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  #21  
Old 03-29-2018, 11:13 PM
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RV8JD RV8JD is offline
 
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Quote:
Originally Posted by TASEsq View Post
Hi all,

Seems like perhaps the place to ask..

EDIT: I take back my question: http://www.vansaircraft.com/pdf/hp_limts.pdf

Why do RVís quote VNE as a TAS? Iíve never really understood that. It would mean, say for a 200 TAS VNE, that I would be indicating a lot less at altitude at my VNE. Do the RVís have some issue at higher altitudes necessitating a reduction in IAS? (Flutter or something)?

All certified airplanes I have ever flown have always quoted VNE in IAS. Meaning the object of the day was to climb as high as you could so your TAS increased for better GS at the end of the day.

As far as I figured, VNE was an aerodynamic airframe strength issue - so it makes sense that it is based on IAS (I.e: the number of bits of air pushing against the airframe).
Yup, to provide adequate flutter margin at altitude.



Instead of stating Vne in terms of TAS, some Certified airplanes and sailplanes show reduced IAS values for Vne as a function of altitude instead.
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Last edited by RV8JD : 04-01-2018 at 11:32 AM.
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  #22  
Old 03-29-2018, 11:34 PM
paul330 paul330 is offline
 
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I am an ex-RAF QFI so, if I can think that far back, here goes:

Instrument Rating Exam Testers are Picked - IRET/PICD.....

IAS - what you see in the cockpit
Rectified AS - IAS corrected for Pressure and Instrument error
Equivalent AS - RAS corrected for Compressibility
TAS - EAS corrected for Density error

Technically, EAS is what you use for stall speed, approach speed, aerobatic speeds, lift, drag etc. It affects how the aircraft "feels" and "flies". It is a measure of the number of molecules going past the airframe and over the wing. So, in that respect, it is what we use for aircraft performance. Since we don't have air data computers to make the necessary corrections then we use IAS which is close enough for our needs.

As altitude increases, air density reduces. So to get the same IAS (number of molecules passing the aircraft), we have to fly faster - this is the so-called density error - that is, our true speed through the air is greater than indicated on the instrument. When looking at range and fuel consumption in still air, this is the number you use. It also affects TO and landing performance in that at high altitude/temperature, you have to accelerate to a higher TAS to get the same IAS for the necessary lift to take-off (plus the engine produces less power so there's a double whammy). Likewise on landing, for a given IAS, your TAS is higher and so the landing roll is longer.

Ground Speed is totally irrelevant to the aerodynamic performance of the aircraft. It is the TAS corrected for wind. It is also what is given by GPS. In that respect, it is what affects navigation performance but unless one is trying to get a Time on Target or similar, one does not "fly" GS. Other than changing altitude to get a more favourable wind, you cannot "fly" a particular GS to improve performance. You get what you get and use that to calculate headings and times.

So, you set your power which gives you an IAS. Using your whizz-wheel, you then calculate your TAS for your altitude/temperature. Using this number, you apply the wind and calculate drift and GS. Using these numbers, you then know your required heading to make good your track and the time to cover the leg distance. With and EFIS and GPS, this is all done for you.

As for the TAS Vne: Vne (in the VANS aircraft at least) is not due to a structural airframe issue, it is do to control surface flutter considerations. Since air density and therefore aerodynamic damping reduce with altitude, the IAS needs to be reduced - hence the TAS limit rather than IAS.
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  #23  
Old 03-30-2018, 12:07 AM
Bevan Bevan is offline
 
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Quote:
Originally Posted by TASEsq View Post
Hi all,

Seems like perhaps the place to ask..

Why do RV’s quote VNE as a TAS?...

...As far as I figured, VNE was an aerodynamic airframe strength issue - so it makes sense that it is based on IAS (I.e: the number of bits of air pushing against the airframe).
Yes, as I understand it... VNE is dictated by airframe strength but the RVs are fast and capable of altitudes higher than other normally aspirated aircraft. So, one of the limiting factors is flutter BECAUSE flutter has to with resonance which has to do with TIMING and natural frequencies of vibration of the structure, particularly the tail feathers. Exciting those frequencies (of airframe vibration) has to do with the ACTUAL speed of the air molecules of air flowing over the surface. That means the airspeed of interest is TAS not IAS. Since there can be quite a difference at the altitudes where our RVs can play, TAS becomes the one to watch. My understanding anyway.

Bevan.
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Last edited by Bevan : 03-30-2018 at 12:09 AM.
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  #24  
Old 03-30-2018, 01:18 AM
hohocc hohocc is offline
 
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Quote:
Originally Posted by Hongie View Post
So who's right (in your opinion)
You, the blue poster.
The other guy is trying to say his bike is faster going downhill than your bike is going uphill. It is, but the argument is nonsense.
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  #25  
Old 03-30-2018, 07:00 AM
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BCP Boys BCP Boys is offline
 
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Wow, isn't this stuff like the ABC's of the Private Pilot training? How could someone argue these points. There is only 1 right answer. If this person was a good friend and I could cut up with them without insulting them I would say, "You can't fix stupid", but If were just an acquaintance then I would respectfully walk away or end this discussion.
Some things are just not meant to be debated. Definitions of TAS, IAS and GS falls in that category.
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  #26  
Old 03-31-2018, 05:13 AM
Hongie Hongie is offline
 
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Wow this blew up! I've been away training in my RV, thanks for your input all. I'm not wanting to insult either of these guys or anything, just wanted to be sure of my arguments. Reassuring that my brain still works sometimes.
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  #27  
Old 03-31-2018, 06:02 PM
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grubbat grubbat is offline
 
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Default Another gee wiz

Another thing that may bewilder the non-Vanís group is that on the Vanís designs, Vno is based on indicated airspeed regardless of altitude and Vne is based on true airspeed. Let them chew on that for a little bit.
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  #28  
Old 03-31-2018, 09:02 PM
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airguy airguy is online now
 
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Quote:
Originally Posted by grubbat View Post
Another thing that may bewilder the non-Vanís group is that on the Vanís designs, Vno is based on indicated airspeed regardless of altitude and Vne is based on true airspeed. Let them chew on that for a little bit.
Well, just like anything else on the interwebz - just because somebody says it, doesn't mean it's accurate.
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  #29  
Old 03-31-2018, 09:24 PM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by airguy View Post
Well, just like anything else on the interwebz - just because somebody says it, doesn't mean it's accurate.
True

But in this case it is accurate.
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  #30  
Old 03-31-2018, 10:07 PM
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RV8JD RV8JD is offline
 
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Default Re: Van's Vne in Terms of TAS

Quote:
Originally Posted by rvbuilder2002 View Post
True

But in this case it is accurate.
Hi Scott,

Re: Van's airplane's Vne (redline) in terms of TAS instead of IAS.

Unfortunately, not even everyone on VAF knows that Van's changed Vne from IAS to TAS, not to mention RV pilots that don't frequent VAF.

This chart on Van's website should be updated to reflect the change of Vne to True Airspeed, similar to Van's V-Speed Letter posted for the RV-14. And the letters/notices could be posted on Van's website on the "Service Information and Revisions" page for each model.

(At the same time, Van's could update Maneuvering Speed (Va) for the RV-8/-8A and RV-7/-7A, if what I've written in the Posts below are correct.

http://www.vansairforce.com/communit...6&postcount=25

http://www.vansairforce.com/communit...9&postcount=37)

Just trying to be helpful here. (I'm a very happy RV-8 owner. In fact I'm on my second one!)
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Arlington, WA (KAWO)
RV-8, 420 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

GlasflŁgel Standard Libelle 201B, N564NS - Sold
Rolladen-Schneider LS1-f, N61MP - No longer owned

Last edited by RV8JD : 03-31-2018 at 11:33 PM.
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