IAS, TAS & VNE

[QNH1013.2]

Fellow pilot and I are discussing VNE.

One says that IAS at altitude is VNE, whereas the other says that TAS at altitude is VNE.

Eg. 9,500ft. IAS 100 knots; TAS is eg 115.
Aircraft VNE is 110.

Is the aircraft exceeding VNE?

Thanks.

[Icarus2001]

No it is not.

Vne is marked on the airspeed indicator as a red radial line. It is therefore indicated airspeed. What is the significance of Vne anyway, think about it.

[Merlins Magic]

Interesting

Vne is a structural speed measured with IAS. That said, the Vne, commonly represented by the barbers pole actually varies with altitude so it cannot be said that the Vne is a fixed IAS.

[Icarus2001]

Vne Vmo?

Barbers pole versus fixed red radial line.

The complexity behind a simple question.:eek:

Oh and there is this...http://www.pprune.org/forums/showthread.php?t=214121

Or this one...http://www.pprune.org/forums/showthread.php?t=226750

[Classic Dick]

Vne is an KIAS limitation. As are most limitations - flaps, gear etc,.TAS is EAS corrected for atmospheric conditions & EAS is IAS corrected for position & compressibility errors.

Vne is a static value eg 350 KIAS. What the "barber pole" does is vary that IAS limitation to a Mach No. limitation for high altitude flight. eg Vne might be 350 KIAS until F270 then M.085.

Icarus2001 is correct.

[djpil]

With that provision for varying the red-line - constant CAS / IAS changing to constant M at altitude - there are some aeroplanes where that M is relatively low so its not compressibility effects. Flutter speed is a function of TAS so one way of maintaining the required margin is to use that M limitation - you'll find that its not much different from a constant TAS line - if you ever see an example of this.
Above would generally apply to aeroplanes with a VMO / MMO not VNE. I note that the initial question mentioned very low airspeeds so the above is probably totally irrelevant to that gentleman.
VNE needs to provide margin from static structural point of view, vibration & buffet plus flutter. EAS, TAS and M are the principal variables. Little, slow aeroplanes its a constant CAS or IAS. OOps, nearly forgot, if its fixed pitch prop there's margin from engine overspeed with throttle closed.

[J430]

Gents and Gals,

I think the original post asked the question about wether an aircraft VNE was refferenced to the IAS or TAS.

He/she asked if his/her aircraft had a VNE of 110kts, and it was up at 9500 feet where on that day the TAS was 115kts, but due to the IAS being only 100kts at that height would it be exceeding its VNE?

Its a simple question with a simple answer...........but gee I love all the responses. Poor QNH is going to be one confused puppy!

Cheers J:ok:

[VH-XXX]

J430, you sound like YOU are the one that's confused!

[Dookie on Drums]

VH-XXX,

J430 was merely repeating the question asked by QNH who by now must be thoroughly confused :ugh:

It is a simple question with a simple answer.

NO. :=

[M.25]

I agree with J430!
I hardly think that an aircraft with a VNE of 110kts is going to have a barbers pole or be referring to Mach numbers!!

The answer is no. Structural speed limits in a light aircraft are IAS. This is why I always climb as high as possible on a very turbulent day to keep the IAS below the turbulence penetration speed, whilst maintaining a reasonable TAS.:ok:

[jack red]

Fellow pilot and I are discussing VNE.

One says that IAS at altitude is VNE, whereas the other says that TAS at altitude is VNE.


You're both wrong. Vne is a fixed KIAS value. It DOES NOT vary with altitude and is not subject to temperature and/or compressibility errors.

Classic Dick has correctly stated the position regarding "barber poles"!

If this is not simple enough, get yourself a copy of Handling the Big Jets and read it on your days off.:ugh:

[J430]

I was wondering how long it would take to get a sensible answer before the thread was derailed.

It seems we have some stute contributors here for once.

Jack Red says it how it is I believe "Vne is a fixed KIAS value. It DOES NOT vary with altitude and is not subject to temperature and/or compressibility errors.":D

Funny how a simple innocent question here gets all sorts of BS some times. But its entertaining to follow.:E

Hope old mate QNH has his/her answer now. But it will be interesting to read all the other gravity defiant transportaion theorists comments.

Cheers J:ok:

[Mr.Buzzy]

I would suggest speed limits are very closely related to TAS and G-limit loadings.
Think of it as like going over a speed hump in your car slowly or at a greater True Speed.

Incoming.....

BBBBBBBbbbbbbbbbzzzzzzzzzzbbbbbbbbbbbbzzzzzzzzzzzzzzzzzzzzz

[BAE146]

Hey Buzzy i think you may have hit your head a couple of times going over those speed humps.
jacky red has summed it up perrrrrrrrfectly:ok:

[Mr.Buzzy]

Vne is a static value eg 350 KIAS. What the "barber pole" does is vary that IAS limitation to a Mach No. limitation for high altitude flight. eg Vne might be 350 KIAS until F270 then M.085.


Hmmmm so why does my twin otter manual show a difference of Vmo of 166kn at sea level to 112kn at 25000feet? Surely we dont risk breaking the sound barrier in a twotter?

Maybe I have hit my head a few too many times but I am pretty sure limit G-loading comes into this and G-loading relates to movement in space, irrespective of how much air passes over the wings.

bbbbbbbbbbbbzzzzzzzzzzzzzzzzzbbbbbbbbbbbbbzzzzzzzzzzzzzzzzzz zzzz

[Andy_RR]

I think there are two questions being answered here:

- how should Vne be interpreted by the pilot - IAS or TAS? (I think the answer will be in the POH)

and

- how is Vne calculated or defined by the aircraft manufacturer? (I think the answer is more complex than even the views expressed so far)

One is relevant to pilots, the other is relevant to aeronautical engineers. In some cases pilots and aeronautical engineers are the same thing.

A

[Squawk7700]

Perhaps someone can answer a relevant question for me.

When I fly my trendy little homebuilt at VNE which is 140 IAS at Sea Level, my ailerons become fairly stiff and they loosen up at lower speeds, say VMO which is 120 knots IAS.

When I fly at say 9,500ft at a 140 K TAS I get the exact same "stiffness;" it's like I'm flying at VNE at the lower level.

If I fly at 140 KIAS at 9,500ft it feels like the wings are gonna fall off and feels like I'm exceeding VNE significantly.

Any suggestions? Is this relevant to the discussion?

[M.25]

Hmmmm so why does my twin otter manual show a difference of Vmo of 166kn at sea level to 112kn at 25000feet? Surely we dont risk breaking the sound barrier in a twotter?


- how is Vne calculated or defined by the aircraft manufacturer?


When an aircraft is taken through flight testing and certification, it must demonstrate a speed called Vdf (demonstrated flight diving speed). This is the highest IAS that the aircraft has been taken to during certification testing. Vmo can not be any higher than Vdf. (Vmo is usually set at a margin below Vdf for airspeed fluctuations and human error)

Maybe when testing the otter the IAS was not demonstrated any higher than 112kts at 25000ft - therefore they cannot publish an Vmo any higher than 112kts?

[ace4bar's girlfriend]

I'm sure 1013.2 asked the time - not how to build the watch :ok:

[AuralFlightRules]

I'm sure 1013.2 asked the time - not how to build the watch :ok:

Well the answer to the original question is probably NO, but if that's all we got, life would be boring and we wouldn't learn much, would we?

edited to add this link, which might be of interest: http://www.auf.asn.au/groundschool/flutter.html

[Squawk7700]

That is an interesting link you have posted, although those RA-Aus boys rarely go over 5,000ft anyways :-)


From the link for those that didn't visit the page... The table and theory suggests that VNE is based on indicated speed, I think as per the paragraph following.

Density altitude/IAS for nominal Vne 122 knots IAS/CAS

Density altitude VNE knots IAS
0 122
3300 116
6500 111
10 000 105
13 000 100
16 500 95
19 700 90
23 000 85
26 300 80

If uncertain about a particular aircraft multiply the (density) altitude, in 1000s of feet, by a factor of 1.5 to get the percentage DECREASE to apply to the nominated Vne for a revised Vne appropriate to the altitude. For example if altitude is 8000 feet and Vne 100 knots then 8(000) × 1.5 = 12%. New Vne IAS = 88% of 100 = 88 knots.

[Sunfish]

With the greatest respect to all you perfessionals, for us little ppl types, the air speed gauge is actually measuring dynamic pressure. Since its dynamic pressure that makes you wings fall off at Vne, it mattereth not if you correct for temperature, altitude, moon phase or whatever.

If the gauge says you are exceeding Vne, you are exceeding the structural limit of the aircraft. Same in reverse with the stall.

As for compressibility and mach numbers .....show offs!

[AuralFlightRules]

Since its dynamic pressure that makes you wings fall off at Vne

All due respect Sunfish, but if you read the previous link, you'll see that Vne is a function of many things sometimes, but not always, related to dynamic pressure.

...and the wings don't generally fall off at Vne

BTW, I too am just a little PPL type, but being an engineer as well makes me interested in the hows and whys. I'm sure others are too

[QNH1013.2]

Aural, can you tell me if the above post from the AUF website is valid? It all sounds convincing and says that IAS VNE decreases with altitude.

[AuralFlightRules]

If you need Vne to fly an aeroplane, use the POH

If you want to know how Vne is calculated, then the link posted earlier gives some indication of what's involved in generating the Vne value. I'm sure it depends on the performance of the aircraft, who's going to fly it and how far the boundaries are being pushed.

There will be dynamic pressure effects (IAS related) and aeroelastic resonance effects; i.e. flutter (related to TAS or maybe more precisely, Reynolds number).

IMO, Squawk's post is a selective mis-quote.

AFR

[desert goat]

A lot of composite gliders are placarded with decreasing IAS for VNE with increasing altitude-the reason given is that flutter likelyhood is related to TAS, and therefore IAS must be reduced with increasing altitude to avoid exceeding the fixed TAS at which flutter becomes a problem. Bit woolly with the theory, there is a better explanation in the manual "Basic Gliding Knowledge" used by the GFA if you can get hold of a copy.

[VMC4ME]

After reading this as an outsider, I'm yet to be convinced.

The glider placquard must have some reasoning behind it. Earlier post regarding stiffness of ailerons sounds mildly feasible too, but the physics and dymanic pressures seem to disagree.

[Andy_RR]

It probably doesn't illuminate this discussion too much, but here's an interesting video demonstrating aeroelastic effects!

http://www.dg-download.de/Videos/dg-300-flatterversuch.mpg

I'd be interested to understand whether it's excited by vortex shedding somewhere on the airframe (TAS-dependant) or is purely caused by the aerodynamic lift-strain-inertia relationship of the wing (IAS/dynamic pressure related)

PS: I think it's been posted on PPRuNe before somewhere.

[gaunty]

Perhaps one needs to consider the effects of controls with increasing total Kinetic Energy equation as a clue to decreasing IAS v Vne with increasing altitude.