Useable Fuel

macrafic · #1 08-20-2008, 07:51 AM

I'm working on the spreadsheet for weight and balance (RV-7A) and had a question for you folks.

I'm wanting to distinguish between useable and non-usable fuel, so that the spreadsheet does NOT allow the fuel amount to dip below the non-usable amount. The full capacity is 42 gallons, but how much of that is usable? How much is left in your tank when the engine stops?

I plan on determining this for my own aircraft, but am simply putting a skeleton spreadsheet together at this point and was wondering about all your experiences. This is also a way for me to check my own plane against the prevailing "normalcy".

Thanks.

Mel · #2 08-20-2008, 08:05 AM

If you build according to plans, pretty much all of the fuel is useable.

scard · #3 08-20-2008, 08:13 AM

I concur with Mel's comment. It is surprising when you get there to do the final testing and find it difficult to actually measure the amount of fuel that is unusable. It was just a few ounces on mine.

rvmills · #4 08-20-2008, 08:28 AM

Mel and Scott,

Do you find that to be true both for an "A" and a TW? I'm getting ready to re-weigh and re-calc my 6's W&B after panel and other upgrades (and re-paint by the previous owner), and was wondering the same thing about useable fuel. Very timely post!

I see you two have one of each, and concur, so just wanted to bounce it off you. Also, my 6 has slightly longer Rocket gear, but I've added a Vince Frazier TW, which lifts the tail slightly, so those two factors may cancel each other. It was not built fully to plans (Super Six), so that may be a wild card. Perhaps it won't be enough to make an appreciable difference, but is there a "best way" to determine unuseable fuel? Is running it till it stops (on the ground of course

) bad for the engine, or is that the accepted method? Thanks!!

Cheers,
Bob

scard · #5 08-20-2008, 08:37 AM

Quote:

Originally Posted by rvmills

Mel and Scott,

Do you find that to be true both for an "A" and a TW. I'm getting ready to re-weigh and re-calc my 6's W&B after panel and other upgrades (and re-paint by the previous owner), and was wondering the same thing about useable fuel. Very timely post!

I see you two have one of each, and concur, so just wanted to bounce it off you. Also, my 6 has slightly longer Rocket gear, and was not built fully to plans (Super Six), so that may be a wild card. Is there a "best way" to determine unuseable fuel? Is running it till it stops (on the ground of course

) bad for the engine? Thanks!!

Cheers,
Bob

"A" or not, usable fuel is measured in flight position.
How to best measure: Put a measured amount in, and pump it out with the in-line fuel pump all the way through the fuel supply hose a the carb. Measure how much came out, subtract.
No problem running it out of fuel. We do it each time we shut the engine down by pulling the mixture

.

Steve Brown · #6 08-20-2008, 09:03 AM

Not a 7, but I've verified in flight that you can virtually get every drop of the 18 gallons of fuel from each wing.

rvmills · #7 08-20-2008, 09:13 AM

Quote:

Originally Posted by scard

"A" or not, usable fuel is measured in flight position.
How to best measure: Put a measured amount in, and pump it out with the in-line fuel pump all the way through the fuel supply hose a the carb. Measure how much came out, subtract.
No problem running it out of fuel. We do it each time we shut the engine down by pulling the mixture

.

Thanks Scott! In-flight attitude makes sense...just wasn't sure if the TW flyers needed to be concerned with a 3-point landing attitude (then again, the tri-gear folks need to keep the NG off as well!)

I've used W&B for years, just never had to engineer the diagram (kinda like computers...I can pound keys with the best of them, but couldn't program to save my bacon). Looking forward to working this piece and getting the know the bird even better!

Good idea on the measuring method, and roger the mixture thing.

Guess I was thinking about not sucking the last of the gas for fear of pulling in yucky stuff, but thinking about it, that's why we drain our tanks before each flight...better be no yucky stuff in there! Still waking up here out west!

Cheers,
Bob

az_gila · #8 08-20-2008, 10:47 AM

Quote:

Originally Posted by scard

"A" or not, usable fuel is measured in flight position.
How to best measure: Put a measured amount in, and pump it out with the in-line fuel pump all the way through the fuel supply hose a the carb. Measure how much came out, subtract.
No problem running it out of fuel. We do it each time we shut the engine down by pulling the mixture

.

...than just in flight position.

For a certified plane, the regulation is here....

Sec. 23.959

Unusable fuel supply.

(a) The unusable fuel supply for each tank must be established as not less than that quantity at which the first evidence of malfunctioning occurs under the most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank. Fuel system component failures need not be considered.
(b) The effect on the usable fuel quantity as a result of a failure of any pump shall be determined.

So it is to be determined in ALL flight positions.... and often (Cessnas?) the worst case may be a climb in a landing go-around.... just the wrong spot to discover that the fuel pick-up is not at the rear end of the tank...

I believe Van has the best spot for the fuel pick-up, and that RV unusable fuel is essentially zero.