03 January 2010

Cold Temperatures And Flying

She asked, "How does the cold affect the helicopter?"
Then she was surprised when I said, "To a point, the colder it gets, performance-wise, the better."
Do you know why?
Do you know why, for a helicopter, the power requirements for a given performance level begin to increase again at a certain point as the temperature drops? (But that is a COLD, COLD temperature!)
In the comments, ddf says:
speed of sound (m/s) = 331.5 + 0.60 T(°C)

The speed of the advancing blade must be kept below .92 mach 1.

And he's right. Remember, helicopters have many parts not flying at the same speed as the airframe.

Back while I was still flying Hueys for the ARMY, I had put in a request on my "Dream Sheet" for an assignment to Alaska. Having heard from others about the severe temperatures they encountered there, I was reviewing the performance charts in the UH-1 operator's manual. I noticed the "required power" to hover "in ground effect" at a certain altitude went down until the temperature reached -40 degrees, (coincidentally the point at which the Fahrenheit and Celsius temperature scales join), and then markedly increase. The reason?
At that temperature, the 48' 3" diameter rotor of the Huey begins to experience the effect of "compressibility" related to approaching Mach 1. (Read the "Forward Speed" portion of the article here. ) As temperature drops, the speed necessary to reach Mach 1 also drops because the air molecules are packed closer together and transmit sound more efficiently.

More modern helicopters have shorter rotor blades, so we can assume the temperature would have to drop even further to experience this phenomena because their advancing blade tip speed is lower, but the principle would still apply.
Colder is better for performance in the helicopter, but only to a certain point.


jinksto said...

Caught my attention with this one. I assume that the increase in performance works the same way that it does in airplanes in that colder air is denser and provides more molecules to affect positive lift. I haven't a clue about why extreme cold would decrease performance though? Does it have to do with fuel combustion? I could imagine that at some point the amount of force required to move a rotor through the air would be more than the gained lift. Sort of a point of diminishing returns as the air grows denser. However, I can't imagine that this would occur at any naturally occurring temperature.

The Old Man said...

I would go with density and vapor pressure.

Greybeard said...

Yeah, air density is at the core of the phenomenon. Vapor pressure? That may have something to do with it too, but that would be "above my pay grade".
Think helicopters guys, and what makes them unique in all the world.

Tim said...

Performance increases as it gets colder because the denser air allows for a better "bite".

However, as it gets "too" cold, bearings in the rotor system start to stiffen up, causing increased system loads and degrading the performance.

Greybeard said...

It DOES have to do with the rotor, Tim,
but it IS NOT mechanical.

ddf said...

speed of sound (m/s) = 331.5 + 0.60 T(°C)

The speed of the advancing blade must be kept below .92 mach 1.

Bloviating Zeppelin said...

I know this:

HEAT + ALTITUDE + WEIGHT = a poorly performing helo.

You can try to PULL all the PITCH you want, but sometimes. . .