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Does Vne vary with altitude?


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Stumbled upon this question when studying for the ATP written:

 

RTC 8404 PLT124

How does Vne speed vary with altitude?

A - Varies directly with altitude.

B - Remains the same at all altitudes.

C - Varies inversely with altitude.

 

I answered C but my ASA prep book says the correct answer is B. Both Robbies and Schweizers have the Vne / Pressure Altitude chart. Error in the prep book?

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The answer was c. I took this test a few months ago and I studied with shepard air because of this reason. There were too many wrong/different answers with other programs. I had the asa book and a dauntless app to use while offshore. They often time conflicted with each other. Shepard guarantees to have the correct answers and back that with money back. The made it worth the $ that I was too cheap to pay originally.

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As a note on this, when the U2 was flying at very high altitudes, the indicated Vne and stall speed came very close to each other. The pilots only had a very small window of pitch attitude to play with. IIRC, it was less than 5 knots indicated between Vne and stall.

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Your absolute Vne is only one number which is the same no matter where you go. For example 102 KIAS in the R22. Maybe this question meant that nothing changes that number. Obviously, other airspeed limits exist at higher altitudes, but the airspeed which you should NEVER exceed is 102.

I think I'm just trying to make sense of the question.

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Stumbled upon this question when studying for the ATP written:

 

RTC 8404 PLT124

How does Vne speed vary with altitude?

A - Varies directly with altitude.

B - Remains the same at all altitudes.

C - Varies inversely with altitude.

 

I answered C but my ASA prep book says the correct answer is B. Both Robbies and Schweizers have the Vne / Pressure Altitude chart. Error in the prep book?

 

 

The answer was c. I took this test a few months ago and I studied with shepard air because of this reason. There were too many wrong/different answers with other programs. I had the asa book and a dauntless app to use while offshore. They often time conflicted with each other. Shepard guarantees to have the correct answers and back that with money back. The made it worth the $ that I was too cheap to pay originally.

 

The answer “C”; It’s a somewhat general answer often limited to retreating blade stall, to say what would happen to VNE with altitude, it would depend on the real reason for selecting the particular VNE. Like the R22, it’s not always retreating blade stall.

 

VNE may vary with altitude, r.p.m., temperature, and/or weight. Design limitations, vibration, flutter, or divergence often limit VNE.

 

VNE is determined under 14 CFR §27.1505 or §29.1505

 

Gleim:

Screenshot2014-05-21at14002PM_zps9522128

Edited by iChris
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Ichris - you are correct. However your question-example is a lot more clear then the op.

 

The written exams are not always clear. Sometimes the answers seem wrong. With the question the op is asking about, the answer for the written exam is, "Varies inversely with altitude." It may or may not be "c" on the test.

 

That's why I liked Shepard air. You know you're studying to pass the test instead of choosing what "should" be the right answer but isn't on the written.

 

Example- hammerhat's asa book says it doesn't change. One might wonder if vne changes directly or inversely

 

Pay the $, use shepard air. You will study the least amount of questions and have no question whether or not you're studying the correct answer(for the test). The test is hard enough without studying the wrong answers.

 

Disclaimer- I'm just a satisfied customer (passed 99%) and no way affiliated with sheppard air.

Edited by rotormandan
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  • 1 year later...

I completely agree that increasing altitude would decrease VNE if we are talking about TAS but if talking about IAS it should remain the same. Many cockpits can have placarded limits or lines drawn on the airspeed indicator for things like VNE or stall speed for this reason. As altitude increases, the pitot tube experiences decreasing air pressure at the same rate as the airfoil. I wonder if this might be what the question was asking.

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Jason, you reckon that the Vne will stay the same?? As an extreme example, go to 40,000' where the TAS is twice the indicated speed. An R-22 at 104 indicated is travelling at 208kt???

 

Doesn't happen, and a Huey at 20,000' needs a minimum of 40kt and a Vne of 60kt, not much to play with. Think about how thin the air is getting, and how much extra pitch is used to get some lift - and the poor old retreating blade is working like a demented ferret to hang in there.

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Jason, you reckon that the Vne will stay the same?? As an extreme example, go to 40,000' where the TAS is twice the indicated speed. An R-22 at 104 indicated is travelling at 208kt???

 

Doesn't happen, and a Huey at 20,000' needs a minimum of 40kt and a Vne of 60kt, not much to play with. Think about how thin the air is getting, and how much extra pitch is used to get some lift - and the poor old retreating blade is working like a demented ferret to hang in there.

 

I understand that power available will decrease with altitude. I was talking about the difference between IAS and TAS. As an example, imagine that a big fancy camera was just installed on the side of your R-22. As a result of structural limitations, your VNE is 80 KIAS. This number does not change with altitude. At sea level VNE is 80 KIAS, at 5,000 ft VNE is 80 KIAS, and at 10,000 ft VNE is 80 KIAS. As I stated in my previous post, your pitot tube sees the same air as your camera. If the air at the pitot tube is really thin because of high altitude then the air at the camera will be just as thin. In this case VNE remains the same at all altitudes.

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This assumes that the Vne you calculate remains HIGHER than the Vne imparted by the camera limitation. Meaning: just because the Vne for camera installed is 80 KIAS, doesn't mean the ACTUAL Vne isn't lower.

 

Using your example: Camera mounted Vne is 80 KIAS. At 10,000' PA and 10° C the Vne for the R22BII is 68 KIAS. In this case, your camera is no longer the limiting factor. So your statement about it not changing with altitude is incorrect.

 

It's not enough to look at one singular Vne and think that's the one that matters. If there are multiple Vne's that apply, the one that is lowest is the one you need to follow.

While your Vne chart may be used to gather TAS data, if you choose to take that extra step, the chart shows limits to be followed in IAS form (at least in my Robinson limited experience). So I'm not really sure why you even brought TAS into this.

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I completely agree that increasing altitude would decrease VNE if we are talking about TAS but if talking about IAS it should remain the same. Many cockpits can have placarded limits or lines drawn on the airspeed indicator for things like VNE or stall speed for this reason. As altitude increases, the pitot tube experiences decreasing air pressure at the same rate as the airfoil. I wonder if this might be what the question was asking.

 

IAS as a ratio to TAS decreases the altitude because the air is less dense with decreasing pressure as you climb. 60 knots IAS sea level standard atmospheric conditions will be pretty close to 60 knots TAS. But, 60 knots IAS at 10,000 feet MSL and standard atmosphere will be faster TAS.

For the wing/blades, the angle of attack to produce the same effective amount of lift will increase with altitude because of decreasing air density, hence the U2 'coffin corner' example previously posted. My limited understanding of jets is that it is possible for mach tuck and conventional stall airspeeds to be identical, so you could be overspeeding and underspeeding the wing at the same time.

If a helicopter had a strong enough engine at altitude you could encounter retreating blade stall with NO IAS even though a stationary hover was possible. You need to consider density altitude for more than engine performance and raw lift.

You should also read the supplement for installed equipment and determine if the VNE is for indicated or TAS.

As posted, use the lower of whatever effective limitation as your actual limit...

Edited by Wally
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Regarding retreating blade stall with increasing altitude and increasing TAS for the same IAS, the advancing blade, like the pitot tube, sees about the same old IAS it normally gets. It will behave pretty much in a normal fashion, though drag rise from mach effects will start to happen.

 

But the poor retreating blade, looking backwards, is still only doing its normal 102% RRPM, but now has a much increased airflow from behind. This limits the area of the blade able to produce lift, and the stall will happen at a lower IAS, because the TAS is much higher.

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1) I understand that power available will decrease with altitude. I was talking about the difference between IAS and TAS. As an example, imagine that a big fancy camera was just installed on the side of your R-22. As a result of structural limitations, your VNE is 80 KIAS. This number does not change with altitude. At sea level VNE is 80 KIAS, at 5,000 ft VNE is 80 KIAS, and at 10,000 ft VNE is 80 KIAS.

 

2) As I stated in my previous post, your pitot tube sees the same air as your camera. If the air at the pitot tube is really thin because of high altitude then the air at the camera will be just as thin. In this case VNE remains the same at all altitudes.

 

The original poster questioned whether letter C, varies inversely with altitude, was a correct answer to the question,” How does VNE speed vary with altitude?” These test questions and answers are often, incorrect, irrelevant, or ambiguous; therefore, they are best answered in general terms. Given that, the best answer is C, varies inversely with altitude, given most pilot’s experience with helicopters in general.

 

1) Yes, you can fly 80 KIAS at each of those altitudes; however, each represents a different TAS, which is what the aircraft and camera actually sees and feel as the actual velocity.

 

2) That’s correct, the pitot tube, aircraft, and camera all see the same airspeed, TAS which is the the actual velocity. However, here’s where the confusion lies, your airspeed indicator is a differential pressure instrument which measures airspeed indirectly by measuring the pressure differential between the static port and the end of the pitot-static tube, dynamic pressure.

 

The airspeed indicator actually measures dynamic pressure, calibrated to display airspeed based on sea level density. TAS = √2q/rho were q equals the dynamic pressure and rho is set to equals sea level density (.00237 slugs/ft.3). Therefore, KIAS (less position error CAS) is only equal to the TAS at sea level on a standard day, thereafter indicated airspeed will read lower than the actual true airspeed if we hold 80 KIAS as the altitude increases. At 10,000 feet, the ISA density would be .00175 slugs/ft.3, 26% less than sea level density.

 

As you can see, the indicated airspeed matters little outside the cockpit, it only stands as a means by which the pilot gauges the true situation, if the airspeed indicator’s limitations can properly be corrected for by placard or other means.

 

In all cases, per §27.1503 an operating speed range must be established that represent the full operating range sought for in the certification of the aircraft. The actual velocity, TAS, more accurately represents the limitations outside the cockpit, therefore indicated airspeed is corrected, via placard or other means, to accurately reflect the true velocity and ensure the pilot remains within the actual limitations. Given the 2% approximation rule (2% of 80 kts. this case), TAS increase 1.6 kts./1,000 ft. over the IAS if no correction is made.

 

So it is with other phenomena outside the cockpit, like retreating blade stall, which is more a tangential velocity (UT = Ωr + V sin azimuth angle) issue i.e. advance ratio, the ratio of the forward velocity to the rotor tip speed. Again, the pilot is the one that sees indicated airspeed, the aircraft only sees and feels the effects of TAS.

 

“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

Edited by iChris
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