Ballast Placement CG Caculation
 

Allright boys, I think I found the right thread for this question. I overpowered an aircraft making it nose heavy. The obvious and easiest choice is to stick some lead on the tail and make it hang at the spot I want. We all know dead weight is not the best answer. I have plenty of weight already on the aircraft in the form of servos, batteries, RX etc. The battery box is aft of CG as I can manage so it is out of the equation. The rudder and elevator servos are forward of the CG and this is what I want to move back to compensate for the larger engine. Obviously I will have to build a mount for a servos somewhere back on the tail boom to move the servo's location. What I want to avoid is randomly punching holes in my airplane and trying and erring my way to a correct servo location. I understand that I really just need to remove my chosen servo, hang the plane, place the servo in the spot I want it to balance, and then go to work. Really, I am looking for more of an academic exercise.

Does anyone have a link or knowledge on how to take the known balance point, the known weight of the ballast (rudder servo), the future balance point, with X (unknown) being the placement of the ballast to achieve the desired future balance point? I am going to go ahead and get my plane balanced the caveman way (putting the servo on the tail until the plane balances).


Any questions? OK 1,2,3, GO!!

Thanks

RE: Ballast Placement CG Caculation
 

What starting data do you have, the unbalance that you created with the heavier engine or the actual location of the CG as is now?

RE: Ballast Placement CG Caculation
 

Don't know of any online applications. They'd need for you to input some numbers, and if you get those numbers you can scribble them into an answer on paper about as fast as you can bring up an online app and type them in.

I don't remember when it was in school they taught the moment arm deal. That's what you use. It'll take more words here to explain, and if I try, it'll sound more complicated than it is.

Let me make a suggestion... or two.
You already have decided to move the servos. And you already know you need to. So move 'em and check to see how far that moved the CG. Try not to break out in tears in front of a woman. The most important suggestion is to accurately mark your starting CG. Unless you do that, you won't have a prayer of even guessing what effect the moves made. Do that and you'll have a good idea how much more weight MIGHT be needed.

I did just that and discovered I could move my battery pack back into the fuselage and get the CG I wanted. Cutting the ultracote out of the bottom of that airplane provided access through a semi-permanent hatch. Simple and quick. I built a balsa box for the battery and glued it in. Easier to do than trying to put braces in that fit the space and the battery. Ironed on matching ultracote and "closed the hatch permanently".... or until I wanted to get at the battery.

Working out the moments really isn't hard, btw.

RE: Ballast Placement CG Caculation
 

To work out the moments you need the total weight of the plane, it's length and how far back the CG is from one end of the length.

and the weight of any components you're thinking of moving
and where they are along that line

(I didn't say it was dead simple and instantaneously quick.)

RE: Ballast Placement CG Caculation
 

BTW, if you think the mass of the servos is enought that they might not need to be moved back to where they are usually placed by almost every mfg, then the CG can probably be moved enough by shifting the battery just into the fuselage aft the wing bolts. Or as far as you can reach to glue in a balsa box for the battery if it's already just aft the wing bolts.

On the other hand, why not just move one servo back under the stab and see how that works.

And if you KNOW moving both servos ain't gonna hack it, then go ahead and move 'em one at a time to find out how much more weight shift is needed.

Do measure the CG locations however. It's often amazing how little we learn from some things that could teach us so much.

RE: Ballast Placement CG Caculation
 

Quick and easy(?) way to do it.

As you describe it, remove the servo, assemble the plane and support it at the intended CG. Then use blue tack and stick the servo on to get the CG right. With the plane at hand this is actually the best way to do it.

Second best, make a full size sketch on the wall or blackboard/witheboard to visualize. Then measure, weight and do the math. Andy Lennons book on model aircraft design outline this procedure.

RE: Ballast Placement CG Caculation
 

From the other thread on this:
It's a simple balance equation. Weight of one thing times distance plus weight of another times distance divided by all up weight gives the location of the balance point. Convention has weights forward of the reference as positive, behind it negative.
Typically for aircraft the datum point is ahead of the nose, this keeps all sign convention simple.
If I were to move the elevator servo back on my FlatFlier AD-5W, the c.g. change would be like this...
Just where it comes out with finger balancing...

RE: Ballast Placement CG Caculation
 

TALL PAUL! Your the berries. Thats the kind of info I'm looking for. The pics are great and I think I'll be able to figure the actual equations. Although I think I'll go in center-meters and grams due to the small scale we're talking about. I am quite surprised at how little the CG moved with such a dramatic change of servo location. HIMAT pretty interesting and hands on way of getting it done. Essentially how I did it so I could get in the air this afternoon. I just ordered Lennon's book.

DA ROCK. I'll "Try not to break out in tears in front of a woman." Thanks. Appreciate the help

RE: Ballast Placement CG Caculation
 

The c.g. won't move far, when the thing being moved is a small proportion of the whole weight of the plane.

RE: Ballast Placement CG Caculation
 

Why c.g. control can be important!
http://www.telegraph.co.uk/news/worl...rks-panic.html

RE: Ballast Placement CG Caculation
 

Yeah I read about that. Too bad for them.

RE: Ballast Placement CG Caculation
 

So everyone rushed toward the cockpit.

Guess the plane didn't follow the rule everyone spouts about, "CG too far foward, plane flies poorly. CG too far aft, plane flies once."

RE: Ballast Placement CG Caculation
 

I have done a my fair share of CG/Weight and Balance calculations for airplanes. But for our RC application, it is easier to just temporarily locate the items in/on the rc model and then balance it on your CG machine(or fingers). However, make sure "ALL" the items are in the plane when you balance it. I remember one of my builds in the last year that was way off when I did the final check before covering. In my initial decision process for equipment placement, I had forgotten to attach the muffler to the motor.

RE: Ballast Placement CG Caculation
 

Getting the airplane to balance by moving heavier items to the extremes of the fuselage is the first consideration. But just as important is minimizing the moment of inertia by keeping the mass of components as close to the CG as possible. This effects how well an aircraft can respond to control inputs.

As far as calculating the amount of distance you have to place an item, you only need to know the total weight, and distance you want to move the CG from it's current position. Multiply that distance by total weight, and then divide that figure by the weight of the object you are moving. That gives the distance it need to move.

RE: Ballast Placement CG Caculation
 

You can think of it in terms of moments about your aircrafts previous CG position:

You need to know the position of the old CG, the weight of both engines and the weight of the servos.

Increasing mass (engine) increases moment.
Increasing moment arm increses moment.

The nose down moment of the engine is it's mass (oz) x distance from CG (inches)
So if your old engine weighs 5oz and is 10 inches forward of CG it's moment is 50in/oz

So if the new engine weighs 8oz and is in the same position it's moment is 80in/oz.
(8oz x 10inches = 80in/oz)

You have increased the nose down moment by 30in/oz.

It follows then that you need to move the servos enough to create a tail down moment of 30 in/oz.

Find out the servos current contribution to the tail down moment: weigh them and measure their distance aft of the CG.

Let's say they weigh 2oz and are 8 inches back from the CG = 16in/oz

You need them to increase that by 30 to 46in/oz so to get 2oz to provide 46in/oz you need to move them to 23" aft of the CG.

46in/oz divided by 2oz = 23 inches.

Then by moving the servos you have increased the tail down moment enough to compensate for the increase in engine weight.


You can put this all into a formula

Servo moment arm required = ((e2W*eMA)-(e1W*eMA)+(sW*sMA))/sW

where
e1W = engine weight 1
e2W = engine weight 2
eMA = engine moment arm (distance from CG)
sW = servo weight
sMA = current servo moment arm (distance aft of CG)

Using the above as a worked example:
e1W = 5oz
e2W = 8oz
eMA = 10 "
sW = 2oz
sMA = 8"

Servo moment arm required = ((8*10)-(5*10)+(2*8))/2
= ((80-50)+16)/2
= (30+16)/2
= 46/2
Servo moment arm required = 23"
Which requires a movement of of 17"
(23-sMA)

Hopefully this should work for you!

Jon

RE: Ballast Placement CG Caculation
 

In your diagram, it looks like your computation only considers the addition of another servo at the tail, not the movement of the servo from around the 17.6 location.

Wouldn't you need to include a - (1x17.6) in the equation?

EDIT: OK, I just noticed the " - 16.52"

I assume that's the servo being removed from it's location which just happens to be on the CG.

RE: Ballast Placement CG Caculation
 

If the servo isn't located at the CG as a start point then two moments need to be calculated and added for the total change. The first would be the moment of the servo's weight and shift back to teh CG. And then the other shift to the tail. The two would then add up to produce the total moment shift.