Explaining density altitude to a student

A

azaviator08

New Member
I was wondering if I could get some advice explaining density altitude to one of my students.

He understands that the higher we fly and the hotter the temperature the less dense the air is. But, what he doesn't understand is why. I told him that there is less air molecules for the prop to work with so we will get less work out of the engine etc. I also talked about it taking a bigger bite out of the air since there is more air molecules.

He seems to think that since there is less air molecules we will get less drag so then we should get better performance. I told him we are getting less drag because we are getting less lift.

Maybe if I show him how the prop is an airfoil?? That's the only thing I can come up with so far. Any help simplifying this would be much appreciated.
 
azaviator08 said:
He seems to think that since there is less air molecules we will get less drag so then we should get better performance. I told him we are getting less drag because we are getting less lift.
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He's partially right. We WOULD have less parasite drag if we flew at the same TAS as we do at lower altitudes. However, we end up flying higher TAS for the same IAS, which cancels out the drag reduction due to less dense air.

And we don't get less lift at high density altitudes for the same reason as above...we're actually flying faster than indicated, so we have the same lift we'd have at lower altitudes.
 
Tell him that an Engine needs three things for it to work, Air, Fuel, Spark. the higher up we go, the less air there is (hence why an engine wont work in space) therefore, if there is less air, we need to have less fuel to keep the fuel/air ratio the same. So less air = less fuel = less performance.

If you have a Turbo engine (one that pressurizes the engine with more air) It will actually go faster when higher because the engine performs at the same power, yet there is less drag because there is less air.
 
tgrayson said:
And we don't get less lift at high density altitudes
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I would be careful claiming that to a student. With speed constant we do have less lift because the air density is less. We know from the lift formula that density has a direct effect on lift.

You seem to assume that the TAS will increase exactly enough to keep the lift the same without IAS changing. I cannot imagine how you can know that.


OP: Ask him how he thinks he can perform at a high altitude. This will work especially well if he has gone skiing/snowboarding above 10,000 feet.
 
shdw said:
You seem to assume that the TAS will increase exactly enough to keep the lift the same without IAS changing. I cannot imagine how you can know that. .
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I don't assume, I know. This is where a naive interpretation of the lift formula leads you to incorrect conclusions.
A little learning is a dang'rous thing;
Drink deep, or taste not the Pierian spring:​
Alexander Pope
​
 
Newton's third law...every action has an equal and opposite reaction.

What makes the plane go forward? The propeller displacing air ahead of the aircraft...If there's less air to move, there's less force acting on the aircraft. With less force, you lose performance.

Obviously, it's an overly simplified explanation, but it should at least theoretically explain the situation. :dunno:
 
element94 said:
If there's less air to move, there's less force acting on the aircraft. With less force, you lose performance.
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As someone else pointed out, turbocharged aircraft do just fine. The loss of performance is due primarily to the reduction in mass flow rate of air through the engine, a problem that a turbocharging system can delay to much higher altitudes.
 
Did I mention it was a gross over-simplification? :rolleyes:

In the cylinders the same effect is what causes a decrease in performance. Less total pressure (as the result of less mass per unit volume of air, in combination with reduced thermal expansion as a result of a less energetic chemical reaction) acting upon the cylinder head driving the crankshaft.

The mechanical inefficiencies caused by friction and design simply amplify its effects, causing the engine to be the most limiting factor rather than the raw aerodynamics of the propeller.

However, it's still the propeller that produces the direct decrease in performance, not the engine itself. I find when explaining esoteric concepts, analogies work best. An engine is like a runner, the higher it goes, the harder it is to breath. Just like the runner, the engine slows down. A slower prop displaces less air, and a prop that displaces less air exerts less force on the aircraft it's propelling. So, in a direct sense, what I said above was correct :D
 
azaviator08 said:
He seems to think that since there is less air molecules we will get less drag so then we should get better performance. I told him we are getting less drag because we are getting less lift.
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He's right about the less drag, but when you consider that not only can you not get as much power out of the engine (assuming a normally aspirated motor), but you're also not going to get as much out of the prop due to the decreased density, the net result is a decrease in performance.

If you have a turbo, then you just have the prop effects to worry about. A constant-speed prop, which most turbocharged aircraft will have, helps out on that front by letting the prop set itself at a more appropriate angle of attack for the less dense air, but even that will eventually run out of effectiveness as it gets up against its pitch limits, and then you'll start to see a decrease in performance.

And you're not getting less lift. Cruising at 9,000ft, your airplane has to produce enough lift to counter its weight, just as it would if you were cruising at 3,000ft. So, assuming the weight of the aircraft at 9,000 and the aircraft at 3,000 are the same, they have to be generating equal amounts of lift in order to maintain level flight.

The aircraft at 9,000ft will be forced to make up for the decreased density by either increasing its angle of attack or (more likely) flying at a higher TAS.

The less drag is of the parasite variety, not the induced variety - you're still generating the same amount of that.
 
shdw said:
Show it?
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Aren't you guys saying the same thing? (That as air density decreases you need a higher TAS to produce the same amount of initial lift, yet since IAS is based on ram air it would indicate the same value in both cases?) Or am I missing something? :)

Edit: No one has mentioned explaining how temperature affects density altitude (maybe too OT). But since temperature is essentially the measurement of molecular vibration (or the speed of gas molecules, Urms etc), you could say that faster moving air molecules spread out due to more frequent collisions with one another and thus air density decreases as temperature increases - just as if your altitude were increased. Haven't had much in the way of thermodynamics so forgive me if it's way off. :)
 
inigo88 said:
Or am I missing something?
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Not exactly. I am saying that with airspeed constant and air density decreasing the lift will be reduced, talking in TAS. I think me and tgray both agree on that part at least.

We disagree on the claim that TAS will increase/decrease exactly enough to counter the change in air density, leaving IAS unchanged. My conflict is this assumption seems to rely on a system, the static system, being perfectly calibrated.

I guess the question I am looking to have answered now is: If we take an aircraft at 1,000 feet and one at 10,000 feet given: AOA constant, lift constant, and TAS changed accordingly to counter the air density change. Given this scenario how do we know IAS will remain unchanged?

Does that help?
 
Xiph said:
The less drag is of the parasite variety, not the induced variety - you're still generating the same amount of that.
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The less drag is only for a given TAS; since IAS would be lower at a higher altitude for a given TAS, you'd have higher induced drag.

For a given IAS, drag will remain a constant at all altitudes, since the increased velocity compensates for the reduced air density to keep the parasite drag the same.
 
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