Scale foam?

There seems to be quite a general feeling that foam makes a poor scale model plane.
Given that the vast majority of even the 'best' scale are not built with the correct construction or materials this criticism seems to me to be a bit unfair or even, dare I say it, prejudiced!

My avatar Skyray was 'built' out of well over 100 pieces in foam with just a bit of balsa. Within the usual limitations of a ducted design it is also to scale and probably more so than some as it has a scale wing section as well.

I shall now take cover!

I think what I am trying to say is we should consider new materials on their merits.
Looking back at model magazines in the 1930s you find balsa and tissue was treated as almost 'cheating' compared to good old bamboo and silk.

I have built with balsa almost all my life and discovered Depron sheet almost by accident just over a couple of years ago.
To me it represented a new building material for lightweight planes, particularly electric, being light and stiff enough to be used as a 'thick' monocoque skin on a conventional built up structure and it was much cheaper too.
It was only a short step to actually build the base structure out of Depron sheet as well and rely entirely on the Depron skin for strength and rigidity.
My first 40" Depron wing (a 12% Clark Y) was made entirely in 3mm Depron and a true monocoque with no spar as such just a web to locate the ribs.

Even the servo mounts

Only the leading edge was balsa simply because at the time I had not found a satisfactory way of doing it in Depron.
This wing proved to be much more efficient than the solid foam one it replaced.

Although Depron has quite a good strength to weight ratio it has such a low density it is difficult to get sufficient material at the right place where stresses are high.
By placing a 3mm hard balsa strip flush with the skin over the Depron 'web' it was possible to build a simple balsa reinforced Depron wing of just 6% t/c ratio. This being the sort of thickness used by many fast jets.

You can see where this was going.
I now had the techniques to build a light, low powered, scale fast jet in Depron.
Why? Just to see if I could!
The duct and centre section

The all Depron fuselage frame work prior to skinning

You have to be a bit daft to build like this but it certainly works.

Hmm - definitely efficient and there is no doubt the model can fly and absorb a bit of a fling around. I also acknowledge that, even in the larger size of model I build there might well be a place for using Depron for non, or low load bearing parts of the construction. But - what about nitro models??? Does it soak up fuel like balsa or just chemically destroy or dissolve itself?? Please be kind to me if these are dumb questions - I'm dumb any way, or so I'm told for not embracing the *lectric, foam wonder route in the hobby. I'm a traditional builder - ply - balsa, etc and I admit it was a long time before I succumbed to things like CA Glue and iron on film - but given another fifty years or so and you just might hear my ghost reluctantly and begrudgingly acknowledging 'plastic fantastics' - or maybe not.

The Skyray makes a good ducted fan as the short fuselage reduces duct losses but this is offset by the relative aerodynamic inefficency of a delta wing.
For comparison I decided to keep exactly the same motor, prop (4.5x4.5) and battery but put in a more conventional fast jet layout.
This is impressive, the Hawker P1121. The so called 'Super Hunter'.

The prototype was never completed but like most Hawker jets it looked nice.
Rather more complex to build than the Skyray with a much longer duct and even thinner wings.
This is how it end up. It is only 1/2oz heavier than the 'Ray'.

To save weight it used an all moving elevon tail although the original had a conventional all moving tailplane and ailerons.
This proved a mistake as its one and only 'flight' showed a serious lack of roll control. Unable to turn the 10 second flight ended in the next field fortunately with no damage.

Ailerons have to be added but this is where my 'built it all in' technique comes back to bite.
Removing the old and adding the new servo cables down the fuselage has needed quite a bit of 'key hole' surgery.

It is proving to be quite a job!

As for Glow fuel and Depron? I believe methanol attacks polystyrene so its really a 'leccy' only material.

Quorneng wrote: As for Glow fuel and Depron? I believe methanol attacks polystyrene so its really a 'leccy' only material.

Hmm - well that seems to be a terminal reason for not using Depron as I'm a dedicated Nitro user, something to leave to the "lecy" crowd. Like Eric I'm also a dedicated balsa user - something about the devil you know as opposed to the devil you don't. It is my PERSONAL opinion but I'm not very impressed by a lot of the foamies I see turning up in parks and at the field - they look like toys, encourage the ARF buyers and add to the decline of proper building that has been the backbone of our hobby (passion?). Not withstanding that I applaud Quorneng for having the skill and interest to take the use of plastic and foam beyond the ARF fad.

There have been a few foam scale 65"winspan warbirds lately at the club recently and they look fantastic.Retracts are weak,and the lack of noise is off putting.

The Beard wrote: There have been a few foam scale 65"winspan warbirds lately at the club recently and they look fantastic.Retracts are weak,and the lack of noise is off putting.

Yeah Stephen but WHY do they all come in bloody gloss finishes? !!

From my days building Airfix kits it is amazing how just 'matt' a finish has to be to look realistic.

In this respect the natural skinned surface of Depron works well for scale types. The problem occurs when you have to sand it as its interior structure is much coarser and like balsa needs several sanded paint coats to fill the 'pores'.

I don't know if its non gloss surface actually improves the aerodynamics but its 'soft' nature, particularly when used as a skin over a framework, does make a plane remarkably quiet in gliding flight. The sort of thing birds have known about for some time!

I certainly would not claim Depron is an all ranging substitute for balsa but if weight is the problem it is hard to beat.

The access holes made good with skin inserts simply glued in.

A job made much easier as the Depron is 3mm thick, gives a bit and is very easy to cut to shape.
Virtually as strong as the original and overall adds very little weight.
I hope its going to be worth all the effort!

Ailerons added to the Hawker P1121.

Overall the 2 servos, filling and painting has added just over 1/2oz.
Now just got to wait for the right weather and try again!

I suppose that part of the argument (discussion) is what the planes are for. Scale to be looked at close up - detail and good finishes are required, including super detailed (and expensive!) engineering in many cases. However, once the thing is in the air, performance is what counts, and the appearance of many a scale model is spoilt for me by the sheer speed in the air, sometimes necessitated by the weight built up by all the super detailing. Stand-off scale (in my case, stand a long way off) works for me, and depron is adequate for this. Polystyrene or blue foam covered with brown paper also works.

Must say I agree with you Tombr. Once the model is in the air, no matter how much detail is put into the model, it cant be easily seen, and as far as I am concerned it the flying that counts.

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