Art_N412SB
Member
I have looked at a few Type Certificate Data Sheets. Velocity Never Exceed (VNE) is usually listed as an Indicated or Calibrated Airspeed. Why is the Van's RV-12 VNE listed as a True Airspeed?
so far this as clear as mud to me. every ias equates to a tas. does the tas number that vans lists now equate to the old ias?
Man, you guys are flying high. But you don't need to be very high to see a significant drop in the Vne number.
The POH Vne for the -12 is 136KTAS. Yeah, I know, she's a slow bird compared to the other RV's.
Here at only 2600 feet the Dynon has dynamically changed the bottom of the red zone to 131KIAS. The red line marker at 136K is the Vne TAS at sea level on a standard day (when IAS equals TAS).
Most of what (little) I know about the physics suggest aero phenomena depend fundamentally on fluid density. Seems not to be the case here.
I?ll take a crack at this.
The problem is, there are many kinds of flutter. Ailerons oscillating up and down. Wings flapping. Wings bending about their lateral axis (torsion mode). Etc.
Climbing towards space the air density would be getting lower and the indicated airspeed would be reducing.
It is the reduction in air density working against (or resisting the movement of) the control surfaces, that begins to make them more susceptible to flutter.
Said in a different way.... the denser the air is flowing past a control surface, the more it works against the movement of the control surface, which has a positive effect on working against the development of flutter.
I think you are saying that as air density is reduced, the damping forces that inhibit (stabilize) flutter are reduced. Makes sense.
But aren?t the aerodynamic forces that excite flutter also reduced in an equal way ? If so it would seem that IAS (sensitive to density) would be more relevant that TAS.
Still curious ....
So, the lack of air resistance over the airfoil can lead to flutter. And that is why we may have a lower TAS VNE at altitude. Did I get that right? Thanks Scott.
I don't think Van's did flutter testing at sea level.
And it probably was done with just IAS (CAS....).
So having Vne as TAS starting at sea level is way to conservative.
So if the flutter testing was done at 8000 ft, to an IAS of 140kts you'd get a Vne of 140kts IAS till 8000 ft and then change to a Vne of 158 kts TAS (140 IAS) above 8000ft.
The key is a what altitude was flutter testing done, and at what IAS.
I think the above is a bit more reasonable than TAS starting at sea level, and still is conservative.
I've seen various aircraft have a Vne as above. It's IAS until some altitude, then either changes to TAS or reduce IAS redline 3 to 4 knots for every 1000'
Actually, this is not totally correct. Some flutter modes have flutter speeds that follow a TAS line with increasing altitude. An example of these are the so-called "explosive" flutter modes, where there is a large decrease in aeroelastic damping for a small increase in airspeed. "Aeroelastic" damping is Structural Damping plus Aerodynamic Damping. Structural Damping is usually a constant, the value of which depends on the construction design and materials used in the structure.
Other modes follow the so-called "half and half" speed line with increasing altitude, roughly midway between EAS (CAS/IAS for us non-Mach challenged RV's) and TAS. An example of these would be the so-called "hump" flutter modes, where there is a small decrease in aeroelastic damping for a large increase in airspeed. It is called a "hump" mode because it looks like a hump when plotted on an Airspeed vs Damping plot.
And some modes follow more of an EAS line with increasing altitude.
But it is correct to say the conservative approach is to assume a constant TAS limit when the critical flutter mode(s) are either not known or not well understood.
For those who may want to know more about flutter, I put this primer together awhile back on Flutter and Aeroelasticity:
https://drive.google.com/open?id=1-BSC6ES-IOiUg37reYwnqsOwgkHG8hr2
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.
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.
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.
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.
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