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  #21  
Old 10-08-2019, 05:27 PM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by Art_N412SB View Post
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.
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  #22  
Old 10-08-2019, 06:03 PM
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RV8JD RV8JD is offline
 
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Quote:
Originally Posted by Reformed SeaSnake View Post
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.
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-B...wnqsOwgkHG8hr2
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(Pic 1),(Pic 2)
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Last edited by RV8JD : 10-08-2019 at 10:48 PM.
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  #23  
Old 10-08-2019, 08:36 PM
svyolo svyolo is offline
 
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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.
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  #24  
Old 10-08-2019, 08:56 PM
rocketman1988 rocketman1988 is offline
 
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Default ^^^^^^^

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  #25  
Old 10-08-2019, 09:03 PM
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emsvitil emsvitil is offline
 
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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'
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  #26  
Old 10-08-2019, 09:21 PM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by emsvitil View Post
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.
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  #27  
Old 10-08-2019, 09:41 PM
rocketman1988 rocketman1988 is offline
 
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Default 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>

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...
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Aerospace Engineer '88

RV-10
Structure - 90% Done
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Wiring...

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  #28  
Old 10-09-2019, 12:34 PM
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pjc pjc is offline
 
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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

Quote:
Originally Posted by RV8JD View Post
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-B...wnqsOwgkHG8hr2
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  #29  
Old 10-09-2019, 11:39 PM
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RV8JD RV8JD is offline
 
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Quote:
Originally Posted by pjc View Post
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, Flutter is a "Black Science", Static and Dynamic Loads are "Black Arts", and Fatigue and Damage Tolerance are, well, just "Voodoo and Black Magic"! (Just kidding, just kidding. No offense to my Loads and F&DT friends!)

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.

And a particular flutter mode may not exist at all altitudes. For example, a flutter mode consisting of a control surface rotation mode coupling with it's parent surface's bending or twist mode may only exist at higher altitudes where control surface aerodynamic hinge moments and aerodynamic damping is reduced. And they may flutter at lower airspeeds, well below Vne, if not properly designed for flutter prevention. Picture the flutter boundary dropping through the top of the airspeed/altitude plot, if that makes sense. The amount, location, and distribution along the span, of a control surface's mass balance weights ("counterbalance" to Stress folks) come into play here. The flutter speeds of these modes don't follow a CAS, "half and half", or TAS line on an Airspeed vs Altitude plot.

That's why Flight Flutter Tests (FFT) should be done at various altitudes. And remember that the purpose of FFT's is not to find flutter, but rather to measure the aeroelastic damping that the airplane exhibits as speed increases to its Design Dive Speed (Vd), and verify that up to Vd there is a proper margin of damping present.

And before FFTs are done, comprehensive flutter analyses should be accomplished. The flutter analyses show that analytically the airplane is flutter free to (usually) 1.2 Vd, and shows a proper margin of damping up to 1.2 Vd.

BTW, A flutter analysis model consists of a structural model coupled with an unsteady aerodynamic model. The structural model consists of a mass model and a stiffness model that produce the "still air" modes and frequencies of the airplane for various fuel loads and payloads.

The flutter analysis model then is used to solve for flutter speeds, flutter frequencies, and mode shapes for the various flutter modes. Then the "critical" flutter modes can be identified, usually the flutter modes with the lowest flutter speeds.

And a Ground Vibration Test (GVT) is done to validate the structural model. The GVT measures the "still air" modes and frequencies of the airplane and these are compared to the the calculated "still air" modes and frequencies of the airplane from the structural model. If there are differences (and there always are) the structural model is adjusted to match the measured results, and the flutter analysis is rerun.

Based on various comments made by Van's employees over the years, my guess is that full-up flutter analyses may not have been done on the earlier models. Just a GVT to gain some knowledge of the "still air" modes and frequencies (and thus their frequency separation), and some limited Flight Flutter Testing. So the critical flutter modes may not be known or well understood. Once they realized folks were flying to higher altitudes (ostensibly due to folks installing larger engines, but folks were already getting to the low 20,000's feet with Van's recommended HP) they simply changed the existing Vne number from IAS to TAS, a conservative assumption. The RV-12 is the exception here, where Vne is a combination of IAS and TAS.

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.
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Arlington, WA (KAWO)
RV-8, 620 Tach Hours
(Pic 1),(Pic 2)
- Out with the Old, In with the New
(Pic)
RV-8, 1938 Tach Hours (Pic 1),(Pic 2) - Sold

Glasflügel Standard Libelle 201B - Sold
Rolladen-Schneider LS1-f - No longer owned

Last edited by RV8JD : 10-10-2019 at 11:35 AM. Reason: Clarifying comments added.
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  #30  
Old 10-10-2019, 02:15 PM
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Art_N412SB Art_N412SB is offline
 
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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?
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