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Helicopter Pace Truck

A pace truck may be used in helicopter flight tests to achieve precise airspeed and direction. A pace truck is a normal truck, but with a boom mounted from the bed and an anemometer atop the boom.

Diagram of a pace truck — Figure 1 - Diagram of a standard pace truck.

This setup provides more precise measurement of total air velocity, including the wind velocity and the motion of the pace truck through the air. The helicopter pilot watches the pace vehicle and closely matches the speed and direction of travel. In this way, the helicopter matches the true air velocity of the pace truck.

Why is this necessary? After all, helicopters are already equipped with pitot systems that measure airspeed. The problem is that these pitot systems (1) are only reliable in higher-speed forward flight (e.g. over 45kt forward) and (2) these systems are typically unable to measure the direction of the airflow (sideslip or angle of attack).

Simple example

Let’s consider a simple example first. Let’s say we want to measure various things like power and control positions of a helicopter at 30 kt of forward speed on a day with 0 wind. A pace truck will drive down the runway and maintain a 30kt reading from its anemometer. The helicopter will fly at a slightly higher altitude than the anemometer, but maintain the same heading and speed as the pace truck. In this way the helicopter will be flying with 30 kt forward airspeed. Various values from the helicopter may be recorded in the steadiest part of this flight.

Helicopter following a pace truck in a simple case — Figure 2 - A helicopter following a pace truck in a simple case, with no wind and measuring 20 kt forward airspeed.

Example with sideward flight

Engineers also need to evaluate a helicopter flying to the right, left and aft. For example, if a helicopter takes off on a windy day, it could be facing any direction relative to the wind. If the helicopter was sitting on the ground facing north, but there’s a 20kt wind from the east, then the helicopter will be in the equivalent of a 20kt rightward flight as soon as it lifts off the gound. Engineer’s must prove the helicopter is safe and stable in such conditions.

With a pace truck, this test may be done even on a windless day. The pace truck drives down the runway holding a 20 kt airspeed (without wind, this means 20 kt groundspeed as well). The helicopter pilot again follows the pace truck, flying the same speed and direction, but in this case keeps a heading 90deg below the pace truck, as shown in the diagram below.

A helicopter following a pace truck on its right side to acheive 20 kt rightward flight with no wind — Figure 3 - A helicopter following a pace truck with no wind and a 90 deg offset heading to achieve rightward flight.

Example with wind

Now let’s say there’s a 10kt wind from the north, and we want to test the helicopter flying rightward with 20kt airspeed. Let’s also assume the runway goes east/west. Assuming the pace truck drives on the runway, it can acheive 20kt airspeed by driving 17.3 kt east (total airspeed will be the square root of 10^2 + 17.3^2 or 20).

We want the helicopter to have this 20 kt airspeed from its right side (air moving right to left across the helicopter). The pace truck’s anemomenter will show that he airspeed is coming from arctan(17.3/10) or about 60 deg (east of north). As a result, the helicopter will need to maintain a heading of -30 deg (330 deg) while following the pace truck, to have a righward 20 kt airspeed.

A helicopter flying east 17.3 kt with a 10 kt North wind to achieve an effective 20 kt rightward airspeed — Figure 4 - A more complicated example with wind. The helicopter maintains a -30 deg heading while flying east with a pace truck to achieve a 20 kt rightward airspeed.

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