c o g

[The Giggler]

Im under the impression using 30% of the root of the wing is a good starting point,Does this apply to tappered wings or just staight wings? Mine are a tappered back leading edge and a straght trailing edge.

 

Cheers All.

 

If this has doubled please exept my appoligies.

[Richard Wood]

Hi Giggler

28 - 30% at the wing root  should apply to tapered wings as well as

straight as far as my understanding goes.

This is usually a good starting point.

I used to use the stubby fingers method of checking the balance point

but have recently starting using a CG balancing machine (Great Planes version)

for better accuracy & repeatability etc.

 

 

Edited By Richard Wood on 12/08/2010 15:10:27

[Lee Smalley]

i was always taught to use MAC  mean aero chord

 

root+tip chord /2 and then 30% of that

[Biggles' Elder Brother - Moderator]

I agree with you Lee. If the taper is only slight it obviously doesn't matter so much. But if there is a pronounced taper you should use the mean chord. It will give you a more "conservative" estimate and you may well find that, on flying the model, you can go back some way from that - but at least its a "safe" starting position.

 

BEB

[The Giggler]

Thanks Richard and Lee,  Question for Lee, When working out the tip chord how would you measure the tip in the link? Would you use a line from the end of the airlon ?? As you can see the tip is also tappered.

[Biggles' Elder Brother - Moderator]

Yes just use the perpendicular line - remember its only an approximate technique to give you safe starting position.

 

BEB

[The Giggler]

Ok, As you know im new to the hobby, The wing root is 325mm And 1/3 of that is 108mm.

 

But if i use Lee's method which i have read before the root is 325 plus the tip 220mm

 

  325+220=545 / 2=272.5 / 3 = 90.8 mm. 

 

For a new to me plane never seen fly with one of my engines i have fitted would you first set it to 91mm  or the 108mm.

 

Sorry for going on with this.

[Richard Wood]

No worries.

That's a fair taper & you would probably be best to set  it at the forward

point to start with - much safer.

You can move it back later like BEB says.

 

[The Giggler]

Ok thanks for the input. I am about to burn the plane ! No not really but that's how I feel about it. On low wing planes the undercarrage can I change the height of the plane ? If I move the C o G near to the 90 mm it will tip forwards very easy with a dry fuel tank the only way around this is to alter the angle of the under carrage,but I will need to make new as.the prop is too close to ground at 6 oclock . I think it will be cutting the grass on take off and landings.

[Erfolg]

I think I have read this thread properly, but may not.

 

Anyway adding tip and root and dividing by 2, only works if of equal taper.

 

The more certain method, was Lee's method,that is find MAC and take 30% of that line.

 

Another proviso, this only works if the taper is constant on LE and TE. Extreme examples are some full size gliders with multi break LE or even a spitfire with an asymmetric parabola wing plan-form.

[Biggles' Elder Brother - Moderator]

Giggler, take a look at this thread. It points onward to a GoG calculator. When using it put your stability margin in as 10 (that's 10%) which will be on the safe side. This will give you a more accurate result.

 

My guess is the mean chord method gives you avery safe answer, the root chord is a bit more "dodgy". You'll probably find that around the middle between them will be OK - say 100mm.

 

BEB

[Biggles' Elder Brother - Moderator]

Giggler, take a look at this thread. It points onward to a GoG calculator. When using it put your stability margin in as 10 (that's 10%) which will be on the safe side. This will give you a more accurate result.

 

My guess is the mean chord method gives you avery safe answer, the root chord is a bit more "dodgy". You'll probably find that around the middle between them will be OK - say 100mm.

 

BEB

[Geoff N]

Try bigger wheels if the prop is to close to the ground .Gives a bit more crearance and a smoother run if strip is rough.

[The Giggler]

Biggles' older bro, i tried that link as a first port of call, But after spend hours measuring the plane it came back with @ 5% margin the C o G was almost 3/4 back from the leading edge, only at 15% it comes up with a 139mm back from the leading edge, The root is 325mm.

Thats the confusing part, I have no idea what model it is ,Who made it and when, Perhaps i should not have bought it??  The guy who sold it on a well known auction site!! said it flys really well, But if i was selling it i would not say it's a load of rubbish!!

 

If you follow the link you just sent and go further down to the bottom of the thread you can see a link to a photo of the plane.

 

My main fear is the further back the  C o G then once fueled up it will just tip forwards.

 

It has a 63" wingspan and a 54" fuzz ,I have put in a .61 2stroke motor in it,So nothing daft proberly under powered but thats all i have being new to the hobby.

 

Thanks guys for your help,You have no idea how much assistance your all being.

 

Cheers The Giggler.

[Biggles' Elder Brother - Moderator]

OK, my advice would be to set it at 95-100mm back from the leading edge at the root, then fly it and see. That should be pretty safe. You might want to dial in a little expo on elevator as a "belt and braces" just in case its a little twitchy - but I really don't think it will be. If you want, get a very experienced pilot to take it up for you, but I really don't think you'll need it.

 

Assess the flight. If you find that the eleator is sluggish and on the glide she's a bit nose down and fast then the CoG is probably too far forward. Take it back 5mm at a time until your happy.

 

All methods of finding the CoG are approximate and intended to just give you a starting point - the real proof of the pudding is in the eating - or in this case the flying!

 

BEB

[Richard Wood]

Hi Giggler,

If the undercarriage legs are piano wire secured to hardwood torque blocks

in the wing you could easily bend up another set to give you more ground

clearance.It could also be raked forward a little to help with the tipping forward

problem.

I had a model prone to tipping forward like this (Galaxy Magician) & bending

the legs forward made a big difference.

The model in that link looks a bit like a Dave Smith type design (Hmmm)

It looks like a nice model though so give it a go!

Good luck.

 

 

[Richard Wood]

It looks a bit like the Dave Smith Smart Move with altered wing tip shape.

Link ??

 

Edited By Richard Wood on 13/08/2010 09:14:05

[The Giggler]

Thanks Richard, After looking at Dave Smith's web site it could be a early DSM kit,I will try to email the with some pics and details.

 

Does anyone on here fly The Exelsior 150/163/177 model ? Just wondering what sort of C o G you have?

 

Cheers again guy's.

 

The Giggler

[Lee Smalley]

Erfolg is correct if you want to get seriously into it, but i will put it into laymans terms

1, this is only ok to do if your wing is fairly standard and the taper is not very high

2, its a very simplified version but to be honest the difference between the two versions is quite small and it will only make your aircrafts c of g slightly more forward, but thats no biggie so here is what i would do to keep things simple

 

measure root chord, measure tip chord, divide that by 2 and then take 30% of that  say for instance that = 2" then i would measure 2" back from root leading edge and set it at that.

 

if you have a highly swept wing then you need to find the MAC (mean aerodynamic chord) and then you set the 30% back from the leading edge at the MAC

 

not too complicated

 

to be honest if you are after setting the c of g on a basic wing then 30% of average chord measured back from the root will suffice, you might be a tad nose heavy but at least you will not bringing her back in a bin bag

[Lee Smalley]

the c of g some people fly the excelsior at will not be suitable for a first flight as they will have this type of aircraft (if they fly it right) right at the back to keep the aircraft neutral go with 30% of MAC and then move it back to suit your flying style, my yak has a rearward c of g and depending on the wind actually slightly climbs on the inverted

[Lee Smalley]

all DSM models did the old nosing over on take off or landing his placement of the undercarrige left a lot to be desired on our rough fields, they were fine for tarmac but rubbish on rough fields i bent my aerostar 62 legs forward quite a bit on mine i suggest you do the same

[Lee Smalley]

oooo richard love the smart move ... very nice !!   i want one !!

[Richard Wood]

Can't take the credit for that one Lee - it's off Dave S's web site!

I think a bloke called Zook built it!

 

A clubmate has had one though & he loved it.

 

 

 

[Lee Smalley]

dman it i was going to make you an offer !!

[Former Member]

[This posting has been removed]