help finding a reference for the C of G…?

[JulianJ]

Dear All,

When setting the C of G, which part of the aircraft should the imaginary horizontal line run through? With the complete lack of flat horizontal surfaces, which part of the aircraft should be referenced as the horizontal?

I wondered about using a large engineers square to ensure the prop is exactly vertical, or would this be a bad idea incase of any amount of down thrust ?

The aircraft in question is a Durafly Tundra.

Thank you in advance for any help!

Kind regards

Julian

[Former Member]

[This posting has been removed]

[JulianJ]

Hi Percy,

Thanks for that. I have no question regarding the balance point, which is detailed in the manual.

The model balances well when supported at the balance point. However by moving the battery for or aft by just half an inch I can make the model balance slightly 'nose up' or slightly 'nose down'. Somewhere in the middle is presumably correct, but *exactly* where? It's easy enough to get the C 0f G 'about' right, but I'm looking to get it 'exactly' right….

BTW I like your old adage a lot !

Thanks for your help,

Kind regards

Julian

[Former Member]

[This posting has been removed]

[The Wright Stuff]

You may be overthinking this a little, Julian. There isn't really a satisfactory definition of 'exactly right' - even the position described in the manual (or marked on a plan) is still only going to be an approximation.

The reasons for this are somewhat involved, but suffice to say that when you measure it, the model is static, whereas in flight, the model experiences lots of other forces, so in effect, the 'perfect' position is a function of the airspeed.

As Percy says, if in doubt, err on the forward side, but once you get down to the last couple of millimetres, the difference will be completely negligible.

[Don Fry]

For exact, you get a spirit level, and do the measurements with the leading edge of the tail plane level with the trailing edge of the tail plane. If it is a airfoil cross section unit you may need a jig to put the level on. Don't forget to remove it when you check the balance point, unless you want to fly with a level attached to the tail plane. But as everyone says, if in doubt, forward C of G is safe.

[The Wright Stuff]

In this regard, there isn't really a definition of 'horizontal', because you are really talking about moments (clockwise or anticlockwise) about a pivot point. The illusion of 'balancing slightly nose up' or 'nose down' is largely because you're inevitably balancing on something (fingertip?) that has a finite area. As the model tips forward, you are actually inadvertently holding it a millimetre or so further forward, so it stops tilting further.

So, to answer your original question literally, you need to balance it upon a tip of a pin: then it either falls forwards, or falls backwards, with no in-between. Unless you invest in a purpose designed measurement rig, the closest you can get in practice to this is to stick on a couple of those little domed rubber cupboard door stops on the underside of the wings in the desired CG location, then balance on your fingernails.

...but as I said earlier, largely of academic interest...

[JulianJ]

Thank you Gents,

I think I may have 'C of G OCD' if there is such a thing……

I have enclosed a picture of the aircraft on my balance jig. The plane is supported exactly on the C of G point measured as per the instructions, on 2mm tack heads, (so not 'quite' a pinhead!)

What is the general consensus? The 'lines' in the garage door are exactly level….

Kind regards,

Julian

[Simon Chaddock]

Julian

That would certainly do for me!

Whether this position is 'exactly' right will take some experience and/or experimentation to determine and is to a degree dependent on personal preference.

Edited By Simon Chaddock on 02/06/2016 18:00:55

[Biggles' Elder Brother - Moderator]

Julian,

with no wish at all to be disrespectful (honest!) I think you really need to lighten up on this one a bit.

The fine tuning of the CoG can only be done when flying the model. The objective at this stage is to simply get it somewhere "there or thereabouts" and preferably on the safe side. You don't need it better than that. Get it about right, but safe (ie slightly nose down) then fly it and see how it feels. Make CoG adjustments in very small increments 1/4" back or forward is plenty.

BEB

Edited By Biggles' Elder Brother - Moderator on 02/06/2016 18:16:10

[Peter Miller]

BEBis quite correct but I will pu my twopennorth in.

Looking at your set up that is great. Now it is commonly accepted in some places that the tailplane should be horzontal. If it has a section other than flat plate then just mark the centre of curve of of the leading edge. Then measure from that mark and the sharp trailing edge. If those measurements are equal the T/P is level.

Now having said all that I just mark the point under or over the wing by making narrow little arrows from three layer of tape. THen I put my fingers under the marks and see if the model hangs roughly level. IT works every time.

Unless you are working right at the rearmost position it won't make a significant difference if it is a fraction either way

[Don Fry]

Fine looking aeroplane, and, having had three HobbyKing offerings, I observe that the stated C of G is forward, and needs to come back with experience. Too small to be an exact sample, but a useful indicator.

[JulianJ]

Thanks Chaps,

I'm genuinely grateful for all your help. I shall indeed take your advice to lighten up & stop being so geeky! Thanks for wording your replies so politely!

Now that it is confirmed the C of G is correct I trust the plane will fly level with minimal trim on about 2/3 throttle?

Kind regards

Julian

[Former Member]

[This posting has been removed]

[Bob Cotsford]

Now that depends on two factors - how well matched the power setup is, and what trim the designer/manufacturer felt it was safe to specify. It won't be a million miles out. Have you tried a Google etc. search for reviews of the model?

[Former Member]

[This posting has been removed]

[Biggles' Elder Brother - Moderator]

The guys are right Julian. What you have will almost certainly be safe. Will it be optimum? Probably not! But that is what tuning flights are for. Manufacturers don't know the skill level of the people who will try to fly theor models - so they tend to err on the safe side and specify the CoG forward of what might later very well proove to be th best position.

If it feels a little nose heavy (especially when inverted), a bit slugish on the controls (particularly the elevator), reluctnt to flair on landing, then its probably the case that the CoG needs to come back a tad.

If its a bit twitchy on elevator - over touchy - the whole thing is a bit of a roller-coaster ride! - then the CoG needs to come forward a tad.

Its unlikely to be in the best position for you straight off, you will need to tune it.

BEB

[The Wright Stuff]

BEB has it spot on, as always. Part of the issue is that without having flown the model, you have no way of knowing whether the manufacturer's specified CoG is neutral or conservative. So if the manufacturer has already built in a margin 'of safety' by specifying a CoG a bit forward of where it should be, but you don't know this, and then add a further margin of your own, then you could end up with a CoG that is significantly further forward than it should be. On the other hand, there may be no margin at all. You don't know. Word of mouth (and forums) is the best bet, here...

'Forward is safer' or '...flies once' are good rules of thumb, but do apply them with caution, too. My own issue was that up to a certain point in my progress, I'd never flown a model that wasn't nose heavy, so it's quite easy to assume that it's the norm. It wasn't until I started to get comfortable and experiment, that I realised I could move the CoG back quite a long way, and that I'd been flying nose-heavy models for two years. Because I'd had it drummed into me that a rearward CoG means instant death, I was too terrified to move it back. The 'dive test' can be quite difficult to interpret for high-wing trainers, since they are designed to recover to some extent, and inverted flight isn't easy if there's a lot of dihedral.

I suspect all of this will make much more sense when you have 10-20 flights on it and it's all trimmed out. Good Luck!

Edited By The Wright Stuff on 03/06/2016 08:14:28

[Peter Miller]

It is intefresting that I like to put my CG at 25% chord but one Ballerina builder has moved it back to 1/3rd chord and finds that the model still flies perfectly.

I still think that is a bit too far myself but he likes it.

[JulianJ]

Gents,

I owe you all a beer….

To conclude, we've just returned from the flying field after a very successful and enjoyable hours flying. As well as fine tuning the C of G, I very carefully measured all the control surfaces and mechanically set the trim with the servos zero-ed.

We found we needed just a few clicks on the ailerons & elevators to get the plane flying straight and level. It's a lot more enjoyable to fly now & I'm grateful to everyone who has contributed on this thread.

Kind regards

Julian

[Chuck Plains]

Heh, it's a foamy. Very unlikely to have the same CG as it's original drawing. Foam density will vary somewhat all over the aircraft. And placement of the wings and empennage could vary by up to 3mm-ish. Just enjoy it.

[OZ e flyer]

So glad I read this thread. Like Julian, I was concerned about the CoG on my newly completed sleeker. It appears to be OK but my research lead to as many questions as answers.

It looks like I might be a tiny bit tail heavy, but I can fix that easily by moving a couple of things around so no biggie. the questions in my head were like Julians, what is nose down when a fuselage doesn't have parallel or truly horizontal lines? It all seemed to be a bit subjective and my technical brain wanted something measurable. So, more research ensued. (I know, lighten up right?)

If my thoughts are correct (please let me know if I'm wrong), what we are really talking about is the angle of attack of the wing surfaces and the effect of weight distribution in the model forward or aft affecting said angle of attack.

That said, is it a fair assumption that a truly "balanced" model will have a horizontal wing angle through the cross section of the aerodynamic line? (Basic wing design with a flat bottom would have the bottom of the wing horizontal for example). Similar to what was said earlier about a horizontal tail plane.

Therefore what we want is a wing that is aerodynamically slightly lower at the leading edge than the trailing edge.... kinda...... I think....... maybe..... lol.

Sorry if I'm causing more confusion but I guess like Julian I want to fly a lovely looking plane without fighting it cause I didn't understand what I was doing.

TP

[TIM Shaw]

Sorry TP

I Really don't think so.

A Plane in flight is a compromise between all sorts of different forces acting on it, and the relative positions of where those forces act.

So a model will balance on its centre of gravity, yes, but it is the relationship between where that is to the aerodyanamic centre of pressure that governs stability in flight.

So while the static measurement of CG is important to determine a "safe" starting point, the model can only really be dialled in while being flown, or rather, between flights, and the way the wing sits relative to the horizontal while on the balancer is not really relevant.

I know it's actually even more complicated than that, but I don't think it's worth going into that just that now.....

[Chuck Plains]

Posted by Tipsy Pilot on 21/04/2017 14:12:36:

So glad I read this thread. Like Julian, I was concerned about the CoG on my newly completed sleeker. It appears to be OK but my research lead to as many questions as answers.

It looks like I might be a tiny bit tail heavy, but I can fix that easily by moving a couple of things around so no biggie. the questions in my head were like Julians, what is nose down when a fuselage doesn't have parallel or truly horizontal lines? It all seemed to be a bit subjective and my technical brain wanted something measurable. So, more research ensued. (I know, lighten up right?)

If my thoughts are correct (please let me know if I'm wrong), what we are really talking about is the angle of attack of the wing surfaces and the effect of weight distribution in the model forward or aft affecting said angle of attack.

That said, is it a fair assumption that a truly "balanced" model will have a horizontal wing angle through the cross section of the aerodynamic line? (Basic wing design with a flat bottom would have the bottom of the wing horizontal for example). Similar to what was said earlier about a horizontal tail plane.

Therefore what we want is a wing that is aerodynamically slightly lower at the leading edge than the trailing edge.... kinda...... I think....... maybe..... lol.

Sorry if I'm causing more confusion but I guess like Julian I want to fly a lovely looking plane without fighting it cause I didn't understand what I was doing.

TP

Ah, yes, and no, in my opinion. I'm sure you're over thinking it all.

What follows is me thinking out loud.

If you have flat plates for the wing and tailplane, zero wing to tailplane incidence angle and undercarriage that keeps them perfectly parallel to the ground, your plane will never take off. It will fly if you hand launch it, but you'll be applying up elevator to maintain level flight.

Having a wing's lift effort lower at the front than the back is not something I've ever heard of and anyway the way an airfoil performs changes as the angle of attack changes during changes in airspeed, if you see what I mean.

If your wing has a flat bottom and your tailplane is either flat or has a symmetrical airfoil, your plane will fly with a zero incidence angle. But it may or may not climb under power. From what I've learned since I started just over 3 years ago, a reasonable climb under power and a horizontal flight at about half power, with the plane flying itself is what we need

An online CG calculator won't ask you about your airfoil, or any wing incidence angles, at least, none of the ones I've seen do.

So, as Tim mentioned, basically, get it up and flying and then you'll find what adjustments might be needed. But, as I mentioned further up the thread, If it's a foamy, don't worry too much, chuck it and see.

Edited By Chuck Plains on 25/04/2017 21:38:55