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 File


Once 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

Interesting

cool, always thought it had to do with the # of commies you could toss out of one w/o landing to get more

A former member here (also a 13er) had a FIL who was one of Bell’s designers and flight test engineers. Bet he had some great and terrifying stories. I never flew rotor, but understand there was always the tension between speed at level flight vs hover/lift performance.

I didn't know I wanted to know that information, yet there it is. Thanks for sharing, OP!

The parts fly in close formation and it always, always leaks oil?

I always thought the 47D/F had pretty decent performance

My beloved OH58D? Not so much...

discuss height required to escape ground effect?

You do a “hover ladder” to determine that and ensure that your OGE test height really is aerodynamically OGE.  
Record the power required at various hover heights and once the power peaks that is oge.  Generally varies between 1 and 2 rotor discs.  

good info, thanks.

I learned something here today. I am in the flight path of Ft. Rucker and watch them fly over almost daily. It is fun to watch. They rent a farmers field a couple of miles away for training

Buy land and get a contract for Rucker to use your land - it’s a nice side income with no work from you besides mowing the field a few times per year.  

When I was in flight school I had an instructor who had a neighbor that knew Hank Williams Jr.  Hank had a gravel ultralight strip in his front yard and I got to land at Hank’s house one day!


Difficult isn’t the right word, it’s just different from any other hovering.  It takes practice to get proficient, but unfortunately it’s not possible to practice it without actually doing it.  US military branches all have requirements to be a test pilot but FAA rules do not.  

I was wondering if risks would come up….  The obvious risk is dynamic rollover.   Equally hazardous are the risks of the cable breaking when under tension or jerking the cable(just like it sounds - have slack in this cable and jerk it tight).  Either one can easily structurally damage the aircraft.  Parts flying upwards through the rotor should any part under tension fail is obviously dangerous, but when you have an instantaneous change in aircraft weight(ie a cable breaks) the rotor is coned for the higher weight, so you can get a vertical oscillation and if that oscillation is near the natural frequency of the rotor it can resonate and destroy the aircraft.