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CG Problems and my Return to Balsa RC building. Part 3. I am not impressed!


Bonzo Moon
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Finally finished the frustrating and annoying build of my first balsa kit construction in several years, and to say the least, I wasn't very impressed with the instructions. And this came on top of the issues I'd had with the first ARTF I'd bought in years just a month or two before. I spent most of this build, having got back into building and covering, in trying to achieve the recommended CG, which wasn't easy even though I was using the recommended motor etc. 🤬. I had to re edit the video cutting out rude words I'd used.

If you're considering buying of these little Real Hawk balsa kits I strongly recommend watching my videos on this first!

 

 

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not got time to watch the video right now, do you have an abridged version of the issue? Adding ballast for c/g is hardly uncommon and indeed is perfectly normal. I work on the basis that up to 10% of the final flying weight as ballast is normal and acceptable. 

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44 minutes ago, Jon H said:

not got time to watch the video right now, do you have an abridged version of the issue? Adding ballast for c/g is hardly uncommon and indeed is perfectly normal. I work on the basis that up to 10% of the final flying weight as ballast is normal and acceptable. 

I think adding ballast say to gliders to help penetrate maybe OK, but adding 30g or 40g to the nose of a 109g model  that I want to be a slow flier, or maybe and indoor flyer, doesn't work for me.  OK, a couple of g to trim maybe. In the end I got away with just 5g as far forward as it could possibly go forward. 

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There is no reason why the Li-Po should not go as far forward as you can get it, putting it on the CofG is not realistic! Add 20g of weight to the nose, you can then fly it! I would also suggest that a model that size needs lightweight covering which I don't think you used, another reason why the tail is heavy.

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I know it's not uncommon and I have done that on many of my own designed builds.  Though I have seen it suggested on forums that the best place for that weigh is as near to the CG as is possible.    I did wonder about weight of the covering,  but in my tests it needed 40g up front to put the lipo further back. And the model was only 109g.   The hollow in the nose was OK for the lipo after my mods, plus just 5g, yes, no problem, but the instructions gave the impression that you could adjust the CG by shifting the lipo forward and aft. No way.  A nd no way you could just push the lipo in the nose without building container for it. 

 I have since posting my video seen threads on forums going back 20 years saying they had exactly the same issues with this plane. I didn't realise it was such and

old design. I don't like adding unnecessary weight to an rc plane, and most people I know who build feel the same? Some suggestions online on the old threads were move the main spar back a bit.  Anyway, it's sorted. I just foolishly though that a sold designed kit would be better thought out. 

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Let me admit I'm having a hard time following your concerns.

 

What does moving the spar aft do? If you are already tail heavy why move mass aft?

 

Are you aware that the CG is placed where it is as a function of the aerodynamic margin of stability and has no bearing on the structure or strength?

 

I as a builder and flyer have no issue adding weight to get the trim I desire.  Proper trim trumps all up weight every time!

Edited by Konrad
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I wouldn't call them 'concerns'  mate.  I was just surprised that a bought designed kit didn't work out as I'd expected. Mostly the last few years I've been building my own design foamie conversions and working out best CG place. 

 

Are you aware that the CG is placed where it is as a function of the aerodynamic margin of stability and has no bearing on the structure or strength?

 

Yes of course I am aware.  

 

What does moving the spar aft do? If you are already tail heavy why move mass aft?

 

 Excuse me if I'm wrong, but moving the main spar aft ,  actually means the nose and motor are further forward which does not make a model more 'tail heavy' as far as CG goes it  means you don't need to add lead at the nose. 

 

And yes of course trim is important , but I didn't much want to add 40g to a nice lightweight 109g model! 

 

I quote this post on RC groups ...

 

https://www.rcgroups.com/forums/showthread.php?103621-How-do-I-get-better-CG-on-the-Slow-Stick

 

 

How do I get better CG on the Slow Stick?


I am finishing up my first electric RC, a GWS Slow Stic. I have mini servos and a 7.2 400 mha GWS battery. I can't seem to get my cg to be where the manual says it should be: 95mm from the leading edge. any ideas?
 
  
 
 Mar 31, 2003, 01:44 PM
Joined Sep 2002
2,098 Posts
If you slide the wing back towards the tail about an inch you will be lengthening the moment of the nose(and shortening the tail moment). That should help balance to 95mm from leading edge of the wing.

You can also locate the battery pack in front of the wing for better balance. This might be too much weight shift though.

As a last resort, you could add some lead right behind the motor. The SS can handle it no problem. I have 1/4 oz. on mine.


Sierra Gold
 
 

 

 

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2 minutes ago, Martin Harris - Moderator said:

Trying to decipher this Bonzo.  Are you moving the whole wing rather than just the spar?  That would make more sense to me. 

Sorry, I used the wrong word!   Ooops, yes THE WING!    😏   I'm not much up on the technical names of the parts of a wing I always thought the main spar meant the big wing!   I just Googled main spar  is just the structural bit .  🙄. BIG apologies. 

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Looking at your video it looks like the plans (your wing dots) are showing a very conservative CG. It looks like they are placing the CG at around 15% to 20% of the Mean Aerodynamic Cord (MAC). For models set up like this I usually aim for a CG placed at 33% MAC. I start with 25% and move aft as the flight trimming dictates.

 

You have indicated that you have experience with ARF but that it has been a while since you built with Balsa. Can you give us an idea of your flying experience?

Edited by Konrad
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17 minutes ago, Konrad said:

Looking at your video it looks like the plans are showing a very conservative CG. It looks like they are placing the CG at around 15% to 20% of the Mean Aerodynamic Cord (MAC). For models set up like this I usually aim for a CG placed at 33% MAC. I start with 25% and move aft as the flight trimming dictates.

 

You have indicated that you have experience with ARF but that it has been a while since you built with Balsa. Can you give us an idea of your flying experience?

 

I think they were different plans , this is an old model and apparently,  I  have since found has been around in a few different guises. 

 

I' d already measured the main wing and it's 115mm.  CG recommended is 32mm  and that calculates as 28% (if my maths is OK)  And I thought conservative.  I'd prefer the usual 30% - 33%   It'll need a bit more lead to achieve that. 😏

 

All the same,  how anyone is supposed to achieve that with this kit, the recommended motor and lipo without lead is hard to see! 

 

Thanks to all for comments.  As I say, last few years I've been building foamboard and foamie conversions and this was a different ball game. And I've had good success with those. 

Edited by Bonzo Moon
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Why are you quoting a 20-odd year old post on a totally different style of aeroplane though? The GWS Slow Stik is a simple stick fuselage, with moveable wing fixing that can easily slide backwards and forwards to position the wing anywhere along that stick, so moving the wing is a viable means of getting the CG in the desired position on the wing.

 

You've built a model with a fixed wing position, exacerbated by having the aileron actuating rods pass through slots on the fuselage sides, so the wing needs to be where the designer of the model put it. Just put the lipo as far forwards as you can manage it and you will reduce the amount of dead weight that you might need to add. It's totally pointless having the battery on the CG - make use of the weight in the battery and start with the CG at something like 25-28% of the wing chord.

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2 minutes ago, Bonzo Moon said:

 

I think they were different plans , this is an old model and apparently,  I  have since found has been around in a few different guises. 

 

I' d already measured the main wing and it's 115mm.  CG recommended is 32mm  and that calculates as 28% (if my maths is OK)  And I thought conservative.  I'd prefer the usual 30% - 33%   It'll need a bit more lead to achieve that. 😏

 

Thanks to all for comments.  As I say, last few years I've been building foamboard and foamie conversions and this was a different ball game. And I've had good success with those. 

If you are needing to add lead to the nose to achieve a CG at 28% of MAC, then you would need to add LESS lead to the nose to get the CG at 33% of MAC.

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13 minutes ago, leccyflyer said:

If you are needing to add lead to the nose to achieve a CG at 28% of MAC, then you would need to add LESS lead to the nose to get the CG at 33% of MAC.

 

I just realised that and was just coming back to my post to correct that!  You beat me to it!  In fact I can afford to move the lipo a couple of mm further aft!  My 77 year old brain was getting a bit confused.  🙄

 

I misunderstood the conservative bit I think. Of course what you meant was it'd be nose heavy, not great, (but better than tail heavy).  For better flight characteristics CG further back, ie less nose weight, would be better right? 

 

I just worked out, the difference to make it 33% is 3 mm  and on that covering it's not actually that easy to figure the actual CG that accurately!  I'll maiden it like it is and see how it flies, making adjustments if necessary 😉

Edited by Bonzo Moon
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Like I said - if you haven't flown the model yet, for a straight sided wing like that, 25-28% is a good starting point for a maiden flight. Once you have flown it successfully you can choose to remove some nose weight, to move the CG back a little, thereby making the model less nose heavy,  more responsive to the elevator and tweak to your personal preference.

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2 minutes ago, leccyflyer said:

Like I said - if you haven't flown the model yet, for a straight sided wing like that, 25-28% is a good starting point for a maiden flight. Once you have flown it successfully you can choose to remove some nose weight, to move the CG back a little, thereby making the model less nose heavy,  more responsive to the elevator and tweak to your personal preference.

Our posts crossed. And yes, that's what I just decided too in my edited post! 

Edited by Bonzo Moon
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