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Vne in the cold


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I believe I just figured out the answer to this, but I'm interested in everyone's response since it baffled me for a while. Why would the Vne of a helicopter be reduced in very cold temperatures? I asked myself this after looking at the Vne chart for the 206L3.

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In high DA, retreating blade stall is an issue at high airspeeds and disc loading. In cold temperatures, the advancing blade can develop transonic flow at high airspeeds and disc loading. Transonic flow is bad because of sharply increased drag and a change in the center of pressure. Helicopter design in particular is a constant battle of tradeoffs.

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The cockpit placard considers altitude, air temperature, and gross weight. Note (Bell 206L3 charts) that at altitudes of 2000 feet and above when starting at the highest temperature listed, 45°C (113ºF) VNE increases as the temperature goes down, but at -40°C (-40°F) the trend reverses and VNE decreases as the air gets colder. This is apparently due to compressibility effects at the blades as the speed of sound decreases with temperature, as referred to in kona4breakfast's and SBuzzkill’s post above.

 

At about Mach 0.92, the effects of compressibility start to appear. The speed of sound at -40ºC is 1004 ft./sec (vs. 1126 ft./sec @ +20ºC). The Bell 206L3’s main rotor tip speed is around 760 ft./sec added to a forward flight speed of 95 knots (160 ft./sec) you’ve got (760 + 160)/1004 for a Mach .916, you’re right there. Link - Calculation of the Speed of Sound

 

F030nnc.jpg

 

“The all-engines-operating VNE is established by design and substantiated by flight tests. The VNE limits are the most conservative value that demonstrates compliance with the structural requirements (§ 27.309), the maneuverability and controllability requirements (§ 27.143), the stability requirements (§ 27.173 and 27.175), or the vibration requirements (§ 27.251). The power-on VNE will normally decrease as density altitude or weight increases. A variation in rotor speed may also require a variation in the VNE.”

 

“The regulation restricts to two the number of variables that are used to determine the VNE at any given time so that a single pilot can readily ascertain the correct VNE for his flight condition with a minimum of mental effort. Helicopter manufacturers have typically presented never-exceed-speed limitation data as a function of pressure altitude and temperature. This information was placarded as well as contained in the flight manual.”

 

Ref: FAA AC 27-1B; page G4; §27.1505

 

Also see - Does Vne vary with altitude?

Edited by iChris
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  • 1 year later...

I am going to elaborate on compressibility real quick. Smart people feel dee to chime in as its been a while.

 

Subsonic air=not compressible

Supersonic air = compressible

 

Local velocities along the blade exceed the actual relative velocity of the blade. This causes the blade to encounter supersonic airflow sooner than we would expect if we did the math without getting into crazy math.

 

When the airflow goes from supersonic (compressible) to subsonic (not compressible) a shockwave is created which can shift the center of pressure well outside the designed limits causing all the stuff that folks above mentioned.

 

Based on air density and all that we encounter supersonic flows sooner in cold temps so they actually effect us.

 

Thats off the top of my head so you might want to bounce it off a book written by someone smarter.

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Subsonic air is considered incompressible, but remember that it's not a matter of subsonic/supersonic. The transonic range exists between the two, and it's in this region that we see the development of the effects of compressibility.

 

Insofar as indicated airspeed, helicopters don't see high speeds, and don't climb to high altitudes. If you get in the cockpit of a turbojet aircraft with high Vne values (and Mmo values), you'll see that indicated numbers change considerably the higher you climb. You might have a limit of 350 knots indicated/calibrated at sea level, for example, but it may be considerably lower at cruise altitude. True airspeed increases, indicated airspeed decreases, and what's important isn't the indicating speed limit at altitude, but the mach limit due to compressibility. One may not be able to reach the airspeed limit at all, but the "barber pole" or mach limit becomes a factor.

 

Put another way, with the increase in altitude, air density decreases, true airspeed increases. With the same increase in altitude, temperature decreases, and with it mach value decreases; one gets closer and closer to the mach limit at lesser and lesser indicated (or calibrated) airspeeds. There comes a point, sometimes called the crossover altitude, where calibrated airspeed and the mach value match. Above that altitude, as one climbs, mach becomes the limiting factor (as temperature continues to decrease) and below it, indicated (or calibrated) airspeed is the limiting factor.

 

As mach, and compressibility, is a function of temperature, at lower and lower temperatures, mach 1.0 occurs at lower and lower velocities. At 40 deg C, M1 is 689 knots. At 0 deg C, M1 is 643 knots. Temperature decreases, the speed of sound decreases, and mach effects will occur at lower and lower speeds (which is why mach limitations occur at lower and lower speeds as one climbs to altitude).

 

Mach effects occur not just as a function of airspeed or velocity through the air, but local airflow over the aircraft will reach transonic speeds in some areas before others. Airflow on the back of the "hump" on the 747, for example, exceeds 1.0 in cruise in some cases at altitude, and can be heard as a snapping sound when approaching the aircraft mach cruise limit in cruise. Likewise, airflow across the top of a blade or wing is at a higher velocity than the free airstream, and compressibility issues occur sooner on some portions of an aircraft than another.

 

In the case of the rotor, the airflow velocity at a particular blade stage or station differ as one moves spanwise; at the tip of the disc, the velocity is of course higher and approaching limiting speeds or mach effects occur there first. Likewise, the advancing blade experiences effects in forward flight that the retreating blade does not. Harmonic issues spanwise due to the variation in these effects may become an issue in some aircraft at some temperatures when exceeding Vne.

 

Where the effects occur depend on the airfoil shape, location, temperature, and speed.

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