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Say you had a runway for light aircraft, but only 700 meters or so of runway. Wouldn't it help to end the runway with a ramp that is ~40 meters above ground and slightly facing the sky at a gradient of 5%. Imagine the runway below was 700 meters long: a RAMP

The point is to put the plane in a position where it is off the ground and in an upwards trajectory so it doesn't have to raise it's own pitch (but still travelling at near-takeoff speed), I think this could be useful with gliders (maybe with some sort of winch to launch it).

If money were no object, what would the flaws be in this sort of contraption? Is there a reason most runways are flat? Is this sort of thing used on any aircraft carriers or other remote-takeoff locations?

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    $\begingroup$ Because of relief, some altiport have convex runway. $\endgroup$
    – Manu H
    Jul 31, 2019 at 7:49
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    $\begingroup$ If the plane does not have enough room to accelerate to speed, the plane would hit the end of the runway and immediately be in a stall condition. $\endgroup$
    – Moby Disk
    Jul 31, 2019 at 16:33
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    $\begingroup$ Defiantly not for gliders. On winch launch a glider takes of in seconds, and most of the runway is used as a place to layout the rope. On aero-tow the glider takes off before the tow plane, and suddenly change the position of one aircraft would be extremely dangerous. Self launchers do not have enough power to take advantage of a ramp, leaving the situation simply dangerous. That leaves auto-tow where a car or truck tows the glider ... well it would be entertaining, but not advantageous. $\endgroup$
    – XRF
    Jul 31, 2019 at 18:37
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    $\begingroup$ I'm on a rush to hop on a plane but please someone write technically correct answer!!! I have never seen so much technical nonsense under a Stack Exchange question! You need AIRSPEED to fly , trading speed for height is useful only in specific cases with extreme W/T ratios. $\endgroup$
    – Caterpillaraoz
    Aug 1, 2019 at 12:28
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    $\begingroup$ To underscore the point that @Caterpillaraoz makes: when you hit the ramp you slow down. The ramp forces you to trade forward speed for vertical speed. If you're not going fast enough to fly, slowing down won't help. $\endgroup$
    – Pete Becker
    Aug 1, 2019 at 13:06

9 Answers 9

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V1.

There are other valid reasons, as have been listed - cost, bidirectional runway use, etc. These would still not completely disqualify ramps, just limit them to very special circumstances. But the one disqualifying factor is that a ski jump takeoff removes the necessary element of safety airliners depend on during takeoff.

Up to a certain airspeed, called V1, an aircraft on a sufficiently sized flat runway can decide to abort. Using all available means, mostly brakes, an aircraft can halt its takeoff run and stop by the end of the runway. With a ski jump takeoff, you don't get that luxury - go off the ramp too slow and you fall off the end, nose first into the ground.

This is not a concern for carrier takeoffs. Every takeoff or landing on a carrier is a close call as it is. There's no spare runway for second thoughts. If things go wrong enough, the pilot has to pull the eject handle before he plunges into the water. In that situation, a ramp adds safety, giving a few more seconds on that ballistic trajectory.

So ski jump ramps are used on carriers that don't have much more expensive and complicated catapults. They are extremely effective there; just a 3 meter tall ramp can cut the takeoff roll in half for a fighter. But this comes at the cost of committing to it completely, do or die (which is the case on carriers anyway).

Since there's no eject handle on airliners, every takeoff has to be a safe one. This means being able to abort the takeoff if you develop a problem on the runway, such as your engines not spinning up to takeoff thrust.

The only way you could get a safe enough V1 with a ski jump is by having a runway of normal length, and then some, before the ramp, which removes the whole point of installing a ramp. You'd never reach it except on aborted takeoffs.

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    $\begingroup$ This. In summary, if you haven't reached the conditions to safely sustain flight by the time you're at the end of the runway, the last thing you want is to be lofted into the air anyway. $\endgroup$
    – a25bedc5-3d09-41b8-82fb-ea6c353d75ae
    Jul 31, 2019 at 19:21
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    $\begingroup$ ^ If I've reached the end of runway and I'm not airborne and I'm still on my takeoff run, I'm dead. $\endgroup$
    – JZYL
    Jul 31, 2019 at 20:20
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    $\begingroup$ I suppose you could build a runout past the jump, but it would defeat the "short runway" purpose. $\endgroup$
    – Joshua
    Jul 31, 2019 at 20:52
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    $\begingroup$ Imagine what'd happen with this sort of configuration if you were stuck in a reject-at-rotation scenario due to say, an elevator jam or disconnect during takeoff... $\endgroup$
    – UnrecognizedFallingObject
    Aug 1, 2019 at 3:57
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    $\begingroup$ I warmly agree with Therac's particularly clear (and exact IMO) explanations. I would add the example of the Harrier fighter, a rare example of really effective and useful STOL (most others are too costly, slow, or too light useful weight). Spectacularly effective with the ramp it most often uses on carriers or on ground. $\endgroup$
    – Michel Merlin
    Aug 1, 2019 at 13:21
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Because then you can only land in one direction.

What do you do if the wind blows exactly along the runway so that departing and landing planes have only tailwind then?

You would waste a precious runway by having such a big obstacle on it.

Moreover, an upward ramp might not be what is needed. Look for example at Lukla (or, as ManuH mentions in the comments, more generally to altiports). Here the mountains force the runway to be used in only one direction, and it is slanted so that departing aircraft will roll downhill, accelerating more thanks to gravity, and arriving planes will have to go uphill, decelerating more for the same reason, and thus both need less runway.

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    $\begingroup$ This is the correct way to use a sloping runway. A departing airplane needs horizontal velocity, not vertical velocity. $\endgroup$
    – Dave Tweed
    Aug 2, 2019 at 20:51
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It exists for some aircraft carriers and it's called a ski-jump. It wouldn't work well for land runways because they're typically used from either end depending on the wind.

Moreover, land planes don't use the whole runway, failures notwithstanding, they takeoff before the end and by regulations must clear a certain height before the end (depends on the plane category). Otherwise you risk colliding with surrounding obstacles (trees, buildings, light poles, etc.).

enter image description here
Source: wikimedia.org

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Federico
    Aug 3, 2019 at 5:34
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This is a smart but unfortunately not very good idea.

It seems quite reasonable to use a ramp at the end of a runway to help launch a plane into the air, and matches everyday experience when we are doing things with say skateboards or kites: an upward kick or movement is what they need to get them aloft.

The other answers have mentioned various reasons why this idea isn't going to work well, but there are two that I don't think I have seen.

Energy has to come from somewhere

Your ramp lifts the plane into the air, which means increasing its potential energy (height). However, that energy isn't free, and it has to come from somewhere. That somewhere is velocity: at the same time your ramp sends the plane into the sky, it reduces its forward speed.

The plane's wing needs that speed to get into the air; if it needed less speed in the first place, it would be flying by that point anyway. Your plane might be in the air, but it will have lost crucial speed and will most likely fall out of the air as a result.

It's a good thing to stay on the ground until you can fly

Your proposal aims to help the plane get into the air, sooner than it would otherwise. However, the reason a plane that's taking off is not yet flying is that it can't yet fly (or fly safely, anyway).

You cannot fly at "near-take-off speed"; gravity will claim you. You have to be at actual take-off speed.

If it's not able to fly, without help, the last thing it needs is to be in the air.

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    $\begingroup$ Energy has to come from somewhere, true. But it can be utilized better or worse. If you're hard-limited to only having X feet of runway (where X is just enough to take off), and at the end of it is certain death (cliff, concrete wall, sea), a ramp is better than no ramp. The problem is that it's still not safe enough. So in practice you'll have Y (Y>X) feet of runway instead, where a ramp isn't needed, and then there's no reason to have one, because the only way you're reaching it is in an aborted takeoff or landing overrun. $\endgroup$
    – Therac - Peace for Palestine
    Jul 31, 2019 at 20:14
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    $\begingroup$ The energy argument isn't completely sound. Energy is not actually conserved in a plane takeoff: in particular at the moment of liftoff, the high angle of attack causes lots of drag losses. Basically, it's a quite inefficient conversion of forward- into upward-kinetic energy, whereas for a ramp that's pretty efficient (close to 100% if you can lock out the suspension dampers). So, the ramp would allow more of the engines' thrust to actually be converted into the kind of energy you need, and less of it into useless air turbulence. $\endgroup$
    – leftaroundabout
    Aug 1, 2019 at 11:30
  • $\begingroup$ Even at rotation, you might have sufficient airspeed to fly, but you might not have airspeed to climb well. I don't know about large aircraft, but that's certainly the case in the little one I fly, where rotation (as per the checklist) begins around 50 km/h IAS (rotation speed target is 70 km/h IAS, which is like one or two seconds later...) but best climb is about 100 km/h IAS. With full flaps (which is more than the normal takeoff flaps setting) and a light load, I believe it's possible to maintain forward flight in that one around 50 km/h IAS, but you'll be right at the edge of a stall. $\endgroup$
    – user
    Aug 1, 2019 at 12:15
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    $\begingroup$ If all of this answer was true, why are ski-jumps so useful on aircraft carriers? $\endgroup$
    – MikeB
    Aug 1, 2019 at 15:14
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    $\begingroup$ @rackandboneman If an aeroplane has enough forward speed relative to the air around it, it will fly. If it does not, it will not fly. If it's on the ground and doesn't have enough airspeed, it will stay there. If it's in the air, it will fall out of it. $\endgroup$
    – Daniele Procida
    Aug 2, 2019 at 23:20
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Nice question. Here are a few reasons and explanations.

  1. As mentioned, it would only allow for take-offs in one direction, and if you put slopes on both ends, it would use up a lot of runway, because landing aircraft would have to overfly the vertical cliff of the ramp on the approach ends. Very dangerous.

  2. One of the purposes of these ramps on aircraft carriers is to prevent launching the aircraft directly into the water. Aircraft carriers pitch up and down especially in rough seas. When an aircraft is being launched it takes a couple seconds and it may not be possible to pefectly time it so that the bow (front) of the ship is pointing up to the sky. If the seas are rough (as anyone with sailing experience can attest) there are moments where the bow is pointing at the surface of the ocean, even if its for just a few seconds, if that is the point when the aircraft is launched it could impact the water. So a slope helps in that scenario. Larger aircraft carriers that are more stable, and have a higher deck above water level can get away without a slope.

  3. Aircraft carriers are not just floating runways, they are catapults! So, aircraft are pushed up the slope and launched into the air. Putting a slope on a runway, means that the aircraft's engine has to use up even more power climbing the uphill slope, which may likely slow it down even more, and the increased pitched means its pitching up and possibly slowing down which is a recipe for a stall. What a small aircraft on a very short runway needs is more speed not more pitch. Because we don't have catapults, a ramp is like taking off up hill, which will eat up some of our airspeed when the aircraft climbs the ramp. If the aircraft has a surplus of speed such that it would be above stall speed by the time it finished climbing the ramp, then it would work, but in that case, the aircraft had surplus speed, and could have just taken off normally before getting to the ramp.

Hope this helps.

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One other consideration: this paper contains experimental results on the use of ramps, and one thing it makes clear is that the length of the run-up to the ramp, for any given airplane, is highly dependent on the gross weight. In a conventional take-off, you can start at the beginning of the runway and rotate when ready, but with a ramp, you have to start at pretty much exactly the right distance from it, which is a function of weight (and also wind speed, and you don't have the guaranteed ~30 kt baseline headwind of an aircraft carrier.) Any miscalculation or malfunction, and you are heading for a crash, especially as, as @Daniele Procida points out, the purpose of the ramp is to get you airborne before you are going fast enough to fly.

Note that starting your run long is not really a viable option (as well as at least partially invalidating the reason for using a ramp), unless you carefully control the speed at which you run onto the ramp. In the studies, the undercarriage loads went up to 90% of the maximum allowed (which also suggests that a ramp takeoff feels like a hard landing.)

This may be moot however: as @jamesqf points out, the question concerns light aircraft specifically. This study did not use any light aircraft, but it did show that ramps were more effective for aircraft with higher thrust-to-weight ratios, giving some quantitative evidence for the proposition that ramps would be of little use to light, low-powered aircraft. This seems quite plausible: a low-powered airplane would have to leave the ramp near to its out-of-ground-effect flying speed in order to continue flying, as a) on account of of its low power, it does not accelerate rapidly, and b) because this is happening at a low speed, the jump off the ramp does not give it much time to reach flying speed.

So using a ramp for light aircraft would be risky in several ways, require a greater level of skills than a typical light-aircraft pilot has trained for, and would be of little benefit anyway.

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One reason would be the simple cost of such a ramp. Either you have to build a structure (and maintain it), or you have to move a lot of dirt and compact it - and then keep it from washing away in the rain.

That said, I have flown at more than one airport where the shape of the land dictates something like a ramp. (The Spanish Springs airport springs to mind, but there are others.) You don't really gain much, since *(unless you're some kind of daredevil) you need extra runway for landing & possible go-arounds.

If you happen to be flying a hang glider, however, you can take this ramp launch thing to a logical extreme, and launch from Glacier Point in Yosemite: https://www.yosemitehg.org/

PS: On re-reading the question, I see that I completely misunderstood the OP's idea. I assumed a launch from the top of the ramp going down, but the OP wants the plane to climb the ramp. With that design, and barring REALLY powerful engines, the plane just gets to the top and falls off the other side.

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  • $\begingroup$ Whatever the engine power, you are ignoring the fact that while on a runway the angle of attack of the aircraft is fixed until rotation, but when it is airborne it is not. Ignoring safety margins for simplicity, a plane can fly straight and level at 1 knot above its stall speed, but it can't take off at the same low speed because the AOA is smaller. $\endgroup$
    – alephzero
    Jul 31, 2019 at 17:47
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    $\begingroup$ I disagree. If an airplane can fly straight and level at one knot above stall it will be able to get airborne because of ground effect. Once out of ground effect it just won't be able to sustain a climb. This fact gets people in trouble... $\endgroup$
    – Michael Hall
    Jul 31, 2019 at 18:34
  • $\begingroup$ Now that Yosemite launch looks like fun! Yet another reason to work towards an H4. $\endgroup$
    – bclarkreston
    Jul 31, 2019 at 18:40
  • $\begingroup$ @Michael Hall: Yes, and I have actually done this. If you take off eastbound on the main runway of Stead airport (near Reno), the ground drops sharply to a (usually dry) lake a couple of hundred feet below. Doing a check ride in a tired old 150, on a hot day, with CFI who must have weighed upwards of 250 lbs (and I'm not exactly small). Lifted off in ground effect, went over the end of the bluff, and had to dive to pick up speed... $\endgroup$
    – jamesqf
    Aug 1, 2019 at 17:24
  • $\begingroup$ Anything you might want to build at an airport costs money, yet airports have all kinds of infrastructure and building any of these things is a major civil engineering project. Cost alone isn't an argument: you have to compare cost versus benefit. $\endgroup$
    – David Richerby
    Aug 2, 2019 at 12:12
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A ramp can help you leverage height and so can lead to increased efficiency so long as you don't miscalculate your lift during takeoff and your landing pattern is amenable to the airstrip's geometry, but this comes at a significant cost of lost safety. This is for the same reasons that early flight experiments involving cliffs or jumps or embankments were also a bad idea. While a ramp does indeed transform some of your forward motion into altitude, altitude is not the same thing as lift. A machine that is not ready to fly from an elevation of zero feet is also not ready to fly from an elevation of 100 feet; the increased elevation instead can be converted into potentially lethal potential energy if the machine does not generate sufficient lift after it has been jettisoned. The case of aircraft carriers using ramps as in @ymb1's answer is an interesting case of economy. There is often not a safe way to stall or maneuver out of a failed takeoff from an aircraft carrier owing to the shortness of the airstrip, so it is make or break. A longer, flat runway is much better for most flights for safety reasons because you are not forced to rely on the aircraft's lift until after it has already proven itself sufficient for flight. A premature lift-and-stall or bouncing could also turn the ramp unintentionally into a very dangerous obstacle.

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From a purely physics/ballistic trajectory point of view, a ramp 40m tall with a 5% gradient will do very little to get you airborne. Suppose you leave the ramp at near-takeoff speed of 100km/h, at a 5% gradient. If you continue on a purely ballistic trajectory, you'll reach level flight only 4m after leaving the ramp. If you had just built the runway just 0.5% longer, you'd be in the same position, with the same pitch, at the same speed, only at 40m lower altitude than with the ramp. This ignores lift generated from the angle of attack, but also the energy needed to climb the ramp, but it illustrates the point.

Building a ramp is far more expensive, impractical, and user-unfriendly than just building a slightly longer runway - an extra 4m of pavement is far preferable to a 40m tall ramp.

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    $\begingroup$ It's actually very effective for a short runway as you find on carriers. A tiny ramp, just 3 meters tall, shortened the takeoff run of an F-18 by as much as 50%. DOD study. This works by using the kinetic energy in a far more efficient way - splitting it between potential energy, vertical and forward momentum, rather than just forward momentum. E=mv^2/2, so that forward momentum is more expensive than it is effective. $\endgroup$
    – Therac - Peace for Palestine
    Jul 31, 2019 at 19:57
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    $\begingroup$ @Therac Good point, ramps are more practical with a steeper angle and a shorter runway. That study tests a 9 degree launch angle, which would give triple the horizontal ballistic distance from the OP's 2 degree angle, and that does become a significant proportion of the runway distance for a runway that's only ~30m long! $\endgroup$
    – Nuclear Hoagie
    Jul 31, 2019 at 20:07
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    $\begingroup$ @Therac True, but let's not forget the F-18 has a thrust-to-weight ratio of almost 1; I suspect that makes the ramp a lot more reasonable than for an airliner with TTW around 0.2. Not to mention the effect on the passengers :) $\endgroup$
    – Luaan
    Aug 1, 2019 at 7:48
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    $\begingroup$ @Luaan You are correct about ramps being more effective for high thrust/weight airplanes - some figures can be found here: apps.dtic.mil/dtic/tr/fulltext/u2/a237265.pdf $\endgroup$
    – sdenham
    Aug 1, 2019 at 13:29
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    $\begingroup$ 100km/h isn't anywhere near take-off speed except for very small aircraft. A typical commercial jet will take off at 2.5 times that, or even more. And what's the relevance of the ballistic trajectory? If ballistics were even close to being the relevant model, the plane wouldn't be able to get up the ramp in the first place. $\endgroup$
    – David Richerby
    Aug 2, 2019 at 12:08

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