If I increase the fuse length by 5 inches over the original plan length, how does this affect the original Centre of Gravity position on the original plan.

Thanks
Charles

It is totally dependent on the length and weight of the fuselage, and the estimated weight you are moving and where is the CG currently. it is a teeter-toter. Things will change a lot, totally dependent on the % of weight you are adding to the model.

The CG will still be in the same place on the wing. Ideally your 5" should be some in front of the CG and some behind. Otherwise it's going to take a lot of ballast to get it right. If this is a kit or a scratch add your length and reposition the wing saddle.

David

A longer rear fuselage will let you reduce the size of the fin, rudder, stab and elevator which can make them a bit lighter. How much, I don't know. You will probably still have to add weight forward to keep the CG in the same place on the for/aft position on the wing.

Charles,
Plug your old and new numbers into this spreadsheet to get the answer
http://adamone.rchomepage.com/cg_calc.htm
Chuck

fantastic link , Charles

If you add 5 inches to the nose of the airplane,
the plane will likely balance at a point well
forward of where the plans suggest the CG should be.

If you add 5 inches to the tail, it will balance farther back.

Neither of these will affect where it SHOULD balance.
Where it should balance is generally somewhere near
the wing spar, regardless of how long the fuselage is.

If it doesn't balance where it should because of the
added fuselage length, it will need either added weight
(easy) weight shifted, (not so easy), weight removed,
(usually harder) or wing moved (harder still).

Jenny

Whenever you add length to the fuse, you should recalculate where the CG will be based on your wing, stab areas and moment arms (tail , nose). You should relocate the wing accordingly. Use one of the online calculators for this task. You will also be prompted to include the percent static margin. I use around 15% typically for aerobatic models. Good luck.

Rodney and Chuck have it right, the CG depends on the location of the neutral point and how much stability you want, so the calculator is the way to go.

Note that the critical part is not the overall length of the fuselage, but the length of the tail arm.

I have done calculations by hand on every plane I have flown for the past 25 years, and guess what I've learned? The aeronautical engineers are not wrong. I don't trust the magazine plans or the kit plans, I always do the calculation.

Jim

CATA's original question is worded so that it's not really clear which answer is needed.

Almost everyone is reading the question in a different way. Some of you are thinking of the added weight and what it will do to the balancing. Others are looking at the effect on the stability and change to the NP and CG location. But since the original post can be read in either way and even in a few others all the replies look good and are right.

Charles needs to come back and give some more insight.

David

given everything (tail feathers, etc, and engine/tank/electronics locations) remains the same other than fuse length itself,.....adding forward length will move the CG forward and adding rearward length will move the CG rearward. the key is to add to both to maintain the location of CG, unless it is specifically CG location you want to change. then, moving the front either way, will have more input on the change of CG location than moving the rear either way, simply because the front of a plane weighs much more than the rear of a plane.

I thought the OP's question was perfectly clear: "how does this affect the original Centre of Gravity position on the original plan"? He wasn't asking how teeter totters work.

In any case he seems to be long gone.

Jim

hi
would this help you the vanessa CG setup http://www.rcuniverse.com/forum/tips...g-rig-how.html

c/g never moves!! Do mods and rebalance with same c/g
for rc planes forget " the entire fuse ' method

YES IT DOES! The relationships between wing area, chord, stabilizer area, and tail arm have been well established for at least 80 years. When you change the design of an airplane, if the design changes affect any of those factors, you can and should use a different CG.

Nope

"flying wing moments please??

the CG will not move "proportionally" in relation to the fuse's length, as long as the lengthening is proportionally distributed. if the lengthening is biased towards front or back, the CG will move proportionally towards the greater distance of bias, unless weight distribution is changed to compensate for the bias.

We're back to teeter totters. The issue is the aerodynamics.

Starting with an airplane with things positioned so the CG is where the suggested CG should be, making the tail longer will add weight aft of where the CG was, which moves where it is somewhat aft, and most certainly changes where it should or could safely be located.

Moving the tail farther aft gives it more aerodynamic "presence" (or power). It's aerodynamic force gets more leverage so to speak. Which is good because the CG is now centered farther aft, and no mention has been made of moving the wing every so slightly aft to keep the CG location positioned in the appropriate place relative to that wing and where it's effects are felt.

As for flying wings.... they absolutely do have pitching moments that need to be controlled. They don't happen to conventional tails, but absolutely do require pitch control. That's why they have "elevators", or surfaces that do elevator functions. And those things have dismal leverage, btw. And they have damn narrow range for their CG to be positioned.

A plank style flying wing doesn't have either a fuselage or tail to use for stability so that's a red herring.

In the case of a strongly swept flying wing the rearward portion of the wing acts a bit like the stabilizer. That's why we put in washout in that area. And if we alter the sweep angle to "make the fuselage longer" by lengthening the wing it most certainly does affect where the CG needs to be located.

Going back to the plank style wing for a moment. Due to the lack of a stabilizer the balance needs to be located quite strongly ahead of the 25% MAC of the wing. Just try to balance back behind that and it'll be like flying an arrow backwards.

If we add a stabilizer at all and put it some distance behind the wing the allowable CG now shifts to some point aft of the 25% MAC. So isn't it reasonable to expect that the further back we shift that stabilizer or the larger we make it that the more the CG can move back? How else do you explain how free flight models with super long tail moments but smaller size stabilizer areas are able to still be stable with a 50 to 70% CG location?

Buzzard bait is right on. All of the variables he described are part of the CG determination.

I have designed a couple dozen planes over the years and know for a fact that changing levers require a change in CG location for proper flight or at least the flight regime I am interested in

No argument, only an addition: A symmetrically foiled plank is even resonably stable when balanced between 25% and 30% (28% recommended) due to down-pitching airfoil moment above 6 degrees AoA at least on thin airfoils (see old thread).

And one could add to your last paragraph that the undercambered wing and tail airfoils develop much down-pitching moment so with a big stab the balance may be even at 100%.