Want Best Technique for Accurate Weight & Balance

[Nedimbek]

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.

[JimWoo50]

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.

[Walt]

Originally Posted by Nedimbek

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"

[Nedimbek]

Originally Posted by Walt

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!
This discussion is just a practice to brain in order to clarify misconseptions.

[Walt]

Originally Posted by Nedimbek

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!
This discussion is just a practice to brain in order to clarify misconseptions.

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.

[az_gila]

Originally Posted by Walt

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...

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.

[Nedimbek]

Originally Posted by Walt

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.

[az_gila]

Originally Posted by Nedimbek

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.

[drone_pilot]

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?

[RV7A Flyer]

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.