Announcement

Collapse
No announcement yet.

Aviation brain teasers

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • TeeVee
    replied
    Originally posted by Gabriel View Post
    Actually, I think he did this "mistake" intentionally to avoid having to post a puzzle
    (but I have no evidence)
    who me????? i would never

    Leave a comment:


  • Highkeas
    replied
    I used to work for BAC and one of the surprises once the Concorde became operational is that fuel usage was reduced slightly due to centrifugal force.

    Leave a comment:


  • elaw
    replied
    Damn! I wish I'd have thought of that...

    Leave a comment:


  • Gabriel
    replied
    Originally posted by elaw View Post
    Well I think 90% of the credit should go to Tee Vee - all I did was read the Wikipedia article!
    Actually, I think he did this "mistake" intentionally to avoid having to post a puzzle
    (but I have no evidence)

    Leave a comment:


  • elaw
    replied
    Well I think 90% of the credit should go to Tee Vee - all I did was read the Wikipedia article!

    As for another puzzle... give me a little time to see if I can come up with something good.

    In the meantime a pretty lame one but let's see if anyone can answer it *without* using an online search engine (honor system): what is the product name (as opposed to model number) that P&W used for many of their early turbine engines such as the JT3 and JT8?

    Leave a comment:


  • Gabriel
    replied
    Originally posted by TeeVee View Post
    Eötvös effect.

    any object co-rotating with the Earth at the equator has its measured weight reduced by 0.34 percent, thanks to the Earth's rotation.
    Yes!!! And thanks for the name, I didn't know that it was called Eötvös effect.

    and the answer is..........the one flying west.
    Hmmmmm.....

    Originally posted by Elaw
    Good call!

    But one teeny thing: the original question is which airplane is using less fuel.

    So the answer would be the *eastbound* one... as the Eötvös effect causes a reduction in the perceived force of gravity when traveling toward the east. Thus the eastbound plane will have less induced drag, thus requiring less power to remain airborne, thus consuming less fuel.
    Correct, you are the winner and the prize goes to you, together with the obligation to present a new puzzle.

    A bit of explanation of the Eötvös effect (non-mandatory reading)

    In simple terms, the centrifugal force of something rotating around the Earth cancels a bit (or a lot) of the apparent weight, and the centrifugal force is greater the faster an object is rotating around the Earth, what an object is already doing if it's static because the Earth itself is already rotating around itself (at about 1000 MPH on the equator, which is about twice the cruise speed of a 777). So if you move "with" the Earth your speed adds to that of the Earth, and if you are flying in the opposite direction you have to subtract your speed to that of the Earth.

    So the airplane flying "with" the Earth will have a lower apparent weight, will need less lift to bear that lower apparent weight, and will then have less induced drag (which is the drag of doing lift). We all know that a "lighter" airplane burns less fuel, right?

    A bit more of detail:

    If you moved fast enough around the Earth you would be in orbit, and apparently weightless. If you go high enough over the Equator, "fast enough" becomes "at the same angular speed than the Earth", and then you become a geostationary satellite. "High enough" provides for a higher tangential speed and a lower gravitational force.

    But at normal airplane altitudes, the altitude itself is almost negligible because it is so small compared to the radius of the Earth.

    So, a 777 at say 500 MPH flying "with" the Earth on the equator will have a tangential speed of 1000+500=1500 MPH, while one flying due West will have a tangential speed of 1000-500=500 MPH.

    Because this centrifugal force goes with the square of the tangential speed, if 1000 MPH cancels 0.34% of the wight as TeeVee correctly said, 1500 will cancel 0.765% of the object's weight and 500 MPH will cancel 0,085% of its weight. That's a 0.68% less apparent wight of the plane flying "with" the Earth than the one flying "against" it, and a 0.68% less of induced drag, which at minimum drag speed is about half of the total drag, so 0.34% less thrust and, if the efficiency of the engine was linear with thrust, 0.34% lower fuel burn. (airplanes fly faster than minimum drag speed so the effect will be a bit lower than that)

    Leave a comment:


  • elaw
    replied
    Originally posted by TeeVee View Post
    and the answer is..........the one flying west.

    Eötvös effect.

    any object co-rotating with the Earth at the equator has its measured weight reduced by 0.34 percent, thanks to the Earth's rotation.
    Good call!

    But one teeny thing: the original question is which airplane is using less fuel.

    So the answer would be the *eastbound* one... as the Eötvös effect causes a reduction in the perceived force of gravity when traveling toward the east. Thus the eastbound plane will have less induced drag, thus requiring less power to remain airborne, thus consuming less fuel.

    Leave a comment:


  • guamainiac
    replied
    TeeVee, back in the 60's I got a 727 out of Honolulu straight into JFK. All the west bound flights had to stop for fuel in CA before going on; I asked and was told by a crew member that if could be done in a 727 using the jet stream.

    I did catch that "no wind" thing later and was going to delete but figured it wasn't fair taking your "dirty laundry" off the line.

    And where the heck did you dig that up? Esoteric as all get out.

    Leave a comment:


  • TeeVee
    replied
    and the answer is..........the one flying west.

    Eötvös effect.

    any object co-rotating with the Earth at the equator has its measured weight reduced by 0.34 percent, thanks to the Earth's rotation.

    Leave a comment:


  • TeeVee
    replied
    Originally posted by guamainiac View Post
    plane going west to east will be in the jet stream.
    tried that. it's a hypothetical and there is NO wind

    Leave a comment:


  • guamainiac
    replied
    plane going west to east will be in the jet stream.

    Leave a comment:


  • Highkeas
    replied
    We all flunked the test - answer please

    Leave a comment:


  • Gabriel
    replied
    Originally posted by elaw View Post
    Well... I could say that whichever plane has been flying longer would have given the fuel more time to cold-soak, thus its fuel temp would be lower. Assuming the autothrottle is maintaining a constant thrust, the fuel flow *by volume* on that plane would therefore be less since its fuel is denser and thus less (again by volume, not mass) fuel is required to produce the same thrust level.

    But I'm guessing that's not what you're looking for.
    Nope, but keep trying!

    The slight difference in fuel burn is measured in mass, not volume. Anyway, for the sake of this puzzle it's safe to assume that the fuel in both airplanes have identical properties.

    We had people that was playing with the right variables to solve this problem:
    - Gravity
    - Rotation of the Earth
    - Direction of flight (one West, one East)

    Aaaaaaand...

    Both airplanes are flying along the ecuator (okay, to prevent that they collide let's have one of them 1 mile North and the other one one mile South of the ecuator, to which they are flying parallel). This was the second reason why Coriolis forces are not a factor. There is no Coriolis forces on the ecuator.

    Come on, it's served in a golden tray!!!

    Leave a comment:


  • elaw
    replied
    Well... I could say that whichever plane has been flying longer would have given the fuel more time to cold-soak, thus its fuel temp would be lower. Assuming the autothrottle is maintaining a constant thrust, the fuel flow *by volume* on that plane would therefore be less since its fuel is denser and thus less (again by volume, not mass) fuel is required to produce the same thrust level.

    But I'm guessing that's not what you're looking for.

    Leave a comment:


  • Gabriel
    replied
    Originally posted by Highkeas View Post
    If the moon and or sun was not directly overhead then there would be a extremely small gravitational gradient between the two aircraft. The moon is in its first quarter today so the Eastbound aircraft would need slightly higher thrust [Except thrust would be different, not drag.]
    Well thought. I had not taken this into account.
    Let's say that it's local midday and there is a solar eclipse.

    We already had the rotation of the Earth and now we have gravity too. We are in the good path. Come on team, last effort!

    Leave a comment:

Working...
X