How does VNE vary with altitude?
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Cap'n Jack
Final Approach
Commercially built aircraft place the redline such that when flown with its envelope, bad things don't happen at Vne. You are correct that air density ( altitude ) plays a role. It's article explains things well http://www.vansaircraft.com/pdf/hp_limts.pdf
warthog1984
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VNE does not change. Too fast is too fast and airspeed is airspeed.
Va changes (increases with increasing weight), but not altitude.
Va changes (increases with increasing weight), but not altitude.
Capt. Geoffrey Thorpe
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Depends.
Flutter tends to be a function of TAS - so you will flutter at lower indicated speeds with altitude, direct aerodynamic loading tends to be a function of IAS - so you can fly at higher TAS at altitude.
So, to say what would happen to VNE with atltitude, it would depend on what is the reason for selecting the particular VNE.
Flutter tends to be a function of TAS - so you will flutter at lower indicated speeds with altitude, direct aerodynamic loading tends to be a function of IAS - so you can fly at higher TAS at altitude.
So, to say what would happen to VNE with atltitude, it would depend on what is the reason for selecting the particular VNE.
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Velocity173
Basic VNE is an indicated airspeed but flutter is affected by TAS. The velocity of air thats actually going over the control surfaces. As you go higher generally the TAS needs to be reduced with an increase in altitude for stability. While you might have the same impact velocity (IAS) youll have less stabilizing static pressure on the sides of the aircraft. Thus a reduction in IAS to keep the aircraft in safe margins. For some aircraft it's not a set VNE TAS either. For instance, in the B407 VNE is 140 KIAS all the way up to 4,000 ft at 0 degrees. Then IAS needs to be reduced with a corresponding TAS.
Also for some aircraft like business jets or helicopters you have Mach limiting TAS as you get into colder air at higher altitudes.
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BillTIZ
Final Approach
Really, I did not know that...
Indicated Vne for some aircraft does decrease with altitude. Read the other posts on the effects of TAS. My glider has lower Vne markings with increasing altitude.
Because the redline is an IAS number and Vne is a TAS number. If you fly a 200kt IAS with a 205kt Vne at 15,000' on an ISO standard day, you are flying 260kts TAS, or 55kts ABOVE Vne.
warthog1984
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Really, I did not know that...
I know you're being sarcastic, otherwise I'd be concerned.
No, VNE does not change with altitude. The IAS corresponding with VNE may change, but the TAS does not; unlike VA which is backed into a range of airspeeds from the forces required to stress the airplane to the limits vs stallling it.
While the TAS for Vne does not change with altitude, the IAS that correlates with that TAS does change with altitude, that is why "redline" IAS speed reduces with altitude and why 'barber poles' replace 'redline' in more sophisticated aircraft. Airspeed is NOT Airspeed, you have IAS, CAS, and TAS, and they all vary from each other with different operating parameters.
kenjr
Line Up and Wait
IAS is IAS - you don't need to adjust for altitude...that's the beauty of the airspeed indicator. IAS is simply the speed of air over the wings - doesn't matter if you're flying over a swamp in Miami or in the mountains around Denver, the pitot/static system will figure it out for you and indicate what the speed of air is over your wings. Your TAS will be higher up there (because you have to go faster, say 120 knots up there to indicate 100kts).
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Right, however Vne is NOT an IAS dependent figure as are stall and flap speeds. Vne is a TAS dependent figure, so if you rely on the Redline painted on the ASI which indicates IAS to keep you safe at 15,000', you will be in deep **** if you match the needle to the redline because you will be greatly in excess of Vne.
Captain
Final Approach
IAS is not the speed of the air over the wing. TAS is the speed of the air over the wing.
Example: You are flying at 130,000 feet with only 3 air molacules per square centimeter at a TAS of 1,000 kts. You fly past a point in the air marked by a floating balloon.
With everything staying constant for one hour the balloon will be 1,000 nautical miles away and the whole time the airspeed indicator will have read 1 or 2 kts. The air molacules go over the wing at a velocity of a 1,000 kts (TAS) but the difference in pressure with only 3 molacules per square centimeter give a very low indication (IAS).
The maximum airspeed an airplane is limited by reg isn't necessarily Vne. As Bruce points out, as you get to higher speed, the never exceed speed isn't some red line on the indicator. However that maximum operating speed *is* not Vne.
Flutter depends on true airspeed, but you'll find that the flutter requirements aren't based on Vne either.
There are some testing that is done on Vne, but you'll find they're not things that are going to be too dependent on TAS. Almost all are pressure effects and not speed through the air.
Flutter depends on true airspeed, but you'll find that the flutter requirements aren't based on Vne either.
There are some testing that is done on Vne, but you'll find they're not things that are going to be too dependent on TAS. Almost all are pressure effects and not speed through the air.
Captain
Final Approach
BTW, with my last example of a plane flying at 130,000 feet, if a plane stalls at 60 kts indicated (level flight) then your TAS would have to be increased from the 1,000 kts (indicating 1 or 2 kts) to a much faster speed to get an indication above the 60 kt stall speed.
wabower
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Bottom line to all this is that pilots really start to understand it when they have the opportunity to fly a high-performance plane with a barber pole incorporated into the ASI. Does anybody know if any of the flight-sim programs include such instrumentation for GA planes?
My buddy the U-2 pilot points out that during much that turns put one wing in mach buffet and the other in stall buffet.
kontiki
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I just looked the TAS altitude correction up for one of those Quizes in IFR magazine.
To find TAS = add 2% to CAS for every 1000ft.
Best quiz yet, i got 50%. It's pathetic! I love that feature though.
To find TAS = add 2% to CAS for every 1000ft.
Best quiz yet, i got 50%. It's pathetic! I love that feature though.
And CAS requires correction from IAS.
kenjr
Line Up and Wait
To clarify:
TAS has nothing to do with the actual speed of the air over the wings. It's the actual speed of the airplane relative to the freestream air.
Planes generally fly using IAS or Mach. Stall speeds, dive speeds, flap/slat/gear speeds, maximum operating speeds, etc. are all given in IAS or Mach, far as I know.
IAS = Speed the plane thinks it's flying.
TAS = Speed the airspeed indicator would be reporting if it took into account atmospheric air density.
GS = Speed the plane is actually going accounting for wind. which one can get by comparing the TAS against a simple weather forecast. GPS solves the calculating, though.
Forget about Equivilant and Calibrated airspeeds, however, because they are trivial for the most part---except for statistics buffs and engineers.
The best way I had it described to me when I was learning was to think of 2 pitot tubes - one in Denver and one in Miami. The air is more dense in Miami so a plane could travel 100 knots to indicate 100 knots. However, in Denver since the air is less dense the plane has to travel faster to get the same amount of air into the pitot tube to indicate 100 knots. That delta is the TAS. So in the Denver example, the plane would have to travel something like 120 knots true to indicate 100 knots.
To take it a step further - groundspeed / TAS.
Think of swimming in a river. Your a good swimmer and can swim at 4mph. But, you're swimming into a current of 3mph. Your "TAS" is still 4mph and your groundspeed (think if someone was on the shore with a radar gun) is only 1mph.
If you flip it and you're now swimming downstream your TAS is still 4mph but now your groundspeed is 7mph.
TAS has nothing to do with the actual speed of the air over the wings. It's the actual speed of the airplane relative to the freestream air.
Planes generally fly using IAS or Mach. Stall speeds, dive speeds, flap/slat/gear speeds, maximum operating speeds, etc. are all given in IAS or Mach, far as I know.
IAS = Speed the plane thinks it's flying.
TAS = Speed the airspeed indicator would be reporting if it took into account atmospheric air density.
GS = Speed the plane is actually going accounting for wind. which one can get by comparing the TAS against a simple weather forecast. GPS solves the calculating, though.
Forget about Equivilant and Calibrated airspeeds, however, because they are trivial for the most part---except for statistics buffs and engineers.
The best way I had it described to me when I was learning was to think of 2 pitot tubes - one in Denver and one in Miami. The air is more dense in Miami so a plane could travel 100 knots to indicate 100 knots. However, in Denver since the air is less dense the plane has to travel faster to get the same amount of air into the pitot tube to indicate 100 knots. That delta is the TAS. So in the Denver example, the plane would have to travel something like 120 knots true to indicate 100 knots.
To take it a step further - groundspeed / TAS.
Think of swimming in a river. Your a good swimmer and can swim at 4mph. But, you're swimming into a current of 3mph. Your "TAS" is still 4mph and your groundspeed (think if someone was on the shore with a radar gun) is only 1mph.
If you flip it and you're now swimming downstream your TAS is still 4mph but now your groundspeed is 7mph.
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Captain
Final Approach
That is a very tight coffin corner indeed, but given the plane I believe it. The tightest I've ever seen was about 40 knots, I think.