TEMPERATURE MEASUREMENT

Drag—total drag DT consists of induced drag DI and parasite drag DP

DT

DI DP

Increases with AOA mainly profile and skin-friction drag resulting from ram air pressure q = V2/2;

increases proportional to V2

Onboard Air Temperature Sensing System includes temperature probe outside the A/C temperature gauge in the cockpit probe-to-gauge hardware/software

Errors in Temperature Sensing result from ram air pressure (q = V2 /2) heating air in the boundary layer near the temperature probe boundary layer air heating the temperature probe itself

The result is a cockpit temperature reading that is higher than the ambient air just outside the boundary layer, unless the temperature sensing system automatically corrects the error.

Ambient Air Temperature (AAT)—the air temperature just outside the boundary layer surrounding the aircraft. A pilot desires the cockpit gauge to read AAT

Indicated Outside Air Temperature (IOAT)—the temperature displayed on the cockpit temperature gauge

IOAT ≥ AAT due to probe heatingIf IOAT = AAT, either there is negligible ram air heating or the system has automatically corrected the error.

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Temperature Ram Rise (TRR)—theoretical expected temperature rise due to ram air heating—proportional to V2 (TAS2) since q = V2 /2

In the figure below, TAS2 M2 for fixed altitude in SA (M = TAS / a)

From the Graph: TRR at Mach 1 = 50O C TRR at Mach 2 = 200O C = 22 (TRRMACH 1) TRR at Mach 3 = 450 C = 32 (TRRMACH 1) TRR at Mach 4 = (Can you figure it out?)

If IOAT > AAT then AAT = IOAT – RC (TRR) RC is the recovery coefficient (0.0 ≤ RC ≤ 1.0) RC reflects the decimal fraction of TRR that must be subtracted from

IOAT to get AAT When RC = 1.0, AAT = IOAT – TRR. In this case, IOAT is called Total

Air Temperature or Total Temperature for short The term Total Temperature (or just Total Temperature) occurs on

several performance charts used in AS310

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A pilot typically uses a table or chart to correct IOAT to AAT: Locate the column corresponding to indicated Mach number Move down that column to observed IOAT (total temperature) Move left across to column 2 (labeled 0 Mach) to read AAT The number in column one is the PA corresponding to AAT in a SA Example: IMN = 080; IOAT = -13oC; AAT = -42o C; PA in SA is

FL290 (flight levels assume std. pressure 29.92) Note: 15o C – 29 (2o C) = (15 – 58)o C =-43o C (as opposed to -42 in

B767 table and -42.455 in SA Table)

Figure 11.3. Temperature Correction Table for Boeing 737-300

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AIRSPEED INDICATION

Pitot Static System T is total pressure; P is static pressure; q is dynamic pressure Computes q = T – P = V2 /2 and displays it as airspeed Thus the displayed A/S is a function of the density of the air mass where

you are flying and the square of your TAS V through the air mass

Indicated Airspeed (IAS)—displayed on the A/S indicator: an indirect measure of ram air pressure q

Calibrated Airspeed (CAS)—IAS corrected for pitot-static system error: Inherent system error (all measuring systems have some error) Installation error (turbulent airflow at pitot tube and static port) Both errors are aircraft system dependent Errors are typically small percentage-wise, or system would be unusable Except for test pilot work, for example, assume CAS = IAS without

resultant intolerable performance errors

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Equivalent Airspeed (EAS)—CAS corrected for air compressibility Often called “perfect IAS” because EAS determines Lift and Drag Air can compress in the pitot tube, causing total pressure T to be

abnormally high Air compression always causes the A/S indicator to read too high EAS = CAS – compressibility correction Even at subsonic speeds, errors can be large and are NOT negligible

Example 1: FL300; 350 KCAS; EAS = 350 – 22.5 = 327.5 KEASExample 2: FL 400; 268 KCAS; EAS = 268 – 18 = 250 KEAS

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True Airspeed (TAS)—EAS corrected for non-standard air density q = V2 /2, so when air density is low, A/S indicator reads lower than

actual TAS—at higher altitudes, MUCH lower! TAS = EAS / = EAS (SMOE) These “errors” can be VERY large; e.g. at FL 400 in SA, TAS = 2.055

(EAS)

Example 1: A/S = 500 KIAS at SL in SA Assume CAS = IAS: CAS = 500 KCAS At SL in SA, no compressibility correction, so EAS = CAS – 0 = 500

KEAS TAS = EAS (SMOESL) = 500 KEAS (1.0) = 500 KTAS

Example 2: A/S = 268 KIAS at FL400 in SA Assume CAS = IAS: CAS = 268 KCAS EAS = CAS – compressibility correction = 268 KCAS – 18 = 250 KEAS TAS = EAS / FL400 = 250 KEAS / 0.24617 = 503.87 KTAS or TAS = EAS (SMOEFL400) = 250 KEAS (2.0155)= 503.88 KTAS

Example 3. Takeoff in air transport jet at 14,000’ MSL in SA. Liftoff speed = 151 KIAS

CAS = IAS = 151 KCAS EAS = CAS – compressibility correction = 151 – 1 = 150 KEAS TAS = EAS (SMOE14,000) = 150 (1.2403) = 186.05 KTAS

o A/C responds to control input like its going 150 KTSo However, ground speed on liftoff is 186 KTSo Higher T/O airspeed (186 KTAS)o Reduced jet thrust. at higher density altitudeo Result is a MUCH longer T/O time and distance compared to SL

takeoff at the same gross weight

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Determining Mach Number Corresponding to Airspeed Know IAS, can find TAS and then use TAS to compute Mach number

M = TAS / a = TAS / (a0 ) However, can also find M from EAS using formula M = EAS (a0 )

(derivation of this equation is in course text) Example:

From previous page, if EAS = 250 KEAS; TAS = 503.88 KTAS at FL400 in SA

M = TAS / aFL400 = 503.88 / 573.80 = 0.878 (rounded) M = EAS / (a0 FL400) = 250 / (661.74 0.18509) = 0.878 (rounded) Note: if you know EAS but not TAS, use the second formula to avoid

having to convert EAS to TAS

Airspeed-Mach Number Chart for Navy F14 Tomcat Example IAS = CAS = 325 KCAS at FL300; IOAT = -40O C From the chart:

o M = 0.845o TAS = 520 KTAS

The chart makes the correction from PA to DA and from IOAT to AAT The chart has “trace” lines (black dashed lines) indicating how to use it

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Figure 3.6. Navy F14 Tomcat Airspeed Conversion Chart.

1. Observe IAS, IOAT, and PA.2. Assume CAS = IAS, or computer CAS from observed IAS using an

aircraft specific chart or graph.3. On the bottom horizontal axis, locate CAS.4. Move upward vertically to intersect the pressure altitude line.5. Move horizontally left to read Mach number on the left vertical axis.6. Move horizontally right to intersect the 0 PA reference line.7. Move downward vertically to intersect the IOAT line.8. Move horizontally right to read TAS on the right vertical axis.

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