Vans ER tank VNE SB

[rocketman1988]

How about a simple thought experiment.

In flutter flight test, the test pilot establishes a specific speed/flight profile, then applies an exciting input (literally) by using a control input.
The lift distribution on each wing BEFORE the exciting input is in a low alpha condition, so pretty benign, and the structure is happy and in balance.
If the air was perfectly smooth, and there are absolutely no external forces changes.... we could probably be way past VNE at this point.

Lets say the input is from the ailerons for a left turn. So, the right aileron is going to increase the lift force and the left decreases.
The secondary effect of the increased lift from the aileron deflection will be to twist the wing.
So if there is increased weight ahead of the spar, it will tend to amplify the right aileron's twist input and dampen the left. Of course there is inertia of that extra mass as well, so lets just say the twisting response due to a different chordwise mass distribution is going to change the wings vibration twist response.

The wing has several natural vibration mode shapes in bending, and in torsion.
But, one of the natural frequencies of the wing structure has this shape... the right leading edge twisting down, and the left twisting up.
The flutter analyst tries to make sure that the external forces at a given flight condition don't get close to the natural frequency of these different mode shapes.

So if the structure was analyzed and tested with "as designed" mass and speed distribution, then it gets changed outside of the design parameters, stuff happens that may be more exciting than one would like.

Today's computer simulations of flutter models is quite good these days. And it's actually pretty easy to add mass to the finite element model at different locations to see the impact of different loading and flight conditions.

Start of totally unsupported speculation:

An engineer somewhere decided to do just that. Looked at the results and said 'Oh S%^t'

Then let’s see the data.

Ken provided data on his er tanks; there is currently nothing to refute except a vague, all encompassing Vne reduction.

It is also an ambiguous statement in that it referencing only total fuel and specifically states the location doesn’t matter.

Since only total extra fuel is referenced, it would be an unsubstantiated assumption to think that the reduction is for half the fuel as in most cases, only half the fuel is carried in each wing section. We really do not know but reading the statement would lead you to multiplying the extra fuel by 2.8 and subtracting it from Vne.

So here is another thought experiment. If the only thing that matters is the total extra fuel carried, and location doesn’t matter, what happens when we put all of the extra fuel in on wing? How does that change the flutter characteristics? Taking it a step further, again considering that location does not matter, let’s put the extra fuel in the back seat area. Could easily fit 20 gallons or more in those areas and still be well within the max gross weight. The way the statement is written, though, I would incur a 56 knots decrease in Vne. If, however, I put a person there instead, I have zero reduction in Vne. Again, location doesn’t matter.

Do you see the problem with the statement that was made?

 

[chepburn]

Do you see the problem with the statement that was made?

Yes, it does read as a sweeping statement. But the location phrase did say "regardless of the location of the extended fuel tank within the wing"
So it is ambiguous between leading edge tanks and possible tip tanks.

 

[grubbat]

it’s amazing to me the changes in the customer base from the 80’s to today. Back then, when Vans issued a change we would say thank you. We were so happy to have the opportunity to build our dream.

Today, when a change is issued, it’s whine and lawsuits.

 

[skirting_virga]

it’s amazing to me the changes in the customer base from the 80’s to today. Back then, when Vans issued a change we would say thank you. We were so happy to have the opportunity to build our dream.

Today, when a change is issued, it’s whine and lawsuits.

I'm curious why you think this is entirely a change in the customers and not at least partially a change in how Vans operates. GA manufacturers became deathly afraid of lawsuits in the 80s and stopped making planes altogether. If Vans perceives heightened risk of lawsuits, they may release overly cautious guidance that the customer base disagrees with.

 

[terrye]

And going back 30 years to a different time, recall Jon Johanson's record flights in his RV-4 VH-NOJ. He consulted Van himself for the modifications he did.
Fuel in wings (standard wing tanks) 31 USG (183 lb)
Forward fuel (custom tank ahead of instrument panel) 11 USG (67 lb)
Fuel in wing tips (fiberglass tips modified to form fuel tanks) 17 USG (102 lb)
Fuel in rear passenger area (custom aluminum tank) 64 USG (385 lb)
Although this was likely before sophisticated FEA mode analysis and ground vibration testing, nevertheless he consulted the design authority and they agreed to the increases. Not only that, but the Australian aviation authority approved it, which required much more documentation than here in North America.
For me, the SB appears arbitrary. But it is troubling in that it puts pilots / owners in a quandary. Vans should put this to rest by publishing the data.

 

[jclark]

Here is one data point for the safety police.
Super 8. 20 years old, 3k hours.
Exceeding 200kts and pulling g's at those speeds for 15 years, hundreds of times per year with 15 extra gallons in the wings smoke oil and 42g fuel.

Flying around not pulling g's, with wet wing fuel and smoke. 58 extra gallons in the wings.
58x2.8=162.4 kts reduction in speed for the Safety Notice if it applied to my plane. Genius math.

Fatigue life reduction? I have no idea. Its why if you want to know, someone has to actually do it. I am.

And he has **LOTS** of videos to prove it!

When I first started reading the thread, this was the very plane that I thought about and its many years of G's and Gallons.

[Newbies might not know what I am talking about but the OG's will understand ]

 

[moosepileit]

it’s amazing to me the changes in the customer base from the 80’s to today. Back then, when Vans issued a change we would say thank you. We were so happy to have the opportunity to build our dream.

Today, when a change is issued, it’s whine and lawsuits.

The counter exists as strongly, in the condition set you selected, to belittle, which showed more hubris than discord.

"We" have 30 plus years since then, to question a change, based on 30 plus years data, "known" vs a tech data set that is unknown.

That never flies well, and it is not about age, year, generation or tribe. That's poor aviation leadership.

If you feel extending range is inherent normalization of deviance- for instance, why is there a dearth of data?

 

[Richard Connell]

it’s amazing to me the changes in the customer base from the 80’s to today. Back then, when Vans issued a change we would say thank you. We were so happy to have the opportunity to build our dream.

Today, when a change is issued, it’s whine and lawsuits.

The irony of course being that the very modifications this SB addresses have been flying since then with no comment from Vans until now.

The customer base has no doubt changed dramatically over the decades. For better or worse. But it was a customer base that was actively pursued by the company.

 

[jclark]

And he has **LOTS** of videos to prove it!

When I first started reading the thread, this was the very plane that I thought about and its many years of G's and Gallons.

[Newbies might not know what I am talking about but the OG's will understand ]

I would be remiss, though, if I did not point out a few things.

1. A highly skilled pilot was doing this.
2. The plane was NOT full of fuel at the time. (The high-G maneuvers)
3. The plane DID "commute" at times full of fuel ( as I seem to recall it being said)
4. VERY IMPORTANT ... This was an RV "Super" 8, ... different fuselage, wing, and spar from the 10 and 14.
5. ALSO IMPORTANT ... With an extended wing tank setup done by the builder (not the ones currently under discussion)

Wanted to be clear that I (nor anyone else) am implying that you would have the same results from your 10 or 14 with similar but different ER tanks.


Just for the record ....

 

[jrs14855]

Post #23:
"whenever an rv breaks up in flight--"
A few months ago I did a personal study of RV4,6,7 and 8 accidents involving US registered aircraft with the following results:
RV4, one horizontal stabilizer detached in flight, attach hardware not found
RV6; one wing failure that involved modified RV3 wings on RV6 fuselage. This was called an RV6 but was not an RV6
RV7: I don't have the numbers in front of me but I believe it was SEVEN inflight structural failures including 2?? non US.
RV8: wing failure on factory aircraft involved massive exceedance of G limits.
I don't have any numbers but low altitude acro and inadvertent stall spin accidents FAR exceed the totals shown above. The "low hanging fruit" for improvement is the inadvertent traffic pattern spin. Avoidance is the solution. For the average Private Pilot the base to final spin out of a skidded turn should be considered non recoverable.

 

[jrs14855]

Regarding flutter:
The first flutter incident is believed to have occurred in 1903
The first formal flutter evaluation was in Germany in 1935.
The first GVT evaluation was in Germany in 1939. (Dornier)

 

[tom_AZ]

In a discussion with people close to the heart of this it emerged that the unknown was not the question of flutter but a potential change in spin characteristics that could make the plane unrecoverable.
Are those of you putting in ER tanks also modifying the tail?
There is a reason most light twins have some modifications to the tail to address this issue.
I don’t know the sky designs people but I do know and respect Ken.
Have they done spin testing with the ER tanks full?
My RV 10 has standard tanks. I can live with those limits.

Has my understanding been wrong? I had the impression that the -10 had not been spin tested.

While altered spin characteristics would certainly be a potential concern with a change in weight across the span, if spins are already prohibited and possibly unrecoverable, that wouldn’t seem to fit with the new SN—at least as it applies to the -10.

 

[dmn056]

Regarding flutter:
The first flutter incident is believed to have occurred in 1903
The first formal flutter evaluation was in Germany in 1935.
The first GVT evaluation was in Germany in 1939. (Dornier)

See Garrick & Reed, "Historical Development of Aircraft Flutter", J.Aircraft, Vol 18 No 11, November 1981, for a more comprehensive history. The first published reports were much earlier than 1935.

 

[glider_rider]

From here
When I started my RV10 project the ER tanks were still a very a new thing and back then I thought that the dual fuel tank configuration was a more mature and flight proven solution. Correct me if I’m wrong, Saint Aviation has been building the dual tank equipped 10s since 2010.
According to the SB all those aircraft should fall from the sky as the "safe" Vne of 32 knots was / is on regular bases exceeded by 140 knots. Yet, no such accidents reports. Dual tank equipped RV10 never experienced structural failure.
In that case, the Vne limitation is incorrect, at least by 140 knots. Or is the flutter margin 140 knots?
In my opinion, it’s all about Vans credibility.

full disclosure: I experienced flutter in a certified glider. I do know how it feels like.

 

[j4cbo]

I'm also pretty baffled by this.

The message from Van's around kit modifications has always been consistent: to pharaphrase, "if we didn't design it then we haven't tested it and can't speak to it; you're on your own". There's language to this effect in Section 2 of all the KAIs that I've looked at. For example, the 12:

If the builder chooses to deviate from the plans and install a non-standard engine or to modify the aircraft to a configuration other than what is called out in the plans, he or she is assuming responsibility for the airworthiness of that modification and any effect it may have on the airworthiness of the airframe and/or powerplant. Technical support may not be available for modifications that deviate from the plans nor for installations that are not specifically recommended by Van's Aircraft.

Keep in mind that insurance companies may not be willing to write a policy on an aircraft that has been modified to a configuration other than that recommended by the manufacturer. Prior to modification, the builder should check with their insurance provider regarding this matter.

If a kit is modified in any significant manner, it should not be considered an RV (type) for registration purposes.

I can't find the source, but I read something from Van's (or Van himself?) that said that if there are any significant modifications, the resultant airplane needs to be tested as an entirely new design.

This was all emphatically restated in SN-00062 a few years ago:

Van’s Aircraft does not evaluate, review, test, approve, or in any other way provide its support for 3rd party deviations from its published and maintained aircraft designs.

But by assigning a specific Vne reduction based on extra weight in the wings, Van's is demonstrating that they have evaluated that deviation from the plans. The suggestion up-thread that it's actually a spin mode issue is even more concerning. I'm no lawyer, but if someone were to get into an unrecoverable spin in an RV-10 with extended range tanks, I could imagine the lawyers arguing that Van's had evaluated the design and found that only a Vne reduction was necessary. Van's could be worse off for having released SN-00108 than if they'd stuck with their previous messaging.

I don't like the idea that Van's would release a service bulletin like this out of sheer spite, but I also don't have a better explanation. If they do have data to support the 2.8 knot figure, it would be a huge benefit to everyone if they would release it.

 

[clarenceoveur]

First off, for amateur-built aircraft, DARs are not obligated to determine the airworthiness of an aircraft. The builder states that the aircraft is "airworthy and is eligible for the airworthiness certificate requested" when he/she signs form 8130-6.

Our responsibility is to determine if the aircraft meets the requirements of 21.191(g). This is the often referred to 51% rule. We are saddled with the determination that the aircraft is "Amateur-Built".

We are NOT responsible for the design of the aircraft. If we determine that the aircraft is unsafe for any reason, in our opinion, we have the option of denying the certificate. After the denial, if the applicant disagrees with the denial he/she is within their rights to seek approval from another inspector.

As an example, I denied an aircraft many years ago. Months later it was signed off by another DAR. Several months after that, the aircraft experienced an in flight failure of the part that I rejected it for.

Hi Mel, maybe you can set me straight on a question I have about submitting a new form 8130-6 to "update the the FAA that I have installed a constant speed prop in place of a fixed pitch. My Conditions and Limitations attached to my current Airworthiness Certificate say I should submit a new 8130-6 after installing a constant speed prop. My question is this; do I just fill out the applicable owner sections of the 8130-6 and send it in, or do I need a DAR/FAA inspection of the aircraft to sign the 8130-6?