Half a croissant, on a plate, with a sign in front of it saying '50c'
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Heat wing elevation

Diff of heat between top and bottom of wing cause elevation
  [vote for,

[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.

pashute, Aug 11 2011

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]


       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).   

       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.   

       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.   

       It would certainly overcome potential problems with wing icing ...
8th of 7, Aug 11 2011

       Would this give more lift? Why?
MaxwellBuchanan, Aug 11 2011

FlyingToaster, Aug 11 2011

       Thought experiment:   

       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.

       After noting failure, apply [marked-for-deletion] bad science.
lurch, Aug 11 2011

       You had us at // Pour gasoline on cardboard. //
8th of 7, Aug 11 2011

       //Would this give more lift? Why?//   

       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".   

       In any event, how much energy would be needed?   

       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.)   

       Assume also that the plane has a wingspan of 50m (a typical long-distance aeroplane), and is travelling at 200m/s (about 500mph).   

       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?   

       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.   

       So, in fact, such heating would be perfectly feasible, energetically.   

       However, I still don't see how heating the air over (or under) the wing will increase lift.
MaxwellBuchanan, Aug 11 2011

       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.   

       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.
Alterother, Aug 11 2011

       wot [2fries] said.
FlyingToaster, Aug 12 2011

       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.
Alterother, Aug 12 2011

       //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.)   

       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.   

       It's just unfortunate that increased density altitude equates to reduced, not increased, lift.
lurch, Aug 12 2011

       // theres quite a lot of spare heat coming out of the jet engines. //   

       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?
Alterother, Aug 12 2011

       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.
Alterother, Aug 12 2011

       That comment, [Alterother], ought to be marked for tagline. Well do I know whereof you speak.
normzone, Aug 12 2011

       Some of this stuff was discussed in one of [Voice]'s ideas. [link]   

       I never saw user [voice] before. Thanks [too shy].   

       I meant lift, sorry for my bad English.   

       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.   

       So, back to the cardboard. See "magic flying paper" link... did you see burning cardboard flying high up into the sky. Ever wondered why?
pashute, Aug 12 2011

       //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.//   

       Can I refer you to the most thorough analysis of the problem by my steamed colleague, [MaxwellBuchanan], in the annotations above?
MaxwellBuchanan, Aug 12 2011

       Am I to understand, [pashute], that you think the video of the burning paper is a demonstration of airfoil lift?
lurch, Aug 12 2011

       [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.
pashute, Aug 19 2011


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