Considering that building runways is expensive and that they take up a lot of space, could you land a large commercial airliner on a circular runway, braking as you go around the curves?
As a matter of physics, there's a certain amount of braking and steering force available to the aircraft after it lands.
Suppose the aircraft touches down moving due north, and continues clockwise around your circular runway. After a quarter-circle it travels due east -- but that means that enough braking force has been applied to reduce the northward component of its velocity to zero, while also giving it some eastward velocity.
If only that eastward force hadn't been applied, the aircraft would be at a halt now -- using less force than what's necessary for it to follow the circle.
So if the plane can keep itself on a circular runway, it will be even easier for it to come to a halt on a straight runway whose length is the radius of the circle.
(Also, the plan ignores that the critical factor in the length of runways is not landing but taking off -- there must be enough runway to brake down safely if the takeoff is aborted just before rotation. And taking off from a curved runway sounds even crazier than landing. If the runway is horizontal, the plane would become airborne in the middle of an uncoordinated turn...)
One major complication would be the shift in relative wind as you work your way around this curved runway.
On approach, the wind would be coming from one direction (ideally straight head), then as you touch down and begin your curve while slowing down, the wind shifts direction to come more from one side. So now you're trying to stay on a centerline that is curving, at somewhat high speed, while the wind moves to one side of your plane.
That is just insane.
Another major issue would be the amount of "float". Although airplanes target touching down right at the end of the runway, that doesn't always happen. Sometimes due to excess speed, gusty winds, or a misjudged flare, the plane will float a distance down the runway before actually touching down. During this time, the plane's nose may be high, and visibility of the runway is limited. Because the runway is straight, it's no problem.
But suggesting that a pilot navigate a curved runway while floating further than anticipated is just nuts. If they don't turn enough, they end up outside the curve of the runway. If they turn too much, they end up inside the curve. Either way, if they overcorrect at low altitude and airspeed, the chance of a catastrophic crash is unacceptably high.
Since there are actual projects for things like this I guess it is possible at least in principle:
From the site:
The main feature of the circular runway is that it will become possible to let an aircraft operate always at landing and take-off with headwind. Whatever its strength and direction, the Endless Runway becomes independent of the wind. When allowing limited crosswind, airspace users can shorten the global trajectory of the flights through optimized departure and arrival routes.
Apart from that they seem to hope that they can start/land more airplanes per time unit by shifting the start/landing point thus avoiding the problem that airplanes must not follow other airplanes to close along the same trajectory.
A circular runway maybe works but this circle can't be small as the turn must be really smooth, talking about just a few degrees per 1000 feet. So this runway would actually be very "long" and need a lot of space.
Let's stop thinking about that! You would probably not be able to maintain 'directional' control if the outboard engine fails (e.g. left turning runway, right engine fails). How would you set up an ILS approach? Aircraft aren't build for this. It would be dangerous or even impossible and would never be approved.
The problem with some of these proposed ideas (like a treadmill) is that they don't account for the inertia of the plane. You can't take a 500,000 lb 747 travelling at 160Mph and simply place it on a treadmill or spinning runway. There is still forward velocity to account for. Even if you could find a way to make the aircraft stop "very fast" without ripping the landing gears off (ie. the arresting gear suggested by Dan), think of the effect it would have on passengers. Consider going from 160Mph to 15Mph in only a few seconds. It's not comfortable, and for many people (think infants and elderly), it's not even safe.
In regards to the aircraft turning, commercial airplanes are quite top-heavy, at least when compared to cars. Cars can turn very fast, even at high speeds (think NASCAR), because they are low to the ground. They have huge surface area on their tires in relation to the size and weight of the vehicle. Airplanes are the opposite. Compared to their size and weight, landing gears are very small, and not very useful for much else than rolling in a straight line.
The cost of building a "circular runway" (probably with banked sides like a Velodrome) would probably be more difficult and expensive to build than a big long flat bit of concrete.
I'd pay extra to land and take off on such a runway, but I may be in the minority :-)
As a pilot, it sounds terrible. Even landing or taking off on a strip with a hump in the middle, so you cannot see the other end, dramatically increases the workload, and the degree of risk. Not to say that it cant be done, when everything goes right, but if anything fails, then you raise the risk of a crash enormously.
And at the moment, the bottleneck is not getting to the airport, its waiting around to go through all the procedures when there that takes most of the time. So its not much advantage to put an airport nearer the city anyway. A better road, catering for airport traffic only, to further away, may well cut the transit time.
Personally I would rather land an aircraft somewhere where there are as few obstacles as possible. Gives a much better chance of success when things go wrong.
As many of these other answers speak to as well, the rate of turn for aircraft on the ground is very low, so the circle would have to be far larger than practical.
However, on a very related note, early airports all had circle landing areas. From wikipedia:
The earliest aircraft takeoff and landing sites were grassy fields. The plane could approach at any angle that provided a favorable wind direction. A slight improvement was the dirt-only field, which eliminated the drag from grass. However, these only functioned well in dry conditions. Later, concrete surfaces would allow landings, rain or shine, day or night. http://en.wikipedia.org/wiki/Airport
Yes. The "long line loiter" method (see https://en.wikipedia.org/wiki/Pylon_turn ) could be used to lower a weighted line from the airliner to the ground. The line would be retrieved by ground crew and attached to a winch mounted on a turntable on top of a tower in the middle of the runway circle, tall enough for the line to clear all obstacles. The winch would be equipped with a system allowing it to apply constant tension to the line, which could be varied to equal any desired tension. As the airliner descends, the winch would start applying more and more tension to the line, so that the line would provide some of the centripetal turning force, allowing both the bank angle and the turn radius to be decreased, after the manner of a control-line model airplane. By the time the airliner reached the surface, the line would be providing all the centripetal force required for the airliner to stay over the circular track with zero bank angle. As the aircraft approaches the surface, a computerized control system linked to a GPS receiver onboard the aircraft could be used to ensure that the line tension was modulated in such a way as to keep the aircraft nearly over the centerline of the circular track, despite variations in wind speed and direction, and then at that point the winch spool could be locked to keep the line length fixed until touchdown. A similar method, but in reverse, could be used for takeoff, with the line being jettisoned (or simply released from the ground winch and reeled back up to the aircraft) after the aircraft is established in a "long line loiter" turn at the appropriate altitude and bank angle.
Different point of views can be derived of such a suggestion:
- Huge turntable-like runways which turn while the landing aircraft rolls out. Turning at the same speed as the aircraft decelerates, result in the aircraft always be facing the same wind conditions during stopping process.
- Development of the first item: a treadmill-like landing runway. At the time of touchdown the aircraft does not require any airstream along its wings. The following deceleration could be performed on a treadmill or assembly line.
But honestly, none of them would financially or even technically pay off the effort of just expanding existing runways in length.
This will work if the circular runway is rotating. The runway should be on a pedestal, and spin up to an to an angular velocity such that the tangent matches the aircrafts speed. then rise up and catch the aircraft, slowly reducing angular velocity and lowering to the tarmac.
Take off would proceed in the reverse manner. with the aircraft speeding up trying to stay on the rotating runway as it rises and increases angular velocity.
In the event of alien invasion we can use the rotating run ways to launch projectiles such as the airport busses.