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  #21  
Old 01-11-2020, 09:08 PM
Greg Arehart's Avatar
Greg Arehart Greg Arehart is offline
 
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We've done a number of long trips in our 9, including to the Arctic ocean this summer (lots of space between airports) and never worried too much about the fuel capacity. Flying at 8-12k, 165 mph cruise we burn about 7.2 gph with an IO360. That translates to easily 4.5 hours plus a half hour reserve, or 4 hours plus an hour reserve. That can be extended by going a bit slower and/or higher altitude. Not too many places where you would come close to running out of fuel before wanting or needing to stop.
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RV-9B (Big tires) Tipup @AJZ or CYSQ
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  #22  
Old 01-11-2020, 11:07 PM
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rvbuilder2002 rvbuilder2002 is offline
 
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Location: Hubbard Oregon
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Quote:
Originally Posted by pa38112 View Post
You could also increase the gross weight of your plane. If I understand right, the Vans numbers are suggestions and the builder can set and flight test the gross. Since the added weight is in the wings, it does not add any addition load to the load bearing components.
Both are common misconceptions but both are untrue.

The published gross weights for all the RV models are the weights used to do all of the structural load calculations, design engineering, and static load tests.
Any deviation higher, though legal here in the U.S., is definitely upping the level of experimental of their aircraft.

It is correct that additional fuel weight added to the wings does not add additional bending load at the wing attach points (it can actually reduce it depending on the circumstances) but depending on where it is added, it will add additional load elsewhere.

For the purpose of simplification, lets say that we were able to add additional fuel to an RV-9A and all of the additional fuel was distributed span wise along the original standard fuel tank. When flying with full fuel the additional fuel weight when pulling G's may actually be reducing the bending load on the wing attach point beyond what it was without the additional fuel (the additional fuel would be inducing a downward load counteracting the additional upward bending load of the wing trying to carry the additional weight).

The problem is that the additional weight is being carried by the entire wing, not just the inboard portion where the fuel tank is. So the portion of the wing outboard of the fuel tank has some level of additional load but the additional mass of the fuel is not there to counteract the additional bending moment, so there is a increase in bending moment, with a specific concentration just outboard of the fuel tank.

This is why additional fuel is sometimes added to aircraft with tip tanks. The mas of the fuel is then keeping the bending moment along the span of the entire wing mostly unchanged.

This would imply that it is a simple thing to add more fuel capacity to any airplane as long as you use tip tanks. Unfortunately that is not the case.
Mass added way out at the end of the wing will have a high polar moment of inertia because of the long arm length relative to the center of gravity. The can have a major influence on flight / handling characteristics. The most serious one being spin recover.... worse case being it no longer having any, if the tip tanks are full of fuel.
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Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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  #23  
Old 01-12-2020, 05:05 AM
SVTPete83 SVTPete83 is offline
 
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Location: Napa, Ca
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I might get flamed for this. Flame suit on. My RV9a weighs in at a portly 1197 pounds with zero fuel. I have the stock 36 gallon tanks and glassed in wingtip tanks that give me an additional 9 gallons per side for a total of 54 gallons. The original builder flight tested it with an 1850 pound gross weight. We fly it at that weight all the time. Just the other day we flew with my wife, me, 35 pounds of baggage, and 44 gallons of fuel. 1840 pounds total. The plane has been flown like this as for over 1100 hours. Front gear is a concern as we are right below max weight on it. I baby it on take off and landing. Overall the plane performs great. IO-360, fixed pitch 3 blade catto, and we use 7.2gph at 9500 hitting 150 knots true. The 9 is a great cross country platform. I am just starting up my IFR but will let you know how it goes with that as well!
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  #24  
Old 01-12-2020, 10:06 AM
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Snowflake Snowflake is offline
 
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If you're looking for an IFR RV under $80K, you'll need to be buying a used one that has old-school IFR hardware in it.

Was going to comment on the gross weight but Scott did it quite eloquently above.
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1996 RV-6 "Tweety" C-FRBP (formerly N196RV)
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  #25  
Old 01-12-2020, 11:55 AM
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RV8JD RV8JD is offline
 
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Quote:
Originally Posted by rvbuilder2002 View Post
This is why additional fuel is sometimes added to aircraft with tip tanks. The mas of the fuel is then keeping the bending moment along the span of the entire wing mostly unchanged.

This would imply that it is a simple thing to add more fuel capacity to any airplane as long as you use tip tanks. Unfortunately that is not the case.
Mass added way out at the end of the wing will have a high polar moment of inertia because of the long arm length relative to the center of gravity. The can have a major influence on flight / handling characteristics. The most serious one being spin recover.... worse case being it no longer having any, if the tip tanks are full of fuel.
Not to mention possible lower flutter speeds of flutter modes involving the wing bending/torsion modes, and wing/aileron flutter modes. That depends on the particular airplane's design, and chordwise location of the tip tank, and if there is any vertical offset.
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Arlington, WA (KAWO)
RV-8, 790 Tach Hours
(Pic 1),(Pic 2)
- Out with the Old, In with the New
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RV-8, 1938 Tach Hours (Pic 1),(Pic 2) - Sold

Glasflugel Standard Libelle 201B - Sold
Rolladen-Schneider LS1-f - No longer owned
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  #26  
Old 01-12-2020, 12:53 PM
airguy's Avatar
airguy airguy is online now
 
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I modified my wing tanks to be full-span on the leading edge forward of the spar with two tanks each side, total of 67 gallons, a-la "Tuckey Tanks". I use them in a partial-to-full state perhaps 12 times per year - and mostly to give me the ability to get somewhere and land with full mains, then return without having to buy fuel. Only 4-5 times per year do I actually make a non-stop that requires the extra fuel, and I fly a LOT of long cross country trips. It's going to be a rather slim percentage of pilots, and a rather slim percentage of that pilots trips, that actually need additional fuel beyond what the standard 9A wing provides.

My nonstop long trips are between 6 and 8 hours so far - well beyond what most people want to spend confined in the cockpit without stretching their legs. This is not something you are going to do often, and I do not encourage builders to make that modification. Those of us that do it have our reasons, and we do it because of them - but it's certainly not "normal" or even necessarily desirable for the main market of 9A pilots.

As Scott pointed out, there are valid engineering considerations that have to be taken into account with such a mod. One of the big ones is bending moment of the wings during ground ops - you greatly increase the bending moment at the wing-fuse joint with more fuel outboard, and taxi/takeoff should be conducted quite gingerly to avoid stressing that area. Landings with fuel in the outboards should be considered verboten, and on my airplane incur a spar inspection prior to further flight if that occurs, just for that reason. The airplane is not designed or intended to take loads higher than full main tanks during normal landing ops. You can't guarantee that you'll make a squeaker of a soft landing on the days when you land with fuel in the outboards, it just doesn't work out like that. I never load more fuel in my outboards than I am planning to burn before my first stop - and even then I've had to divert for weather twice, once landing with fuel in the outboards (and inspecting the spar before leaving again) and once spending an extra hour burning off fuel before landing back home again. Being in a scenario where you CAN'T land is almost as bad as being in a scenario where you MUST land, and requires additional flight planning and thought.

Weight and balance is also an obvious one to consider - that fuel is forward of the center of lift, and you may be forced to carry weight in the baggage compartment to offset it, especially if flying solo. The nosegear (old school standard) from Vans on the 9A also has a total weight limit of 325 pounds, which is not difficult at all to encroach upon with more than standard fuel in the wing leading edge. This can also require baggage compartment weight or even tailcone ballast to shift the CG, but also raises total weight and can easily exceed max gross weight.

Short version is you're painting yourself into a corner by doing this - your reasons for doing it should be very good, your planning and engineering and testing should be better, and you must be able to accept more "experimental" in your experimental aircraft.
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Greg Niehues - SEL, IFR, Repairman Cert.
Garden City, TX VAF 2020 dues paid
N16GN flying 700 hrs and counting; IO360, SDS, WWRV200, Dynon HDX, 430W
Built an off-plan RV9A with too much fuel and too much HP. Should drop dead any minute now.
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  #27  
Old 01-12-2020, 09:20 PM
OneTwoTree OneTwoTree is offline
 
Join Date: Nov 2019
Location: DFW
Posts: 14
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Quote:
Originally Posted by 9GT View Post
It was not an EZ modification to do on tanks already fabricated. I would not recommend it unless it was a new wing build. I cut off most of the outboard bay and spliced on a new partial tank skin and baffle with an additional rib. Not for the faint of heart, but I am glad I opened up the QB tanks .
It would be a slow build

Quote:
Originally Posted by Greg Arehart View Post
We've done a number of long trips in our 9, including to the Arctic ocean this summer (lots of space between airports) and never worried too much about the fuel capacity. Flying at 8-12k, 165 mph cruise we burn about 7.2 gph with an IO360. That translates to easily 4.5 hours plus a half hour reserve, or 4 hours plus an hour reserve. That can be extended by going a bit slower and/or higher altitude. Not too many places where you would come close to running out of fuel before wanting or needing to stop.
Interesting. I was seeing so many 8.5 to 9 gph burns.

Quote:
Originally Posted by SVTPete83 View Post
I might get flamed for this. Flame suit on. My RV9a weighs in at a portly 1197 pounds with zero fuel. I have the stock 36 gallon tanks and glassed in wingtip tanks that give me an additional 9 gallons per side for a total of 54 gallons. The original builder flight tested it with an 1850 pound gross weight. We fly it at that weight all the time. Just the other day we flew with my wife, me, 35 pounds of baggage, and 44 gallons of fuel. 1840 pounds total. The plane has been flown like this as for over 1100 hours. Front gear is a concern as we are right below max weight on it. I baby it on take off and landing. Overall the plane performs great. IO-360, fixed pitch 3 blade catto, and we use 7.2gph at 9500 hitting 150 knots true. The 9 is a great cross country platform. I am just starting up my IFR but will let you know how it goes with that as well!
Very interesting. Thanks for this.

Quote:
Originally Posted by Snowflake View Post
If you're looking for an IFR RV under $80K, you'll need to be buying a used one that has old-school IFR hardware in it.

Was going to comment on the gross weight but Scott did it quite eloquently above.
Quote:
Originally Posted by airguy View Post
I modified my wing tanks to be full-span on the leading edge forward of the spar with two tanks each side, total of 67 gallons, a-la "Tuckey Tanks". I use them in a partial-to-full state perhaps 12 times per year - and mostly to give me the ability to get somewhere and land with full mains, then return without having to buy fuel. Only 4-5 times per year do I actually make a non-stop that requires the extra fuel, and I fly a LOT of long cross country trips. It's going to be a rather slim percentage of pilots, and a rather slim percentage of that pilots trips, that actually need additional fuel beyond what the standard 9A wing provides.

My nonstop long trips are between 6 and 8 hours so far - well beyond what most people want to spend confined in the cockpit without stretching their legs. This is not something you are going to do often, and I do not encourage builders to make that modification. Those of us that do it have our reasons, and we do it because of them - but it's certainly not "normal" or even necessarily desirable for the main market of 9A pilots.

As Scott pointed out, there are valid engineering considerations that have to be taken into account with such a mod. One of the big ones is bending moment of the wings during ground ops - you greatly increase the bending moment at the wing-fuse joint with more fuel outboard, and taxi/takeoff should be conducted quite gingerly to avoid stressing that area. Landings with fuel in the outboards should be considered verboten, and on my airplane incur a spar inspection prior to further flight if that occurs, just for that reason. The airplane is not designed or intended to take loads higher than full main tanks during normal landing ops. You can't guarantee that you'll make a squeaker of a soft landing on the days when you land with fuel in the outboards, it just doesn't work out like that. I never load more fuel in my outboards than I am planning to burn before my first stop - and even then I've had to divert for weather twice, once landing with fuel in the outboards (and inspecting the spar before leaving again) and once spending an extra hour burning off fuel before landing back home again. Being in a scenario where you CAN'T land is almost as bad as being in a scenario where you MUST land, and requires additional flight planning and thought.

Weight and balance is also an obvious one to consider - that fuel is forward of the center of lift, and you may be forced to carry weight in the baggage compartment to offset it, especially if flying solo. The nosegear (old school standard) from Vans on the 9A also has a total weight limit of 325 pounds, which is not difficult at all to encroach upon with more than standard fuel in the wing leading edge. This can also require baggage compartment weight or even tailcone ballast to shift the CG, but also raises total weight and can easily exceed max gross weight.

Short version is you're painting yourself into a corner by doing this - your reasons for doing it should be very good, your planning and engineering and testing should be better, and you must be able to accept more "experimental" in your experimental aircraft.
air guy I actually read your entire post on this modification. Very cool. While I have no problem sitting in one place for over 8 hrs on a regular basis, I do see that a whole bunch of thought is going to have to be put into this! Thanks
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  #28  
Old 01-12-2020, 09:26 PM
OneTwoTree OneTwoTree is offline
 
Join Date: Nov 2019
Location: DFW
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Quote:
Originally Posted by rvbuilder2002 View Post
Both are common misconceptions but both are untrue.

The published gross weights for all the RV models are the weights used to do all of the structural load calculations, design engineering, and static load tests.
Any deviation higher, though legal here in the U.S., is definitely upping the level of experimental of their aircraft.

It is correct that additional fuel weight added to the wings does not add additional bending load at the wing attach points (it can actually reduce it depending on the circumstances) but depending on where it is added, it will add additional load elsewhere.

For the purpose of simplification, lets say that we were able to add additional fuel to an RV-9A and all of the additional fuel was distributed span wise along the original standard fuel tank. When flying with full fuel the additional fuel weight when pulling G's may actually be reducing the bending load on the wing attach point beyond what it was without the additional fuel (the additional fuel would be inducing a downward load counteracting the additional upward bending load of the wing trying to carry the additional weight).

The problem is that the additional weight is being carried by the entire wing, not just the inboard portion where the fuel tank is. So the portion of the wing outboard of the fuel tank has some level of additional load but the additional mass of the fuel is not there to counteract the additional bending moment, so there is a increase in bending moment, with a specific concentration just outboard of the fuel tank.

This is why additional fuel is sometimes added to aircraft with tip tanks. The mas of the fuel is then keeping the bending moment along the span of the entire wing mostly unchanged.

This would imply that it is a simple thing to add more fuel capacity to any airplane as long as you use tip tanks. Unfortunately that is not the case.
Mass added way out at the end of the wing will have a high polar moment of inertia because of the long arm length relative to the center of gravity. The can have a major influence on flight / handling characteristics. The most serious one being spin recover.... worse case being it no longer having any, if the tip tanks are full of fuel.
I wanted to reply and thank you for this write up specifically. It made me realise that I have a bunch of work to do if I'm going to do this safely. Thank you for your time and knowledge.
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  #29  
Old 01-12-2020, 09:31 PM
OneTwoTree OneTwoTree is offline
 
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Location: DFW
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Quote:
Originally Posted by Snowflake View Post
If you're looking for an IFR RV under $80K, you'll need to be buying a used one that has old-school IFR hardware in it.

Was going to comment on the gross weight but Scott did it quite eloquently above.
Oh great ......... what am I missing now! 35 for a slow build RV9 (includes lights servo's ect.) 25 for engine and prop. 20 for avionics.

I left out 10 to 20 for paint because I may do it myself. I have painted vehicles and while it's not the same, I am willing to learn and put the time into it.
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  #30  
Old 01-12-2020, 10:41 PM
BobTurner BobTurner is offline
 
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Location: Livermore, CA
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Quote:
Originally Posted by OneTwoTree View Post
Oh great ......... what am I missing now! 35 for a slow build RV9 (includes lights servo's ect.) 25 for engine and prop. 20 for avionics.
$25K will get you a used engine and prop. Also-opinions will vary-but if you are really concerned about range/endurance a fuel injected engine can easily be tuned for lean of peak operation-extending range. But they are more expensive.
20K will get you minimal ifr avionics. Most ifr pilots will demand redundant backups, increasing the cost significantly.
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