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EXPANDED POLYPROPYLENE

Known as EPP, it is a synthetic rubbery type of material that first became commercially available in the early nineties. The denser varieties are used behind the bumpers on cars and for side impact support.
The blocks we use come from moulds primarily produced for industrial prototype work. Density can vary anything from 15 g/lt up to 110 g/lt. After much experimenting from 17g/lt up to 42 g/lt, we concluded that 25 g/lt was the best compromise between strength, lightness and longevity.

We were the first in the UK to produce a full kit, and are currently with our seventh supplier!!
   

CARBON FIBRE

We consider carbon fibre to be the best stiffening for our wings and have standardised on internally fitted pulltruded tubes 6 mm od, 4 mm id., more expensive but better than flat strips which we abandoned even before we sold our first epp kit, since these give adequate stiffness, whilst supplying some flexibility which is desirable whilst absorbing impact stresses.

GLASS REINFORCED CROSSWEAVE TAPE

This tape is used commercially for high strength packing purposes. The adhesive is put on either by a hot melt or a solvent basis - we have opted for the latter since it has better adhesion and is of a higher quality, obviating the need for a base coat.

COLOURED TAPE

A perfectly opaque tape would be ideal as this would virtually eliminate u.v. degradation of the latex based adhesive on the crossweave tape. Signwriters vinyl meets these requirements, but can crack if flexed on impact. Solartrim or equivalent products are excellent, but expensive. Coloured vinyl tape (most easily identified because the adhesive side is white, is far superior, and although the much cheaper polypropylene tape might look good on a new model, it gives little or no protection to the reinforced tape as it is transluscent, and SHOULD BE AVOIDED.

FINS AND CONTROL SURFACES

We have standardised on 4 mm microflute Correx, since we have prioritised on strength, which is why you are probably reading this article.

CONCLUSION That was quite a lengthy dissertation into materials which now leads on logically to model design.

MODEL DESIGN CONSIDERATIONS

A few simple and obvious facts to start with. Epp absorbs impact loads,and we have found the optimum density to be about 25 grams/litre range. Though much stronger than polystyrene, it is more flexible, hence much better at absorbing impact loads which it can take without denting permanently. It can be sheered, twisted or torn apart, hence the need to cover it with the crossweave tape. We prefer to stiffen the wing with internal carbon fibre tubes as this results in a much better sirface finish whilst still retaining some flexibilty which is useful in spreading impact stresses. Monocoque construction is by far the best for these types of model since point forces such as wing attachments are minimised - that is why we have specialised in flying wings, apart from the fact that I have always preferred wings!!

Theoretically, a wing with an elliptical lift distribution is the most efficient, the higher the aspect ratio, the less important are the tip vortices which probably represent about 20% of the drag of a typical model wing. However it would be most boring flying a wing at it's most efficient - a straight line, so we have to make sacrifices in theoretical efficiency to improve the handling and manoeuvrability. The designer who makes the best and most acceptable compromises is on to a winner. It is also necessary to impose design limitations on the model for safety reasons - the BMFA have sensibly put limitations on the epp combat class rules, but these are not enforced with the moulded models which could cause severe damage with over sharp noses etc!!          

 We have taken three approaches :-                                                                                            1)          A straight wing such as is used on the FUSION and WILDTHING models.         2 )         A swept wing as used on the VENOM and MAMBA models.                                   3)          A model with a tailplane such as the COBRA

Wing taper is important to give a pleasing roll which can be more accurate with a central fin and many pilots find it easier to orientate on. Pitch response s generally better.
It is necessary to place the battery in front of the wing in order to get the  centre of gravity correct. This generally makes for a better general purpose sports / aerobatic model.

Both wing sweep and taper is critical. Tip fins are theoretically more efficient because they reduce tip vortices, but produce more drag when rolling..Too much taper and the model will roll nicely but not loop as well, too little and it will loop nicely but not roll as well.

Choice of wing sections is of paramount importance and as far back as 1981 we used those of our own design for our flying wings, since those available were generally quite efficient for upright flying and good speeds could be obtained with some of them, but this was generally obtained by sacrificing inverted performance, if that was even considered in the section development.

WHATEVER YOU DO - ENJOY YOUR FLYING - BUT NOT AT SOMEONE ELSE'S EXPENSE.

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