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-   -   Vne (https://vansairforce.net/community/showthread.php?t=199436)

rgmwa 09-22-2021 06:08 PM

Vne
 
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?

12vaitor 09-22-2021 09:30 PM

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

rgmwa 09-22-2021 10:14 PM

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?

RV8JD 09-22-2021 10:32 PM

Asked & Answered ...
 
... by "rvbuilder2002", Scott McDaniels of Van's.

https://vansairforce.net/community/s...1&postcount=28
The RV-12 POH specifies the Vne as 136 Kts IAS at or below 16,000 ft.
As long as you never go above 16000 (would be rather rare for an RV-12 I think) you do not have to consider whether it is TAS or IAS.
and:

https://vansairforce.net/community/s...8&postcount=33
From (the middle of the) page 2-3 of the RV-12 POH

Never Exceed VNE red line below 16,000 feet 136 IAS

AIRSPEED DESIGNATION KTAS
Never Exceed VNE red line 136


This means that if you below 16,000 feet, VNE is to be referenced as an IAS.
If you at 16,000 ft or higher, it is referenced as a TAS.

(This limitation is poorly written in the POH, leading to some confusion. Not to mention the large discontinuity in Vne at 16,001 feet due to the Vne schedule chosen.)

rgmwa 09-22-2021 10:47 PM

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?

12vaitor 09-23-2021 06:04 PM

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/...1/hp_limts.pdf

John Salak
RV-12 N896HS

RV8JD 09-23-2021 06:30 PM

Quote:

Originally Posted by 12vaitor (Post 1557531)
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/...1/hp_limts.pdf

John Salak
RV-12 N896HS

This information predated the RV-12 series, and was taken into account when Van's Aircraft set the Vne that is in the RV-12 series POH.

Scott McDaniels ("rvbuilder2002"), whom I referenced and quoted in my Post #4 above, works for Van's Aircraft. His interpretation of the RV-12/12iS POH is what Van's Aircraft intended for Vne for those airplanes.

12vaitor 09-23-2021 10:02 PM

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-cont...4_V_speeds.pdf

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

John Salak
RV-12 N896HS

RV8JD 09-23-2021 11:17 PM

Quote:

Originally Posted by 12vaitor (Post 1557568)
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.

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.



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.

Taltruda 09-23-2021 11:59 PM

Quote:

Originally Posted by RV8JD (Post 1557582)
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.



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.


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