Guestimating CG during a build - any tips?

[solentlife]

We could of course start calculating the bare airframe at design stage by using the area and density of the foam / material used. Create a mathematical model that then JetPlaneFlyers component moments can be applied to - arriving at CoG desired.

It's not as hard as you think ... use graph paper with scaled down sketch to arrive at area ... (counting the squares ....) Then using density - you can work out the bare foam moments and it's initial CoG. You then know what moments are needed to bring it to correct. Use the weight of each component as JPF showed at start of thread to do this ...

Yeh - I'm a daft b*****r at times !!

Nigel

[mclarkson]

Originally Posted by JetPlaneFlyer

The thread is not about how to check where the CG is on a model. It's about how to decide where to place components during design of a scratch built model in order to achieve the desired CG. The assumption is that you already know where you want the CG to be.

Thanks for noticing that!


Solentlife, now I'm gonna drive myself nuts trying to think of a censorable 7-letter word that starts with B and ends with R.

[edjahar42]

mclarkson, if you haven't already gotten past that point, here's a trick I came up with.
Get a board or something longer than your designed fuselage, mark the dimensions of the plane- prop line, rudder hinge line, wing location, and proposed cg point on the wing. With these marked on the board, place the board on a piece of triangular stock, at the cg mark, of some kind for a fulcrum, then because the tail moment will be longer than the nose you'll need to add weight to the board at the front until the "empty" board is perfectly balanced.
Now place the actual components on the board and move them around until they allow the board to balance. If you don't yet have the actual component, use weights that you have weighed to match what you intend to use. I do this planning to use my battery over cg to fine tune after the build.
Hope this helps someone,

Ed

[Flite-Metal]

I am afraid we don't typically design something then try to figure out where CG is... Well most of us don't.

Try this out for size:
http://www.scaleaero.com/CG_Calculator.htm

[Bill G]

Originally Posted by edjahar42

mclarkson, if you haven't already gotten past that point, here's a trick I came up with.
Get a board or something longer than your designed fuselage, mark the dimensions of the plane- prop line, rudder hinge line, wing location, and proposed cg point on the wing. With these marked on the board, place the board on a piece of triangular stock, at the cg mark, of some kind for a fulcrum, then because the tail moment will be longer than the nose you'll need to add weight to the board at the front until the "empty" board is perfectly balanced.
Now place the actual components on the board and move them around until they allow the board to balance. If you don't yet have the actual component, use weights that you have weighed to match what you intend to use. I do this planning to use my battery over cg to fine tune after the build.
Hope this helps someone,

Ed

The common sense, simple model test method has been discussed in the past, which is probably why it wasn't specifically stated in terms. Errors occur when people do not adequately account for the aft weight of materials, or simply put not properly modeling the situation. Knowing and determining what they are, and adding them to the "stick airframe test model" is key. Any weight such as pushrods, hinges/clevises, sheeting, covering, etc has to be properly modeled in the test model. Obviously the errors are more critical in a tail heavy plane with a short nose moment, as the nose ballast needed to offset error becomes a higher multiple. One of my past oversights was the fact that considerably more than 50% of most wings is aft of the cg. If the cg is worked out before a wing is sheeted and then covered with materials heavier than Solite, it can be substantial. The same scenario is true for wing mounted aileron servos, as most are centered aft of the CG. When the nose length is short, the ballast required to offset what normally would be a small fraction of the servo weight, becomes closer to the weight of the servos themselves. The subject that I'm currently building fits perfectly into the short nose/tail heavy tendency scenarios described.

[mclarkson]

Thanks for the helpful info.

[edjahar42]

OK, should have been more specific, given more details. I've already built my horizontal and vertical stabilizers and placed them on my balance board along with my tailwheel assembly and while the sticks and sheeting used in the tail of the fuselage will be slightly heavier than the wood of the front, I think not enough so that a 5000 mah battery can't make up for it with slight movement.
As the OP stated his problem is CG placement. If you're trying to place your CG then you must already have a fuselage to place it in!
Just trying to help. It works for me.

Ed

[Flite-Metal]

I believe this discussion is under the faux assumption that the physical size of a given design will change its CG. This is clearly
a faux assumption given CG is aerodynamic balance point based on physical layout of a given design plus its airfoil design.

I don't care how many P-51 "sizes" you design, they all have the same CG location. The CG's sweet spot will be effected by
the airfoil chosen with respect to fore to aft CG width.

More often than not, a model will exhibit identical flight issues as the 1:1 except in the area of high aspect ratio wings and
horiz stabs. Angle of attack tends to offset negative effect of narrow tapered wings. Typically, high angles of attack are
accompanied by strakes and saw tooth breaks in leading edges to delay encroachment of lateral flow down wing toward tips.

Angle of attack can effect your control functions in a negative way in as much that high angles of attack reduce air speed
@ ROG.

[Old 'N Slow]

Well, if you want to get that technical, the static Centre of Gravity and dynamic Centre of Pressure can be in 2 different locations on an airframe. It can be very difficult to calculate the dynamic Centre of Pressure. I've never bothered to do it, but I've only scratch built basic high- or low- wing .40 sized trainers or sport airframes. So I haven't felt the need for that kind of precision. I just picked (stole/plagiarized/etc. from a similar plane that someone else had already done all the hard research work on) a basic known airfoil, kept the thrustline/wing incidence/stab incidences at roughly zero, picked a ratio of stab/wing area, and built.

(Downsizing an airframe from a full-sized airplane plan invariably leads to undersized wing area resulting in high wing loading, undersized vertical and horizontal stabilizers, and undersized control surfaces. You have decreased the size of the plane, but the air it flys in stays the same size... It can be done, but it usually leads to a very twitchy plane.)

One can modify the Center of Gravity by shifting static weight on the airframe fore and aft, up or down, left or right.

One can shift the Center of Pressure by varying the aerofoil of the wing or control surfaces, or the angle of incidence of any of the air-affected surfaces. Or the speed of the plane itself...

When I scratch build, I do use a rudimentary calculation to predict how my CG is going to come out. In general, I know where I want it to be on a finished model.

I do the Weight and Balance calculations based on the weight of material used and the equipment I plan on installing in the plane. I use a full-sized plan, a digital scale, some graph paper, a calculator, and a pencil. I know the weights of the servos, engine/motor and auxilliary equipment.

It isn't that hard. Find out how many square inches of balsa/ply/foam/covering etc. you will be using, when weigh an actual sample. I use grams/square inch, in a true mix of measurement units.
Now figure out how many square inches of that material will be fore or aft of the intended CG. Try to get the difference as close to Zero as possible.
I set the actual component of the radio/power system on the plan, and play with the locations a bit. Do some more math.

Yes, you can move some of the installed equipment around a bit after the airframe is built to get the CG where you want it, dialed in perfectly for the type of flying you're into. If you remember to leave room for that.

Even with these rough "calculations", I don't remember ever having to add ballast. I do recall making some small adjustments to the radio installation tray or the battery/fuel tank placement.

Sure, it can be more complicated than this - and it needs to be, for example, if you are building a very large (or very small), complex, scale bird. But for sport flying, it doesn't need to be exact, just as good as you can get it. There will always be some variance, even in high-dollar kits.

Go for it.

[mclarkson]

Originally Posted by Flite-Metal

I believe this discussion is under the faux assumption that the physical size of a given design will change its CG. This is clearly a faux assumption given CG is aerodynamic balance point based on physical layout of a given design plus its airfoil design.

There are two or three threads here working at cross-purposes. I started the discussion to ask for tips on computing where the CG would end up, in a build, not where it belonged.

The question was: If I'm building a plane, be it P-51 or whatever, where do I need to put the components to assure the CG of the finished plane falls where it belongs. Let's say I'm building a P-51 profile. Let's say I know that the CG belongs at 30%.

So I don't need to compute where the CG ought to wind up. What I need is to compute where the components ought to go to give me a final CG where I want it. What size battery will I need? What size motor? Where do I need to put the battery compartment to achieve that CG? Should I shove the servos and ESC all the way forward? Or all the way backward?

[Old 'N Slow]

No cross purposes here, unless I am misunderstanding the question..

Since you must know where the CG should be on the finished aircraft, it is up to the builder to determine where to put the components of the plane in advance of the build, so the CG ends up where it should be.

There's no shortcuts here - there are some basic calculations, some tools on the internet, some spreadsheets, etc.

You pick the plane you want to build, then select the components and materials you want to incorporate into the plane - get the weights, then do the calculations. That will tell you where it is possible to install your power and radio equipment. These are the moveable bits. It's tough to re-configure a built airframe.

"What size battery will I need? What size motor? Where do I need to put the battery compartment to achieve that CG? Should I shove the servos and ESC all the way forward? Or all the way backward?"

All good questions. I try to have the answers in advance of building anything. I know how much wing I have, so I know what sort of power I'll need. Only the calculations can tell you that.

A very basic way to do this is to consider the CG point to be the fulcrum of a balance beam.

If, for example, you have 300 grams of fuselage/stabs/hinges/blobs of glue/etc. hanging out behind the CG point, you need to hang X grams of something off the front so it balances. Could be 600 grams of something strung out at different points, hung to make it balance... only doing the math will tell in advance.

[N74tg]

Here's a CG technique I learned from the coroplast guys. Build your whole airplane with everything installed basically where you want it... servos, control arms, receiver, motor, ESC, battery, prop...everything. Then rubber-band the wing to the fuse so that you can slide wing forward and backward along the fuse until it balances on the spar (if that's where you want it to balance). Then if you have to cut into the fuse to mount the wing, at least you know where to cut.

Since all my planes are "rough scale" the wing final location doesn't have to be any particular place; an inch forward or back really won't change how the plane looks.

I use this technique on all my planes and it's never failed me yet.

If this idea has already been posted on this thread, my apologies.

[Flite-Metal]

I am sorry folks. There seems to be considerable minsunderstanding about determining
where CG is on an airframe you have no earthly idea where it is supposed to be.

Answer part 1 for method 1: Calculate CG as 25% of MAC on a traditional wing and horiz
stab with vertical fin airframe. To accomplish this first acquire the formula. Based on the
directions provided with the formula to plug in values enter all data into formula to reach
an tinal value.

Answer part 2 for method 1: Place a small mark on fuselage at the resulting locaation
discovered with formula above. If you perfer a slightly nose heavy feel to your model when
landing and to reduce "float", all weight to nose until you are satisfied with the landing results.

http://www.scaleaero.com/CG_Calculator.htm Click to view the two calculation methods.