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Does the nominal C of G work for you?


The Wright Stuff
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I'm currently wondering how conservative ARTF manufacturers are when they specify the centre of gravity position in the instructions, or mark it on a plan. Is it deliberately too far forward to make it 'more forgiving' for beginners? Perhaps there are some designers on here who can comment?

I'm currently fettling a HK Night Walrus (taking advantage of the darker evenings), but here is how my last four builds have gone:

  1. Assemble aircraft according to the build plans/instructions.
  2. Mark the recommended balance point on model wing with a permanent marker 'dot'.
  3. With battery installed, balance the model on fingertips with battery as far forward as it will go. Find it is tail heavy.
  4. Spend the next two weeks trying to carve out the inside of the nose cone in order to make room to stuff lead.
  5. The model now balances level when suspended on the marked points.
  6. Fly the model. Find I need arm loads of up elevator trim to maintain level flight. Aircraft then swoops up out of a dive test, and porpoises horribly when slowing it down. Climbs like a homesick angel on half throttle.
  7. Land.
  8. Find the elevator is 5 mm up from the centre line of the tail-plane.
  9. Take all of the lead back out of the nose again.
  10. The model now trims nicely, but with the centre of gravity 10 mm behind the marked points.

Unless I am completely missing something, I can only assume that many model manufacturers or designers want me to start off with the CofG in a 'safe' place (i.e. forward of the ideal position). It could be the way I measure it, or the way I fly. Does anyone else suffer from this, or (to quote the late Mr Wogan):

is it me???

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Regrettably, yes, many kit manufacturers suggest CG locations which are way too far forward. . Presumably they do it to be on the safe side, but it can result in a very dull aeroplane.

Models definitely fly better and have a more precise response to control inputs when they are balanced properly.

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Yes, the OP is undoubtedly right. I've gone through exactly that sequence many times (today even!).

The problem is that you only find out by flying the thing, and not many of us dare to risk a first flight with the CG a long way aft of the specified point...

My partial "solution" is to make sure that any mods made to achieve the CG position are very temporary and easily reversible. For example, where TRS says he carved away a load of foam, I would have firmly taped-on external ballast, just for a quick flight (less than one minute) to check the trim.
I only make any ballast permanent (built in) when the best flight CG is found and, very often, you find that you don't actually need any!

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I sometimes wonder if some manufacturers - particularly the 'cheap n cheerful' ones - actually fly & develop their models before writing the instructions & putting them into production.

In my experience Sebart, E-flite, Hangar 9, Multiplex, Chris Foss, ST Models and the latest from Arrows Models & FMS have settings which can be relied upon. Others I approach with a degree of caution & scepticism.

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Not always. I bought a model Sukhoi on eBay as a bare, used airframe in quite reasonable condition but without instructions. In these days of th internet that wasn't a problem and I downloaded them OK. I fitted an engine (ASP 53? - it was befiore I went all electric, so a while ago) and set the CoG as per the instructions.

The maiden was a bit exciting! The elevator was very sensitive but I managed a circuit and landed OK (I was quite proud of myself). Further investigation on-line revealed that it was well known that the CoG in the instructions was much too far back. I corrected it and all was OK. I've still got the moddel. It's not been flown for years so really needs to be rehomed

So I'd advise a little investigation and check on other flyers' experiences before commiting to a CoG further back than the instructions state.

Geoff

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Unless your model has a very unusual planform or layout it's relatively simple to check whether the CG in the instructions/plan is in the right ballpark - there's lots of calculators online, or you can do it manually on a bit of paper. I never blindly set it based on the instructions (I've been burnt by that in the past, in both directions) with the exception of plank flying wings where I know the designer will have paid studious attention to the CG position over many, many test flights (you can't make a treue plank fly well without optimising the CG).

Edited By MattyB on 01/10/2020 17:05:04

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The Max Thrust Riot gives a CoG of 75-90mm in the instructions, cgCalc gives a figure of 78 to 92mm. With the CoG towards the front it's very docile, with the CoG towards the rear it's still rather docile but will fly inverted with very little down elevator although you won't get a flat spin.

On the other hand the Dancing Wings Spacewalker I built shows CoG at 70mm and set at that it was a wild ride on the first flight. cgCalc gives a value of 45 to 55mm which was much better.

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What about - as a starting point if you're unsure of just how reliable the instructions are - basing your initial CG on the thickest part of the wing (adjusting for swept planforms etc) with a little erring towards an initial nose-heaviness? Then test-fly, being ready to land as soon as necessary to add temporary weight to the tail or even sometime the nose, then progressively fine-tune by carrying out the usual inverted tests etc?

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Posted by Shaun Walsh on 01/10/2020 17:14:09:

The Max Thrust Riot gives a CoG of 75-90mm in the instructions, cgCalc gives a figure of 78 to 92mm. With the CoG towards the front it's very docile, with the CoG towards the rear it's still rather docile but will fly inverted with very little down elevator although you won't get a flat spin.

On the other hand the Dancing Wings Spacewalker I built shows CoG at 70mm and set at that it was a wild ride on the first flight. cgCalc gives a value of 45 to 55mm which was much better.

Case in point: exactly where is the thickest part of the Spacewalker wing? 50mm-ish or 70mm?

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THis has been said on this forum and many times elsewhere

"With the CG too far forwards the model will fly badly. With the CG to far back it will fly ONCE"

I use a CG located at 25% wing chord on mmy designs.

Someone who shall remain nameless complained that one of my desings was horrible to fly but they admitted that they had the CG at 30% chord and once they put it at 25% the model was great!!!.

Edited By Peter Miller on 02/10/2020 09:12:31

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Posted by Jonathan M on 01/10/2020 21:06:11:
Posted by Shaun Walsh on 01/10/2020 17:14:09:

The Max Thrust Riot gives a CoG of 75-90mm in the instructions, cgCalc gives a figure of 78 to 92mm. With the CoG towards the front it's very docile, with the CoG towards the rear it's still rather docile but will fly inverted with very little down elevator although you won't get a flat spin.

On the other hand the Dancing Wings Spacewalker I built shows CoG at 70mm and set at that it was a wild ride on the first flight. cgCalc gives a value of 45 to 55mm which was much better.

Case in point: exactly where is the thickest part of the Spacewalker wing? 50mm-ish or 70mm?

Unsurprisingly the thickest part of the Spacewalker wing is 50mm approx. from the LE.

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Kits and plans often seem to be marked on the safe side, if in doubt or even always check out with a CG Calculator as above. I never trust what a plan or instructions say and always double check it for myself. for 90% of airframes this is really pretty basic thing to do. After initial flights you can then tweak for personal preference if required and of course different airframes and styles of flying will required differing set ups but for general sport flying 25% is usually really safe and 25-30% not unusual. Linds

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Posted by Peter Miller on 02/10/2020 09:11:22:

THis has been said on this forum and many times elsewhere

"With the CG too far forwards the model will fly badly. With the CG to far back it will fly ONCE"

I use a CG located at 25% wing chord on mmy designs.

Edited By Peter Miller on 02/10/2020 09:12:31

Thanks Peter,

as you say, it's been said many times and I appreciate the take-home-message that a margin of safety is represented by a slightly forward C of G.

However, let me ask you this. Who is responsible providing that margin? The designer, or the builder? Or put it another way, you design a plane, and indicate a C of G on the plan, either from 25% or from calculations. Do you consider that to already contain a margin (degree of safety by being slightly forward), or should I add it myself by starting with the C of G slightly forward of your marked position?

Thanks everyone for responding!

TWS

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It is a brave man that assumes the manufacturers stated C of G errs on the safe side! Many moons ago I flew a Durafly Vampire with it's C pf G at the recommended position. The maiden was a wild ride to say the least and only became tame after moving the c of g forward 20mm (quite a bit forward of 25% MAC for some reason). After a quick internet search I discovered that I wasn't the only one to find this out the hard way. Hindsight is a wonderful thing, so the moral is, always check the net before flying a new model! smiley

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Posted by Zflyer on 01/10/2020 22:16:35:

I have HK walrus and Night Walrus and didnt have any problem with c of g as I recall, but will check over weekend as they are both my ladies . In any event they are cracking to fly.

Hello Zflyer,

My OP was a general rant, but I'd love to take advice regarding the Night Walrus specifically. With the same 2200 mAh Turnigy 3S shown in the manual, I found I had to add 40g behind the motor mount in order to get it to balance according to the instructions. It would be good to know where yours balances when trimmed...

Did you add any down-thrust to the motor?

It's been a while since I flew a glider - most of my recent maidens have been low wing sports models. Trimming elevator for power off glide is a slightly different technique than I'm used to!

Cheers in advance for advice!

TWS

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TWS, My Walrus was my first aileron model, 7 years ago. I still have it and use it whenever I haven't flown for a while, just to "get my hand in".

Be advised that a 3S 2200 battery (a Wot 4 foamie or Riot battery) just fits between the servos and gives a CG that's dead right for me. It needs to be pushed back into the fuselage for about half the battery length.

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I'm currently building a vintage patternship called Clipper III the CG on the plan is shown at around 65%, this is clearly ludicrous because it has trike u/c as the nose wheel would never touch the ground. It is safe to assume whoever drew the plans measured it from the trailing edge not the leading edge. I shall start with it at about 30% and work back from there.

The Acrowot I built in 1986 ended up with the CG set at 1/2" behind the most rearward recommended position where it was the most enjoyable to fly.

A.

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When it comes to manufactures recommended Balance Points, there should be a special mention for Multiplex, both my FunCub and my Funjet have small "bumps" on the underside of their wings, close to the l/e which allow you to balance them on your fingertips - if they balance level or a few degrees nose down you're good to go. I have cribbed this idea for everything else that I've built or assembled since then - once they've been flown and trimmed out to suit me I pop in a couple of map pins, secured with Cyano', where she balances on my index fingers, behind the l/e on the underside of the wing, Then lift her up with your index fingers on the pin heads - she balances level or very slightly nose down - check all controls work in the correct sense - fly.

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I tend to err on the side of caution in regard to CG. If it looks a bit 'odd' on a plan or kit, then unless the layout is particularly unusual, starting at 25% MAC is going to be a safe bet. Sure, it can be moved back from there if required. I do think there is sometimes an element of 'bravado' though, as if your skill as a pilot us reflected in how far behind the suggested CG you have your model.

The 'optimum' CG depends on the pilot as much as anything, as well as (especially with gliders) the prevailing conditions.

Same goes for control throws. I find suggested aileron throws a but tame. It's all down to personal preference ultimately.

Cheers

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If you are not into aerobatics such as inverted flight or knife-edge then it probably doesn't matter too much to have a forward CG as you trim for mostly upright flight. Aerobatic models are designed so that the fight against unhelpful flight characteristics is minimised as far as possible so it's crucial to get the CG in the best possible position which almost always means being a bit adventurous.

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OK, so going back to the Night Walrus example. I took the advice and used cgCalc to estimate the C of G position based on the geometry. It was a bit rough and ready - using two trapezoids for the wing and tail section. The result for a static margin of 15-10% is 84 to 92 mm back from the LE. This corresponds to 44% to 49% of the chord at the root).

Wow. Given the wing is only slightly swept, that's much further back than I expected.

I'm not sure if I completely believe this - this is much further back than for most models cited as 'typical', and way further back than the 71 mm stated in the manual. I guess the model may be regarded as slightly unusual in that the tail-plane is pretty far back - behind the fin.

cgcalc.jpg

I may of course, just be overthinking this. The irony is that if I'd just ignored the manual and built it stock, stuck in a battery and gormlessly chucked it, it would have flown just fine!!!

It's clear that I can be confident in moving back the C of G in stages, and test flying each time.

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Posted by The Wright Stuff on 06/10/2020 09:53:31:

I may of course, just be overthinking this. The irony is that if I'd just ignored the manual and built it stock, stuck in a battery and gormlessly chucked it, it would have flown just fine!!!

Exactly!!!

It's only a beginner's electric glider. You can even start with just a hand glide into long grass (I did).
Get the glide right, and then try gentle power so that it just climbs away.
It was designed to be "no problems / dead easy", and it is!
Mine is by far my oldest electric foamie (7 years old), and it's never been crashed.

My advice would be to stop all this fancy computing, install a 3S 2200 mAh pushed back between the servos, and start gently from there.

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Thanks, brokenenglish,

Yes, I agree of course - it's an easy beginner's model and I've flown it many times. I was just using it as an example in this case to experiment with cgCalc, and illustrate my rant about the incorrect manual value!

But I stand by my OP - a bad value for the C of G in the manual is literally worse than useless, because it would be better to not state a value at all, than to state one so far out...

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