True Airspeed

S

ssoll7

Filing Flight Plan
Joined
Jul 1, 2021
Messages
2
Display Name
Display name:
kiwen7
Why is true airspeed used in determining lift? I understand why TAS varies with altitude/density -- it is intuitive that for a given CAS, TAS will increase as altitude increases due to a decrease in density.

Let's use an example: Two cubic meters of air, one at S.L., one at 10,000 ft. MSL. Given the same true airspeed in each block of air, I understand that IAS will be much higher for the block at S.L. But why would lift theoretically not differ between the two (assuming all else equal)? Isn't density the very basis of performance?

Am I thinking of this all wrong?
 
ssoll7 said:
Why is true airspeed used in determining lift?
Click to expand...
Let’s start with this…where are you getting this? If we dig further into that resource, we might be able to answer your questions more appropriately.
 
Mostly it's a benchmark so as to compare apples to apples. It's like in population statistics where we break out results "per capita" so that we can compare the populations of Anton, Texas to Houston, Tx.
 
Equivalent (or calibrated if you're slow enough) airspeed is also used in calculating lift. TAS *can* be used but requires additional data. Lift is a function of dynamic pressure, as are equivalent airspeed and true airspeed, but to calculate dynamic pressure from true airspeed requires knowing (or calculating) the local density...which can be calculated from CAS/EAS and OAT. EAS is a *direct* measure of dynamic pressure, so why take the additional (unnecessary) step?

For performance or flutter, sure, TAS is a better measure, but for aero characteristics (lift, drag, etc) it's more work than necessary.

Nauga,
who feels pressured
 
The Lift formula factors in air density. So you are correct, the same wing will not generate the same lift at the same TAS at SL vs. 10,000 MSL. But that's built into the formula. If you start with IAS, you'll get a bollocks result.
 
Lindberg said:
The Lift formula factors in air density. So you are correct, the same wing will not generate the same lift at the same TAS at SL vs. 10,000 MSL.
Click to expand...
The air density term in the lift formula, which is really part of the dynamic pressure equation, uses a density term appropriate to the airspeed being used in the equation. If you use equivalent airspeed, which is calibrated airspeed corrected for compressibility, the density term is sea-level standard day density. If you use true airspeed it is density at the current conditions and altitude. With a little algebraic manipulation you can show that true airspeed is just equivalent airspeed with a temp correction*, so why not skip the whole TAS step and use EAS and SSL density?

*which if rearranged can be shown to be mathematically identical to using EAS and converting density at test conditions to SSL density.

Lindberg said:
If you start with IAS, you'll get a bollocks result.
Click to expand...
Depends on whether or not you apply the position error correction to get it to CAS, and how big that correction is. It's just as true to say if you apply a "TAS" calculated from your airspeed indicator the result is bollocks because (a) it probably doesn't have a good position error correction, and (b) it doesn't have a compressibility correction, but in reality most anyone doing real analysis has the information to do (a), and the compressibility correction in (b) is insignificant at most GA speeds and altitudes and calculable at others. A wing *will* generate the same lift at the same equivalent airspeed at any altitude, and the difference between equivalent and calibrated is less than 1 kt for speeds below 200kt and below 10k feet, and decreases as altitude and airspeed decrease.

Nauga,
ambient
 
Aero engineers use TAS because, as said above, the lift equation includes the density, and we plug in whatever the standard density is at whatever altitude we're considering. And some things, like flutter analysis, have to be done in TAS, and it's best to use consistent units throughout.

IAS (or CAS or EAS), by nature of how it's measured, includes a correction factor for density. Because density affects lift and drag the same as it affects the pitot-static system, IAS is more useful in flight to the pilot, who can expect the airplane to behave a certain way at a certain IAS. TAS is for flight planning and navigation.

How many people in the GA world actually correct IAS to CAS? I haven't needed to do it since my private checkride, and probably before that. But then I only fly low and slow airplanes.
 
Dana said:
Aero engineers use TAS because, as said above, the lift equation includes the density, and we plug in whatever the standard density is at whatever altitude we're considering. And some things, like flutter analysis, have to be done in TAS, and it's best to use consistent units throughout.
Click to expand...
If V-n diagrams and stall speeds used true airspeed they'd change with altitude, and if aero engineers analyzing flight test data used standard density at altitude they'd get garbage. ;) I'm with you on TAS for flutter though, so we got that goin' for us. Which is nice.

It's best to be careful and consistent with units, but it's also wise to use data appropriate to the analysis you're performing.

Nauga,
and horses for courses
 
During flight test, yes, of course you have to use the actual density, and depending on what you're doing you may or may not convert IAS to TAS. But I've never worked in flight test so I don't know what the protocols are.
 
ssoll7 said:
Why is true airspeed used in determining lift? I understand why TAS varies with altitude/density -- it is intuitive that for a given CAS, TAS will increase as altitude increases due to a decrease in density.

Let's use an example: Two cubic meters of air, one at S.L., one at 10,000 ft. MSL. Given the same true airspeed in each block of air, I understand that IAS will be much higher for the block at S.L. But why would lift theoretically not differ between the two (assuming all else equal)? Isn't density the very basis of performance?

Am I thinking of this all wrong?
Click to expand...

Why would anyone give a darn about indicated airspeed when calculating lift?
 
You are thinking from a false perception. We as pilots use airspeed indicators, but indicated airspeed is not airspeed at all. It is a pressure gauge calibrated to read airspeed at one air density, sea level and standard temperature. Every other situation means the ASI pressure pseudo airspeed is lying to you all the time. TRUE Airspeed is actual airspeed and that is why it is used in calculations of lift. To an engineer the ASI does not exist... because it doesn't. The ASI is a convenience for the pilot, and has no relevance to the engineer. The cockpit does not exist and is unnecessary to calculate aircraft performance.
 
William Pete Hodges said:
TRUE Airspeed is actual airspeed and that is why it is used in calculations of lift.
Click to expand...
*sigh*
These equations are equivalent:

L = 1/2 * rho_ssl * Ve^2 * S * CL
L = 1/2 * rho_amb * Vt^2 * S *CL

L is lift
rho_ssl is sea level standard day density
rho_amb is local density where the measurement is taken
Ve is equivalent airspeed, which is calibrated airspeed corrected for compressibility, which is very close to calibrated airspeed at light GA speeds and altitude
Vt is true airspeed
S is reference area and CL is lift coefficient

Many engineers use equivalent airspeed to calculate lift because it does not require knowledge of the atmospheric conditions where a measurement was taken - handy for things like flight test data correction and model validation.

Aero engineers use measured, observed, indicated, calibrated, equivalent, and true airspeed in different ways for different things, and occasionally for the same thing...but they sure as heck don't think anything other than true has no relevance.

Nauga,
who is amused when people tell him how he does his job
 
Last edited:
nauga said:
The air density term in the lift formula, which is really part of the dynamic pressure equation, uses a density term appropriate to the airspeed being used in the equation. If you use equivalent airspeed, which is calibrated airspeed corrected for compressibility, the density term is sea-level standard day density. If you use true airspeed it is density at the current conditions and altitude. With a little algebraic manipulation you can show that true airspeed is just equivalent airspeed with a temp correction*, so why not skip the whole TAS step and use EAS and SSL density?

*which if rearranged can be shown to be mathematically identical to using EAS and converting density at test conditions to SSL density.

Depends on whether or not you apply the position error correction to get it to CAS, and how big that correction is. It's just as true to say if you apply a "TAS" calculated from your airspeed indicator the result is bollocks because (a) it probably doesn't have a good position error correction, and (b) it doesn't have a compressibility correction, but in reality most anyone doing real analysis has the information to do (a), and the compressibility correction in (b) is insignificant at most GA speeds and altitudes and calculable at others. A wing *will* generate the same lift at the same equivalent airspeed at any altitude, and the difference between equivalent and calibrated is less than 1 kt for speeds below 200kt and below 10k feet, and decreases as altitude and airspeed decrease.

Nauga,
ambient
Click to expand...
I am sure your answer is accurate and complete, but I read the OP's question as being why air density isn't taken into account when calculating lift if TAS is used. That question is simply based on a false premise. If TAS is used, then air density is input as a separate term.
 
Lindberg said:
I am sure your answer is accurate and complete, but I read the OP's question as being why air density isn't taken into account when calculating lift if TAS is used.
Click to expand...
Fair enough, we interpreted the OP in different ways. Subsequent posts, however, seem to state that using TAS is the *only* way (or the only way that matters), which is more how I interpreted the OP and what I took issue with.

Nauga,
whose density is variable
 
All this talk about Lyft reminds me I have some expense reports to finish.

nevermind.
 
Dana said:
How many people in the GA world actually correct IAS to CAS? I haven't needed to do it since my private checkride, and probably before that. But then I only fly low and slow airplanes.
Click to expand...

I do that, not for cruise, but for calculating stall margin. 1.3*Vso can be way off (in the wrong direction) if you use IAS instead of CAS.
 
A lot of complicated answers when it’s simple. TAS is not used for calculating lift.
 
sarangan said:
I do that, not for cruise, but for calculating stall margin. 1.3*Vso can be way off (in the wrong direction) if you use IAS instead of CAS.
Click to expand...
Can you provide a material example?

Tim

Sent from my HD1907 using Tapatalk
 
You must log in or register to reply here.
Back
Top