Weighing your aircraft
 

I have a single load cell and would like to get an acurate weight on my already-flying aircraft to check the validity of the mfg's numbers. I have a single load cell - I assumed it as simple as putting the load cell under each main and then the tailwheel - Anyone have any tips or traps to avoid to get a good number?

Just make sure its level
 

As in exactly the same attitude with the load cell in each spot.

The level will be in flying attitude assuming you want to calculate your CG.

Frank

Also remember to raise the other wheels to the same level as the scales so that the aircraft is level for all weighings. To do this you will need 2 blocks the same thickness as the scales. If the aircraft is tilted toward one wheel, the weight will shift to that wheel. And as Frank said the aircraft must be in level flight attitude for W&B calculations.

What about the gas?
 

When it comes to weighing an aircraft, I understand that it's ideal that the gas tanks be empty. However, once the plane has been flying, it's a bit of a pain to drain the tanks for weighing.

Probabaly a silly question, but is it possible to weigh the plane with the tanks FULL, knowing the capacity and the weight of avgas, then just subtract the weight, or does this adversely affect the process by shifting the CG?

Not really very hard to empty RV tanks, espeically if they are less than half full. But you most certainly can weight the aircraft with full fuel and then subtract it.

If you know the weight and location of the fuel "EXACTLY" (or any component), then it is easy to back the numbers out of the total. However, the weight and location are often as a result of rounding, and will therefore corrupt your true empty weight. So if your baseline is off, and then you start rounding with passengers and baggage down the line, where does that leave you? It's best to start with an accurate baseline - the actual ZFW.

Also, remember that "unusable fuel" becomes part of the empty weight of the aircraft - there is no need to soak up evey drop with towels. If the engine can't get at the last bit of fuel, then it's part of the aircraft.

My 18 gal tanks....
 

Left one hold 18.1 and the right one hold 18.2 gal.
That is if I fill to just before over-flow.

I would recommend draining. Just fly to to point of getting low and then drain into some cans from the removed drain plugs. This will also give you a change to recalibrate the tanks (capacity and fuel gauges).

When I did this I also made notes of the fuel level inside the tank. IE where did I see the fuel at 4 gal, 6 gal, etc. I made myself a sight gauge with notes where on the baffles and first tank rib the fuel came to.

If any 9A driver wants to gauge, I can post it. It should be close enough for any 9A.


Kent

Usable vs. Drainable
 

One thing to keep in mind when draining fuel is the difference between what the pickup can get to and what the drain can remove. Sometimes there is no difference (usable and drainable are the same), but in some cases, such as with a flop tube, the drain can be lower than the pickup. This is very apparent in the flop tube equipped aerobatic tank in my Hiperbipe ? That?s why on long cross countries, I usually run this tank dry early in the flight when I?m up nice and high. Even with a sight gage, I have no way of knowing exactly when the flop tube is going to start sucking air, and it seems to change a little from flight to flight. The main tank on the other hand, draws from the bottom so it gets every last drop. In fact the sight gage will be dry for some time before the engine quits.

Some of you have flop tubes in your tanks, so make sure that you have identified the ?unusable? fuel to the engine, and have that added back in as basic aircraft weight and moment. A good test to see what your true ?usable? is would be to fly one tank to a dry condition, and then see if you can get any out of the drain once on the ground.

My -9 holds slightly more, almost a full gallon because with the tail down, the fuel cap is at the very top. On the -9A's, they are slightly forward.

Ditto

Also, don't forget to close your canopy, be it a tip-up or slider. They both impact the W&B.

Weighing it while it is level fore and aft as well as sideways has already been mentioned, so I say it again.