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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?

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"Breaking" is the most likely result, yes. :) –  egid Feb 24 at 18:49
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I was thinking of airports like Berlin-Tempelhof, which is in the middle of the city and therefore small, but which has circular taxi-ways on the perimeter around the runways. One reason this airport was closed was that the runways didn't support large aircraft. –  Sebastian Henckel Feb 24 at 20:15
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Circular runways are called helipads but they work better for helicopters. –  p1l0t Feb 24 at 20:15
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Circular Runways: Because if there is one thing I want during my landing roll is to turn away from the headwind –  Radu094 Feb 24 at 21:07
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The short runway problem has already been solved: en.wikipedia.org/wiki/Aircraft_arresting_system. Brace! –  Igby Largeman Feb 26 at 4:57
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10 Answers

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...)

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What if the runway was slanted? :P –  anarcat Feb 25 at 5:02
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The logic seems reasonable, but is it actually true? That is, if a vehicle in general can make an unbanked turn with radius r, then it can necessarily come to a halt in distance r? –  Steve Jessop Feb 25 at 9:04
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@SteveJessop: There are some differences being swept under the carpet here because a turning force does no work whereas a braking force has to dissipate some waste heat. But it looks like the limiting factor in actual fact is not energy dissipation but the available friction between the landing gear and the runway anyway, so yes, it ought to be true. –  Henning Makholm Feb 25 at 9:35
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Also, the question assumes that braking and turning is done by the same force, but isn't an airplane better at turning than stopping because it can use the tail rudder which it wouldn't be able to use for braking? –  Superbest Feb 26 at 13:01
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@Superbest at the low speeds involved, the rudder is not very effective at turning, so you'd have to rely on differential braking and thrust instead (talking airliners here, not your C172). –  jwenting Feb 26 at 13:18
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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 flair, 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, its 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 over correct at low altitude and airspeed, the chance of a catastrophic crash is unacceptably high.

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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.

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This would also be disastrous with anything less than great braking action. It would also destroy our ability to do reduced vis (300 or 600 RVR) takeoffs. Where would the approach lights go? Would there be an extension like a running track? –  casey Feb 24 at 18:47
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Since there are actual projects for things like this I guess it is possible at least in principle:

http://www.endlessrunway-project.eu/

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.

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yes, this thing would have a runway be a very large circle, with a radius large enough that you can always run along it (nearly) straight and thus always have optimal wind. In reality of course that'd be so large as to be impractical. But it's essentially possible using a circular platform that's 10x10km in size, not dissimilar to the meadows used for WW1 and early WW2 airfields where aircraft could take off and land on a roughly circular patch of grass around the periphery of which was constructed the airfield. –  jwenting Feb 26 at 13:21
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They envisage a circular runway 3000 m in diameter. By my back-of-the-envelope calculations, the lateral acceleration necessary for following that circle at a takeoff speed of 140 knots would correspond to a bank angle of about 20 degrees. Getting airborne will be exciting ... –  Henning Makholm Feb 27 at 12:21
    
How does one calculate/estimate the bank angle? –  Jens Schauder Feb 27 at 12:55
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@Jens: It's arctan(speed²/radius/g) –  Henning Makholm Feb 28 at 11:38
    
Not to mention that, at a 3,000 m diameter and 140 kt takeoff speed, the wind would be changing direction relative to your aircraft very quickly. Where there really becomes a problem is as you're turning through the area of the runway where you have a headwind. As you turn into that area, your airspeed increases, potentially to the point that you leave the ground. As you turn out of that area, you're airspeed drops again, potentially stalling the wings. Bad news. –  reirab Mar 28 at 15:40
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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.

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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.

Another way to solve the same problem would be to use arresting gear and catapults like they do on aircraft carriers (which have the same problem - too expensive to build a big enough runway).

I'd pay extra to land and take off on such a runway, but I may be in the minority :-)

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A banked runway sounds like a recipe for grounded wingtips... –  David Richerby Feb 25 at 23:06
    
@DavidRicherby - not if you're very very careful :-) –  Dan Pichelman Feb 26 at 0:39
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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.

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Yeah, most larger cities have tall enough buildings that you don't want the airport too close to the city center, anyway. –  reirab Mar 29 at 15:15
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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

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The landing area might have been circular or oval shaped, but the actual landings and takeoffs were straight. –  BeowulfNode42 Feb 25 at 13:36
    
... yes, I am aware. Thats the rate of turn restriction I mention. As explained in the quote, the advantage of the circle is not to shorten the field but to give the pilot the ability to always land and takeoff into the wind. –  woody121 Feb 25 at 19:17
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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.

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I can't tell if this is a serious answer or not...technically it might work, but it's just so ridiculous it sounds sarcastic –  SSumner Feb 25 at 21:05
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Aircraft fly due to lift created from air moving over the wings, not because the ground moves quickly under the landing gear. Even if there was a rotating disc under the air craft, there'd be no lift created. –  CJBS Feb 26 at 7:39
    
@CJBS, You're correct, of course, however, I assumed he meant that the aircraft would be stationary on the runway and let the runway accelerate it up to takeoff speed. Technically, that could work, but it would require far, far more energy input than just letting the aircraft take off under its own power (not to mention the obvious problems with trying not to get thrown off the side of the runway, changing wind direction, etc.) –  reirab Mar 29 at 15:22
    
@reirab - having a set of stationary fans blowing huge amounts of air at the craft is potentially more practical than having a spinning disc... –  CJBS Mar 30 at 4:41
    
@CJBS, Well, that wouldn't really help anything, as it wouldn't actually accelerate the aircraft forwards. You could get some air flowing over the wings while it's behind the fans, but that's not going to help you once the aircraft rises out of the stream of air being blown at it by the fans. It would then immediately stall and plummet back down into the ground. Of course, the important thing to note here is that neither solution is practical. - lol –  reirab Mar 30 at 4:58
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Different point of views can be derived of such a suggestion:

  1. 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.
  2. 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.

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A treadmill wouldn't help. If you landed without using wheel brakes, it would obviously take the same distance (relative to a stationary point on the ground) to stop, whether the runway was moving or not. So the only way you could stop in a shorter distance would be if you could dissipate more kinetic energy per second in the wheel brakes, i.e., by having more powerful brakes and grippier tyres. But then you could use those brakes and tyres just as well to get a shorter stopping distance on a static runway! –  David Richerby Feb 25 at 23:01
    
Sure, without wheel brakes it takes all the same distance. But was thinking about using wheel braking + treadmill for a decreased need for landing space on an airport. –  LRT Feb 26 at 7:58
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The version without wheel braking was a thought experiment to set things up. With wheel braking, a treadmill wouldn't help because the plane would need more powerful brakes and stronger, grippier tyres or it would just skid against the moving pavement. To avoid that, you would need better brakes and tyres than existing planes have but, if you had that, you could stop shorter on existing runways, without needing the expense of the treadmill. –  David Richerby Feb 26 at 8:35
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