airspeed indicator
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 |
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 |
Yeah. What Genghis said. :}
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Thanks Ghengis.
I'll "chew" on that for a while ;) |
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