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ALL Airplanes Are Low-Speed Airplanes

By Craig Hange

May 15, 2000

I'm very fortunate that I can sit at my desk and look out the window (instead of actually doing work) and watch airliners setting up to land at San Francisco International Airport. They are easy to watch at this point in their flight because they are closer to the ground, and are flying a lot slower than they did earlier in the flight. Some of the big airliners like the 747 or 777 seem like they are just hanging there, and it sure looks like they are going too slow to remain airborne.

Of the different flight regimes airplanes fly in, the one they all have in common is take-off and landing. Not all airplanes go fast, and not all go high, but they all at some point have to go slow and come down to the ground. No matter how long an airplane can stay in the air, they all have to start and end their day on the Tarmac at zero speed. So, no matter what the mission or objective an airplane has, low-speed characteristics will always need to be addressed.

Of course, low-speed is a relative term. A small general aviation airplane whose maximum speed is less than 100 mph doesn't come close to the "low-speed" of 160 mph that a jet airliner has during take-off. At NASA, we generally consider low-speed to be between zero and 300 mph, so relative to other types of vehicles such as race cars, our low-speed can still be very fast. However for aircraft, low-speed flight is characterized by the need to generate a lot more lift than is needed during the faster portions of the flight. And, aerodynamically speaking, that can be a big challenge.

The amount of lift an airplane generates needs to be equal to the weight of the airplane. There are two ways to get a given size wing to generate more lift. One is to use more air by flying faster, and the other is to deflect the air going over the wings through a greater angle. At high-speed the first choice is easy to do, because the airplane is moving fast enough, the wings can push a lot of air through a small deflection and generate a lot of lift. This amount of lift is more than enough to balance the weight of the airplane. At low-speed though, there is less air flowing over the wings. Now, to generate enough lift, the airflow will need to be deflected much more. This is accomplished by increasing angle of attack or deploying flaps and slats on the wings. Usually a pilot does a little of both. If you have ever sat behind the wing next to the window in an airliner you have seen these flaps and slats deploy just before the take-off and landing. On some airplanes these are so cleverly concealed that the wing looks like it's coming apart when these pieces start to move. By deploying these flaps and slats, the wing now has more camber, or curvature. By increasing angle of attack and using flaps and slats the wing can generate more lift through the greater deflection of the airflow. This makes up for the loss in lift caused by the aircraft going slower. On a typical jet airliner, the landing and take-off speed is one quarter to one third of the cruising speed. On supersonic airplanes, the ratio is even bigger.

This seems easy enough, so where's the challenge? Well, low-speed flight is characterized by flying a lot closer to what's called the stall. A stall occurs when the airflow over the wing separates from the wing and the wing no longer generates any lift. This separation occurs for a variety of reasons, but the primary causes are due to flying too slow, and flying at too high of an angle of attack. You may ask, "Aren't these also the conditions an airplane is flying at during take-off and landing?" Yes! That is exactly the problem. Also add in the possibility of a wind gust upsetting the airplane when it's close to the ground and you have a very challenging flight regime indeed. It's no wonder you have to wear your seat belt during take-off and landing.

This is where the airplane designer has to make some choices. They have to make a trade off between low-speed handling and high-speed requirements. The airplane will be profitable only if it can fly at fast speeds with minimum fuel usage. This means minimizing drag and this in turn means using small wings with minimum camber generating minimal lift. However, the airplane must take-off and land, so it has to fly at low-speed during a portion of its flight. That means it needs wings with flaps and slats that generate a lot of lift, and it must also be stable at these speeds and be responsive so the pilot can correct for problems such as the wind gust. The designer has to come up with an airplane that can do both, and has to be good at both.

At NASA, there are people and organizations to help with such problems. Many of the NASA engineers specialize in low-speed aerodynamics. We understand the problems and requirements for flying an airplane during take-off and landing. We understand how to fly close to the stall, and what needs to be done to keep the airplane from going beyond stall. We can help design good flap and slat systems that help during low-speed, but fold out of the way for high-speed. We also conduct tests in the low-speed wind tunnels to investigate low-speed flight. The largest wind tunnel in the world, the 80 ft X 120 ft Wind Tunnel at NASA Ames is a low-speed tunnel. In addition to testing helicopters, we also test STOL airplanes in it. STOL stands for Short Take-Off and Landing. These are airplanes that can fly so slow that they can take-off and land in less than 1000 ft. Some even use jet thrust to make up for the loss in lift from the wings. Someday these airplanes will be taking off from much smaller airports that are more convienient to your home.

In our modern lives of high-speed computing and travel into space, it's easy to get excited about flying higher and faster. It may not seem glamorous or interesting to study low-speed flight. There are however, plenty of challenges that need to be addressed in the regime of low-speed flight, and these challenges will still be around when you are ready to enter the workforce. And don't forget, not all airplanes are high-speed airplanes, but ALL of them are low-speed airplanes.

 
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