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Hot/High-Why A Problem?

Posted: Tue Oct 24, 2000 10:43 am
by Guest
Hello there;
I constantly read the "difficulties" of hot and high operations ie. La Paz, Cuzco etc. I have a few ideas as to why extra thrust/lower payload is required at certain elevations, but could anyone provide a brief reason as to why operational requirements to such airports are different?


RE: Hot/High-Why A Problem?

Posted: Tue Oct 24, 2000 10:46 am
by modesto2
Hot air lacks density. High elevations lack dense air. That's it in a nutshell! With less air, it takes more airflow to achieve the same lift and performance.

RE: Hot/High-Why A Problem?

Posted: Tue Oct 24, 2000 1:53 pm
by fr8tdog
Hello Capt. Picard,
As Modesto2 said When it is hot out, the air molecules are more "excited" so to speak (have more movement) and bumps into on another moving them farther apart. ie' less air molecules for a given amount of space.(less density) than cooler air.
If you take a column of air (from the surface of the earth to the upper edge of the atmosphere) 1 sq inch in diameter it will weigh approx 14lbs (If I remember correctly).
Air near the surface of the earth will be more dense because of the weight of the air above it. (packing the air molecules closer together)

An aircraft wing generates lift by a combination of factors ( I will try and keep it simple)
#1 Bernoulli's Principle-states that when a fluid (Gas "air" is a type of fluid) is accelerated in a vessel its internal pressure will drop and vise versa if its slowed the pressure will increase. (mathmatically speaking)

Slow flow = high pressure
Fast flow = low pressure

slow slow
--------------- Fast -----------------
Flow---> > -------------- > > ^^^ > >
> > > > >>>>>>>>>> > >Pressure > >
> > > >>>>>>>>>> > > VVVV
> > > > -------------- > > > >
--------------- ------------------

even though there is a constriction in this model, the same volume of water that is in the wide part of the tube is passing through the small part of the tube, so if there is the same volume going through the tube the velocity of the water must increase and lowers the pressure that is pushing out against the sides of the tube, after the tube the velocity decreases or slows the pressure increases.
another way to look at it is, the Static
(outward pushing pressure) and the Dynamic (speed of the fluid) must equal 4 (arbitrary number that I just made up) one or the other must go up or down
D= dynamic, S= static
D -- 2 then S is 2 also, in order to equal 4
if S=1 then D=3 to make 4
the same applies if D=1 then S=3 to make 4
now for the airplane wing.... if you look at the top half of the wing (generally speaking) it is curved and the lower half is flat. what happens is the air is accelerated over the top (acting like the lower half of the model that I just described) causing lift (slightly lower pressure than the bottom of the wing.

Ok, so why doesnt the air just move up and out of way of the wing ?

The upper half of the model is the weight of the atmosphere above the wing. causing a squeeze like effect on the air flowing over the wing.

Another factor is Newtons 3rd law - for ever action there is an equal and opposite reaction.
very simply put ( lift is a very complex issue ) there is also some deflection of air downward causing the wing to go upward.

now to get to the question at hand,

Why does high, hot and humid conditions affect airplanes ?
if the airplane is at a low temp and low alt. lets say std cond. 59F and at S.L. the air is fairly dense (allowing for a lot of air molecules flowing over the wings) and creates lift quite rapidly. Also the indicated airspeed is more closely related to the True airspeed.
Now if we have a runway up at let say 8000' MSL (mean sea level) and the temp is high lets say around 85F the density of the air is going to be less. (less wt overhead and thermal expansion) if water vapor is present (humidity) then you will have even less dense air. This is what we call Density altitude.(I dont have a flight computer with me so I cant give an accurate figure but I will give an guess-timate) the density altitude of an airport, lets say has an elevation of 5600'MSL and the temp is 88f is going to be around 9200' (density altitude is an height at which the density of air most closely resembles)
any way at higher alt. airports airplanes must have the same amount of air passing over the wings to produce lift as the lower airports. This means the airplanes must travel at a higher rate of speed, using up more runway..
The indicated speed will read the same as a lower elevated airport but the ground speed will be higher ......
The reason for this is similar to how the wing works
True airspeed is a speed that is corrected for nonstandard temp and pressure.
Airspeed indicators measures Pressure via the Pitot tubes in pounds/sq ft. and calibrated to miles or kts/hr
at std temp and pressure.
Lets say that an aircraft rotates at 80kts and we are at S.L., the speed that the airspeed indicator tells you and the actucal (lets say the aircraft is on the runway) ground speed is very close to the same.

If you are at a higher elevation airport the aircraft must travel faster (ground) in order to pack the same amount of molecules in the Pitot tube to read the same speed.
Basically thats the reason for longer runways and longer takeoff rolls when you have High, Hot and or humid conditions.
hope this helps and didnt confuse anyone to badly.....

RE: Hot/High-Why A Problem?

Posted: Tue Oct 24, 2000 1:58 pm
by fr8tdog
Hmmm my Model did not come out the way that I wanted... I appologize.... use your imagination and stretch it out a bit.

RE: Hot/High-Why A Problem?

Posted: Tue Oct 24, 2000 7:12 pm
by Guest
Many thanks for your informative responses.

RE: Hot/High-Why A Problem?

Posted: Wed Oct 25, 2000 2:18 am
by Aer Lingus
With regard to height:
As the air is much thinner at 13000 ft. than on sea level, less air is being sucked into the engine and thus a longer runway is required aswell as higher thrust


RE: Hot/High-Why A Problem?

Posted: Thu Oct 26, 2000 11:57 pm
by fr8tdog
Aer Lingus, you are absolutely correct on that!
in addition, That is the reason that there is a mixture control in piston aircraft, so the pilot can lean the fuel air mixture (for more power and fuel economy)
Propellers are not as efficient at higher altitudes.
turbine powered aircraft have a temp limitation verses torque or N1 that is more noticable in the higher elevations.

RE: Hot/High-Why A Problem?

Posted: Fri Oct 27, 2000 12:08 am
by MD80
Recently flew out of Cuzco Peru. Quite exciting watching how much runway those 737-200's need to get off the ground. They only took a penalty of about a dozen seats but I'm sure they're not "topping off" the tanks for an hour flight to Lima.