by Dick Roemer Senior Project Engineer EAA Flight Research Center

How often have we flown airplanes that performed worse than advertised? When discussing performance, are we talking about indicated airspeed, calibrated airspeed, true airspeed, ground speed or maybe even equivalent airspeed? Is it really necessary to understand these terms? Let's answer the last question first. In order to measure the performance of an airplane and to be able to communicate this information to others, an understanding of the terms is essential. What we see on an airspeed indicator is not necessarily a true representation of the speed of the airplane. It does not indicate how fast the airplane is going over the ground or through the air except under certain conditions. With an understanding of the airspeed system, we can accurately determine the speed of the airplane relative to the air and ground. Diagram A represents the relationship of the various airspeeds. Remember ICE TEA: I C E T, the first letter of each of the airspeeds. IAS — Indicated Airspeed — the speed of the airplane relative to the wind read from the airspeed indicator. This speed may not be accurate due to instrument and position errors. CAS — Calibrated Airspeed — the IAS correct for instrument and position errors. EAS — Equivalent Airspeed — the CAS corrected for compressibility error. This error, caused by high airspeeds and/or high altitudes, is normally less than 1 mph for homebuilts and is disregarded. We will use CAS instead. TAS — True Airspeed — the CAS adjusted for altitude and temperature. This airspeed measures the actual airspeed of the airplane relative to the ground under no wind conditions. G. S. — Ground Speed — the speed of the airplane over the ground. This speed can be determined in several ways including measurement of time over a known distance or adjusting the TAS for wind at altitude. In the "perfect" airplane there would be no errors and the IAS would be the same as CAS. CAS is the speed that the airplane "feels" and is the speed engineers use in designing the airframe.

When an airplane stalls at 50 mph or has a dive speed of 185 mph, these represent CAS to the engineer. But what about the pilot? Per FAR 23.1545, the airspeed indicator must be marked in IAS. This is logical since the pilot is dependent on the airspeed indicator, even with its errors, for information. These errors cause the IAS to differ from the CS due to both instrument and position errors. Instrument error is caused by friction in the diaphram, gears and levers. For an IFR qualified instrument, these errors are typically less than 2 mph below 140 mph and can be as much as 4 mph above 200 mph. Position error is related to the location of both pitot head and the static source. The airspeed indicator determines the airspeed by measuring the difference in air pressure between the pitot head and the static air source. The pitot head senses total pressure which is both moving air pressure and the static pressure (the ambient atmospheric pressure) while the static source tries to sense only atmospheric pressure. By mechanically subtracting the static pressure from the total pressure, the airspeed indicator measures the moving air pressure, dynamic pressure, or airspeed as we call it. However, if the pitot head is located near disturbed air such as in the prop wash, an erroneous reading will result. Also the location of the static air source is critical since any air pressure other than ambient atmospheric pressure will

cause airspeed deviation. This is why many airplanes have a static air source on either side of the fuselage to compensate for side slips where one source will be pressurized while the other side will be below atmospheric pressure. The net result of which is atmospheric pressure. Depending on the location of the pitot head and static air source, changes in power setting and/or flap setting cause changes in airspeed. In a flight test airplane, a trailing bomb is used to measure the true undisturbed atmospheric static pressure. This is a bullet shaped metal device that is trailed several wing spans in distance behind the airplane. Also, a wing tip mounted, gimbled pitot head is used to measure the true undisturbed total pressure. A gimble is used to keep the pitot head in line with the relative wind during flight at high angles of attack or large yaw angles. Various pitot head/static pressure locations are compared to the true readings of the flight test instruments. The chosen location is generally a compromise since there is never a position on the airplane that is totally free of local air disturbances. A less expensive alternative to elaborate flight test instrumentation is to install and calibrate a pitot static system by first locating the pitot head below the wing but outside the propeller arc. Then place static air sources on the right and left side of the fuselage. Try the aft fuselage first. These installations should be temporary since relocation may be necessary if gross errors result during flight tests. To calibrate the system, we will use the measured course method that consists of flying an exact IAS over a measured course and timing with a stop watch. This method will calibrate the whole system, taking into account both instrument and position errors. Lay out a course of exactly 1, 2 and 3 miles that can easily be seen from the air. Fly over the one mile course at indicated airspeeds from 1.2 V stall to 80 mph, the two mile course between 80 and 120 mph and the three mile course above 120 mph. The height above ground should be low enough for the course end markers to be easily seen but high enough for safety of flight. The airspeed should be held exactly as should be the altitude. Use a 1/2 mile lead in to the course to stabilize the 63

airplane. A two way pass is necessary to minimize any wind correction. Deviation in time of more than 10 percent indicates too much wind for testing. Use 10 mph increments to establish a good curve when plotted. Remember that speed equals distance divided by time. For a stop watch calibrated in minutes and seconds, use these equations to determine airspeed: 1 mile course: Speed (mph) = 60 divided by minutes 2 mile course: Speed (mph) = 120 divided by minutes 3 mile course: Speed (mph) = 180 divided by minutes Hint: To convert minutes and seconds from stop watch to minutes, divide seconds by 60 and add to minutes. Example: 1 minute 34 seconds = 34 divided by 60 = .57. Thus, 1.57 minutes. If this was timed in a 3 mile course, calculated speed equals 180 divided by 1.57 = 114.6 or 115 mph.

Before the flight test, set up the following table from 1.2V stall to maximum speed in 10 mph increments. As an example, V stall is estimated at 50 mph, 1.2V stall = 60 mph.

Upon completion of flight test, plot the data so that a calibrated curve of your airspeed system can be developed. The calculated speed will now be called calibrated airspeed or CAS. Once the data is plotted, the curve can be extended to V stall (50 mph on 64

sure is high and CAS is the same as TAS. But as we climb in altitude, the pressure decreases. If we were to maintain the same TAS, the CAS would continue to drop as we climbed. We can see from the chart that 120 mph TAS is equal to 120 mph CAS at sea level but drops to 95 mph CAS at 15,000 feet.

rated airspeed system, we can determine our TAS. The TAS is calculated from the CAS by using pressure altitude (Kollsman window set at 29.92) and outside air temperature (OAT). For a given CAS, the higher the airplane flies or the warmer the OAT, the higher will be the TAS. Conversely, the higher the airplane flies for a given TAS, the lower will be the CAS. If the example airplane could fly to 47,000 feet, it would stall at

the CAS scale in the example) as shown by the dotted line on Chart A. You now have an indication of what your airspeed indicator reads at stall speed without having flown at that speed. What you actually read at stall may be different due to errors introduced at high angles of attack. To use this chart, enter the bottom of the chart at an indicated airspeed and read the corresponding CAS. Or enter the chart from the left with CAS and read the matching IAS. As an example, for 70 mph IAS read 65 mph CAS. Now that our airplane has a calib-

120 mph TAS (50 mph CAS)! This is

illustrated in Charts B and C. Chart B demonstrates the effect of altitude on TAS. At sea level, the air pres-

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Chart C illustrates how TAS increases with altitude if a constant CAS is maintained. As altitude is gained and air pressure drops, the airplane has to fly faster to maintain the same pressure at the pitot head. At 15,000 feet, although indicating only 120 mph, the airplane would actually be flying at 151 mph relative to the ground (assuming no wind). Finally we address the ground speed, G. S., which is TAS adjusted for the wind at altitude. The airplane will fly the same TAS regardless of the wind but the effect on ground speed is directly proportional to the wind. Through the use of the venerable E6B computer or newer electronic flight calculator, the wind direction and velocity, airplane ground track and TAS are used to determine G. S. This speed is used to estimate time to fly known distances. Hopefully now when you hear someone say, "I can sell you a 150 mph airplane," you will realize that you'll never see 150 mph on the airspeed indicator in level flight unless at standard sea land conditions with an error free airspeed system. You will also realize why two airplanes flying side by side might have different IAS. Ice tea anyone?

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