Vne is based mostly on flutter, AIUI.
And flutter is based mostly on airflow velocity i.e. TAS (not IAS).
So the Vne marking on the ASI (which is marked in IAS) is done by taking the max TAS, at the certified ceiling, and translating it back down to the IAS.
So for example a plane with a Vne of 190kt and a ceiling of 20k feet will have a Vne of 253kt TAS (ISA i.e. -25C at 20k). The manufacturer needs to check for freedom from flutter at 253kt TAS (plus some margin).
If you now put in a turbo engine and the ceiling goes up to 25k then the Vne would be 274kt TAS. The manufacturer now needs to check for freedom from flutter at 274kt TAS (plus some margin).
But if you were doing 190kt IAS at just 5000ft, the TAS is just 203kt. So you now have a huge flutter margin of 274-203kt.
Is the above correct?
I know that Vne takes into account other factors besides flutter, but it does suggest that overspeeding (in IAS terms) an airframe at low levels is a lot safer than overspeeding it (in IAS terms) high up.
TAS is only of use for calculating you speed over the ground (still air) Aerodynamics is based upon IAS. VNE normally increases with altitude.
No, wrong – Flutter speed is a function of TAS, not IAS … as strange as it seems, first. Structural limits, for example are given in IAS, but flutter in TAS. I know it’s right, but can’t explain it well …
I find the idea of flutter being a function of TAS intuitive – because what causes flutter is an oscillatory excitation of the aerofoil, and that (like the sound made by a whistle) is going to be a function of the air velocity, not its dynamic pressure which is what IAS is (well not directly anyway).
Is Vne really mostly a function of flutter for typical GA aircraft? I don’t know much about it but aren’t there a lot of other factors as well? I have never seen a GA aircraft flutter in tests but seen it several times with larger jets (remember the Grob disaster?) and gliders.
The Malibu’s red line is basically the JetProp’s cruise speed and there aren’t any changes to the airframe that would influence flutter to my understanding.
No, no changes in the airframe in the JetProp. Vne closer to Cruise. That’s what was critisized often, and even more so with the C210 turbine conversions!
I think the low Vne for the Jetprop is basically a result of nobody wanting to pay for certification tests (and fly wearing a parachute, no doubt) at its ceiling which must be ~ FL300. So the FAA forced them to set it at the top of the yellow arc…
You still get ~ 260kt TAS out of it, at FL260, which is not bad at all, considering the fuel flow at that speed and the half-price fuel.
Is Vne really mostly a function of flutter for typical GA aircraft?
What else could the major factor be? The wings are hardly going to get ripped off by the extra parasitic drag.
Actually the wing drag loads can be a significant factor on some designs, but I believe on many or most planes its flutter.
As Peter says, the way the TAS issue is often visualized is to say its the relative speed of the air that matters in relation to exciting vibrations, not the dynamic pressure associated with IAS.
Other control issues like stick force and aileron reversal might also be factors in determining VNE, as I understand it.
I think you are generally correct. Flutter excitation is related to the compliance of the trailing edge and the flow Strouhal number which is an inverse function of flow velocity (high velocity gives smaller vortex at higher frequency).
there may be other stagnation pressure related considerations with seals, cowlings, aerials etc..
Dick Van Grunsven put together a nice article explaining why he won’t sanction turbo charging of his RV designs : http://www.vansaircraft.com/pdf/hp_limts.pdf
The issue with the Jetprop is that for legal reasons the red line gets set to the speed where in the piston plane used to be the end of the green arc. The only reason I can think of is that in the piston plane you have no chance to reach the orange arc in cruise so over the life of the airframe flight in this speed range is an exception. With the turbine you could fly in there all the time which could sum up to more strain on the airframe than anticipated for the piston plane.
Besides I think we piston pilots always see power as the limiting factor on cruise speed. But in many bigger planes it often is aerodynamics. So they often cruise right below redline with enough power available to exceed either the IAS, TAS or Mmo limit.
