Catapult Assisted Airplane Take-off

[Grey_Wolf_c]

So, I was just reading about current state of the industry for electric planes, and one of the things I learnt is that the take-off is proportionally the most energy expensive segment (one of those facts that becomes obvious in retrospect, I suppose), and in current airplanes, forces them to make wings bigger, which impacts the rest of the flight.

So, I was wondering, how feasible would it be to off-load some of the energy requirements of take off by assisting it with a catapult? And, if so, what kind of design would work? I am vaguely aware that the major limitation of such a system is that a catapult would accelerate the plane much quicker than humans would like, or would require a very long runway, but is there any other issues?

Grey Wolf

[halfeye]

It's been done, but it barely gets you to flying speed so you need full power anyway:

https://en.wikipedia.org/wiki/Aircraft_catapult

[Grey_Wolf_c]

It's been done, but it barely gets you to flying speed so you need full power anyway:

https://en.wikipedia.org/wiki/Aircraft_catapult

I'm aware of that type, but it's purpose is not to save energy for the plane, but to allow it to take off at all from a very short runway. I've skimmed the article, and I don't see it mention using it on land - only on ships (if I missed it, please do let me know). The Electromagnetic variant, however, could probably be repurposed to be used on commercial aircraft?

Grey Wolf

[halfeye]

I'm aware of that type, but it's purpose is not to save energy for the plane, but to allow it to take off at all from a very short runway. I've skimmed the article, and I don't see it mention using it on land - only on ships (if I missed it, please do let me know). The Electromagnetic variant, however, could probably be repurposed to be used on commercial aircraft?

Grey Wolf

It's mainly a military system, which means they don't have to worry too much about user complaints.

Planes have a lot of retractable stuff, which is not in use in the main when in steady flight. Saving on that complexity would be good, but it's also used when landing, even if the power use at that time is often much less (if a landing is aborted, all the power of takeoff is needed to get up to flying speed again). If you made the plane to fly at no less than 300mph (for a semi random example), then you'd need much longer runways to deal with landing at that speed, or arrester wires and an aircraft structure strong enough to withstand the extra strain, which I suspect would be heavy enough to negate any gains made on the takeoff.

Apparently airlines get complaints if passengers are subjected to accelerations equivalent to more than 1.2g.

[LordEntrails]

For commercial aircraft ...

Well, yes full engine power is used for take off. It's also used for climb because the faster you get to cruise altitude, the less fuel/energy overall you use, even if you use a bunch during climb. As for wings, they are optimized for cruise. Need low drag. Wing geometry is changed for take-off and landing to provide higher lift at lower speeds and lower stall speeds.

So, sure, you could use a catapult to launch an electric plane. But electric planes/engines are not yet able to replace combustion engines during cruise for commercial aircraft, so ... what's the use case?

As for the catapult itself, steam was the traditional method, but the newest carriers use electro-magnetic. And at one time their was at least one US Navy airfield with a catapult system for training. Their are videos of a catapult launched plane racing a top fuel dragster, and winning.

[Fat Rooster]

Something like this?

The alternative might be vehicle launched. Put your electric plane on an electric car, and use that to get you up to speed. It would require some extra run off to slow down again, but otherwise could use conventional runways.

[factotum]

I can see one very big problem right to start with. The stated objective of this launch system is to make the plane's wings smaller. Let's assume you succeed in doing that. What happens when you come to land? Smaller wings means less low-speed lift, which means your landing speed goes up, requiring longer runways and better tyres and brakes on the plane. The sort of net and tail-hook arrester arrangement they use on aircraft carriers might help that, but they'd have to be a LOT stronger to stop a 200+ ton airliner than a 10 ton fighter aircraft--and you have the excessive acceleration issues again, only in reverse.

[Fat Rooster]

I can see one very big problem right to start with. The stated objective of this launch system is to make the plane's wings smaller. Let's assume you succeed in doing that. What happens when you come to land? Smaller wings means less low-speed lift, which means your landing speed goes up, requiring longer runways and better tyres and brakes on the plane. The sort of net and tail-hook arrester arrangement they use on aircraft carriers might help that, but they'd have to be a LOT stronger to stop a 200+ ton airliner than a 10 ton fighter aircraft--and you have the excessive acceleration issues again, only in reverse.

I believe the problem was more to do with electric aircraft wanting to cruise at full power, leaving very little reserve power for accelerating on your take-off run. Getting to any respectable speed would require a long runway, with the only answers being very low take off speeds or more power than you need. Watching a Ryanair aircraft climb at 45" gives you an idea how much power a typical jet has in reserve.

[wumpus]

More issues:

Take-off/landing is from anywhere from sea level to commonly 1-4km (1 mile in the US, possibly 2 miles at Leadville, CO), while level flight can be up to 10km (30,000ft). Expect a lot more power to be available thanks to all that air (this might work against you with jet engines tuned for altitude).

Flaps already allow much smaller wings. Have you taken them into account in your "plane designed for a catapult"? As mentioned above, while takeoff may require more power, landing has the same lift requirements as take-off.

[Yora]

Something like this?

The alternative might be vehicle launched. Put your electric plane on an electric car, and use that to get you up to speed. It would require some extra run off to slow down again, but otherwise could use conventional runways.

I just got the idea of somehow supplying power for the takeoff from the ground and cutting the connection when the wheels lift off the runway. Assuming the engineering problems are taken care of, would this significantly improve the battery power for the flight?

[gomipile]

The sort of net and tail-hook arrester arrangement they use on aircraft carriers might help that, but they'd have to be a LOT stronger to stop a 200+ ton airliner than a 10 ton fighter aircraft--and you have the excessive acceleration issues again, only in reverse.

Another problem is that arresting cables are dangerous. At least with an aircraft carrier everyone on board knows they signed up for being in a potentially dangerous environment. But if a cable at an airport snaps, whips in an unexpected direction and takes out a fence and a few cars on a road just past the end of the runway.....

And lots of runways are inside cities now. If they weren't at the beginning, often the city expanded and closed in around them over the years.

[tomandtish]

Another problem is that arresting cables are dangerous. At least with an aircraft carrier everyone on board knows they signed up for being in a potentially dangerous environment. But if a cable at an airport snaps, whips in an unexpected direction and takes out a fence and a few cars on a road just past the end of the runway.....

And lots of runways are inside cities now. If they weren't at the beginning, often the city expanded and closed in around them over the years.

Not to mention redesigning passenger plane seats to handle a harness restraint system that can handle passengers of all sizes. And even then you are probably going to have someone who gets hurt when their head whiplashes.

[DavidSh]

If you aren't doing extremely short runways, but just using a catapult because the plane is underpowered, you don't need arresting cables, as long as you willing to forego the possibility of a go-round if the landing is bad. After all, dead-stick landings (with no power at all) are possible.

I think it is interesting to note that the Wright brothers used catapult launches on some of their early flights, giving Europeans an excuse to claim that those flights didn't count.

[GloatingSwine]

I'm aware of that type, but it's purpose is not to save energy for the plane, but to allow it to take off at all from a very short runway. I've skimmed the article, and I don't see it mention using it on land - only on ships (if I missed it, please do let me know). The Electromagnetic variant, however, could probably be repurposed to be used on commercial aircraft?

Grey Wolf

EMALS could be repurposed to rip commercial aircraft in half...

Catapult launch puts a lot more stress on an airframe than normal takeoff, and planes have to be specifically designed and built to account for it.

EMALS puts even more stress on than steam catapults, overstressing even planes designed to be launched by catapult.

It's also expensive and failure prone because it's new.

So catapult takeoff for land based commercial aircraft is probably a non-starter. It would drastically increase maintainance cost and reduce airframe life.

[Grey_Wolf_c]

EMALS could be repurposed to rip commercial aircraft in half...

Catapult launch puts a lot more stress on an airframe than normal takeoff, and planes have to be specifically designed and built to account for it.

EMALS puts even more stress on than steam catapults, overstressing even planes designed to be launched by catapult.

It's also expensive and failure prone because it's new.

So catapult takeoff for land based commercial aircraft is probably a non-starter. It would drastically increase maintainance cost and reduce airframe life.

All of which I'm sure its true, but I suspect it is in large part because of the length of takeoff. The EMALS (which I hope is the US military catapult) is trying to solve a problem different from the one I'm asking about. Since what I'm wondering is if this approach could be used to reduce fuel needs of commercial planes by off-loading the energy costs of take-off to land-based sources, rather than allow them to take off in less distance, I don't think that the adapted version for commercial airlines would need to put as much stress on the plane.

I never wanted to suggest we use the jet-fighter catapult on commercial planes, in other words, I understand that the needs of one and the other are very different.

Grey Wolf

[Sean Mirrsen]

Several thoughts on the matter.

1. Wireless power. How much power would an airfield be able to deliver through a typical wireless power setup (induction coils? microwave beamed power? tesla towers? :P), given a typical aircraft's wingspan? Yes, horrendous setup costs, but in principle? Assuming it works with a little bit of altitude to spare (i.e. at least some ten meters of range, generously assuming a low-wing design of the aircraft) it could supply both takeoff and go-around power. There's also the possible issue of frying every bit of consumer electronics onboard, but I don't know how probable an issue it would be.

2. In an EM catapult scenario, what if the aircraft itself is the moving part, in some form? I.e. the runway itself is a series of alternating electromagnet strips, and the aircraft is propelled along with that, rather than being towed by its landing gear. In this setup the airfield itself could also slow down the aircraft in a controllable manner, removing the need for heavy brake systems, and reducing the danger of high-speed landings with a smaller wingspan.

3. None of these really address the actual problem with introducing such systems, and that is that with any infrastructure at all being needed at both takeoff and landing, these aircraft would have a very limited number of possible departure and destination points, and could not divert to somewhere else if the situation makes a landing at the destination impossible, at least until the system becomes more widespread.

4. Re-reading the first post, it appears the question never concerned actual electric planes. Derp.

5. For conventional aircraft, it may be a better idea to redesign the airframe to a resemble more of a box-wing. Sort of a modern take on a biplane I suppose. Less wingspan for same lift power.

6. As for using assist systems to reduce fuel expenditure on takeoff, I'd assume that the aircraft in question would be a standard aircraft, and thus able to land normally at the destination, and also take off normally anywhere regardless of infrastructure present (just with more fuel used). In that case, I suppose a takeoff assist system could indeed reduce the fuel used for takeoff, but I have doubts as to whether it would be an overall sound investment. It'd involve a large amount of engineering work to rebuild runways, as well as design changes to existing aircraft to reinforce and adapt their landing gear to the booster system, all for fairly nebulous gains.

[LordEntrails]

Several thoughts on the matter.

1. Wireless power. ... There's also the possible issue of frying every bit of consumer electronics onboard, but I don't know how probable an issue it would be.

Don't forget frying the occupants of the planes themselves.

2. In an EM catapult scenario, what if the aircraft itself is the moving part, in some form?

And assume none of the occupants have pace makers or electronics with them? Or build a Faraday cage into the plane and not worry about the extra weight?

Their is a reason that #1 & 2 are not on the drawing boards or concepts by any engineers or engineering companies.

5. For conventional aircraft, it may be a better idea to redesign the airframe to a resemble more of a box-wing. Sort of a modern take on a biplane I suppose. Less wingspan for same lift power.

Not quite the way aerodynamics work. And of course completely ignores the aerodynamics for cruise flight. There is a reason the only modern bi planes are stunt/competition planes, and they don't use a straight wing either.

[Fat Rooster]

All of which I'm sure its true, but I suspect it is in large part because of the length of takeoff. The EMALS (which I hope is the US military catapult) is trying to solve a problem different from the one I'm asking about. Since what I'm wondering is if this approach could be used to reduce fuel needs of commercial planes by off-loading the energy costs of take-off to land-based sources, rather than allow them to take off in less distance, I don't think that the adapted version for commercial airlines would need to put as much stress on the plane.

I never wanted to suggest we use the jet-fighter catapult on commercial planes, in other words, I understand that the needs of one and the other are very different.

Grey Wolf

Yeah, that thing doesn't work well. That's what happens when you get a new contractor that has underbid the last one and promised their tech is magic. They run into the same problems that made the last one so expensive, and have to find new solutions.

Electric aircraft are limited by power, but turbofans are not in the same way. At the altitudes the airframes want to cruise at the turbofans aren't providing nearly as much thrust as they are capable of at lower levels. They are not very efficient at low speeds, but take-off thrust is not exactly low, and has good margins. We have gotten really good at multi-spool turbofans that can handle take-off without compromising cruise. The compromises in order to make a jet take off are less than required to let it land, despite the fact that it lands considerably lighter (an electric aircraft would not). The gains for a modern jet would be minimal, as you need the runway to land on anyway (with emergency margins).

The scale of catapults is what will dominate the cost, and low accelerations mean large size. It is also a point of failure that can both kill, and have huge economic consequences. Basically, it cannot be allowed to fail, so has to be shut down if it looks like it might. If it is shut down, billions of £ of aircraft are stranded. That means horrifically expensive.

[Ravens_cry]

Well, given all the new gear and safety equipment that would be needed on the aircraft, my guess is you'd end up losing out, even if there was a savings on fuel for take off in comparison, as all that extra stuff would be dead-weight between take off and landing.

[Sean Mirrsen]

Not quite the way aerodynamics work. And of course completely ignores the aerodynamics for cruise flight. There is a reason the only modern bi planes are stunt/competition planes, and they don't use a straight wing either.

I didn't mean a literal 'box' wing, more a design of wing where the entire lifting surface assembly is one continuous loop. I tried (and I know I'm about to say the most damning thing here) this sort of design in KSP, with the main wing and the tail connected together into one surface. I'm not a great aircraft designer (even in KSP), so my attempts weren't terribly successful, but given what I read about closed-wing advantages it might be something to look into.

fake edit: looking around for pictures, I found some people who apparently built my KSP design IRL:


Sure it's a tiny seaplane rather than a big passenger liner, but I suppose the concept at least works.

[Vinyadan]

There are catapults for drones, although I don't know the exact reason:

[Lord Torath]

There are catapults for drones, although I don't know the exact reason:

Probably to save energy on lift-off, which extends the range of the drone.

Drones don't have on-board human pilots, and thus can be accelerated more quickly. They're relatively small, so the Square-Cube law doesn't hit them as hard as it does passenger jets, allowing a sturdier airframe.

[LordEntrails]

I didn't mean a literal 'box' wing, more a design of wing where the entire lifting surface assembly is one continuous loop. ...
Sure it's a tiny seaplane rather than a big passenger liner, but I suppose the concept at least works.

Ah yes. I've seen similar in college design competitions and such. I suspect such a design choice is still an anomaly because of issues. I would expect these to include;
- Manufacturability
- Fuel storage (remember, win
- Cost to provide long-term fatigue and behavior studies. If you want to put a design feature into a commercial airplane, you have to prove to the FAA it's safe, and that it will be safe tomorrow, and that you know how it will fail, and you know how to inspect and repair it.

[Son of A Lich!]

Another problem is that arresting cables are dangerous. At least with an aircraft carrier everyone on board knows they signed up for being in a potentially dangerous environment. But if a cable at an airport snaps, whips in an unexpected direction and takes out a fence and a few cars on a road just past the end of the runway.....

And lots of runways are inside cities now. If they weren't at the beginning, often the city expanded and closed in around them over the years.

This was my thought immediately - I was a corpsmen on an aircraft carrier, and we never had an incident with the arresting cables (We did lose an Eagle eye and three pilots, but that was technical failure when it was in flight), but Airmen get drilled all the time on proper safety for working on the flight deck.

Similar issues for Bos'nmates, but for docking lines. Synthetic Lines stretch and can snap like a rubber band.

I think it would probably be better for a psuedo-lift to "Drop" the plane from where the air resistance is less of an issue, but I don't know how 'Commercial' of an idea that is. I dunno, just spitballing.

Damnit I'm a Doctor, not an aerospace engineer!

[khadgar567]

I think it's cheaper to invest ion engines then trying to find a way to create catapult for commercial planes. as the main problem is the fuel and thrust to weight ratio. discovering better fuels and lighter engines means less cost on each flight rather than each time gambling with a plane full of human lives. as catapult like that drone begs the unique question of intercepting rockets with planes as it becomes kinda crazy enough that Ian Fleming might want to write a novel for it and let's remember the catapult needs lots of calculations to make sure each throw is perfect which means it needs a computer to assist to do it. and hello security risk as hackers can access the system and launch planes to each other in flight with perfect collision course your company now has a perfect tool for terrorist attacks.

[wumpus]

I think it's cheaper to invest ion engines then trying to find a way to create catapult for commercial planes.

I don't think anybody has come remotely close to developing an ion engine that can be used to escape Earth's gravity. They are amazing for going from Earth Orbit to just about anywhere (possibly leaving propellant in orbit so people can go just about anywhere while the propellant s l o w l y gets there) but don't compete at all with catapults and air-launch.

[factotum]

I suspect that "ion" there was a typo for "in"...

[khadgar567]

I suspect that "ion" there was a typo for "in"...

nope no typo as investing better engines ion or regular means you can shorten the road need to get into top speed and have longer flights possible due to better fuel economy which op wants to archive via catapult and one of the risks of catapult is needs to complex tech to and a lot of crew to work which can be used to research better fuels and engine designs that can archive same result without needing to double the ground-based personal and inviting disaster each attempt. call me pragmatic money grubber but I prefer current tech to experimental catapult for planes. hence the ion engines comment of mine.
EDIT: even with the possible typo my point stands

[halfeye]

nope no typo as investing better engines ion or regular means you can shorten the road need to get into top speed and have longer flights possible due to better fuel economy which op wants to archive via catapult and one of the risks of catapult is needs to complex tech to and a lot of crew to work which can be used to research better fuels and engine designs that can archive same result without needing to double the ground-based personal and inviting disaster each attempt. call me pragmatic money grubber but I prefer current tech to experimental catapult for planes. hence the ion engines comment of mine.
EDIT: even with the possible typo my point stands

You can edit posts for typos and grammaros on this website, which is a big relief.

[Sean Mirrsen]

I don't think anybody has come remotely close to developing an ion engine that can be used to escape Earth's gravity. They are amazing for going from Earth Orbit to just about anywhere (possibly leaving propellant in orbit so people can go just about anywhere while the propellant s l o w l y gets there) but don't compete at all with catapults and air-launch.

My first thought was he meant ionocraft, ionizing air and propelling it electrically to create thrust. Nothing exists yet in terms of manned flight, but theoretically a large and efficient enough ionocraft could fly and carry cargo.