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Want Best Technique for Accurate Weight & Balance

drone_pilot

Well Known Member
When it comes to technique in performing Weight and Balance on my RV-7A, I have a few questions, while I wait for my scales to arrive....

A) Is it acceptable practice to take the arm measurements to the nose wheel and each main wheel prior to putting the airplane onto the scales?

I feel I can get more accurate distance measurements off of the scales with the aircraft in a natural stance on the floor but that creates a new question....

B) The aircraft has to be perfectly level while on the scales but normally sits slightly nose high in the natural stance on the floor. Will the datum to wheel measurements change significantly with the airplane sitting perfectly level?

I will have each wheel on its own scale. The scales are 3" high so I thought I'd construct looooong wooden ramps to roll the airplane onto the scales as opposed to jacking it up and letting it down on the scale. I feel rolling it onto the scale is better to prevent side loading of the scale. I also feel that the arm measurements taken prior to rolling on the scale will remain fairly static if I can roll the airplane as opposed to jacking it up.

My plan is to roll it onto the scales and check for level. If it is out of level, roll airplane off of scales, add shims under appropriate scales and repeat until I can achieve a level stance.

Is my thinking okay on this? Any way to simplify? Are there better techniques or more steps that I should consider?
 
For the measuring only bit you can level the plane with the nose wheel removed.

I found this, and a minor tire pressure adjustment in the mains, got my -6A level. With no scale platforms in the way it was much easier to drop plumb bobs and use squares to translate the axle positions to marks on the floor.

For the final weighing (not yet done) I can now use my wing jacks and lift the plane and just use blocks under the main wheels to remove the approx. 5 degree aft down stance of the -6A.

I did find the construction manual dimensions to be a bit off.

http://www.vansairforce.com/community/showthread.php?t=142975
 
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...

B) The aircraft has to be perfectly level while on the scales but normally sits slightly nose high in the natural stance on the floor. Will the datum to wheel measurements change significantly with the airplane sitting perfectly level?

...

The datum is defined and correct only while the plane is exactly level.

For the 5 degree longeron slope I mentioned in the previous post the wing leading edge plumb bob mark on the floor would have an error of around 2 inches.
 
The datum is defined and correct only while the plane is exactly level.

For the 5 degree longeron slope I mentioned in the previous post the wing leading edge plumb bob mark on the floor would have an error of around 2 inches.

Typically you have to raise the mains about 2 inches to level a 7A model. Assuming the NLG stays in the same position, I'm not a math major but I guarantee you that raising the mains 2" does not move the wing LE (or datum) anywhere near 2".
 
Typically you have to raise the mains about 2 inches to level a 7A model. Assuming the NLG stays in the same position, I'm not a math major but I guarantee you that raising the mains 2" does not move the wing LE (or datum) anywhere near 2".

I think it does...:)

Not 2 inches on the floor, but an effective 2 inches.

If the fuselage is sloped, then the real LE datum would be where a 5 degree sloped plumb bob would fall - if gravity could be defied....:)

It would be a major error in the datum line measurement.
 
I think it does...:)

Not 2 inches on the floor, but an effective 2 inches.

If the fuselage is sloped, then the real LE datum would be where a 5 degree sloped plumb bob would fall - if gravity could be defied....:)

It would be a major error in the datum line measurement.

Excuse my ignorance but what it the world is "an effective 2 inches" ??
Is this some kind of virtual measurement you are talking about?

We use plumb bobs to make marks on the floor and then take our ARM measurements from there, I'm not aware of any other method to measure ARMS.

From the FAA W&B hand book the method is:

Determining the Center of Gravity
When the aircraft is in its level flight attitude, drop a
plumb line from the datum and make a mark on the hangar
floor below the tip of the bob. Draw a chalk line through
this point parallel to the longitudinal axis of the aircraft.
Then draw lateral lines between the actual weighting
points for the main wheels, and make a mark along the
longitudinal line at the weighing point for the nose wheel
or the tail wheel. These lines and marks on the floor allow
you to make accurate measurements between the datum
and the weighting points to determine their arms.
 
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Excuse my ignorance but what it the world is "an effective 2 inches" ??
Is this some kind of virtual measurement you are talking about?

We use plumb bobs to make marks on the floor and then take our ARM measurements from there, I'm not aware of any other method to measure ARMS.

From the FAA W&B hand book the method is:

Determining the Center of Gravity
When the aircraft is in its level flight attitude, drop a
plumb line from the datum and make a mark on the hangar
floor below the tip of the bob. Draw a chalk line through
this point parallel to the longitudinal axis of the aircraft.
Then draw lateral lines between the actual weighting
points for the main wheels, and make a mark along the
longitudinal line at the weighing point for the nose wheel
or the tail wheel. These lines and marks on the floor allow
you to make accurate measurements between the datum
and the weighting points to determine their arms.

This is the way I have done it. Only reason Plumb Bobs are in my box.
 
Excuse my ignorance but what it the world is "an effective 2 inches" ??
Is this some kind of virtual measurement you are talking about?

We use plumb bobs to make marks on the floor and then take our ARM measurements from there, I'm not aware of any other method to measure ARMS.

From the FAA W&B hand book the method is:

Determining the Center of Gravity
When the aircraft is in its level flight attitude, drop a
plumb line from the datum and make a mark on the hangar
floor below the tip of the bob. Draw a chalk line through
this point parallel to the longitudinal axis of the aircraft.
Then draw lateral lines between the actual weighting
points for the main wheels, and make a mark along the
longitudinal line at the weighing point for the nose wheel
or the tail wheel. These lines and marks on the floor allow
you to make accurate measurements between the datum
and the weighting points to determine their arms.
i

Walt, that is the correct way.

If you check my post I was trying to explain the errors that would occur if the plane is NOT leveled fore and aft.

The approx. 5 degree slope of a -6A would give a very significant error if it was not leveled. The plump Bob mark from a non-leveled LE would be about 2 inches off.
 
i

The approx. 5 degree slope of a -6A would give a very significant error if it was not leveled. The plump Bob mark from a non-leveled LE would be about 2 inches off.

I know the 6A has a little bit more deck angle than the 7A sitting on the floor. I have done the 7A both ways (level and not level) and found the change in datum to be negligible, certainly no where near 2", so I can't imagine the 6A being that much different is all.
 
I think what he is trying to say is that if you think of the plane level and the string for the plumb bob is perpendicular to the cord of the wing, if you rotate the wing 5 deg and the string were to remain perpendicular to the cord, the point that it would touch would change by more than 2 inches. Probably 5 inches or something.
 
Make sure the tires are centered on the scales, both fore and aft as well as side to side. Maybe if I had a better set of scales it wouldn't be critical, but I found this can make a significant difference in the readouts.
 
I think what he is trying to say is that if you think of the plane level and the string for the plumb bob is perpendicular to the cord of the wing, if you rotate the wing 5 deg and the string were to remain perpendicular to the cord, the point that it would touch would change by more than 2 inches. Probably 5 inches or something.

I can see that, but that's not how it works in real life.
 
I think what he is trying to say is that if you think of the plane level and the string for the plumb bob is perpendicular to the cord of the wing, if you rotate the wing 5 deg and the string were to remain perpendicular to the cord, the point that it would touch would change by more than 2 inches. Probably 5 inches or something.

Exactly..... I simply estimated the -6A LE at 24 inches above the floor and that gives about a 2 inch error.

In reality the error would be affected by the vertical heights of the C of G and would be less, but still measurable.

Just for fun I'll do it both ways when I do my final W & B. :)
 
Dusting off some math cobwebs here... using good ole Pythagoras theorem to find the difference in the datum arm, for the 6A raising the mains 3" would cause the datum to move from 60" to 59.93", less than a tenth of an inch.

According to those same calculation raising the 7A MLG 2" would cause the datum to move from 70" to 69.97"

Someone please correct me if my math looks incorrect.
 
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if you jack the plane up and set it on the scales instead of rolling up on them, you may be putting a lot of side load on the scales when the gear wants to load and flex outward, a side load on some scales may be inaccurate?
 
Exactly..... I simply estimated the -6A LE at 24 inches above the floor and that gives about a 2 inch error.

In reality the error would be affected by the vertical heights of the C of G and would be less, but still measurable.

Just for fun I'll do it both ways when I do my final W & B. :)

Take into account the plumb bob suspended from the spinner tip will be at the same "wrong angle" if airplane not leveled, which helps reduce the overall arm measurement error somewhat, but by no means completely.
 
Using my sheetmetal calculator.

A 24" run the (vertical part) and 5 degree pitch will give you a rise (the horizontal part) of (2") inches. This would be the difference where the plumb bob lands, plane being level vs.5 degree tail low. Im in Gils camp.:)
 
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Hello all,

In my math this trigonometry is not that simple. The plum bob will always be perpendicular to a level surface. What matters is: how much the leading edge, axle centers etc move forward when you raise the mains to compansete 5 degrees off level. Hence, in my calculations that deviation for 100" of arm will be equal to 100-cos5*100=.38". Also remember your datum will shift as well as your front wheel and everything else. Therefore the error is not anywhere close to 2 inches but still needs to be taken into account if you dont want a very rough cg calculation with an error of like +/- 3/8 of an inch.
After all if you are putting your life in risk, you will definitely want to do it accurately, complied with the recommended proper procedure IMHO.
Another heads up: An "arm" measurement on a non-level floor ( with some slope for drainage) also has a similar effect but much subtle! Here we go again!:D

Nedim
 
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Also remember that your DAR is not gonna accept your W/B with such discrepancy ( i.e doing it without leveling your airframe) unless you lie and say you did it properly!

Happy thankgiving.
 
I'm reweighing now.

Very timely for you to start this thread as I am now reweighing my 0-320 powered fixed pitch prop 6 for the first time since being painted. To answer one of your questions IMO it would be preferential to roll the plane onto the scales from a ramp; I tried it but the ramp was not secured to the floor well and slid away. So I just jacked it up one main at a time and slid scales under the tire. Lifted the tail by hand onto a table and leveled it there.

My main gear numbers were pretty close to the pre-paint numbers but my tail wheel weight increased by 20lbs.! Doh! This had the effect of moving my empty cg aft by 3.75 in. This put me VERY close to aft limit when carrying a pax in most flight conditions. I think the fact that my pre-paint cg was 69.12 helped me avoid being aft of the limit.

Tomorrow I will move the elt from the shelf under the empennage fairing to the area up against the firewall-where the battery should be in the original design. That moves about 3 lbs forward which alleviates about .75 in of my aft cg situation. The only minor problem is where to remount the antenna? I would like to avoid making another hole in the forward fuselage near the firewall. Can the antenna be mounted inside the plane behind the instrument panel? Horizontally?

For the rest of the weight transfer; I have access to a old scuba divers belt-2 of the actually-one of which is a sealed canvas unit weighing about 4 pounds. I am considering wrapping it in heat proof tape and securing it to the top of my engine just forward of the lifting eye. Laying it flat laterally across the engine case. Would not interfere with airflow.

Another idea is to fabricate another smaller lifting eye and securing a length of threaded rod between them to which I could attach lead weights. I'm not real sold on this idea but it would allow me to vary the weights.

One more question. Should I adjust the cg to be as far forward as possible? Knowing that under any loading scenario the cg will always move aft? Thanks. Jim.
 
Hello all,

Hence, in my calculations that deviation for 100" of arm will be equal to 100-cos5*100=.038". Also remember your datum will shift as well as your front wheel and everything else. Therefore the error is not anywhere close to 2 inches but still needs to be taken into account if you dont want a very rough cg calculation with an error of like +/- 3/8 of an inch.

Nedim

I think you meant .38" not .038"

Using your formula but substituting 60" (RV6A datum) would yield and error of 0.24" for a 5 deg angle.

The 7A has less of a deck angle change and a longer arm so if we assume 3 deg with the 70 arm that would yield an error of 0.1"
 
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0.038" equals about 1/25", not 3/8".

That was a typo. Sorry, my bad!

Actually there is more into it. As you try to level an A model, you shim the mains w.r.t. Front wheel, front wheel being the pivot, therefore effective arm that shifts the measurements is the distance between front and main wheels. One has to do a geometrical analysis to properly model the geometry and calculate the exact deviations however it is nowhere close to 2 inches. It will be very wise to do it properly using the standart procedure. Bottom line; you just level your airframe properly and do your w/b and take no chances. It shouldn't be that hard! :D
This discussion is just a practice to brain in order to clarify misconseptions. :D
 
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That was a typo. Sorry, my bad!

Actually there is more into it. As you try to level an A model, you shim the mains w.r.t. Front wheel, front wheel being the pivot, therefore effective arm that shifts the measurements is the distance between front and main wheels. One has to do a geometrical analysis to properly model the geometry and calculate the exact deviations however it is nowhere close to 2 inches. It will be very wise to do it properly using the standart procedure. Bottom line; you just level your airframe properly and do your w/b and take no chances. It shouldn't be that hard! :D
This discussion is just a practice to brain in order to clarify misconseptions. :D

We agree, the trouble is practically speaking, its more difficult to level the aircraft and then try to get the center lines of the mains and nose accurately transcribed to the floor, so there is more room for error using that method.

With the tires on the floor you can get a pretty accurate centerline of the wheel marked on the floor but that's not so easy when the mains and nose are up on scales.

Which one provides more accuracy is hard to say, not hard to be off a few tenths of inch no matter which way you do it! The good news is these small errors have very little effect on CG.

I think my previous calculations, which come close to your method, may be more accurate only because we are guessing at the angle change while I know that raising the MLG of a 7A 2" levels the aircraft. Not positive on the 6A but I'm pretty sure it's no more than 3" up on the MLG to level.
 
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We agree, the trouble is practically speaking, its more difficult to level the aircraft and then try to get the center lines of the mains and nose accurately transcribed to the floor, so there is more room for error using that method.

With the tires on the floor you can get a pretty accurate centerline of the wheel marked on the floor but that's not so easy when the mains and nose are up on scales.

.......

Which brings us full circle back to my first post...:D

Level the plane off the scales by removing the nose wheel just for the measurement step. In the case of my -6A it got it almost exact. The axle hole becomes the measurement point.


As a separate subject....

I managed to borrow some old race car scales from a local builder that I were described as inoperable.

I took the case part and found that it had a 12+ year old NiCad internal battery. It was a standard RC receiver type, and a replacement got the scales fully operational. If you hear of any local old failed race car scales, check them out.
 
I think my previous calculations, which come close to your method, may be more accurate only because we are guessing at the angle change while I know that raising the MLG of a 7A 2" levels the aircraft. Not positive on the 6A but I'm pretty sure it's no more than 3" up on the MLG to level.

You're right, 5 degrees requires to lift the mains about 4" which is a little too much. A 7A usually levels out in 2" which correspond a little more than 2 degrees and a 6A should not be much different. Therefore a rough estimate of the deviation is within 1/10th of an inch.

With all of the above, the answer of the OP's question will be no.
 
You're right, 5 degrees requires to lift the mains about 4" which is a little too much. A 7A usually levels out in 2" which correspond a little more than 2 degrees and a 6A should not be much different. Therefore a rough estimate of the deviation is within 1/10th of an inch.

With all of the above, the answer of the OP's question will be no.

My -6A measures at 5.6 degrees just sitting on the flat hangar floor.

MORE

Thinking about this whole issue over a few Thanksgiving beers and Walts comment on the 2 inches at the floor marking getting sort of cancelled out and not much error added to the final C of G location....

While that is probably true on the CG actual location the big error from that 2 inch difference will come in the position of where you mark the RV datum line 60 inches ahead of the wing LE.

With an incorrect datum line, all of the moment arms given in the plans will be off by that 2 inch error, and all future W and B calculations will be off... And off in the "bad" direction, giving you an optimistic location (more forward) of the C of G.
 
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Okay. The post about removing the nose wheel and measuring the arms with the airplane leveled on the floor makes sense assuming I cannot adjust air pressure enough in the tires to get to level. I would also take from this that the airplane needs leveled on the scales or the nose wheel weight will be off if the airplane is not leveled identical to what it was on the floor. (Yes, I'm assuming to put the nose wheel and fairings back on the airplane before putting it on the scales).

Now thinking more about this, if I had a way to jack the airplane up at the mains, I may be able to prevent side loading of the scales vs building ramps. This is because tension would be held on the mains while raising the aircraft.

Is there a jack point that allows the 7A to be jacked up at the main wheels?
 
I just don't recall this being that hard...roll the plane onto the scales, let a little air out of the nosegear as necessary, bingo bango bongo. Done.

You could always put the scales for the mains up on a couple of pieces of plywood or something to get it even closer to start with.
 
i


The approx. 5 degree slope of a -6A would give a very significant error if it was not leveled. The plump Bob mark from a non-leveled LE would be about 2 inches off.

As the saying goes, a picture is worth a thousand words....

When I was preparing to to my W&B, I ran across this thread and was trying to visualize what was being discussed here. When I did my W&B the concept is quite clear & decided to capture a few pics and upload here.

Gnd= Normal ground attitude (RV-14A)
Flt Level= Level flight attitude (accomplished by deflating nose tire until level)
 

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The error in the errors

Gil was talking about the change in each individual position when changing the stance, as depicted in the post above, and correctly characterized this as “about 2”.

Walt was talking about the change in distance between the reference points between GRD and FLT. Since each point moves in the same direction and by only slightly varying amounts, the relative change is small.

They are both correct, just not talking about the same measurement.
 
Does the engine weight noticeably affect the stance of the mains on the tailwheel models? I have the engine off my -4 right now and was considering going ahead and getting the datum and arm measurements now while the plane is a bit easier to maneuver around.
 
I'll be doing a final end-of-phase-1 tinkering & mods W&B soon, and when I do, I'll carefully re-measure the datums (data?) and report back how different they are in ground and flight attitudes for the RV-10 so that posterity will have this information. I will get to do this a third time after paint/wrap, but that won't change the plumb-bob and tape measure data...

Remember, for those of us who have scales of a thickness that exactly levels the (nose-gear) plane when under the mains alone, there's an easy way to do the weighing in flight attitude that avoids having to put the plane on scales plus blocks while in level attitude.

Weigh plane on all three scale platforms, in ground attitude. Record total weight.

Weigh plane with scales only under the mains (now in flight attitude because of scale thickness) and record the weights on each main wheel.

Subtract the sum of the main wheel weights from the total weight to determine weight on nose wheel in flight attitude.

Calculate W&B.

I didn't figure this out before I had built a pair of wood platforms equal in thickness to the scales in order to jack the mains up way higher than I was liking using an under-wing jack. Won't ever bother using them again! :D
 
Remember, for those of us who have scales of a thickness that exactly levels the (nose-gear) plane when under the mains alone, there's an easy way to do the weighing in flight attitude that avoids having to put the plane on scales plus blocks while in level attitude.

Weigh plane on all three scale platforms, in ground attitude. Record total weight.

Weigh plane with scales only under the mains (now in flight attitude because of scale thickness) and record the weights on each main wheel.

Subtract the sum of the main wheel weights from the total weight to determine weight on nose wheel in flight attitude.

Calculate W&B.

I didn't figure this out before I had built a pair of wood platforms equal in thickness to the scales in order to jack the mains up way higher than I was liking using an under-wing jack. Won't ever bother using them again! :D

Came in here to post this method. And i'll go one further: If your scales are a little thin, you can let air out of the nosewheel to get it down lower.

(and as an aside, for Tailwheel aircraft the same process applies... Just lift the tailwheel off the rear scale until the plane is level and have someone read the mains)

I'll also add that I would be surprised if the datums differed between aircraft builds by enough that your CG measurements would be off in any significant amount. Use the book values and save yourself a lot of measuring and plumb-bobbing. It's a lot more important to get the plane levelled and to get accurate weights.
 
Weigh plane on all three scale platforms, in ground attitude. Record total weight.

Weigh plane with scales only under the mains (now in flight attitude because of scale thickness) and record the weights on each main wheel.

Subtract the sum of the main wheel weights from the total weight to determine weight on nose wheel in flight attitude.

! :D

I think the plane has to be in the same exact attitude with both weighings, otherwise the subtraction is not valid. JMHO
 
Dimension change of 5 degree pivot

Typically you have to raise the mains about 2 inches to level a 7A model. Assuming the NLG stays in the same position, I'm not a math major but I guarantee you that raising the mains 2" does not move the wing LE (or datum) anywhere near 2".

Assuming that the dimension between the nose gear axle to the axle line of the mains is around 36";
A 36" horizontal line pivoted 5 degrees from horizontal; the pivoted line's horizontal plane dimension is 35 7/8".

A slight, somewhat insignificant dimension difference in the horizontal plane distance. Percent change between the two dimensions (36" and 35 7/8") is 0.34722 percent.

There would also be a difference (very slight) between the weights found at the main gear wheels and the nose gear depending if the plane was in level flight configuration or 5 degrees nose high.

The total weight in either leveling configuration surely would be the same; assuming that you are holding your tongue the same way and the wind hasn't shifted too much between weigh-ins.

The differences are small. Depending on everything else going on in the weighing process, the degree of 'insignificance' or confidence is not the same for everyone. I went to some extra lengths to be as accurate with wheel axle distances from the datum and the weigh-in. I started with three $40 Chinese digital scales but ended up having an local IA do the weigh-in with his calibrated and certified scales. I had calibrated the Chinese scales and they were 'close' between each other but the total weight difference compared to the IA's certified scales was about 6 pounds higher.

After investing as much as I have in the 14A, the IA's fee for getting it done with some degree of confidence was chump change. Hey, it's an airplane I'll be flying - sometimes with another person.
 
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