Crash Resistant 60in EPP Pylon Racer / Sports Aerobatic Slope Soarer
Designed by Stan Yeo Produced by PHOENIX MODEL PRODUCTS
Fast-bak is the fifth model PMP have developed for the popular 60in EPP Pylon racing class. Needless to say each model has seen an improvement in performance on its predecessor. Fast-bak is no exception. Not only is it faster, but it is easier to build and more robust in the world of model on model pylon racing. Despite its pedigree, Fast-bak is also an excellent sports model, more than capable of performing any manoeuvre that could be expected of a flying wing, continuous loops and rolls, sustained inverted flight and with practice rolling circles.
As with all EPP models, building time has been kept to a minimum; typically a quarter to a third that of an equivalent wood foam model depending on the skill and care taken
Tools / Materials Required
The only tools required are a modelling knife and spare blades, 180 grade Wet & Dry sanding block, a set square and soldering iron. Glues etc. required are spray impact adhesive (Stikatak 3M77), runny super glue, 12 minute epoxy, No Sanding Polyfilla and Sellotape Diamond plus 'top covering' (Profilm recommended). Titebond Polyurethane glue can be used for fixing the mainspar assembly if preferred. Please observe safety precautions for the glues!
The R/C equipment used in the prototypes consisted of two metal gear micro servos (HS81MG /85MG or Futaba S3150 Digitals) a AAA size square Rx battery pack, a JR R700 / RS70 (highly recommended) / Futaba 147F / Hitec DC Slimline Receiver and a switch harness or Jack Switch assembly. All the items are available from PMP at competitive prices.
Building the Wing
Fin Assembly & Fitting
Building Stub Fuselage
Note: The components for the stub fuselage are 20mm longer than shown on the plan. The model can either be built with a longer nose to reduce the amount of nose weight required to achieve the correct balance point or built as per plan.
To achieve the design performance of any model care must be taken in setting up the controls and balancing the model both laterally (wing tip to wing tip) and longitudinally (nose to tail). The Fast-bak is no exception. Control movement must be symmetrical i.e. the same for both Elevons.
wishing to further improve the performance of the Fast-bak Andy Ellison,
a leading exponent of 60 inch pylon racing and well known magazine columnist,
has kindly written an addendum to these instructions giving tips on how
to make the Fast-bak a more competitive racing machine. Thankyou Andy.
Optimising your EPP model for race use or high performance sport flight.
By Andy Ellison
So you want to turn your new PMP sport model into a full on competitive 60" race machine do you?
Firstly you'll have to sit mulling over your new pride and joy as it sits in it's box, totally untouched and consider this carefully. If you want to produce a model with a surface finish to rival a moulded carbon race machine, and if you want to get the best possible performance from these simple pieces of EPP foam lying before you, you're going to have to make your mind up right now - before you build anything at all.
To obtain a model capable of taking on the best; to optimise the airframe for maximum speed and agility, it's going to have to be built clean, true, accurately and slippery. You're going to have to consider modifying the model and also the use of digital servos to obtain the necessary resolution as the CG comes back to the optimised point for the airframe. You're going to have to revise the radio installation, begin to use materials unfamiliar to you and adapt to building techniques you may never have considered before.
Sounds like this is for you?
We'll look at the digital servo thing first. As the CG comes further and further back to optimise the speed potential of the airframe and produce tighter turns, servo resolution plays a bigger and bigger part. On some 60" slope racers the total travel of the elevators with a race tuned balance point is no more than 2mm from full up to full down!
In order to help achieve this good resolution across the entire stick movement, very short servo arms (7mm or so) are used in conjunction with overlong horns on the control surfaces. It is NOT OK to just rate the movement down at the transmitter. This just wastes useful servo resolution...
The use of digital servos assists this optimal resolution and also ensures better centring of the control surface after a movement to full deflection.
To turn out a PMP model in race trim you may also like to consider some of the following common modifications.
One of the most obvious is the replacement of the supplied Correx fin with a shaped balsa version. You can do this in a couple of ways. Either replacing the Correx fin in it's entirety or by cladding it with balsa sides before re-forming. If you'd like to keep the Correx but reduce the drag somewhat you can remove some of the fluting around the edges and then bring them together into a point. This can be retained by gluing or more commonly, with fibre tape. The reduction in drag is audible in flight.
PMP models often feature the fitting of a tube up the front of the fin which is utilised to route the aerial from the receiver. Whilst this does dangle things out in the breeze nicely it does nothing for the top end speed of the model. If you are to optimise the aircrafts performance you need to tuck the antenna away into the wing. A shallow scalpel cut snaking around the foam of the wing will hide it nicely. You will need to do this before you even think about covering the model.
A further popular modification is the 'bagging' of the elevons with unidirectional carbon fibre and epoxy to eliminate twisting and to finish them in a knife sharp trailing edge. Bagging is achieved with the use of a vacuum pump but good results can be achieved by pressing the carbon on against a plastic or glass surface. Carbon cloth can be obtained at a number of outlets in the UK but for best results the weave should be heavy (at least 200g) or sewn tows should be used instead. The latter is more difficult to source and has the appearance of straight, flat carbon tows sewn together by clear monofilament or glass cloth. You will also of course need to fit any ballast tube, ensuring easy access to the lead which may need to be changed in a hurry at a race.
So with these points in mind it's time to build the model. You should follow the instructions implicitly varying only where you are considering a 'racing' modification.
When you have the radio gear installed and a bare model ready for tape you need to fully consider the race tuned finish.
Spackle is a term popularised on American forums and relates to nothing more than interior wall filler. It is used to provide a glassy surface finish free of the bumps EPP foam can leave under the model covering.
The product I use for this task is Polycell quick drying filler from B&Q or similar DIY outlets. When you pick up the large tub and think it's empty, you have the right stuff.
To proceed you must sand the wing in its entirety and make sure that the servos, leads, receiver, antenna and battery (if it is housed in the wing) are all in place. Bear in mind that you will need access to the receiver if racing to facilitate removal of your crystal or switch frequency.
When you are happy that the model is sanded smooth, liberally coat it in spackle as if you were icing a cake. Go over everything except the receiver. That includes the servo's which, if you have built the model correctly, may only have their arms poking above the surface of the wing. Don't forget to spackle over the ballast tube leaving easy access to the hole!
Now, quick drying filler it may be but it's not going to set hard in 15 minutes in your cold shed at the bottom of the garden. Get it in the house overnight and let it really bake dry. Now you can cut it back with glass paper to achieve a smooth finish. You will note that there may be some cracking and that when sanded this first coat has only really filled the patina of the foam. That's why you must now spackle the whole model once again. This coat however can be much thinner, paying particular attention to areas you might have missed properly first time around. Again you will need to let this dry very hard overnight before cutting back to a fine, smooth finish.
The model is now at the stage where we need to use fibre reinforced tape (CW Tape). Before this is applied some form of additional adhesive must be applied to the surface of the wing. Many readers will probably be using foam backed carpet spray adhesive. The kind that comes out of the can like silly string and produces lumps under the covering if you spray from too far away. A much better product (it's also more expensive) is 3M77 adhesive spray. This again is a product used more in America than here but this glue is available in the UK from an increasing number of outlets. It always sprays in a controllable fine mist and dries much quicker than the carpet adhesive. Some users may try permanent spray mount from art suppliers but in relation to 3M77 it is quite weak.
When the model is dry the tape can be laid onto the model in accordance with the kit instructions. Taping on a biase of 45 degrees increases tortional rigidity on some models and others often require no tape outside of the servo arms. Follow the manufacturers lead but do not overlap the tape. Instead the strips should be butt joined together to preserve the smoothness of the surface finish.
The fibretape we use has a release agent on the backing. How else would we ever get it off the roll? This release agent must be removed before you progress to the next stage of covering. This can be achieved by light sanding of the tape. Just enough to get the agent to spall off in little balls but not enough to sand through to the fibres of the tape. Go lightly until the entire surface has a matt finish. The model is now almost ready for final covering.
In my experience Profilm has produced the best results for a racing finish. It does not de-laminate like older Solarfilm and is generally tougher. Many pilots have tried products such as gloss or Solartex but whilst this does contribute more to the rigidity of the model it seems to produce slower flight. I have no hard proof of this except the experiences of pilots flying different models covered in both materials. I generally however use Solartex on the fuselages of the models as it tends to wrap compound curves much better than film.
PVC Tape is not a serious finish for a racing 60" model. Great for combat but not for speed. Many pilots however have started to achieve good results with sticky backed 'Fablon' type material.
You must also consider your chosen colour scheme. At a 60" race a distinguishable colour scheme is a must. Not only for the flagmen but also for you as pilot in the heat of a race. Consider this carefully as a yellow model with red wing tips is sure to be one of many on the slope.
You also seriously need to think about solid colours for each wing panel which can be applied with one piece of covering. As we are applying the covering to a foam model, the iron temperatures must be lower and the adhesive may not grab as well. The scheme can be spruced up with sticky trim later but the less edges capable of lifting, the better. Many pilots actually tape over the leading edges with clear Sellotape Diamond after completion to alleviate this further.
Before you apply the covering one more light coat of 3M77 adhesive is required. When this has set dry you will find that the film can be laid out and removed for repositioning easily before ironing. A luxury not afforded to you if you use a carpet spray adhesive. Take care to cover the undersides of the model first to ensure that leading edge overlaps end up the right way round and consider how you are going to hinge your control surface. If your elevons are film covered you can also film hinge them. If however you carbon bagged your elevons the best results can be achieved by hinging with fibretape.
So now you have a model ready for flight, but in need of race tuning in the air. Generally if you push the CG back to the point where the model becomes un-flyable and then shift it forwards slightly you'll be somewhere near the money. Some plank type wings benefit from negative differential on the elevons (i.e. - more down that up) to overcome the reflex and track better through the turns and don't forget your control throws, especially the elevator which should be set to avoid a high speed 'flick' if you over pull in the heat of the moment.
When you have the model optimised for yourself you're ready to race...
I hope you can use the above notes to your advantage and look forward to seeing you in the national leagues.