Why does heat affect the operation of a plane?

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flightmedic

Pre-takeoff checklist
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flightmedic
Pardon my ignorance, but I have not yet started my training. I read many posts that talk about how the heat (air temperature) affects the plane. Is there a simple explanation as to why or would it be to complicated to explain on a forum?
 
flightmedic said:
Pardon my ignorance, but I have not yet started my training. I read many posts that talk about how the heat (air temperature) affects the plane. Is there a simple explanation as to why or would it be to complicated to explain on a forum?
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Simply that temperature increases reduce the density of air which reduces the lift at the wings and props, and reduces the power of normally aspirated engines.
 
Thank you. That certainly makes sense. I guess I assumed it would be more complicated than that. By the way, I just looked at your slideshow and you have a beautiful plane there and the photographer did a great job.
 
flightmedic said:
Thank you. That certainly makes sense. I guess I assumed it would be more complicated than that. By the way, I just looked at your slideshow and you have a beautiful plane there and the photographer did a great job.
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To reinforce,
The thinner air reduces engine power, while at the SAME TIME reducing the efficiency of the prop, and at the SAME TIME reducing the lift of the wings (which requires more power at the same time you have less to work with).

That's why so many pilots get themselves in trouble with density altitude, because of the compounding losses.
 
Henning said:
Simply that temperature increases reduce the density of air which reduces the lift at the wings and props, and reduces the power of normally aspirated engines.
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Not just normally aspirated. With turbo engines, it depends on the turbo controller used. Most just have a rated boost level, so regardless of temperature they produce that same maximum boost. Some (the Navajo comes to mind as the main one) have density controllers for the maximum boost, and those will vary your boost for takeoff to produce the same power regardless of temperature, but they still have limits.
 
PilotAlan said:
To reinforce,
The thinner air reduces engine power, while at the SAME TIME reducing the efficiency of the prop, and at the SAME TIME reducing the lift of the wings (which requires more power at the same time you have less to work with).

That's why so many pilots get themselves in trouble with density altitude, because of the compounding losses.
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And, at the same time reduces aerodynamic drag.
 
Ideal Gas Law:
PV=NkT where P is the absolute pressure of the gas; V is the volume; N is the number of particles in the gas; k is Boltzmann's constant relating temperature and energy; and T is the absolute temperature.
In SI units, P is measured in pascals; V in cubic metres; N is a dimensionless number; and T in kelvin. k has the value 1.38·10−23 J·K−1 in SI units.
Sometimes this is expressed as PV=nRT where n is the amount of substance of gas and R is the ideal, or universal, gas constant, equal to the product of Boltzmann's constant and Avogadro's constant. In SI units, n is measured in moles, and T in kelvin. R has the value 8.314 J·K−1·mol−1.
The temperature used in the equation of state is an absolute temperature: in the SI system of units, kelvins; in the Imperial system, degrees Rankine.[4]
 
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flightmedic said:
Thank you gentlemen.

What do the letters stand for in the formula PV=nRT stand for?
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P Pressure
V Volume
n quantity (number of molecules)
R constant factor
T Temperature

For more than you ever wanted to know about the ideal gas law:

http://en.wikipedia.org/wiki/Ideal_gas_law

Simply stated for a given number of molecules of a gas, the pressure times the volume is proportional to the absolute temperature. Raise the temp while holding the volume constant and the pressure goes up. Raise the pressure in the same container and the temp goes up.

And for the issue of atmospheric density, let the gas expand while maintaining the same pressure and raising the temp, the number of molecules goes down.
 
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Henning said:
Ideal Gas Law:
PV=NkT where P is the absolute pressure of the gas; V is the volume; N is the number of particles in the gas; k is Boltzmann's constant relating temperature and energy; and T is the absolute temperature.
In SI units, P is measured in pascals; V in cubic metres; N is a dimensionless number; and T in kelvin. k has the value 1.38·10−23 J·K−1 in SI units.
Sometimes this is expressed as PV=nRT where n is the amount of substance of gas and R is the ideal, or universal, gas constant, equal to the product of Boltzmann's constant and Avogadro's constant. In SI units, n is measured in moles, and T in kelvin. R has the value 8.314 J·K−1·mol−1.
The temperature used in the equation of state is an absolute temperature: in the SI system of units, kelvins; in the Imperial system, degrees Rankine.[4]
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You just had to go there didn't you?
 
That reminds me of the humorous email that makes its rounds regarding if Hell is Endothermic or Exothermic.

I'll try to find it and post it on Friday is Joke Day thread unless it's already there and I've fogotten
 
I think sometimes I should just keep my mouth shut. I haven't even heard the word "Pascals" since college. (Other than Johnny Pascals that I used to work with)
 
The ideal gas law can also be stated as: pressure is proportional to density times temperature. There's a constant in there, but that's not important if you just want to understand how air 'works'. So, say the air pressure stays constant. Then, if you raise the temperature, density has to decrease to compensate. As others have noted, it's not the heat specifically that kills your performance, it's the density, but the air temperature affects the density.

Of course, you can use the ideal gas law in other ways. Supposing the temperature doesn't change, if you increase the pressure, the density must increase as well.
 
OK so you can use the ideal gas law to see that if the temperature increase then the air density decreases. (Note that the atomosphere is not actually an ideal gas, but its good enough for government work)

The lift equation includes density:

L = 1/2 * Cl * rho * v^2 * A

Where:
L = force of lift
Cl = coefficient of lift (which is a constant for the wing airfoil)
rho = density of air (can be approximated with the ideal gas law)
v = airspeed
A = planform area of the wing

So you can see if the density goes down then the velocity must go up to achieve the same amount of lift. An increase in velocity requires an increase in power.
 
flightmedic said:
I think sometimes I should just keep my mouth shut. I haven't even heard the word "Pascals" since college. (Other than Johnny Pascals that I used to work with)
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And you won't again, in flight training. The unit used in aviation (at least in the States) is inches of mercury - on the altimeter settings, vacuum gauges, manifold pressure gauges, etc.
 
chucky said:
Of course, you can use the ideal gas law in other ways. Supposing the temperature doesn't change, if you increase the pressure, the density must increase as well.
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Yeah, but... that's Boyle's Law....:yikes::D
 
Jeanie said:
That reminds me of the humorous email that makes its rounds regarding if Hell is Endothermic or Exothermic.

I'll try to find it and post it on Friday is Joke Day thread unless it's already there and I've fogotten
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Well, you know heat is faster than cold,

because you can always catch cold.
 
Man, there are a lot of hot wind bags in this thread
 
If you cool us down, we get dense.
 
Now Boyles Law I know. It is something we need to understand when flying patients at altitude. Not that we fly patients much higher than 500 feet in Florida.
 
Wiki seems does pretty good and simple explanation :

The density of air, ρ (Greek: rho) (air density), is the mass per unit volume of Earth's atmosphere, and is a useful value in aeronautics and other sciences.

Air density is perhaps the single most important factor affecting aircraft performance. It has a direct bearing on:http://en.wikipedia.org/wiki/Density_altitude#cite_note-A-0

  • The lift generated by the wings — reduction in air density reduces the wing's lift.
  • The efficiency of the propeller or rotor — which for a propeller (effectively an airfoil) behaves similarly to lift on wings.
  • The power output of the engine — power output depends on oxygen intake, so the engine output is reduced as the equivalent "dry air" density decreases and produces even less power as moisture displaces oxygen in more humid conditions.
 
flightmedic said:
Now Boyles Law I know. It is something we need to understand when flying patients at altitude. Not that we fly patients much higher than 500 feet in Florida.
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That's interesting - that hadn't occurred to me as an issue, but of course it makes sense. But if you understand that, you understand all you need to know about the ideal gas law, at least as far as you need it to understand airplane performance.

There's another issue hiding in the ideal gas law that you'll need to understand, though, and that is the effect of temperature, density, and pressure on the altitudes we measure.
 
flightmedic said:
Now Boyles Law I know. It is something we need to understand when flying patients at altitude. Not that we fly patients much higher than 500 feet in Florida.
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Interesting...explain please.
 
flightmedic said:
Now Boyles Law I know. It is something we need to understand when flying patients at altitude. Not that we fly patients much higher than 500 feet in Florida.
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Right, Ideal Gas Law is a combination of Boyle's (pressure) and Charles's (temperature) Laws so it takes care of both in one equation.
 
Jaybird180 said:
Interesting...explain please.
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If someone has a chest injury and a collapsed lung, that means the space where the lung was is now full of air (sometimes blood). As you climb in altitude those gases will expand, thereby collapsing the lung even smaller and causing a respiratory compromise.
 
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