airspeed indicator

[mstram]

Given "standard day" conditions, what is the relationship between impact pressure on a pitot tube and the indicated airspeed ?

I found this formuala :

velocity = square root of (2 x [impact pressure - static pressure] / air density).

To solve for Vel = 1, (1 what ?? .. ft/sec ?), I used Static= 14.7 lb/in^2, air_density= .00237 slug/ft^3

got impact_press = 14.701185 lb/in^2

Is that correct ? (14.701185 lb/in^2 pressure difference = 1 ft/sec ?

**** edited, note started with 14.7 ***
Is that correct ? (.001185 lb/in^2 press diff = 1 ft/sec ?
****

If so, how would I solve for Vel=XX. I realize it's a "basic" math question, but basic math is not one of my strong points ;)

Mike

[Genghis the Engineer]

The difference between Pitot and Static is q, which is calculated as ½.Rho.V²

So, V in this case is identical to TAS.

So, q = ½.Rho.TAS²

Re-arranging this, you get TAS = (q / ½.Rho)^½


So that is the relationship between TAS and q. But the difference between TAS and CAS is given by...

CAS = TAS * Sigma^½

or

CAS = TAS * (Rho/Rho_0)^½

or

TAS = CAS * (Rho/Rho_0)^-½

[Reminding you, Rho = local air density, Rho_0 = sea level air density]


So substituting this into the earlier result, you get...

CAS.(Rho/Rho_0)^-½ = (q/½Rho)^½

This is easier to digest if you square the lot...

CAS² * Rho_0 / Rho = 2q / Rho

Multiply through by Rho and you've eliminated anything altitude dependent from the equation, and you get

CAS² = 2q / Rho_0


Which is fairly close to a relationship for IAS, since for any CofA aeroplane CAS and IAS are supposed to be within 5 knots of each other from 1.3Vs to Vne.

Hope this helps, the formula will work with either of the two main unit systems, viz...

Airspeed in m/s, pressure in N/m², density in kg/m^3
or
Airspeed in fps, pressure in lb/ft², density in slugs/ft^3 (Pretty certain I got the units right in the latter, I tend to think in metric when doing sums)


G

[Onan the Clumsy]

Yeah. What Genghis said. :}

[mstram]

Thanks Ghengis.

I'll "chew" on that for a while ;)