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Why is VNE TAS in the RV-12

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?
 
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?

IAS and TAS don't have a constant a=b relationship. Basically, the higher you go the further they are apart, due to less dense air having fewer molecules of "stuff" going into the pitot tube at a given true airspeed.
 
I refer the OP to a post I made a short while ago.

During flight testing, I took our 12 to 15,000'.

At that altitude, the speed scales referenced to TAS, the Vne was lower etc. As I started to descend, the Vne gradually increased back to normal.

Think that this is a combination of Vans and the Dynon Wizards :D

I was most impressed............

2019100322463945--4949481272925733982-IMG_6941-M.jpg
 
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.
i-6J5sprK-L.png


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).
 
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.
i-6J5sprK-L.png


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).

Tony, is this pic on a Skyview D1000 Touch or a HDX model?
 
Just in keep in mind that Vne for the RV12 is 136 KIAS below 16,000' and 136 KTAS 16,000 and above.

I don't know how many folks are flying their -12's above 16,000, but I'd guess for the vast majority, 136 KIAS is the Vne to be concerned with.
 
True

Decending out of 17,000 ft in the RV-9 which I rarely do but just a case in point, I?ve got to hold Indicated to around 130mph which trues out to around 180 mph. The modern EFIS makes this a cakewalk.

My legacy old inefficient airplanes have the old antique steam gauges but wider margins and a bit simpler. Decending through same altitude, I hold Comanche at 180mph indicated (243mph TAS) and the Aerostar (Deathstar to the newbies) to around 250mph indicated (337mph TAS). 🙃

I LOVE the efficiency of the -9 and I bet the -12 is even better but those old engineers from yesterday sure knew a thing or two about stuff.
 
Without going into the reasons why, Some of the factors that relate to establishing Vne are directly related to TAS. In older part 23 airplanes, where all that we had available was a simple mechanical airspeed indicator, a red line was drawn and that was it. By the CAR?s and part 23, Vne was determined for standard day sea level and marked on the IAS indicator.

Airplanes that go to higher altitudes under Part 25 and now Part 23, have always had Vmo in place of Vne. This is a value based on TAS. A sliding scale or flag moves to indicate Vmo based on the current pressure altitude. Even for older mechanical instruments, the Vmo value changed with altitude on the IAS indicator. They also have Mach limitations but that is an aside.

Because gliders can go to high altitudes (25000 ft and above), the Europeans required the Vne to be indicated for the altitude conditions, typically with fixed markings that showed the Vne for various altitudes. It is interesting to note that the US never required this until recently. As a glider pilot we just used the rule of thumb of 2% Vne reduction per thousand feet, or at least some of us did.

With electronic displays we have the ability to see Vne in terms of TAS. Now for even small airplanes we can easily abide by the Vne as it varies with pressure altitude, and that?s a good thing.

I?d like to note that Vans isn?t deficient on this. He followed the standard and continues to follow the standard for Vne that was established by the FAA over the years. There is lots of published info for pilots on the need to reduce the fixed Vne in small airplanes as altitude increases, hence the 2% rule of thumb.
 
I think what is lost in the IAS verses TAS debate for setting VNE is every airframe is different. You really have two curves. You have a limit based on dynamic pressure or IAS that is fixed or valid at any altitude. You then have a flutter limit based on where and when you can expect some portion of the airframe to flutter which is TAS dependent. You need to overlay those two curves to see the big picture but sadly that information is rarely available to us.
As a example some manufactures set VNE as a IAS up to the certified altitude of the aircraft. This is because they know the onset of flutter is above the highest true airspeed the aircraft can obtain at that altitude using the dynamic presssure limit (IAS). A different airframe May have the exact same dynamic pressure limit (IAS) but a much lower flutter limit (TAS). In that case the VNE will need to be set lower on aircraft two. A glider is a excellent example of the 2nd aircraft. They often have low flutter limits and fly very high. Most high performance gliders use TAS exclusively for VNE for that reason.
It’s possible because there are two different curves to set VNE as IAS below a certain altitude and TAS above that altitude. That changeover point could be the same for IAS or dynamic pressure for two different aircraft but very different for TAS or flutter margin leading to two different changeover points. That point could be sea level for some aircraft or 16,000 feet for others as Vans seems to have set at one point for the 12. To complicate all this the TAS is of course pressure not actual altitude.
I think what I have posted above is the reason there is so much confusion on if VNE is IAS or TAS based. The answer is it could be either or both. If both each aircraft type will have different crossover points where one becomes limiting. The above is from my aero courses years ago and there are other factors. It’s a simple explanation.
G
 
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Aerodynamics of Flutter

I know this is a very complicated subject, but can any of our experts explain in simple terms why flutter is excited as a function of TAS (velocity of the flow field) rather than IAS (dynamic pressure) ?

Most of what (little) I know about the physics suggest aero phenomena depend fundamentally on fluid density. Seems not to be the case here.

Pointers to readable references welcome.

Thanks, and apologies if this takes us too far off topic ...

Peter
 
Most of what (little) I know about the physics suggest aero phenomena depend fundamentally on fluid density. Seems not to be the case here.

In simplest terms, air is a fluid.

In simplest terms of an explanation....
Think of a fixed wing airplane orbiting the earth in the vacuum of space.
There would be zero air molecules to exert any force on the control surfaces or dynamic pressure in the pitot tube so the control surfaces would be free to move with zero (aerodynamic) resistance and the airspeed indicator would read zero (but you would still be moving quite fast).

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.

So by using TAS, instead of IAS as the limit, it automatically adds in a correction for the reduction in air density working against the control surface movement.

This is a greatly oversimplified explanation that doesn't expand on all of the different influences related to the occurrence of flutter but hopefully gets the idea across.
 
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. Now throw in the complication that damping (think of air resistance) is, at best, approximated. Often as rho v squared. But for some low speed stuff, like wings flapping, rho v is a better damping model. So you end up with a bunch of possible modes. For some, the parameters group together as rho v squared, and so IAS is the appropriate single parameter. But for other modes the equation may work out rho V. Then, neither IAS nor TAS is a correct, single parameter characterization. Generally speaking, using TAS is the most conservative, IAS the least, and the ?truth? is usually in-between.
 
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.

Good point. For further simplicity let?s limit to the Van?s designs where (I believe) control surface flutter is the first concern. (I wouldn?t expect the short thick wings of the RVs to be susceptible to aero-elastic twisting at anything close to the published Vne).

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.
 
It gets complicated

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 ....

There are a number of forces at play in flutter, but lets say there are only two to start off with; aerodynamic excitation (density times velocity squared - which is what our airspeed indicator measures) and aerodynamic damping. The damping is proportional to air density but not forward airspeed. Now lets say you are flying at exactly the critical flutter airspeed at sea-level. The excitatory force is exactly in balance with the damping force. If you suddenly increase altitude but maintain velocity, TAS stays the same, aerodynamic excitation (IAS) is reduced and aerodynamic damping is reduced by the same amount. However, you are still at the critical flutter airspeed because damping still exactly balances excitation.

That is a drastically over-simplified case. Critically, in real airfoils there are internal damping forces due to the construction materials and assembly methods (riveted aluminum) which are effectively constant.

Bob hit the nail on the head in saying critical flutter airspeed is neither TAS or IAS but somewhere in between. The conservative approach is to use TAS.
 
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.

Partially correct

It is not a lack of air resistance/pressure interacting with the control surfaces at altitude, it is just that it is less than what it is at lower pressure altitudes.

The TAS VNE isn't a lower VNE either. It is the same VNE value, but that value is read as a TAS instead of an IAS.

Example - On my RV-6A, at higher altitudes (12,500 -14,500) I can cruise at TAS of 160-165 Kts, but my IAS would be only show about 125-130 Kts (approx).

Using my IAS for reference of how far below VNE I am, could put me way over TAS VNE.

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.
This is only a factor operationally, when an RV is capable of reaching speeds at high altitude that would make the TAS above the rated VNE. For most RV's this is not possible other than in a power on decent.

That is the reason for the caution against using turbo normalized engines. The additional power at high altitude could easily allow an RV to operate in a steady state cruise condition, at a speed well above VNE based on TAS, with the IAS showing that the aircrafts speed is no where close to VNE.
 
I never understood Vans stance on Vne based on TAS either, and just assumed being most conservative.

I can't say I totally agree with "most conservative" all the time, but in the absence of a multi-thousand hour test flight program, and destructive static and fatigue testing, I am not sure it is not appropriate.

Flutter can go from zero to destructive in fractions of a second.
 
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'
 
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'

Van's has made a recommendation regarding VNE, based on design details and testing, though only a limited amount of that testing was in actual flight. The rest was done safely on the ground with GVT (ground vibration testing).
So it is probably correct to say that the current spec is a bit conservative but when a highly detailed, full blown in flight flutter mode test hasn't been done on every model, it is a sensible choice in the interest of safety for the entire RV community.
BTW, For those not aware, in flight flutter mode testing can be a very risky business.

RV's are experimental aircraft. Therefor this recommendation doesn't have to be followed. Owners are free to do what ever the want if they feel sure that the spec is way overly conservative.
Better yet, they could do their own testing at a wide range of altitudes and then readjust the limits for their airplane based on the results.
 
yeah...

"... Therefor this recommendation doesn't have to be followed. Owners are free to do what ever the want if they feel sure that the spec is way overly conservative..."

Yeah, what do the engineers know, anyway...<sarc>:rolleyes:

One of my college professors worked for one of the big three years ago. They were doing inflight flutter testing. He watched the aircraft get destroyed and the test pilot killed as it made a smoking hole in the ground.

So back to the above comment. You need not follow the recommendations but beware of the potential consequences...and don't blame Van's if you get a firsthand look at flutter...
 
All,
Thanks for the excellent discussion. I?m more interested in the basic physics, aeroelaticity, and aerodynamics than Van?s published guidance and whether others choose to disregard it.

Carl,
Thanks for the excellent primer. My precise question using what I learned there is: What ?airspeed? is correct on the horizontal axis of the graph of damping vs ?airspeed? on p32? (I ask in the context of classical torsion/bending and control surface coupling flutter. Both are presumably relevant to wing and tail surfaces of ?RV like? aircraft well below transonic speeds.) My other reading suggests this should (in theory) be EAS (equivalent airspeed-which is simply calibrated airspeed adjusted for Mach effects) or simply CAS (essentially dynamic pressure) for low Mach numbers.

Please do weigh in if I still have more to learn before we get to ?flutter is black art?.

Thanks,
Peter

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
 
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?
 
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 ...
 
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".
 
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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.
 
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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.
 
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.
 
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.
 
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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
 
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


Doesn't this make it a step function?

At 15999 you can go 136 KIAS = ~ 174 KTAS
At 16000 you should slow to ~ 106 KIAS for a 136 KTAS
 
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