I read the Vne definition from wikipedia (which you posted) and your
observations that Vne is always given in IAS, and am having a difficult
time reconciling the two. If I am reading the wikipedia definition
correctly, it is saying that if the IAS is held constant (damping held
constant), as TAS increases with altitide, the inertia-induced
disturbances (derivative of TAS) will get stronger and predisposes the
airplane towards flutter.
I agree however that every airplane I have flown also had Vne given in
IAS. However, if you do a quick search on various discussion groups
(search for "Vne and flutter") the answer seems to be anything but
straight forward. Many sailplane pilots who fly up to 30-40k ft claim
Vne and stall can become the same. Others say no. The only thing that
most people seem to agree on is that Vne is difficult to pin down, and
it is neither IAS nor TAS.
--- Mike Parkin <mikenjulie.parkin@btinternet.com> wrote:
> Here is another definition for the debate.
>
> VNE
> The VNE , or the never exceed speed, of an aircraft is the V speed
> which refers to the velocity that should never be exceeded due to
> risk of structural failure, due to calculated factors such as wing or
> tail deformation or due to aeroelastic 'flutter' (unstable airframe
> or control oscillation). VNE is specified as a red line on many
> airspeed indicators. This speed is specific to the aircraft model,
> and represents the edge of its performance envelope in terms of
> speed. Well below the speed of sound, the VNE is read as Indicated
> Air Speed (IAS), since the pitot indication is a direct measure of
> the dynamic pressure for any given airspeed. At altitude, where TAS
> is higher than IAS, aerodynamic damping is weaker than at lower
> levels (damping is proportional to IAS) whereas inertia-induced
> disturbances are stronger (inertia grows with acceleration, which is
> the time derivative of TAS). This condition, if continued beyond
> tested limits, pre-disposes to unstable oscillations or 'flutter'.
> For instance, the TAS/IAS ratio at 40,000 ft on the ICAO ISA is 2:1,
> that is, TAS is approximately twice IAS.
>
> Every aircraft I have ever flown stated the Vne as indicated
> airspeed. The Falcon 20 I'm away flying at the moment has its Vne
> and Vno stated as an indicated airspeed.
> Just been to the bar to get a beer and a colleague of mine answered
> the question - Vne is in indicated airspeed always. How else would
> the pilot know if he were exceeding it. I have some respect for his
> opinion as he recently retired from British Aerospace as a Senior
> Test Pilot, he flew harriers and did much of the flight testing on
> the new Typhoon 2 fighter.
>
> I don't know why everyone is getting so hot under the collar about
> the flight at Vne. Ivan and Pete Clark did a lot of testing of the
> europa. I do it every year at permit renewal as required and it
> really is a non event - I must confess however that I cannot see the
> point of that particular part of the flight test schedule.
>
> Incidentally, at medium to light weights G-JULZ will achieve Vne
> straight and level at 2000 feet (indicated airspeed) 914/Airmaster
> 'with the pedal to the metal'. Andy Draper found the ASI to
> reasonably accurate - I don't worry about it too much anyway.
>
> I don't think we Rotax chaps need to worry too much anyway. It would
> be different if we all fitted Jabiru 6s - we could then climb at some
> mind spinning rate towards the tropopause and cruise at a Mach No -
> just to be cool. Of course we would have to fit a larger fuel tank -
> wouldn't we Bob? One distinct advantage of the Jabiru is that at
> those dizzy altitudes it can reach you will be able to keep the cabin
> warm - apparently many of them overheat or have some type of cooling
> issues.
>
> Oh! I have slipped the surly bonds of earth
> And danced the skies on laughter-silvered wings;
> Sunward I've climbed, and joined the tumbling mirth.................
>
> It makes me want to sign on again.
>
> Now don't get upset, just having a bit of a leg pull - you are all
> getting far too serious.
>
> regards,
>
> Mike.
>
> "TAKE COVER!!!!..........INCOMING"
>
>
>
|