aftcg

My Feedback: (1)

I'm building another fuse for this plane. I want the fuse to be a little longer so it will be a little more stable. I'm considering adding 1" to length of fuse in front of wing and 2" to fuse behind the wing (stretching the fuse a total of 3").
Total length now (including rudder) is 62", WS is 84", weight is 16#, 1800 sq in of wing area.


The CG now is fine, but it is a little on the tail-heavy side.

How do I stretch the fuse without changing the CG? Is there a formula or rule of thumb?

What will the change be if I add 3" to fuse as described above?

aftcg

My Feedback: (1)

I think I'd be ok if I add 1" to nose and 1" to tail-end of fuse.

I need someone with experience confirm before I start cutting because it will be too late once it's built.

Tall Paul

I'd be surprised if an inch change would be noticeable in the flying characteristics.
Looking at the photos, the horizontal can be made much larger. This would improve the stability more than a stretch alone.

Cowboy_Bob

for what its worth....if you change your fuse length the cg WILL change as it is a function of weight over distance from the original cg. I would think recalculation would be complicated. There is a forumula for calculation of cg( do a search for calculation of aircraft cg and see what comes up) but i'm aware of only use from a given reference point and known cg. I would also think that the effectiveness of your fix would be unpredictable. Is your instability due to torqe, p factor or is it a control instability? I'd guess and put a bigger stabilizer surface as appropriate. GeeBees were funky to fly anyway i suppose. Also, im not sure what you mean by cg is fine but a little tail heavy....cg is never fine tail heavy...maybe nose heavy a little is ok. That may contribute to your instability right now.

Good luck!

aftcg

My Feedback: (1)

I think Cowboy pointed out something I was over looking. The aft cg is probably the main reason for my instability. This plane is obviously short-coupled though, so instead of adding weight to nose I want to stretch the nose (at least, maybe the tail too - for looks).

Here are some of my thoughts:
If I add an inch to the nose, I realize that will move the CG forward. That is not much different than moving an engine forward one inch. One inch is quite abit of adjustment forward. Somewhere in the back of my mind - I seem to remember that there is a rule of thumb that says basicly 8 ounces of weight to the nose has the equivalent affect of 1 once at the tail (something like that).

I am hoping to discover some kind of principle that will guide me in what effect lengthing the nose & tail will do. I'm going to add 1 inch to the nose on this rebuild. I'd like to add an inch on the tail too. I'd love for the net effect of that to move CG forward by a half inch or no more than one inch.

Any thoughts?

BMatthews

I'm with Tall Paul on this one. The fuselage is not the problem so much as the horizontal tail. That's a way too small for the wing area. Especially if you try to use coupled flaps with the elevator travel. From the looks of the photos I'd suggest making the horizontal area twice as large. Increase the span and chords by 1.45 to do that. There are many models flying with that short or even shorter coupled but not many with that small a tail.

In the end though it may just be that you got too greedy in moving the balance back. Adding a bit of lead is a LOT easier than making new parts and in truth is the proper action. You can't mask a too far back CG with longer moments. But if for some reason you actually need to build a new fuselage I think adding an inch to the nose wouldn't be a bad thing. You're right about the relative nose vs tail weight so it's much easier to add a small amount of weight to the tail than a large amount to the nose.

Dzlstunter

Maybe I missed something here, but if it is already a little tail heavy and you add more to the tail than to the nose, won't you be making the problem worse? Why not first get the cg right with what you now have, and then make alterations (such as larger tail surfaces) as needed. Adding a bit of weight temporarily will tell you if that is the answer. Good luck, Dzl

aftcg

My Feedback: (1)

There's been quite a bit of good comments here and I greatly appreciate it.

Two things:

1. Yes, adjusting the CG forward will make it more stable. I have a giant heavy-hub that I used for the first several flights and it makes it more stable. I also realize that (apart from proper CG) the longer the fuse, the more stable the pitch is. You can take that too far, but with this is a short-coupled plane - I've got room to spare. I'm building a new fuse (broke the original one in half last week). So I'm going to lengthen the nose so that no dead-weight will have to be added to bring CG forward (about 1"). Maybe I shouldn't have mentioned lengthening the tail in the same breath as needing to bring CG forward (I can see how that could be confusing). Sounds like I should scrap any thoughts of lengthening the fuse on the tail side (making the horizontal stab larger another issue). I guess the real issue on this question is if I want to bring the CG forward between .5" and 1", how much do I add to the nose - 1" (does anyone have any idea)?

2. Making the horizontal stab larger - I hadn't thought of that. I don't really understand the affect it would have. For instance, will it make a hammerhead stall-turn less violent or can that only be obtained by lengthening the fuse? Maybe moving CG alone is all that is needed. I'm familiar with how it flew with the horizontal stab that is in the picture. FYI - the stab is 36" wide, compared to the 84" WS - it seems large to me.
What kind of flight tendencies should I look for in order to determine if more horizontal stab is needed?

Thank you so much for your brain power. I ordered the materials to build fuse last week and it should be here by Tuesday, so I need to get my game plan together rather quickly.

BMatthews

Typically the stab on fun fly types should be around 30% of the wing area from what I've seen of the more successful models around. For a super short coupled model like this one I don't think 35% would be too large either. I wouldn't want to see an aerobatic model with any less than 25% and only that when combined with a longer moment arm. Hot sailplanes and specialized racers can often get away with as low as 12 to 15% but for aerobatics you want the tail area for the leverage if offers. A larger horizontal will also increase the tail volume so the model will/should be more tolerant of having the balance closer to the neutral point.

The hammerhead issue is another story. Lighter short coupled models have very low moments of inertia so they can swap ends very quickly. Adding nose moment and making the tail larger, and presumably heavier, will increase the inertia and slow down the rotation.

Setting your CG. Put the model into a 45 degree dive at medium throttle and neutralize the controls. The model should maintain the dive or only very slightly tend to nose up back to level. If it tries to tuck to a steeper angle then you're a smidge to the rear of where you want to be.

Rudeboy

To calculate the effect on the CG when lengthening the fuse you have to know the following parameters:
The weight of the fuse portion aft of the CG
The CG of the fuse portion aft of the CG
The weight of the fuse portion in front of the CG
The CG of the fuse portion on front of the CG

Make a drawing with the moment arms of the respective fuse portions CG's in relation to the planes' CG.
If the moment arm aft of the planes' CG is twice that of the moment arm in front of the planes' CG you can add 2" at the back and 1" at the front without changing the CG...

Since your fuse broke in half, and you need to build a new one this is feasable... On a plane still intact it is a litte harder...

But I agree with the other posts: the problem isn't so much the tail moment arm, it is the combination of a too small tail surface and a slightly aft CG. The bulky fuselage isn't helping you either probably...

The rule of adding 1 ounce to the back equals adding 8 ounces to the front is only correct if the distance of the rear weight to the planes' CG is eight times that of the front weight to the CG... the same principal applies here... it's all about moment arms...

DanSavage

My Feedback: (5)

Someone correct me if I'm wrong, but the CG is related to the static margin of stability of the airfoil/wing, not the length of the fuselage.

Granted lengthening the fuselage will change the moment arm of the components as well as the moment arm of the tail feathers, but by moving the tail feathers aft, for example, their effectiveness will increase.

So, if by lengthening the nose/tail, the CG shifts, then you need to move the components or add lead to get the CG (of the wing) back into the allowable limits of the CG (of the wing) shown on the model's plans.

Dan

feihu-RCU

In reply to your Post no. 1
If you add 1" to the nose and 2" to the tail for a total stretch of 3", the cg will move .5" AFT of the original location. This assumes that the fuselage stretch is at the the original cg location but the airfoil is not stretched.
The cg cannot be fine if it is "a little on the tail heavy side" and this change will make it worse.
The formula for this change is that the summation of moments about a balanced point is zero; and I will not go into that.

In reply to your Post no. 2
There would not be any change in the location of the cg; assuming that the stretch is at the original cg location and the airfoil is not stretched.

In reply to your Post no. 5
If you only stretch the nose 1", then the cg will move FWD .5" with the same assumptions as above.
If you plan to rebuild and stretch the nose and tail, why don't you move your components (batteries, receiver, and even servos) around and locate the cg where you want?

In reply to your Post no. 8 part 1.
Same as reply to your Post no. 5

I agree with the other replies you received that you should consider enlarging the horizontal tail.