Centre of gravity

[Paul Williams]

I picked up a seconded hand sports model this weekend, I have no idea what make it is only it has a super tigre 51 two stroke engine. I was checking it over when I thought about C/G. Where is a good place to start, it has full symmetrical wings and is a mid winger.I was alarmed to find 275grams of led strapped to the engine mounts! any ideas how to the calcs for this type of plane?

[Tim Mackey]

Start at around 25% of chord from the leading edge back - then adjust as rerquired after test flights

[Paul Williams]

Tim, with out sounding stupid where is the wing chord? Are we talking mean aerodynamic chord? how in layman's terms do I find that. Sorry to be a pain its a long walk to the flying field I would like to be some what confident it will fly when I get there.

Have a look at this web site which I just found CG

[Brian Cooper]

Would it be possible to post a picture of the model?

 There are many people on here who could probably recognise the model and then be able to give the C.G. information.

B.C.

[Tim Mackey]

If it is fully symmetrical in both section and span chord ( a la WOT4 early versions etc ) then just use any point along the span... if its a tapered wing, then I usually just go in approx 25% of span also from the root, and take my 25% of chord at that point. It may not be a scientific way of doing, but then I am not a scientist, or even an engineer.... but it has always worked OK for me.

Of course with a swept wing things do get slightly more complicated

I would say 25% of MAC should be OK...but others may have other thoughts - in fact I will be very surprised if they dont!

[Paul Williams]

I would be interested in anybody's view of the web site which does the calc's for you. Here

will post piccy later.

[Paul Williams]

Right just entered the aircraft measurements into the spread sheet on web site and its come up with 151mm back from leading edge at root chord. This has me confused, remove the 275g of lead from the nose and the plane balances bang on this point. Why did the previous owner have the lead in? Surely it must have flown.

I did the exercise with my twister edf entered the figures and it gave me a 109mm figure which is bang on where multiplex put the little knobs for you to feel when setting up.

The site gives a variation of 5-15% static margin. even explaining how the margin will affect its performance. Brilliant.

Will some body else try it confirm it works please.

[Tim Mackey]

It works yes.... nice little site

As to the 275g of lead in the nose... some peeps just like a forward COG

Why not leave it in, fly her and then adjust if you feel comfortable with it. Also, maybe the previous owner had a different battery pack or whatever - with consequential weight difference.

[Paul Williams]

thanks Tim, I tried it this afternoon using the figures from the web site as a start point and was well pleased, It needed just 50g of lead and all was straight and level. Sorted.

[Jo]

That "C of G calculator" site mentioned above is another misleading one. Like nearly everything else published on this topic - books and current magazines -  it contains two errors: 1) The NP (given by 'Vbar') is wrong. 2) Stability has nothing to do with the length of an MAC. Suggest looking at my method. Another site includes correct NP in an easy to follow diagram here, scroll to para "The simplest way of locating the aircraft's NP..." 

However, the usual error in NP location depends on the ratio of tail to wing area and is quite small when that is small - as for the usual conventional model. But, for such as a tandem-wing design the error is enormous. 

[Jo]

Thankyou too! May I say that my experience is of about 20 years of getting 'funny' models in the air - and back again - based on the work of the late David Fraser, an American glider designer of note who died young in a Cessna crash. I think he got 'CG position' right because my only real maiden flight failure in some 20 odd different model launches since then was due to my incorrect data entry in my computer program application for a (pusher) tailless model. Happily, the error was corrected, and it flies today.