Helicopter Flight Test Challenges

Flight test data is critical for helicopter design, evaluation, and even creating pilot training simulators. Despite technological advancement, flight test data is fraught with a number of significant problems. In this article, we discuss common problems with helicopter flight test data.

Altitude

A number of systems can measure altitude in a helicopter. A radar can measure the distance from the helicopter to the ground (radar altitude), pitot ports measure air pressure (which correlates with altitude), and a GPS system measures altitude via the distance from several orbiting satellites. Each system has strengths and weaknesses.

Radar altitude is most useful when flying low over level terrain. Above a certain altitude, the receiver won’t get a strong enough signal reflection from the ground to estimate height above ground. Additionally, radar altitude must be used carefully. A change in radar altitude does not imply a change in helicopter altitude—a helicopter maintaining altitude will show decreasing radar altitude when approaching a mountain or upslope. Even around a “flat” airport radar altitude will jump discontinuously as the helicopter flies over buildings or other aircraft.

Pressure altitude is based on air pressure measurements from one or more pitot-static systems onboard the helicopter. While this is often the primary altitude measurement used in cockpit displays, it’s prone to errors. Changes in rotor downwash, ground effect, or angle of attack (among other things) may cause pressure fluctuations that offset pressure altitude over 100 ft.

Airspeed

Airspeed is one of the most fundamental parameters in flight test. It’s simply the speed of the helicopter relative to the surrounding air mass. For example, a helicopter hovering at a fixed position has a 10 kt airspeed if there’s a 10 kt wind, and a helicopter flying 100 kt relative to the ground has a 100 kt airspeed if there’s no wind.

Even airspeed measurement suffers from many limitations. Like pressure altitude above, it’s measured by one or more pitot-static systems. These systems typically cannot measure airspeed below about 35 kt.

Above 35 kt there are still issues. Close to the aircraft, where the pitot system must be located, the presence of the aircraft disturbs the flow of air. For example, this is most notable under the main rotor where downwash in excess of 50 fps can exist. Even under the fuselage, the presence of the fuselage creates significant disruption to airflow, moreso at higher speeds. Engineers account for all this when deciding where to place pitot ports, but there’s no perfect solution. Worse yet, the error introduced by this phenomena changes with airspeed, sideslip, angle of attack, ….

The raw airspeed measured by the pitot system is referred to as indicated airspeed (IAS). IAS also has errors as a function of the air temperature and pressure. These errors are easily corrected for if those quantities are known. Calibrated airspeed (CAS) is the value obtained after correcting IAS for temperature and pressure. We have calculators that show you the math and can perform this correction automatically.

True airspeed (TAS) differs from CAS due to the errors we mentioned initially (local flow anomalies). This can be more difficult to account for, but engineers often collect data to facilitate this correction. To do this, a trailing bomb is attached to a test aircraft via a long line and pulled behind the helicopter. This device measures airspeed free from the disturbances caused by the fuselage, rotors, …. Flying with this is dangerous, so it’s only done in well controlled test scenarios. Engineers can then make tables or plots relating the CAS (from a pitot system) to TAS (from the bomb) as function of airspeed, angle of attack, sideslip, …. This allows engineers to subsequently estimate TAS from CAS, angle of attack, sideslip, … without the need for the trailing bomb.

A pace truck can also improve airspeed measurement in flight test, particularly at lower speeds.

Wind

The last thing we’ll include is wind. While wind may be accurately measured at a tower on the grounds of an airport, the wind at the helicopter may be quite different. Wind can vary significantly with time and position. For example, the wind is typically stronger at higher altitudes. Gusts or shear can cause the wind at one location/time to substantially differ from a nearby location/time.

Lack of wind knowledge in flight test causes significant issues. For example, wind during a maneuver could cause a helicopter to misleadingly pass or fail a test required by the FAA for safety or performance. Another example is a helicopter simulator. Simulators are rigged to match flight test data. A simulator designed to match the roll or torque of a helicopter with significant (unknown) wind could provide negative pilot training.