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Can I take Weights for W&B calculation w/ Full Fuel

lr172

Well Known Member
When I did my W&B calc's with the tanks empty, I did not have access to quality scales. I believe that the scales were decent and got me close, but I now have access to high end scales and would like to do the weights again, as I have a 6A that will live near the aft CG every time I have a passenger and baggage. My problem is that I don't want to drain the fuel tanks.

Can I take the weights with the tanks full (known qty +/- 1 gallon) and then back out the fuel for empty weight / CG? It seems feasible from a math stand point, but I am sure there is a reason it is done empty

Thanks for your guidance on this.

Larry
 
Short answer is yes, that is an acceptable way. You then subtract unusable fuel at the fuel station from the "as weighed" condition. Your Engine oil should be full and you should have only the aircraft equipment in the airplane. If you have a flight manual, it should be included. As you point out there is the possibility that the fuel density isn't the nomonal 6 lbs/ gal but if you want to be detailed you can use a hydrometer to get the fuel density. If you are near 68 deg. F is should be very close.

Don't forget to level the aircraft when weighing.

Hope that helps.
 
IMHO you are just replacing one "close enough" W&B for another "close enough" W&B. As an old A&P friend of mine used to say: "If it is important to do it, then it is important to do it right."

:cool:
 
IMHO you are just replacing one "close enough" W&B for another "close enough" W&B. As an old A&P friend of mine used to say: "If it is important to do it, then it is important to do it right."

:cool:
Hopefully Mel will jump in here. I agree with the previous post. Take the extra time and do it right, this won't be the last time you drain your tanks. It's easy to do, unlike the rv12.
 
As said before, you CAN do like you suggest. However, you must know the exact amount and CG of the fuel.

Personally I would drain the tanks and do it right. It's not that difficult.
And as also stated above, be sure to level the aircraft, flaps in flight configuration, canopy closed, etc.
 
You need to weigh the aircraft with a known quantity of fuel etc.

In some ways weighing with full fuel is better as it is a better representation of the gross weight condition.

That may even be more accurate than weighing with just unusable fuel as that varies with attitude, flop tube variability etc.

To weigh in the zero usable fuel condition, you need to put the aircraft in the max climb pitch attitude and pump remaining fuel out using the electric pump, so full tanks may be a more accurate measure.
 
You need to weigh the aircraft with a known quantity of fuel etc.

In some ways weighing with full fuel is better as it is a better representation of the gross weight condition.

That may even be more accurate than weighing with just unusable fuel as that varies with attitude, flop tube variability etc.

To weigh in the zero usable fuel condition, you need to put the aircraft in the max climb pitch attitude and pump remaining fuel out using the electric pump, so full tanks may be a more accurate measure.

Lots of people who have taken the time to test it (and have properly installed fuel pink-ups) will confirm that unusable fuel is just about zero (typically a pint or less).

I also recommend that weighing be done with tanks empty. Any other way will by just an approximation (which you have already).
 
My two cents: initial weighing, plus measurement of moment arms to the wheels, will not be perfect but will be as good as it gets. Anything after that, like a different fuel configuration, will involve more calculations with moment arms that have some error. So the best way to do the initial measurements is in a configuration as close as possible to the limit in question. For weight, most pilots will be concerned with a full fuel -full load situation, so to minimize the errors measure the weight with full fuel. But for cg, at least for the -10, it's most critical at low weights. So measure empty. Since I base at an airport with a fuel truck, it was easy to get set up empty, record the numbers, fuel, record the numbers. Actually I just put in 5 gal at a time so I could calibrate the fuel floats at the same time. When all was said and done, the numbers calculated from empty, and from full, were in good agreement. I forget the differences, but they were probably due to the scale errors more than anything else. So I'd say either way, full or empty, is okay. Remember the scales aren't perfect so don't push the limits right up to that last quarter inch.
 
I agree that weighing with full tanks and subtracting is just an estimate, but then weighing with empty tanks then doing a W&B with full tanks is no less of an estimation. This way you will know exact weight and CG with full tanks, and you fly much more often this way then with empty tanks.
 
Thanks for the help guys. Lot's of good ideas here. The more I read and the more I thought it seems the only major downside here is possibly getting the fuel qty wrong. But I need to trust the Fuel CG datum provided by Van's at some point as I am using it for every calculation except the exact one that weighed.

Larry
 
Do it both ways, full and empty - then you can calculate the arm to use for fuel for your airplane.

==Dave==
 
Some here say it is important to do it correctly and I would agree, but levelling the plane will not get the correct answer. We normally level the longerons on an RV, but does an RV fly with the longeron exactly level or is it slightly tail down.
Whatever we do it is an approximation and luckily it works, but trying to be 100% theoretical is just about impossible.
 
There is a method to remove the fuel from the tanks that is fun! You just have to remember to not call the fuel truck to fill the tanks up after the flight.

If not already done, you can also accomplish another task while doing this and that is finding out when one of the tanks is actually empty versus what it indicates while in level flight. (edit; Note: You do not have to wait until the engine stops to find when the tank is empty, just monitor your fuel pressure and when it drops off switch tanks. There will be enough fuel in the lines and fuel bowl to keep the engine running until get the boost pump on and change tanks.)
 
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Opportunity!

There is a method to remove the fuel from the tanks that is fun! You just have to remember to not call the fuel truck to fill the tanks up after the flight.

Draining the tanks is a great opportunity to learn some vital information about your plane. You can refill them a few gallons at a time and take note of what your fuel gauges read. Analog fuel gauges are prone to some wild inaccuracies. When I bought my -6, 1/2 tank on one side meant 'a few gallons left', and the other side meant '3/4 full'. I've since replaced those gauges with a calibratable digital unit (great enhancement). The calibration was done, adding one gallon at time, after reweighing the plane with empty tanks.

You can use a small pair of visegrips on the quickdrain to drop the fuel into a bucket... just don't make any sparks.

Lastly, if you have weepy or otherwise unhappy quickdrains., this is a wonderful opportunity to replace them.

-jon
 
Some here say it is important to do it correctly and I would agree, but levelling the plane will not get the correct answer. We normally level the longerons on an RV, but does an RV fly with the longeron exactly level or is it slightly tail down.
Whatever we do it is an approximation and luckily it works, but trying to be 100% theoretical is just about impossible.

It doesn't matter.

The weight and balance computations are done to determine if the C.G. of the airplane is within the limits recommended by the designer. All that is important for that is that it be measured the same way the designer did (that is what the fwd and aft C.G. limit specs are based off of).
So if the designer said to measure with the longerons at a 20 degree nose up pitch angle, and the fore and aft C.G. limits were based on that reference, that is what should be used.

For RV's, longerons level is used as the reference because in cruise flight they are relatively level (it will always have some level of variability because of different speed and weight), because RV's have wing positive incidence built in.

In a nut shell..... If your goal is to use the designers recommended fore and aft C.G. limits (highly recommended) then you need to use the designers recommended pitch reference when weighing to compute the empty C.G., otherwise the numbers are meaningless.
 
Some here say it is important to do it correctly and I would agree, but levelling the plane will not get the correct answer. We normally level the longerons on an RV, but does an RV fly with the longeron exactly level or is it slightly tail down.
Whatever we do it is an approximation and luckily it works, but trying to be 100% theoretical is just about impossible.

Yes leveling the RV aircraft does get the correct answer. That is the way the DESIGNER used to set the limits and tested the prototype. Doing it differently is WRONG and will get incorrect results that could be deadly.
 
Draining the tanks is a great opportunity to learn some vital information about your plane. You can refill them a few gallons at a time and take note of what your fuel gauges read. Analog fuel gauges are prone to some wild inaccuracies. When I bought my -6, 1/2 tank on one side meant 'a few gallons left', and the other side meant '3/4 full'. I've since replaced those gauges with a calibratable digital unit (great enhancement). The calibration was done, adding one gallon at time, after reweighing the plane with empty tanks.

You can use a small pair of visegrips on the quickdrain to drop the fuel into a bucket... just don't make any sparks.

Lastly, if you have weepy or otherwise unhappy quickdrains., this is a wonderful opportunity to replace them.

-jon

I calibrated my gauges by adding 2 gallons at a time and they are pretty accurate. My problem here is that I don't have a bucket to hold 20 gallons of gas. I suppose I could run both tanks very low but that also brings risk that I would rather not take. I don't run my tanks much below 5 gallons indicated..
 
I calibrated my gauges by adding 2 gallons at a time and they are pretty accurate. My problem here is that I don't have a bucket to hold 20 gallons of gas. I suppose I could run both tanks very low but that also brings risk that I would rather not take. I don't run my tanks much below 5 gallons indicated..
Sounds like a reason to go fly to me! :D
 
Watch that aft CG

Howdy,

I've run both of my RV-6A tanks dry (on separate flights, of course) and found that each took exactly 19 gallons to fill. The easy way to do this would be to fill them to the top and weigh it like that, calculating the empty weight from there.

That said, the most adverse aft CG will occur with nearly empty tanks, 2 up, and a lot of baggage. When it happens, you'll know it. You won't like it. The airplane feels mushy at approach speed and requires large stick movements. When it happened to me, as soon as we landed I immediately started rooting around for something heavy to move forward. I made a (now permanent) spot for my toolkit to live up forward just behind the battery. We've had big loads back there since without issues.

The heavier you are loaded, especially with camping gear/baggage, the more likely you are to run into it. Land sooner, with more fuel reserve, and carry more speed in the pattern to give yourself some more elevator authority.

Ed Holyoke
RV-6A O320 Sensenich FP
RV-6 nearly done
 
Another gotcha is that with many airplanes, you don't necessarily know that the tanks are full because of air bubbles behind ribs, bladders that don't behave, etc. BTW, somebody in the know should discuss how many factory built planes are individually weighed, and what the procedures were in the 50s and 60s. Another way of looking at this is, how accurate is enough? 100 lb is too coarse, but measuring to 1 gram is not required.
 
Howdy,

I've run both of my RV-6A tanks dry (on separate flights, of course) and found that each took exactly 19 gallons to fill. The easy way to do this would be to fill them to the top and weigh it like that, calculating the empty weight from there.

That said, the most adverse aft CG will occur with nearly empty tanks, 2 up, and a lot of baggage. When it happens, you'll know it. You won't like it. The airplane feels mushy at approach speed and requires large stick movements. When it happened to me, as soon as we landed I immediately started rooting around for something heavy to move forward. I made a (now permanent) spot for my toolkit to live up forward just behind the battery. We've had big loads back there since without issues.

The heavier you are loaded, especially with camping gear/baggage, the more likely you are to run into it. Land sooner, with more fuel reserve, and carry more speed in the pattern to give yourself some more elevator authority.

Ed Holyoke
RV-6A O320 Sensenich FP
RV-6 nearly done

Thanks for sharing your experiences! Also, GREAT idea with the tool bag. I have it sitting behind the seat now and that is 5lbs moving way forward to help move the CG.

Larry
 
Howdy,

I've run both of my RV-6A tanks dry (on separate flights, of course) and found that each took exactly 19 gallons to fill. The easy way to do this would be to fill them to the top and weigh it like that, calculating the empty weight from there.

How do you define "Full?'" When I had the line guy filling during my calibration, he couldn't get much more than 18 gallons, but I can't remember how high he got them. The plane was also level (2" blocks under main).

Larry
 
All the way full

Howdy Larry,

I call it full when the fuel touches the ring. If I dribble it in, I can get it to go most of the way around the ring. Gotta make sure and leave room for the cap, though, or there's gas on the paint. I've never had a line guy tell me that I can't pump my own gas.

Ed Holyoke
 
Thanks for sharing your experiences! Also, GREAT idea with the tool bag. I have it sitting behind the seat now and that is 5lbs moving way forward to help move the CG.

Larry

5 lbs! I haven't weighed my tools, but I'd guess closer to 12 lbs. I hate not being able to fix something on the road. A good kit has come in handy a few times.

My wife is tiny and since we've got to jack her up somehow so she can see, I tend to put heavy stuff under her seat cushion. Full size airbed weighs about 12 lbs. and moves her up about 4 inches and the CG forward. We've found that she gets a lot less nauseous if she can see over the panel and I'm happier with the CG. :)

Ed Holyoke
 
Draining the tanks is a great opportunity to learn some vital information about your plane. You can refill them a few gallons at a time and take note of what your fuel gauges read. Analog fuel gauges are prone to some wild inaccuracies. When I bought my -6, 1/2 tank on one side meant 'a few gallons left', and the other side meant '3/4 full'. I've since replaced those gauges with a calibratable digital unit (great enhancement). The calibration was done, adding one gallon at time, after reweighing the plane with empty tanks.

You can use a small pair of visegrips on the quickdrain to drop the fuel into a bucket... just don't make any sparks.

Lastly, if you have weepy or otherwise unhappy quickdrains., this is a wonderful opportunity to replace them.

-jon

I 100% agree with the learning opportunities, just differ on the method to empty the tanks. I personally believe that emptying the tanks by making lots of noise out the exhaust while out flying is the only way, as long as the plane is airworthy. Then you can do the scientific refilling of the tanks.:)
 
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