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Vans ER tank VNE SB

I completely understand Van's wanting to distance themselves from these kinds of modifications.

But it's strange to provide "generic" guidance - 25 gallons max for acrobatics, -2.8kt VNE per gallon extra - that depend on the specific design.

Given normal (non-acrobatic) flight only has the VNE limitation, the acrobatic limit must be related to yawing / rolling moments of inertia, which affect things like spin recovery.
For the SkyDesigns ER tanks, the manfucturer calculated an acrobatic fuel limit so that the moments of intertia would not exceed that of a stock RV14 with full tanks, assuming fuel in the ER tanks migrates outboard. Full details are in the engineering report SkyDesigns provides.
It just so happens that this limit is also 25 gallons! But if the extra fuel was in tip tanks, the moments of intertia would be much higher, so the limit would have to be lower!

I understand a lot less about flutter / VNE limitations, but it also seems like the location of the fuel would affect this. The RV10 Modal Impact Testing report SkyDesigns put out reports that typically fuel forward of the elastic axis is stabilizing to wing flutter, but fuel aft can increase the danger of flutter.

In both cases it seems impossible to calculate these limitations without knowing where the extra fuel is! What happens if I put 12.5 gallons in a hypothetical tip tank and enter an unrecoverable spin? Or put extra fuel in the ailerons which, as I understand it, wouldn't be good for flutter? Can i now blame Van's?

For the SkyDesigns case specifically, their engineering report and the ground vibration testing they caried out seem to justify the limits they set.
 
lr172 said:
In fairness to the counter point, I am pretty sure that vans did not run those tests again with ER tanks installed, so how did they arrive at the 3 kts per gal. reduction? Seems to be poor engineering to hypothesize on how weight distribution across the wing would change the results they got without them and then provide specific guidance to mitigate. Again, I fully concur with their warnings. "We tested and got X. If you change things, we don't know what you'll get and are at the mercy of your own testing." as they have always said. But that is not what they said this tim
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Walt said:
The point is Van’s can’t possibly afford to test/evaluate every mod that builders incorporate into their designs.
However, There is little doubt that ER tanks would be considered a major alteration.
I have no idea how they came up with the numbers, but I’m pretty sure they didn’t just pull them out of thin air.
Now you get to think about that every time you fly, and hope they weren’t right.
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If you have no idea how they came up with the numbers, the assumption that they didn’t pull them out of thin air is a faulty assumption.

A better assumption would be that they did not perform the engineering, as they cannot possibly test every condition and are certainly not required to. So instead, they make a blanket decrease in vne that will assure that there will never be an issue, and in doing so, effectively prohibit that particular mod.

From their perspective, I get it but a simple statement that the engineering analysis has not been done on er tanks and installing them is on the builder would have sufficed.

You really like to get that final jab in, too. During phase 1, I tested to a bit over 200 ktas, and in phase 2 I don’t go above about 185 ktas. The aircraft has been rock solid within that envelope both with and without the extra fuel. As the -10 isn’t designed for aerobatics, that is not a concern.
 
rthur said:
I completely understand Van's wanting to distance themselves from these kinds of modifications.

But it's strange to provide "generic" guidance - 25 gallons max for acrobatics, -2.8kt VNE per gallon extra - that depend on the specific design.

Given normal (non-acrobatic) flight only has the VNE limitation, the acrobatic limit must be related to yawing / rolling moments of inertia, which affect things like spin recovery.
For the SkyDesigns ER tanks, the manfucturer calculated an acrobatic fuel limit so that the moments of intertia would not exceed that of a stock RV14 with full tanks, assuming fuel in the ER tanks migrates outboard. Full details are in the engineering report SkyDesigns provides.
It just so happens that this limit is also 25 gallons! But if the extra fuel was in tip tanks, the moments of intertia would be much higher, so the limit would have to be lower!

I understand a lot less about flutter / VNE limitations, but it also seems like the location of the fuel would affect this. The RV10 Modal Impact Testing report SkyDesigns put out reports that typically fuel forward of the elastic axis is stabilizing to wing flutter, but fuel aft can increase the danger of flutter.

In both cases it seems impossible to calculate these limitations without knowing where the extra fuel is! What happens if I put 12.5 gallons in a hypothetical tip tank and enter an unrecoverable spin? Or put extra fuel in the ailerons which, as I understand it, wouldn't be good for flutter? Can i now blame Van's?

For the SkyDesigns case specifically, their engineering report and the ground vibration testing they caried out seem to justify the limits they set.
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Yes, they do.
 
rocketman1988 said:
If you have no idea how they came up with the numbers, the assumption that they didn’t pull them out of thin air is a faulty assumption.
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Is it? Do you know how Van's came up with ANY limitation numbers for their air frames? Do you assume they pulled all their specifications from thin air? I don't think Van's Aircraft has ever publicly disclosed any of their engineering analysis and data related to their aircraft designs and limitations. The one exception may be the information they released regarding laser cut parts.

Skylor
 
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skirting_virga said:
Just to hijack for a moment: the 7 and 8 are popular older models which use the same wing (as each other, not as the 10/14) and have a few thousand examples flying. Why does nobody (SD?) make extended tanks for these models? 42 gallons is OK, but could be better. One more rib on each wing would be approximately 50-52 gallons, which is in the ballpark of what a fair number of similarly-engined certified planes might have (like an R-model 172 or some of the older models with extended tanks).

EDIT: at 10gph, 50 gal is "only" 2h out, 2h back and 1h reserve for someone who wants to visit a place that doesn't have fuel, or has expensive fuel. If you fly more frugally at 8gph it's still only about 2.5h out and 2.5h back with 1h reserve... not a totally bladder-busting amount like 5-6 hours of endurance would imply.
Click to expand...

And how does the actual range in NM of a -7 or -8 compare to an R Model 172?

Skylor
 
skylor said:
Is it? Do you know how Van's came up with ANY limitation numbers for their air frames? Do you assume they pulled all their specifications from thin air? I don't think Van's Aircraft has ever publicly disclosed any of their engineering analysis and data related to their aircraft designs and limitations. The one exception may be the information they released regarding laser cut parts.

Skylor
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So you are saying that they analyzed every different location and capacity for every different mod and came up with a blanket 2.8 knots/gal reduction to prevent flutter of something...mmm, ok.

or

They put a blanket 2.8 knots/gal reduction on, knowing that at those speeds it wouldn't ever be a problem AND new builders would likely NOT install the ER tanks due to the performance penalty.

Which choice is more plausible?
 
Here’s Sky designs GVT analysis.

On the question of 7/8s. I’d say the issue here is that the z brackets are made by the builder and not pre punched (certainly weren’t on early ones) so there’s too much variation to be able to provide a plug and play solution.

Down here id say the majority of 10s have some form of ER tanks simply because of the large distances and minimal refueling options. Plenty of the other models do too.

It is an amazing coincidence that all combinations of spanwise or chord wise additional fuel in various tank designs yield the same knot/gal reduction in VNE.
 

Attachments

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.
 
Mel said:
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.
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WelI guess i will say it - you really didn't answer the question....would you refuse to sign off a RV10 ER solely on the basis of the addition of the added Sky Design ER tanks?

I recall several months ago you posted a mysterious post about wanting to discuss something with other RV10 DARs.

I guess I am asking because I am about to hit the point of 'no return' on ER versus standard tanks.

Vans Safety Letter basically makes ER tanks untenable from a practical standpoint regardless of how one wants to interpret the EAB regs.
 
Someone correct me if I am wrong -My understanding is that if the builder has ER tanks, fills them with fuel, does a VNE test (at 200kts), places a log book entry about the successful test flight (no flutter or other adverse flight characteristics) doesn't that put this issue to bed?
 
Strikhedonia said:
Someone correct me if I am wrong -My understanding is that if the builder has ER tanks, fills them with fuel, does a VNE test (at 200kts), places a log book entry about the successful test flight (no flutter or other adverse flight characteristics) doesn't that put this issue to bed?
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Not so. In the certified world, Part 23 requires VNE to be no more than 0.9 of demonstrated dive speed. If you fly to 200kts, VNE would be no more than 180. Of course, that’s in a certified world—-
Then there’s the IAS vs TAS with regards to VNE.
 
rvbuilder2002 said:
I fail to understand why everyone is so riled up over this.

People expect kit manufacturers to own their design and take responsibility for everything related to that design, but they also want the freedom to deviate from that design and execute modifications as they wish.

I am all for that as well. That’s what makes the experimental amateur built category so great here in the USA.

But now it seems a lot of people think that a kit manufacture should own responsibility for any modification that they or others design and/or choose to incorporate. 🤨

This publication is just Van’s way of emphasizing to you that choose to modify, that you need to own responsibility for modifications that you choose to make. They also included recommendations based on their engineering, data and experience, to help keep modifiers safe when they have chosen to deviate from the standard design.
Are those recommendations made based on physical testing and analysis?
I doubt that it is and why would it be? Why would Vans invest the dollars to test modifications designed by someone else. Not to mention all of the different configurations that have been available over the years.

The bottom line is, if you choose to build your kit, configured differently from what Vans supplies, you need to own everything related to that modification you made.

If you don’t have the engineering skills to determine whether the modification might have changed the Vne or other performance aspects, then you need to either rely on an outside source, such as the designer of the modification, or accept whatever information the kit manufacturer has provided. You get to choose… that is what owning your decisions entails.
Remember
These aircraft are experimental. If you choose to make modifications, then whether you think so, or not, you are making it even more experimental.
Own those decisions!
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Scott,

I think the issue here is almost all of us agree that once you deviate from the plans and original kit, we (the builder) accept responsibility for the changes and what may happen as a result thereof. That has always been the case, and we'd all be foolish if we did not expect some type of statement from Van's washing their hands about builder modifcations. But as builders we all tend to put the Experimental in our Experimental planes to a certain degree. Some more than others. After all, Van modifying the Stits Playboy design is how it all started!

But what is different in this case is a huge change to operating specs without any engineering data provided to back it up. Will there be changes in Vne to the DeltaHawk equipped RV-14s? If not, why not? In the DH case, almost everything firewall forward has been changed? What about those aircraft equipped with ZipTips (that actually somewhat change the airfoil shape at the tip)? And would I expect Vans to test each of these mods with an engineering analysis? Of course not!

This almost has the feeling of sour grapes between Sky Designs Engineering and Vans Aircraft. And so far I've seen way more data from SD than Vans. In my opinion there's a huge difference between a "you accept responsibilty for changes" statement and a huge change in allowable operating specs.

Just my two cents . . .

Rick
 
Rick RV-4 said:
Scott,

I think the issue here is almost all of us agree that once you deviate from the plans and original kit, we (the builder) accept responsibility for the changes and what may happen as a result thereof. That has always been the case, and we'd all be foolish if we did not expect some type of statement from Van's washing their hands about builder modifcations. But as builders we all tend to put the Experimental in our Experimental planes to a certain degree. Some more than others. After all, Van modifying the Stits Playboy design is how it all started!
Click to expand...

I would hope all builders would have the same thoughts, but it doesn't seem that all do.

Rick RV-4 said:
But what is different in this case is a huge change to operating specs without any engineering data provided to back it up. Will there be changes in Vne to the DeltaHawk equipped RV-14s? If not, why not? In the DH case, almost everything firewall forward has been changed? What about those aircraft equipped with ZipTips (that actually somewhat change the airfoil shape at the tip)? And would I expect Vans to test each of these mods with an engineering analysis? Of course not!
Click to expand...

I wont disagree about the point that Van's chose to deal with this subject differently than they have others, but I don't see that it matters.
As has already been pointed out numerous times already in this thread string, nothing published by Vans in service bulletins or service notices is mandatory for compliance.
The only time anything they have published comes into play at all is when an accident investigation infers that an un complied with SB or SN may have been a cause factor for the accident.
So the only valid reason I see for someone to be upset about this SN is if they are not wanting to fully own their decision to make a given modification, by doing all of the research and testing to validate the modification. In simple terms, they are choosing to in essence put that burden on Vans by referring to their published info on the subject, but not liking what Van's published.

If you choose to modify but do not have they knowledge, skills and/or finances to validate your chosen modification(s) I don't think you should disparage others for taking a position. Regardless what the reason for their position is.
So if you don't like what Vans has published, why not trust Sky Designs published GVT report? Particularly since I assume most that purchased the extended fuel kit, did so because they did not have the knowledge, skills, and/or finances to get it done on their own, but they were willing to trust Kens design. If that is true, then I would think you should be willing to trust his validation testing.

If you are going to modify... Own your decision.

Rick RV-4 said:
This almost has the feeling of sour grapes between Sky Designs Engineering and Vans Aircraft. And so far I've seen way more data from SD than Vans. In my opinion there's a huge difference between a "you accept responsibilty for changes" statement and a huge change in allowable operating specs.
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SD is the one that has something to gain by validating the design so it is no surprise that effort has been made to do so, and that is what any good designer should do. I might be wrong but I don't think any of the advanced testing was done until way after the kit was made available, and someone provided a financially feasible means of doing it. Van's has nothing to gain by doing so other than that some may argue that they would sell more kits because of people that would buy a kit if it "just had a little more fuel capacity". Probably a very small number of people.

As for the feeling of it being retaliatory against Sky Designs, I have not heard a single thing that would make me think that, but if it was true, particularly if it has had an impact on kit sales, who could blame them?
 
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A few years ago I installed some extended range tanks in a Cessna 182 , they fit behind the spar , the end rib was modified and the tank went in the end rib and stopped at the second rib . Clean easy install… I designed a set for my Rv8 , 9 gallons per side … sent the drawings to Steve Smith … “ Flutter Issues “ was his concern and answer…. Extra Range Tank Design was put in the trash .. Problem with Flutter and structure failure is it can happen quickly with bad consequences… Be Careful.
 
cactusman said:
WelI guess i will say it - you really didn't answer the question....would you refuse to sign off a RV10 ER solely on the basis of the addition of the added Sky Design ER tanks?
I recall several months ago you posted a mysterious post about wanting to discuss something with other RV10 DARs.
I guess I am asking because I am about to hit the point of 'no return' on ER versus standard tanks.
Vans Safety Letter basically makes ER tanks untenable from a practical standpoint regardless of how one wants to interpret the EAB regs.
Click to expand...
And I will NOT answer the question the way it is asked. That's how "Flame Wars" get started. As I said before, we are certificating aircraft to meet the requirements of §21.191(g). We are certificating the "airplane".

As Scott and others have said, If you modify your aircraft, then own your modification. If you don't like the designers operational limitations, either build something else or come up with your own ways of testing to prove your ideas.

I look at every aircraft I inspect as a one off. I see VERY few (EAB) aircraft built exactly per plans. Each individual has his/her own ideas. YOU set your operational parameters. If you deviate from the designer's parameters such as a gross weight increase, be prepared to "show your work."

If I am convinced that it meets the requirements of §21.191(g) and it is safe for operation, I will sign it off.

And BTW, the discussion we had about the RV-10 was a question about a common practice meeting the regulations. It was a very minor thing and we ALL agreed. It had nothing to do with safety or fuel systems.
 
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rvbuilder2002 said:
I would hope all builders would have the same thoughts, but it doesn't seem that all do.



I wont disagree about the point that Van's chose to deal with this subject differently than they have others, but I don't see that it matters.
As has already been pointed out numerous times already in this thread string, nothing published by Vans in service bulletins or service notices is mandatory for compliance.
The only time anything they have published comes into play at all is when an accident investigation infers that an un complied with SB or SN may have been a cause factor for the accident.
So the only valid reason I see for someone to be upset about this SN is if they are not wanting to fully own their decision to make a given modification, by doing all of the research and testing to validate the modification. In simple terms, they are choosing to in essence put that burden on Vans by referring to their published info on the subject, but not liking what Van's published.

If you choose to modify but do not have they knowledge, skills and/or finances to validate your chosen modification(s) I don't think you should disparage others for taking a position. Regardless what the reason for their position is.
So if you don't like what Vans has published, why not trust Sky Designs published GVT report? Particularly since I assume most that purchased the extended fuel kit, did so because they did not have the knowledge, skills, and/or finances to get it done on their own, but they were willing to trust Kens design. If that is true, then I would think you should be willing to trust his validation testing.

If you are going to modify... Own your decision.



SD is the one that has something to gain by validating the design so it is no surprise that effort has been made to do so, and that is what any good designer should do. I might be wrong but I don't think any of the advanced testing was done until way after the kit was made available, and someone provided a financially feasible means of doing it. Van's has nothing to gain by doing so other than that some may argue that they would sell more kits because of people that would buy a kit if it "just had a little more fuel capacity". Probably a very small number of people.

As for the feeling of it being retaliatory against Sky Designs, I have not heard a single thing that would make me think that, but if it was true, particularly if it has had an impact on kit sales, who could blame them?
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I guess my problem with this comes down to this:

Before this notice, Van's position is they have not tested this modification and disclaim any liability on this, as they should. It is up to the builder to be the owner of the modification and make a decision. Skydesigns has tested the design and I've reviewed the report and I'm satisfied with my decision to install ER tanks and the analysis passes the smell test to me.

Now, Van's has come out and published hard numbers on VNE reduction. I don't claim to be infallible. While I still think the testing Skydesigns has done is sufficient, I am left wondering what have I missed? What does Vans know that I missed in my analysis? Since Vans has not published any reasoning for this or even indicated that ANY engineering analysis was done on this, I'm just left in limbo second guessing myself. If a reasoning is provided, something even as simple as "we're doing this to be conservative", then I can make a decision based on my personal risk tolerance and knowledge. As an aerospace engineer myself, and as many others upthread have pointed out, the stated reason in the notice about flutter margin makes no sense without accounting for location.
 
It makes you wonder why Vans decided to spend the engineering time (and thus funds) on this at this particular time. There must have been a reason for concern, concern enough to deviate from the normal course of advising against modifications with an extra warning.
 
rvbuilder2002 said:
As for the feeling of it being retaliatory against Sky Designs, I have not heard a single thing that would make me think that, but if it was true, particularly if it has had an impact on kit sales, who could blame them?
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The pilots and builders could blame them. They put out a very specific restriction, which i still believe is fiction with no engineering basis, that pilots must now deal with. While it may be legal to ignore it, it is still a sticky wicket to be dealt with. The manufacturer of the kit is generally considered the authority on its design engineering and setting operating limits, therefore anyone who ignores it without solid engineering proving it wrong, runs the risk that someone like the faa or an attorney claims it is negligent to ignore. This was not guidance or a recommendation. It was a safety notice from the design authority stating, not speculating, or wondering, that it is UNSAFE without a vne restriction. Words and tones matter in these type of things.

An example - there is an sb for checking and removing slosh from tanks. If a pilot ignores this and crashes due to fuel exhaustion with fuel in those tanks, both the faa and the insurance company are going to think hmmm, he was told about the problem and ignored it, how or if do we want to penalize him for that negligence? And there is no doubt that is negligent behavior. Without supporting engineering work to counter these things, i believe there is some liability for just ignoring it. VERY different when the vans doc says we haven’t tested these things and therefore have no idea if performance will change or how much. There is not much negligence in ignoring that.

Sorry, but this was not the typical “you’re on your own with this mod.” Instead it was a blanket statement condemning the product as unsafe. This is especially irritating because they likely just guessed and have no data supporting the condemnation.

There is some saving grace for the er tank crowd, as kens test data is solid evidence proving vans was wrong (vans apparently has no data to contradict this) or at a minimum takes negligence off the table, as the mod has plenty of evidence showing its safety. And this is why this is all so weird and spiteful. The one time that vans takes this kind of stand, they have no data supporting their claim while the mod maker has extensive, real data.
 
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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.
 
I would have loved to own my decision ! I spend 6 months building my own RV-10 ER tanks. I started documenting the process 11 years ago and found, at that time, that 14 RV-10 were flying with a 90 to 120 gallons capacity. As far as I know none of them have lost a wing until now. As mentioned also in the “Safety” Notice : No flutter events have been reported with aircraft incorporating aftermarket extended-range tanks.

Not even a single little event to base this SN on…

The way Van’s dealt with this “issue” matters a lot. To publish such an extreme limitation based on not much and hide behind the fact that it is not mandatory to comply is not the proper way to increase safety in my opinion.

I really hope Van’s will come up with a revised SN and gives us proper way to mitigate the risks we happily accept and not this generic cover the worst-case limitation.

I only intend peaceful cruising at 160 kts, respecting gross weight.
 
Given the level of testing that was done on an RV-10 wing during the LCP fiasco I can’t imagine it was too much of a stretch to run the computer models on this and come up with the limitations they have.
Just because they aren’t sharing doesn’t mean they don’t have evidence.
Just my 2 cents. I don’t have ER tanks, my bladder won’t take the strain.
 
Jim Frisbie said:
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.
Click to expand...
How does Vne affect spin characteristics?
 
Jonnyb said:
Given the level of testing that was done on an RV-10 wing during the LCP fiasco I can’t imagine it was too much of a stretch to run the computer models on this and come up with the limitations they have.
Just because they aren’t sharing doesn’t mean they don’t have evidence.
Just my 2 cents. I don’t have ER tanks, my bladder won’t take the strain.
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Why would they? LCP is chiefly fatigue analysis for crack initiation and growth. This is flutter. Structural vs Aerodynamic. Completely different calculations and analysis.

Unless you're saying the ER tanks cause cracks and that's why they imposed....VNE limitations? Instead of imposing a service life on the spar?
 
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.
 
cactusman said:
Yes that would be interesting considering the EAA Grand Prize winner was an RV-10ER at OSH.

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Response from my neighbor with the RV-10 you are referring to -

Cactusman,

I’m struggling to understand your question regarding a DAR’s willingness to sign off an airplane and specifically what winning Grand Champion has to do with this particular issue.

A couple of clarifications. 1. My DAR was extremely thorough (and the FAA was observing). The airplane had standard tanks at the time of the AW issuance. The ER tanks were retrofitted later during Phase I. I was not trying to hide the ER tanks from the DAR, but the kit wasn’t available at the time of the inspection. This same DAR has signed off RV-10’s and -14’s since then with the SD tanks although this latest SN did not exist at the time.

There is a float equipped RV, a twin engine RV and a turboprop RV-10 flying. Obviously some DAR or the FAA signed these off. How about the RV-10 with a 330hp IO-540? Should the DAR have refused to sign this off?

In the specific case of the Sky Designs tanks, there is a thorough analysis provided by SD indicating that there is no reduction in Vne with full fuel. Given the pedigree of the engineer responsible for the design of these tanks, I have absolutely no concern about using them. If I ever do, I can either reduce the fuel load and/or reduce the Vne.

And for further clarification- we won Grand Champion at SNF 2025, not OSH. We are still thrilled with the honor. It took a village and Sky Designs was a part of that.
 
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280FX said:
Response from my neighbor with the RV-10 you are referring to -

Cactusman,

I’m struggling to understand your question regarding a DAR’s willingness to sign off an airplane and specifically what winning Grand Champion has to do with this particular issue.

A couple of clarifications. 1. My DAR was extremely thorough (and the FAA was observing). The airplane had standard tanks at the time of the AW issuance. The ER tanks were retrofitted later during Phase I. I was not trying to hide the ER tanks from the DAR, but the kit wasn’t available at the time of the inspection. This same DAR has signed off RV-10’s and -14’s since then with the SD tanks although this latest SN did not exist at the time.

There is a float equipped RV, a twin engine RV and a turboprop RV-10 flying. Obviously some DAR or the FAA signed these off. How about the RV-10 with a 330hp IO-540? Should the DAR have refused to sign this off?

In the specific case of the Sky Designs tanks, there is a thorough analysis provided by SD indicating that there is no reduction in Vne with full fuel. Given the pedigree of the engineer responsible for the design of these tanks, I have absolutely no concern about using them. If I ever do, I can either reduce the fuel load and/or reduce the Vne.

And for further clarification- we won Grand Champion at SNF 2025, not OSH. We are still thrilled with the honor. It took a village and Sky Designs was a part of that.
Click to expand...


My concerns are that we have competing analysis's and I'm a dumb pilot presented with a quandary. I am struggling to understand why some can't see the problem this presents - both from a safety standpoint as well as from a insurance and regulatory standpoint. Everyone wants to pooh pooh it...no big deal...what is all the fuss about.....but frankly after BK, waiting on laser cut parts, 4 years from a non responsive Lycoming I am about ready to pack it all in. Frustrated beyond all belief right now.
 
cactusman said:
My concerns are that we have competing analysis's and I'm a dumb pilot presented with a quandary. I am struggling to understand why some can't see the problem this presents - both from a safety standpoint as well as from an insurance and regulatory standpoint. Everyone wants to pooh pooh it...no big deal...what is all the fuss about.....but frankly after BK, waiting on laser cut parts, 4 years from a non responsive Lycoming I am about ready to pack it all in. Frustrated beyond all belief right now.
Click to expand...
Direct message sent. Call if you want to.
 
Liability? Lawyer Speak? Serious Unsafe Condition? or just a poor business decision to publish this SN? It seems to me the effect of this SN will only serve to reduce the value and resale potential on a very large percentage of the Vans RV10 Fleet. And I own a wonderful "RV10ER" with NO intent to Sell, but this once again hurts my opinion of the Vans Aircraft Administration.
 
Jim Frisbie said:
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.
Click to expand...
Jim,

This is not an attack on you, but if that’s the case, why does the SB not mention spin characteristics at all? The only thing the SB mentions is flutter. If true, this lends even more doubt to the methodology of this Safety Bulletin. And as another poster pointed out, how does slowing down help prevent a spin in the first place?

Too many holes in this process so far…
 
Rick RV-4 said:
Jim,

This is not an attack on you, but if that’s the case, why does the SB not mention spin characteristics at all? The only thing the SB mentions is flutter. If true, this lends even more doubt to the methodology of this Safety Bulletin. And as another poster pointed out, how does slowing down help prevent a spin in the first place?

Too many holes in this process so far…
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+1

you don't improve spin resistance or spin recovery by decreasing Vne.
 
The problem I have with this is that Vans published a very specific number to derate the wing without any data to back it up. Is it their right to do so? Yes, but it stinks of politics in this particular scenario.

The stated reason for the change is this - "Mass, including fuel centered away from the wing's elastic axis, will decrease the flutter margin. This will lower the speed at which flutter will occur."

But two paragraphs later, in the only justification for the change, they specifically say it doesn't matter where the extra mass is - "For both the RV-10 and RV-14/14A, to maintain tested flutter margins, Vne must be reduced by 2.8kts for each additional gallon of fuel (regardless of the location of the extended range fuel tank within the wing)."

So the extra mass either matters, or it doesn't - but this is written both ways.
 
airguy said:
The problem I have with this is that Vans published a very specific number to derate the wing without any data to back it up. Is it their right to do so? Yes, but it stinks of politics in this particular scenario.

The stated reason for the change is this - "Mass, including fuel centered away from the wing's elastic axis, will decrease the flutter margin. This will lower the speed at which flutter will occur."

But two paragraphs later, in the only justification for the change, they specifically say it doesn't matter where the extra mass is - "For both the RV-10 and RV-14/14A, to maintain tested flutter margins, Vne must be reduced by 2.8kts for each additional gallon of fuel (regardless of the location of the extended range fuel tank within the wing)."

So the extra mass either matters, or it doesn't - but this is written both ways.
Click to expand...
I'm not an engineer but here's what I think. I'd guess the "elastic axis" is on the spar. So mass either in front of, or behind the spar will change the speed that flutter can happen. Is 2.8kts per gallon appropriate for the current common methods (sky designs) of adding fuel capacity? I don't thinks so given the amount of real world testing by owners that has been done.
 
airguy said:
"For both the RV-10 and RV-14/14A, to maintain tested flutter margins, Vne must be reduced by 2.8kts for each additional gallon of fuel (regardless of the location of the extended range fuel tank within the wing)."
Click to expand...

Am I correct in thinking that the max reduction is about 31 kts for the -10 and 25kts for the 14? It seems like a) the plane probably won't cruise that fast anyways, and b) after a climb to cruising altitude some of the added fuel is gone and the limitation is reduced. Once you burn off 18-23 gallons (evenly L/R) the limitation is gone completely?
 
skirting_virga said:
Am I correct in thinking that the max reduction is about... 25kts for the 14?
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It depends on which ER tanks we're talking about and Vans doesn't mention that. Could be tip tanks, Sky Designs tanks, baggage tanks, etc.

The Sky Design tanks for the RV-14 are 68 gallons vs 50 gallons stock. At 2.8 knots per gallon, that's a 50 knot reduction in Vne for full tanks.
 
skirting_virga said:
Am I correct in thinking that the max reduction is about 31 kts for the -10 and 25kts for the 14? It seems like a) the plane probably won't cruise that fast anyways, and b) after a climb to cruising altitude some of the added fuel is gone and the limitation is reduced. Once you burn off 18-23 gallons (evenly L/R) the limitation is gone completely?
Click to expand...
Perhaps we are thinking about this the wrong way. Van's has through their means of compliance, just approved using larger tanks with fuel placed anywhere in the wing so long as we adjust our VNEs. (yes I know that is not the intent)
 
bertschb said:
It depends on which ER tanks we're talking about and Vans doesn't mention that. Could be tip tanks, Sky Designs tanks, baggage tanks, etc.

The Sky Design tanks for the RV-14 are 68 gallons vs 50 gallons stock. At 2.8 knots per gallon, that's a 50 knot reduction in Vne for full tanks.
Click to expand...
Wouldn't the reduction be for half that amount? 25 vs 34 gallons per side. I seriously doubt the right wing flutter margin decreases because of fuel in the left tank...
 
I've got 5 gals in each tip, won't spin or land with fuel in there, but would there not be a balance, in flight, that relieves bending stress and makes the wing stronger? Sure works that way in other aircraft, jets on down to-

"Tip Tanks by D'Shannon
D'Shannon Aviation is synonymous with Beech Bonanza tip tanks for a reason: For over 50 years, D'Shannon has done tip tanks best. D'Shannon Aviation auxiliary tip tanks give you the added range and safety of 40 additional gallons of fuel for Bonanza or Debonair. The advantages of :

D'Shannon tip tanks include:
Gross Weight Increase approved for all models.
Extended range for fewer stops and quicker block-to-block trip times.
Ability to "tanker" fuel to take advantage of low-cost fuel and bypass expensive fuel stops.
Over two hours of additional fuel for IFR flexibility and reserves.
Enhanced stability, lower stall speed and increased aileron authority for safer operation and smoother ride. Tip tanks enhance overall stability, improve spin characteristics and lower landing and stalling speeds at the aircraft’s original gross weight. Or take advantage of the FAA approved gross weight increase, which you can in either added fuel or cabin weight."
 
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rvbuilder2002 said:
But now it seems a lot of people think that a kit manufacture should own responsibility for any modification that they or others design and/or choose to incorporate. 🤨

This publication is just Van’s way of emphasizing to you that choose to modify, that you need to own responsibility for modifications that you choose to make.
Click to expand...

It's perfectly reasonable for Vans to say "We know nothing about these modifications, we haven't evaluated them, we can't give any information on whether they are safe."

But this seems like the opposite - by issuing a SB they are taking responsibility for the safety of the modifications. By implication, Vans has said they are OK if you observe the VNE limits. I don't think they intended to do that. (They have also removed a lot of liability from the producers of the tanks - in case of problems the vendor can legitimately point to the Vans SB and ask whether it was complied with.)

Far better to stick to the original line where they give no information and put all the onus on the builder or seller of the modifications. I somehow doubt this was done on legal advice.
 
Jjackh10 said:
I'm not an engineer but here's what I think. I'd guess the "elastic axis" is on the spar. So mass either in front of, or behind the spar will change the speed that flutter can happen. Is 2.8kts per gallon appropriate for the current common methods (sky designs) of adding fuel capacity? I don't thinks so given the amount of real world testing by owners that has been done.
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Read SD's analysis. The elastic axis isn't on the spar.
 
I think this SB may be a bit misunderstood, honestly. I get the controversy and the blanket recommendation from Vans, but I think people are mistakenly attributing the correction factor to total fuel and not fuel per tank/wing. It makes no sense, IMO, to assume that they are talking about extra fuel added to the plane and not extra fuel added to the individual wing. A right wing with 100% of the factory-intended fuel should have 100% of the factory-intended Vne. Once you go above 100%, the flutter margin reduces proportional to the extra in that wing. Now, a debate around whether the decrease is linear, or whether it is dependent on location in the wing, seems valid to me.

RV14: 25 gal -> 34 gal = +9 gal
9 gal * -2.8 kts/gal = -25.2 kts
200kts - 25.2 kts = 174.8 kts


RV10: 30 gal -> 41.5 gal = +11.5 gal
11.5 gal * -2.8 kts/gal = -32.2 kts
200kts - 32.2 kts = 167.8kts


in the case of the RV-14, that is ~3.5 kts slower than the factory-demonstrator 75% power cruise speed at 8k feet with the IO-390 EXP 119

in the case of the RV-10, that is ~7 kts slower than the factory-demonstrator 75% power cruise speed at 8k feet with 260hp


In both of these cases, that assumes 100% full tanks and no fuel burn for startup, taxi, takeoff, or climb. I figure in both cases this is a couple gallons minimum. For the RV-14 you would need to burn off 1.25 gallons per side, 2.5 gal total to make Vc (75% @ 8k feet) = Vne. For the RV-10 you would need to burn 2.5 gallons per side, 5.0 gallons total to make Vc (75% @ 8k feet) = Vne.

Obviously your specific plane may vary. Some have more power or less drag than the factory demonstrator. Some people fly higher at slower TAS. Some fly lower at higher power settings and higher TAS. Some people are going to be upset that they can't blast through a full-power descent after 30min of cruise starting with full tanks.

IMO, this restriction, while arbitrary and seemingly overly-broad, has minimal real-world effect on the limitations of the plane. The amount of additional fuel is 2h tops (being generous) with the commonly installed powerplants on the 10/14, and you can't get close to Vne in cruise anyways.
 
skirting_virga said:
…Once you go above 100%, the flutter margin reduces proportional to the extra in that wing. Now, a debate around whether the decrease is linear, or whether it is dependent on location in the wing, seems valid to me.
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And where does this assumption come from?

You seem to be jumping to the same conclusions that vans did, also without any testing or strong evidence from credible scientific knowledge. I am not an aero engineer, but strongly doubt there is a guiding principle stating that increased fuel in a wing reduces flutter margins. There is simply no way it can be that simple. If it was, guys like you and i would be designing wing structures; just read the book and follow the instructions.

If we took this literally, those of us with std tanks could start increasing our vne as we burn off fuel. if adding fuel weight reduces flutter margins , then reducing fuel weight must increase flutter margin. Can’t have it both ways. Just an example of how this kind of stuff is not simple and why none of it means anything without testing or at least using accepted design principles.
 
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lr172 said:
And where does this assumption come from?

You seem to be jumping to the same conclusions that vans did, also without any testing or strong evidence from credible scientific knowledge. I am not an aero engineer, but strongly doubt there is a guiding principle stating that increased fuel in a wing reduces flutter margins. There is simply no way it can be that simple. If it was, guys like you and i would be designing wing structures.
Click to expand...

It’s actually worse than that because it would seem that extensive research would suggest that adding mass ahead of the elastic axis is actually stabilizing wrt wing flutter.
So is there something unique to the RV design that the factory knows and the rest of us test pilots don’t? Or is it actually that Ken’s GVT and assumptions regarding the ER tank mass distribution are on the money?
I suspect I know which side most with an ER tank are falling but it’s an unseemly position for everyone to be placed in given the history of the designers and the ubiquitous nature and long service history of ERs of all shapes and sizes.
 
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lr172 said:
And where does this assumption come from?
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I'm just taking Vans analysis at face value, not attempting to establish some ground truth about aeroelasticity. If their analysis has any merit and didn't get, say, a 2x reduction factor applied because lawyers said so, you might have to cruise a couple knots slower for a few minutes until some fuel burns off.

Richard Connell said:
It’s actually worse than that because it would seem that extensive research would suggest that adding mass ahead of the elastic axis is actually stabilizing wrt wing flutter.
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If true, this is fascinating.


I'm not trying to increase the controversy here, just playing what-if assuming their analysis is exact and you want to operate with an abundance of caution. I think the limitations, even if they are overstated, have minimal practical effect on the use of the plane.


And I still want ER tanks on my 8. And it's a bummer that nobody offers them.
 
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Richard Connell said:
It’s actually worse than that because it would seem that extensive research would suggest that adding mass ahead of the elastic axis is actually stabilizing wrt wing flutter.
Click to expand...


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'
 
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