FX3 Wing Fuel Tank Recommendation?

[Schwbo]

I got data from CC and it suggests the following
- standard tanks are 25 gallons and 24 useable.
- long range tanks are 44 gallons and 40 useable. They are 42 lbs heavier than the normal tanks. I think this in only the tanks and additional equipment alone.
- Fuel pod carries 25 gallons all useable. It weighs 12.5 lbs.
- net net you gain additional 9 gallons of fuel and save 30 lbs of weight by going with standard tanks a Fuel pod. There is a budget penalty of few thousand that goes with it as well.


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[Daveembry]

See post #131 here regarding usable fuel on an EX3. I’ve done this testing on 3 EX3’s. There was a fairly large difference in the last 2 and I have no idea how that could be as the test were done identically with the exception on 1 test I drained it at the gasolator with the fuel selector only on 1 tank and in the 2nd test airplane, I did it with the fuel selector on BOTH so it was draining from both tanks? I suppose this would be important to know if in low fuel situation (leave it on BOTH?)

You can see what the actual “static” useable fuel is. As I say, that will change (probably increase) with air pressure during flight into the tank vents in the caps and decrease during certain maneuvers such as full power climbing turn.

In my instances, there has always been much more usable fuel in level flight attitudes than advertised. I also mark my fuel sight gauges on the first fillings of fuel into the tanks in level attitude so. I know when I really reach 2.5 gallons/5 gallons etc.

http://forum.cubcrafters.com/showthr...ight=Ex3.+Tips

The problem is that I don’t know how one could “easily” determine this after fuel has been placed in the fuel tanks for them first time. You can drain fuel but not be able to determine unusable fuel except to possibly refuel and see how much the tanks hold and then do the math. The problem with that as you will read, is that is knowing the TRUTH total fuel the tanks will hold. (Is it really 44 gallons??)

The actual weight (I weighed them) of 1 of the “double tanks” that go into 1 wing on the extended fuel (44 gallons) is 12 lbs total as installed as 1 unit, so 24 lbs is the total weight of all the fuel tanks in both wings. Much less than I’m seeing posted here.

[hawgdrvr]

Great info, as always Dave, thank you. FYI if you right-click on the post number such as #131 then right click on that number and choose Copy Link Address you can send people directly to a post, in case anyone didn't know. So here is your post 131:

http://forum.cubcrafters.com/showthr...ll=1#post22995

@Schwbo, what is having you choose a FX3 over SS based on the modifications you're considering? Curious...

[Schwbo]

I wanted to enjoy the better control harmonization, 2,000lbs MTOW, variable pitch and other goodies.

My plan is to take it to AK, ID and MT for mountain flying.

I’m not comfortable flying the SS with two persons and luggage and be overweight. I know the SS is designed for 1,850 MTOW, but equally I don’t want to start proving it to the FAA.


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[turbopilot]

See post #131 here regarding usable fuel on an EX3.

Dave, excellent well documented information. Sure wish we could get this from CubCrafters. So is the quote below from post 131 of thread your current bottom line for the 44 gallon tank option. Given the variability you have seen in your airplanes do you have "fail safe" bottom line the would account for the variability seen in all FX's? Two issues now emerging. What are the "dead" weight issues associated with the 44 gallon versus 24 gallon systems and how much fuel is really available for both tanking systems.

Did you ever weigh the 10 gallon tank and hardware alone? CubCrafters did tell me they think it is 17 lbs or 8.5 lbs per tank but based on your weights for both tanks I wonder.


THE FULL FUEL CAPACITY THEN IS 42.5 WITH ADVERTISED 5 GALLONS UNUSABLE IN ALL CONFIGURATIONS THAT MAKES 37.5 GALLONS USEABLE AND NOT 39 AS ADVERTISED. I do know from my test though that actually there was only 2.5 gallons unusable in the static, level configuration for 40.0 useable.

[Andy]

Several references in Dave's linked posts to "turning". A coordinated turn makes no difference to usable fuel. What may make a difference to usable fuel is slipping or skidding. Either can happen with wings level or while banked.

It may also be important to distinguish between level flight attitude and fuselage leveled. They are unlikely to be the same pitch attitude.

However, it's good to have Dave's confirmation that real world unusable fuel is likely to be less than that established by the worse case flight test point. (Yes, it was flight tested not calculated according my CubCrafters source).

I stand by my earlier statement. If you want to get the most fuel out of the tanks then don't select both. Pick a wing to burn from, select that tank, and fly with that wing slightly raised. When that tank is exhausted swap sides.

My lawyer says you should ignore what I say and follow AFM procedures.

[turbopilot]

My lawyer says you should ignore what I say and follow AFM procedures.

You have a good lawyer.

This thread is teasing out the differences clearly existing in the 44 gallon tank "option". I say option because it is an option on the SS and can be deleted on the FX.

We all understand the 24 gallon system. It seems pretty straight forward though I have never performed a static test on the 24 gallon system like Dave. On my list.

The question I still have is given that you have (or can have) only a 24 gallon system in an SS or FX. What is the most efficient option to carry more than 24 gallons in an SS or FX?

The belly tank option seems very compelling. What are the downsides of using a belly tank versus in wing additional fuel storage?

Here are the upsides as I see it.

1. It has virtually no unusable fuel penalty. You pump to the left tank till dry.
2. it is very light. I have heard 19 lbs but I have seen other weights quoted.
3. You can remove it depending on the mission. Four point attachment.
4. It can carry more fuel than the 44 gallon tank option. Meaning more than the 20 gallons in the two 10 gallon wing tanks (minus unusable you pick the number).
5. It MAY improve the overall aerodynamics of the airplane.

It seems like a no brainer unless you all ready have an airplane with the 44 gallon tank system.

[40m]

Of all the options I considered and then had built into my EX2 long range tanks are by far the most useful. The 24 gallons in my first CC aircraft required constant monitoring and then topping off to be sure I was ready to go. I personally haven't met a CC owner who hasn't wanted more fuel, some like myself are willing to sacrifice pounds for the convenience of wing tanks others are happy with bags and or a pod. To each his own.

[Daveembry]

Andy, I'm just reporting what I found..... as your lawyer would suggest, might I add, "Your mileage may differ".

The wings already have 3" of dihedral (slope) built into them from wingtip to over the cockpit, so I would assume all the fuel drains from the outer tank to the inner tank as the crossovers are at the very bottom of the fuel tanks. You can see this in my post #110 as well as how the "washout" is in the wings.

(Thanks Neal for the info on how copy the link to a specific post. I learned something new!)

http://forum.cubcrafters.com/showthr...ll=1#post22769

As I noted, I was able to get more fuel to drain when BOTH was selected than when on 1 tank. I understand what you are saying and maybe that's the best way??? I'm just reporting the facts and I don't really know why that would matter....but it did.

What I try to do myself, (as I have always done in my life) in to question everything and instead of all the speculation.......do the test and see for myself. So I test and then I report......you will note I question any suppositions by even myself in regards to them.

When I say in a turn, what I was told from Cubcrafters was that in a "full throttle, hard climbing turn" ....was where the most fuel was needed. I would also assume an "uncoordinated" turn would be required (thus more "unuseable"). You can take it from there. I can only assume that statement was correct. CC engineers can correct and inform us maybe if it's wrong.

Also to be clear, as I stated....it was in a "level flight attitude" as prescribed by CC and not "fuselage level".

Several references in Dave's linked posts to "turning". A coordinated turn makes no difference to usable fuel. What may make a difference to usable fuel is slipping or skidding. Either can happen with wings level or while banked.

It may also be important to distinguish between level flight attitude and fuselage leveled. They are unlikely to be the same pitch attitude.

However, it's good to have Dave's confirmation that real world unusable fuel is likely to be less than that established by the worse case flight test point. (Yes, it was flight tested not calculated according my CubCrafters source).

I stand by my earlier statement. If you want to get the most fuel out of the tanks then don't select both. Pick a wing to burn from, select that tank, and fly with that wing slightly raised. When that tank is exhausted swap sides.

My lawyer says you should ignore what I say and follow AFM procedures.

[Andy]

It seems like a no brainer unless you all ready have an airplane with the 44 gallon tank system.

You make arguments that may well apply to an SS but the OP was asking about an FX-3.

How many people ordered an FX-3 with less fuel capacity than standard? I only heard of one but there could be more. Would you want to be trying to sell an FX-3 that has less fuel capacity than standard?

The thing about the FX-3 is that you can have the larger tanks AND the belly pod. It isn't a choice of one or the other. My FX-3 has 850 lb useful load. After I add me and full fuel there is still 446 lb remaining. The compartment limits are 150 total. That leaves 296 lb that could be used for the rear seat (probably aft CG limited) and/or a belly pod.

Weight simply isn't an issue for me.