What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Vne: Indicated -v- True Airspeed

Bill Wightman

Bill Wightman

Well Known Member
Hi all,

In recent months, there have been a few discussions of how Vne (Never Exceed Speed) is defined and determined. Unfortunately, there have been several posts and one article published in the RVator that have muddied the waters in helping to correctly understand what factors lie behind determining Vne for an airplane.

To put it simply, the issue of defining Vne for an airplane IS NOT a one-variable problem. The RVator article accurately points out one facet: that of aerodynamic flutter, and how it's sensitive to True Airspeed, not Indicated Airspeed. This is well known by aero engineers. Unfortunately, we've now found that its possible to fly well into a possible flutter situation while still under what we think are published airspeed limits for our airplanes. This would NEVER work for a certified aircraft... so we have a special situation here.

What I want to highlight is that in conventional GA aircraft, Vne is almost exclusively driven by aerodymamic limits. But the Vn diagram in the RVator article clearly shows the region over Vne labeled as "structural failure". To date, I've been under the assumption that our airplanes were found to be flutter-free for operation within the Vn envelope. For higher performance aircraft, a Mach limit sometimes can come into effect to address compressible flow around the airframe. But its important to note, that for ALL certified aircraft with no special limitations, you can safely operate the airplane within the prescribed INDICATED AIRSPEED limits and AT ANY ALTITUDE within the altitude limits for the aircraft. Just to be clear, all operating speeds as described in the FAR's are "Equivalent" airspeeds. Equivalent speed is simply indicated airspeed corrected for compressibility, and installation/calibration errors.

FAR 23.335 defines this:
23.335 Design airspeeds.

Except as provided in paragraph (a)(4) of this section, the selected design airspeeds are equivalent airspeeds
Click to expand...

A certified airplane must be shown to be free of flutter within its entire flight envelope at any airspeed (less than redline) and any altitude (under absolute ceiling).

FAR 23.629 talks about this:
(a) It must be shown by the methods of paragraph (b) and either paragraph (c) or (d) of this section, that the airplane is free from flutter, control reversal, and divergence for any condition of operation within the limit V-n envelope and at all speeds up to the speed specified for the selected method. ... (snip, emphasis added)
Click to expand...

We've now found out this isn't true for our RV's, which I think was an unintended limitation on the design. Keep in mind, you don't necessarily need a turbocharged engine, or lots of installed HP to go too far into the Safe TAS window: all a pilot needs to do is point the nose down hill from high altitude, and we all know the airspeed needle will wind up pretty quickly. This was reported on in the same RVator article.

About airplane design, and Vne:

The airplane's structure must react all primary forces acting on it. The structure is designed to handle only so much force in any direction, with a safety margin applied over that. Loads applied to the structure create stress levels that must be controlled to remain under safe levels for the materials used.

The point is: structure in our aircraft, and all certified aircraft, is designed around STRESS LIMITS (sometimes deflection limits). Exceed the ultimate stress (loads) and you fail the structure.

Stresses in an airplane are generated by AERODYNAMIC FORCE acting on the aircraft. Aerodynamic force is directly tied to something we call "dynamic pressure" , or denoted by the letter "Q" in engineering equations. Dynamic pressure is a function of the density of the air you're flying in, and the speed of that airflow. Its essentially "how hard" (not "how fast") the air is pushing against the airplane. The equation looks like this:

Q = 1/2 x (rho) x (V squared) -or- (one half times air density times speed squared)

The airspeed indicator we read in the cockpit is actually a dynamic pressure indicator. It reacts to the speed and density of the air we fly in, and thus it's important to understand the FORCES acting on our airframes are coming from the Q (dynamic pressure) shown to us as INDICATED AIRSPEED.

So, airframe stress limits represent one area of the Vn diagram limiting what the airplane can do safely.

OK, so how do we distill all of this into something every RV'er can use to fly safe?

--> You must know and understand that we fly airplanes that have known limitations in two important areas: Structural load limits, AND flutter limits.

To obey both of these two limits, we must always stay UNDER "Vne", or the red-line on the airspeed indicator (IAS), AND we must obey the (now published) TAS limit that was accurately pointed out in the RVator.

BOTH APPLY to our airplanes!

Sorry for the rant, but this is a "hot topic" for me...
 
Last edited:
I'm thinking you are going to see some major disagreement here as applied to RVs. Van himself won't agree with you on this point. Air loads don't seem to be the main factor on RVs.

While most normally aspirated aircraft can safely enough use IAS for Vne flying generally below 10,000 feet, flutter becomes a more important concern at higher altitudes due to reduced damping and higher TAS. Forced induction engines allow much higher altitudes to be used routinely and most WW2 aircraft for example had limiting dive speeds published in IAS for the pilots to refer to at different altitudes but actually based on TAS.
 
Disagreement?

rv6ejguy said:
I'm thinking you are going to see some major disagreement here as applied to RVs. Van himself won't agree with you on this point. Air loads don't seem to be the main factor on RVs.

While most normally aspirated aircraft can safely enough use IAS for Vne flying generally below 10,000 feet, flutter becomes a more important concern at higher altitudes due to reduced damping and higher TAS. Forced induction engines allow much higher altitudes to be used routinely and most WW2 aircraft for example had limiting dive speeds published in IAS for the pilots to refer to at different altitudes but actually based on TAS.
Click to expand...

I'm not sure why anyone would disagree with Bill, if i read his rant correctly (well, I might disagree with the tone, but the argument is correct):

1) most aircraft have Vne's based on structural considerations, and that can be translated into a dynamic pressure, ie, and equivalent airspeed related directly to indicated airspeed.
2) The RV's upper end is defined by flutter limits, which are related to True airspeed (the actual speed of the molecules going past the airframe).
3) Not everyone has a direct indication of TAS in the cockpit (although more and more do, through the magic of EFI!).
4) You need to be careful not to exceed the appropriate limit!

Aren't both of you guys essentially saying the same thing?

Now, as to whether or not you LIKE this limitation, that is a matter of personal taste and opinion.;)

Paul
 
A subtle, overlooked aspect of flutter vs altitude

In sailplane design and operation, flutter does most often determine Vne because of the much higher aspect ratio wings. The discussions about limiting the TAS to avoid flutter at altitude have led to some very over-restricted flying by people.

The classical flutter equations contain forcing terms that are proportial to dynamic pressure ( q = 1/2 rho V^2) and damping terms that are proportional to just density and velocity ( rho V). This produces the altitude dependence, BUT it is not strictly related to TAS either. I have found, and others also have shown, that a good approximation to a realistic flutter boundary as a function of altitude is a line roughly half way between a constant EAS line and a constant TAS line. On a plot of speed vs altitude, you can lable the X axis as EAS or ( or IAS) and the y axis as altitude. So a constant EAS line is vertical. A constant TAS line curves over to the left, corresponding to lower EAS at higher altitude. Now, draw a second curve about half way between the two. This has been found to be a fairly good approximation to typical flutter boundaries vs altitude.

To show how this provides a less restrictive adjustment to the indicated Vne, consider that at 18,000 ft at a TAS of 200 kts, the EAS is only 150 kts. If you used that as your personal adjustment to Vne, it would be pretty restrictive. Based on my arguments above, a reasonable reduction in Vne to maintain about the same flutter margin would be to not exceed 175 kts EAS. Thats not so bad.

I'm sorry that I don't have some actual data or references to back this up right now. I did the analysis quite a long time ago using some flutter vs. altitude data from a helicopter rotor test, and working through a classical torsion-bending flutter model problem. This topic comes up periodically, especially in the soaring community, and I really should try to put together a little white paper on it.

You can certainly choose to follow or ignore my suggested adjustment approach, at your own risk.
 
Bill Wightman said:
. . .OK, so how do we distill all of this into something every RV'er can use to fly safe?

--> You must know and understand that we fly airplanes that have known limitations in two important areas: Structural load limits, AND flutter limits.

To obey both of these two limits, we must always stay UNDER "Vne", or the red-line on the airspeed indicator (IAS), AND we must obey the (still officially unpublished) TAS "issue" (limit?) that was accurately pointed out in the RVator.

BOTH APPLY to our airplanes!
Click to expand...
Well, I don't necessarily disagree with you on this point of staying under Vne and the "red-line". However, as the builder of my aircraft I have this question. What is the "red-line on the airspeed indicator" for my aircraft if I am the one building it, installing instrumentation for it and ultimately making performance determinations for it? I am the one who has to make that determination as opposed to relying on some other manufacturing authority to tell me what this "red-line" setting is to be?

Where do I find the red line indicator on this IAS?

horizonhx.jpg


As the builder, I am going to have to figure out what the "red-line" number is going to be then make sure it is configured accordingly on my instrument. Granted, this number will be based upon a great deal of collaboration between me, the kit manufacturer, and many others who have gone ahead of me on this airplane building process. However, the truth still remains that determining what and how this number will be displayed on instrumentation is still my responsibility. Then, and only then, will I, as the pilot, be able to rely on the red-line on my IAS to guide me.

Because of this I feel, as the builder, it is very important for me to know what the TAS needs to be for me to be able to also determine what the IAS needs to be for me, my passenger and my aircraft to fly safely.

Live Long and Prosper!
 
I guess when we run out of things to debate about and nothing remains unresolved we go dig up old topics that have been resolved and get two members to jump in the ring and duke it out both taking the same side.

Don't go up to 1X,000' with your IO-3X0 C/S RV, do an aileron roll, get the nose pointed straight down, and scare yourself on the pullout going 3X0 mph TAS. Not good.

http://www.youtube.com/watch?v=5SzI5EJ8C6c
 
My Mini-Nimbus sailplane is...

scsmith said:
In sailplane design and operation, flutter does most often determine Vne because of the much higher aspect ratio wings. The discussions about limiting the TAS to avoid flutter at altitude have led to some very over-restricted flying by people.
....
I'm sorry that I don't have some actual data or references to back this up right now......
Click to expand...

...a possible reference in it's certification documents - if I wanted to make it Standard instead of Experimental - I don't...:)

From the Type Certificate -

Airspeed limits (I.A.S.)
Maximum Airspeeds In Calm Air
Never Exceed (Vne)

0 - 9800 ft. alt. 135 kts 155 mph 250 km/h
9801 - 19700 ft. alt. 120 kts 139 mph 223 km/h
19701 - 32800 ft. alt. 96 kts 110 mph 177 km/h

If I wanted to change it to standard airworthiness, I would have to have change the airspeed marking to reflect the 3 different red lines based on altitude - and yes... I have had the glider into the highest altitude band...:)

My german is non-existant, but here is the original German Type Certificate, and I don't think is has altitude vs. Vne restrictions in it - perhpas a german-speaking RV person could verify?

http://www.gliding.co.uk/bgainfo/technical/easa/tcds/3937_6.pdf

I think the above mentioned restrictions were added by the FAA for the US type Certificate, which makes me think that the European (JAR?) standards may have been more lax in the late 70's.
 
Last edited:
Gil has a nice example here

Using Gil's data:
If sea level Vne is 155 mph, and you decided to restrict yourself to 155 mph TAS at all altitudes, that would limit you to 113 mph IAS at 20,000 ft, whereas the certificated restriction allows 139 mph IAS. If you follow my rule of thumb to go half way between the 113 and 155, that gives 134 mph IAS.
Pretty close!

Thanks Gil.

Gil is right, most early certificated gliders from europe had no info on altitude reductions on Vne. Some of the companies, rather than calculate or predict or demonstrate an altitude-dependent flutter boundary just took a very conservative approach, and started the whole 'constant TAS' limit. It looks like in this case, Shemp-Hirth determined a more realistic placard limit to satisfy the FAA.
 
If Van is worried about flutter related to high TAS, it might be good to consider his advice. One should also consider the possible consequences of pushing this too far. Flutter has killed quite a number of test pilots over the years. Not a topic to ignore IMO.

On the other side of the coin, there is nothing to stop you from testing and validating your own flutter limits and Vne. Well there is that one point- that nobody has ever bailed out of an RV and lived yet...

No doubt there is some margin there and that is what has kept some of us alive who strayed over Van's recommendations.
 
Ross, Actually I'd agree with most of what you said (other than that airloads aren't the main issue on RV's). I do think we need to respect BOTH envelopes (flutter and structural) as I said at the end of my post. Van certainly could point all the structural design limits/features of the RV aircraft and how they work with designed stress levels.

The source of my exasperation: I'm currently building an RV8. The instruction manual says exactly nothing about flutter limits. It also doesn't specify whether the airframe speed limits are TAS, IAS, EAS, CAS, or anything else.

And as Paul noted, the tone of my "rant" was a bit perturbed. I expect more than an article in a newsletter from the kit manufacturer to warn me about the real big problem that my airplane might have a flutter envelope that's overlapping the published V-n envelope.
 
Last edited:
Why I posted this in the first place...

Perhaps I should give some explanation of why I went to the trouble to air my views on this subject.... I've lost more friends due to stupid aviation accidents than I would like to admit. All but one were avoidable. I don't want to go into details, but one of the problems was years back when an RV3 shed wings. The pilot, a friend of mine, didn't even know there was a design flaw in the airplane. He had wife and kids. Last night, I got about 4 hrs sleep just thinking about shedding more wings/ailerons/tail surfaces due to a lack of information. I know a few things about airplane design, and have worked a couple jobs in the industry. I'm an airline pilot now, but still keep my hand in engineering, as I always have.

Honestly, I don't think Vans supports these airplanes like they should. The airframe limits should be known: nailed down tight, specified in exact terms, and ANY problem areas should be aggressively dealt with. Just my honest-to-God opinion, but it has a cost when ignored.

To sum it all up, I just want everybody here to be acutely aware of the possible pitfalls out there. As we know, RV's are pushing farther, faster and higher than Van ever probably thought they would. We need to be very careful in stepping from the known to the unknown.

This issue of flutter is serious business. The RVator attempted to talk about it in the article referenced above, but that has (IMHO) left some of us with misguided ideas of what the actual limits are. I've actually been told more than once here that the RV's are TAS limited. Well, now that answer to that is "yes and no". I simply stepped in to point out that we now have TWO areas to think about: structural limits and flutter.

That's all...
 
Bill Wightman said:
The source of my exasperation: I'm currently building an RV8. The instruction manual says exactly nothing about flutter limits. It also doesn't specify whether the airframe speed limits are TAS, IAS, EAS, CAS, or anything else.
Click to expand...

I'm just finishing up my RV-9A and was reading Section 15: Final Inspection and Flight Test in Van's manual. There's a table on page 15-23 that gives airspeed indicator markings for the -4 through the -9. The last line is labeled "Red Line: VNE TAS (Maximum permissible speed under any condition)". The reference to TAS is not limited to the -9 and I assume this section of the manual is common to all the RV's. Additionally there's quite a bit of information about Vne and flutter limits on page 15-21 and 15-22 under the section called Flutter Testing.
 
Bill Wightman said:
Ross,
The source of my exasperation: I'm currently building an RV8. The instruction manual says exactly nothing about flutter limits. It also doesn't specify whether the airframe speed limits are TAS, IAS, EAS, CAS, or anything else.

And as Paul noted, the tone of my "rant" was a bit perturbed. I expect more than an article in a newsletter from the kit manufacturer to warn me about the real big problem that my airplane might have a flutter envelope that's overlapping the published V-n envelope.
Click to expand...

You are upset because no one will give you a number. The number changes based on atmospheric pressure, temperature, the balance, finish, and paint job on your specific airplane. No one can give you that number. It's out there. A line from an old movie "Don't look for it, Taylor. You may not like what you find."

If you want, climb wide open, cruise wide open..... just don't point the nose down and descend wide open at the (IAS) redline passing through 15,000'. You might not like what you find. If you can't sleep without a number, just don't exceed the published Vne (even though it's meant to be IAS) in TAS.

If you have no fancy TAS indication you can use this gross conservative wag....

for indicated airspeeds around 150........

ALT x 3 + IAS = TAS

Sample problem

Flying along...... 10,000' 150 IAS


10 x 3 + 150 = 180

Now before you get you wiz wheel out remember I said I was a wag. :D
 
This just seems to be going around and around. In my view, Van's has set the limit at 200 knots TAS for 7s, 8s, and 10s, 182 knots for 6s. This is his recommendation allowing plenty of margin for construction differences.

Observing TAS limits is not that hard. You can paste a small placard on the panel showing an IAS limit at 5000 foot increments. You can dial your IAS/TAS airspeed indicator to the density altitude you are at and read off TAS. Use a whizwheel or your fancy glass. Pull the power back a bit if you are pointing the nose down.



I'll stand by my statement that air loads are a secondary consideration on RVs. Aileron buzz has been encountered by a few RV pilots who stepped over the boundary. Nobody to my knowledge has ever suffered even minute structural damage from exceeding Vne from air loads. At 200 knots, if you calculated the drag load on the wings, I think you'd find it is only in the hundreds of pounds, not thousands. No way the wing is going to snap off backwards with those loadings. RV windshields are similar in thickness and design to other much faster aircraft. One of you AEs could run the numbers. It would be interesting.
 
200 knots at altitude is very fast. To maintain the same TAS, do the higher you go, the less the structure load?
 
Aviator168 said:
200 knots at altitude is very fast. To maintain the same TAS, do the higher you go, the less the structure load?
Click to expand...

Yes, because the structural load FROM AERODYNAMIC FORCES is driven from Q, the dynamic pressure. As altitude increases the dynamic pressure for a given TAS will decrease. The IAS is a direct measure of dynamic pressure and you will see a lower IAS for the same TAS at higher altitude.

Structural load from the airframe itself and how's it loaded/occupied will remain the same and still contributes to total load.
 
asav8tor said:
...If you have no fancy TAS indication you can use this gross conservative wag....
Click to expand...
Given the fact that Vne is based on TAS, does it seem a little strange to anyone but me, that the red line on the only TAS indicator that Van?s sells is 15 knots beyond the adjustable window?
 
rv6ejguy said:
Nobody to my knowledge has ever suffered even minute structural damage from exceeding Vne from air loads.
Click to expand...

I know of at least one incident (thankfully) many years ago when a passenger in the back seat of an RV-4 was allowed to have the stick for a while. After a while the pilot got a bit too complacent and before he could stop it the passenger started the execution of a roll. He botched it badly, and by the time the pilot got things sorted out enough to save it, they ended up well above VNE with the stick shaking during the pull out. Both elevators had damage.
Side note: if you are going to allow friends/passengers to try aerobatic maneuvers [in this case it had not been allowed] you better be relatively proficient at aerobatics and know how to take the shortest escape route from any botched maneuver, regardless of the airplanes attitude. Anyone involved in any level of aerobatics should be well trained to understand that the shortest path to level flight (if the airplane is anything other than nose pointed straight at the ground), is a roll to upright and then a pull out as needed to level flight. In the above incident it was basically a split-S maneuver started at cruise speed. In an RV, this will usually always have a scary outcome.

Low n Slow said:
Given the fact that Vne is based on TAS, does it seem a little strange to anyone but me, that the red line on the only TAS indicator that Van?s sells is 15 knots beyond the adjustable window?
Click to expand...

This is standard with all airspeed indicators. It would be rather confusing to have VNO, VNE etc labeled in multiple places on an airspeed indicator. The other issue is that not all of the import speed depictions on an ASI are TAS related. Stall speeds and manuvering speed, for example, are always indicated airspeeds.
 
The next step, certification....

Bill Wightman said:
Honestly, I don't think Vans supports these airplanes like they should. The airframe limits should be known: nailed down tight, specified in exact terms, and ANY problem areas should be aggressively dealt with. Just my honest-to-God opinion, but it has a cost when ignored.

To sum it all up, I just want everybody here to be acutely aware of the possible pitfalls out there. As we know, RV's are pushing farther, faster and higher than Van ever probably thought they would. We need to be very careful in stepping from the known to the unknown.
Click to expand...

I think the only way Vans could support this further would be to go through the certification process, elliminating the variables of the builders, and producing them in a factory at certified airplane costs. They have set conservative limits and your point about pushing farther and faster is the builders and pilots choice, not Vans. I hope I have not missed your point.
 
rvbuilder2002 said:
I know of at least one incident (thankfully) many years ago when a passenger in the back seat of an RV-4 was allowed to have the stick for a while. After a while the pilot got a bit too complacent and before he could stop it the passenger started the execution of a roll. He botched it badly, and by the time the pilot got things sorted out enough to save it, they ended up well above VNE with the stick shaking during the pull out. Both elevators had damage.
Side note: if you are going to allow friends/passengers to try aerobatic maneuvers [in this case it had not been allowed] you better be relatively proficient at aerobatics and know how to take the shortest escape route from any botched maneuver, regardless of the airplanes attitude. Anyone involved in any level of aerobatics should be well trained to understand that the shortest path to level flight (if the airplane is anything other than nose pointed straight at the ground), is a roll to upright and then a pull out as needed to level flight. In the above incident it was basically a split-S maneuver started at cruise speed. In an RV, this will usually always have a scary outcome.
Click to expand...

Was the damage due to G loads, air loads or elevator flutter? Been a few come apart that I can think of due to yanking too hard way over maneuvering speed.
 
rv6ejguy said:
Was the damage due to G loads, air loads or elevator flutter? Been a few come apart that I can think of due to yanking too hard way over maneuvering speed.
Click to expand...

I don't know if that was ever determined with any certainty. They way the story was told it seemed the pilot had thought it was with they way the stick was shaking in the pitch direction. I have always assumed it was a form of low freq flutter (even though I believe in aircraft, flutter is usually at a high freq. and almost instantly destructive). My feeling is, that regardless of speed, as long as the pilot isn't over G'ing the airplane, I don't think he would be applying enough load on the elevators to cause damage from air loads, but that is just my opinion based on testing control surfaces.
 
JonJay, I understand what you're saying, but reality is these airplanes aren't certified, so "we have what we have" so to speak.

I'm a repeat customer of Vans Aircraft, having built an Rv4 prior to this, so that says something about my approval of their product and company in general. Today, I'm a paying customer with an RV8 in the shop. My kit was delivered in 2003, along with the manuals. In the assembly manual, there's a section talking about operating speeds, but no mention of flutter at all, and the operating speeds are published as plain numbers - not even a mention of whether they're IAS, nautical or statute units, or otherwise. Keep in mind, this manual was shipped to me something like 10 years after the RV8 was put on the market.

Recently, I became aware of the RVator article from 2006. Late to discovery on my part, but hey I was just surfin the web! I was stunned to see it mentioning flutter being a problem for us, since we've had Rv's of various types pushing so freaking far into the TAS envelope I thought that was enough of a real world test to totally put any issues to rest, at least as far as excursions into the known Vn diagram are concerned.

So it was a jolting revelation (to me) that Van now publishes Vne as a TAS limit rather than an (assumed) IAS limit like 99.9% of the rest of the GA fleet. I guess this change happened sometime after the 2006 article. This is a major change in the operating limitations for the airplane, one that has direct and possible grave consequences if violated, and I never got as much as a post card from the kit manufacturer about it.

I'm going to use Ross's idea about posting a placard on the instrument panel with TAS limits -v- altitude. And, Ross, I have to apologize for misreading your first post. I know what you're talking about, and I agree there shouldn't be any structural issues that limit Vne. Sorry for the misunderstanding - I really hate the internet sometimes. (first post edited now)
 
maybe I need a new section 15

MTBehnke said:
I'm just finishing up my RV-9A and was reading Section 15: Final Inspection and Flight Test in Van's manual. There's a table on page 15-23 that gives airspeed indicator markings for the -4 through the -9. The last line is labeled "Red Line: VNE TAS (Maximum permissible speed under any condition)".
Click to expand...
Hi Mike - I checked my manual and the table is on the top of page 15-22, and the last line reads "Red Line: (Maximum Permissiable seed under any condition)" The entry for the -9 is 210 SMPH... no mention of indicated or true.

What number was in your table for the maximum VNE (TAS) for the -9?

BTW - does section 15 ever get refreshed? Mine is the original one from the preview plans... does it maybe come with the finishing kit?

Thanks,
 
Bill Wightman said:
JonJay, I understand what you're saying, but reality is these airplanes aren't certified, so "we have what we have" so to speak.

I'm a repeat customer of Vans Aircraft, having built an Rv4 prior to this, so that says something about my approval of their product and company in general. Today, I'm a paying customer with an RV8 in the shop. My kit was delivered in 2003, along with the manuals. In the assembly manual, there's a section talking about operating speeds, but no mention of flutter at all, and the operating speeds are published as plain numbers - not even a mention of whether they're IAS, nautical or statute units, or otherwise. Keep in mind, this manual was shipped to me something like 10 years after the RV8 was put on the market.

Recently, I became aware of the RVator article from 2006. Late to discovery on my part, but hey I was just surfin the web! I was stunned to see it mentioning flutter being a problem for us, since we've had Rv's of various types pushing so freaking far into the TAS envelope I thought that was enough of a real world test to totally put any issues to rest, at least as far as excursions into the known Vn diagram are concerned.

So it was a jolting revelation (to me) that Van now publishes Vne as a TAS limit rather than an (assumed) IAS limit like 99.9% of the rest of the GA fleet. I guess this change happened sometime after the 2006 article. This is a major change in the operating limitations for the airplane, one that has direct and possible grave consequences if violated, and I never got as much as a post card from the kit manufacturer about it.

I'm going to use Ross's idea about posting a placard on the instrument panel with TAS limits -v- altitude. And, Ross, I have to apologize for misreading your first post. I know what you're talking about, and I agree there shouldn't be any structural issues that limit Vne. Sorry for the misunderstanding - I really hate the internet sometimes. (first post edited now)
Click to expand...

No worries. When I dive tested my 6A back in 2004 I assumed Vne was IAS as well just like every other airplane I'd ever flown. I was taken aback when I found out on RVs a couple years later that it was TAS. Ummm I didn't feel too clever afterwards. I got out the whiz wheel and dialed in from memory altitude and approximate OAT. 195 knots true or something. Oops. I was wearing a chute and a helmet and had actually practiced a procedure for getting out on the ground several times. Good thing I didn't have to take to the silk! I have my doubts you could open the slider canopy anyway in flight- even with lots of adrenaline pumping.

Not a bad idea to keep this topic up front once in while. It could save someone's bacon some day with so many new RVs being completed each year now.:)
 
Barber Pole

Is there anyway we could get a barber pole installed on the airspeed indicators? I'm sure mechanical indicators would be tough but I would think it would be "relatively" easy on a glass panel.
 
Vne: Indicated -v- True Airspeed

Here's a scan of the airspeed limitations table from section 15 of my manual.

Airspeed_limitations.JPG


Note: the page is labeled "SEC 15r7 1/27/06" at the bottom.
 
Mike...

We have similar table, but SEC 15r8 6-08 at the bottom. Crucially to this debate it seems the same as the table you show except The last line says Red Line: VNE IAS :rolleyes:

What the above debate is missing, IMHO, is that whilst Van's might set a (guideline) VNE in IAS (?), translating that to TAS is difficult since he has not expressed at what Altitude / Pressure this is to be done. The point of the article he wrote was to point out that getting outside a "normal operating envelope" e.g. with a Turbo Charged engine, I am not sure you can directly translate his comments to operating "within the envelope" i.e. at less than say 10,000'?

The article talks of "flutter margins", and shows them well in excess of 10% @ MSL. The RV-4 sub-article was above VNE in IAS terms, and greater than 10,000'. It would seem the VNE (IAS) is fairly valid at 10,000', albeit with a <10% margin. These quotes seem to sum it up to me:
RVs are designed presuming the installation of naturally aspirated engines (and pilots). Van?s flutter analysis is conservative, but not so conservative as to allow for the true airspeeds that might occur using an engine that can develop 75% of rated power up to altitudes of 20,000 feet or more.
Click to expand...
With an 260 hp engine capable of maintaining rated power up to 24,000 feet, the flutter margin goes negative somewhere below 12,000?.
Click to expand...

Summary: I do not read the Manuals/Article as stating VNE is an MSL TAS value. It is an inexact science, especially with a "homebuilt", and that the quoted VNE is valid in IAS terms in the normal envelope (<10,000'?) - albeit with reduced margins at the upper end.

Andy Hill
RV-8 G-HILZ
 
danger will robinson

Wait a minute, I'm confused (not unusual..)

My table (from the "15r5 doc 7/17/02") says VNE for the -9 is 210 SMPH but doesn't talk to IAS or TAS.

Then the 2006 version in Mike's posts #27 shown above specified TAS?

Then the 2008 version that Andy describes in post #28 specifies IAS?

Do I have this right? Andy, are you sure it says IAS?
 
Last edited:
Missing input

MTBehnke said:
I've added a little table to my checklist that shows the equivalent IAS for various altitudes. I used this site to do the conversions: http://www.csgnetwork.com/tasinfocalc.html
Click to expand...

This is OK for a "gouge" but remember the you are not accounting for the temperature variable with a chart. It would be easy to be off 10 to 20 knots if the temperature is well off ISA. Modern glass will do it automatically or my 30 year old "whiz wheel" will give an answer in about 10 seconds.

John Clark ATP, CFI
FAAST Team Member
RV8 N18U "Sunshine"
KSBA
 
Dave - I'm assuming the RV-9A manual has it as a TAS limitation while Andy's RV-8 manual has it as an IAS limitation
Click to expand...
No... that seciton of the Manual is common to all types. The RV-8 manual we have from a few years back is the same as the one above i.e. TAS and "SEC 15r7 1/27/06".

It is a new manual (for all types) that has changed it to IAS with the SEC 15r8 6-08 references.

Do I have this right? Andy, are you sure it says IAS?
Click to expand...
Yes!

Andy
 
Manuals have changed

Andy and Mike: My 8 manual is alot earlier than yours. The page I have talking about speeds is from March 2001. And I need to add - I was wrong - it DOES specify speeds in statute miles per hour. It does not specify indicated or true speeds, so I assumed indicated since I've never seen otherwise in any GA piston single acft.

Thanks Mike for your post; that proves to me the manuals have gone through some significant updating. But I sure would like to see some good solid clarity from Vans on this subject... I'm confused now too!
 
How fast is too fast

There is a really good article about indicated airspeed, true airspeed and altitude in the Pipistrel aircraft manual.

http://www.mcp.com.au/pipistrel-usa/manuals/SinusAUSmanual.pdf

look to the section called how fast is too fast and it may help you with some of your discussion

Sorry, I can't work out how to cut and paste an image into this e-mail that you have a look at the following information it will give you an idea of the table which is inside every aircraft cockpit.


SINUS VNE AT ALTITUDE
Altitude IAS Knots
0ft 122
3300ft 116
6500ft 111
10000ft 105
13000ft 100
16500ft 95
19700ft 90
23000ft 85
26300ft 80

if for example you are flying at an altitude of 20,000 feet than the maximum indicated airspeed should be around 89 kn otherwise you will be exceeding the aircraft VNE of 122 kn
 
Bill Wightman said:
<snip>
Honestly, I don't think Vans supports these airplanes like they should. The airframe limits should be known: nailed down tight, specified in exact terms, and ANY problem areas should be aggressively dealt with. <snip>/QUOTE]

Since these are experimental airplane kits and each airplane is unique and each certified alone vs a certified aircraft built to a type certificate each certified to match a test plane, your above desired "nailed down tight, specified in exact terms..." thoughts are simply not possible. The variations in production prevents that from ever being the case. Even the paint job of one airplane to the next could change "exact terms" you seek.
Click to expand...
 
Ops limits are mandated by FAR

It is a fact: operating limitations for all US registered civil aircraft MUST meet the provisions of FAR 91.9 “Civil Aircraft Flight Manual, Marking, and Placard Requirements”, experimentals included. Note this FAR speaks not only to the aircraft markings, it also addresses the flight manual requirements. In the case of our RV experimental aircraft, the airframe and flight ops limits are given to us by Vans Aircraft. The overwhelming majority of these airplanes are marked and flown with the standard flight envelope and weight/balance limits.

It's ridiculous to assert that something like differences in paint jobs would invalidate the published ops limits, as set forth by the engineers who designed the airplane. To say that its “simply not possible” for Vans Aircraft to publish technically accurate operating limitations is way off the mark. Developing such ops limits is germane to the design process!

I stand by my statement: The ops limits must be published (as required by FAR) and they should be accurate, of course. Vans Aircraft, being the designer, is the First Source for these limits, not withstanding major modifications to the aircraft. To publish important parameters such as V_ne without specifying reference to IAS or TAS is just sloppy technical work.
 
Last edited:
Michael Coates XCOM said:
There is a really good article about indicated airspeed, true airspeed and altitude in the Pipistrel aircraft manual.
(snip)
Click to expand...

Mike, thanks for the pointer there. Interesting little airplane.

I'm finding more references to V_ne being tied to TAS in aircraft like this: sailplanes, motor gliders, etc. With their very high aspect ratio wings, structural flutter jumps into the picture much more readily.

In the case of what I've called "GA aircraft", structural flutter is almost a non-issue due to short torsionally stiff wings. Control surface flutter is much more likely in our type of airplane. Every standard GA type airplane I've ever flown has referenced IAS as the source for V_ne (which also is mandated by FAR in lieu of alternate limitations). Other high time pilots I've queried have said the same thing (with the sailplane exception).

It is important to know, though, that with some of us flying RV's so high and fast, Vans has made additional information known by way of their newsletter. They also have updated the flight manuals for most of their kit planes.

Thanks!
 
Last edited:
GA types

Bill,
I wouldn't get too hung up on the aircraft differences. Aerodynamics works for the same for "GA" guys too.

I think the reason that the typical 172 doesn't have a Vne table and the sailplane does is because the test pilot can easily climb up to the service ceiling in typical GA airplanes and check the performance and then establish a reasonable IAS for Vne. Slap it on the airspeed and the pilot is done. No thinking required.

You just can't do this on a glider (I've been over 30000' and the records are WAY higher) or on homebuilts (every one is different even if its the same model).

Also, talk to the Jet guys, they understand this issue (or should) because of their large performance envelops.



Bill Wightman said:
Mike, thanks for the pointer there. Interesting little airplane.

I'm finding more references to V_ne being tied to TAS in aircraft like this: sailplanes, motor gliders, etc. With their very high aspect ratio wings, structural flutter jumps into the picture much more readily.

In the case of what I've called "GA aircraft", structural flutter is almost a non-issue due to short torsionally stiff wings. Control surface flutter is much more likely in our type of airplane. Every standard GA type airplane I've ever flown has referenced IAS as the source for V_ne (which also is mandated by FAR in lieu of alternate limitations). Other high time pilots I've queried have said the same thing (with the sailplane exception).

It is important to know, though, that with some of us flying RV's so high and fast, Vans has made additional information known by way of their newsletter. They also have updated the flight manuals for most of their kit planes.

Thanks!
Click to expand...
 
Dave, I am one of the "Jet guys", being that I fly the Big Twin (the B777). Yeah, it does have a pretty wide envelope, speed-wise anyway. IAS limited to 340 and mach limited below about .87 or so. We don't deal with TAS at all - the airplane is operated mostly relative to compressible limits.

Thks for your concern, but I'm not really hung up on sailplanes -vs- RV's; just trying to point out real-world reasons why the sailplane would be TAS limited much more readily than the RV would.

GA type aircraft may have airspeed limits imposed by TAS/flutter issues, but the certification process requires the airplane to be safe to operate anywhere in its EAS (for the pilot: IAS) envelope. The airplane then would have to be designed/tested to be safe to operate under the worst-case TAS situation, given that the pilot stays within the prescribed IAS/altitude envelope.

I speculate that the RV's weren't designed to accomodate hitting V_ne at higher altitudes. So, we have some learning happening here, and a retroactive change to the operating limits, as I see it.

BTW, you got some Kahunas going to FL300 without an engine, or pressurization... Whoa!
 
Last edited:
Bill Wightman said:
I stand by my statement: The ops limits must be published (as required by FAR) and they should be accurate, of course. Vans Aircraft, being the designer, is the First Source for these limits, not withstanding major modifications to the aircraft. To publish important parameters such as V_ne without specifying reference to IAS or TAS is just sloppy technical work.
Click to expand...

Bill,
You are obviously very educated in aviation with your background, but the above statement is not correct
Their is no FAR that requires any experimental kit designer to document any performance numbers or limitations. There is also no requirement for them to provide a customer with any kind of a flight manual. This is because they are certificated in the experimental category. By its very nature, this category is setup to allow for Experimenting and their is no expectation that any two RV's will be built the same (though most are).

If you would like to look at the actual rules and guides lines that that have to be followed when a DAR or FAA airworthiness inspector are evaluating an airplane that someone has built and has applied for an Experimental / Amateur Built Airworthiness Certificate, you can read it in the FAA order 8130.2F located here http://rgl.faa.gov/Regulatory_and_G...184B11BB25FD020C862572C3000B21AB?OpenDocument

Go to Section 9 - EXPERIMENTAL AMATEUR-BUILT AIRWORTHINESS
CERTIFICATIONS
These 16 pages pretty much spell out what is required to be eligible for the E-AB certificate and what operating limitations are required to be issued with an E-AB airworthiness certificate.

I am not aware of any interpretation of the rules for the requirement of a flight manual. Remember, this experimental category, besides being for kit built aircraft, is also for one of a kind, never before flown aircraft that someone has designed and built them self. Their is no way to supply a operating handbook when it has never flown before. That has to be developed with methodical flight testing. Such is the same with any other airplane certificated as E-AB, even an RV.
As for Van's supplying performance information...in a sense they do. They tell customers that this so and so RV was designed for these engines with these propeller choices. If the customer uses the engines that the designers designed it for, they do not need to know any other information. The airplane will for the most part not capable of flying outside of the design envelope. If the builder chooses to be involved in the design process by installing a different engine other than recommended, then they get to fully utilize the freedoms that we are fortunate to have here in the USA with our experimental category, but they must also be ready to take on teh full responsibility that goes along with that freedom.
 
Bill Wightman said:
It's ridiculous to assert that something like differences in paint jobs would invalidate the published ops limits, as set forth by the engineers who designed the airplane. To say that its ?simply not possible? for Vans Aircraft to publish technically accurate operating limitations is way off the mark. Developing such ops limits is germane to the design process!

I stand by my statement: The ops limits must be published (as required by FAR) and they should be accurate, of course. Vans Aircraft, being the designer, is the First Source for these limits, not withstanding major modifications to the aircraft. To publish important parameters such as V_ne without specifying reference to IAS or TAS is just sloppy technical work.
Click to expand...

Vans numbers would be for the aircraft Vans built. Your airplane is NOT built to a type certificate tied to Vans airplane. Your airplane is NOT built under a production certificate. A type certificate that specifies every detail of construction and materials used. A production certificate that specifies who can do the work. And yes the paint job could have an effect on the airplane. You might want to check with Boeing on that one.

You can stand by your statement all day and all night. It doesn't mean a thing because you the builder/manufacturer of the experimental aircraft determine the numbers for your individual airplane. Vans has no way to know how you built your airplane. This concept extends far beyond performance numbers and limitations.
 
altitude record

Bill,
Not all that much really! Out in the Sierras in the winter, guys do it every day that conditions allow. Great wave conditions there near Tahoe. The absolute record was around 49500' set there 30 years ago until Steve Fosset broke it in the Andes (50k and change)

FL300 is plenty high enough for me though. COLD!!!!!


Bill Wightman said:
BTW, you got some Kahunas going to FL300 without an engine, or pressurization... Whoa!
Click to expand...
 
Scott, thanks for your reply. I understand the experimental amateur built guidelines fairly well. I've worked as a primary aero and structural engineer on "one-off" design efforts in the past. You are absolutely correct.

My point is that the great majority of these RV's are built pretty much to plans. They have no major structural or aerodynamic mods that would require a re-work of the ops limits we get from Van. So, my statement that Van is the "First Source" is just exactly that: they're where 99.9% of the builders/flyers go for reference to the ops limits.

Its been a few years since I've looked at 8130.2F, but the document does refer to "predetermined data" as an appropriate source for wt/bal data if the airplane was built from a kit. For "self designed" aircraft, they suggest the wt/bal should be determined by the builder. Ref (9)(f)(1). That same paragraph references FAR 91.9 which requires all US registered civil aircraft to have ops limits marked, a flight manual, wt/bal, etc.

With that in mind, Vans does give us the basic limits for the airplane. Its really a matter of product responsibility, but as you explained not regulated by law. My commentary on this from the beginning was to explain the need for supplying accurate and well presented information.

Once again, Scott, thank you for a well written and informative post.
 
Last edited:
Bill Wightman said:
Honestly, I don't think Vans supports these airplanes like they should. The airframe limits should be known: nailed down tight, specified in exact terms, and ANY problem areas should be aggressively dealt with. Just my honest-to-God opinion, but it has a cost when ignored.
Click to expand...

I find this very interesting when coming from an engineer.

I realize that this is an old thread but I'm glad I was pointed at it. Good points have been made. Lots of food for thought.
 
Last edited:
Quote:
Originally Posted by Bill Wightman View Post
Honestly, I don't think Vans supports these airplanes like they should. The airframe limits should be known: nailed down tight, specified in exact terms, and ANY problem areas should be aggressively dealt with. Just my honest-to-God opinion, but it has a cost when ignored.

How can you say this when Van's does not build the planes. Each plane built will be different because each is built different. There are not two of his planes built the same.
 
Liability

Norman CYYJ said:
How can you say this when Van's does not build the planes. Each plane built will be different because each is built different. There are not two of his planes built the same.
Click to expand...

I agree, this isn't an engineering issue as much as it is one of liability. Van's just cannot guarantee or "certify" things that were built by other people. Sadly, supplying hard data would just encourage plaintiff's attorneys. Based on their track record and customer service, I have no problem with Van's approach to this issue. Several times in my airline career I tried to get information on a systems problem from a major airframe manufacturer. Each time I was referred to their legal department. I have not had that experience with Van's.:rolleyes:

John Clark ATP, CFI
FAA FAAST Team Member
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA
 
Differences in the build are usually considered small enough or not relevant to changing the ops limits for most E-AB aircraft. In lieu of flight test / engineering evaluation of a specific airplane, the kit manufacturer's numbers are to be used. Ref 8130.2F, section 7 and 9.

The enlightenment provided in the RVator about flutter and TAS was significant to the safe operation of our aircraft. In my view, the article contained a change - of sorts - to the ops limits for our aircraft. But it did so without any kind of mandate, without any kind of compulsory language. And in a company newsletter... ?

Clearly, I found the whole process to be substandard to how the a kit manufacturer should support their product, especially when critical safety of flight data is being changed.

My view is not shared by many here, and that's fine.. At least we've talked about it and I think we all understand what's being discussed.
 
Last edited:
You must log in or register to reply here.
Back
Top