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Assisted takeoff is one of several technologies that can possibly boost efficiency of aircraft, as said:

Listen to the changing sound of engines during flight and it’s obvious: an aircraft draws on its power reserves more during takeoff than at any other time. The power needed to take off is determined based on a number of factors - including runway length, wind speed, temperature, and the weight of the aircraft itself.

However, this takeoff power only is required for a very brief portion of the total flight. Once cruising in the sky overhead, an aircraft doesn’t need as much to maintain altitude. So why not source the energy required at takeoff from an innovation installed on the ground? Can the burden (and weight) be removed from the aircraft itself?

An assisted takeoff – using some form of propelled acceleration – would mean aircraft could be lighter, with smaller engines consuming less fuel. …
(source: http://www.airbus.com/innovation/future-by-airbus/smarter-skies/aircraft-take-off-in-continuous-eco-climb/)

future Airbus
(Original image source: http://www.airbus.com/fileadmin/processed/csm_1b._Aircraft_take-off_in_continuous_%E2%80%98eco-climb%E2%80%99_e7b9d930ac.jpg)

Nevertheless, in a later part of the article, they suggested using normal landing gear or similar cart also for landing. However, I started to wonder, because such cart can be somewhat hard to land on. Technically speaking some cases a different place was being considered as much more forgiving:

Apollo 15 landing
(Original image source may be http://www.spacefacts.de/graph/drawing/drawings2/apollo-15_landing.jpg)

So I want to combine those two ideas. Would an aircraft that uses assisted takeoff and water landing be realistic? (I am especially asking whether water landing would provide some weight savings, instead of carrying whole landing gear. By realistic, I ask whether it would provide any saving in weight in comparison to landing on ground.) I assume that aircraft would have engines high enough so their contact with water would not be an issue. I also assume that hauling this aircraft or getting compatible infrastructure is not a problem.

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    $\begingroup$ Would a valid answer be "water landing does the exact opposite of weight savings?" Water landing gear has to be a lot more rugged and has to take up a lot more space. $\endgroup$ – Cort Ammon Apr 14 '17 at 16:09
  • $\begingroup$ @ Cort Ammon If explained a bit in to details - unfortunately yes. $\endgroup$ – Shadow1024 Apr 14 '17 at 17:05
  • $\begingroup$ It's funny how you use an Apollo CM landing to illustrate your post. In my book, Apollo take-offs definitely qualify as assisted. However, parachute landings come with a whole host of problems and difficulties. (Let's just say there's a reason why few spacecraft these days soar through the air suspended in parachutes during final landing approach.) And of course, while it's easy to forget, hitting water at any appreciable velocity results in some serious forces. People were terrified about Apollo 13's parachutes because even if the CM survived reentry, without parachutes the crew was doomed. $\endgroup$ – a CVn Apr 14 '17 at 17:47
  • $\begingroup$ Water landing would seem to present problems if you want to fly to Denver, Phoenix, Las Vegas, or many other places where there are no large bodies of water handy. You might also consider the desirability of having power reserves for reasons other than takeoffs. $\endgroup$ – jamesqf Apr 14 '17 at 17:49
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Water landing is going to introduce a number of problems and complications. You can do it two ways with an aircraft: via pontoons like a float plane or by a specially designed hull like a flying boat. Float pontoons are going to introduce a massive amount of drag (and lots of associated fuel use) because they won't be 100% retractable no matter what. A flying boat type hull will also introduce some aerodynamic compromises because of the need to shape the hull to handle hydrodynamic loads at landing speeds and be stable on the water. In addition to this, the hull will have to be made stronger, and you are going to add costs in terms of issues like corrosion protection, dealing with water ingestion in engines, etc.

Generally, it would not save anything to use water landing with this concept... EXCEPT that there is a potential use for this.

There is a known aerodynamic effect of flying at low altitude over the ocean. Air is denser, lift is increased, and an aircraft needs to expend less fuel and requires less aerodynamic surfaces to stay in the air. This "ground effect" has been exploited with some interesting designs like the Russian Ekranoplan project. Boeing had a concept a few years back to build a massive cargo hauling flying boat designed to take advantage of "ground effect" to radically lower the cost of airborne cargo.

If you are using ground effect for most of your trip, then the costs of a "flying boat" hull in terms of weather and corrosion proofing, structure, and aerodynamic compromises becomes well worth it. Such an aircraft could definitely take advantage of a ground-based "assisted takeoff" technology of one type or another. If these were combined; the cost of aerial transport could be RADICALLY reduced.

One warning: it is always very complicated to engineer something to be able to operate in two different environments (air and water) AND ocean going aircraft have always been very much subject to bad weather at sea causing major problems for them.

Ground Effect Vehicles

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Assisted takeoff: definitely not realistic.

An large airliner uses the highest amount of thrust during these scenarios:

  1. Takeoff
  2. Climbing to cruise altitude
  3. Go-around (i.e. abort a landing)

With a reduced engine rating:

  1. The aircraft may not have enough power remaining to maintain altitude (let alone climb) in the event of an engine failure during takeoff
  2. On long range flights, the aircraft is forced to stay at a low altitude due to insufficient engine power to climb higher, where fuel efficiency suffers
  3. The pilots may be unable to abort a landing in an urgent situation, e.g. another aircraft accidentally enters the runway

Water landing: possible, but a worse choice than landing on land

The size of pontoons is larger than wheels. Therefore even if it is fully retractable, it would take up more room. Moreover, aircraft experiences a stronger deceleration when touching down on water body. Therefore the airframe would need to be reinforced.


The "ground-effect on water" idea is plausible though, as it solves all the above problems:

  1. In the event of an engine failure, you're just a few feet above the water.
  2. There is no need to climb to 30,000 feet for cruise.
  3. There is no landing, so no need to go-around either.

You would end up with an large floating "aircraft" with a cargo capacity more than a typical air freighter but less than a container ship, with a groundspeed between the two. Therefore you can target the market where air transportation is too costly but ship transportation is too slow.

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  • $\begingroup$ I think the point of the Airbus article is not necessarily that the aircraft would have a reduced engine rating, but that the aircraft would not have to use as much fuel, because it would not be going to full power at takeoff. Full power would still be available in case of emergency. $\endgroup$ – JBiggs Apr 14 '17 at 19:27
  • $\begingroup$ @JBiggs the fuel burnt for accelerating down the runway, is, quite minimal. For typical values, the fuel flow of an engine is 12,000 lbs / hr, and it takes 45 seconds to accelerate to takeoff speed. That saves 150 pounds of fuel. $\endgroup$ – kevin Apr 14 '17 at 19:34
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Yes and Yes

Background

The atmosphere changes properties with altitude. This change in properties is called the lapse rate.

Atmosphere Lapse Rate
Atmosphere Lapse Rate

Different types of turbine engines perform better at different altitude regimes. Turbofan engines get better fuel efficiency than similar power rated turbojets.

This change in engine performance with altitudes (confusingly) also called a lapse rate:

Engine Lapse Rate
Engine Lapse Rate This complicated chart shows decreased engine thrust as altitude increases.

Because the relative engine performance changes with altitude, aircraft with different engine types must be designed to different specifications.

For aircraft with turbojets (no fan, only compressor), the design constraint is the engine thrust needed during take-off. A turbojet engine designed to be sufficient for the aircraft take-off has excess thrust available when flying at cruising altitude.

For aircraft with high bypass turbofan engines (most of the air is not combusted), the design constraint is the engine thrust needed at high altitude cruise. A turbofan engine designed for this design constraint has plenty of excess power at low altitudes and take-off.

Almost all current airline aircraft now use high bypass ratio turbofan engines. Pilots very often use much more power from these engines than is strictly required for takeoff so they can clear the crowded and dangerous airspace around the airport.

So I suspect that using assisted take off system would not improve efficiency by very much.

Assisted Takeoff

However, you didn't ask whether the idea was better than the current system, you asked whether it was realistic.

Yes, it is realistic. The USAF experimented with JATO (Jet Assisted Take Off) & RATO (Rocket Assisted Take Off) for aircraft for many years. The original idea was to get US bombers away from their airbases before Soviet Nuclear weapons destroyed the base and the slow bombers. The USAF also experimented with them for other uses too but they were never widely used.

This is an example of this technology in current use to aid cargo craft trying to take off from Antarctica. C-130 taking off from Antarctica with the assistance of a JATO package

Water Landing

During WWII aircraft able to land at sea were used widely across the world by all of the belligerents. The ability to perform water landings was extremely useful for flying into and out of regions with few or no aircraft facilities. The aircraft just needed some fuel and an open stretch of water to be operational.

In the current era, most regions of the world have sufficient aviation infrastructure to not need this capability. However, you can still see float planes in use in wilderness areas such as Siberia and Alaska which do not have widespread aviation infrastructure.

Yes, building and using an aircraft capable of water landing is realistic.

Alaskan Float Plane still in use

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Yes, it was already used a great deal.

The system is probably not very practical for large airliners. However, in the interwar period large naval vessels desperately needed aircrafts for recon/fire control purposes. But helicopters were jet to be invented, and the decks were occupied by the great cannons.

So they used catapults to launch small floatplanes, which landed on water, and were recovered by cranes:

https://en.wikipedia.org/wiki/Aircraft_catapult#/media/File:HMS_Bermuda_aircraft.jpg

Many cruisers and battleships used this method, including legendary units like Yamato, Missouri or Bismarck.

But the savings manifested on the ship (more place for weapons) and not on the planes, whose performance was usually inferior to carrier and ground based aircraft.

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  • Assisted take-off: I don't know. It depends on how much fuel is saved by having the plane be accelerated to take-off speeds by an external device instead of using the plane's engines. The engines cannot be made less powerful than those of a normal plane, because you still want the plane to be able to remain airborne if one engine fails immediately after take-off.

  • Water landing: A flying boat is quite a bit heavier than a normal aircraft, and the boat-shaped hull induces quite a bit more drag, making flight less efficient and requiring more fuel.

    The great advantage of flying boats was that they did not require airports; this is why they dominated transoceanic air transport between the two world wars. Once suitable airports were built all over the world, the inherent disadvantages of flying boats made them obsolete for most use cases.

    There are some flying boats still in service for special uses; for example the Bombardier 415 is used for fighting forest fires; its boat-shaped hull enables it to land of a lake, scoop 6 tonnes of water in a very short time, then fly over the fire and dump the water.

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In addition to what was said, water landing requires a large, accesible body of water. This limits your aircraft's use to such places (a bit), as having both normal landing gear, and a boat hull is somewhat complicated and prone to problems in an event of water landing (if the gear systems intersect water hull, you can get leaks due to stress/fractures).

Also, water landing requires facilities / additional craft for easy boarding on/off the aircraft, and especially in any emergencies, access to the aircraft is limited (so. i.e. engine fire cannot be tended to as fast if you stop at the middle of the lake, compared to the runway service firemen rolling next to you / right after, the moment you touch the ground)

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