COG measurement

[Vecchio Austriaco]

Just because of using my tool at the moment to set the COG on my Seagull Extra I thought I could make a picture or two to show what it is about.

You can adjust the width to take different fuselage sizes as well as the height - to stay close to the ground should your expensive model slip off.

The actual reading is done from the two ply strips with the printed cm sale. As you read on both sides it helps avoiding misalignment.

Now you see it in action

Here you see how the reading is done. Works well, if the producing companies give you values for the leading edge centre. For this plane they had the brainwave to give leading edge values at the wingtip....
Which means you have to transfer the wing tip position with a bit of sewing thread to the wing centre.

Material: light ply, 8mm and 3mm (was very cheap and available in every DIY market in Italy...), 4 screws for adjustment, white wood glue.
Done in one evening if you have a small circular saw.

VA

[Danny Fenton]

looks good VA, and simple to make.

Cheers

Danny

[Devon Flyer]

Well done VA.

This is, however, a balance point measurer and not a CoG guage.

Balance point and CoG are entirely different things.

Edited By Devon Flyer on 19/05/2012 12:56:48

[dav84]

I have made something similar. Mine the balance frame is seperate to a measuring tool that I made on a laser cutter. The tool is much like a engineers square however on the 'arm' part of it i have engrave a scale.

The advantage of this is that you push the stock up to the leading edge so there is no guess work due to the curved nature of the leading edge.

You could always add a slider that moves along the rulers to give an accurate reading of the CoG.

Hope that makes sense, if you want i can post a pic of my tool I made.

Dave

[Bob Moore]

This is, however, a balance point measurer and not a CoG guage.

Think you meant gauge!

But I don't understand what you're getting at. Having just put a Wot4 wing on my old Peppi trainer (bit of an experiment and mentioned in another thread.) And balanced it on my finger tips at the recommended CoG on the as described on the Wot literature . 82mm from the leading edge, I'm wondering if my experiment is doomed to failure!

Edited By Bob Moore on 12/06/2012 01:18:16

[Martin Harris - Moderator]

The C of G is actually a 3 dimensional point but we usually refer to the balance point as the C of G.

The balance point is a vertical line in the flying attitude which passes through the C of G

I suspect that it's a little pedantic though..although the point is correct.

Bob, the tail moment is the critical factor in determining whether the WOT4 C of G is suitable. If the tail area and distance from the wing are similar then all should be well. Opt on the forward side and move it back if it's very sluggish in pitch - or look at some of the on-line C of G calculators which mostly seem to work OK for me...

One example is this one (which I think is one I've used successfully in the past) and enter a static margin of 15% to be on the safe side - best bet is to search around and try a few different ones and see if you get general agreement!

For some more detail, have a look at the "Stability Concepts" section here.

Edited By Martin Harris on 12/06/2012 10:18:16

[Bob Moore]

Thanks for the interesting comments Martin. I had wondered if it would work. And comments on the other thread said it should be ok. The tail on the Peppi is smaller and the fusi a fair bit shorter. I can increase the tail area easily but to extend the fusi very difficult.

I haven't test flown it but I'd balanced it at the same position on the wing as recommended for the Wot4, with a slight nose down bias. But you're suggesting tail heavy is better if I read that right? I'll use the calc you link (very useful thanks) and see how that works out.

[Martin Harris - Moderator]

Nooooooooooo!!!!!!

Don't go tail heavy whatever you do!

If the tail effectiveness (area x distance) is less (which seems to be the case) then, depending on the positioning of the wing, it's very likely that the C of G will need to be further forward on the wing than it was on the WOT4.

An extreme example is a tailless model where the C of G may be pretty much on the leading edge - the tail effectively being the trailing edge with reflex giving the longitudinal dihedral (I think that's BEB's preferred term over the often used decalage?)

Edited By Martin Harris on 12/06/2012 11:50:39

[Bob Moore]

Thanks. That's the best CoG calculator I've seen and I have tried a few in the past. It calculates to 53.2mm as opposed to the 82mm which is a considerable difference and would make it very tail heavy. Shame you didn't see my other post where I asked if it was a bonkers idea to put this wing on the Peppi!   To achieve balance I'd need to add a load of lead and hang the engine a long way out front. I could increase the tail area quite easily which by calculation moves the CoG a bit further back. But not a lot. I guess a combination of all might do it?

 

Any suggestions please!

 

 

Edited By Bob Moore on 12/06/2012 13:11:40

 

Edited By Bob Moore on 12/06/2012 13:12:26

Edited By Bob Moore on 12/06/2012 13:13:55

Edited By Bob Moore on 12/06/2012 13:14:52

Edited By Bob Moore on 12/06/2012 13:26:43

[Martin Harris - Moderator]

Can you play with the wing position? Perhaps move it forward a bit?

Try the tool with a lower stability margin - 5% should still be flyable...but a lot twitchier.

Edited By Martin Harris on 12/06/2012 14:13:52

[Bob Moore]

Not easy to move the wing forward, in fact I'd actually had to move it a few mm further back. I'll sort it with a bit of fiddling about! Thanks for the help and advice. Appreeciated.

[Bob Moore]

Out of interest I put the Wot4 specs into the CoG calculator and at 15% it came up with 5.85 . Quite a difference from the Chris Foss recommended 8.2. @ 10% it's 7.8 which is closer.

I think I will increase the tail area and move the engine a bit further forward and see how that calculates?

[Martin Harris - Moderator]

That seems a good way to go. 15% is playing very safe so the 10% margin may be more than adequate. How about reverse engineering Mr Foss's C of G to establish the static margin and try inputting that to your new layout?

[Bob Moore]

Good idea Martin. Will give me something to do while this rubbish weather continues!

[Paul Adams]

With regards to C of G balances, I use two pins laid on the wing surface at the required C of G location fixed with some low tack masking tape. I apply these early on in the build, so when I have located all the unmovable items I can check the C of G by just picking up the model, feeling where the pins are with my fingers and do a quick balance. I then lay on the servo's, receiver, batteries etc and move them about to try and obtain a balance without adding additional dead weight.

Bob, I would recommend the calculator on this Website:- **LINK** I use it on every model I have and found it to be very good.

[Bob Moore]

Thanks for the link Paul, that is in fact the one Martin linked too, and it 's the best one I've ever seen for sure.

[Garbo]

For mid size models I tape two cocktail sticks at the suggest COG and this gives a better "feel" when using my fingers

[John Privett]

To accurately measure the C of G of any model you could try the "Vanessa CG machine" designed by the late Jim Archer, and documented here.

Jim was a keen aeromodeller and active on the usenet newsgroups in the earlier days of the internet before forums like this really existed - I'm sure there are several members on this forum who were also active on the old newsgroups and will remember him from there. I reckon that if he was still with us Jim would be an active, and valuable, member on this forum too.