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RV 7A weighed with fuel for weight and balance

danastoker

Member
I just got my RV 7A back from an avionics shop. I had a full Garmin panel put in removing Dyson equipment. They asked for the weight and balance information from the build manual and I sent them section 14 with detailed instructions.

I picked up the aircraft and discovered that they had weight the aircraft with full fuel and then deducted the fuel weight [42 gal - 252 lbs] to complete the weight and balance. I am not real comfortable with that as Van's says to weigh the aircraft with oil but without fuel. Any thoughts?

I also found it interesting that my prior weight and balance information showed the aircraft total weight at 1,174 Lbs and their weight claims that the weight is now 1,260 Lbs. The weight of the new Garmin equipment is less than the Dynon equipment and with their new weight I cannot carry a passenger over 200 Lbs.... I am 245 Lbs. So, either the prior weight was wrong or the new weight is wrong.

I suppose the only resolution is to reweigh the aircraft after draining the fuel. I do not have a scale capable of handling the weight so I am going to ask my local chapter of EAA for help as well.
 
There can be a lot of variation in scale accuracy. You're probably experiencing some of that. As for the fuel, uggh.

Their numbers are probably reasonable, but I'd fly off most of the fuel, drain the remainder, and re-weigh.
 
I’m pretty sure the new weight reflects reality much closer than the old one. Most aircraft, as humans, do pick-up weight with age. And it is one reason that in EASA land, which we know you’re not, a weighing is required every 10 years...
Re weighing with fuel on board, is perfectly acceptable if the fuel weight is known, since the moments are a known factor.

As for the weighing itself, almost a science, e.g. most people don’t even recognize the need to relieve the side loads on the main gear, use bathroom scales, etc.

Re-weight, but don’t be surprised for an even higher empty weight, or mass in today’s world.
 
DC is right,
But I just can't believe your RV gained 86 lbs. Weight it again, tanks empty, with the oil in. If you can find scales.
 
Don't know about that shop scales but accuracy on my aircraft scales are:

Accuracy ± 0.1% of Reading or 0.02% of Load Cell Capacity– Whichever is Greater (Load cell capacity is 1500#)

Basically .5 lb or better.
I think most "aircraft" scales would meet the same accuracy limits.
I think the 42 gal weight subtraction is ok when it comes to weight calculation, but I don't like doing it that way for CG reasons mostly.
 
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It is not uncommon to weigh an aircraft full fuel, if you have a good fuel capacity for the tanks. I doubt that fuel density or qty errors would make up your weight gain.

Do you have a good equipment list for the previous weighing and the new weighing? Differences in weighed configuration can result in otherwise unexpected results.

If they used certified scales, typically worse case is .1% full scale, so if the scales are rated at say 5000 lbs per scale, that could represent 5.0 lbs per scale, still not that large of an error.
 
Considering your sig shows you purchased the plane----you have no idea how accurate the original weighing was.

I would suggest you redo things with good scales, and use that number.
 
CG location

Just got to the point in the build where I am about to finish balancing the surfaces after painting and get a final weight and balance empty weight and CG. While waiting I built a contour map of the CG locations for a range of loading conditions based on where I hope the empty weight and CG fall out. So, some observation that are pretty obvious looking at the data. Fuel, pilot/passenger and baggage all load the CG aft. The aircraft empty CG needs to be as far forward as will allow the minimum weight flight crew, no fuel and no baggage to still be within the forward CG flight limitation while staying within the nosewheel weight limitation (RV-7A). (Shouldn't be a problem). Clearly it is very easy to be in a situation taking off with full baggage, fuel restricted to meet max gross limit and at max gross with the CG well in range. However, the landing with low fuel will likely result in out of limit or on limit aft CG due to the minimum fuel condition.
Given the importance of getting the empty CG location accurately known I would be inclined to drain the fuel and redo the empty weight and balance. Maybe do a couple of intermediate points as well just to get a more accurate moment for less than full tanks.
A higher empty weight and a heavier pilot certainly puts some limitations on range and CG that need to be carefully evaluated and understood.
KT
 
I used the local EAA chapter’s UNcertified scales for my initial Airworthiness Inspection. 1213 lbs. Then got it painted. Then weighed it by local A&P with CERTIFIED scales. 1180 lbs. 33 lbs LESS, with paint.
I suggest u get it weighed using certified scales, recently calibrated. I did it with FULL tanks. EZ to deduct the weight of 42 gallons. And I know that’s what they hold.
 
What RV model are you talking about?

All RV’s except for the 12 have a shift of the C.G. aft-ward as fuel is used.

Scott, I don’t think that is correct. On the -10, it is true for most loadings. But if the CG is very close to the forward limit, then, as you burn fuel, it moves more forward. So, for the -10, you need to check the weight and weight limit prior to takeoff; but you need to calculate and check the cg and limits for the plane in its landing configuration.
 
You had a great results after painting. One thing I noted when looking at logbooks for possible aircraft purchase is a lot of experimental aircraft are weighed before painting and not weighed again after. Paint can be heavier than many think.
 
CG position clarification

What RV model are you talking about?

All RV’s except for the 12 have a shift of the C.G. aft-ward as fuel is used.

Correct- with pilot weight and/or baggage the CG will move aft as fuel is reduced since the moments from pilots and baggage now have more influence. The point I was trying to make was that the empty airplane (with just engine oil and brake fluid) CG was forward of any other condition so getting the empty airplane CG as far forward as practical with the nose gear weight restriction in mind has some potential benefit.
The attached back of the envelope plot shows a number of conditions to get a feel for where CG moves as a function of fuel,pilot weight and baggage for one likely empty weight CG position. When I get the final weight and empty CG (after paint) I will put together a cleaned up “pretty” version of this to put in the POM.
BTW - rebalancing the flight control surfaces after painting is equally important as getting the final weights and moments.
KT
 

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I used the local EAA chapter’s UNcertified scales for my initial Airworthiness Inspection. 1213 lbs. Then got it painted. Then weighed it by local A&P with CERTIFIED scales. 1180 lbs. 33 lbs LESS, with paint.
I suggest u get it weighed using certified scales, recently calibrated. I did it with FULL tanks. EZ to deduct the weight of 42 gallons. And I know that’s what they hold.

I’ve always had my 170 weighed with full fuel, so there’s no guessing about what the amount of unusable fuel is, and I KNOW what it weighs full. That’s OK in that airplane, as the tanks are rectangular and flat-bottomed so the the arm of the fuel doesn’t change as it’s burned. As Scott M. once mentioned, the ARM of the fuel in the tanks of the wet leading edge RV tanks moves aft as fuel is burned. I think it might be a good idea while the airplane is on the scales to weigh it at various fuel loadings from empty to full and see just how much the arm moves.
 
I had a similar experience with my -8. The only W&B data available when I bought it last year said the original total empty weight was 1175. (200HP IO360, CS prop, etc. etc.) Since then the aircraft was painted with a beautiful multi-layer clearcoat surface, a nice interior installed, avionics updated, all with no log entries. I had it weighed last summer, guess what, it's now 1275 empty. Ouch. Still performs like an RV though.

It's important to weigh the plane in its level flight attitude so you get accurate numbers for each wheel, especially a tail dragon, which for mine meant lifting the tail about 3 feet and putting the tailwheel scale on a table. While you have it in that attitude is also a great time to recalibrate fuel gauges.
 
IIt's important to weigh the plane in its level flight attitude so you get accurate numbers for each wheel, especially a tail dragon, which for mine meant lifting the tail about 3 feet and putting the tailwheel scale on a table. While you have it in that attitude is also a great time to recalibrate fuel gauges.
You never need to weigh a tailwheel when it's off the ground.

Weigh the aircraft in 3-point attitude and total all three scales. That's your total weight.

Lift the tailwheel to flying attitude, and read the scales at the two main wheels again. Record the readings, add together, and subtract from your total weight, and you have the tailwheel weight.
 
Scott, I don’t think that is correct. On the -10, it is true for most loadings. But if the CG is very close to the forward limit, then, as you burn fuel, it moves more forward. So, for the -10, you need to check the weight and weight limit prior to takeoff; but you need to calculate and check the cg and limits for the plane in its landing configuration.

You are correct but this is only true for a very much atypical load condition.

It is not true if the airplane is being used for its designed purpose... transporting people and baggage.
In the rare case where it is true, the mean arm position of the fuel, and the "as loaded" C.G. are very close to being the same, so the reverse C.G. shift influence is small but can exist.

So to clarify, in a typical load situation, an RV-10 will have the C.G. shift aft with the usage of fuel just like the other 2 seat RV's, but in an unusual case it can do the opposite.
 
Correct- with pilot weight and/or baggage the CG will move aft as fuel is reduced since the moments from pilots and baggage now have more influence. The point I was trying to make was that the empty airplane (with just engine oil and brake fluid) CG was forward of any other condition so getting the empty airplane CG as far forward as practical with the nose gear weight restriction in mind has some potential benefit.

I agree but the level of benefit depends on the primary mission usage goals for the airplane.

Empty C.G. as far fwd. as practical is great for broad utility in the context of aircraft loading and baggage hauling capability, but it will have a somewhat negative impact on the fun to fly aspect of the airplane.

I.E., if aerobatics and fun to fly handing were higher on the desires list, an empty C.G. that resulted in a C.G. that landed more in the middle of the allowable range when loaded with partial fuel and a moderate pilot passenger load, would be a better choice.
 
Choosing CG location

I agree but the level of benefit depends on the primary mission usage goals for the airplane.

Empty C.G. as far fwd. as practical is great for broad utility in the context of aircraft loading and baggage hauling capability, but it will have a somewhat negative impact on the fun to fly aspect of the airplane.

I.E., if aerobatics and fun to fly handing were higher on the desires list, an empty C.G. that resulted in a C.G. that landed more in the middle of the allowable range when loaded with partial fuel and a moderate pilot passenger load, would be a better choice.

Agreed. When I owned an ASW19 sailplane I arranged the CG to be close to the aft limit to maximize soaring performance when I flew it. When my son flew it I made sure he had the CG closer to the forward limit to provide better handling and spin performance. Knowing the flavor and effects of the poison is always good.
KT
 
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