Explaining density altitude to a student

[Douglas]

This has really helped. Thanks everyone.

You lie!
(just joking)

 

[shdw]

The airspeed indicator measures 1/2(rho)V^2.

Yes and it is measured by the static port, right? That has to be calibrated doesn't it?

My conflict is this assumption seems to rely on a system, the static system, being perfectly calibrated.[/quote

 

[inigo88]

Does that help?

Very much. Thank you!

 

[tgrayson]

My conflict is this assumption seems to rely on a system, the static system, being perfectly calibrated.

No, it doesn't, it just assumes that the system is equally in error in high density altitude conditions as it is in low density altitude conditions. That's a reasonable assumption.

The point is this: we fly indicated airspeeds. For a given indicated airspeed, the airplane exhibits a particular aerodynamic behavior regardless of density altitude. The increased velocity exactly compensates for the reduced density, because the airspeed indicator is affected in exactly the same way as the wing.

The density symbol in the lift equation is an attractive nuisance and causes people to draw some bizarre conclusions. A local, very experienced instructor teaches people to fly higher approach speeds when it's hot outside because "we get less lift", which is absurd to an alarming degree.

 

[CDNPilotDave]

A local, very experienced instructor teaches people to fly higher approach speeds when it's hot outside because "we get less lift", which is absurd to an alarming degree.

... and for the student, an alarming amount of float.

 

[CDNPilotDave]

I agree with what previous posters have said re: lift, prop efficiency, drag etc. Add to that, the appetite of the engines (assume recip). When air density is reduced, there are fewer oxygen molecules so the amount of power the engine(s) can produce is reduced. It's all so limiting (pun intended),

 

[Mike H]

Originally Posted by shdw
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.

I don't assume, I know. This is where a naive interpretation of the lift formula leads you to incorrect conclusions.

When I was planning my long X-Country for my Commercial cert. I considered taking a Diamond DA40 to Rapid City until I noticed on the performance chart that at the expected temperature (+20C) at the planned altitude (9000 MSL), TAS was actually a lot slower than if the temperature was closer to standard. I took the 182RG instead

 

[tgrayson]

TAS was actually a lot slower than if the temperature was closer to standard

Was this a comment on what you quoted?

But anyway, your observation is what one would expect. There is a sweet spot in altitude for a normally aspirated aircraft which seems to be around the 8,000 ft mark. After that, the aircraft can't maintain cruise power and the IAS starts falling, resulting in a lowering in TAS.

 

[msmspilot]

I agree with what previous posters have said re: lift, prop efficiency, drag etc. Add to that, the appetite of the engines (assume recip). When air density is reduced, there are fewer oxygen molecules so the amount of power the engine(s) can produce is reduced. It's all so limiting (pun intended),

Doesn't have to be a recip.

 

[shdw]

seems to be around the 8,000 ft mark. After that, the aircraft can't maintain cruise power and the IAS starts falling, resulting in a lowering in TAS.

This is why I don't think you can make the assumption that the "system is equally in error." I personally don't know the inner workings of an airspeed indicator. However, I suspect it is at least a mechanical instrument. Pressure changes would have some effect on any mechanical instrument, even small changes. This might explain the discrepancy you identify here?

 

[tgrayson]

This might explain the discrepancy you identify here?

What discrepancy? I haven't identified one here. And what pressure changes are you talking about with regard to the airspeed indicator? It's a pressure instrument, so being affected by pressure changes is part of its nature. Any error in the instrument is captured by the IAS to CAS conversion chart, but I still don't see the relevance in this discussion.

My original point remains valid, mathematically so. If you have the proper airspeed, it doesn't matter how dense the air is, except for engine performance.

 

[Fly_Unity]

This is why I don't think you can make the assumption that the "system is equally in error." I personally don't know the inner workings of an airspeed indicator. However, I suspect it is at least a mechanical instrument. Pressure changes would have some effect on any mechanical instrument, even small changes. This might explain the discrepancy you identify here?

I'll have to disagree with you. 140 IAS at 2,000 msl will be identical to 140 IAS at 10,000 MSL according to the pitot tube and airspeed indicator.

 

[SuperCubRick]

Good discussion. TGrayson & Shdw, I get a kick out of seeing you two go back and forth in these technical threads, heh.

The density symbol in the lift equation is an attractive nuisance and causes people to draw some bizarre conclusions. A local, very experienced instructor teaches people to fly higher approach speeds when it's hot outside because "we get less lift", which is absurd to an alarming degree.

Wow, that's terrible :laff: