Hot or high?

[chornedsnorkack]

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?

[Brett_Henderson]

Those are some good questions. For piston engine planes, density altitude is a triple-edged sword. Less air for the wings to generate lift; Less air for the prop to 'bite' .. and less oxygen for the engine to generate horse-power.

Or is the air density all that matters, with temperature and pressure being irrelevant for performance at a given air density?

That statement kinda answers the question by trying to simplify it. Density altitude

[chornedsnorkack]

Those are some good questions. For piston engine planes, density altitude is a triple-edged sword. Less air for the wings to generate lift; Less air for the prop to 'bite' .. and less oxygen for the engine to generate horse-power.

Or is the air density all that matters, with temperature and pressure being irrelevant for performance at a given air density?

That statement kinda answers the question by trying to simplify it. Density altitude

[Brett_Henderson]

Does it mean that a plane at 1800 msl and ISA conditions (+5 Celsius) performs exactly the same as a plane at 300 msl and +52 Celsius, since the air density is equal?

(I'm not doing density altitude calculations here, so I'll take your word on these figures.. let's just keep using the term "density altitude", so we're on the same page)

In terms of take-off roll, rate-of-climb, true airspeed, that sort of thing... yes. Density altitude takes in all those variables. You can isolate a variable as far as making it constant, but it's still relevant.

When you say, "hot and high"...

[Mobius]

Air density, temperature, and pressure are all idealy related using the formula P=dRT (and many derevations of it), where P is the pressure of the air, d is the density of the air, T is the (absolute) temperature, and R is the gas constant of air (287.0 J/kg*K), so you can now plug in various values of P (in pascals), d (in kg/m3), and T (in Kelvins), and see how other values are affected.  

As for a useful answer, see what everybody else wrote...

[Brett_Henderson]

Does it mean that a plane at 1800 msl and ISA conditions (+5 Celsius) performs exactly the same as a plane at 300 msl and +52 Celsius, since the air density is equal?

OK.. I just ran density altitude calculations on those two scenarios, and they aren't even close...

If you're trying to make a comparison between air density ( Kg/M^3 ) and density altitude.. I can appreciate your point.. But I'm not sure of its relevance. A calculated density altitude of 3000ft at an airport field elevation of 5000ft.. is the same thing as a calculated density altitude of 3000ft at an airport field elevation of 50ft.

[beaky]

[quote]Air density, temperature, and pressure are all idealy related using the formula P=dRT (and many derevations of it), where P is the pressure of the air, d is the density of the air, T is the (absolute) temperature, and R is the gas constant of air (287.0 J/kg*K), so you can now plug in various values of P (in pascals), d (in kg/m3), and T (in Kelvins), and see how other values are affected.

[Mobius]

Hey, now... leave Pascal and Kelvin out of this... they are dead, and won't be able to help you make that go/no-go decision at that short-runway airport on a hot, humid day...and that is what understanding density altitude is all about- not spouting formulae and numbers, and dropping names of famous physicists.

That's why I said to see everyone else's answer, mine was totally useless, but somewhat informative none-the-less.  That's all engineers are good for, they can tell you everything about something, but in the end, it will still be a useless load of information. [smiley=grin.gif]

Changes in density altitude and pressure altitude are frighteningly obvious.  I fly out of and airport that's 928 ft above sea level, and in the summer it can get up around 100o F, and in the winter, it will get down to 0oF or less, and there is an obvious drop in performance when it's outrageously hot, and an increase when it's outrageously cold.  Last Saturday I went flying with my dad, and we took off, and had climbed to 2,000 ft AGL almost within three or four minutes (helped by a healthy headwind, but still...), doing the same thing on a hot, humid day in the summer, it might take seven or eight minutes just to get up to 2,000 ft AGL (with a similar head-wind).  The distance used on the runway changes very obviously as well.  I can take off with half the runway when it's cold, and just climb like a rocket, then come back down to land, and just float down the runway.  If I flew the same  approach on a hot day during the summer, I'll bring it down to land and be able to turn off the first taxi-way, which doesn't happen during the winter (it did once, on my BFR when the instructor had me jump on the brakes and we skidded all the way to the taxi-way entrance, I'm still not quite sure why he wanted me to do that ).  In Wyoming (KCOD), there are aircraft at the flight school near us that can't be flown during the summer, because they would never be able to get off the runway on a 110o F day at an airport that's only at 5100 ft.  That's why there's all those handy performance charts in your POH.

[beaky]

Hey, now... leave Pascal and Kelvin out of this... they are dead, and won't be able to help you make that go/no-go decision at that short-runway airport on a hot, humid day...and that is what understanding density altitude is all about- not spouting formulae and numbers, and dropping names of famous physicists.

That's why I said to see everyone else's answer, mine was totally useless, but somewhat informative none-the-less.  That's all engineers are good for, they can tell you everything about something, but in the end, it will still be a useless load of information. [smiley=grin.gif]

Changes in density altitude and pressure altitude are frighteningly obvious.  I fly out of and airport that's 928 ft above sea level, and in the summer it can get up around 100o F, and in the winter, it will get down to 0oF or less, and there is an obvious drop in performance when it's outrageously hot, and an increase when it's outrageously cold.  Last Saturday I went flying with my dad, and we took off, and had climbed to 2,000 ft AGL almost within three or four minutes (helped by a healthy headwind, but still...), doing the same thing on a hot, humid day in the summer, it might take seven or eight minutes just to get up to 2,000 ft AGL (with a similar head-wind).  The distance used on the runway changes very obviously as well.  I can take off with half the runway when it's cold, and just climb like a rocket, then come back down to land, and just float down the runway.  If I flew the same  approach on a hot day during the summer, I'll bring it down to land and be able to turn off the first taxi-way, which doesn't happen during the winter (it did once, on my BFR when the instructor had me jump on the brakes and we skidded all the way to the taxi-way entrance, I'm still not quite sure why he wanted me to do that ).  In Wyoming (KCOD), there are aircraft at the flight school near us that can't be flown during the summer, because they would never be able to get off the runway on a 110o F day at an airport that's only at 5100 ft.  That's why there's all those handy performance charts in your POH.

There ya go... a little anecdotal data...we'll make a non-engineer out of you yet.

Now, why don't you explain how there's "less air" in humid air... that's usually not so critical, but I love driving people crazy insisting that no matter how hard might be raining, it's water vapor in air that spoils performance (aside from rain spoiling lift on some very-high-aspect wings)...

[Mobius]

Now, why don't you explain how there's "less air" in humid air... that's usually not so critical, but I love driving people crazy insisting that no matter how hard might be raining, it's water vapor in air that spoils performance (aside from rain spoiling lift on some very-high-aspect wings)...

The same number of molecules must always be in a volume, no matter what kind of molecules they are.  If you take air, which is made up of diatomic Nitrogen and diatomic Oxygen, and add water vapor, made up of two molecules of Hydrogen and a molecule of Oxygen, you would have to displace three molecules of air for every two molecules of water added, so as more water is add (higher humidity) the number of molecules in the volume decreases (lower density).  That's why dry air is more dense than the resulting gaseous mixture, which is why nobody flies to a chili cook-off.

Can you tell I've been studying for the last 8 hours straight for the four finals I have tomorrow. [smiley=2vrolijk_08.gif] [smiley=2vrolijk_08.gif] [smiley=2vrolijk_08.gif]  I just want to go home....


What if it rained so hard that the force of all the raindrops pushed you out of the sky....? :-?  Maybe not...

[Brett_Henderson]

I love it when these discussions get more practical and less formulatic (almost as good as the plural for formula, hu ? )

The main runway at KOSU is just over 5000ft. As at most airports, when you're flying closed traffic they want you at 500agl before turning crosswind. On a hot, summer day (using 27L) you can be a full runway-length past the departure end before you're at 500agl... and lucky to be at pattern altitude by the time you turn downwind. On a brisk, winter day, you can be at pattern altitude before Bethel Road (red arrow). Those residents hate summer  ::)

[Brett_Henderson]

Has anybody ever figured out how much more a 172 weighs, when it's soaking wet ?  It's got to be at least 10, if not 20 lbs.

[Mobius]

Has anybody ever figured out how much more a 172 weighs, when it's soaking wet ?  It's got to be at least 10, if not 20 lbs.

I bet it's more like 2310 or 2320 lbs, assuming you have full fuel, and all the twinkies you can fit without going over 2300 lbs.  So being wet limits aircraft performance by increasing the weight, meaning you have to decrease your twinkie load, and 10 or 20 pounds is 800 or 900 twinkies or so (give or take a few ), so you should wait until it's dry out to fly, so you can carry as many twinkies as possible.

My brain is mush............. [smiley=2vrolijk_08.gif]

[Brett_Henderson]

LOL  :D ...  But I said,  "how much   M  O  R  E  "...  *snicker*

[Mobius]

Did you really?

Oh well, I guess studying for the last nine hours has actually made me partially illiterate, maybe I need a break....


The twinkie example still holds true though...[smiley=thumbsup.gif]