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Simon's 'Petiterina'


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I have been thinking about this for, well, 9 months but I can resist no longer!

The question I asked myself was not so much how much weight could be saved using Depron but how much would it weigh if it was simply made in Depron?

First there is the question of size. At some point Depron needs significant reinforcing so it starts to loose some of its weight advantage.

My experience suggested that 40" span is about right and has the big advantage (for me anyway) that it is then small enough that it can be built in one piece. Thus it would be 40/53 or 3/4 the size.

That means it will only have 9/16 the wing area and given 'smaller' wings tend to be less efficient it is going to have to weight better than half the weight. The original came in at 71 oz so I don't really have a target weight except lower than 35 oz!

So off we go with a light weight Depron Ballerina 'look alike' .

First the wing.

40depron2s

Basically 2 mm Depron skins and close spaced ribs with wide but tapered 1 mm balsa spar flanges flush with the wing surface to give maximum beam depth.

That about as far as my thinking goes at the moment.

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Just realised I have misspelt the title! It should be Petiterina with an "e" in the middle rather than "a". Perhaps a moderator could do the honours at some point.

Next job is to shape a "rib master"

Ribmaster

The original wing section looks to be a standard 11.7% Clark Y.

I have kept to Clark Y but in truth as the wing loading and chord come down a thinner section actually gives a better L/D ratio.

By placing the spar flange within the wing skin means the ribs are 'simple' and have no cut outs.

So 26 identical ribs in 2 mm Depron.

Edited By Simon Chaddock on 08/09/2016 19:19:50

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Hello Simon,

If you don't mind, I don't think it's a 'pure' Clark Y as Peter's airfoil seems to have the flat bottom section starting closer to the maximum thickness and the Clark Y is more 'flat bottomed' than this one.

Maybe he modified it a bit to have some better 'inverted' behavior than a 'standard' one... or this could be another part of my mental insanity... But I guess Peter will jump in & clarify it all...

A question, if allowed. If you skin the wing with 2mm Depron and your tapered balsa spar is 1mm, how do you compensate the thickness difference, please? Or do you foresee your foam beam to be tapered the same as as your spar flange?

'Petiterina' sounds definitely better in my opinion, especially in French... yes

Hakuna matata

Chris

BRU - BE / CTR Clark Control

 

Edited By McG 6969 on 08/09/2016 20:28:18

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Peter

Thanks for clearing that up.

Thus using the Clark Y I will, albeit unintentionally, be using a slightly thinner section that is better suited to its lower Reynolds number! wink 2

Chris

The wing skin is made in 3 pieces

Skin pieces

When the Depron is added over the 1 mm balsa it does of course stand proud above the rest of the skin so it is carefully 'slotted' by 1 mm with a 2 mm wide needle file.

Rib slots

Doing it this way, rather than cutting a notch in the rib, means it also accurately locates them as they are glued in.

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The wing ribs glued onto the lower skin and the shear webs added.

Wing ribs1

The spar as such will not be complete until the top element is added so it is all still pretty flexible. That is the next job. wink 2

In fact the wing section follows the original pretty closely. It is truly flat bottomed which the NACA 3414 is not quite but at 14% it has exactly the same thickness to chord ratio.

Edited By Simon Chaddock on 09/09/2016 14:02:52

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The top flange added along with the front top skin.

RH wing 1

It is now plenty strong and stiff enough to be handled so a 6 mm Depron leading edge is added and sanded carefully to profile.

6mm LE

In this state the LH wing weighs just 0.91 ounces (25.8 g).

So far so good so a start can be made on the LH wing - before I forget how it was done!wink 2

The next stage is to cut out the aileron and fix in its servo.

.

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Started thinking about the fuselage.

I think it can be made entirely from Depron, probably 3 mm at least for the flat side panels and is might end up with more formers.

It will look something like this.

40depron1s

A 'soft' long stroke undercarriage is the best approach to reduce the shock loads and then to spread the forces over a large an area of the Depron as possible.

The solution I evolved for my Depron Super Cub works very well. It uses acetate sheet side plates glued to the fuselage and carries a braced undercarriage made of relatively fine wire.

U/C mounting

With this method the wire itself is not fixed directly to the Depron at all

Depron tyre

The result is light, holds the wheel securely in line but allows considerable vertical travel.

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Hi Simon,

If you don't mind, there was a mistake - more than one, in fact - on the published plan of the Ballerina.

Following Peter's original drawing and later remarks, the stabilisator plane should 'butt' against the last former (F10) and not leaving that small gap that you reproduced on your Petiterina drawing. It then brings the elevator joiner just on top of the fuselage at the extreme end of it. I don't think it is of extreme importance but 'originally' it was designed that way.

I really like your acetate 'load distributor'. Very clever. yes

At this tempo, your build will be maidened before I manage to finish my tailplanes... blush

Hakuna matata

Chris

BRU - BE / CTR Tempo Control

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Peter and Chris

Yes the drawings are a direct electronic copy of the originals so faithfully reproduce the inaccuracies but the errors are noted, thanks.

My building tends to be of "make it up as you go along" technique so if something does not come out quite right it is altered regardless of what the plan may show.

In fact I tend to use the plan only to mark out the components. The wing is actually built directly off the lower wing skin. The only time it was really used was to create the wing tip shape and to mark the ribs positions.

I have decided to go for the 'silly' aileron option on the grounds the Petiterina is intended to be light so it might as well use the lightest possible option - a tiny direct coupled aileron servo.

Ail cut out

As the servo is so small the aileron is a bit shorter but has to be slightly broader to get the servo positioned within the wing skins.

In the best model making tradition the aileron is literally cut out of the wing. A 6 mm Depron leading edge and the top skin added. The nose is then carefully sanded to shape and the servo horn 'inset' into the aileron.Servo horn

The outer will literally be a pin hinge.

I must say I had forgotten just how tedious this is to do and I doubt it saves more than a few grams.

It probably made sense with the Endurance where I wanted minimum drag but in this application I fear it is a case of "effort for efforts sake"! wink 2

 

Edited By Simon Chaddock on 11/09/2016 15:33:37

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The fuselage bits.

Fuselage bits

All 3 mm Depron. The sides are the good flat stuff, the formers are cut from 'aero'.

All the curves have been marked from the the plan by the 'pin' method held up flat against a window.

The 'parallel' section of the fuselage.

Fuselage 1

Just the same as if it were in balsa! I guess it will end up slightly 'thin' as there are no fuselage doublers.

As it looks like it will be seriously light., I have downsized the motor a bit to a humble Emax 2822 (1200kV).

2822 7 9x5

Turning a 9x4.7 slow fly.

On a 3s it draws 11.4 A giving exactly 120W. Quite a conservative set up as the motor can handle up to 200W.

We shall see.wink 2

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Hello Simon,

I think I'm close to get totally demotivated now.

As opposite to what I wrote before, you will be maidening yours even before I 'start' the covering of my tailplanes... blush

Doing great, sir. yes

Hakuna matata

Chris

BRU - BE / CTr Motivation Control

 

Edited By McG 6969 on 12/09/2016 20:58:37

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Today I managed to get the motor and ESC installed in the fuselage. This allowed a short test to make sure it all still worked. Important when there will be very little (actually none!) access once the fuselage is complete.

It demonstrates a good example of motor/air frame resonant frequency!

The 20A ESC is placed in the bottom of what is likely to be the battery bay..
ESC1
It has a 'fingered' heat sink that protrudes into the airflow so should get adequate cooling.
ESC heatsink
Oh and by the way I have placed all the bits on the kitchen scales and .........lets wait and see! wink 2
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Please, don't worry, Simon.

I'm not 'really' getting demotivated, but just had a 'dippie' when I saw your speedy progression.

I know you're planning that huge Comet but then really wondering when your EDFs will be at home... smiley

Two things are sure by now: you love to burry your components into your fuselages and secondly, you must have a serious provision of 'finger-heat-sinks'... wink

Hakuna matata

Chris

BRU - BE / CTR Funeral Control

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Putting bits in so not much actual building.

The elevator (5g) and rudder (3.7g) servo mounted right at the back so they only need short direct links.

Ele Rud servos

The undercarriage wire formed with the acetate mounts.

U/C wire

Actually redundant push rod wire which gives quite a bit of 'suspension' travel.

The tail wheel uses the same principle but in a triangular arrangement.

Tail moun

At last sitting on it wheels.

Wheels 1

The axle stubs are too long so will have to be cut off with a Dremel.

Yes, even the wheels are made of Depron! wink 2

Edited By Simon Chaddock on 14/09/2016 20:46:29

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