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Vne

rgmwa

Well Known Member
The RV-12 POH says Vne is 136 KIAS below 16000 feet. Then it also says Vne is 136kts KTAS. How is this meant to be interpreted? Why have both KIAS and KTAS?
 
It means which ever one gets to 136 kts first is the governing Vne. I regularly push up near the 136 Kts TAS in descents from 6,000+ ft with IAS in the 120 kt region. The Dynon Vne is set to both ISA and TAS mode, so the redline adjusts the IAS tape to the correct value to match the 136 TAS number. As I live near sea level, I have seen the 136 kts IAS limit at low altitudes.

John Salak
RV-12 N896HS
 
OK, but since IAS is always less than TAS the first to read 136 kts will be TAS, which the Dynon or Garmin will tell you. If you only have steam gauges and have to rely on IAS, then I assume Vans is saying it's OK to use the IAS value of 136 kts up to 16,000. However, since the 12 would struggle to get any higher than 16,000' it seems an odd limitation. Also, at 16,000' there would be a significant difference between IAS and TAS, which would put you at risk if the real limit was 136 kts TAS. What am I not understanding here?
 
Perfect, thanks Carl. Should obviously have done a bit more research before asking the question. Perhaps Vans could clarify this a bit better in the POH?
 
I do not think it would be wise to exceed 136 KTAS at any altitude in an RV-12. If you follow the logic proposed for using a Vne of 136 KIAS at any pressure/density altitude below 16,000 ft, that works out to a Vne TAS in the 174-184 knot region at 16,000 ft (changes with actual DA). During Phase I testing I had the Dynon configured for 136 KIAS Vne and managed to post 140+ KTAS (148 KTAS was the peak) at 130 KIAS and 8,000 ft, I can tell you the airplane was not happy at that speed. The Dynon got changed to IAS/TAS Vne after that flight.

Although not much help in explaining the RV-12 POH wording, you may find this Van's article helpful in applying density altitude to IAS based Vne limits. The sailplane KIAS discussion is a good indicator of why a KIAS Vne number should change with the density altitude. Perhaps Van's should consider a KIAS Vne altitude table like the one in the article as the POH standard.

https://vansaircraft.com/wp-content/uploads/2019/01/hp_limts.pdf

John Salak
RV-12 N896HS
 
The problem with the RV-12 POH is the 136 KIAS Vne is negated by the 136 KTAS Vne except in a vary narrow instance of a colder than standard day at low altitude (negative density altitude) where the TAS calculation would be lower than IAS. The 16,000 ft reference is also odd considering the published service ceiling in the POH is estimated to be 12,000 ft. Has Van's actually flown an RV-12 at 16,000 ft at 136 KIAS, or a better question would they fly it to validate that statement.

The technical discussion in the Van's paper is valid regardless of when the RV-12 came about. The issue is the actual speed of the flow over the control surface at any altitude and the structural implications of the loads generated. It is interesting that Van's published V speeds for the RV-14 are all IAS except for Vne, which is TAS only number. The RV-12iS POH also has no reference to a KIAS Vne, the only Vne number is 136 KTAS.

https://www.vansaircraft.com/wp-content/uploads/2019/01/RV-14_V_speeds.pdf

With good reason, Vne is not something you should be playing with.

John Salak
RV-12 N896HS
 
Not sure why you say that "The RV-12iS POH also has no reference to a KIAS Vne, the only Vne number is 136 KTAS." That is incorrect. Below is a screenshot of page 2-3 of the RV-12iS POH, Rev 7.

i-nzm4CQ2-M.jpg


BTW, I spent my professional career in flutter, aeroelasticity, and aeroservoelasticity; as an engineer, DER, AR, manager, etc. So I know a little bit about flutter (and loads!).

Oh yea, I'm done here.

I think the confusion (for me at least) is that if you were to do 135 indicated at 15000, you would be within the INDICATED limits but way exceeding the TAS limit.
 
Many certified manufacturers have similar limits where you can use IAS up to a specific altitude. Above that you revert to TAS. VNE is a combination of flutter margin and structural strength. Flutter margin is generally a true airspeed function. Structural issues are IAS related. It’s quite common to have a aircraft where structural failure will occur before flutter. In that case IAS is the appropriate measure for VNE. From everything posted here it sure appears that Vans is comfortable with flutter limits up to 16,000 PA and 136 knots indicated.
 
Yes, but the POH doesn’t make this very clear. Also the big step change in the speed limit switching between IAS and TAS at 16,000’ is hard to understand. I’d be interested to know how that’s derived. Not that it’s much of a limitation since I’d probably have passed out well before I got that high anyway.
 
If you climb at 136 knots TAS will exceed 136 knots as soon as you lift off. The difference grows as you climb but it starts at liftoff or even before if your airport is not at sea level and it’s not a standard day.
 
Well just to keep things simple, I'm going to say that Vne for my plane is 136KTAS. If that means that according to the POH, my safety margin increases as I go higher that's a good thing. Then I only have to look at one speed readout and it doesn't matter if the margin magically disappears at 16,001' for no apparent reason while I’m on my way to a new altitude record. Anyway, since I can only get to 136 kts in any system of measurement in a dive, if I'm forced to descend at an angle somewhat less than vertical, I can live with that too. I'll just enjoy the view for a bit longer.
 
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