James W. Gregory Introduction to Flight Testing

In traditional flight testing methods at the university level, many of the desired parameters are hand‐recorded from cockpit indicators such as the airspeed indicator (ASI), attitude indicator, heading indicator, and altimeter, which are illustrated in Figures 3.1 and 3.2 for a traditional “steam gauge” cockpit. The traditional instruments are arranged in a “six pack” configuration (Figure 3.2): starting at the upper left and moving clockwise, the instruments are the ASI, attitude indicator, altimeter, vertical speed indicator (VSI), heading indicator (also known as the directional gyro), and turn/slip indicator (also known as the turn coordinator). We will group these instruments into two types. The first group, consisting of the attitude indicator, heading indicator, and the turn/slip indicator, is based on gyroscopes. The second group, consisting of the ASI, altimeter, and VSI, is based on physical measurements of pressure. We will discuss each group of instruments in more detail as follows. Interested readers can also consult various publications such as Chapter 7 of the Pilot's Handbook of Aeronautical Knowledge (2008), Chapter 5 of the Instrument Flying Handbook (2012), and the Advanced Avionics Handbook (2009) for more details.

Figure 3.1 Overview of aircraft cockpit instrumentation for traditional “steam gauge” instruments.

Source: https://upload.wikimedia.org/wikipedia/commons/thumb/e/e7/Slingsby.t67c.panel.g-bocm.arp.jpg/1200px-Slingsby.t67c.panel.g-bocm.arp.jpg.

Figure 3.2 Detailed view of the six pack of key instruments in a traditional cockpit.

Source: Modified from photo by Mael Balland on Unsplash, https://unsplash.com/photos/V5hAryReZzo.

3.1.1 Gyroscopic‐Based Instruments

Before discussing the first group of instruments, it is helpful to briefly review the ideas behind gyroscopes. The operation of a gyroscope is based on the Newtonian principle of conservation of angular momentum. Due to conservation of angular momentum, the axis of rotation of a spinning object with a given angular momentum (L = Iω, where I is the moment of inertia and ω is the angular speed) will remain fixed unless acted upon by an external applied torque. (Think of how the axis of a child's spinning top remains upright as long as the top is spinning.) Over time, small external forces and moments that are present in any practical implementation of a physical gyro will lead to gyroscopic precession, which results in small changes in the axis of rotation that can grow over time.

The three gyroscopic instruments in a traditional six pack of instruments – the attitude indicator, heading indicator, and turn coordinator – all depend on gyroscopic principles. The rotational speed of the gyros in the attitude and heading indicators are traditionally powered by a vacuum system that flows air over a small turbine connected to the spinning disks in order to maintain rotation. The turn coordinator, on the other hand, is typically powered by the aircraft's electrical system. The attitude indicator provides an artificial horizon that provides an indication of the amount of bank and pitch that the aircraft has at a given moment. Essentially, the internal gyroscope (to which the artificial horizon is mounted) maintains its rigidity in space and the aircraft pitches and rolls about the internal gyro. Arcing along the top of the indicator, the first three white lines on either side of the centerline indicate increments of 10° bank angle (up to 30° bank). The next two lines on either side represent a 45° and 60° bank angles, respectively. The heading indicator is based on a geared gyro and simply indicates the magnetic heading of the aircraft (as long as the gyro has been set to match the magnetic compass). Both the attitude indicator and the heading indicator experience errors due to gyroscopic precession that must be occasionally corrected. For example, periodically in straight and level flight, the pilot may need to reset the heading indicator to match the indication on the magnetic compass. The turn coordinator, also based on a gyro, indicates the instantaneous rate of turn. The two white lines below the level lines each indicate a standard rate of turn of 3°/s to the right or left, which would require two minutes for the aircraft to complete a 360° turn.

3.1.2 Pressure‐Based Instruments

The second group of instruments is based on measurement of total pressure and static pressure on the aircraft. Total pressure is measured by a pitot probe, which is often mounted under the wing on GA aircraft (see Figure 3.3(a)). Static pressure is measured by flush‐mounted static pressure ports on the side of the aircraft fuselage (see Figure 3.3(b)). The pitot probe and static port are connected via tubing to the ASI, altimeter, and VSI mounted on the instrument panel (Figure 3.4).

Total pressure from the pitot tube and static pressure from the static port are fed into the ASI. The ASI is calibrated based on the assumption of standard sea‐level conditions with the isentropic Mach relation to convert a measured “impact pressure” (difference between total and static pressure) into indicated airspeed. For low‐speed, low‐altitude flight this is equivalent to converting a measured dynamic pressure to velocity by the Bernoulli equation. Further details on the functioning of the ASI are provided in Chapter 8 on calibration of the ASI.

Figure 3.3 Examples of the pitot tube (a) and static pressure port (b) on an aircraft.

The VSI, connected to the static pressure port, measures the time rate of change of pressure and converts this to a vertical speed. It is based on a mechanical comparison of the rate of change of static pressure with a known rate of change coming from a calibrated leak. Indications of vertical speed on the VSI tend to fluctuate so it is not as useful for measuring rate of climb or descent in a precise manner. Instead, for measurement of vertical speed in flight test, it is best to establish a steady rate and directly measure altitude from the altimeter and time with a stopwatch.

The altimeter (Figure 3.5) is based on a measurement of static pressure, which is converted into an indicated altitude. Altitude is displayed on the traditional altimeter by three hands, much like an analog clock displays time. The long, thin pointer with a triangle at the end indicates ten thousands of feet; the short, thick hand displays thousands of feet; and the medium length, slender hand displays hundreds of feet. So, the medium, slender hand makes a full revolution of the dial every 1000 ft; the short, thick pointer makes a complete revolution every 10,000 ft; and the long, slender pointer makes a full revolution every 100,000 ft. For example, the altimeter depicted in Figure 3.5 indicates an altitude of 10,180 ft.

Figure 3.4 Schematic of the pitot‐static system.

Source: Flight Instruments, Federal Aviation Administration.

Figure 3.5 Diagram of an aircraft altimeter. The reference pressure is set by the knob on the lower left side and indicated in the Kollsman window on the right side of the instrument.

Source: Bsayusd, A 3‐pointer pressure altimeter. Originally from en.wikipedia.

The calibration of the altimeter is based on the pressure lapse rate of the standard atmosphere (assuming standard temperature). As discussed in Chapter 2, the pressure lapse rate is extremely consistent, which enables the altimeter to provide a highly accurate measurement. However, the local barometric pressure can change significantly, which would have a first‐order impact on the indicated altitude provided by an altimeter. Thus, an altimeter must account for variations in local barometric pressure reading, which is done by setting a reference pressure. This reference pressure appears in a small window on the right side of the altimeter and is set by a knob on the lower left side of the instrument (see Figure 3.5). It is important to note that the reference pressure is not related in any way to the actual pressure at some arbitrary altitude. Instead, the reference pressure simply shifts the base of the calibration curve, providing an offset above or below standard sea level pressure in order to accommodate the actual local barometric pressure reading and providing a more accurate reading of altitude.

This type of altimeter – the so‐called sensitive altimeter – was first invented by Paul Kollsman, leading to the naming of the Kollsman window where the reference pressure is read on the altimeter (e.g., the reference pressure shown in Figure 3.5 is 29.92 inHg). The Kollsman setting essentially shifts the altimeter's calibration curve in order to account for local variations in sea level pressure, which is routinely encountered due to weather variations (e.g., low or high pressure regions moving through a geographical area). Since accurate measurement of altitude is critical in most flight testing work, we will devote some attention to the calibration and correct setting of the altimeter.

The most typical use of the altimeter under routine flight is to set the Kollsman reference pressure to the local barometric pressure, such that the indicated altitude is the aircraft's height above mean sea level (MSL). A pilot can easily obtain up‐to‐date readings of the local barometric pressure by listening to broadcasts from local weather stations (such as the Automated Weather Observing System, or AWOS, found at many airports). Alternatively, air traffic control (ATC) will often report the local barometric pressure setting, especially when aircraft are transitioning from one controller to another with responsibility for different geographical areas. Reporting of the local barometric pressure is an important function for ATC, since they aim to maintain consistent vertical separation between all aircraft. Thus, for a pilot to ensure an accurate reading of MSL altitude, the altimeter must be set to the correct local barometric pressure reading (viewed in the Kollsman window) by adjusting the Kollsman knob.

However, in flight testing, we often wish to know the pressure altitude, or the pressure at our flight altitude. This can be accomplished by setting the altimeter to a reference pressure of 29.92 inHg (1013 hPa) instead of the local barometric pressure reading. Under this situation, the altimeter will not provide an accurate indication of height above sea level; rather, it will indicate pressure altitude. A flight test engineer can readily take the measured pressure altitude and convert this to a value of the local freestream static pressure via the standard atmosphere, using the theory described in Chapter 2[3].

3.1.3 Outside Air Temperature

Knowledge of ambient (freestream) temperature of the flight environment is important for calculating the freestream density from the ideal gas law and for determination of true airspeed. The challenge of measuring outside air temperature (OAT) from a moving aircraft is related to the frame of reference of the air (the desired temperature) versus the aircraft reference frame (where temperature is measured). Since the air is moving relative to the aircraft, the temperature measured in the aircraft frame of reference will be higher. If a temperature probe completely stagnates the flow (i.e., with no loss of energy), the isentropic Mach relation,

(3.1)

may be used to find the freestream temperature (T∞) from measurements of the stagnation temperature (T0) and the freestream Mach number (M∞), where γ = 1.4 is the ratio of specific heats for air. However, most temperature probes do not fully recover the full stagnation temperature, so a recovery factor (k) must be introduced. Furthermore, there may be differences between the local Mach number (Mℓ) and the freestream Mach number, due to local acceleration or deceleration of the flow around the aircraft body. Thus, we can modify Eq. (3.1) to accommodate these two factors,

(3.2)

where Mℓ and k can be found by calibration (Gracey 1980).

3.1.4 Other Instrumentation

Other relevant instruments on a traditional cockpit panel include a clock (for timing various events), the engine tachometer (for measuring engine speed in RPM), manifold pressure for the engine air intake, fuel flow rate, and total fuel burned. In traditional flight testing at the university level, readings from the “steam gauge” instruments must be manually recorded via pen and paper. These manual data recording techniques can be effective, as long as the time scale of any transient characteristic of the flight maneuver is long relative to the speed at which data can be manually recorded. For example, the rate of climb in a sustained, steady climb can be easily recorded by hand with a stopwatch and the altimeter, while the level acceleration of an aircraft would be much more difficult to manually record since velocity is changing quickly over time.