Ebay sellers...

[PMIKEY]

Forget about weight!! C of G is about balance. The more weight you place on the tail the further back the balance point will travel. Therefore to move the balance point forwards you must add weight to the nose or preferably remove weight from the tail. On a side note that model looks to have a parallel chord wing along with a short tail and a shoulder mounted wing. I would go 30% from the leading edge.

[cymaz]

Balance the plane on the wing about 25% of wing chord, add weight to the nose to get the plane to balance level if needed.

The cg needs to be just on front of the centre of lift to enable you to have pitch control. The cl is about 1/2 -2/3 back on a typical wing. 

Edited By cymaz on 01/05/2015 06:39:43

[cymaz]

Posted by brfc7 on 01/05/2015 01:11:42:

Sorry but yous have got me all confused, surely the further back the CofG the more nose heavy ie 33% is more nose heavy than 25% so for a first flight is rather more nose heavy.

Baz

Try here Baz? They are talking of full size but the principles are exactly the same....air is air and lift is lift !

Edited By cymaz on 01/05/2015 06:46:33

[Colin Leighfield]

A plane is stable in level flight at any particular speed when the centre of gravity co-incides with the centre of pressure on the wing, i.e. the point where lift is focused. That is a function of the combination of effects of wing section and thickness, angle of incidence/decalage primarily. Classic free flight designs with flat bottomed or under-cambered wing sections and large tail planes, often themselves with lifting sections, have the centre of pressure occurring much further back, combined with a deliberate wish to have the aircraft tend to climb under power and glide level, have a centre of pressure which is further back, so the cg has to go back as well. Otherwise, the plane would dive into the ground.

A typical radio model with semi or fully symmetrical wing section and neutral lift section tailplane, plus the wish for a more neutral stability to make the plane controllable, develops the centre of pressure further forward and 25% cg typically will co-incide. I've never found that to be far off as a safe starting point. Taking Pmikey's point about 30%, I'd go further and say that it could be in the range 20/30%. Personally I would consider 30% back to be the limit and on the margin, but experienced aerobatic pilots who are comfortable with and indeed want a "twitchy" plane, will push the cg back deliberately to a point that lesser mortals like me would find hard work and probably dangerous. For me, the safe starting point is 25% and as Pete said, the smaller the tailplane area as a function of wing area, the further forward the cg needs to be for safe pitch stability.

Make no mistake about it, the further forward the cg is, the more nose-heavy the plane will be and vice-versa. Concluding the opposite is a fundamental error and dangerous. Don't forget either that the calculation must be made on the mean of the wing chord position. i.e. for a parallel chord wing with a chord of 10", a 25% cg will be at 2.5" back from the leading edge. However, for a wing with a swept back leading edge and a straight trailing edge, a root chord of 10" and a tip chord of 5"' the 25% cg will at be the mean of (2.5" back from the l.e. at the root and 1.25" at the tip), which gives you an actual position at the root of (2.5+6.25) divided by 2. i.e. 4.375" back from the l.e. at the wing root. You have always got to take the wing shape and relative taper/sweep of leading and trailing edges into account when you make the calculation. If the wing is tapered but the taper is equal at both l.e. and t.e, the cg calculation will be exactly the same as for a parallel chord wing, but if the tapers are different, the cg measured at the root will have to go forward or back with the dominant taper to achieve the designed position. There is loads of advice available on the ways to do the calculation for different wing lay-outs, but the basic principles are always the same. Don't get this one wrong.

Edited By Colin Leighfield on 01/05/2015 07:04:33

Edited By Colin Leighfield on 01/05/2015 07:07:52

[Peter Miller]

I work on this principle. If the CG is too far forwards you can always move it back. If it is too far back you can always build a new model.

A forward CG make a model stable. Arearward CG will makeit anyhting from twitchy to uncontrollable.

But then, those are just my opinions.

You will find that these days all my designs have the CG at 25% chord. THey all fly well and do nice aerobatics.

[The Wright Stuff]

Posted by brfc7 on 01/05/2015 01:11:42:

Sorry but yous have got me all confused, surely the further back the CofG the more nose heavy ie 33% is more nose heavy than 25% so for a first flight is rather more nose heavy.

Baz

Hi Baz,

The chaps above are absolutely correct, but I think I understand what you are saying, so perhaps I could offer a slightly different way of looking at it...

It is true to say that the further back the point at which you test the balance the model, the more nose heavy it will appear to be. Imagine the CofG is at 25%, then it balances if you support it 25% behind the leading edge. Now if you try supporting it 33% back from the leading edge instead of 25%, it will result in the nose appearing to be more heavy. To restore balance, you would have to add weight to the tail to get it to balance again. Hence if it is balanced at 33%, it will be more tail heavy.

Cheers,

TWS

[Pete B]

This is getting way off-topic, folks - please continue elsewhere if necessary. ta.....

Pete

[Peter Miller]

I agree,

I see that the Cox Stuka went to £310.!!!

I can think of much better ways of spending £310.

[onetenor]

Just as a reverse. I bought some bits that were slated by most of you guys. O ne was the wire cable and bits 99p ($1) Exactly what I wanted . Stranded steel ( light Laystrate ? ) with screw clevis at one end and a nice loop at the other nicely crimped ans a servo arm The other was a 35 Mhz mini Rx and 4 servos . all working £2 . One mans junk Eh!i

[David P Williams]

I think this one has the decimal point a couple of places out

[..]

The aviation fuel looks more like orange squash to me. Also how's he ever got to that postage amount, is he planing on flying it to the buyer ?

[Phil Winks]

while I can't imagine how he can envisage selling what at a cursory glance looks like an sc40 for that price the postage is what parcel force 48 charge for a standard small parcel

[onetenor]

It is O.S next to Blackcurrant juice if you look closely. The description is an attempt to say what fuel he thinks the engine takes I think

[Peter Miller]

Posted by David P Williams on 01/05/2015 18:42:03:

I think this one has the decimal point a couple of places out

I did send him a question asking what make and what size it is. I also commente that at that price it must be very rare and special No reply yet.

But then he seems to sell olive trees and scrap car bits.

[Peter Miller]

Posted by Peter Miller on 02/05/2015 08:25:16:

Posted by David P Williams on 01/05/2015 18:42:03:

I think this one has the decimal point a couple of places out

I did send him a question asking what make and what size it is. I also commente that at that price it must be very rare and special No reply yet.

But then he seems to sell olive trees and scrap car bits

At least the price has dropped.

I sent another question again asking for size and make and pointing out that the photo could be any one of about 40 engines.

No reply. I shall try again.

[adrian lawrence 1]

How about this **LINK** may be of use to someone.

[PatMc]

Posted by adrian lawrence 1 on 06/05/2015 19:16:10:

How about this **LINK** may be of use to someone.

It looks like a Cloud Airmaster from the Ben Buckle plans.

[Peter Miller]

You thought that EBay sellers were nuts. Well I have just been told about this modern art auction in Germany

THis auction features a model aircraft, Look at the bid and the estimated sale price.

**LINK**

Get out your old models, sell them as works of art

[Martin Whybrow]

Posted by Peter Miller on 08/05/2015 08:23:07:

Get out your old models, sell them as works of art

That's crazy!!! I see it's made using bits from those rubber powered kits you used to get in a plastic bag in the local toy shop, so there's not a great deal of creativity in the design.

[Erfolg]

Call that art, pah!

If only I had taken my art installation (about 6" down), of my Me 110 in a muddy field, titled "oh well" to the Tate Modern. They would now have an additional asset worth millions.

Edited By Erfolg on 08/05/2015 12:20:45

[DH 82A]

"Get out your old models, sell them as works of art" My models are works of art.

[onetenor]

Came in paper bags in my day

[Kevin Wilson]

The website wouldn't accept my bid of 50p, which I thought was quite generous considering the rubber band is broken.

[Pat (rActive) Harbord]

As I recall it was part of an installation in Tate Britain. There was a long mechanical production line that was supposed to take the raw components and assemble, then launch a plane every 2 minutes. I don't think it ever worked. There was one single example on a plinth under glass that turned out to have been made by hand. Tate never admitted it, but the common view is the artist was being ironic and it was never meant to work.

[Steve T]

I have now estimated my collection of hangar queens at about £500,000 if I call them art rather than models I'm trying to find time to fly. Then I can buy a real plane and get my licence.