AMONG THE MINIMUM instrumentation required for VFR (Visual Flight Rules) flying are two flight instruments - the airspeed indicator and the altimeter. While a small number of builders might think a rate of climb (vertical speed) indicator is essential, it is not one of the mandatory VFR instruments. Nevertheless, the three instruments do have something in common - each must be vented to a source of atmospheric (static) pressure for proper operation. In addition, the airspeed indicator requires a source of ram air (pitot) pressure. These two air pressure sources, and the interconnecting plastic tubing, constitute the aircraft's pitot-static system.
Calling it a "system" when little more than a bent aluminum tube and some plastic tubing embody the main elements may be a bit grandiose. However, you can make your own installation of the pitot-static system into a big deal or keep it on a modest minimum effort level.
Static Pressure Sources
Some pitot tubes contain both an inlet for the pitot or ram pressure, and another for the static pressure. These pitot tubes are, therefore, more correctly identified as "Pitot-Static" tubes. In such installations the pitot-static tube is a dual-function unit providing both ram (P) air and static (S) air for the instruments connected to it.
Although the static pressure is not always obtained at the pitot tube head assembly, it would seem that such an arrangement where both ram air and static air are taken from the same source area is a good one. It could assure a higher degree of accuracy in instrument performance than might a static air pressure source remotely located. On the other hand, one fairly reliable source location for a static pressure vent, remote though it might be from the pitot tube area, is the fuselage . . . somewhere in the side of the fuselage, that is.
There you will often find one to three small vent holes, in a flush-mounted fitting of sorts, serving as a static air inlet. Actually, static ports are ordinarily installed on both sides of the fuselage to minimize the effects of slipping or skidding flight resulting from sloppy or unintentional rudder inputs. Brief transgressions from coordinated flight will affect your instrument readings only momentarily unless you habitually fly the airplane in a constant skid.
Both static vents, when located on opposite sides of the fuselage, must be manifolded (connected together by a "T" or a "Y" fitting).
One advantage of a separate static source remoted from the pitot tube is the theoretical assurance that your altimeter and vertical speed indicator will continue to function even though the pitot tube may become blocked by mud daubers, ice, dirt or a forgotten pitot tube cover . . . and this is worthy of your consideration.
The remoted static pressure vents, when installed, should be located in an area of air flow unaffected by wing junctures or fuselage bumps and irregularities. Even though the fuselage sides may seem to be uniform in the area selected for the static vents, a future relocation of the vents could become necessary because of erratic or unreliable instrument readings.
If the static source is located in an area producing higher than true (surrounding atmosphere) static pressure, your altimeter will read lower than it should. Conversely, a static source producing lower than true static pressure will cause the altimeter to indicate higher than it should.
A poor static source also affects the airspeed indicator's readings. For example, a static source co-located with the pitot head positioned just ahead of the wing but too close to its under surface could be in an area of slightly higher pressure than that of the surrounding air. This is an area of higher pressure induced by the flow of air around the leading edge of the wing. In such a case the airspeed indicator will be induced to read slow since the pressure differential between the ram and static air would be less in that area.
A location too high could induce the reverse effect causing a too fast indication by the airspeed gauge. Similar errors can occur with the remoted fuselage static ports if they are located in a disturbed pressure field.
Carrying this static error subject a bit further, we can see where the same errors could result in a biplane installation where the pitot-static tube assembly is mounted too high or too low on an interplane strut. If too high the static port could pick up a slightly higher pressure under the top wing and give a resultant slow reading. If the static tube is located too low on the strut, the static source will pick up a lower than true pressure because of the induced airflow over the top of the bottom wing. This would result in an airspeed indication that is too fast. Remember, the airspeed indicator measures the differential between the ram air and the static air.
Pitot or Ram Pressure Sources
As previously stated, only one of the three VFR flight instruments, the airspeed indicator, requires ram (pitot) pressure. Its source of ram air pressure is a pitot tube mounted parallel to the longitudinal axis of the aircraft and in line with the slipstream (relative wing). The location of the pitot tube is no less important than its orientation on the aircraft.
All this really means is that the pitot tube should not be located inside the propeller blast area or any place where its pick-up opening might be in air disturbed by the influence of proximate aircraft structure. Although some airspeed error is no big thing for the weekend aerial putt-putt, it can be a serious matter for a fast high flying homebuilt or one used occasionally in IFR excursions.
It seems as though the only thing standardized about pitot tube head locations is that, for some reason unknown to me, more pitot tubes are located somewhere in the left wing than elsewhere. But you are just as apt to see pitot tubes installed almost anywhere on the aircraft.
In a twin the fuselage nose location is quite suitable because it is not within the propeller blast area.
The most serious boo-boo you can make in locating your pitot tube is to place it in the leading edge of the wing. Yes, in spite of the effectiveness of that location. It is so easy to install a pitot tube there that it is almost irresistable for the first time builder to ignore. Unfortunately, it is such a good location that almost anyone walking by will notice it after he has bumped into your pitot tube or has broken it off. Even more unfortunate - the builder himself may be the very first one to ruin it. One builder told me his was broken off so often that now, just before he flies, he slips a drinking straw into the hole once occupied by the original pitot tube.
Of course, the leading edge location is not too bad in a high wing aircraft . . . if that wing is high enough off the ground.
Maybe Molt Taylor has the solution for vulnerable pitot tubes sticking out where they can get bent or broken. He has installed, in his MicroIMP, a flexible pitot tube that merely gives way under impact and twangs back good as ever. (I wonder how he does that?)
It becomes obvious then that the safest location for a pitot tube is beneath the wing. Almost anywhere under the wing is O.K. for aircraft utilizing the older airfoils (Clark Y, etc.), just so long as the pitot tube opening is about 5" or more below the surface of the wing. An aircraft with a laminar flow airfoil should have its pitot tube located further aft to some point where the maximum camber of the wing occurs.
Pitot-Static System Installation Notes
Some pitot static tubes are made with built-in heater elements. These are electrically powered and must be hooked to the electrical system. The pitot heat would then be controlled by an appropriately labeled instrument panel switch . . . but why a heated pitot-static tube if you don't fly IFR?
When the static source is to be obtained at the pitot tube head, you can make your own assembly using two aluminum tubes attached to a mounting bracket.
One of the tubes in the pitot head assembly will have an open end for ingesting ram air pressure. This tube should be connected to 1/4" polyethelene (plastic) tubing routed to the "P" (pressure) opening in the airspeed indicator.
The other tube (static) of the pitot head assembly must have its end sealed (pinch, weld, insert a screw or otherwise close the opening in the tube). In addition, it will have at least four small holes (use a #60 drill bit) equally spaced, around its circumference to provide the ambient (static) air pressure needed by the airspeed indicator, altimeter and the rate of climb. This static source will also be connected to each of the VFR flight instruments with 1/4" plastic tubing. See Figure 1 for a few ideas and typical hook-ups.
Before completing the connections at the instruments, blow out the tubing to assure yourself that none of the lines are plugged. Do not, however, blow into any of the instruments as that may cause internal damage.
Aircraft used in instrument flying should have an alternate source for static pressure in case the primary static source becomes inoperative. This alternate source can consist of a single valve that opens the line to the cabin atmosphere – an unreliable source at best but better than inoperative instruments.
A few builders of uncomplicated aircraft often don't bother with a static source and simply leave the "S" ports of the instruments open behind the instrument panel.
This is a poor installation if for no other reason than the abuse the instruments will be subjected to from ingested dust and dirt floating around in the unfiltered air. Another drawback to using the cabin atmosphere as a static source is the wild fluctuation induced in the instrument readings every time the cabin ventilation vents are opened or closed.
Ever hear of adjusting your airspeed by modifying the static tube in a pitot-static installation? Well suh, the story goes like this.
If your airspeed reading is slow, it is because the static port is in a slight high pressure zone. This can be corrected by slipping a small "O" ring over the end of the static tube ahead of the tiny drilled holes (vents, that is).
Moving the "O" ring aft on the tube (in very small increments) will increase the airspeed - on the gauge (not for the airplane, amigo . . . sorry). Conversely, moving the "O" ring forward, away from the ports, will decrease the indicated airspeed in much the same manner. Very small changes in the "O" ring position make a noticeable difference in the indications.
What happens is that you are attempting to lower the static pressure "felt" by the static ports with the "O" ring interrupting the airflow, thereby causing a slight lowering in air pressure behind (downstream) the ring.
The closer the ring is to the ports, the lower the induced pressure.
If on the other hand the static-tube port happens to be located in an area of low pressure, moving the ring to a position behind the ports should provide a slight increase in the pressure felt by the static ports with results opposite to that described above.
Because a rubber "O" ring will deteriorate, it should be replaced with a metal ring having identical dimensions after the proper location is determined. A dab of paint or epoxy should hold it in place. One gent who does some wild gyrations in his modified Starduster has a big ol’ set’screw securing his "scientifically" located ring.
There you have it – much more than you wanted to know about pitot-static systems. And thanks to my Technical Advisor for this month, Frank Luft of Central Point, Oregon.