This place has been awfully quiet for a while, so I figured it was time for something interesting: How do you measure the performance of a helicopter?
A helicopter has to be able to hover and hovering requires more power than forward flight, so hover performance is by far the most important thing for helicopter performance.
In summary, you wind up hover testing by changing only two variables: hover height and referred weight(weight divided by density ratio).
So you tether the helicopter to the ground and hover while pulling against the cable. The cable length sets your hover height and the tension on the cable effectively adds to the aircraft weight, this lets you collect data for a range of weights very quickly. To get a range of referred weight large enough to cover all possible ambient conditions you will usually do the testing at a low altitude location in winter (lower referred weight) and a high altitude location in summer (higher referred weight). You have to repeat the testing for each aircraft configuration(for example, a blackhawk with wings and without wings or cabin doors open/closed both make a significant difference in hover performance).
Most people who do this do the low altitude testing at home station and travel to Colorado for the high altitude testing. Buena Vista, Leadville, and Gunnison are common locations with tethered hover pads - if you're passing through those places at sunrise in the summertime with no winds you might get to see some very interesting aircraft.
From a pilot perspective, you have to hover perfectly stable with the cable perfectly vertical. Engineers will be watching time histories of flight control positions, cable tension, cable angle, and engine parameters and looking for short periods of time (3-5 seconds or more) where all those parameters are as stable as possible.
For a tiny helicopter and short cable like below, you have to maintain the aircraft position within about 6 inches in order to keep the cable angle acceptably close to vertical. For an OGE length cable the allowable position increases greatly, a chinook on a 150 foot cable has to hold position within about 20 feet.
Attached File Attached FileOnce you're done, you get a chart like below for each aerodynamic configuration of the aircraft. The lower line is for normal hover height and the upper line is for OGE.
Nerds take this data and generate the hover charts that are in the operator's manual of your helicopter.
Note on the graph that the lines do not pass through the origin. The helicopter this data is for requires about 280 horsepower just to spin the rotors at flat pitch.
Attached File