Ballerina sans Tutu!

[Biggles' Elder Brother - Moderator]

Well, here we are, after selection, debate, discussion, analysis, we're about to pick up some wood and build a model aeroplane!

Like the rest of you I'm looking forward to the adventure of going from a few "planks" to a fully finished and flying Ballerina! But I have a slight delay on starting as I still have a model to finish off! Still I shouldn't be that far behind the rest!

My Ballerina will be electric powered and I've choosen a set up that should give me the option at least of fairly sparkling performance. Obvioulsy I don't have to fly it like a hooligan - but its nice to have the option!

I'm planning on going for a 1930's Kings Cup Racer style finish. Possibly silver solartex with blue trim - not firm on that at the moment but that's the plan.

Anyway - I'm keen to get started and to seeing all the other builds. As John says one of the great things about Mass Builds is the ways people find of building the same model but with so many variations and individual aspects!

Happy building

BEB

[McG 6969]

Hello BEB,

Thank you for visiting the Bella Ballerina blog.

Are you going for a 250 W/lb-version on 6S now then??? ... .

Can't miss that for sure.

Already subscribed.

Have a great New Year's Eve & a successful 2016 for you and your family.

Happy powering

Chris

Brussels, Belgium

[Biggles' Elder Brother - Moderator]

Well - I'm planning on around 150W/lb. Which should be plenty lively enough for me! First job - which I have done today and will post later - is all about working out the approximate battery position to get the CoG in the right place!

BEB

[Biggles' Elder Brother - Moderator]

Those of us using electric power have a bit of a quandry where the Ballerina is concerned and it involves the CoG. In an electric powered model by far the heavest component is the battery. The battery is always located forward of the desired CoG, but the unknown question is: how far forward does it need to go?

Often this is not a pressing issue and we don't worry about it too much. Being so heavy, relatively small shifts of the battery's position produce significant shifts in the position of the CoG. So provided we have a bit of length-wise "wiggle room" we can be confident that a battery position can be found that will balance the aircraft correctly. But there's the rub, in the Ballerina as designed there isn't at lot of scope for moving the battery forward because F1 gets in the way. We should be concerned here because of the possibility that the battery may have to go further forward than F1 to achieve the desired CoG.

Now other builders are well aware of this and have come up with various solutions you will see on other build blogs. Chris (McG 6969) for example has opened F1 up to allow the battery to pass through it if necessary. KC has altered F1 so he can position it further forward thus giving him wiggle room, this has the added benefit that he can fix his motor directly to F1.

Both of these solutions are fine - perfectly good. But I got to thinking - what if we found that we didn't need to locate the battery forward of F1? What if by using, for example, a bigger (slightly heavier), battery that was possible? If that was so then we could build the model "as is" and also our access problems would be simpler as the battery hatch could be positioned further back.

But how to know? Well we can't know, but we might be able to make a sound educated guesstimate! So, I'm going to do a few back of a fag packet calculations today - just to see if there is any milage in this idea. I'll report back later.

BEB

Edited By Biggles' Elder Brother - Moderator on 05/01/2016 16:05:44

[Biggles' Elder Brother - Moderator]

OK, I've done the calcs - and the results (assuming they are correct!) are very surprising.

Our starting point is Peter's original IC set up. He used an OS FS40 Surpass with a 6oz tank and the model balances on the CoG. So basically we want to replicate that - but with an electrical set up. The first thing to note is that all the structure (ie airframe) is assumed to be the same in both models - so we don't need to include that in our analysis, we only need to focus on the things that will be substantially different.

The engine/tank provide a certain moment to the front of the aircraft - we need to calculate what that moment is and replicate it.

An OS FS 40 weighs 12.74oz (with silencer) according to the spec. That's 0.36Kg. Examining the 3-view we find that the centre of the engine is 47mm back from the drive plate (we will assume that the engine's centre of mass is approximately at its geometric centre). Taking a rule to the plan and setting off 47mm from the nose ring to align the engine centre that tells us that the centre of the engine is 0.245m forward of the aircraft's CoG. So the moment created by the engine is:

Me = 0.36 x 0.345 = 0.0882Kgm.

(Before any fellow engineers jump on me, yes I know that mass should be multiplied by 'g' - acceleration due to gravity - but so will every mass and so all the g's will eventually cancel out - therefore we can ignore them for the purposes of this.)

Regarding the tank - well the CoG must still be OK with it nearly empty - or Peter couldn't land the model! So let's go with a weight of 3oz (ie a bit of fuel, the weight of the tank itself, the clunk and the linkage etc.) 3oz is 0.114Kg and the centre of the tank bay is 0.135m forward of the aircraft CoG. So the moment due to the tank is:

Mt = 0.114 x 0.135 = 0.0154Kgm

There isn't really anything else - there is a throtle servo, but that's very close to the CoG so we can ignore it. Therefore the total nose-moment Peter's gubbins creates, what we have to replicate with the electrical stuff, is:

M = Me + Mt = 0.0882 + 0.0154 = 0.104Kgm (rounding)

So next - to look at the electrical system,.....

BEB

[Biggles' Elder Brother - Moderator]

First we need to establish the motor position. Here is my motor on the plan:

Now as I said I'm going for a fairly powerful set up so that motor (a Turnigy SK3 4240 740kV) is quite beefy. It weighs in (with collet and mounting spider) at 0.222Kg. Again, taking a rule to the plan we can measure that the motor centre is 0.285m forward of the CoG. So the moment that gives us is:

Mm = 0.222 x 0.285 = 0.0633Kgm

I'll try to get the ESC in under and slightly behind the motor - looks quite possible. The ESC weighs 60g, that's 0.06Kg and lets say it would be 0.2m in front of the CoG. So the moment due to that is:

Ms = 0.06 x 0.2 = 0.012Kgm

Finally we need to account for the motor mount. I will use M4 studding for this back to F1. I won't bore you with the details but sufficient to say I estimate the total weight of that (studding, washers, nuts etc) to be around 50g and its centre would be 0.215m forward of the CoG. So the moment due to the weight of the mount would be:

Mf = 0.05 x 0.215 = 0.011Kgm.

Now we need to work out the total moment of that lot:

M = Mm + Ms + Mf = 0.0633 + 0.012 + 0.011 = 0.0863Kgm.

From the post above we worked out that Peter's set up created a total moment of 0.104Kgm., so we can work out how short we are and what the extra moment needed is:

Mn = 0.104 - 0.0863 = 0.0177Kgm.

Where are we going to get this extra moment from? Easy, the battery - remember we haven't included that yet. The battery must be positioned to give us 0.0177Kgm of anticlockwise moment, then we would have replicated the weight distribution Peter has. Let's do that....

BEB

[Peter Miller]

The tank is always empty. Everything on my drawings is based on the very scientific principle TLAR.

My last design (not test flowm) was a slight enlargement of an old design with some structural changes and a 1" longer moment arm. Needed 3 ounces of lead in the nose. Goyt it right first time

[AVC]

Posted by Peter Miller on 05/01/2016 15:11:07:

The tank is always empty. Everything on my drawings is based on the very scientific principle TLAR.

My last design (not test flowm) was a slight enlargement of an old design with some structural changes and a 1" longer moment arm. Needed 3 ounces of lead in the nose. Goyt it right first time

I do like your "very scientific approach (I had to google TLAR principle, by the way...)

BEB, wise approach !

[Martyn K]

Following - a nice engineering approach to the problem..

[Biggles' Elder Brother - Moderator]

I have identified three candidate batteries for my Ballerina:

1. A Zippy Flightmax 3000mAh 4s. Dimensions: 139x46x25mm. Weight: 313g. I already have these batteries. The problem is they are just a little low on capacity - I'd only get 4mins at full throttle. But, as stated, this is quite a powerful set-up so I wouldn't need full throttle except in bursts. So while its less than ideal - it's OK and should get 7-8 mins with a bit of care.

2. A Zippy Compact 3300mAh 4s. Dimensions: 145x44x25mm. Weight: 350g. Slightly narrower, but longer and with 10% more capacity which would be welcome.

3. A Turnigy Nanotech 4000mAh 4s. Dimensions: 155x49x27mm. Weight 427g. Bigger and heavier, but with 33% more capacity, maybe the best choice if it will fit.

So, I drew these out full-size on some graph paper.

The crosses mark the centre point.

Now let's consider the 3000mAh Zippy Flightmax first. This weighs 0.313Kg and we need it to generate a moment of 0.0177Kgm to put the CoG of the model in the right place. So:

0.0177 = 0.313 x d.

Where 'd' is how far in front of the aircraft's CoG the battery centre has to be to give us the desired result. Working this out:

d = 0.0177/0.313 = 57mm.

So now we can put our paper cut-out Zippy Flightmax battery on the plan so that its centre is 57mm forward of the CoG. Here it is:

Wow - now that is a surprise! Its not even close to F1! Its well back, in fact the "problem" former is not F1, its F2.

If we do the same calculation for the 3300mAh Zippy Compact and the 4000mAh Turnigy Nanotech we get distances forward of the CoG of 51mm and 41mm respectively. This makes sense of course that these progressivley heavier batteries need to be progressively less far ahead of the CoG to generate the necessary moment.

Here are pictures for the position of the 3300mAh and 4000mAh bateries:

Well this is quite a surprise. I had suspected that we might not need to get the battery so far forward that it protruded through F1, but even I didn't think it would be this far back. It changes a lot of intentions. First the hatch needs to be not in the nose, but around the cockpit area. Second, we need to modify F2 - but, except possibly for cooling purposes, F1 can remain unchanged in shape and position. Also, those of you lightenng the tail with built up structures may want to reflect on this result! The Ballerina, as designed, does not seem to have a "tail heavy" nature.

A brief couple of thoughts on interpreting this analysis:

1. remember many people have lighter motors than I do - but even taking that into consideration I still doubt the battery will need to go through F1. You could of course follow through this calculation (using these posts as a model) but with your own motor/mount/battery weights to see.

2. There are obviously assumptions in these calculations. Some weights have been estimated (although most are based on published specs or actual weighing) and approximations such as 'the engine's CoG is at it geometrical centre' have been made. But even allowing for this - if my sums are right (and I intend to double check them!) - then I can't see us being more than say 10-15% in error. That sort of error would not change the main conclusion that, for this set up anyway, the battery needs to be positioned considerably aft of F1.

BEB

[Martyn K]

5 sharp photos on the trot.. BEB must have a hired a photographer

Looks like the hatch could go between F2 and F4 with the LiPo sliding into a box where the fuel tank was

Martyn

[Biggles' Elder Brother - Moderator]

I have a new smartphone Martyn - with a camera! And its so clever that even I seem to be able to take in-focus photographs with it!

BEB

Edited By Biggles' Elder Brother - Moderator on 05/01/2016 16:42:15

[Chris Barlow]

Way too many numbers there for me. I prefer TLAR

[kc]

The weight of an OS 40FS Surpass with silencer is 13.2 oz not 12.5 ( that's without silencer!) acccording to the OS manual here

Not much different but it may change the calculation a little.

The weight of a glow motor may be mostly well forward. All the steel parts -crankshaft, bearings etc are forward. I don't have a 4 stroke handy to check but a quick check on an OS 46AX two stroke shows it balances just behind the carb despite a large silencer hanging out back beyond bulkhead position. Could make difference in the calculations as electric motors usually have alloy prop adaptors and their weight is therefore at the rear end. Also if BEB is using Solartex and a coat of paint instead of the Supershrink Solarfilm used by Peter then maybe that will move the Lipo forward a bit......

However if BEB is right about CG then the electric builders problem is the top part of F2 which gets in the way of inserting a Lipo and needs to be eliminated or minimised if a full length hatch is proposed. A crosspiece between F3 and F4 might be needed to stiffen fuselage instead of F2.

Edited By kc on 05/01/2016 17:16:00

[Biggles' Elder Brother - Moderator]

Different manuals must show different weights! But as you say KC it isn't going to make that much difference. We gain a bit here, we loose a bit there. It isn't going to change the basic conclusion that, as you say, we need to have a good look at F2 and how we engineer a hatch that far back without compromising the structure.

BEB

[Biggles' Elder Brother - Moderator]

Posted by Chris Barlow on 05/01/2016 17:13:06:

Way too many numbers there for me. I prefer TLAR

I like TLAR as well!

The problem is that when a number of people's TLAR is suggesting one thing (ie get the battery forward and lighten the tail) and your TLAR is suggesting something else (ie this model has a long nose the battery doesn't have to be far forward) you need to resolve it someway!

You could build the model and trust to being able to being able to "fudge it" when it comes to balancing time - and as I say I do that a lot. But here we don't really have room for that. If we do need to go through F1, or as it turns out modify F2, then we need to know now - before we build the fuselage. And there is only one way to do that that I know - and that's try out a few numbers.

BEB

[Lucas Hofman]

Thanks BEB, i should have doen this calculation before cutting à big hole in F1! However, ESC and battery do benefit from cooling, at least in summer.

As a reconciliation for those doing build up tails: less weight at the end, or more around the CG is never bad for flying characteristics.

[john stones 1 - Moderator]

I figured battery very similar by looking at other models I have, and knowing I'd have room to move slightly forward/back, so the confirmation's good also means the lipo will keep pilot warm in Winter

John

[Danny Fenton]

I reckon if you do the tailplane from 6mm aluminium, you will be okay

Otherwise how will you get a full length pilot and fully detailed cockpit in (runs away) ((quickly))

You could go IC.........

[WolstonFlyer]

Thanks for all the calculations BEB, very interesting that the LiPo will be so far back.

[McG 6969]

Hi to all & to BEB specially,

Could someone - please - translate all this for a French-native Registered Newbie... ???

This reminds me of having to learn a lot of 'acronyms' for punishment... or the NATO alphabet for my first radio license...

As I'm supposed to build my 'hybrid' version with an - hopefully light - aft fuselage & tailplanes in Depron, should I then consider placing my Lipo on top of the servos... ???

Happy gravitating

Chris

BRU - BE / CTR Ground Control

[kc]

TLAR = that looks about right

[Biggles' Elder Brother - Moderator]

We're having a bit of fun Chris! As kc says, TLAR stands for "that looks about right" - a time honoured design philosophy perhaps summed up by the expression "if it looks right, it will fly right"!

Building model aeroplanes is mainly a TLAR approach. But just now and then its worth checking in with the numbers to be sure - it very reassuring when the numbers confirm what you felt to be right anyway.

BEB

[kc]

Of course they also said TLAR when designing the Edsel........

[Biggles' Elder Brother - Moderator]

Definitely not such a good advert for TLAR!

BEB