Saturday 25 May 2013 04:14 UTC
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Can anyone satisfy my curiosity?
Is there any margin designed in to Vne (as with SWL on cranes etc) or is it an absolute? By absolute I mean structural damage is possible from the specified Vne.
Although my understanding is that it is an absolute it appears to me this is not in keeping with most engineering practices.
Clarification would be appreciated.
When we design an aeroplane all of the various structural and stability considerations and determine a theoretical maximum speed which we call Vd. That will have structural and speed margins, but the margins will vary depending upon what factor it is.
In flight test we then try to get to Vd without anything nasty happening, but often we do reach a lower speed that nonetheless seems an acceptable maximum. The maximum speed set in flight testing is set to Vdiv.
Then Vne is set a margin below Vdiv. Almost invariably it's set to 90% of Vdiv.
Much the same is done for high performance aircraft with Mach number as well.
So saying that aeroplane's are tested to 110% of Vne is true, but simplistic. And also it took that when brand new and being flown by a test pilot, not years later flown by somebody without the detailed knowledge and backup that TP had. So particularly with older aeroplanes, Vne should really be regarded as a hard limit.
It is expected that aircraft will be flown at VNE (I've flown mine to VNE many times on CofA air tests) - therefore there has to be a fairly generous safety margin - otherwise any aircraft - particularly an older aircraft, that is flown to or near VNE would be in immediate danger of coming apart. The manufacturers couldn't take on that liability.
Edit: Genghis has it!
Edit 2: It is surprisngly hard to get a "chunky" aircraft like a Cherokee six up to VNE - takes about a 4000 ft dive at full power and 45 degrees down bubble. Believe me - the pull out is alway very very gentle
Eye off the ball for a few moments in even a very gentle descent and the RV4 busts VNE (210mph) very convincingly. 75% Cruise at 8000ft is only 10 mph slower.
You do need to keep on top of it (as my mentor Hatz warned me)
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Very true - "slippery" aircraft can accelerate to & through VNE very easily. Mr Piper doesn't build very many in this category.
There are other requirements that there's a safety margin between the maximum level speed and Vne, and that it has a noticeable push to reach Vne. However those margins can vary quite a lot more between classes and types.
One extra bit of food for thought:
Even though Vne is marked on an ASI as a fixed line, it is more related to TAS than IAS in so much as flutter will occur at a fixed velocity.
I believe that the margin from certification testing is good up to 10000', but above there you need to reduce Vne according to TAS.
Anecdotally, there are reports of RVs doing cruise descents from high altitude and getting the onset of flutter with the ASI safely below the red line, thankfully without disastrous consequences.
Also control surface balance is critical and repainting without re-balancing can have a significant effect on flutter margin.
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