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RV-12 Wing Tanks

joedallas

Well Known Member
RV 12 Wing Tanks
The RV12 is a great little airplane and removing the wings is a nice feature for some.
I believe that the number of people removing the wings more than once or twice a year is less 20%.

I also believe that if a wing tank option would be available from vans that they would sell more than 50 % with wing tanks.

There is no question that Vans could make a wing tank option better than what I came up with.

I have been working on this for over two years and have changed my mind more than a few times.

This is what I think is my final design. ( this is not for anyone to try )

http://joesrv12.com/Builder Log/al_06_04.htm

If anyone can see anything I have missed your comments are welcome

Thanks Joe Dallas
 
Looking good Joe! I of course went with the tubular wing tanks, not necessarily the BEST solution, but regardless it is ONE solution. I just did not feel I had the expertise or time to design and build tanks like yours.
 
Nice Work

Joe,

I do not have the expertise to comment on your design, but I recognize excellent craftsmanship when I see it!
 
Just curious: Is there a reason you chose to go with a discrete tank mounted within the wing structure, rather than making a wet wing tank like the other RV's have? The quality of your work in those photos suggests to me that you could probably have done that with equal difficulty. Just wondering if there is a CG or other structural consideration.
 
Good Question

Good Question

My first plan was to do that however after looking at the way Vans built the
wings with the spars in a scissor configuration and the thin skins, I felt it was
a little more than my ability could handle.

Also I wanted the fuel behind the spars to help with the W&B problem that I will have with using a Jabiru 3310 ( I Hope ).

With The fuel at the back of the spars the W&B will be almost the same with the tanks full or empty.

Like I said in my post There is no question that Vans could make a wing tank option better than what I came up with.

The RV12 Wing was not designed with Fuel Tanks in mind as all the other RV's were ( Vans could fix that ) it's more than I can handle.

Thanks for your question

Joe Dallas




Just curious: Is there a reason you chose to go with a discrete tank mounted within the wing structure, rather than making a wet wing tank like the other RV's have? The quality of your work in those photos suggests to me that you could probably have done that with equal difficulty. Just wondering if there is a CG or other structural consideration.
 
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Fuel in the wing would be very attractive for me as a buyer.
It is a project Van's could do much more easily and faster than most other projects they may contemplate.
It would significantly enhance the appeal of SLSA RV-12 to non builder buyers.
There is a lot of likely return for the invested development buck, probably much more than any other project they may contemplate.
 
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A better RV 12

I totally agree
This is the reason I built the wings last, I was hoping - waiting that Vans would see the light.
If Vans offered a wing kit with Fuel Tanks, I think some builders would sell their wings and build the new wings.
The RV12 is a great it would be better if it had wing tanks

MY View

Joe Dallas




Fuel in the wing would be very attractive for me as a buyer.
It is a project Van's could do much more easily and faster than most other projects they may contemplate.
It would significantly enjance the appeal of SLSA RV-12 to non builder buyers.
There is a lot of likely return for the invested development buck, probably much more than any other project they may contemplate.
 
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I totally agree
This is the reason I built the wings last, I was hoping - waiting that Vans would see the light.
If Vans offered a wing kit with Fuel Tanks, I think some builders would sell their wings and build the new wings.
The RV12 is a great it would be better if it had wing tanks

MY View

Joe Dallas

Joe, FWIW (probably not much) my view is that the RV-12 would be better if it had a bigger fuel tank rather than wing tanks per se. I know that like all aircraft the RV-12 is a compromise, and I'm very happy with mine. I don't particularly care whether the tank is in the fuselage or the wings, but having recently completed a 6,500 km trip, the 20 gallon tank is a limitation for long trips in a country as big as ours. It's manageable, but a 30 gal tank would be much better. What I don't really understand is why go to the trouble of designing wing tanks if they don't carry a lot more more fuel than the fuselage tank? The tubular tanks that fit in the wing rib cutouts seem to me to be even worse due to their adverse weight distribution as well as their limited capacity. As far as the design of your wing tanks goes, I can't add anything to what has previously been said about the loss of stiffness and strength in the affected wing ribs, which I agree with. Your workmanship is not in question, and I admire your determination to go your own way, but what advantage are you gaining with these wing tanks?
 
I also have felt that 20 gal of fuel was a bit on the short side for our Rotax powered birds. Not that I would ever consider flying a longer leg than 20 gals would support, but the lack of the availability of a lead free fuel while traveling is an issue. I would prefer not to deal with issues that using 100LL creates. However, as soon as 100LL is replaced with a lead free aviation fuel, I think that our 20 gal tanks will be more than adequate, and I personally can't wait!!

We are still probably about 2 years away from seeing the new fuel at our local FBOs............Tom
 
Please remember, Guys, the primary purpose of light sport aircraft is for the weekend flyer. They are not specifically designed for long cross countries.
Everything is a compromise.
 
Point taken Mel, and in spite of being primarily a weekend flier, the RV-12 does a good job of cross-country too. More fuel would be nice, but a 20 gal tank is OK most of the time.
 
To Answer your question

To Answer your question - what advantage are you gaining with these wing tanks?

First of all I enjoy design and building and working in my shop more than I do flying.
Spending 10 doing something that everyone else can do in 5 hours gives me 5 more ours in the shop.
Also anyone can build or buy a aircraft that someone else has designed.

Having something different and maybe a cut above helps my individuality.

1. The Tanks now hold about 23 gallons ( 20 all usable ) with 3 gallons of reserve.
2. The baggage compartment will now hold larger items.
3 The baggage compartment will now hold 100lbs.
4 The chance of having a fire in the cabin in a hard landing is less
5 The inertia of the tanks and fuel may make the ride a little less bumpy
6. I believe that my wings are now stronger than they were.
7. All other RV's Have wing tanks
8 . And I wanted to.

My view

Joe Dallas



Joe, FWIW (probably not much) my view is that the RV-12 would be better if it had a bigger fuel tank rather than wing tanks per se. I know that like all aircraft the RV-12 is a compromise, and I'm very happy with mine. I don't particularly care whether the tank is in the fuselage or the wings, but having recently completed a 6,500 km trip, the 20 gallon tank is a limitation for long trips in a country as big as ours. It's manageable, but a 30 gal tank would be much better. What I don't really understand is why go to the trouble of designing wing tanks if they don't carry a lot more more fuel than the fuselage tank? The tubular tanks that fit in the wing rib cutouts seem to me to be even worse due to their adverse weight distribution as well as their limited capacity. As far as the design of your wing tanks goes, I can't add anything to what has previously been said about the loss of stiffness and strength in the affected wing ribs, which I agree with. Your workmanship is not in question, and I admire your determination to go your own way, but what advantage are you gaining with these wing tanks?
 
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I have a 7 gallon aux tank to suit me. What many forget is that for some of us the 12 is the ONLY cross country machine we are allowed to fly! Since that is the FAA choice for me, then I intend to do as much cross country work as my body will allow. Sure I would be safer in a Cherokee or a Centurion, but that is not allowed and I cannot change the rules.
 
T
6. I believe that my wings are now stronger than they were.


Joe Dallas

Joe, I have several things for you to consider about your design. I am not trying to ?rain on your parade? here, but in my opinion what you are doing here is not a good idea without the ?blessing? of the designers who know the critical design loads and load paths in the airframe and I am reasonably sure they will not discuss this with you for reasons of liability. My opinion here is based on my many years of airframe structural analysis experience with the military. I am aware that there is at least two RV-12 variant aircraft now flying with fuel in the wings, but that does not make it a good design mod.

First the wing spar caps were never designed to take these increased bending loads. Remember the potential max 3g load factor has to be included on the increased mass you have put into the wing. On top of that the designer has to add a factor of 1.5 to assure no structural rupture at ultimate load. Say you are carrying 10 gallons in one tank. Do the math. That would be, just for the fuel, 10 x 6 lb/gal x 3 = 180 lbs for limit load and 180 lbs x 1.5 = 270 lbs for ultimate load. I imagine your spar caps are now undersized.

How about the added shear and bending moments you are now introducing into the attachment fittings for the wings into the fuselage. Again they could be now undersized.

How about the two shear webs (wing ribs) you have now dead ended into your fuel cell rather than into an intercostal (inboard to outboard) connecting the two wing ribs that bound your fuel cell. If any portion of the flight control loads flow through these stub ribs, where do these loads go now? You have broken the rib cap continuity.

The clips you show abutting the fuel cell show no attachment to the cell. Are they containment barriers or do they attach to the fuel cell? What holds this heavy mass, when filled, in place in the wing - the physical straps that appear to go over the tank tops in the assembled picture?

Wing skins are thin. Can they take the localized increased wing skin shear flow with the same rivet size and pitch of the original design for the life of the airframe?

I am surprised to see that you are still using the light weight ribs at the two vertical ends of the fuel cell. I would expect thicker gage webs and perhaps now a full T cap with no scalloping to take an increased shear and bending moment around the contained cell.

So far I have only discussed the bending stiffness of the wing, or the EI related design issues. There are also wing torsional stiffness concerns or GJ impacts which also need to be considered. You have probably changed the torsional stiffness of an inboard portion of both wings. How do you consider these impacts? I will not go into a discussion of these design issues.

In short, you and others that have or are modifying or adding structure that has significant primary structure and critical load path impacts without consideration of at least these basic design concerns are inviting serious degradation of airframe structural strength and airframe life. The airplane could fly safely for years without suffering a visible structural failure but you could, and I am speculating here, end up with internal (buried) fatigue cracks from repeated cycles beyond the natural design capabilities of the one-of-a-kind configuration you have built into the aircraft. Only if you understand the critical design loads, load paths, and run a detailed stress analysis will you really know what you have.

George
 
Please remember, Guys, the primary purpose of light sport aircraft is for the weekend flyer. They are not specifically designed for long cross countries.
I'm left scratching my head a little over that comment. Are you talking about LSA in general, or the RV-12 in particular? I'm curious as to what aspect of the RV-12 you would find unsuited for cross country travel. Sixty additional knots would be nice, but certainly not a requirement.

Dismissing light sport planes, and especially the RV-12, as a "weekend flyer" is all well and good when you're still getting your medical issued without hassles. That can change in a hurry, and I find that my requirements have not changed because of it.
 
I don't know about you guys, but my bladder doesn't last longer than about 15 of those 20 gallons in my tank!😜

For a three hour X-country my 12 is great. I don't know if I would like to fly three such legs in one day though. My Cherokee is more comfortable for all day flying.
 
I never said that the RV-12 or any other LSA is not suitable for cross country or that you cannot modify it. My comment was to explain why these aircraft are designed the way they are.
SLSA have a maximum empty weight based on hp vs useful load and ELSA kits must be based on that SLSA.
Yes LSA are very beneficial to us old farts that may not be able to, or choose not to take a class III medical, but that's was not the original purpose for the classification.
 
Things to consider

George
Thanks for your review
I have a structural engineering background in multi story building design, not aeronautical design.
The reason I made the statement (6. I believe that my wings are now stronger than they were. )
was to get someone to challenge me on my design before I close up my wings.
I read your post a few times to understand your concerns.
I believe we can agree that the math is easy, understanding the dynamic load path is more difficult in Aircraft, ( for me more than you )
The math I used for the load (10.5x6)+ 10 =63 X3 =189 x1.5= 283.5 lbs I used 280 in my math knowing that I will not see these loads after takeoff.
I assumed the load would be uniform on the spar between 12" to 47" from the outer wing pin.
The Spar no longer has to take this load from the fuel in the baggage compartment at a longer arm
I have tested the joints of the rib to spar in a test mockup and added some support at these locations.

At the center two ribs I added a row of extra rivets

The things that concern me is the long term affects on the skin rivets from repetive loads.

My annul will include a close watch on all rivets around the wing tanks

In military these aircraft take some real test of their design and come back with wings shot up so bad and still fly. My father was a aircraft mechanic on a aircraft carrier and the photos and stories are amazing
Thanks four review

I will take this as a chance for me to review my design before I close up my wings

Thanks Again

Joe Dallas




Joe, I have several things for you to consider about your design. .

George
 
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They are not specifically designed for long cross countries.

Uh, someone forgot to tell my LSA that. I've been to every state in the lower 48 except 5 (MN, ND, WA, NV, and MS). I think it's a great cross country machine!! Very capable and very comfortable.

On the fuel question, I don't even bother with auto fuel on cross country flying. Just use 100LL and some Decalin and forget about it. Most auto fuel at airports is too low an octane anyway to use without some booster and that's more harmful than 100LL IMO.

I looked closely at the fuel tank capacity issue and understand the engineering difficulties of changing location and/or changing capacity. I understand why we are where we are. I think Joe is on the right track all things considered.
 
Uh, someone forgot to tell my LSA that. I've been to every state in the lower 48 except 5 (MN, ND, WA, NV, and MS). I think it's a great cross country machine!! Very capable and very comfortable.
On the fuel question, I don't even bother with auto fuel on cross country flying. Just use 100LL and some Decalin and forget about it. Most auto fuel at airports is too low an octane anyway to use without some booster and that's more harmful than 100LL IMO.
I looked closely at the fuel tank capacity issue and understand the engineering difficulties of changing location and/or changing capacity. I understand why we are where we are. I think Joe is on the right track all things considered.

I said, and I repeat, They are not SPECIFICALLY designed for cross country.
 
Mark,

Flying solo in the 12 vs My Cherokee 180 shows about an 8 knot advantage for the Cherokee in cruise although it's probably closer to 5 knots if I push the 12 to 5500 RPM vs 5100.

The Cherokee is just a little more comfortable for long X-countries.

Rich
 
Wing and Header Tanks

Joe, I took a thorough look on your website. The wing tanks look good to me and I agree with your logic about spreading the wing load out.

Regarding the spar cap discussion - physically, the mass is the same to the landing gear, however, with the fuel spread out in the wings, the fuel load is carried fully by the wing spars in flight, which are pinned to each other (which is important for load transference) and on landing, from the spars to the spar box to the gear. There should be less stress on the spar box with the fuel spread out in the wings vice the baggage compartment, where the load is always transferred between the wings and gear via the spar box.

I'm guessing the airplane will ride a little smoother.

Your header tank looks great. I'm having a welded aluminum tank being made up by a nice guy with skills in that area. And then I'll be mounting my fuel pump and filters in the same bay on the opposite side connecting with an AN8 line.
 
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Remember the potential max 3g load factor has to be included on the increased mass you have put into the wing. On top of that the designer has to add a factor of 1.5 to assure no structural rupture at ultimate load. Say you are carrying 10 gallons in one tank. Do the math. That would be, just for the fuel, 10 x 6 lb/gal x 3 = 180 lbs for limit load and 180 lbs x 1.5 = 270 lbs for ultimate load.

Maybe I've misunderstood this, but don't the components in the wing that support the tank see a 4g design load? The LSA load factors are +4 (and -2), so as I understand it 3g effectively represents an additional load applied to the structure. Sitting on the ground the wing has to support a static 1g tank and fuel load of 60 lbs. Adding the additional 3g brings the effective weight up to 240lbs. Applying the 1.5 factor on top of that brings the ultimate load up to 360lbs. Or have I got that wrong?
 
In answer to questions and comments raised, I have this to say. Even though the wing is pinned to the fuselage, there is a direct increase in shear load across the pins due to there being added weight in the wing. These pins were designed to take a certain wing weight with lift under a specified G loading, as determined by the designer. Even though LSA specification may be as high as 4G I think, but am not sure, that this is a 3G airplane. The designer controls this by giving you an operational envelope. Let us go with my 3G ascertain here. That means those wing attach pins were sized for certain wing weight under a max 3G loading times a factor of safety of 1.5. The added weight in the wing is taken out as shear (the load supported by the spar web) and a bending moment (a “couple’ consisting of a tension load and compression load on the flanged spar caps) which is created by the reaction at the fuselage attach points. The caps of the spar were not sized to take an additional axial and compressive load caused by the moment generated by hanging a potential 180lbs lbs out X inches from the fuselage, period! This added couple at the fuselage increases the shear across the pin surface which means the shear margin of safety for these pins have to go down. Also the pins transfer load into the fuselage by bearing. Is the bearing surface now adequate? Also this new load has to be absorbed by the fuselage by added shear and probably moments into the surrounding structure of the fuselage. And remember you have “double the pleasure” here since you have this being applied to the pins from both wings. See, this is not a trivial design change.

One other thing to consider is the torsional impacts of this change. You have one spar component going into the fuselage. This new fuel bay, I would think, causes significant torsional effects on the wing. This has to play out in that single attachment to the fuselage. Again, this is another reason not to put fuel in these wings, or so it seems to me.
 
See, this is not a trivial design change. /QUOTE]

I totally agree. Here's a simple way of looking at it. I'm not sure what an RV-12 wing weighs, but two people can easily lift one, so let's say it's around 100 lbs, give or take. Adding 60 lbs of tank and fuel is a very significant increase in weight, irrespective of where it is placed in the wing. Personally, this is not a modification I would attempt without expert engineering analysis.
 
It Works Fine!

Joe,
I don't have a dog in this hunt. As you know, I completed my RV-12 in Texas and sold it to a gentleman in Iowa. I removed the Jabiru and delivered it (via U-Haul to Iowa). He installed a new Rotax and it is now flying actively up there.

Please tell everyone about our other Jabiru buddy who has fuel in his wings. And probably a very similar design. There's no telling how many hours he now has on that aircraft, and no problems that I've been made aware of?

I am NOT an engineer. But as always, your work looks flawless. And since you'll be strapping yourself into this aircraft on an active basis, I am certain you have calculated most everything imaginable to be certain it is within the safe flight envelope!? It has already been done safely, and tested by someone a few years ago, folks. Just my .02-cents. :)
 
Lets think about this wing weight question with fuel moved to the wings:

When the aircraft pulls 3 g's in flight, most all the lift is provided by the wings. The fuselage is dead weight at 3g load factor pulling down on the wing attach locations. (Actually worse than that due to large tail downloads.)

If you remove the 20 gallons (120 lb at 1 g) of fuel from the fuselage and place it in the wings, you have reduced loading at the wing attach points accordingly. The wing attach point flight loads due to this mod are not a concern.
...

There could be a flight load concern with wing internal structural design with this mod.
...

The bigger issue with this mod would occur during landing transients, when the wing weight is now increased substantially during a hard landing incident as the wings are transiently pulled downward relative to the fuselage. My suspicion, given that there have been many instances of modding aicraft wings with tip tanks, that this increase in load is not going to be enough to cause problems, however, a check by a structural expert would be desireable. Most certified aircraft have, seems to me, higher design margins than the somewhat flimsy design forced on one by the LSA certification limits.
...

I am all in favor of Van's coming out with a wing tank design for the RV-12. This seems like Aircraft Safety 101: Get the fuel tank out of the cockput.

-Paragon
Cincinnati, OH
 
Gary
Thanks for your comment
The reason I started the Wing Tank post was to see if missed anything after over two years of designing and testing.

I am a little slow, from me grandmother ( the oxen is slow but the earth is patient )
The comments I received just help me to see if I missed anything.
From my Grandfather ( listen and learn )

There is no advantage in me convincing anyone that they may be wrong
They have to convince me that I may be wrong for me to make a change.
I did receive some information that will cause me to add a doubler to the rear spas behind the tank.
71 years old and still learning.
The two ribs at the center of the tank do not have enough sheer strength, this load needs to go across the back spar to the reinforced ribs at the outer ends of the tank.
Thanks to David Paul’s constructive concerns, this is not an approval of my design by David Paul.
We need try to help each other, this is what makes the EAA great.
My View
Thanks again Gary and David






Joe,
I don't have a dog in this hunt. As you know, I completed my RV-12 in Texas and sold it to a gentleman in Iowa. I removed the Jabiru and delivered it (via U-Haul to Iowa). He installed a new Rotax and it is now flying actively up there.

Please tell everyone about our other Jabiru buddy who has fuel in his wings. And probably a very similar design. There's no telling how many hours he now has on that aircraft, and no problems that I've been made aware of?

I am NOT an engineer. But as always, your work looks flawless. And since you'll be strapping yourself into this aircraft on an active basis, I am certain you have calculated most everything imaginable to be certain it is within the safe flight envelope!? It has already been done safely, and tested by someone a few years ago, folks. Just my .02-cents. :)
 
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The landing is a concern

Paragon

I agree 100%
The landing is a concern, however most of the times the tanks will not be full.

I have the wing internal structural design handled

Joe Dallas


Lets think about this wing weight question with fuel moved to the wings:


When the aircraft pulls 3 g's in flight, most all the lift is provided by the wings. The fuselage is dead weight at 3g load factor pulling down on the wing attach locations. (Actually worse than that due to large tail downloads.)

If you remove the 20 gallons (120 lb at 1 g) of fuel from the fuselage and place it in the wings, you have reduced loading at the wing attach points accordingly. The wing attach point flight loads due to this mod are not a concern.
...

There could be a flight load concern with wing internal structural design with this mod.
...

The bigger issue with this mod would occur during landing transients, when the wing weight is now increased substantially during a hard landing incident as the wings are transiently pulled downward relative to the fuselage. My suspicion, given that there have been many instances of modding aicraft wings with tip tanks, that this increase in load is not going to be enough to cause problems, however, a check by a structural expert would be desireable. Most certified aircraft have, seems to me, higher design margins than the somewhat flimsy design forced on one by the LSA certification limits.
...

I am all in favor of Van's coming out with a wing tank design for the RV-12. This seems like Aircraft Safety 101: Get the fuel tank out of the cockput.

-Paragon
Cincinnati, OH
 
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