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Further questions about the Earth’s rotation/jetstream effecting the airplane

Asked by: 8146 views
General Aviation

How accurate is this depiction?  (be sure to play the animation)

http://www.classzone.com/books/earth_science/terc/content/visualizations/es1904/es1904page01.cfm

Can North/South travel truly be effected by rotation that much?  If this link IS accurately depicted, it would seem that a Westward journey would be quicker because the Earth is passing underneath us, but an Eastward one is slower because the Earth is going the same way. 

We know this is not the case b/c the atmospheric rotation and Earth rotation exist within the same frame of reference, so the only thing pilots need to compensate for from point A to B is wind.  It would seem as if the Coriolis force would also deflect the plane along with the winds.

The Coriolis Force causes the jetstream, right?  Therefore, is it correct to assume that in general, flights heading West take longer than flights heading East due to the jetstream?  Does most weather come from the West due to the jetstream?

 

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5 Answers



  1. Wesley Beard on Feb 02, 2011

    First off, I admire your willingness to learn and find out the answer.  It is quite an admirable trait.  To say the least these are quite difficult questions to answer at a high enough level that it makes sense to the reader.  This is my attempt and may not quite be correct but hope to provide some keywords you may be able to research further.
     
    I am going to make some statements to premise this discussion.
     
    Statement One:  We are moving at around 5000 m/s (1100 mi/hr) which is the rotational speed of the earth.  I’m pretty sure this is at the equator but am not certain about it.
     
    Statement Two:  When the air is calm, it is moving at the same rotational speed as the Earth.
     
    Statement Three:  When we takeoff from the airport and enter the air, we either add speed to the rotational speed or we subtract speed from the rotational speed.  This means that if we can fly west at the rotational speed of the Earth, from outer space it would appear that the Earth is moving underneath the airplane.  From our perspective, it will be just a normal flight except that the sun or moon appears not to move in the sky.  If we go slower than the rotational rate of the Earth, then it will appear that we are moving eastward onlooking from outer space as compared to moving westward from within the airplane.
     
    The best way I can think of to expain this phenomen is an analogy that you are standing in the caboose of a very long train that is moving at 100mph.  As you start walking towards the front of the train, your actual speed as compared to the ground will be the train speed plus your walking speed.  If you were to walk back towards the caboose, your actual ground speed will be the train speed minus your walking speed.  It is really all about perspective.
     
    How does this answer your question?   Reviewing your linked videos, it would appear to me that the author chose not to include the effect of winds on the airplane.  I absolutely agree that if there are no winds (0 mph versus the normal rotational speed of the Earth) the airplane would travel in the depicted path.  If you were to add the effects of the winds at the rotational speed (calm to our perspective in the airplane), we would get to our destination fairly simply.  As stated in your previous question, the winds and coriolis force pretty much cancel each other out.
     
    Your question:  It would seem that coriolis force would deflect the airplane along with the winds brings up an interesting topic for long distance travel and that is the shortest distance is not always a straight line.  Because the Earth is not flat, the shortest distance is a curved path from point A to point B and it is called the Great Circle Route.  I am not going to pretend on how this formula works but the route takes a northerly curvature utilizing the slower rotational speeds of the in the northerly latitudes.  So in essence, even though the airplane is traveling at the same ground speed it is travelling faster over the longitudes of the Earth (This is from the perspective of outer space).
     
    Now you can use the great circle route for either easterly or westerly routes.  It doesn’t really matter but most airlines will use the great circle route to travel westward (shortest distance) and the jet stream with it 50+ mph and usually much higher tailwinds easterly.  The end result is that the airplane travelling easterly usually takes less time than it does travelling westerly even though it covered more ground.
     
    A search on Orbits reveals that a flight from JFK to LHR (London Heathrow Airport) takes 7:20 hours and the return trip takes 8 hours.
     
    http://www.newton.dep.anl.gov/askasci/phy00/phy00385.htm
     
    I hope this helps you answer your question or aid in your research.

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  2. Anonymous on Feb 02, 2011

    Great answer!  Thank you for taking time to write out such an in-depth answer!
    Great information and VERY informative and ineteresting!  I hope others read it and learn as well…

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  3. John D. Collins on Feb 02, 2011

    I agree with Wesley’s answer up to the point of describing the great circle route.  The great circle route gets its curve from the projection of the direct route on a globe which is an ellipsoid (close to being a sphere) onto a flat surface map. It has nothing to do with the speed of rotation of the earth. 
     
    You can see this effect on a flight between my home in Charlotte, NC and London, UK. Find a nearby globe and stretch a rubber band between the two cities.  The rubber band will stretch in the shortest direction between the two cities.  Note where it goes, up the east coast, across Nova Scotia, then Newfoundland and across the Atlantic, then southern Ireland and into London.  Now plot these points on a map and connect them with a smooth curve.
     
    Any straight line in space projected onto a sphere will be a great circle route.  Examples of great circle routes that we frequently come in contact with are VOR radials or GPS direct routes.  The VOR curve is hard to see because of the short distances involved except in some cases on High Altitude IFR Enroute charts where the distance between VOR’s can be much greater on the Jet airways.
     
    Although I have never studied it, “pressure pattern navigation” takes advantage of the Coriolis effect indirectly by planning a flight to use a single heading that takes advantage of the winds around high and low pressure areas. You can google it if you are interested.
     

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  4. Brian on Feb 02, 2011

    Here is another way to think about Coriolis:
     
    Consider what it is we are trying to understand. We have a massive sphere (earth), placed inside a liquid (air), and we start spinning it. To help me understand this I like to picture a golf ball, or any other ball, placed under water and spun. What happens to the water near the ball? What about the water further from the ball?
     
    Well the water right next to the ball, especially that water contacting it, will be effected most by the ball spinning. The water will rotate in the direction of the ball, but at some lesser speed. As we move further from the spinning ball we find the water is less effected by its rotation. 
     
    Similar principals are in action with Coriolis. The earth spins and the air immediately next to the surface is most effected by this spinning and the further we get from this spinning object the less this fluid (air) is effected by the earths spin.
     
    Notice, at no point is the air being spun at a speed equal to that of the earth. In other words, the earth, like the golf ball, will cause the air to spin with it at some lesser speed. The further from the earth we get, the less the air is effected. Thus, Coriolis and wind corrections cannot possibly be canceling each other out.
     
    However, like others have said, the degree of impact, especially for short flights (couple hundred miles or less), is practically non existant. To my knowledge there is no technique to use for determining the impact of Coriolis. You might look for some balloon pilot forums if you are really curious to find a rule of thumb. If anyone had one I’d suspect it would be them as they see far more impact from Coriolis than we ever will as fixed wing guys and gals.
     
    Good luck.

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  5. Wesley Beard on Feb 03, 2011

    Thanks John for the clarification.  That makes sense.  Always learning something new.

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