The A4b - a V-2 with wings on!

[Simon Chaddock]

mightypeesh

Its virtually all UHU POR with just a tiny bit of foam safe cyano.

It may sound bizarre but I am blessed with an oily skin so I can fairly easily 'rub off' any excess POR from my fingers before I have to touch anything.

With body heat any POR on your fingers quickly become a brilliant contact adhesive so you stick quite firmly to anything you touch, usually the piece of Depron you are trying to work with! .

[Ian101]

That is fantastic Simon.

It is always good to see something a bit different, especially when it is as good looking as that.

[mightypeesh]

Mmmm. Oily skin eh. I turn into a 'post it note' using por - i stick to everything by the time i have finished (or rather it sticks to me!) I might try a bit of barrier or hand cream next time i use it - it could be the secret to happy building. V2 looking great by the way!

Thanks, Simon

[Simon Chaddock]

Many thanks for the kind words.

With the surfaces complete and the mixers in circuit a full control test was possible, however you would't believe the number of combinations of "which plugs to what" before they all moved appropriately and in the correct sense!

This test was done using a 10 year old GWS crystal Rx and an even older Futaba Skysport Tx to show no computer capabilities are required to achieve it.

[Simon Chaddock]

This shows the wiring used on the control test.

The four servo wires are connected into the 'vertical' and 'horizontal' mixers. The 'aileron' connections are combined using a "Y" lead leaving just conventional Ail, Ele and Rudd plugs to the Rx.,

All this will have to be tidied up for the flight version.

wWth such a small span I fear it may be very twitchy, particularly on the maiden!, so I have decided to instal a gyro rx from the word go. For best effect it needs to be installed on the CofG with a hatch to adjust the gain pots.

To simplify installation the Orange Rx and the mixer units have been combined into a Depron box.

The four control surface servos plug into the front, the ESC plugs into the top.

When the wing and tail fuselage sections are finally glued together the Rx box will also be glued directly on top of the wing as it passes through the fuselage.

The next task is to instal those long 24" (600 mm) motor wires.

[Stephen Jones]

Then it`s Blast off i guess

Steve

[Simon Chaddock]

Blast off? Not quite.

I nave no idea where the CofG should be and it is far too delicate to risk 'trial and error'!

The problem is not the wings but the relatively huge fuselage and fin area which will undoubtedly contribute some lift but acting where?

The only way of getting even close will be to build and test a 1/2 size profile chuck glider out of 6mm Depron.

Hopefully that will give an indication (and perhaps more important how it behaves with it too far back ) so that the battery position in the RC version can be determined and the nose actually finished.

It could take some time.........

Edited By Simon Chaddock on 11/12/2015 23:49:55

[Simon Chaddock]

So progress as everything is now permanently built in. No easy going back if something has been over looked!

Next the motor wires.

They are long (24" 620 mm) so to save weight they are magnet wire. This is insulated with a polyurethane varnish and for the same copper area is about half the weight of the normal multi strand silicone insulated stuff.

A short length runs externally along the root of the lower fin.

Before entering the fin and into the fuselage.

The wires run along the bottom supported by the fuselage formers.

A bit ungainly as the wires have to be long enough to reach the nose where the ESC and battery will be.

The middle and rear sections joined.

Now to get an idea of where the CofG might be.

Big and little!

The elevators are 'bendable'.

As the whole thing only weighs 23g (0.81 oz) you don't have to worry too much about it damaging itself during testing.

It appears to glide reasonably with the CofG at 42% of the root chord with just a touch of up elevator and handles a stall quite well just sort of "mushing" with little tendency to auto rotate.

As a cruciform fuselage is likely to give a bit more lift than a round one I will set the EDF version to say 40%.

My biggest concern at the moment is the size (weight) of battery it will need in the nose to actually achieve 40%!

[Simon Chaddock]

Happy at establishing a reasonable CofG location the nose can be glued on.

This means the A4b is now structurally complete although the nose will have to cut open to install the battery box and ESC.

I then ran into a 'brick wall' problem - the intended battery (1500mAh 3s) was nowhere near heavy enough to achieve the required CofG! Even a 3000mAh is not quite enough and using that would result in an unacceptable all up weight.

The problem is the 70 mm EDF is just too heavy for the airframe so it will have to be replaced with a smaller lighter one.

A 55 mm EDF is about half the weight but it will require some serious 'surgery' to change them over.

The joys of an unconventional own design. .

[Stephen Jones]

Simon,

How much longer will the nose have to be to archive the correct CoG

Steve

[Simon Chaddock]

Steve

To achieve the required CofG with the intended battery would require it to be moved forward as far again as the current nose is long!

So unless the existing EDF itself is moved significantly forward (which would require a complete rebuild of the whole rear fuselage adt of the wings) the only option is to change the EDF for a lighter one.

As the EDF is right at the back hopefully the structural alterations required are not that great.

In addition a lighter EDF and smaller ESC along with keeping the 1500mAh battery should result in a significant deduction in the all up weight so even a 1:1 thrust ratio may still be possible.

[Simon Chaddock]

The heavy EDF cut out.

Could have been worse with only limited damage to the rest of the structure.

Now just have to wait for the 55 mm EDF to be delivered.

The complete airframe with servo, radio and long motor wires weighs 7.2 oz. The smaller EDF, ESC and 1500mAh 3s will weigh about the same. It looks like a target of just 15 oz (425 g) is achievable.

[Simon Chaddock]

Made a start installing the smaller 55 mm EDF so allowing the control surface inner hinges to be refixed.

As the 55 mm EDF fan has only 55% of the area of the 70 and by using the same inlets the smaller fan should have slightly lower duct losses.

HK claim the 55 can generate 470g at 11V.

It looks like the A4b will weight just over 400g so it should be able to fly.

[Colin Leighfield]

Simon, do you reckon that you will get a good vertical take-off? It look as if you've got enough margin in the thrust to do it and with the gyro, it should stay upright. Is that the plan?

[Simon Chaddock]

Colin

It is at the back of my mind but certainly not initially.

If it proves to fly well enough and have suitable control characteristics the route to a vertical take off would be to use a very high C 4s 1200mAh as that would be more or less the same weight as a 3s 1500.

With a 4s the EDF should give a thrust to weight of nearly 1.5:1 so would lift off quite smartly but it could only stand the current for a few seconds.

The intention would be to extend the control surfaces into the EDF exhaust so the gyro would have influence from launch.

Nearly finished putting in the 55 mm EDF.

Although from this angle you cannot really tell the difference!

[Colin Leighfield]

Well thought through Simon.

[Simon Chaddock]

This view shows the difference better.

The 70 mm exhaust occupied the entire of the rear fuselage area.

On the basis you can never have enough access the nose cut out is substantial.

The battery is squeezed right up into the nose although it will have a 'battery box' but this is more to give extra strength to the nose than support the battery.

On such a light Depron structure I actually prefer the battery to break away as crushing the Depron gives it considerable cushioning. Its much cheaper to rebuild the Depron than replace a battery. 

Once the ESC and battery box are installed virtually all the cut out will be simply glued back on.

Edited By Simon Chaddock on 24/12/2015 23:38:04

[Simon Chaddock]

Even managed to just a little bit despite having the kids and grand kids round.

As the ESC will be right at the front with little positive airflow it has been given a new much better 'fingered' heat sink.

The tips of the fingers have been profiled to follow the fuselage contours and will stick out about 2 mm into the air stream.

This type of heat sink has many times the heat dissipation capacity of a flat plate, let alone one that is completely buried under thick shrink wrap!

[Simon Chaddock]

The ESC installed in the side of the nose almost along side the battery as it need every gm of nose weight to achieve the desired CofG.

The ESC to EDF wires secured along the bottom of the fuselage.

The Depron battery box.

It is always a bit of a challenge to arrive at a battery hatch layout that does not detract too much of the skin's strength but still leaves sufficient spaces to actually connect the battery!

[Colin Leighfield]

As you said though, if it noses in the damage will be easy to fix, so the light weight is far more important than ultimate strength. It should slow down to virtually nothing, so as long as it is controllable how can you go wrong? The fuselage/rocket body probably contributes quite a lot of lift anyway, it could be a pussy-cat and won't share the self-destruct characteristics of its' murderous prototype!

[Simon Chaddock]

Apart from painting it is now complete.

With a 1500mAh 3s, but not the intended high C one, the figures are:-

18.0 A

10.9 V

208 W

It weighs 14.75 oz

With 12 oz thrust

Now I have got to work out a way to launch it as there is nothing to hold onto!

But no rush with the current weather forecast!

Edited By Simon Chaddock on 30/12/2015 00:53:31

[Simon Chaddock]

Now with its black paint.

As expected the paint has pushed the weight up to 15 oz (425 g) exactly.

[Colin Leighfield]

Something of a contrast to the Hamilcar Simon?

[Simon Chaddock]

Colin

Ah! the Hamilcar.

It is still hanging on the wall right in front of me rather badly damaged.

My first (2010!) big scale Depron plane. It should have flown much better than it ever did but despite 4 major rebuilds it never got properly sorted and I am still not really sure what was wrong.

On the other hand the full size was noted as having very predictable handling, for an 110 ft span glider that weighed as much a fully loaded Wellington!

[Simon Chaddock]

The A4b does actually fly!

Although the "landing" was not what i had intended.

Actually quite a gentle impact but the field is still extremely waterlogged and very muddy so there was no ground slide just a firm splat!

It is not a bad as it looks as the Depron has done its usual 'crumple' so the rest of the structure is undamaged and all the electrics are operational.

For the maiden I elected to have the gyro 'on'. In hindsight this may not have been such a good idea as it tends to swamp any of the planes true aerodynamic characteristics.

I suspect the CofG is too far back as it tended to lift the nose readily and slow down with the result the roll control to become very marginal. I felt I was always "chasing the correction" so the flight was pretty erratic. I tried to 'land' before things got any worse.

Of course without the gyro the flight might well have been even shorter!

I will not attempt a direct repair but actually build a new slightly longer (1/2" so not really noticeable) nose section to move the battery and thus the CofG frward - a process that has already started.

The new formers cut out.

I never said it was going to be easy!

 

Edited By Simon Chaddock on 04/01/2016 15:31:29