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Brett82

Fuss ballast effect on COG check

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Brett82

Hi guys 

Just wondering how you guys check the effect on COG when loading ballast. Do you only do it with wings on or do any of you find the COG of just the fuss and check if it stays the same with ballast?

In my mind, if the COG of just the fuss stays the same then it should be exactly the same with wings on. If it does move slightly forward or back then the effect with wings on will need to be checked as there could be a difference by how much forward or back it has moved it.

I've just got some ballast for my Skorpion and it needs to be loaded with the plane upside-down, but trying to turn the glider around in my dinning room is a nightmare. I would rather do my checks with the wings off (if it works) because I keep hitting the wingtips every time I flip it to load then flip it to check.

It has no ballast in the wings, just the fuss. Any help or advice would be greatly appreciated. Thanks. 

Brett 

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mikef

In general, you need to check the ballasted CoG with the model fully assembled.  [The only case where this would be unnecessary would be if the wing CoG was at the same chord-wise position as the CoG of the rest of the model (fus, tail etc.)]

I don't know how your ballast works but it should be possible to make sure that the CG of the ballast always coincides with the CoG of the unballasted model.  

My DLG system sits in a tube.  The picture below shows two amounts of weight.  The piece of wood at the right side packs the ballast forward to get the CoG in the correct place (same ballasted as unballasted.). The piece of wood with the locking bolt at the left end of the top set is the right length to complete the set and lock it in.  The front and back bits will belong to a particular model.

The lighter ballast set at the bottom of the picture has the rear packing piece on the right hand end from a different model.  The amount of ballast can be varied by replacing the weights with wood spacers - all the bits are the same length - you need to maintain symmetry of course so the CoG of the ballast/spacers stays in the middle!

image.jpeg

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satinet

I try not to make a fuss over it........ drum tish.

You can work out the CG from actually measuring it on a balancer or you can do it with simple maths. But to work it out with maths you do need to know the location of the ballast tube relative to the CG. This is easier in the wing than the fuselage.

What you will find is it makes less difference than you think. Assuming your skorp weighs 2.4kg and the ballast is 900 is stainless steel. Assume the CG is 105mm and the CG of the ballat tube is 100mm (the ballast tube is 5mm forward of the CG).

The CG fully ballasted would be ((2400 * 105) + (900* 100))/(2400+900) = 103.64mm.  I don't know if you will be able to tell the difrerence. Obviously models feel and fly differently when heavily loaded anyway. 

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Brett82

Thanks Mike, I didn't think of the fact that if the COG of the fuss is not the same as the full model then adding weight to the fuss will move the COG. I have spacers but because I got the model second hand I didn't install the tube so im not sure where the centre of the tube is relative to the COG.

Tom your brain is just in another league, how the heck did you work that out? Haha. I guess if I first find out where the centre of the tube is and compare it to the COG then I will be able to work out if it will have an effect. Thanks guy.

Any further advice is still welcome. 

Brett 

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Brett82

Forgot to mention Tom, I like what you say in terms of it doesn't effect it that much and with my inexperienced thumbs 2mm won't be a huge difference. As long as it doesn't push it to far back for my skill level. 

Thanks again mate 

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pete beadle

Hi Brett

Mark the current CG you are using on the outside of the fuselage behind the ballast tube front opening

Insert a stick/dowel/rod at least 12" long into the mouth of the ballast tube until it can go no further

Mark the stick exactly at the point where the stick exits the front lip

Withdraw the stick, measure the distance from one end of the stick to the mark you made where it exits, halve that distance

Lay the stick on the outside of the fuselage horizontally level with where you've marked the CG position

Mark the fuselage side at the mark where the halfway point is.......that's hallway down your ballast tube.....which is the CG point of your ballast tube

Hopefully that shouldn't be too far away from your CG, if it is, adjust the balance spacers to suit the Skorp's CG......OK?

Several American competition flyers measure their CG's with the fuselage, and wings, inverted.......they say measuring "right way up" is indistinguishable from upside down....I haven't tried it myself but I see no reason why they're wrong......

Regards

Pete

BARCS1702

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Brett82

Thanks Pete 

I didn't think of sticking a dowl down and marking it then measuring halfway. I put the Willow slugs down, counted how many, took them out then lined them up and measured. Dowl would have been so simple. I will do it tonight. 

Never thought of checking the COG inverted, might try it and see if it works. 

Thanks mate 

Brett 

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satinet

Don't you have the steel ballast that they come with?

The front of the ballast is well inside the nose area - you can measure off that.  The slugs are 20mm stainsless steel IIRC.

If you have the factory ballast you would need to cut down one of the slugs to move the ballast further back. I'm pretty sure the ballast tube will be at a fairly conservative CG on the model. 

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Brett82

Hi Tom

No, when I got it second hand it came with 5 brass ballast slugs, 32mm long each. I checked them and they do fit into the loading gap so I ordered 9 more on ebay (more than I need but at least I will have spare). I tried the lead slugs I have for the willow which were same length but 1mm thicker. They are very tight in the tube so I decided not to use them and to get properly cut brass slugs. 

I also need to check the sequence of slugs and spacers to use for half ballast. For now I will only be flying with either 0%, 50% or 100% ballast. Im not at the level where making small changes to ballast will have a more significant effect to times compared to the effect my thumbs will have. Im going to do it similar to Mike but with the spacers in the middle and the slugs at the ends. That way there is always a slug behind a spacer to so the weight of the slug can push it out. Dont know why I have a fear of things getting stuck in the tube.

I was going to do it on the weekend but my step brother surprised us by flying in on Saturday morning for business so I ended up spending the whole weekend with family. Ive got my head around your formula and either tonight or tomorrow night I am going to crack on with it. Once Ive got the Skorpion set up Im going to re-look at the Willow. When I did my ballast checks on that I simply checked COG empty and COG fully ballasted. I would rather do it with the formula and use the new brass slugs as my old lead slugs are all different weights (some as much as 30g) and I have to put them in a particular order or it totally messes up the COG. I will lose a bit of all up weight but thats ok for now. 

Thanks for the help mate, I can always rely on BARCS for some good advice.

Brett

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satinet

I think part of your issue is that you have non standard ballast.

From the factory all skorps, extremes, rotmilans and stribogs all have pretty much the same ballast. Slugs might be different length but it's always 900g afaik. Which is way under what you want on a modern plane but that's by the by.

Not really sure how the skorp takes ballast. I had one but i was less experienced at the time and that was considered normal at the time. Aerofoils have probably changed but the skorp is pretty fast.

Anyway the factory ballast is steel slugs that fill the tube and they are held in place with a small rubber bung at the front.  I would be very surprised if the centre of the tube is at a racy balance point. It wasn't on mine.

As i recall the front slug should be right up to where the servos are. The bung is only about 5mm thick. Think that's right?

 

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pete beadle

Hi Brett

If you look back at the contemporary reviews of the Skorpion, such as the very comprehensive one by Kev Newton, you'll find that much was made at the time of the Skorp's ability to carry its momentum through turns,and, in some cases, appear to come out of the turn faster than it went in(!)

Unfortunately none of the early Skorps were fitted with telemetry, nor any on-board devices whose data could be downloaded from the plane at the end of the flight either, so the use of ballast was usually a lot less scientific than today. There were no descriptions of the physical placement of ballast in relation to the CG either, for example,it was just assumed that "competition flyers would know best" and ballast was usually loaded either all or none, because its total weight wasn't as dramatically effective as the tungsten stuff currently in use

Having said that, everyone over a certain age knew the stories of how the Skorp could "bite" you if you allowed yourself the use of too much elevator in the turn. Much was made of the of the "fact" that, if you went past a certain point, 7mm EW being the most quoted, you couldn't be certain of the direction the Skorp would be facing on its way out of the turn. Several explanations were put forward at the time, the favourite being the high speed stall, as I remember, but the very important factor of how much ballast you were carrying at the time wasn't commented on

All I knew was that my Skorp at the time didn't misbehave, and I put this down to my much slower average speed and/or lack of ballast.....if I knew then what I know now, I think I'd have looked more closely at the possibility of a CG change at full ballast....but then, who can say?

Personally, my CG positions at the time were more "conservative" than nowadays, and I believed that a slightly forward COG produced improved smoothness.....but then, what did I know?

I believe you're going about it the right way in sorting one thing at a time so I believe that you will get provably better as you remove as many variables as you can.......best of luck in the search....keep at it:yes:

Pete

BARCS1702 

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Brett82

Thanks once again guys, some very good points there. 

Before I even got into F3F I read about the Skorpion on Keven's blog and thought the amount of ballast was conservative compared to a number of newer models but I have also read many times that the Skorpion doesn't need more ballast. That extra 900g or so is plenty and when you consider it was a world record holder I'm certainly not going to try add more. It may have been 10 years ago now but even so, 27.79 sec is no slouch by today's standard. I'm not saying I can get that time, just pointing out the glider is capable of it so the plane doesn't need improvement. 

I have the COG fairly conservative at the moment and I am hoping the ballast keeps it conservative. I've only flown her a couple of times and I don't have much throw on the elevator so even though I pulled full up a few times there was no flick. That's how I want to keep it and why I'm looking closely at the COG with ballast as I am more worried about it moving back than anything else. 

Anyway I still have yet to try checking the COG with the glider inverted so I can load ballast without flipping it over each time and hitting the wing tips on the dinning room walls. It may make me sound careless but trust me, it would get really messy if you tried to swing a cat in there. :whistle:

Thanks guys

Brett

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Scram

Brett, seems to me your original question has, by and large gone unanswered - or only partly.

Seems to me that if you have a CoG position in mind, this will apply to both loaded and unloaded (or the other way around  :whistle:  )

If you check your assembled model CoG is where you want it and then take the wings off and set the fus alone on the CoG checker, you may find the balance point has moved.  That's OK.  All you need do now is load your ballast and adjust its position till you get the same balance point.  Put the wings back on and you should have your preferred model CoG.

No need for calculations or marking or measuring your tube length.  You can do the same with 50% ballast, distribute the slugs how you like as long as the pattern is even about the GoG yo want.

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mikef

I'm not sure if Scram is suggesting you make

1 the 'fuselage' plus ballast CG the same as the 'fuselage' only CG

or

2  the 'fuselage plus ballast CG the same as the required overall model CG (same ballasted and unballasted)

Note: 'fuselage' here means the model less the wing.

Note that in 2, the ballast CG position will generally vary with the ballast amount.

Both methods will give you a CG movement between ballasted and unballasted unless the wing CG is at the model CG.  The amount of movement will be small in the typical case - it will increase the further the wing CG is from the overall CG.  Formula will be provided if required but you don’t need it to achieve the original aim.

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oipigface

The CG of the wing is a concept with no relevance at all for the performance of an aeroplane. 

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mikef
31 minutes ago, oipigface said:

The CG of the wing is a concept with no relevance at all for the performance of an aeroplane. 

Agreed (as long as it isn't in a daft place like near one tip).    But it does matter if you take the wing off and try to calculate the overall weight and CG from the weights and CGs of the separate bits.  The concept of measuring the overall CG with the wing off was introduced in the first post.

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Brett82

Thanks Jerry, I was beginning to think it wasn't a good idea as it hadn't been touched on much. Mike pointed out something I didn't think of, that if the COG of the fuss is not the same as the full model then adding weight will move it, if only slightly. I just don't want to move it back, even slightly. 

Pete mentioned some guys in the states measure the COG of the model inverted so if that is possible then it solves my problem of having to flip the model over. 

With all the comments and support I have been given I should get it sorted, to be frank, the advice from everyone has covered all aspects so I would have to be a complete idiot to not get it right. 

Brett 

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isoaritfirst

Just mark the required CG onto the fuselage wing roots with a felt pen.

then measure the length of your fuselage ballast weights.

Ideally you have an odd number of weights, as this makes it easier to position them centrally around the Cg.

Position them into the ballast tube centrally around the mark on the fuselage wing roots.

You may need to make an end filler piece for your ballast tube to position the ballast weight correctly to suit your CG position.

Or you may need to ditch one weight and add a filler etc etc. 

It only gets complicated when you have wing ballast to consider which the Skorpion doesn't. 

 

With models with wing ballast I mark the wing root at my desired position and measure to the centre of each wing ballast pocket. using this dimension and the ballast weights its easy to calculate their effect and position accordingly. 

I make up a pictorial excel spread sheet where entering the weights into the boxes calculates their effect, I then print out the finished sheet to keep with the model.

 

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mikef
15 minutes ago, isoaritfirst said:

With models with wing ballast I mark the wing root at my desired position and measure to the centre of each wing ballast pocket. using this dimension and the ballast weights its easy to calculate their effect and position accordingly. 

If your wing ballast shifts the overall CG (due to offset from the model CG) do you have to put a suitable ’correcting weight' in an appropriate place in the fuselage?

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isoaritfirst

Untitled.thumb.jpg.f2274f282928a5952d137934a35a8218.jpg

This is a snap shot of my FS4 sheet.

The layout represents the 4 wing joiner pieces and also the 8 wing slot pieces.

colours show either brass or tungsten, numbers show their weight

Blue block shows total ballast weight

Centre vertical block shows distance ballast is from CG position and also calculated Cg position with weights added.

In the last one with full ballast you can see the CG changes from 97 to 99. All others stay at 97.

 

In a wing like this it should be remembered that, you don't have to mirror weights into identical fore/aft positions across the wing as long as you keep the balance across the wing. See the 375 weight picture for an example of this.

If you have to ballast with different weight across the wing, provided you don't go mad it is unlikely that In F3f style flying you will ever notice. 

 

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