It’s a scenario that’s not at all useful in the real world, but the theoretical debate rages on. In this situation, picture an airplane – the type is irrelevant – on a gigantic treadmill. The treadmill spools up its belt and the airplane powers up its engine(s), travelling in the opposite direction of the belt, like you would imagine a person walking on a treadmill at the gym. So the question that’s so heavily debated: can the airplane take off?
I’m amused to read people’s perspectives on this topic as the conversation gets more and more technical, with formulas involving friction and all sorts of factors being thrown into the fray. As a pilot who must constantly consider the forces at work in this scenario, I find it much simpler of an answer than many try to make it. To get there, though, we need to explore a few concepts.
What Makes An Airplane Move
Imagine you are at an airshow. An airplane and a race car are lined up next to each other on the runway. A signalman fires a shot and the race is on. The car floors the gas and the engine provides power, ultimately to the wheels, turning them faster and moving it along. The airplane also blasts full throttle, spinning up its propeller. Both are accelerating, but there is a key difference in the mechanics of the acceleration of the airplane. While the car’s power is torquing the wheels to provide the acceleration, the airplane’s propeller is creating thrust through the air, pulling the airplane along. The wheels aren’t serving to accelerate the airplane – they are just rolling along for the ride, faster and faster as the airplane’s propeller accelerates it through the air.
What Makes An Airplane Fly
Now that we understand what creates the movement of the airplane, let’s dig into what makes it fly. Why does it take off? Well, to oversimplify it, we know that an airplane has to get to get going fast enough to take off. Basic experience on an airliner tells you that happens. Think a little deeper with me, though. What has to be moving fast for the airplane to fly – the wheels? No – it’s the wings. With enough air flowing over the wings, the airplane can generate lift and fly.
Let’s Invite Wind To The Party
So how does this relate to our treadmill? Well, in an airplane, the speed at which it’s traveling over the ground is irrelevant to flight. On a day with no wind, an airplane traveling a certain speed over the ground on takeoff is also traveling the same speed through the air. But consider that when there is wind of any kind, airplanes try to take off into the wind. Here’s why. Imagine you are in an airplane, on a runway, waiting to take off. There is a 10 knot headwind coming at the nose of the airplane. In this case, there is already 10 knots of air moving over the wings. Powering up and taking off will lead to a shorter takeoff roll because of the headwind. That means a shorter distance rolling on the ground, speeding up for takeoff. The opposite is true. If there is a tailwind, the airplane will have to roll further on takeoff to achieve the same airspeed over the wings. Feel free to dig deeper into this concept in this video exploration of wind using X-Plane.
To illustrate this concept in action, let’s take a look at an airplane taking off and landing in a strong headwind. There is almost no ground roll needed because the wind is flowing fast enough over the wings to create lift. If you look at the wind sock in the background and try to picture the wind, you realize that the airplane is still traveling at a decent airspeed through the air, but in relation to the ground, it’s barely moving.
Again, what is important is the airflow over the wings, not the speed of the wheels. The wheels just roll along – they don’t provide any power or speed.
Let’s look at a slightly different approach to understand the big concept here. Sometimes, an airplane that has floats needs to get airborne from the ground. Maybe they had some work done at a shop and were brought in on a trailer. They can take back off from the trailer, as long as that trailer is moving fast enough to give the airplane enough air over its wings. Watch it in action:
You might have made your own assessment about the treadmill situation by now, but let’s put our examples on the treadmill and apply what we know.
If you put the floatplane on the trailer onto a treadmill, the movement is being driven by the wheels of the truck. If the truck was driving at 60 miles per hour and the treadmill was matching it’s speed, the whole truck/trailer/airplane would appear stationary on the treadmill – there would be no airflow on the wings unless the truck drove faster than the treadmill. If it sped up to 90 miles per hour, the truck would drive off the front of our theoretical treadmill but the airplane would have 30 miles per hour of wind over its wings.
So back to the original scenario. Our airplane is on the treadmill and throttles up. The treadmill fires up but the airplane generates it’s thrust from the propeller. As the propeller begins to pull the airplane through the air, it begins to move forward relative to the treadmill. Remember, it’s the propeller “gripping” the air that is providing movement. The wheels are just rolling along with the airplane. Mind you, the wheels are spinning very fast – the speed of the treadmill PLUS the movement of the airplane as it moves forward on the treadmill. Now the only important factors are how long the treadmill is, and if there is any sort of wind. Let’s assume no wind for simplicity and say that this airplane usually requires 500 feet of runway to take off and lifts off at 60 knots. The airplane will roll ahead, accelerating much like it was on a stationary runway, and it’s wheels would be spinning along at breakneck speed. If our treadmill is short, the plane will run off the front of it. Let’s hope it has one of those emergency belt stop tethers clipped to the wing. If, hypothetically, the treadmill was 500 feet long, the airplane would accelerate to the very front, achieve 60 knots and take off.
So does that mean an airplane can take off from a treadmill? The classic scenario seems to allude to an airplane matching the speed of the treadmill and just lifting off, seemingly without moving forward. We now know that wouldn’t happen unless there was enough headwind match the takeoff speed like our video from above.
The bottom line is, as it relates to an airplane taking off on a treadmill, the belt moving underneath really only affects the speed that the wheels spin, which doesn’t have any relation to how or when the airplane takes off. That’s all about the airspeed.
Said another way, you couldn’t use a treadmill to get an airplane to take off from a stationary position in lieu of using a normal runway. Sorry, gigantic treadmill entrepreneurs.