Weight and Balance gut check...
02-03-2012, 09:29 PM
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Weight and Balance gut check...
Guys....I am moving weight around in my flash. Trying to work the movements using math....
What I have done is move my ECU pack aft as the jet was nose heavy. I balanced her perfectly at 225 aft of the wing/fuse joint as indicated by instructions. When I test flew the other day I found her to very nose heavy. We added two ounces of lead weight to the tail as far aft as I could get it. This improved the situation but she is still nose heavy.
So, without moving or removing any weight I put her back on the balance stand to see where she balanced with the additional two ounces on the tail. 228 back from the wing joint...
With that I moved the ECU pack aft as much as i could without a total rework of the install. I then removed the lead from the tail. She now balances @ 228....although if you get it nose high on the balance stand she will fall aft, same with nose low but not as aggressively....
I wanted to work this weight movement mathematically just to confirm my results...
The two ounces of lead I removed was 29 inches aft of the balance point giving it a moment of 58in/oz (2oz*29"=58in/oz)
The Battery WAS located 36 inches forward of the balance point and weighs 12oz giving a moment of 432in/oz (12oz*36"=432in/oz)
I moved the battery aft so it was located 29 inches forward of the balance point giving it a new moment of 348in/oz (12oz*29"=348in/oz)
So if I remove the lead, I loose 58in/oz aft, and if I move the battery to the new location I loose 84in/oz for a net GAIN of 30in/oz forward of the CG. So I would have made the aircraft more nose heavy but will have reduced overall weight by 2oz....correct?
Now I can hear you all saying all that for two ounces???? And, I agree, however I am concerned that I am balancing aft of the recommended balance point so I want to sneak up on this slowly.
What I have done is move my ECU pack aft as the jet was nose heavy. I balanced her perfectly at 225 aft of the wing/fuse joint as indicated by instructions. When I test flew the other day I found her to very nose heavy. We added two ounces of lead weight to the tail as far aft as I could get it. This improved the situation but she is still nose heavy.
So, without moving or removing any weight I put her back on the balance stand to see where she balanced with the additional two ounces on the tail. 228 back from the wing joint...
With that I moved the ECU pack aft as much as i could without a total rework of the install. I then removed the lead from the tail. She now balances @ 228....although if you get it nose high on the balance stand she will fall aft, same with nose low but not as aggressively....
I wanted to work this weight movement mathematically just to confirm my results...
The two ounces of lead I removed was 29 inches aft of the balance point giving it a moment of 58in/oz (2oz*29"=58in/oz)
The Battery WAS located 36 inches forward of the balance point and weighs 12oz giving a moment of 432in/oz (12oz*36"=432in/oz)
I moved the battery aft so it was located 29 inches forward of the balance point giving it a new moment of 348in/oz (12oz*29"=348in/oz)
So if I remove the lead, I loose 58in/oz aft, and if I move the battery to the new location I loose 84in/oz for a net GAIN of 30in/oz forward of the CG. So I would have made the aircraft more nose heavy but will have reduced overall weight by 2oz....correct?
Now I can hear you all saying all that for two ounces???? And, I agree, however I am concerned that I am balancing aft of the recommended balance point so I want to sneak up on this slowly.
02-04-2012, 05:01 AM
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RE: Weight and Balance gut check...
Hi Inbound
Just done some maths using your numbers. I calculate that if you remove the 2oz from the rear, you will need to move the 12oz battery back by 10.83 inches to keep the CG in the same place. As you've only moved it back by 7 inches, your CG will be forward of the position by a bit - Can't tell you by how much without knowing the AUM of the jet. If you let me know the AUM (with the 2oz still in), I can work out where your new CG is.
It's easier to do the calculations using a datum forward of the nose. This way, all the moments work in the same direction, so there are no plus/minus problems.
Cheers - Pat
Just done some maths using your numbers. I calculate that if you remove the 2oz from the rear, you will need to move the 12oz battery back by 10.83 inches to keep the CG in the same place. As you've only moved it back by 7 inches, your CG will be forward of the position by a bit - Can't tell you by how much without knowing the AUM of the jet. If you let me know the AUM (with the 2oz still in), I can work out where your new CG is.
It's easier to do the calculations using a datum forward of the nose. This way, all the moments work in the same direction, so there are no plus/minus problems.
Cheers - Pat
02-04-2012, 05:29 AM
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RE: Weight and Balance gut check...
sounds about right Dave. to make life easier, I always use the tip of the nose as the datum though 
As far as balance point, i was once told by an older gentleman. "put it where you like the way it flies. If its not there yet, keep moving it"
As far as balance point, i was once told by an older gentleman. "put it where you like the way it flies. If its not there yet, keep moving it"
02-04-2012, 06:37 AM
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RE: Weight and Balance gut check...
Dave,
Just don't go too far back like I did. The plane flew absolutely BEAUTIFUL, but it was the hardest plane to land with the aft CG. It was many years ago and I don't know if things have changed, but from my experience, it likes a slightly forward CG.
Just don't go too far back like I did. The plane flew absolutely BEAUTIFUL, but it was the hardest plane to land with the aft CG. It was many years ago and I don't know if things have changed, but from my experience, it likes a slightly forward CG.
02-04-2012, 07:55 AM
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RE: Weight and Balance gut check...
Thanks for the input guys, I know that overall I am still going to be nose heavy....just wanted to make sure my numbers are right....
02-04-2012, 10:41 AM
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RE: Weight and Balance gut check...
Adding moment torque of lead weight = 2 oz * 29 in = 58 oz-in (pitch up)
(sign between "oz & in" in toque is not "minus" either "/" but dash or dot to separate units between the product of distance and force)
Removing lead weight will cause -58 oz-in, pitch down
Battery old moment = -432 oz-in, New = -348 oz-in, Difference = +84 oz-in
Therefore, moving the battery from 36 to 29 will have effect of 84 oz-in, pitch up
Overall net moment torque = -58+84 = 26 oz-in (pitch up) from original balance point
If you want the same effect from balance point (equilibrium) the summation of all forces and moments are zero therefore using simple equation where nose up produce + moment and opposite is negative. Actually it's easier to understand when there's diagram, you can see the distance, force, moment and directions, including variables. Aside from balance point, you can also use other point as reference for moment circle so that you can include distance and the weight of your new CG. However as per your data and OP the following are example that you can use:
From summation of al moments equals zero,
Wo * dw + Wn * dw + B * dbo + B * dbn = 0
where:
Wo = original weight of lead
Wn = new weight of lead
dw = distance to weight and balance point
B = weight of battery
dbo = original distance of battery
dbn = new distance of battery
Transfering B*dbn to the other side
-B*dbn = Wo*dw+Wn*dw+B*dbo
distance of battery new position = dbn = -(Wo*dw+Wn*dw+B*dbo)/B
let's try to use the formula:
Wo = 2
Wn = 0
dw = 29
B = -12
dbo = 36
dbn = - (2*29+0+12*36)/(-12) = 40.8 in
40 inches forward of CG without gaining moment effect while reducing weight of 2-oz.
let's test, battery weight x change in distance = 12 * (40.8-36) = 57.6
Therefore moving the battery 4.8 in backward is equivalent to removing 2-oz loacted at 29 in aft.
(sign between "oz & in" in toque is not "minus" either "/" but dash or dot to separate units between the product of distance and force)
Removing lead weight will cause -58 oz-in, pitch down
Battery old moment = -432 oz-in, New = -348 oz-in, Difference = +84 oz-in
Therefore, moving the battery from 36 to 29 will have effect of 84 oz-in, pitch up
Overall net moment torque = -58+84 = 26 oz-in (pitch up) from original balance point
If you want the same effect from balance point (equilibrium) the summation of all forces and moments are zero therefore using simple equation where nose up produce + moment and opposite is negative. Actually it's easier to understand when there's diagram, you can see the distance, force, moment and directions, including variables. Aside from balance point, you can also use other point as reference for moment circle so that you can include distance and the weight of your new CG. However as per your data and OP the following are example that you can use:
From summation of al moments equals zero,
Wo * dw + Wn * dw + B * dbo + B * dbn = 0
where:
Wo = original weight of lead
Wn = new weight of lead
dw = distance to weight and balance point
B = weight of battery
dbo = original distance of battery
dbn = new distance of battery
Transfering B*dbn to the other side
-B*dbn = Wo*dw+Wn*dw+B*dbo
distance of battery new position = dbn = -(Wo*dw+Wn*dw+B*dbo)/B
let's try to use the formula:
Wo = 2
Wn = 0
dw = 29
B = -12
dbo = 36
dbn = - (2*29+0+12*36)/(-12) = 40.8 in
40 inches forward of CG without gaining moment effect while reducing weight of 2-oz.
let's test, battery weight x change in distance = 12 * (40.8-36) = 57.6
Therefore moving the battery 4.8 in backward is equivalent to removing 2-oz loacted at 29 in aft.
02-04-2012, 04:39 PM
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RE: Weight and Balance gut check...
You got me re-thinking the maths problem, RCTech - I've been re doing my calcs and I agree with you. Moving the Battery aft by 4.8 inches counters the loss of the 2oz at the rear of the jet. Moving the Battery back 7 inches as you've done will make it tail heavy. Let us know where it balances, Inbound! Calcs are great, but there's nothing like a practical measurement! From the 3mm change in CG with 2 oz of lead in the tail, I calculated your jet weighs around 30lbs (30.8lbs was the exact figure I got, but depends how accurate the 3mm measurement was!) - let me know how heavy it really is!
02-04-2012, 09:42 PM
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RE: Weight and Balance gut check...
I initially had mine at the 225mm mark. Moved my Fromeco 5200 four cell from the tip of the nose to behind the UAT.. FLYS SWEET !!! Elevator trim is flush.
02-04-2012, 10:11 PM
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RE: Weight and Balance gut check...
ORIGINAL: AFTERBURNER1
I initially had mine at the 225mm mark. Moved my Fromeco 5200 four cell from the tip of the nose to behind the UAT.. FLYS SWEET !!! Elevator trim is flush.
I initially had mine at the 225mm mark. Moved my Fromeco 5200 four cell from the tip of the nose to behind the UAT.. FLYS SWEET !!! Elevator trim is flush.
02-04-2012, 10:13 PM
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RE: Weight and Balance gut check...
ORIGINAL: rctech2k7
Adding moment torque of lead weight = 2 oz * 29 in = 58 oz-in (pitch up)
(sign between ''oz & in'' in toque is not ''minus'' either ''/'' but dash or dot to separate units between the product of distance and force)
Removing lead weight will cause -58 oz-in, pitch down
Battery old moment = -432 oz-in, New = -348 oz-in, Difference = +84 oz-in
Therefore, moving the battery from 36 to 29 will have effect of 84 oz-in, pitch up
Overall net moment torque = -58+84 = 26 oz-in (pitch up) from original balance point
If you want the same effect from balance point (equilibrium) the summation of all forces and moments are zero therefore using simple equation where nose up produce + moment and opposite is negative. Actually it's easier to understand when there's diagram, you can see the distance, force, moment and directions, including variables. Aside from balance point, you can also use other point as reference for moment circle so that you can include distance and the weight of your new CG. However as per your data and OP the following are example that you can use:
From summation of al moments equals zero,
Wo * dw + Wn * dw + B * dbo + B * dbn = 0
where:
Wo = original weight of lead
Wn = new weight of lead
dw = distance to weight and balance point
B = weight of battery
dbo = original distance of battery
dbn = new distance of battery
Transfering B*dbn to the other side
-B*dbn = Wo*dw+Wn*dw+B*dbo
distance of battery new position = dbn = -(Wo*dw+Wn*dw+B*dbo)/B
let's try to use the formula:
Wo = 2
Wn = 0
dw = 29
B = -12
dbo = 36
dbn = - (2*29+0+12*36)/(-12) = 40.8 in
40 inches forward of CG without gaining moment effect while reducing weight of 2-oz.
let's test, battery weight x change in distance = 12 * (40.8-36) = 57.6
Therefore moving the battery 4.8 in backward is equivalent to removing 2-oz loacted at 29 in aft.
Adding moment torque of lead weight = 2 oz * 29 in = 58 oz-in (pitch up)
(sign between ''oz & in'' in toque is not ''minus'' either ''/'' but dash or dot to separate units between the product of distance and force)
Removing lead weight will cause -58 oz-in, pitch down
Battery old moment = -432 oz-in, New = -348 oz-in, Difference = +84 oz-in
Therefore, moving the battery from 36 to 29 will have effect of 84 oz-in, pitch up
Overall net moment torque = -58+84 = 26 oz-in (pitch up) from original balance point
If you want the same effect from balance point (equilibrium) the summation of all forces and moments are zero therefore using simple equation where nose up produce + moment and opposite is negative. Actually it's easier to understand when there's diagram, you can see the distance, force, moment and directions, including variables. Aside from balance point, you can also use other point as reference for moment circle so that you can include distance and the weight of your new CG. However as per your data and OP the following are example that you can use:
From summation of al moments equals zero,
Wo * dw + Wn * dw + B * dbo + B * dbn = 0
where:
Wo = original weight of lead
Wn = new weight of lead
dw = distance to weight and balance point
B = weight of battery
dbo = original distance of battery
dbn = new distance of battery
Transfering B*dbn to the other side
-B*dbn = Wo*dw+Wn*dw+B*dbo
distance of battery new position = dbn = -(Wo*dw+Wn*dw+B*dbo)/B
let's try to use the formula:
Wo = 2
Wn = 0
dw = 29
B = -12
dbo = 36
dbn = - (2*29+0+12*36)/(-12) = 40.8 in
40 inches forward of CG without gaining moment effect while reducing weight of 2-oz.
let's test, battery weight x change in distance = 12 * (40.8-36) = 57.6
Therefore moving the battery 4.8 in backward is equivalent to removing 2-oz loacted at 29 in aft.
You beat me to it.....I was JUST getting ready to type all this. I have recently taken up Greek and was going to write this same essay. In english, the above says (rough translation) "You're fine, Dave."
C
02-04-2012, 11:44 PM
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RE: Weight and Balance gut check...
ORIGINAL: InboundLZ
Where do you think your balance point is now?
ORIGINAL: AFTERBURNER1
I initially had mine at the 225mm mark. Moved my Fromeco 5200 four cell from the tip of the nose to behind the UAT.. FLYS SWEET !!! Elevator trim is flush.
I initially had mine at the 225mm mark. Moved my Fromeco 5200 four cell from the tip of the nose to behind the UAT.. FLYS SWEET !!! Elevator trim is flush.
02-05-2012, 07:07 AM
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RE: Weight and Balance gut check...
I understand that, that's why it's easier to put it on drawing, I only used sign and direction because numbers may look complicated, so I test it at the end. With the latest post this is becoming more interesting and provide us jet owners how easy to work with CG and balancing since most jets are huge and heavy...
Now, I give you the chance or anyone to calculate the location of new CG and also the weight of the jet using the above data provided by the author on the original post. I know you can, just let me know if you need help. Pat also has good idea in calc becasue he came up in the same decimals and also weight but I don't know what method did he used.
Now, I give you the chance or anyone to calculate the location of new CG and also the weight of the jet using the above data provided by the author on the original post. I know you can, just let me know if you need help. Pat also has good idea in calc becasue he came up in the same decimals and also weight but I don't know what method did he used.
02-06-2012, 04:51 AM
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RE: Weight and Balance gut check...
ORIGINAL: InboundLZ
What I have done is move my ECU pack aft as the jet was nose heavy. I balanced her perfectly at 225 aft of the wing/fuse joint as indicated by instructions.
What I have done is move my ECU pack aft as the jet was nose heavy. I balanced her perfectly at 225 aft of the wing/fuse joint as indicated by instructions.
W1 = weight of the jet w/o battery (ECU pack)
B = weight of the battery = 12 oz
Wt1 = total weight of the jet = W1 + B
02-06-2012, 05:05 AM
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RE: Weight and Balance gut check...
The jet is sitting on a balancer somewhere between W1 and ECU pack. The distance from the balancer to the battery is 36".
The distance of W1 from the balance pt we call it dw1, we can get equation from there. W1 * dw1 = 12 * 36
ORIGINAL: InboundLZ
The Battery WAS located 36 inches forward of the balance point and weighs 12oz giving a moment of 432in/oz (12oz*36''=432in/oz)
The Battery WAS located 36 inches forward of the balance point and weighs 12oz giving a moment of 432in/oz (12oz*36''=432in/oz)
02-06-2012, 05:16 AM
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RE: Weight and Balance gut check...
The second CG is located at 228 mm or 8.98" with added 2-oz at 29" aft.
ORIGINAL: InboundLZ
We added two ounces of lead weight to the tail as far aft as I could get it. This improved the situation but she is still nose heavy.
So, without moving or removing any weight I put her back on the balance stand to see where she balanced with the additional two ounces on the tail. 228 back from the wing joint...
We added two ounces of lead weight to the tail as far aft as I could get it. This improved the situation but she is still nose heavy.
So, without moving or removing any weight I put her back on the balance stand to see where she balanced with the additional two ounces on the tail. 228 back from the wing joint...
02-06-2012, 06:54 AM
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RE: Weight and Balance gut check...
The difference between CG1 and CG2 is 0.12” therefore the distance between existing W1 and 2nd balance point (BL2) is dw1-0.12”. The second distance between BL2 and present location of battery is 36.12”.
