Appendix 8
ANGULAR DISPLACEMENT FORCE Multiply By To Obtain Degrees -21.745 x 10 Radians Radians 57.3 Degrees Multiply By To Obtain Pounds 4.448 Newtons A8-6
Appendix 9 Appendix 9. Airspeed Calibrations Introduction The airspeed and altimeter systems on an aircraft depend upon accurate measurements of ambient static pressure and total pitot pressure. Static and pitot pressures are sensed by the pitot static tube, which gives true readings in an undisturbed freestream when aligned with the flow streamlines; however, when attached to the aircraft, which generates a pressure when flying, the pitot and the static reading will be affected by the aircraft pressure field and the flow angularity. The errors, caused by the pressure field and by flow angularity are called position errors due to the sign and magnitude of the errors, which are a function of the position of the pitot-static probe on the aircraft. The position errors are a function of aircraft angle of attack and Mach number and are determined from flight test.
In this text, corrections are used rather than errors. Normally, errors are subtracted and corrections are added with the result that the position error corrections (PEC) are added to the aircraft pitot-static data to get the ambient conditions of static and pitot pressures. The ambient static pressure is defined as PSref and the ambient pitot pressure is defined as PPref. The position error correction of the static source PS is defined as P P PS Sref SA / C and PP the position error correction for the pitot pressure is defined as P P Pp Pr ef PA / C The total position error correction for a pitot static system to be used for an airspeed system is Pd where Pd PP PS General Discussion of the Various Flight Test Techniques Each of the flight test techniques (FTT's) that are described in this appendix have certain limitations and instrumentation accuracy criteria that must be considered prior to selecting a flight test technique.
The trailing bomb method only calibrates the aircraft static source. The bomb must be stable when flying below and behind the aircraft, any oscillations will make the reference static pressure invalid. At high speeds the bomb tends to rise up into the wake of the aircraft, which causes bomb oscillations; therefore, the trailing bomb has an upper airspeed limit. The trailing bomb is useful for most speeds up to approximately 200 knots and is particularly useful for helicopters. In addition, the trai ling bomb can also be used down to stall speeds. The trailing bomb deployed behind and below helicopters tends to keep the bomb and the attaching tube clear of the tail rotor; however, care must be taken when expanding the speed envelope.
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Appendix 9 The trailing cone method is capable of a much higher speed range than the trailing bomb and is a favorite method with the large aircraft manufacturers. The trailing cone method only calibrates the aircraft static pressure system.
The speed course method calibrates the airspeed indicator and considers the position error correction for both the static and pitot pressures. Use of the speed course data to calibrate the altimeter makes the assumption that the total position errors of both the pitot and the static sources are in the static source only. This assumption may not be correct. The main source of error in the ground course FTT is in timing since a stopwatch is used to record the time. Figure A9-1 shows the effect of aircraft airspeed on airspeed error with various length ground courses due to a 0.5 second timing error. Obviously, if the maximum error is limited to one knot, then the maximum speed for a three mile ground course would be about 120 knots. Essentially, the ground course method is suitable for slow moving aircraft.
The Global Positioning System (GPS) method is essentially the same as the speed course method except that the GPS unit replaces the stopwatch and, therefore, the timing errors are not an issue.
Satisfactory results have been obtained with a wide variety of inexpensive, handheld GPS units.
The best indication of the validity of data gathered using this method is the calculated wind.
Wind speed and direction are calculated as a by-product of the method. If the calculated wind is constant over a range of test points, then the calculated true airspeed can be considered valid. A significant advantage of this method is that it can be done at higher altitudes, allowing tests near stall speed. A disadvantage is that the relatively small speed error obtained will be the result of subtracting two larger numbers (actual true airspeed less the indicated airspeed correct to true).
At higher speeds, small percentage errors in true airspeed will have a larger impact on the position error.
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i I o~~::::::::::]:_ __ ..,L ___ ,L_ __ _L ___ ~f ISO 100 Ampeed(kts) 11/16/2011 AC 23-8C Appendix 9 Figure A9-1 - Error Analysis of Ground Course Method The pace aircraft technique for pitot static calibration is often the initial calibration method for the first flight of a new aircraft or the first flights of extensively modified aircraft. The problem with the pace aircraft method is the accuracy of reading both the altimeter and airspeed indicators in both aircraft simultaneously, and any errors in the pace aircraft are transferred to the test aircraft.
The pitot-static boom method is a standard for small aircraft; however, prior to use, it must be established that the boom static source is outside the pressure field of the aircraft and the pitot tube is unaffected by the flow angularity at the boom.
The tower fly-by method only calibrates the aircraft static source and, if the data are used to calibrate the airspeed systems, the assumption is that the pitot has no errors. Accuracy problems exist with the tower fly-by method if altimeters are used in the tower and in the aircraft. The reading accuracy of an altimeter is generally + 10 feet; therefore, the combined error of both altimeters could be + 20 feet, which is very close to the FAR/JAR limits of + 30 feet per 100 knots. The use of sensitive pressure transducers in the tower and the aircraft considerably improve the reading accuracy. An additional improvement in accuracy can be obtained by taking aircraft ground block data at the base of the fly-by tower, i.e., record the altimeter and temperature and compare the tower data taking into consideration the height o the tower.
The tower fly-by method is also useful as measuring the recovery factor of temperature measuring systems. The serious limitations of the tower fly-by method are: the requirement for an instrumented tower and a fly-by line, the hazard of flying near the stall speeds and the Mach limits of the aircraft close to the ground and the time consuming procedure of one data point per aircraft circuit.
A summary of the speed ranges for various PEC flight test techniques is shown in Figure A9-2.
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-100 0 100 200 300 V, (kts) 1100 Trailing Static Bomb Appendix 9 Figure A9-2 - Summary of PEC Test Methods
1. Trailing Bomb or Cone Method. A trailing bomb or cone, as depicted in Figure A9-3 is used to measure the static pressure of the ambient air about the aircraft. The trailing bomb is sufficiently behind and below the aircraft and the trailing cone is sufficiently far behind the aircraft to be unaffected by the pressure field around the aircraft and can, therefore, be referred to as the reference static pressure (PSref).
Figure A9-3 - Sketches of Trailing Static Bomb and the Trailing Static Cone (not to scale) A trailing bomb or cone can be used to calibrate the aircraft static source or to determine the Position Error Correction (PEC's) for the altimeter. The use of the reference static sources to calibrate the airspeed systems assumes that the errors in the total head (pitot tube) are zero. The reference static sources could be connected to the altimeter, which would read the pressure altitude of the aircraft. The difference between the reference altitude from the trailing cone or bomb and the aircraft altitude, both corrected for instrument errors, would be the position error correction for the altimeter H pec for a particular aircraft configuration and speed.
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--- Appendix 9 H pec (H ref Hic ) (HiA / C Hic ) Where Href is Reference altitude Hic is the instrument correction to the altimeter HiA/c is the indicated aircraft altitude The above altimeter method is simple but suffers from the difficulty of accurately reading an altimeter, with altimeter calibration errors and hysterisis. Hysterisis is the difference in altimeter calibration with the altitude increasing and decreasing.
A more accurate technique is to connect the trailing static source and the aircraft static source to a pressure differential gauge so that the pressure difference Ps can be read directly, i.e., P P Ps sref sA / C where Psref is the reference static pressure and PsA/C is the aircraft static source pressure Note that the ( PS ) as expressed above is a correction which must be added to the aircraft static pressure (PS) to get the reference static pressure. The ( P ) data in lb/ft2 can be converted to H data in feets pec by the use of the pressure static equation:
PSP gH pec orH pec S g Units PS in lb/ft2 H in ft in slugs/ft3 Where g is the gravitational constant 32.2 ft/sec2 and is the density of the air in which the aircraft is flying. H pec can be determined throughout the speed range of the aircraft in all configurations and plotted as shown in Figure A9-4:
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Appendix 9 Figure A9-4 - Typical Position Error Correction Data for an Aircraft The FAR/JAR 23.1325 limits of +30 feet per 100 knots are also shown on Figure A9-4.
The Trailing Static bomb and cone can be used to calibrate the airspeed systems, if it is assumed that the total head (pitot tube) has no errors. The total position error correction for a pitot-static system is defined as Pd where P P Pd p S where Pp is the pressure correction for the total head due to flow angularity A9-6
Fiapso• Appendix 9 P p PPref PPA C/ If PP is assumed to zero, then Pd PS 1 2 2V 1 C 2M C 4 4M C 40 1 2 2V 1 ic 2M ic 4 4M ic 40 where Vc and Vic are in ft/sec.
For low speed aircraft that fly at speeds of less than 200 knots and at altitudes less than 10,000 feet, the compressibility corrections can be ignored and the above equation reduces to:
1 2 2 P P oV Vd S c ic 2 Where Vic is the indicated airspeed of the aircraft corrected for instrument errors and Vc is the calibrated airspeed corrected for instrument and position errors.
V V Vpec c ic Knowing the Ps for each indicated speed of the aircraft (Vi), then plots of position error corrections for the airspeed system can be generated as shown in Figure A9-5.
Figure A9-5 Typical Position Error Corrections Data for an Aircraft The FAR/JAR 23.1323 limits of +5 knots or +3 percent, whichever is greater are also shown in Figure A9-5.
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Appendix 9
a. Test Conditions.
(1) Air Quality. Smooth, stable air is needed for calibrating the airspeed indicating system using a trailing bomb or trailing cone.
(2) Weight and C.G. Same as speed course method. See paragraph 2a(2) of this appendix.
(3) Speed Range. The calibration should range from 1.2 V STALL to VMO/VNE or maximum level flight speed, whichever is greater. If the trailing bomb becomes unstable at high airspeed, the higher airspeed range may be calibrated using another accepted method; that is, trailing cone or pace method.
(4) Use of Bomb. Care should be exercised in deploying the bomb and flying the test to ensure that no structural damage or control interference is caused by the bomb or the cable.
At higher speeds, the bomb may become unstable and porpoise or oscillate. A means for a quick release of the trailing bomb should be provided, in the event an emergency arises.
Flight tests using a bomb should be conducted over open (unpopulated) areas.
(5) Free Stream Air. The bomb hose should be of adequa te length to assure bomb operations in free stream air. This should include consideration of all airplane test configurations that could possibly impart body interference upon the bomb. It will usually require that the bomb be at least one-half wing span away from the airplane.
(6) Qualifications For Use. Under stabilized flight conditions at constant airspeed and altitude, trailing cones and airspeed bombs are considered excellent airspeed reference systems. See paragraph 17b of this AC for additional discussion.
b. Test Procedures.
(1) Stabilize airplane in level flight approximately 30 seconds just above stall with flaps and gear retracted. Record data.
(2) Repeat step (1) at sufficient increments to provide an adequate calibration curve for each of the configurations.
c. Data Acquisition. Data to be recorded at each test point:
Altimeter Method Pressure Differential Method
1. Airplane Airspeed (V i) 1. Airplane Airspeed (V i)
2. Airplane indicated altitude (H iA/C) 2. Airplane indicated altitude (H iA/C)
3. Trailing Cone/Bomb altitude (H iref) 3. Pressure Differential P P PS Sref SA C /
4. Flap position 4. Flap position
5. Landing gear position 5. Landing gear position
6. Fuel used 6. Fuel used A9-8
Appendix 9
d. Data Analysis. The data are analyzed according to the methods and equations presented above. The data could be presented in the form as shown in Figures A9-3 and A9-4. Data that fall outside the FAR/JAR limits fail the airworthiness codes.
2. Speed Course Method. The speed course method consists of using a ground reference to determine variations between indicated airspeed and ground speed of the airplane. An accurately measured ground course is required. The course distance should be se lected to be compatible with the airspeeds being flown. Excessively long times to traverse the course will degrade the test results.
Generally, airspeeds above 250 knots should be flown over a 5-mile course. Below 100 knots, limit the course to 1 mile. Perpendicular "end lines" (roads, powerlines, etcetera) should be long enough to allow for drift and accurate sighting of end line passage. A one-second error at 200k is 6k on a 2-mile course.
a. Test Conditions.
(1) Air Quality. The air should be as smooth as possible with a minimum of turbulence and wind. The wind velocity, while conducting the test, should not exceed approximately 10 knots.
(2) Weight and C.G. Airspeed calibrations are usually not c.g. sensitive but may be weight sensitive, especially at low airspeeds (higher angles-of-attack). Initial airspeed calibration tests should be conducted with the airplane loaded at or near maximum takeoff gross weight.
Additional tests should be conducted at near minimum weight and at low airspeeds to spot check the maximum weight airspeed calibration results. If differences exist, an airspeed system calibration should be accomplished at minimum weight.
(3) Altitude. When using a visual reference on the airplane for timing, the altitude throughout the test run should be as low as practical but should be maintained at least one and one-half wing spans above the highest ground elevation so that the airplane remains out of ground effect. When conditions permit using the airplane shadow for timing, speed course altitudes of 500-2000 feet AGL can be used. All run pairs should be conducted at the same altitude.
(4) Speed Range. The speed should range from 1.3 V S1 to the maximum level flight speed, to extrapolate to VD.
(5) Run Direction. Reciprocal runs should be made at each speed to eliminate wind effects, and the ground speed obtained in each direction should be averaged to eliminate wind effects.
Do not average the time flown in each direction.
(6) Heading. The heading should be maintained constant and parallel to the speed course throughout the run, allowing the airplane to "drift," if necessary, so that the effect of crosswinds can be eliminated.
(7) Configuration. The airspeed system should be calibrated in each landing gear and wing flap configuration required in §§ 23.45 through 23.77. This normally consists of gear up/flaps up, gear up/flaps takeoff and gear down/flaps down.
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Appendix 9
b. Test Procedures.
(1) Stabilize airplane in level flight at test speed, with gear and flaps in the desired configuration, prior to entering the speed course.
(2) Maintain constant speed, altitude, and heading through speed course. Record data.
(3) Repeat steps (1) and (2) of this paragraph on the reciprocal speed run.
(4) Repeat steps (1) through (3) of this paragraph at sufficient increments (minimum of five) to provide an adequate calibration curve for each of the configurations.
c. Data Acquisition and Reduction. Data to be recorded during each run:
(1) Time to make run;
(2) Pressure altitude;
(3) Total air temperature (airplane indicator) corrected to static air temperature (SAT);
(4) Indicated airspeed;
(5) Wing flap position;
(6) Landing gear position; and
(7) Direction of run.
d. Sample Speed Course Data Reduction (Refer to Table A9-1).
DistanceSpeed = Time 1 Knot 6076.1 feet/nautical mile 1.6878 feet/second = = 3600 second/hour Ground Speed 10560 .5925 (10560) 132.8 knots = = =
(1.6878)(47.1) 47.1 G S AVG (TAS) = 132.8 + 125.6 = 129.2 knots 2 Sample Speed Course Data and Data Reduction.
a . W e i g h t C.G.
b. Course Distance 10,560 Ft.
c. Pressure Altitude 1,600 Ft. (Altimeter set to 1013 m.b.)
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J-- I I I I 11/16/2011 AC 23-8C Appendix 9 Flap Position Degrees Gear Position (Up or Down) Time Seconds I.A.S.
(Knots) Pressure Altitude Feet SAT.
Degrees Fahrenheit Ground Speed* Knots Average Ground Speed Knots Factor** Calibrated Air Speed Knots Average I.A.S.
Knots Airspeed System Instrument Position 0 Fixed 47.1 128.0 1610 55 132.8 129.2 0.975 126.0 128.5 2.5 1 1.5
49.8 129.0 1600 55 125.6
44.5 135.0 1600 55 140.5 136.7 0.975 133.3 136 2.7 0 2.7
47.1 137.0 1600 55 132.8
40.5 148.0 1600 55 154.2 149.3 0.975 145.6 148 2.4 -1 3.4
43.3 148.0 1600 55 144.3 C Course Distance (Ft)* Ground Speed = Time (Seconds) C = 0.5925 for course speed in kts or use: C = 0.6818 for M.P.H.
** Factor = chart) F from read (oreTemperaturObserved 7 . 559Hg.) (In. PressureObserved16 . 4 / O o Table A9-1 - Sample Speed Course Data and Data Reduction
(1) Density Altitude. TAS is greater than CAS if density altitude is above sea level. For density altitudes below 5000 feet and calibrated airspeeds below 200 knots, it is considered acceptable to use the term CAS = EAS = TAS o ./ In this case, density altitude is obtained from figure A7-4 in Appendix 7. At 1600 ft. pressure altitude and SAT 55 °F, we read a density altitude of about 1700 feet. This density altitude intercepts o at a value/ of .975. CAS = 129.2 (0.975) = 126.0 knots.
AVERAGE
GS ERROR (TAS) CAS IAS System = Instrument + Position
129.2 126.0 128.5 +2.5 +1 +1.5 (CAS-IAS) V inst Vpos
(2) Required Accuracy. Instrument error is determined by applying standard pitot and static pressures to the airspeed instrument and developing a calibration curve. IAS corrected for +1 instrument error = 127.5 knots. The position of the static source is causing +1.5 error.
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