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The Associated Press reports via KTAR-FM in Glendale, Arizona
At Phoenix Sky Harbor International Airport, American Airlines regional jets sit on the tarmac as American Airlines says seven regional flights have been delayed and 43 have been canceled because of a heat wave as temperatures climb to near-record highs Tuesday, June 20, 2017, in Phoenix.
[...] It's the air density.
Hotter air gets thin, making it harder to take off and land safely, mostly for smaller jets. That's what has kept some planes grounded in Phoenix this week where temperatures have been pushing 120 degrees. Airplanes take off and stay aloft because of lift, the force from the movement of air underneath the plane's wings that push it upward.
"As air warms up, it expands and there's fewer molecules to be under your wing", said Lou McNally , professor of applied meteorology at Embry-Riddle Aeronautical University. With less lift, "you need more of everything. You need more thrust to take off. You need more distance (on the runway) to take off. You need more distance to land. You need more speed to land. It gets to a point for some aircraft that it gets just too much", he said.
High heat also means a plane climbs at a lower rate, said pilot Patrick Smith, author of the book "Cockpit Confidential".
To compensate, planes have to generate more thrust or power and have larger wings. Smaller jets that generate less thrust, like Bombardier's CRJ regional jets, which have a 118-degree limit at Phoenix's elevation, are more likely to be stuck in the heat.
At Dubai International Airport and other Gulf airports, which are used to hot weather, many flights--but not all--arrive at night and early morning to get around the heat problem. Gulf carriers also tend to operate longer flights using larger planes that aren't as limited by high heat.
[...] Airlines can take other steps when the temperature climbs too high. They can lighten the plane's load by selling fewer seats--a tactic American Airlines is using in the Phoenix heat wave--or reducing cargo. They can take off with less than a full tank of fuel and then stop somewhere cooler to refuel.
(Score: 1) by pTamok on Thursday June 22 2017, @12:48PM (1 child)
Guess why it's called lift? because it's lifted up and not pushed. Air velocity is higher on top of lift surfaces such as wings, therefore pressure is less and the craft is *lifted*
You are referring to the Bernoulli effect (or possibly Coandă effect). Neither of those explain how aeroplanes fly, especially aeroplanes with symmetric wing sections. Most illustrations of aerofoils omit showing the angle of attack, so end up being more confusing than they should be.
The main (but not only) reason aerofoils work is that they are tilted at an angle to the direction of motion, so divert the airflow towards the ground. A side effect of this is that air in front of the angled surface is compressed, and air behind the angled surface is rarefied - which is pretty obvious. A large mass of air is forced downwards, so good old Newton tells us there is an equal and opposite reaction on the aeroplanes wing, pushing it up, and with it, the rest of the aircraft. With sufficient power, you can use a flat plate as a wing, so long as it is tilted at the correct angle.
The characteristic shape of a classic aerofoil is to improve efficiency by encouraging laminar flow of the air past its surfaces (flat plates produce lots of energy-sapping vortices).
If anyone ever tells you that air molecules have to take a longer path over the top surface of an aerofoil, so therefore have to move faster, and by the Bernoulli effect reduce the pressure above the wing, then they are deeply incorrect. It can be shown by experiment relatively easily that a block of air going over the top surface of an aerofoil arrives at the trailing edge after a block of air that starts at the leading edge and goes under the aerofoil. There's no magic 'forcing' it to arrive at the same time.
So, a tilted plate in an airstream generates a force, one component of which is perpendicular to the airstream. Which is glaringly obvious to the meanest intellect*. There are other, secondary effects in play, but can be ignored in the first approximation.
*Take a sheet of cardboard. Waft it in front of your face to cool yourself. In this case a flat plate is generating an airflow towards your face by you pushing alternately on each side of the cardboard. Your hand is not alternated getting closer to and further away from your face. If you reverse the motions, you can see that an airflow directed towards an angled sheet of cardboard will generate a force at right angles to the flow (and, also, try to push the cardboard in the direction of the airflow). That's how a wing works.
(Score: 1) by pTamok on Thursday June 22 2017, @01:29PM
By the way, the above is very much 'lies to children' - the explanation for lift generated by free-flying aerofoils (i.e. nowhere near the ground) is a little more complicated.
I also didn't proofread properly - a block of air going over the top surface of an aerofoil arrives at the trailing edge BEFORE a block of air that starts at the leading edge and goes under the aerofoil. (I recast the sentence several times and didn't make all the relevant corrections. Sigh). In a nice hand-wavey way, you can think of the air under the angled aerofoil 'piling-up' and slowing down relative to the air above the aerofoil.
I recommend these two texts:
http://www.eskimo.com/~billb/wing/airfoil.html [eskimo.com]
http://www.av8n.com//how/htm/airfoils.html [av8n.com]
You can describe lift in terms of Bernoulli's equations (or something like them), but it is non-intuitive, and easy to get wrong. They are actually equivalent to a Newtonian approach, and mathematically, Bernoulli's description is more useful to aerofoil designers. So from a mathematical point of view, both ways of looking at lift are correct, but when over-simplified, the Newtonian approach is more intuitive and easier to understand.