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[edit: meant - lift]
One side (top?) is hot, so air nearby is expanded, while the
other side is cold so air passing nearby is dense.
If needed, perhaps the hot and cold air would be released
earlier on, somewhere ahead of the wing, or perhaps this
would work on a long but thin streamlined
wing.
Airfoil lift
http://xkcd.com/803/
If you work forward from a bad assumption, you get worse results. [lurch, Aug 11 2011]
Lowering density/pressure above airfoil
http://www.grc.nasa...irplane/wrong1.html
Please note that this is an *incorrect* theory. [lurch, Aug 11 2011]
Flow redirection
http://www.grc.nasa...irplane/right2.html
Because inquiring minds say "WTF!?", here's the actual process. (From a fairly good source, too! Cool, huh?) [lurch, Aug 11 2011]
You might be interested in the Ayak link Pashute.
External_20combustion_20aircraft_20engine
[2 fries shy of a happy meal, Aug 12 2011]
Magic flying paper
http://www.youtube....watch?v=UxYOBIzZ8C4
No magic. No bad science. [pashute, Aug 12 2011]
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This would require gargantuan amounts of heat; and the air would need to be raised to maximum temperature just after the leading edge, certainly well before reaching the balance point (typically. 1/3 of the chord). |
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Having a turbojet exhaust in the form of a long rearward-facing slot just above and behind the leading edge would be possible, although what the effect of throttling the powerplant up and down would be on the aerodynamics would be a tricky thing to quantify. |
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It could be done by positioning a curved scoop below the trailing edge, directing air into a turbine mounted in the wing but facing "backwards", with the exhaust exiting as described above. |
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It would certainly overcome potential problems with wing icing ... |
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Would this give more lift? Why? |
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Pour gasoline on cardboard.
Place cardboard on block of ice. (Not perfectly flat - allow airspace underneath.)
Ignite top surface of cardboard.
Observe closely to see if cardboard lifts into air. |
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After noting failure, apply [marked-for-deletion] bad science. |
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You had us at // Pour gasoline on cardboard. // |
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//Would this give more lift? Why?// |
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I agree - excellent question. I could equally well
say "heat the air underneath the wing, thereby
causing it to expand. This expansion will result in
an increased pressure underneath the wing, which
is lift". |
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In any event, how much energy would be needed? |
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Let's assume that the heating can be confined to
the centimetre of air next to the wing surface (I
think this is a reasonable guess), and assume also
that we need a temperature change on the order
of 20°C to have any effect. (20°C is on the order
of a 10% increase in absolute temperature, and
10% seems a sensible threshold.) |
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Assume also that the plane has a wingspan of 50m
(a typical long-distance aeroplane), and is
travelling at 200m/s (about 500mph). |
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This means that, in one second, 100 cubic metres
of air have to be heated. However, the plane is
also at an altitude of about 40,000ft, where the air
is only 20% as dense as at sea level. So, how much
energy will it take to heat 20 cubic metres of [sea-
level] air by 20°C? |
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The answer is about half a megajoule, meaning
that you need half a megawatt of power for this
heating. However, the power output of a typical
jet engine is around 50-100MW. |
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So, in fact, such heating would be perfectly
feasible, energetically. |
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However, I still don't see how heating the air over
(or under) the wing will increase lift. |
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Heating the air silghtly in front of a moving wing would have the desired effect if the wing met the expanded volume of air just as the air began to lift and contract, and if the leading edge of the wing encounterd this volume of air just below its mid-point as the air contracted. |
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There is always the question of whether the energy
required to create/re-route the heat is less than that
saved by the increased lift. I'm gonna hazard a guess and
say no. |
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Yes, but how much extra weight would this forward-edge
heating system add to the wing, and how much extra
power would it consume to produce such tremendous heat?
Let's not forget that the air that must be heated is
approaching and rushing past the wing very fast, leaving
very little time in which to heat it. |
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//However, I still don't see how heating the air over (or under) the wing will increase lift.// By decreasing the density of the air, either under or over the wing, the lift decreases. (Look up "hot and high conditions" for a full explanation.) |
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However, I note that [pashute] never mentions "lift" at all - he only refers to "elevation". Heating the air near the wing does indeed increase "density altitude" - a measure of the air's ability to interact with the airfoil. The wing will act like it's at a higher "elevation" - closer to (or perhaps beyond) its service ceiling. |
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It's just unfortunate that increased density altitude equates to reduced, not increased, lift. |
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// theres quite a lot of spare heat coming out of the jet
engines. // |
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Right, but to use it for this purpose requires special
insulated ducting to route it from the exhaust ports to the
leading edge of the wing, plus equipment to open and
close those ducts when necessary, plus special exhaust
ports built into the wing (unless you plan to bring it all the
back to where it started from), and once we've addressed
all of these practical concerns, is it still of sufficient
temperature to get that onrushing air hot enough to make
a significant improvement in lift? |
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I'm not in a position to know these answers, I'm just playing
devil's advocate. Too often (in real life) have I seen (and
sometimes had to fix) the results of somebody assuming
their idea would work without questioning its fundamental
properties. |
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That comment, [Alterother], ought to be marked for tagline. Well do I know whereof you speak. |
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Some of this stuff was discussed in one of [Voice]'s ideas. [link] |
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I never saw user [voice] before. Thanks [too shy]. |
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I meant lift, sorry for my bad English. |
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Lurch, I happened to have studied that in the air
force, before I was thrown out at the beginning of
my army service. That's why I allowed for the top
or bottom exchange. Just because of the 'Equal
Transit Time Fallacy' it does NOT mean that there
is not more pressure on the bottom of the wing.
Of course there IS a pressure difference at least at
some part of the wing, and this is interpreted into
lift. But the exact equations are more
complicated. |
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So, back to the cardboard. See "magic flying
paper" link... did you see burning cardboard flying
high up into the sky. Ever wondered why? |
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//how much extra power would it consume to
produce such tremendous heat? Let's not forget that
the air that must be heated is approaching and
rushing past the wing very fast, leaving very little
time in which to heat it.// |
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Can I refer you to the most thorough analysis of the
problem by my steamed colleague,
[MaxwellBuchanan], in the annotations above? |
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Am I to understand, [pashute], that you think the video of the burning paper is a demonstration of airfoil lift? |
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[lurch] No. Not airfoil lift. But heat under cardboard
does lift it. So heat MAY be used, to change the
airflow around a wing, and perhaps could make for a
shorter or thinner or different shape of wing. |
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