A PITOT/STATIC SYSTEMFOR THE HOMEBUILT

AIRCRAFT

By Richard Jimenez (EAA 8535)6319 Glenwood Dr.Mentor, Ohio 44060

N,EEDLESS TO SAY having accurate readings on theair speed and altitude are of paramount importance. Onemight say it could be so important under certain condi-tions as to be a matter of life and death. Having inaccuratereadings for the rate of climb while not as important, canstill have disastrous results. So it is obvious that the pilotneeds accurate readings from those instruments.

The obvious question is, from where do the inaccuraciesoriginate? Basically from three areas; DThe inaccuraciesdue to local atmospheric conditions. 2) The inaccuraciesof the instruments. 3) The inaccuracies of the pilot staticpneumatic system. The next question is how much canthese three areas contribute to the inaccuracies of the en-tire system? Let's take them in the same order as listedabove.

The inaccuracies due to the local atmospheric condi-tions can, for example, make the altimeter read incorrectlyby as much as 500 ft. or even more. That is the reason con-trol towers give the atmospheric pressure to pilots whentaking off and landing. This allows the pilot to take correc-tive action by reseting the altimeter to compensate forthe effect change in the atmospheric pressure has onaltimeters. Not all atmospheric changes can be compen-sated for in this fashion. It may be that the intended land-ing site does not have a control tower and the atmosphericpressure is different than it was at the departing airport.It is also possible that the intended flight path is overmountainous territory. Theairblowingovermountainscanhave the same effect as air blowing over the upper surfaceof a wing, that is reduce the air pressure. And we knowthis reduced air pressure gives false altimeter readings.Changes in atmospheric pressure also affect the airspeedand rate of climb but to such a negligible degree we willnot discuss it here. In short, from the amateur airplanebuilder/pilot point of view all that can economically bedone is being done.

The inaccuracies of instruments can be substantial. Forexample, if an instrument is inoperative and stuck at agiven reading, the percent of error can be enormous.Naturally, this instrument should be nowhere near an air-plane. This type of situation is not much of a concern since

the inoperative condition is so obvious the pilot will notbe likely to believe the instrument's reading. What is ofconcern is the situation wherein the instrument is opera-ing but of the prescribed limits of error. This is obviouslya very dangerous situation and for this reason instrumentsare calabrated periodically to insure they are readingwithin prescribed limits. For example, a prescribed limitfor airspeed accuracy is plus or minus \c/c of full scalereading. This means an instrument with 200 mph as itsmaximum reading can deviate from true airspeed by plusor minus 2 mph. Another example is the altimeter. A pre-scribed limit for the altimeter is to have the altimeter readwithin given accuracies for given altitudes. For the altitudeof minus 1,000 ft. to plus 1,000 ft. the altimeter must readwithin plus or minus 20 ft. of actual altitude, from 1,000ft. to 1,500 ft. within plus or minus 25 ft., from 1,500ft. to 3,000 ft. within plus or minus 30 ft, from 3,000to 5,000 ft. within plus or minus 35 ft. etc.

These procedures just described have been in existencefor some time and involve the amateur aircraft builder/pilot to the degree he avails himself of these services. How-ever, the amateur aircraft builder/pilot has not been giventhe tools to improve the pneumatic system of his aircraftover what has been the practice for many years — namely,an aluminum tube for the pitot tube and very possible justexposing the static port of the instruments to cockpitpressure.

Exposing the static port of instruments to cockpit pres-sure is actually using the cockpit air pressure for the staticair pressure source.

This would be o.k. if the cockpit air pressure wereequal to static air pressure. In all likelihood the cockpitair pressure will be above or below static air pressure de-pending on: the shape of the aircraft in the cockpit area,whether it is an open or closed cockpit and ratio of air leaksinto and out of the cockpit. For example, if cockpit pressurewere as little as 1 inch of water pressure different thanactual static air pressure the airspeed would have an in-correct reading of approximately 11 mph. and the altimeter

(Continued on Next Page)

SPORT AVIATION 31

9" dia. plastic funnel with 1/4" dia.holes drilled as shown.

65 Ft.

A PITOT/STATIC SYSTEM .(Continued from preceding Page) 1/4" O.D. x 1/8" I.D. Polyethylene

tubing plugged at cone end.

Six .045 dia. holes drilledas shown.

Fig. 2 Basic Components

Vertical speed indicator

Altimeter

Fig. 1would indicate the wrong altitude by approximately 80 ft.It is interesting to note that 1 inch of water pressureequals only .036 psi. The author is quite certain that ifmeasurements of cockpit air pressure versus static air

.pressure were made of the homebuilt aircraft a significantpercentage of homebuilt aircraft could be found withpressure differentials far in excess of the 1 inch of waterpressure just used as an example. Needless to say, usingthe cockpit air pressure as static air pressure source is notat all advisable. The static air pressure source should betaken from somewhere on the side of the fuselage prefer-ably the aft end. If more accurate results are required a sta-tic pressure source should be placed on both sides of the •

- Airspeed indicatorPress in placeclip

Tee 93-2

Elbow 93-1

Plastic tubingPitot tube

Fig. 332 FEBRUARY 1974

Wing leading edge

MountingPlate

Center line of pitot tube

Flat washer

Locknut

3°

Washer shaped towing configuration

Plastictubing

Fig. Showing pitot tubemounted to wing leading edge.Plastic tubing not shown forclarity.

Fig. 4

Fig. Showing pitottube mounted tomounting plate

——— Fuselage skin

Drill thru using static portas a drill jig.

Outboard

Static port

Inboard

Fig. 5

Fig. 6 Pitot Tube InstalledFig. 7 Pitot Tube cover shownon Pitot Tube.

fuselage. This gives more accurate readings when the fuse-lage is yawing. Sensing the static pressure on the side ofthe fuselage instead of using cockpit air as static source isa very large step in the right direction. Adding a secondstatic pressure sensing source on the other side of the fuse-lage is a good step also but not nearly as important asadding the first static pressure sensing source. If one wantsto really fine tune the static pressure sensing location thefollowing procedure can be used. However, we would liketo mention that for the tremendous amount of effort neces-sary to accomplish this, the added accuracy is probably notworth the effort unless there were a group of identicalaircraft and once the most accurate location was found onone airplane it would then be used for all the other air-craft.

The location where static pressure is to be measuredis somewhat critical and sometimes difficult to find. Amethod of finding the proper location is by comparing thestatic pressure some distance away from the aircraft withthe system used in the airplane. To find the proper locationsome method of measuring the static pressure away fromthe aircraft is necessary. One method is called a TrailingCone Static System, see Figure 1. Using a trailing cone ahomebuilder can obtain an accurate static pressure source.By trying various static locations on the aircraft the home-builder can locate a static source that is equivalent to thetrue static pressure. This would then give accurate read-ings.

Most pitot tubes found on homebuilt aircraft are justaluminum tubes sticking out in the airstream. While ade-quate, it is far from the best. An aluminum tube normallygives accurate readings at high speeds and low angles ofattack. At low speeds/high angles of attack the aluminumtube pitot head, in effect, stalls out. This gives a pulsatingeffect on the ram air pressure which, in turn, gives in-accurate readings to the airspeed. Unfortunately, lowspeed/high angle of attack is a very undesirable place tohave incorrect air speed readings, due to the fact that theselow speeds are encountered at low altitudes during landingand takeoffs. This is not the place to stall an aircraft be-cause the airspeed is reading incorrectly due to the wrong

(Continued on Next Page)SPORT AVIATION 33

A PITOT/STATIC SYSTEM . . .(Continued from Preceding Page)configuration of the pitot head. Pitot tube heads should bedesigned so that the outside and inside configurations lendthemselves to be flown at high angles of attack with a min-imum of ram pressure distortion. The location of the pitottube is also important. The best location for the pitot tubeis far out in front of the aircraft, about one wing chord outin front. While this can be done, the weight and economicsof the situation does not normally warrant this drastic ameasure. Without sacrificing much accuracy in ram airpressure the pitot tube head can be brought in muchcloser proximity to the aircraft. Generally, pitot heads arelocated just in front of the wing leading edge or just infront and slightly below the wing leading edge. These loca-tions offer a minimal of positional error characteristics.One must remember that no matter where the pitot tube isplaced, some positional error will be encountered.

Well, we hope the amateur airplane builder/pilot is say-ing, that is nice information — but how does this effectme? Well, here comes the commercial. We are proud toannounce that Aircraft Development Co. is introducing apitot tube/static port pneumatic kit. The kit consists of apitot tube, static port and enough materials and supplies toconnect the airspeed indicator, altimeter, and rate of climbto the pitot tube and static port. For an illustration of thebasic components included in this kit, see Fig. 2. This kitis so simple to install we recommend that those homebuiltaircraft that do not have a professional pneumatic systemconsider retrofitting their aircraft with this kit. Naturally,we also recommend that anyone now building an airplaneconsider this kit for their aircraft. We are so proud of howsimple it is to install that we would like to describe itsinstallation to you. Fig. 3 shows a schematic of the locationof the components in the airplane. The first step is to screwall the especially designed barbed fittings to the instru-ments. A hole is then drilled in the wing leading edge, orbracket, so that the pitot tube's aft end can be placedthrough the hole. If the pitot tube is being mountedto the wing leading edge a washer filed to the shape of thewing's leading edge is slipped over the aft end pf pitot tube(see Fig. 4) then a locknut is tightened until it clamps thepitot tube assembly to the wing. That is the hardest part ofthe job completed. The plastic tubing is slipped over thebarbed end of the pitot tube and routed through the wing.At about every foot place an adhesive backed clip overthe tube and press in place so that the tube is supportedproperly. Continue this process until the tubing is broughtto the airspeed instrument. Cut the tube to the properlength with a knife and slip the tube over the barbedelbow on the airspeed. Be certain that once the tube isinstalled to the barbed fitting it will not have to be re-moved because once the tubing is installed it is nearlyimpossible to remove it from the fitting. At the selectedlocation press the adhesived backed static port to the in-side of the fuselage. Drill a hole through the fuselage usingthe static port as a drill jig (see Figure 5). Slip theplastic tubing over the barbed end of the static port andwork the plastic tubing to the proper instruments placingthe clips as was done with the wing. It will take about 3or 4 hours of work to install this kit, which will give a veryprofessional, accurate and good looking pneumatic systemto your aircraft. One you will be most proud of. And to capit off, figuratively and literally, a pitot tube cover can bepurchased as optional equipment to protect the pitot tubeorifice while the aircraft is on the ground. This pitot tubecover is made of a material that repels insects as do all ofAircraft Dev. Co. pitot tube covers. This cover will pre-vent bugs and moisture from clogging the pitot tube andcausing erroneous readings. Fig. 6 shows the pitot tubeinstalled and Fig. 7 shows the pitot tube cover on the pitottube. For more information see Aircraft Dev. Co.'sadvertisement in the back of this issue.34 FEBRUARY 1974

COVER STORY

THEMILLERJM-2

F,OR NEARLY THREE decades into the jet age, sport/general aviation pilots have been standing on the sidelines,tongues hanging out in anticipation, waiting for the fall-out from high speed technology to shower them with newand exciting lightplanes . . . really modern lightplanes.

So far it has been a long and not very fruitful vigil,however, in recent months the atmosphere of the sportaviation world has become electric . . . sparks are danc-ing this way and that as new, heretofore radical shapesshimmer and dazzle their way into the public eye. It issomewhat ironic . . . but not very surprising . . . that thisactivity is springing up from the increasingly fertile fieldof homebuilding. Nowhere else, it seems, is there the open-mindedness to new concepts, the enthusiasm and the will-ingness to plunk down hard earned dollars for wings thatare something other than . . . "evolutionary, based on 25years of proven service" . . . and all those other wordsad agencies spill out to obfuscate "old fashioned."

The latest mind-wrencher is Jim Miller's tiny ducteddart, the JM-2. Most widely known as the designer of theimminently successful Formula One racer "Little Gem"and for his modifications to various stock aircraft, JimMiller is now preparing to step into the homebuilding"kit" market and, ultimately, into the manufacturing oftwo and six place developments of the JM-2.