IAS, TAS & VNE

[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]

Originally Posted by Sunfish

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-...terversuch.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.