At a minimum, an airplane needs lift, thrust, stability, and control. Learn how they work together. This is a text, video, and audio post!
I’m trying something new this time. There are three versions of this presentation. First, a 25 minute audio-only version is part of this week’s Crash Cast. A link is at the bottom. A ten minute video is at the bottom, too. Finally, a shorter text summary follows. Which version do you prefer? Let me know!
What makes an airplane?
- lift – without it, it is not an airplane!
- thrust – gliders use gravity as thrust
- stability – three axes
- control – a big deal in the early days, not so much anymore
I won’t say much about control. Nowadays it is not a hard problem to solve.
The Wright brothers believed that an airplane should be hard to control. I am not kidding! It would keep the pilot focused on what he was doing. This is part of the reason why their company lost the huge advantage it had in the airplane industry.
Not much is said about the Wright Flyer engine. It was not the most sophisticated of its day, but it got the job done. I would argue that the airplane could not have been invented ten years earlier, simply because the internal combustion engine technology was too new.
Propulsion was a key problem that had to be solved in order to have a working airplane. The Wright brothers, in typical style, had an engine built that simple and innovative at the same time. It was the first aluminum airplane engine ever.
Airplane engines are all about horsepower to weight ratios. The Wright engine weighed 170 pounds. A modern electric motor with the same HP rating weighs just 5.5 pounds (2.5 kg). Batteries are approaching the weight of a gas tank.
Lift is a surprisingly complex phenomenon. The key is forcing the air to speed up as it goes around the front of the wing. You end up with a higher air pressure right in front of the wing. But the air has to speed up to get around the nose of the airfoil. That leads to lower air pressure above the wing. Viscosity keeps the air flowing around the wing to the back. This is called the Coanda effect.
The air pressure has to come back to normal at the trailing edge of the wing. That means that the air pressure first goes down then comes back up over the top of the wing. It normally starts increasing about 25% of the way back, and this is where separation and other bad things start to happen. In fact, in a model airplane, it is safe to assume that separation has occurred past the 25% chord point.
Dihedral is not too hard to understand. The wing tips are raised. When one wing gets low, its projected horizontal area increases. More area leads to more lift, which pushes the wing back up.
Two other effects contribute to roll stability. Low center of gravity helps because of the pendulum effect.
Wing sweep helps because roll and yaw are tied together in airplanes. This is a surprisingly complex mechanism. I will not try to explain it here.
The size of the vertical stabilizer is proportional to the wing span and the wing area. That is why gliders have relatively large vertical surfaces.
Sizing the vertical stabilizer is surprisingly similar to sizing the horizontal stabilizer. Instead of the center of lift, like we do for pitch stability, use the center of lateral area (CLA). Measure the distance between the CLA and the center of gravity. This is an indicator of how much lateral stability the airplane has.
The hardest to understand. Watch my earlier video where I just talk about this!
The size of the horizontal stabilizer is proportional to the wing area and the wing chord. Airplanes with short stubby wings need bigger elevators.