The performance of aircraft deteriorates in thin air – in “hot-and-high” conditions. Wings have less air for lift, and engines have less air for thrust.
But is there any difference between hot and high? Do aircraft perform differently in low-but-hot conditions and cold-but-high conditions? Or is the air density all that matters, with temperature and pressure being irrelevant for performance at a given air density?
By: paulc - 11th December 2006 at 14:31
According to a well known test pilot, the thrust from a jet engine is reduced by approx 100 lbs for each 1 degree above ambient. This might not sound a lot but in a vtol situation (where this pilot had his experience) it is cruical.
(ie exhaust gas being taken back into the engine)
By: chornedsnorkack - 11th December 2006 at 09:32
didn’t know we were talking about take offs. Doesn’t matter though. lets say that by some chance air temp at some particular airfield at 14000 is 40 deg below zero celsius. (below 40 something fahrenheit) air density would be around 0.9 kg/m3.
Now if we take some arfield at 0 feet at 50 degrees celsius (around 120 F), air density would be around 1.1 kg/m3. Still higher.
In conclusion, it would be almost impossible to have weather conditions that would make a high and cold take off better than low and hot. If by some miracle temperatures do drop to more-than-arctic values that’d provide you with nice air density, i doubt one could even start the plane in the first place.
Which actually happens. You don´t have runways there, but you do have snow for skis.
The South Pole is slightly over 9000 feet above sea level. It is said to be 50 years ago that a first ski-equipped plane landed there – in summer. There has never been over -15 Celsius or so observed on the South Pole – typical polar summer day temperatures are -20, -30 Celsius range.
In winter, the absolute minimum is -78,9 Celsius.
Vostok camp is not on South Pole, but it is higher on the ice dome at about 11 500 feet and farther from sea, so it is colder year-round. Summer maximum -22 Celsius. Winter minimum observed -89,2: the minimum ever observed anywhere on surface of Earth. This does not mean it is the coldest place on Earth… for example, the summit of the ice dome is just over 13 000 feet, but there is no camp there and the camps in Antarctic interior are not very many, so not sure what exactly is the coldest place.
Anyway, -89,2 is record, but -70-s and low -80s are common polar night weather. And an important point is that it is impossible to reach Vostok by air in winter. The reason is said to be not that engines would not start, but that the snow is too soft and thick in winter and would not allow ski planes to reach takeoff speeds.
By: totoro - 8th December 2006 at 18:33
didn’t know we were talking about take offs. Doesn’t matter though. lets say that by some chance air temp at some particular airfield at 14000 is 40 deg below zero celsius. (below 40 something fahrenheit) air density would be around 0.9 kg/m3.
Now if we take some arfield at 0 feet at 50 degrees celsius (around 120 F), air density would be around 1.1 kg/m3. Still higher.
In conclusion, it would be almost impossible to have weather conditions that would make a high and cold take off better than low and hot. If by some miracle temperatures do drop to more-than-arctic values that’d provide you with nice air density, i doubt one could even start the plane in the first place.
By: chornedsnorkack - 8th December 2006 at 16:46
Air density IS all that matters. However, since air density depends on temperature and pressure, then those two certainly are quite important.
Low and high are relative terms, but lets say we’re talking about 10 000 feet and 30 000 feet. Difference in air density for those two is so huge that no local temperature difference can influence it. With certainty, air density will always be much lower at ‘high and cold’ than at ‘low and hot’, as the possible high and low values of hot-cold are much closer together than possible altitude values.
Well, while there is some terrain at 29 000 feet, no runways I know of. The highest runways are in the region of 14 000 feet.
But yes, the highest runways are not simultaneously the coldest… the coldest runways are near sea level.
By: totoro - 8th December 2006 at 15:46
Air density IS all that matters. However, since air density depends on temperature and pressure, then those two certainly are quite important.
Low and high are relative terms, but lets say we’re talking about 10 000 feet and 30 000 feet. Difference in air density for those two is so huge that no local temperature difference can influence it. With certainty, air density will always be much lower at ‘high and cold’ than at ‘low and hot’, as the possible high and low values of hot-cold are much closer together than possible altitude values.
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December 8, 2006 at 2:48 pm