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08-11-2015, 01:21 PM
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Join Date: Jan 2005
Location: Louisville, Ga
Posts: 7,840
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Quote:
Originally Posted by Palamedes
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,
__________________
Pierre Smith
RV-10, 510 TT
RV6A (Sojourner) 180 HP, Catto 3 Bl (502Hrs), gone...and already missed
Air Tractor AT 502B PT 6-15 Sold
Air Tractor 402 PT-6-20 Sold
EAA Flight Advisor/CFI/Tech Counselor
Louisville, Ga
It's never skill or craftsmanship that completes airplanes, it's the will to do so,
Patrick Kenny, EAA 275132
Dues gladly paid!
Last edited by pierre smith : 08-11-2015 at 01:27 PM.
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08-11-2015, 01:40 PM
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Join Date: Apr 2008
Location: Lake Havasu City AZ
Posts: 2,393
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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.
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08-11-2015, 03:21 PM
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Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,035
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Quote:
Originally Posted by pierre smith
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.....
__________________
Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.
Scott McDaniels
Van's Aircraft Engineering Prototype Shop Manager
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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08-12-2015, 07:36 AM
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Join Date: Aug 2010
Location: Wichita, KS
Posts: 1,958
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Quote:
Originally Posted by rvbuilder2002
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?
__________________
Kurt W.
RV9A
FLYING!!!
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08-12-2015, 08:45 AM
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Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,035
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Quote:
Originally Posted by krw5927
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.
__________________
Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.
Scott McDaniels
Van's Aircraft Engineering Prototype Shop Manager
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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08-12-2015, 11:06 AM
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Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,035
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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.
__________________
Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.
Scott McDaniels
Van's Aircraft Engineering Prototype Shop Manager
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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08-14-2015, 10:54 AM
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Join Date: Jun 2015
Location: Alberta
Posts: 84
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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.
__________________
RV-6A slow build,
serial # 21681. First flight March 1993
Dec 2019 donation paid
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08-22-2015, 09:04 AM
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Join Date: Aug 2015
Location: Wisconsin
Posts: 3
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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
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08-22-2015, 09:24 AM
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Join Date: Aug 2015
Location: Wisconsin
Posts: 3
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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
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08-22-2015, 09:24 AM
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Join Date: Jan 2005
Location: Kansas
Posts: 384
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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.

Last edited by curtis : 08-22-2015 at 12:34 PM.
Reason: Added images
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