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I was writing an anno for the "Blow-
assisted take-off" idea, but it became way
too long, and such a different concept that
I just decided to put it into it's own idea.
Here it is:
The boeing 747 has 16 main wheels in
which to take off and land. Why not use
these wheels? Install in every
wheel an
electric motors, like the ones coming out
that put out over 100 horsepower per
wheel. Use this power to propel the plane
to sufficient speeds that they don't have to
put their engines on full throttle to get
airborne.
Powering these monsters is another thing.
It would be to much, in my opinion, to
store batteries on board so a different
system is what I suggest. An aircraft
carrier catapult-type system is perfect for
the job. It does not apply much pulling
force, only enough to safely stay ahead of
the airplane. What it does do is supply the
electricity needed to those electric wheels.
The advantages of this setup are:
- Less noise at takeoff as less power is
coming from the jet engines.
- Possibly shorter runways, depending on
howmuch additional power these powered
wheels can provide.
- Saves wheels, if you can give store
enough power on board to get them
rotating as fast as the ground is moving
before touchdown, you can save them
from the brutal treatment they receive
whenever landing.
Disadvantages are:
- Hard landings could quite possibly and
often destroy in-wheel electric motors and
it's probably not a simple thing to change
a wheel on an airplane.
- electricity-providing-catapult would run
the length of the runway and cost a lot.
It's a very large capital investment, as well
as airplane retrofit.
I believe the pros outway the cons though,
IMO.
Not for take-off, but getting closer to your idea
http://www.earthtim...elease,173780.shtml
"<snip> is designed to allow airplanes to taxi without using their main engines, saving expensive jet fuel and reducing harmful emissions." [Klaatu, Sep 11 2007]
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Could be a runner. I would foresee some difficult practicalities that might make it hard to implement. Since this is the half-bakery though, the bun is yours! |
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There are two things that rule this out, I think. The extra weight of motors capable of contributing significantly to accelerating the mass of a plane to take-off speeds would be considerable. Firstly, this has a bad effect on fuel consumption throughout the flight. Secondly, this weight is in precisely the worst place, being part of the unsprung weight. |
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On the other hand, you could use the motors to spin the wheels up before landing, reducing tyre wear and the risk of blowouts. You wouldn't need such big motors for this purpose, but they still probably wouldn't pay for the fuel cost of the extra weight. |
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Why not use an electric winch to actually pull the plane along, as on aircraft carriers? Yes, that would be an expensive winch, but not as expensive as electric motors on every plane - and its weight wouldn't need to be carried with every plane. |
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I am worried about a full-strength catapult
putting pressure on the frame of the
airplane. I don't know how strong they
make them. Maybe you can have the
wheels jettison after take-off, taking care
of the weight problem, but then there's the
whole landing thing. |
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//I am worried about a full-strength
catapult putting pressure on the frame
of the airplane.// If you want a given
acceleration, it doesn't really matter
how you apply it, as long as you
transmit the force intelligently. |
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A long gentle winch/catapult would be
the best solution. |
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Or convert the plane into a giant railgun
projectile. |
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Like the railgun idea. Run a big one up the side of Mt. Kenya and shoot a 777 into low earth orbit. |
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Strap down the drinks trolleys first though.
Or could it be arranged so that the trolleys
experienced the same accelerative force as
the airframe? It would be quite nice to see
128 small packets of peanuts shoot
towards the back of the aircraft, whilst the
trolley itself remained perfectly poised in
mid-aisle. |
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//Run a big one up the side of Mt. Kenya//
Actually, that is not such a stupid idea, el.
Just build all runways on 30 degree slopes.
Downhill for take-off, uphill for landings.
Win win. |
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Or how about one of those magnetic-induction systems they use now to shoot off roller coaster trains without the use of hills with chain drives? Fun. I'm trying to imagine the announcement that'd precede such a takeoff: "Ladies and gentlemen, we're just about to be abruptly flung 20 miles into the air. Please be sure to secure all loose objects that you want to keep and that you don't want to become part of someone's head." |
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First as someone already mentioned, weight. Airlines consider even the mass of the PAINT on the airplanes. A hundred horsepower motor can easily weigh more than a thousand points. Adding sixteen thousand pounds to an aircraft is not going to make them happy. |
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Second is structure. Landing gear is designed to withstand thrust in the braking direction. Withstanding thrust in both directions would require redesign, and surely even more weight. |
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Third there is a small flaw in your reasoning. Airliners usually do not use 100% thrust during takeoff roll. Sometimes they are at 80% or less, depending on the length of the runway, total load, wind profile, etc. Full thrust is usually engaged for a short time just after take off to get the aircraft up to a safe altitude as quickly as possible. |
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//depending on the length of the
runway// If the runways were extended
sufficiently, the need for takeoff could be
completely eliminated, at least for
domestic flights. |
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I'm told by experts in the field that it costs an airline operator, on average, about $600/year per pound of added weight. |
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My rough calculations say that if you use electric motors on the wheels to accelerate your plane, these motors, generators, associated wiring and structural reinforcement would weigh almost as much as the rest of the plane. |
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Even assuming that you're only providing a fraction of the total thrust through the wheels, this added weight very quickly negates any potential benefits. |
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Spinning the tires prior to touchdown wouldn't save much on tire wear. The overwhelming majority of wear occurs due to scrubbing in turns. |
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There have been studies into putting a small electric motor in the nosewheel so that a plane can perform pushback and general ground maneuvering without a tug, but even that small amount added more weight than the airlines were willing to carry around. |
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Not a bad idea, just unworkable in the real world. Sorry, [-]. |
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Weight on the airplane is bad, and electric motors are HEAVY, so the idea isn't a good one. |
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A catapult could provide any rate of acceleration desired, including a gentle push over two miles, which would be less force than most airplanes experience. |
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In particular, a catapult could provide the same pull, at the same point of application, as the engines provide - so obviously the plane's structurally able to take it. This would reduce fuel used on the plane, and consequently reduce the initial mass (or extend the range). But only VERY slightly. |
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Unsprung weight? On a plane? Seriously? |
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[GutPunchLullabies] Of course. All the bottom end of the landing gear is unsprung weight - wheels, axles, brakes, the lot. And these motors, if you instal them down there. Alternatively, the transmissions would be horrible (and heavy) if you installed the motors as sprung weight at the top end of the gear. |
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Yeah, but how much difference could this possibly make towards fuel efficiency? It won't make any difference the 99% of the time the plane is ariborne. |
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Unsprung weight isn't a fuel efficiency issue, it's an airframe stress and passenger comfort issue. |
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Any weight, sprung or unsprung, is a fuel efficiency issue. |
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It's a lovely idea, if only for the sheer impracticality and cost of the issue. |
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(If the runways were extended sufficiently, the need for takeoff could be completely eliminated, at least for domestic flights.) And another bun for [Maxwell Buchanan]'s anno. |
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This is way safer and easier to do than the blow-assisted take off (which I mercilessly shot down), so you get a bun. |
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You would only need these motors for take-off so there is no reason for them to go with you, nuff said. |
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So, the wheels would drop off the plane and be recovered by the ground crew for the next launch? Perfect! |
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The main engines on something like a 787 provide tens of thousands of horsepower. A few 100hp electric motors are not going to add anything of significance except weight. |
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