TESTING YOUR HOMEBUILT:_________PITOT-STATICS_________

by Eric Hansen, EAA 53042 USAF Test Pilot, 38214 7th St., West, Palmdale, CA 93551_________

HOW TO TEST YOUR PITOT STATICSYSTEM ... AT THREE DIFFERENTLEVELS OF COMPLEXITY

Pitot-statics is a term used to describe the method of usingpressures to measure the altitude and airspeed of an aircraftin our atmosphere.

Because there are always errors in any measuring system,it's necessary to define and measure the errors as closely aspossible to have any reasonable accuracy. But then themeasuring system used to measure the errors will have er-rors, so, theoretically, no measuring system will ever be per-fect.

We can choose the amount of error we want to accept,however, and for homebuilt aircraft, it's probably okay to aima little lower than perfection. Three methods of widely varyingaccuracy will be presented in this short discussion to calibratethe pitot-static errors in your airplane. One method will beincredibly crude, another just fair, and the third will be prettygood. We'll next discuss where the errors come from, thengo into how to do each of the three calibration methods.

First, let's look at a typical pitot-static system and seewhere the errors orignate. The primary instruments are theairspeed indicator and the altimeter. The airspeed indicatormeasures the pressure difference between the pitot and sta-tic source, and the altimeter measures the pressure at thestatic source. The primary sources of error are the instru-ments themselves, the pitot source and the static source(see Figure 1).

Late model, new instruments are usually pretty good. Ifyou have them and are willing to trust their accuracy, thetesting discussion that follows will assume no instrumenterror. Older, used instruments are very suspect: they mayhave leaks, worn gears, worn pivot pins and weak springs.If you're serious about having accurate airspeed and altitude,buy new or overhauled/certified instruments, or have an in-strument shop overhaul and calibrate yours.

Ask the instrument shop to provide you with a calibrationsheet for each instrument. Apply this instrument correctionto every cockpit reading you take in testing before makingcalculations.

The pitot system derives its name from the engineeringnotation, "PTOT" for P — total, or total pressure. It is the sumof the ambient air pressure and the dynamic air pressure inflight. This sum, or total pressure, is collected in the front ofa tube placed in the airstream and carried to the airspeedindicator with tubing. As long as there are no leaks and thetube is placed in the free airstream, errors are usually neglig-ible. For subsonic light aircraft, the size and shape of the tubehave no effect at all on its function, and it can be misalignedup to 20 degrees from the free airstream without noticeableerror.

The static system is always our big problem. It's supposedto supply the free air ambient pressure to the instruments.That would be-easy if the airplane could sit still in the air.Since most people build airplanes that move through the air,and any shape moving through the air has a variable pattern

of pressures around it, the static source has to be locatedsomewhere on the airplane where the pressures remain asclose to ambient as possible without variations due to speed,angle of attack or sideslip.

Aircraft manufacturers do engineering analysis, wind tun-nel work and some cut and try to find a suitable static sourcelocation. Some manufacturers just guess and cut and try —and do pretty well. Homebuilt designers sometimes find agood place for a port and sometimes they don't. Most often,finding a static source location is left to the builder — withresultant wide variations in airspeed and altitude errors, evenfor the same basic aircraft designs. A frequently selectedlocation is on the sides of the fuselage, manifolded togetherto equalize sideslip errors. Another location is on the sidesof the pitot tube or another tube sticking out into the freeairstream. Holes are drilled around the periphery of the tubesome ways back from the tip. Some typical pitot-static set-upsare shown in Figure 2.

In flight tests, more reliable static sources are used to cali-brate the aircraft's proposed pitot-static system. One methodis to place the static source well out ahead of the aircraft'spressure disturbances, usually on a long pitot boom eitherhalf a chord ahead of the wing leading edge or half a fuselagelength ahead of the nose. Another is to use a bomb shapeor a cone at the end of a tube trailing behind and below theaircraft, outside of the aircraft pressure disturbances. Thesemethods require additional equipment and instrumentsbeyond the scope of most homebuilders.

RAM AIR

AIRSPEED INDICATOR ALTIMETERFIGURE 1.

Basic Pitot Static System

The test procedures presented here require no elaborateequipment, and most people can handle the calculations ona good hand calculator.

To decide on which method is suitable for you, considerthe accuracy you require. If you want to compare your perfor-mance to another aircraft, you'd better be accurate. Somebuilders are greatly disappointed that their machine doesn't

SPORT AVIATION 37

MOST TYPICAL LIGHTAIRCRAFT ARRANGEMENT

WITH THIS ARRANGEMENT, THETUBES MUST BE LOW ENOUGHAND FORWARD ENOUGH TO BE OUTOF THE WINGS PRESSURE DISTURBANCES. THIS ARRANGEMENT, OR SIMILAR

WITH THE PITOT "BOOM" ON THENOSE IS LESS SUBJECT TO ERRORS.

PITOT

(CLOSED END) \. STATIC HOLES STATIC HOLES

FIGURE 2.Typical Pltot Static Systems

fly as fast as the designer claimed (which may not be apitot-statics problem) or they may be wildly ecstatic that theircreation cruises 25 mph faster than everyone else's. Actualperformance variations will result from differences in weight,rigging and attention to aerodynamic detail, assuming thesame thrust. The only way to make fair comparison is tomake sure the numbers are right.

YOU MUST HAVE A GOOD PITOT-STATIC SYSTEM —Assuming you have a reasonable pilot-static system with auseable pilot tube location and a fairly stable static location,no leaks, and a good set of instruments, let's look at someways to test and calibrate the system for flight.

YOU HAVE TO FLY — There's no way to get the informa-tion for your unique airplane without flying it. You have to flya flight test period for the FAA anyway, so here's somethingto do after you've established your engine cooling and relia-bility, refined your rigging and flight controls, and feel fairlyconfident that your airplane is safe and airworthy.

METHOD ONE (INCREDIBLY CRUDE)If you don't really care to compare to anything or anyone

else, and your airplane is a day VFR hedgehopper, thismethod is for you: Do nothing at all and accept your errors.As long as your cruise speed is the same number and yourstall speed is the same number, you don't need to know whatthe number represents. You could even rescale yourairspeed indicator without numbers (see Figure 3).

Although the FAA requires you to have a sensitive altime-ter, the FAA doesn't say how accurate it must be for dayVFR. As long as you're below 3000 ft. AGL, you don't haveto comply with VFR hemispheric altitudes.

METHOD TWO (FAIR)If you want to get in the ballpark but don't need a dugout

seat, this method is easy and fun: fly formation with a pace

aircraft and find the differences.This method requires another aircraft whose airspeed and

altitude errors are known or are negligible for the precisionyou desire. For you, flying formation may not be so easy orsafe; either don't do it or find someone who can. Also, findinganother "calibrated" pace aircraft can be tough.

FLYING FORMATION — It doesn't matter whether you flyformation off the pace aircraft or the pace flies formation offof you, as long as whoever is leading establishes the testpoints on his instruments. Whoever is leading should flystraight and smoothly, precisely on altitude and airspeed foreach test point. The lead should choose an area of uncon-gested airspace in which to fly, and be responsible for clear-ing for the flight. Each of you should have thoroughly dis-cussed the details of your formation flight and safety proce-dures, have a good method of plane to plane communication,and have identical test cards showing each airspeed andaltitude test point.

THE PACE AIRCRAFT — An aircraft with known, precisecalibration is not usually found outside the test community.For this reason, this method probably won't result in betterthan 3 or 4 mph accuracy. The closest substitute for cali-brated pace aircraft is a late model production aircraft with aspeed range similar to your aircraft. The FAR's require aproduction aircraft to have at most a 3% or 5 knot error,whichever is greater, and better than +30 ft. of altitude errorat sea level. Most production aircraft do much better thanthat. Many aircraft operating manuals supply an airspeedcorrection chart that you can use to get more precision.

COLLECTING DATA — The pilot in the lead aircraft muststabilize as closely as possible to the altitude and airspeedfor each point. The formation pilot must stabilize relative tothe lead, exactly level, with no fore or aft movement. Theformation pilot calls "ready, read" and both pilots note andrecord their actual airspeed and altitude indications at thatinstant. Establish points every 10 mph from slow flight up to

38 MAY 1987

your maximum compatible speed. One altitude will do if youwant to make some simple calculations to establish the al-timeter error for all altitudes. If not, do the same airspeedpoints — within the performance capabilities of the two air-craft — every 2000 ft. up to the maximum altitude you wantto calibrate. Both aircraft should use a 29.92 altimeter settingso that actual pressure altitudes are recorded. Be sure toreset the local altimeter setting when you are finished. Figure4 shows an example of a typical test card to fill out as you fly.

( 1 ~~ *̂S^X < -̂̂, , AIRSPEED i '

\^ jf 3JiJILL>̂ ^

ALTERNATIVE AIRSPEED INDICATORFIGURE 3.

Alternative Airspeed Indicator

DATA REDUCTION — Correct the pace aircraft speedsfor its known errors if you have a calibration sheet. Thesespeeds then become your calibrated airspeeds, or Vc. Sub-tract the corresponding airspeeds read in the test aircraft (V,).Your answers, with the appropriate algebraic sign, are yourairspeed position errors (A Vpc). Plot A Vpc versus V,, andyou have a convenient chart for finding the number to add toany indicated airspeed to get your calibrated airspeed (seeFigure 9).

For each test altitude, subtract the test aircraft's indicatedaltitude, H,, from the pace aircraft's "calibrated" altitude, Hc,and plot your answers, A H ,̂ for each airspeed at the al-titude. Because density changes with altitude, you'll have adifferent altitude position curve for each altitude. The resultsshould look like Figure 5.

If your lowest altitude was not sea level, and you want tocorrect your numbers to sea level, use the following relationto compute a set of sea level points, or any other altitude forthat matter:

AHpc = AH,.,, x ii (known altitude)(altitude desired) (known altitude) cr (desired altitude)

Obtain a ("sigma") for a given altitude from the U. S. StandardAtmosphere table (Figure 10).

If you have calibration sheets from the instrument shop foryour instruments, you can be a bit more rigorous in yourcalculations by applying the instrument errors before youmake your plot of A V^ versus V.and A H^ versus H,. Insteacof using V, and H,, use V,c and Hic, defined below:

V,c (instrument corrected airspeed) - V, (indicatedairspeed) + A V,c (airspeed instrument correction).

Then your actual calibrated airspeed, Vc, is your indicatecairspeed. V,, plus the instrument correction, A V,c, plus theposition error correction, A V^.

Correspondingly,BE CAREFUL TH

A Vpc, A H,c and AYOU "ADD" THEMRECT.

METHOD 'This method will

you're careful and athe "speed course"low altitude to calcfurther compute alti

TEST REQUIREI\curate outdoor themU. S. Standard Atrrcourse you can precThe ground coursea minute to fly at y<the time on the courlong should do, butcisely than to the n<cision errors are cumaking computationearest 50 ft. is onl)100 seconds is als<than 2%, which re|mph and 3 mph aienough of obstructsabove the groundreasons: so that ycwatch crossing thecourse and so thatthermometer on thaltitude ambient ten

Many areas of thelines in flat rural arhighway constructiclong runway is a gdistance measurerrand need to land, apeople or vehicles,light or calm and csteady heading —

Vaim (mph)

150

140

130

120

110

100

90

80

70

60

50

Vc = V, + A V,c + A VpcHc - H, ^ A HIC « A Hpc.

AT THE ALGEBRAIC SIGNS OF A VIC,Hpe ARE CORRECT, SO THAT WHENTOGETHER, Vc AND Hc ARE COR-

THREE (PRETTY GOOD)get you some pretty good numbers ifre pretty handy with a calculator. Calledmethod, you fly a measured course at

ulate actual calibrated airspeed and toude position errors.dENTS — You need a stopwatch, an ac-nometer, a good calculator, a copy of theosphere (see Figure 10), and a groundisely measure and fly at very low altitude,should be long enough to require around)ur fastest level flight speed. The longerse, the less error you have. About 2 milesyou need to measure it a lot more pre-

;arest 110 mile on a car odometer. Pre-nulative in measuring time, distance andns. A 5000 ft. course measured to the/ 1% precision. One second errors out ofD 1%. Together, you can't do any better)resents a possible 2 mph error at 100

150 mph. The course should be clearjns to be able to fly 1 -1 12 to 2 wingspanssafely. You need to fly this low for two>u can more precisely "hack" your stop-beginning and ending lines of your speedthe temperature you measure with youre ground can be considered your flightnperature.; country have very precise 1 mile sectioneas free of trees and power lines. New>n is usually very precisely surveyed Areal place, not only to have an accuratelent, but in case you develop problemss well as avoiding flying within 500 ft. ofFly early in the morning while winds areround heating is nil. Fly crosswind on aallow the aircraft to drift. (See Figure 6)

Vi (indicated)

148

140

129

118

107

96

85

74

66

57

48

/ indicated \Hi ^ altitude J

5000

5000

4985

4950

4940

4900

4870

4825

4800

4720

4700

FIGURE 4.Sample Pacer Data Card

SPORT AVIATION 39

150

V, (mph)FIGURE 5.

Altitude Position Error vs. Airspeed

COLLECTING DATA — Measure the ambient temperatureaccurately — within one degree. Measure in the shade, ashigh above the ground as is reasonable to avoid measuringground heating. Set the altimeter to 29.92 so that it readsactual pressure altitude. Fly the same very precise indicatedairspeed in each direction. Be stablized prior to crossing yourstart timing point and remain stable until after passing yourstop timing point. Make runs in both directions for indicatedspeeds from maximum speed down to safely above a stallat even 5 or 10 mph increments. Record the time, speed andindicated pressure altitude for each run. If the temperaturechanges, record the temperature to the nearest degree foreach pair of runs.

SPEED COURSE METHOD

1. FlY IOWJ. FlY CROSSWIND3. FlY COURSE HEADING AND AILOW AIRCRAFT TO DRIFTt. GET PRECISE TIMING IN IOTH DIRECTIONS

WIND

«.NO - - FIGURE 6.Speed Course Airspeed Calibration

Use a card such as depicted in Figure 7 to record yourinformation after each run. Climb up to a safe altitude to doyour recording. You might also try calling your data into atape recorder or radioing the data to a friend on the ground.

After performing diligently as a test pilot, now it's time toput on your engineer's hat and go home to crunch somenumbers.

For each run, you have a list of things to compute in orderto arrive at the final answers — the airspeed and altitudeposition errors. This work is tedious and some enterprisingperson out there might do well to write a program for homecomputers to make it easier. Figure 8 is the list with thenumbers computed for the first run. Following the list is pre-sented the equations and a sample calculation for each step.

EQUATIONS AND SAMPLECALCULATIONS

Vu + Vib + 2 ( A V J C X1.47

The 1.47 factor changes mph to ft./sec. If your airspeedindicator is in knots, use 1.69 to change to ft./sec.

A V,c is the instrument calibration correction for the indi-cated airspeed at this test point. If you don't have a calibration

sheet for your airspeed indicator, assume zero error and ig-nore this term.V,c = 149 + 149 x 1.47 = 219.0 ft/sec

2

2 .V T = 1 / D + D \ = 1 / 12450 + 12450\2 ^At, AtjJ 2 ^ 56 52 J

= 1 / 647400 + 697200 \2 ^ 2912 J

= 23.9 ft/sec

3. For this information, H, (indicated altitude) is closeenough to Hc (calibrated altitude) to not make any difference.Enter the U. S. Standard Atmosphere table with your H,forthe nearest "Hc" and find ft ("delta"). Interpolate to refine thevalue of 5. In this case, the average of both runs was about850', so for an Hc of 800', ft = 0.9714, and for 900', 8 =0.9679. Halfway in between would be 5 = 0.9679 + 0.9714 =0.9697 2

4. °K = °C + 273°C° = 5/9 (°F-32)C° = 5/9 (74-32) = 5x42 = 23°C

9°K = 23° + 273° = 296°K

5. The test altitude temperature ratio, "t, is simply Ht =Ja (°K) = 296.30 = 1.027Ta (°K) 288.16SEA LEVEL

(TaSL = 288.16°K)6. find <j, ("sigma-tesf )

<r, = 8t CT, = 0.9697 = 0.94418, 1 .027

7. Vc = VT = 230.9

8. Vc(mph) = Vc (ft/sec) -

0.9441 = 230.9 x 0.9717= 224.4 ft/sec

1.47Vc (mph) = 224.4 H- 1.47 = 152.6 mph

9. AVp,. = Vc - V1C = 152.6 - 149 = + 3.6 mphThis airspeed correction is good for all altitudes.10. APS = -0.001188 (Vc

2 - V1C2) (Vs must be in ft/sec)

= -0.001188(2282-2192)= -0.001188 (2394) - -2.845 Ibs/ft2

11. AHPC,.*

x 13.07

= - (-2.845) x 13.07 = 38 feet0.9697

12-AHPCSEALEVELx (T test altitude

u sea level

Obtain a ("sigma") from the U. S. Standard Atmospherefor each altitude.

= 38x .9754 =37feet1

13. AH, = AHPCSE« LEVEL = 37 = 40feet

—(f^o-— 0.9428

14. Similarly, A H, = 42 feet.15. AHix™ = 44 feet16. AHpc,ooo = 47feel17- A Hpe,̂ = 50 feetAfter calculations are completed for all test airspeeds,

create plots of A V^ versus V, and A Hp<. versus V, for severalaltitudes. Figure 9 is a representative plot of A V^ vs V, withour one point circled. Note that it might have turned out thatA Vpc could be a negative value, so it would be subtractedfrom V, in that case.

40 MAY 1987

Aim Speed

V, (actual indicated mph)

Hi (average indicated altitude ft)

Ta (ambient temperature, °F or °C)

t (run time, seconds)

D (distance, feet)

Direction of run

RUN1

a

150

149

860

74°F

56

12450

West

b

150

149

850

74°F

52

12450

East

RUN 2

a

140

140

870

74°F

63

12450

West

b

140

140

870

74°F

59

12450

East

RUN 3

a

130

131

850

74°F

68

12450

West

b

130

130

840

74°F

66

12450

East

FIGURE 7.Sample Data Card, Speed Course Method

For our sample point, assuming no instrument error (A V,c),if we are flying at 150 mph indicated, we must add 6 mph toget our actual, or calibrated airspeed, 156 mph.

If the actual speed is faster than the indicated, looking atour pitot static system diagram, there must be a slight positivepressure at the static port because the airspeed indicator, adifferential pressure gauge, is not reading as large a differen-tial pressure as it should. If there is a slight positive pressureon the static port, what does this do to the altimeter? Havinggreater pressure corresponds to a lower altitude, so the al-timeter reads lower than actual altitude. For our test point,we found A Hpc of 65 ft. at sea level.

(calibrated altitude) = (indicated altitude) + (instrumentcalibration correction) + (altitude position error correction), or

position correction. A typical altimeter position correction isplotted for various altitudes in Figure 5.

Use a french curve to draw a smooth line that seems tofollow the points for each altitude the best. One data pointthat you can assume to be good for all altitudes is that theposition error is zero at zero airspeed, so all lines shouldconverge on zero.

Now for any altitude and airspeed, you can find prettyclosely your position error from this chart.

H = H, A Hic AHL

1. (true alrapeed. fi/»ec)

5. A pc (airspeed position correction, aph)

11. AHpc. t (altitude petition correction, ft)

1). AHpe 20°0'

!<.. AHpc 4000'

IS. A Hpc 6000'

lt>. AHpe 8000'

[ T . A Hpi. 10000'

2)0.9 ft/srr

2«°K

6 "ph

64 fe«t

W ft

70 (t

74 ft

79 ft

B* ft

6

(mprT) 4

2

-2SO 100 150

V, (mph)

FIGURE 9.Airspeed Position Correction vs. Indicated Airspeed

FIGURE 8.Speed Course Calculations Results

If you have the correct altimeter setting and are flying anindicated zero altitude at 150 mph, assuming no instrumenterror, you are really flying 65 ft. above sea level. The correc-tion could go the other way: if the static source is located inan area of slight vacuum, you'd be subtracting the altimeter

SUMMARYWe've reviewed the basics of a pilot-static system, seen

where the errors come from and covered three methods ofdealing with the errors. There are other ways to test anddetermine pilot-static errors, but they're for the big boys withthe fast machines and lots of money. In presenting themethods I've covered here, I've made what I consider safeassumptions to simplify things considerably. Those of youout there who are well grounded in aerodynamics, flight testand pitot static theory will recognize these assumptions andunderstand their validity. For those of you who aren't, trustme.

Next time someone claims his T-18 is faster than yours,ask him what his A V.*. is.

SPORT AVIATION 41

HC « e o( fee t ) ( P / P ) (T /T ) ( p / p )

a U r i « a r t ol.

0. 1.0000 1.0000 1.0000100. 0.9964 0.9993 0.9971200. 0.9928 0.9986 0.9942300. 0.9892 0.9979 0.9913400. 0.9856 0.9972 0.9883500. 0.9821 0.9966 0.9855600. 0.9785 0.9959 0.9826700. 0.9750 0.9952 0.9797800. 0.9714 0.9945 0.9768900. 0.9679 0.9938 0.9739

1000. 0.9644 0.9931 0.97111100. 0.9609 0.9924 0.96821200. 0.9754 0.9917 0.96541300. 0.9539 0.9911 0.96251400. 0.9504 0.9904 0.95971500. 0.9470 0.9897 0.95681600. 0.9435 0.9890 0.95401700. 0.9401 0.9883 0.95121800. 0.9366 0.9876 0.94841900. 0.9332 0.9869 0.94562000. 0.9298 0.9862 0.94282100. 0.9264 0.9856 0.94002200. 0.9230 0.9849 0.93722300. 0.9196 0.9842 0.93442400. 0.9163 0.9835 0.93162500. 0.9129 0.9828 0.92892600. 0.9095 0.9821 0.92612700. 0.9062 0.9814 0.92332800. 0.9029 0.9807 0.92062900. 0.8996 0.9801 0.91793000. 0.8962 0.9794 0.91513100. 0.8929 0.9787 0.91243200. 0.8896 0.9780 0.90973300. 0.8864 0.9773 0.90693400. 0.8831 0.9766 0.90423500. 0.8798 0.9759 0.90153600. 0.8766 0.9752 0.89383700. 0.8733 0.9746 0.89613800. 0.8701 0.9739 0.89343900. 0.8669 0.9732 0.89084000. 0.8637 0.9725 0.88814100 0.8605 0.9718 0.88544200. 0.8573 0.9711 0.88284300. 0.8541 0.9704 0.88014400. 0.8509 0.9697 0.8774-4500. 0.8477 0.9691 0.87484600. 0.8446 0.9684 0.87224700. 0.8414 0.9677 0.86954800. 0.8383 0.9670 0.86694900. 0.8352 0.9663 0.86435000. 0.8320 0.9656 0.86175100. 0.8289 0.9649 0.85915200. 0.8258 0.9642 0.85655300. 0.8227 0.9636 0.85395400. 0.8197 0.9629 0.85135500. 0.8166 0.9622 0.84875600. 0.8135 0.9615 0.84615700. 0.8105 0.9608 0.84355800. 0.8074 0.9601 0.84105900. 0.8044 0.9594 0.83846000. 0.8014 0.9587 0.83597000. 0.7716 0.9519 0.81068000. 0.7428 0.9450 0.78609000. 0.7148 0.9381 0.7620

10000. 0.6877 0.9312 0.738511000. 0.6614 0.9244 0.715512000. 0.6360 0.9175 0.693213000. 0.6113 0.9106 0.671314000. 0.5874 0.9037 0.650015000. 0.5643 0.8969 0.6292

FIGURE 10.

PARTIAL TABLEU.S. STANDARD ATMOSPHERE

Note: For those who have scientificcalculators or home computers,the following equations may beused:

A _ | V O9 ' I - K! «c

where

K t • 6.87535 x 10"6

« - M r H )5-25610 • (I - K. H )

a - (1 - K, H )4'2561c

Explanation of Greek Characters

Engineers f ind It very convenientto use another alphabet to describenumerical relations or functions. Inthis case the following Greek charactersare used:

6 "delta" (lower case)Used to describe the ratio of ambientatmospheric pressure to standard sealevel atmospheric pressure.

A "delta" (upper case)Used as a p r e f i x to indicate that thenumber Is a small Increment of thebase quantity.

o "sigma" (lower case)Used to describe the ratio of ambientair density to standard sea level airdensity.

0 "theta" (lower case)Used to describe the ratio of ambientair t empera tu re to standard son levelair tempera ture (° Ke lv in ) .

42 MAY 1987