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Dense exoplanet atmosphere, choice zeppelin vs. plane

  1. Feb 12, 2015 #1
    Would on a exoplanet with dense atmosphere zeppelin/blimp better idea than a plane?

    If I get it right there would be:
    -much higher buyancy (argument for zeppelin)
    -higher drag (argument against plane)
    -higher lift (argument for plane)

    (but I'm not good enough to know what would matter)

    Assumptions:
    -contemporary technology;
    -dense atmosphere (let's say 3 atmospheres), mostly nitrogen, molar concentration of oxygen similar to that on Earth.
     
  2. jcsd
  3. Feb 12, 2015 #2

    mfb

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    Higher lift and buoyancy are directly related so they kind of cancel each other.

    Zeppelins might be interesting for shorter distances. They can be smaller than on earth.
    An airplane can simply fly higher to get earth-like conditions again - certainly better for long distances as they are much faster.

    Flying cars could be smaller, so those could be more interesting as well.
     
  4. Feb 12, 2015 #3
    Agreed
    Cancel in a way... but something important will not cancel.
    May I slightly hone the assumptions?
    4,0 bars, mostly nitrogen. Convenient for computations because the square root is 2, and also plausible. Venus also has 3,2 bar nitrogen (3,5 % of 92 bar, mostly carbon dioxide) so you might terraform Venus by somehow removing surplus carbon dioxide, adding water and oxygen, but leaving nitrogen exactly in place. Say 3,2 bar nitrogen and 0,8 bar oxygen - exactly Earth air at 4 times density.
    Also, let us assume 1,00 g, not the 0,95 g of Venus.
    And so do higher lift and drag.
    Yes. Contemporary common planes on Earth do climb to roughly 1/4 the sea level air pressure, upward of 1 bar.
    On such a planet, you could perform theoretically exactly the same exercise in the other direction - from 1 bar downwards to 4.
    Note that this is not a "simple" exercise. A plane that climbs has to function at different air densities.
    In my opinion, interesting for longer distances. But this depends on weather which I cannot immediately work through.
    Yes. And birds could be heavier. Bigger, too.
    A longer analysis I´ve thought of previously, but don´t have time to present it at the moment.
     
  5. Feb 13, 2015 #4
    "Flying cars could be smaller, so those could be more interesting as well."

    Do I get it right?
    Big passanger plane - 800km/h, altitude of 10 km, roughly 1/4 atmosphere.

    Small Cessna - 200-300 km /h, altitude of 3 km, roughly 0,7 atmosphere.

    So smaller, slower - like car... at almost 3 atm. How should they look like?


    Anyway, climbing to a higher altitude (20km?) for plane to get low enough pressure shouldn't be a fuel consuming nightmare? (worth a try only for intracontinental connections?)
     
  6. Feb 13, 2015 #5
    Like Sky Baby?
    http://www.aerospaceweb.org/question/design/q0214.shtml
    Wingspans 2,1 m and down.
    The aim of Stits and Starr was minimum span, though. Not the best plane they could make inside an accomplished target.
    How about, accepting wingspan 2,5 m - matching the width of trucks and buses in a highway lane - and building the best plane that works inside that box?
     
  7. Feb 13, 2015 #6
    How to adjust that they work in denser atmosphere? Less wingspan? Smaller engine in relation to mass?
     
  8. Feb 13, 2015 #7

    mfb

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    Drag comes from both wings and the body, lift only from wings. Reducing wings to 1/3 their area gives an airplane with the same lift but more drag.

    I think we start at 3-4 bar, and we want to reach ~1/3 like on earth. That means we have to go to ~20km - short flights won't do that but for longer flights this would be interesting (and fuel-saving). The speed advantage over zeppelins is just too massive - if you have a choice between getting there in 10 hours or in 2-3 days, what do you choose?
     
  9. Feb 13, 2015 #8
    Both of these work.
    Consider the equation for aerodynamic forces
    F=rho*uˇ2*A*C

    Since C depends on geometry and rho is dictated by the height where you climb, you have various options:
    First 2 are:
    1) Change uˇ2. Meaning that a plane has to increase speed to provide same lift at decreasing density.
    The drag remains constant, because the lift does stay constant. But although the engine is providing the same thrust, the power - product of thrust and speed - has to increase in a climb.
    And vice versa - a plane in a denser atmosphere would find that although it encounters larger drag, the wings provide lift at a lower speed. At 4 times the air density, a plane of the same weight and geometry will fly at 1/2 the speed, and need 1/2 the power to overcome the same drag. Therefore a smaller engine can be used to propel the plane.
    2) Change F. You can take the same plane, propel it to the same velocity, and make it carry 4 times the load. Sure, you will then need the wings of the same size to carry 4 times the lift. But then you just have 4 times the weight, so you can make the wings stronger and heavier. You need engines to provide 4 times the thrust, but since the speed is the same, you still need just 4 times the power.
     
  10. Feb 13, 2015 #9
    Zeppelins have the advantage over planes that they do not need power to remain airborne.
     
  11. Feb 13, 2015 #10
    "uˇ2" is a square root, right? (sorry, if it sounds silly)

    So with 4 atm, I get a plane like Cessna 172 (145-180 hp), with engine of passenger car, like Fiat Punto 1.4 litre (95hp), and it flights with half of speed of Cessna, so roughly counting 110km/h? (in very rough calculation)
     
  12. Feb 13, 2015 #11
    Cheap airline? :D
    (that how I behave in RL)

    And more seriously: cargo delivered in any place.

    Concerning planes I have got one more are where I'm uncertain - oxygen. Under assumption that there is "the right" concentration near surface, then at 20 km there should be right pressure for passenger jet but 1/3 or 1/4 of amount of oxygen. Wouldn't it be a problem for engines?
     
  13. Feb 13, 2015 #12

    mfb

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    Just staying airborne is not useful for transportation, unless you have really strong winds.
    It has to be u2 in that context, u squared.
    Hmm, true. And the combustion is not as oxygen-rich as I hoped, according to rough estimates. That could be solved with larger turbines I guess.
     
  14. Feb 13, 2015 #13
    Square was the correct interpretation.
    What you can do with airframe of a similar size...
    Look at DC-3. Fairly developed, mass produced commercial plane.
    Wingspan 29,0 m. Wing area 92 square m. Cabin wide enough to accommodate 4 seats abreast at a squeeze, or 3 in comfort.
    That wing lifts a total weight (structure, payload and fuel) of 11 400 kg.
    How?
    A loaded DC-3 under other standard conditions (sea level, +15 Celsius), with full flaps, stalls at 58 knots airspeed (108 km/h). For landing, 20 % faster than stall, so 70 knots being 130 km/h. Takeoff a bit faster, 84 knots or 155 km/h.
    This is propelled by 2 engines at 1100 or 1200 horsepower each.

    And now compare Embraer 190.
    Wingspan 28,7 m. Wing area 92 square m. Cabin wide enough for 4 seats abreast.
    So far, just like DC-3. Right?
    Well, the detail is: the total weight of Embraer 190 is up to 51 800 kg (for commercial passenger liners) and even 55 000 kg (for the private jet Lineage 1000). Almost 5 times the weight of DC-3 (exactly 5 times would be 57 000 kg).

    In 1 bar and 1 g, I should not advise attempting to slow down a Lineage 1000 to 130 km/h to land.

    So... in 4 bar air, an E-jet would fly much like DC-3 does at 1 bar (in terms of stall speed and landing strip length needed). And in 4 bar, the DC-3 should be able to stall at 29 knots (54 km/h), land at 35 knots (65 km/h) and take off at 42 knots (78 km/h).
    And do all of this with 2 engines a under 600 horses, not 1200.

    How long airstrip would you want cleared from boulders (like the few pictures of Venus´ surface show) in order to land you DC-3 there, and take off again at 78 km/h (and then climb 11 km to return to the Maxwell Montes Airport)?
     
  15. Feb 14, 2015 #14
    I somewhat dislike this idea of climbing to 11 km. ;)

    If I get it right:
    http://www.ana.co.jp/eng/aboutana/corporate/galleryclassi/1953/dc3.html

    It needed 840m for take off. Needs half speed with half of engine power? Would mean 420m.

    For giving smaller engine something like 10% of total price should be saved.
    "Typically, for civil transports the costs average 50 percent for structure and integration, 20 percent for engines, and 30 percent for avionics."
    http://www.britannica.com/EBchecked...dustry/225669/Engine-and-avionics-manufacture

    Concerning those planes that should have a cruise speed somewhere around 130 km/h: they would have to be really aerodynamic. With low stall speed shouldn't they be able to work in case of emergency as gliders which can land on any meadow?

    Would such low start speed facilitate starting from water?
     
  16. Feb 14, 2015 #15
    "Planes powered by jets are faster, but for flights of less than around 500 nautical miles, the shorter time spent in the air is insignificant compared with the fuel savings to be made by flying a slower turboprop."

    http://www.economist.com/blogs/gulliver/2012/02/air-travel-and-turboprop-revival

    So the break even point with 4 time denser atmosphere (longer climb) could be assumed somewhere around... 800 nautical miles? 1480 km?
     
  17. Feb 17, 2015 #16
    Oh - one more thing - the most aerodynamic zeppelin would have a shape of a teardrop?
     
  18. Feb 17, 2015 #17
    Gliders may land on meadows but this does not mean they can take off from there.
    This
    http://byrongliding.com/flying/outlanding-procedures/
    suggests that glider pilots with stall speeds 35 to 40 knots (65 to 74 km/h) take outlanding as an acceptable risk.
    A DC-3, stall speed 58 knots in 1 bar, should stall at 29 knots in 4 bar. And the DC-3 has more space in fuselage for crumple zones (in which case the plane does not fly off, but people do walk off). So at 4 bar, the Embraer 190 jet still cannot make a safe outlanding, but a DC-3 sized plane with say 20 t weight would meet the 40 knot stall speed limit.
    Yes - just like in 1 bar.
     
  19. Feb 20, 2015 #18
    I would say neither... I'd go more along the idea of "swimmers" or submarines. Create a lighter than air bladder with a Plane around it to gain the maneuverability. I think this would be more likely to develop than a plane or zeppelin in a denser atmosphere.
     
  20. Feb 22, 2015 #19
    Compromise solution :D
    (but for sure to think about it)

    Two more questions:
    1) Would it be feasible to try to improve buoyancy thorough using exhaust fumes from engines to heat up content inside? (I imagine inside big aerodynamic envelope and huge bags with actual lifting gas)?

    2) Would it be possible to change altitude to get wind blowing in the right direction? (in passenger version or after dropping cargo, because presumably with full load it would just fly low) Or the change would be minuscule?
     
  21. Feb 22, 2015 #20
    1) You could, but I'd think high speed travel to be less likely as you'd need more weight on the vehicle to protect it. Either from the heat or the cold that would start really doing the number on you the faster you went... also if your talking really high speeds, you're defeating the point a bit as the exhaust is what moves the jet lol. I suppose you could run the jet under the bladder, but then the faster you go the more you have to release from the bladder to maintain altitude...and then you're screwed when you slow.

    2) I imagine so if you created a bladder where you had a heater on the bottom and cooler at the top and then also had a wind sail that could hold whatever wind speed. I'm pretty sure with me limited mechanical knowledge I could cook something up myself to work like this that is highly inefficient but would work primitively.
     
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