Why is VNE TAS in the RV-12

[RV8JD]

Originally Posted by Art_N412SB

Let me read this back. Aeroelastic dampening decreases with gain in altitude. Structural dampening mostly remains constant. So Vf (velocity flutter) decreases with altitude. Is this correct?

Unfortunately, flutter is not that simple. It depends on the type of flutter mode involved, the structural dynamics of the airplane (i.e., its modes and frequencies), and its aerodynamic characteristics. Reducing aerodynamic damping (going high) is more adverse for some flutter modes, and increasing dynamic pressure (going too fast) is more adverse for other flutter modes. They both play a part. But those other variables are also at play, not just dynamic pressure and aerodynamic damping, and it's kind of a balancing act among those in the flutter equation to see what will bite you in the butt in the end.

The designer/manufacturer sets Vne to insure that the airplane will be flutter free and that proper margins of damping exist to Vne (with some margin) up to the Absolute Ceiling of the airplane. And that is whether Vne is stated as a constant IAS/CAS, constant TAS, or a combination of both. Also, just because a designer/manufacturer sets Vne as a constant IAS or constant CAS doesn't mean they have not taken flutter into account. It means that they just set Vne in terms of a constant IAS/CAS number that ensures that flutter will not occur within the operating envelope of the airplane.

Hope that helps little.

[pjc]

Originally Posted by RV8JD

Actually, Flutter is a "Black Science...?

So to your question. Since the calculated speeds occur at a certain density (i.e., altitude), codes usually spit out the speeds in CAS, EAS, and TAS. The speeds can then be plotted on the Velocity vs Damping plot in any airspeed unit of interest.

As a side note, many flutter points need to be calculated to get a flutter boundary on an Airspeed vs Altitude plot for a given flutter mode.

But, that doesn't mean the flutter modes that an RV might exhibit are a strictly a function of CAS (or TAS) with increasing altitude, as I mentioned in my post above. For example, a flutter mode consisting of the first wing bending mode coupling with the first wing torsion mode could be an "explosive" mode and would follow more of a constant TAS line with increasing altitude.

.... lots more good stuff clipped ....

I'm not sure if any of this answered your original question, so feel free to clarify. But I hope some of the above helps.

Again very helpful. Thanks. Based on this and some other outside reading, I?ve (reluctantly) come to the understanding that the aerodynamic forces (driving and damping) that interact with the stiffness and mass properties of the aircraft structure and result in flutter are not simply a function of dynamic pressure (q) or velocity (TAS) but are indeed a more complex non-linear function of both density and velocity (or equivalently altitude and velocity). So the hope that a single observable parameter (perhaps IAS or TAS) would provide a reliable indication of flutter margin (independent of air density/altitude) is mistaken ? even for ?simple? structures at low Mach numbers.

So, yeah, it?s complicated ...

[Art_N412SB]

Scott said....
"As already mentioned, using TAS for VNE reference is inducing a compensation factor that changes in a mostly linear rate with the reduction in air density as you climb to high altitudes".

Thank you all for your responses to my original question. The quote above from Scott is the answer for the original question of "Why does Van's use TAS rather than IAS for VNE"?

That said, I feel that I have a better understanding of "flutter".

[NinerBikes]

Originally Posted by Art_N412SB

Scott said....
"As already mentioned, using TAS for VNE reference is inducing a compensation factor that changes in a mostly linear rate with the reduction in air density as you climb to high altitudes".

Thank you all for your responses to my original question. The quote above from Scott is the answer for the original question of "Why does Van's use TAS rather than IAS for VNE"?

That said, I feel that I have a better understanding of "flutter".

Kind of sucks that I am such a visual learner, vs reading.

I wonder if Scott or someone else, might have a graph laying around that might diagram this set of functions. TAS vs VNE vs elevation of flight, plus density altitude.

Am I assuming correctly that the Dynon D1000 Skyview and HDX take all these parameters into account with the Kt per hour ribbon and color of the backround from green to yellow to red for IAS?

I have no intentions of trying to push the envelope to anywhere near Vne, but I would like a better understanding of what the Dynon Skyview is really telling me for variables that it accounts for in a E-LSA RV-12. Sorry, so, so, so much to learn.

[rvbuilder2002]

Originally Posted by NinerBikes

Kind of sucks that I am such a visual learner, vs reading.

I wonder if Scott or someone else, might have a graph laying around that might diagram this set of functions. TAS vs VNE vs elevation of flight, plus density altitude.

Am I assuming correctly that the Dynon D1000 Skyview and HDX take all these parameters into account with the Kt per hour ribbon and color of the backround from green to yellow to red for IAS?

I have no intentions of trying to push the envelope to anywhere near Vne, but I would like a better understanding of what the Dynon Skyview is really telling me for variables that it accounts for in a E-LSA RV-12. Sorry, so, so, so much to learn.

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.

I am pretty sure that as long as the EFIS software is up to date with the currently available files on the web site -
https://www.vansaircraft.com/downloads/#RV-12-12iS

the EFIS will automatically adjust the VNE marker on the airspeed tape in reference to TAS if above 16000 but I am not certain because I never go that high.

[emsvitil]

About sailplanes, but might help

http://journals.sfu.ca/ts/index.php/...ewFile/607/570

[Art_N412SB]

Originally Posted by rvbuilder2002

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.

Scott,
Just now looking at the POH REV 7 and it seems VNE is both 136 Kts TAS and 136 kts IAS. That can be a little confusing. I suppose one should respect the most limiting of the two speeds. I started this conversation thinking that the POH specificity stated VNE as 136 kts TAS and now I find it states both IAS and TAS. Joke is on me.

[rongawer]

Originally Posted by Art_N412SB

Scott,
Just now looking at the POH REV 7 and it seems VNE is both 136 Kts TAS and 136 kts IAS. That can be a little confusing. I suppose one should respect the most limiting of the two speeds. I started this conversation thinking that the POH specificity stated VNE as 136 kts TAS and now I find it states both IAS and TAS. Joke is on me.

The change over point is at 16,000'. Below that, it's IAS, above that, it's TAS; it's not intended to observe the most limiting all the time.

[Art_N412SB]

Originally Posted by rongawer

The change over point is at 16,000'. Below that, it's IAS, above that, it's TAS; it's not intended to observe the most limiting all the time.

I went back and read this link http://www.vansairforce.com/communit...86&postcount=2

I am now convinced that VNE is 136 IAS below 16000 ft no matter what the indication is on the Skyview. VNE is 136 KTAS above 16000.

Thanks much

[rvbuilder2002]

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.

BTW, the RV-12 POH is currently at Rev. 15
You can download a digital copy HERE