28 more gallons of fuel on an RV-9A

Palamedes said:

My concern is with how it might add an untested load to the wing spars..

Vans themselves say don't do this.. (or that's what they said on the 10 anyway)..

How do you know its safe? This is math that is beyond me =)

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Years ago when I often ferried Cessna Agwagons, we often got to talking with their engineers. We were told that the Cessna 310 with its main tanks in the tips allowed them lighter spars because the weight out there actually lessened the spar loads. The added weight is not on the center section and causes less bending force.

In this particular case, the added 168 lbs would mean that you A) Either have that much less useful load, or
B) Raise the weight on the paperwork to include that extra weight. There is another caveat..spins. The Grumman American was/is placarded against spins because the fuel would be slung out toward the wingtips (In its tubular spar/fuel tanks), and spin recovery may not be possible, because of the outboard moment of inertia..kind like spinning a barbell from its center with the weights outboard.


Best,

Tip Tanks

I realize we are not talking about tip tanks per se, but these are very similar. The Cessna 310 production started around 1954, there were two different shape tanks used over the years. All the 310 tanks hold 50 gallons per side. The 310 wings bend visibly in turbulence and when taxiing over the slightest bump. The structure supporting the tip tanks on the 310 is very fragile looking.
Add on tip tanks have been in use for many years on Bonanzas, Commanche's, Twin Commanche's, Navion's etc., without issues. In the case of the Commanche the fuel gets a free ride. A special gross weight increase only for the weight of the tip tank fuel. The Commanche wing tips do not visibly bend nearly as much as the Cessna, of course the Commanche carries much less fuel in the tips.
It is generally accepted based on over 60 years of experience that the full tip tanks reduce the wing bending loads in turbulence. The VISIBLE bending on the 310 is rather meaningless, it just looks scary.

pierre smith said:

There is another caveat..spins.

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I agree.

This would be my biggest concern with this type of modification.

The RV-9A went through an extensive spin test program with a well regarded and well know professional test pilot.

With fuel located out near the wing tips, the results of all of that testing is null and void.....

rvbuilder2002 said:

The RV-9A went through an extensive spin test program with a well regarded and well know professional test pilot.

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Is there a documented test report you can share?

krw5927 said:

Is there a documented test report you can share?

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The only thing to share, is what has always been shared....
That the results of the testing were very favorable, and that if built per the plans and operated within the recommended weight and C.G. perimeters, the The RV-9(A) will have good spin recovery performance using traditional spin recovery control inputs.

It is true that making a change to the wing tip could have an effect on stability (which could in turn have an influence on spin recovery) but the primary focus (of my comment anyway) was on the tips containing fuel (and the resultant mass that goes along with that).
It is the mass of the fuel, and the resulting change in the wings moment of inertia, that would likely have the most influence on spin recovery.

Any modification to an RV wing tip that also adds span could potentially be a problem also. If this additional wing area is producing lift that is farther away from the fuselage than occurred with the unmodified wing, then the bending moment at the attach point of the wing has been increased...... longer lever arm equals higher moment with the same amount of force.

I used to have a Cherokee 235. If you want factory built, that's the way to go. Anyway it has standard 84 gallons fuel tanks. 25 in each main, and 17 gallon tip tanks.
I too always worried about so much weight on the tips especially landing. I only filled them on a trip that would burn out that weight before my scheduled landing. On short trips I only filled the mains.
To my common sense mind, I am certainly not an engineer, I always burned out the tips first, after taking off on the mains and switching at a safe altitude because of that weight.
Upon reading my 235 owners handbook it says, paraphrase: to take off on a main tank, burn the tank for an hour, switch to the other main and burn till nearly exhausted, then back to the original main until it is exhausted, then begin to alternate burning the tips. This is for lateral stability.
It would also have you landing as a tip tank being your fullest tank.

I understand that an RV is not a Piper but after reading pierre smith, and jrs14855, I now kind of understand the engineering behind those directions.
I suppose the weight out there does not effect the spar the way it seems to a non engineer mind.

the setup on and RV is just going to be a one way feed from the tip draining into the main, so that's going to be emptied first regardless.

Still, if you want to spin you should probably not fill the tips.

awesome awesome

AAAAAA +++++++ Thank u. Vary much. I really appreciate people like u doing out of the box thinking then actually doing it .

I talked to over 20 owners at eaa including vans they told me it can not be done then laughed at me.

I explained to them I am a student piolt and we were doing some investigating on what makes the rv which is a amazing plane and my plane of choice witch I am putting the finish touches on mine as we speak !!! So awesome HAS ANY ONE ELES DONE ANY OTHER CAN NOT DO MODS !!!!! I WOULD LOVE TO KNOW ABOUT !!!! TNKS TO ALL OF U ON THIS GROUP ps some one told me there was a wing mode.cut 45deg

awesome awesome

AAAAAA +++++++ Thank u. Vary much. I really appreciate people like u doing out of the box thinking then actually doing it .

I talked to over 20 owners at eaa including vans they told me it can not be done then laughed at me.

I explained to them I am a student piolt and we were doing some investigating on what makes the rv which is a amazing plane and my plane of choice witch I am putting the finish touches on mine as we speak !!! So awesome HAS ANY ONE ELES DONE ANY OTHER CAN NOT DO MODS !!!!! I WOULD LOVE TO KNOW ABOUT !!!! TNKS TO ALL OF U ON THIS group

The plane pictured in post #56 of this thread is not used for storage or fuel. I know Jim who owns this plane, who worked with Paul Lipps on the design. If I remember correctly, the tip helps keep the nose lower at altitude by moving the lift at the tip aft.



All of Paul's designs are way above my head, all I do is follow the manual and build what Van has designed.

Note: The three bladed prop is also a Paul Lipps design.

Truthinaction said:

AAAAAA +++++++ Thank u. Vary much. I really appreciate people like u doing out of the box thinking then actually doing it .

I talked to over 20 owners at eaa including vans they told me it can not be done then laughed at me.

I explained to them I am a student piolt and we were doing some investigating on what makes the rv which is a amazing plane and my plane of choice witch I am putting the finish touches on mine as we speak !!! So awesome HAS ANY ONE ELES DONE ANY OTHER CAN NOT DO MODS !!!!! I WOULD LOVE TO KNOW ABOUT !!!! TNKS TO ALL OF U ON THIS GROUP ps some one told me there was a wing mode.cut 45deg

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So, if I put an 800 HP turboprop engine on the front of my RV-9A and it results in a way out of spec C.G. position and breaks all kinds of standard engineering practices, but I successfully fly it without hurting my self or anyone else..... does that mean it is a good idea?

Experimenting is good. But if you are going to experiment you should get educated on all of the engineering disciplines that could come into play with what you are designing / experimenting with.

A significant one has been pointed out in this thread. No comments provided whether it was considered or whether testing is being done to verify it is an issue.....
If that is the case, that is not the type of experimenting this peer group should be encouraging IMHO.

Wingtip

curtis said:

The plane pictured in post #56 of this thread is not used for storage or fuel. I know Jim who owns this plane, who worked with Paul Lipps on the design. If I remember correctly, the tip helps keep the nose lower at altitude by moving the lift at the tip aft. All of Paul's designs are way above my head, all I do is follow the manual and build what Van has designed.

Note: The three bladed prop is also a Paul Lipps design.

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The late Paul Lipps did a lot of great design work, and I don't wish to take away from that, only to set the record straight.
The tapered wingtips as seen on the green RV6 were designed by Steve Wittman in the mid 70's. The tips as designed and tested by Wittman resulted in a substantial reduction of the power off sink rate at speeds in the approach speed range. High altitude cruise performance is also improved because of reduced loss due to wingtip vortices and reduced span loading.
Boeing also tried to take credit for this wingtip design but ultimately abandoned it in favor of winglets.

Not saying Paul was the initial designer, just did the work for making them work on an RV. In reality most all wing designs are old and someone has already done something with them, unless you are talking Scaled Composites which is always breaking new grounds with their designs.