Pressure altitude vs Density altitude

[cocolos]

Could someone give me a quick explanation.The way I see it density altitude is the important one because it gives you the pressure altitude adjusted for non standard temp, which tells you what performance you would actually get. Where pressure altitude just gives you your altitude at standard pressure which could be different depending on the temp, right?

 

[PilotAlan]

Pressure altitude is a function of pressure. Density altitude is a function of density.
I know that sounds pedantic, but follow me here. They are measuring different things.

Hydrogen and oxygen have different densities, right? You can compress hydrogen to a certain pressure, and oxygen to the same pressure, but their densities will be different, right?
The same is true for air. Cold air will have a higher density than warm air, at the same pressure.

You altimeter reads pressure only, density is irrelevant. Your engine, prop, and wing perform based on density, pressure is irrelevant.
So both are important, as one effects the reading of your altimeter, and one effects your performance.

 

[denverpilot]

Alan! Good to see you!

Remember, pressure altitude is the altitude above sea level that you would be at on a day when the atmosphere is also standard. That rarely happens.

But... In order to understand the relationship...

We set the altimeter to 29.92 and read it. Let's say today truly is a perfect standard day. The altimeter matches our map and we are at ground level sitting in an airplane on the ramp.

Pressure and temperature are the same everywhere today, and the Earth is a boring place with no weather at all.

The air all around the globe today is a perfect spheroid that matches the planet's shape. A Standard Atmoshphere.

Pressure is (essentially) the weight of the air stacked on top of you. Gravity pulls it down on your head. Heavy isn't it? ;-)

Now today you have a magic knob that you can turn to heat or cool all the air around the planet in one twist instantly.

Turn that knob and heat up that air. What happens?

The air molecules become excited and push each other further apart. The column of air above you has nowhere to go but up. (Let's pretend for a sec, it can't move sideways. We'll get to that in a minute.) All the air around the planet expands upward because it can't go down or sideways.

So it goes up. The column of air above you becomes taller. The spheroid gets bigger instantly.

Here's the kicker: It's weight on your head doesn't change. Still the same amount of air molecules above you as there were before you turned up the temperature knob. Pressure remained the same.

But those molecules are further apart now. When you accelerate a wing (or just walk) through them, there's less of them flowing over the wing (and you) in the same distance, and lower performance results.

That's showing the difference in density.

That's pressure vs density. Raise the air temperature the air molecules get further apart. They don't change their mass/weight.

Think of it like a slinky. The slinky can either be tightly together and held in your hand, or hung from your hand off the ground. It still weighs the same amount.

Pressure measures how heavy the air is above you. Density measures whether or not you can see space between the molecules like between the rings of the hanging slinky.

Now you're saying, in the real world if something magically heated the air above my head, it would move upward, but it would also start flowing outward away from the heated area!

And you're right. That's the key to all weather. Uneven heating of the Earth's surface! Figuring out where that unevenly heated air (and the water vapor it contains) will go, is a whole lifetime of thought, welcome to Meteorology.

Cool, huh? Weight vs space between molecules.

 

[murphey]

Or simply put - the hotter it is, the lower performance (lower HP) because the engine is "operating" at a higher altitude.

Turbo doesn't count in this situation.

 

[denverpilot]

Or simply put - the hotter it is, the lower performance (lower HP) because the engine is "operating" at a higher altitude.

Turbo doesn't count in this situation.

Engine, prop, wings. Not just engine.

Turbo-normalized aircraft still take more runway in higher density altitude than they do in lower, even if you set them so they're putting out identical horsepower in each scenario.

That's the effect of less molecules flowing over wing and prop. Have to accelerate to a faster speed to compensate, and make more molecules flow over the wing, which means a longer ground roll.

For us normally aspirated folks we can simulate it. Do a 65% power takeoff on a cold day and compare it to a 65% power takeoff on a hot day. (Hard part is accurately setting 65% power.) On the cold day, the throttle won't be all the way in. On the hot day, it will.

If power is made to be a constant, the airplane wing will still perform more poorly on the hot day when the molecules are spread apart.

 

[MAKG1]

Engine, prop, wings. Not just engine.

Engine, prop, wings, and pilot.

You'll feel the effects of altitude (including hypoxia) according to density altitude as well.

Basically, physics = density altitude. Class A = pressure altitude. Everything else is indicated altitude.

Note that none of these gives you the actual geometric altitude above the ground (absolute altitude) or MSL (true altitude) unless the temperature is standard, or you're at an airport with an up to date altimeter setting. This can be an issue if using a barometric altimeter to judge terrain clearance.

 

[poadeleted20]

To put it simply, pressure altitude is important to people while density altitude is important to airplanes.

 

[bbchien]

Yes, and that is because in the alveolar lung sac, the temperature is always 37C and the realtive humidity is 100%.

 

[PilotAlan]

Engine, prop, wings, and pilot.

You'll feel the effects of altitude (including hypoxia) according to density altitude as well.

Doc said it, but I'll reinforce it.
Physical (human) effects of altitude are based solely on pressure altitude, not on density altitude.
The partial pressure of oxygen in the atmosphere determines the amount of oxygen permeating across the capillaries in the alveoli.

As the Doc said, it's always a set temp and humidity in the alveoli. So the only remaining variable is pressure.