fly if you bolt a big enough engine to it.” The answer lies in efficiency. Aerodynamicists have spent over a century investigating which airfoils are best for any given situation. Which are best for slow speed flight and which are best for high speed flight? Which are best when the wings are dirty; which are best when wings are clean? Which are best for the stubby wings of the Space Shuttle and which are best for a skinny winged sailplane? Which are best in the confined housing of a jet engine, and which are best at the tip of a spinning windmill on the plains of Kansas?
Sadly, nobody has definitively answered any of these questions. Nature is far too complex and elusive to give us a confident 100% rating on what shape is best for any particular airplane or situation. The curvature of the bird’s wing gave the curious their first clue as to how to shape a wing. However, from there we have determined where some shapes do better than others. If you took the airfoil shape of a Piper Cub and bent an F-16’s wings to match, it would never be able to break the sound barrier and outspeed its enemies. If you took the airfoil shape of an intercontinental passenger liner and tried to force it upon a Short Take Off and Landing (STOL) airplane it would never lift a fisherman and his gear off a sandbar marooned out in the middle of a desolate river.
As has been the theme of this series, however, we must ask ourselves how flight would be different with one of the four fundamental forces “turned off” and the other three left to fend for themselves. I can’t even begin to imagine what would be different about air’s behavior if it couldn’t partner with a wing to create lift. However, let’s just assume for the sake of this case that we humans had never stumbled across the idea of airfoils. Would the possibility of flight be completely skunked? To answer this question, we have to go back to the basic physics of what an airfoil accomplishes.
On the first day of one of my last classes in college, Jet and Rocket Propulsion, our professor made a bold claim--in the first five minutes of class, he would teach us all the theory we would need to succeed in his class. “Picture this,” he said. “You’re floating in a rowboat in the middle of a perfectly calm lake. You don’t have any paddles, and you need to get to shore. However, you do have a 50 pound box of rocks aboard. The concept of equal and opposite reaction from school comes floating back and you decide to throw a rock astern. You notice your boat develops a slight forward drift so you keep chucking rocks to the rear. Before long you’ve built up and can keep up enough speed to get to the dock.” He then explained how this story is analogous to how a rocket engine works. It takes fuel and oxygen already on board, burns it, and kicks it out the tailpipe in the opposite direction the rocket needs to go.
Since our class was about both rockets and jet engines, he went on to extend the analogy. “Imagine now you're in the same boat but without any rocks aboard. You still need to get to the dock but this time your rocks are on the dock. You convince your friend to toss you a rock from the pile so that you can use the same trick as the last time. However, you notice that when you catch the rock, the boat drifts slightly backwards, farther from the dock. You turn around and throw the rock back even faster than it was going when you caught it. Your progress is much slower this time. Your friend decides to mess with you a bit and starts chucking the rocks to you harder and harder. His throwing arm is just as good as yours and as long as he keeps this up you won't be making any progress at all.” This analogy connected with how airbreathing jets and propellers work. They take air coming right at the airplane and chuck it backwards harder than it was received.
Through this roundabout analogy, we’ve arrived at the crux of what an airfoil does. It deflects and influences air to travel in the opposite direction that lift is needed, thus providing the required reaction. Hopefully, you can now see that when speaking of wings, propellers, and blades, the idea of lift and thrust are interchangeable. However, for our purpose of imagining flight without lift, hopefully you can see we still have options. Even if airfoils don’t work, rocket engines are still available to us. Another viable candidate for a lifting force is buoyancy, the same kind of buoyancy used by blimps, party balloons, and boat keels.
So, with enough imagination, I think we can see a path back to powered human flight. A rocket powered airship would be quite the sight to behold but since both of these technologies exist in their own right, the coupling of them is not too far fetched. In fact, if you go back in history before the Second World War the bright optimism of the day held airships, helicopters, and airplanes in equal esteem. No one was sure which technology would win out.
Sadly, the explosion of the Hindenburg airship in May 1937, and the proverbial explosion of airplane technology in the early days of World War Two, ensconced our world with the domination of airplanes as we know them today. Without the need to trap lifting gasses inside a bag or expel rocket fuel with fury, the wing had found its place in the subduing of air. Alternative forms of air travel have still continued on and have found their own niche markets but the airplane, with the force of ever outstretched wings, will continue lifting us into the future.
