Effects of moving weight fore and aft of CG?
10-19-2004 | 01:50 AM
#1
Thread Starter
Senior Member
Effects of moving weight fore and aft of CG?
I want to do some moderate deviations from the standard setup on my Slow Stick. Before I do this, I’d like to have a general idea of how it will work out. (I could just take everything apart and see if it works, but I’d rather spend 5 minutes with a calculator, if I can.)
How do you calculate balancing a ‘scale’ using more than one weight on each side of the scale with the weights at different lengths from the pivot point? And can I use this formula to give me a general idea of whether my deviations are feasible?
Secondly, how will moving the weight away from the CG effect the way an airplane flies? So rather than the heavier stuff located almost under the CG, it will be moved fore and aft. I’m thinking the plane may be slower to react and to stop when it is moving about the point of balance, but also may make it more stable at the same time.
The main battery weighs about 3 oz, the two servos about 0.3 oz each and the second pack will weigh in at about 1.5oz. I plan on moving the main pack about 6 – 8 inches forward, and the servos as far back as I can with the second pack under the wing.
Thanks
How do you calculate balancing a ‘scale’ using more than one weight on each side of the scale with the weights at different lengths from the pivot point? And can I use this formula to give me a general idea of whether my deviations are feasible?
Secondly, how will moving the weight away from the CG effect the way an airplane flies? So rather than the heavier stuff located almost under the CG, it will be moved fore and aft. I’m thinking the plane may be slower to react and to stop when it is moving about the point of balance, but also may make it more stable at the same time.
The main battery weighs about 3 oz, the two servos about 0.3 oz each and the second pack will weigh in at about 1.5oz. I plan on moving the main pack about 6 – 8 inches forward, and the servos as far back as I can with the second pack under the wing.
Thanks
10-19-2004 | 11:52 AM
#2
Member
Joined: Jul 2004
Posts: 39
Likes: 0
Received 0 Likes
on
0 Posts
From: Carmarthen, UNITED KINGDOM
RE: Effects of moving weight fore and aft of CG?
There is a simple formula. tage the existing cg as the center of all measurements.
so, using the formula weight(W)xdistance(d)=force you can work out how far back to mowe the servo st rebalance the plane. for example, if you move the pack by 6in then the force to balance = 18oz. so the servos will have to move a very long way back, 30in to be precise!! If you move the second pack back by 6in the servos need only move by 15in. By moving the masses away from the cg the 'plane will be slower to respond and also slower to stop moving.
Hope this helps, Tom
so, using the formula weight(W)xdistance(d)=force you can work out how far back to mowe the servo st rebalance the plane. for example, if you move the pack by 6in then the force to balance = 18oz. so the servos will have to move a very long way back, 30in to be precise!! If you move the second pack back by 6in the servos need only move by 15in. By moving the masses away from the cg the 'plane will be slower to respond and also slower to stop moving.
Hope this helps, Tom
10-19-2004 | 01:06 PM
#3
Senior Member
RE: Effects of moving weight fore and aft of CG?
ORIGINAL: Time Pilot
I want to do some moderate deviations from the standard setup on my Slow Stick. Before I do this, I’d like to have a general idea of how it will work out. (I could just take everything apart and see if it works, but I’d rather spend 5 minutes with a calculator, if I can.)
How do you calculate balancing a ‘scale’ using more than one weight on each side of the scale with the weights at different lengths from the pivot point? And can I use this formula to give me a general idea of whether my deviations are feasible?
Secondly, how will moving the weight away from the CG effect the way an airplane flies? So rather than the heavier stuff located almost under the CG, it will be moved fore and aft. I’m thinking the plane may be slower to react and to stop when it is moving about the point of balance, but also may make it more stable at the same time.
The main battery weighs about 3 oz, the two servos about 0.3 oz each and the second pack will weigh in at about 1.5oz. I plan on moving the main pack about 6 – 8 inches forward, and the servos as far back as I can with the second pack under the wing.
Thanks
I want to do some moderate deviations from the standard setup on my Slow Stick. Before I do this, I’d like to have a general idea of how it will work out. (I could just take everything apart and see if it works, but I’d rather spend 5 minutes with a calculator, if I can.)
How do you calculate balancing a ‘scale’ using more than one weight on each side of the scale with the weights at different lengths from the pivot point? And can I use this formula to give me a general idea of whether my deviations are feasible?
Secondly, how will moving the weight away from the CG effect the way an airplane flies? So rather than the heavier stuff located almost under the CG, it will be moved fore and aft. I’m thinking the plane may be slower to react and to stop when it is moving about the point of balance, but also may make it more stable at the same time.
The main battery weighs about 3 oz, the two servos about 0.3 oz each and the second pack will weigh in at about 1.5oz. I plan on moving the main pack about 6 – 8 inches forward, and the servos as far back as I can with the second pack under the wing.
Thanks
Moving the weight "away from the c.g.", while maintaining the same c.g., will make the plane a bit less eager to respond to controls, and after responding return to stable flight more slowly. Keeping the massy stuff near the c.g. relieves the load on the plane to maintain stability.
10-19-2004 | 02:52 PM
#4
RE: Effects of moving weight fore and aft of CG?
ORIGINAL: Time Pilot
Secondly, how will moving the weight away from the CG effect the way an airplane flies? So rather than the heavier stuff located almost under the CG, it will be moved fore and aft. I’m thinking the plane may be slower to react and to stop when it is moving about the point of balance, but also may make it more stable at the same time.
Secondly, how will moving the weight away from the CG effect the way an airplane flies? So rather than the heavier stuff located almost under the CG, it will be moved fore and aft. I’m thinking the plane may be slower to react and to stop when it is moving about the point of balance, but also may make it more stable at the same time.
I like to do this to make the plane "look" more stable in the air- note that not only will the plane react more sluggishly to control inputs but also external influences, such as bumps in the air. This is good for scale looking flight, not good for aerobatics. You can, by the way, also add weight to each wingtip (if you can tolerate the overall weight increase) to increase the lateral moment of inertia if the plane gets bumped around the roll axis too much for your desired flying appearance.
Also, if you have a plane that is marginally stable, increasing the moment of inertia will exacerbate the stability issue (although the stability itself does not change), as you have to fight the controls a bit more as the plane's reaction lags the input a bit.
10-19-2004 | 06:42 PM
#5
Senior Member
RE: Effects of moving weight fore and aft of CG?
I think when you are figuring moments of inertia the distance is squared, whereas it is not when figuring torque. Sometimes referred to as the 'dumbbell effect'.
Jim
Jim
10-20-2004 | 01:06 AM
#6
Thread Starter
Senior Member
RE: Effects of moving weight fore and aft of CG?
Jim,
Are you suggesting I need a different formula than what Sergeant suggested?
If not, an easy calculation. To complicate matters a bit though, I've rebuilt the tail from 1/8" balsa stick and it seems heavy compared to the foam. How much, I'll find out once I'm finished. I should have asked some of the builders here how to do it more lightly
. I love the signature of one of RCU's members: Hindsight is always 20/20. That's why they don't make glasses for your butt. Anyway, nice to have that big battery pack to rely on to counter some of that weight.
I'm turning my Slow Stick into a night flyer, so that any air bumps are flattened out will be a bonus.
Thanks all for the advice.
Are you suggesting I need a different formula than what Sergeant suggested?
If not, an easy calculation. To complicate matters a bit though, I've rebuilt the tail from 1/8" balsa stick and it seems heavy compared to the foam. How much, I'll find out once I'm finished. I should have asked some of the builders here how to do it more lightly
. I love the signature of one of RCU's members: Hindsight is always 20/20. That's why they don't make glasses for your butt. Anyway, nice to have that big battery pack to rely on to counter some of that weight.I'm turning my Slow Stick into a night flyer, so that any air bumps are flattened out will be a bonus.
Thanks all for the advice.
10-20-2004 | 09:30 AM
#7
RE: Effects of moving weight fore and aft of CG?
Time Pilot,
Sergeant's formula is the one to use to relocate weights on either side of the "scale" and maintain the CG location.
Frankly, I'm not sure what Jim was stating, but I don't suggest you try to calculate the moment of inertia- just doing the things we've discussed will give you the qualitative results mentioned. Just for fun, here are some basic equations on the subject of rotational motion.
moment = control surface force * its distance from cg
OR whatever external forces * whatever relevant distance
mass moment of inertia = SUM (mass of each distinct weight * its distance from cg ^2)
angular acceleration = moment / mass moment of inertia
So, you can see through the formulas the effect of slowing down the responsiveness of the plane by spreading weights out toward the end points. Of course, you can apply this in each of the plane's axes.
Sergeant's formula is the one to use to relocate weights on either side of the "scale" and maintain the CG location.
Frankly, I'm not sure what Jim was stating, but I don't suggest you try to calculate the moment of inertia- just doing the things we've discussed will give you the qualitative results mentioned. Just for fun, here are some basic equations on the subject of rotational motion.
moment = control surface force * its distance from cg
OR whatever external forces * whatever relevant distance
mass moment of inertia = SUM (mass of each distinct weight * its distance from cg ^2)
angular acceleration = moment / mass moment of inertia
So, you can see through the formulas the effect of slowing down the responsiveness of the plane by spreading weights out toward the end points. Of course, you can apply this in each of the plane's axes.
10-20-2004 | 12:50 PM
#8
Thread Starter
Senior Member
RE: Effects of moving weight fore and aft of CG?
I will stick with the simple calculations, then, and post my results when done. One of my servos is in for service right now, so should have everything up in the air by the middle of next month. Still have to cover the tail surfaces and wire up the LED's....
Thanks again.
Thanks again.
