MAKING YOUR OWN PROPS:

This is an article that was detailed to me by Trevor Henderson. He has been making propellers for a considerable length of time, and he does not have any failures.

  His main warning is .  Do not skimp with materials - it is dangerous.When making propellers, insufficient Resin or insufficient Carbon Fibre or Glass content will lead to a weakly constructed propeller, which will have a real risk of breaking when running under load.

   If you are not willing to follow his instructions both in the creation of the mould, or propellers, you should not even attempt the manufacture your own propellers.

 For example I have made my own props for some time.  It is quite a bit of trouble to get them right.  The hardest part is making sure that you have the fibres of the Carbon Fibre completely wetted through with the resin whilst you are building up the carbon fibre prop in the mould.  If you are a bit lazy here and you do not wet the Carbon Fibre properly you will get a very weak prop and I can assure you it is not at all nice to be hit with a prop blade that is thrown from an engine.  I actually had this happen to me when another model nosed over on the flight line and the broken propeller blade hit me in my face.  I actually saw it coming at me - anyhow it hit me on the cheek  - stung a bit - but I was very lucky - could have lost a eye

 So the risks are there in prop manufacture. The construction techniques are time consuming and very messy.  I eventually stopped moulding my own props, and now buy them from people who are experts in this field.

MAKING THE PROP MOULD:   By Trevor Henderson.

Items required:

• You must have a perfect prop as a pattern, it must be highly polished (Must be thoroughly checked for pitch – even blade thickness – ensuring that both blades are exactly the same. Make sure that you use an accurate pitch gauge to validate this.
• Wax release agent
• Aluminium filled epoxy
• Plasticine or modelling clay
• Aluminium channels- for F3d props I use 40mmx20mmx200mm long
• Metal base plate slightly bigger than the Al. channel
• Silver steel pin the size of the desired propeller shaft size
• Wood to form a box around the channels
• Refrigerator/ deepfreeze............

 Pattern (Propeller to be copied)

•  A ‘perfect prop’ is required to be sanded very smooth and coated with 6-8 coats of thin cyano, then sanded with 1200 wet and dry paper to a very smooth finish- any imperfections here will certainly be transferred to the finished product. I use a full length prop BUT be very certain that the hub and the hub hole are central to the blades. This can be checked by marking the tips of both blades, marking a baseline /reference line on a base and swinging thru 180 degrees, any variance will show up and should be rectified before you proceed any further.

• An out of centre hole can be over drilled to relocate the centre, and then sleeved using aluminium tubing to the desired size. The alternative is to chop off one blade, leaving the complete hub, of course some work is required here to form a perfect hub, and use of a milling machine and end-mill tool comes in handy to get complete curvature and angle suitable for release from the mould. Again one has to mark the tip, and when forming the mould spin the prop through 180 degrees on a baseline to give an exact copy - this is by far the most accurate method but does take a lot more time. One cannot stress enough that the perfect prop must be perfectly matched in pitch and more importantly the location of the highpoint! This, I believe is one reason for propeller flutter.

Mould

Chamfer and holes - The rear face of the mould showing where the epoxy has been poured through the countersunk holes. The photo shows the chamfer filed after the moulds have been made - this ensure correct alignment of the moulds.

•  Position the prop over the silver steel pin and onto the base plate and with modelling clay/ plasticine, pack lightly, but tightly under each blade ( twin prop blade method ) ensuring there are no gaps anywhere to either the blades or the hub, and ensuring a reasonably wide spread of modelling clay. I then cut with a scalpel around the total prop perimeter and against the outer extremities of the blades, to an angle of around 5 - 10 degrees off vertical. This angle I find very important as the carbon and resin has to be forced into the mould face, angles in excess of 15 degrees tend to push the ‘matrix’ out of position and to the sides of the mould face. There is a trick to getting the hub correct but if you look at any propeller at the hub area you will get an idea how others have formed the split line. At this point I then put the work completed into the refrigerator or deepfreeze, for a period of 12 hours. I do this because the ‘chill’ tends to stabilise the modelling clay available in NZ- maybe warmer climates will need to do it for a longer period of time because of the heat!

• I then drill a series of holes in the Aluminium channel, and then build a box around the 4 faces of the aluminium cannel using balsa or similar.

 Mould Stage One  - here is shown the silver steel pin is used to mould around to form the hole for the crankshaft. NEVER DRILL THE HOLE as in will never be accurate enough. Note the Plasticine  prior to trimming off before moulding.

• I ensure that there are no obvious leak points in the balsa box and form plasticine fillets to ensure no leakage. I then apply lots of release agent to all exposed faces, all applied as per manufacturers instructions( yes my ego isn’t always correct)- use a lot of release agent as its cheap and very gently polish as required. Now the messy part is to pour the aluminium filled epoxy thru the holes in the channel to totally/ overfill the formed cavity- a gentle tap on the channel, or holding the mould on a slight downward angle and pouring from the top will tend to eliminate any air bubbles. Ensure the Aluminium filled epoxy rises out of each hole then you know that it has flowed everywhere. I let cure for at least 48 hours and generally put the components in direct sunlight- don’t use the heat box as it will melt the modelling clay............

• After 48 hours open the balsa box to reveal the modelling clay to the back face of the prop- then very carefully remove all the clay. This is helped or made easier by the chilled modelling clay, but the final removal and cleaning up process may require the use of Johnson’s ear buds (A lump of cotton wool on the end of a small plastic shaft) dipped in a weak solvent eg meths,or methanol- keep way from acetone. Do not remove the prop/ pattern at this point!

• Mould Stage two - here we see the trimmed plasticene, on one  propeller blade - Note how at the hub the plasticene is significantly wider than the hub to allow for sculpturing of the split line - Note that the other blade is yet to be trimmed.

• Repeat the above process using another balsa box and the other Al channel complete with holes. This time you have to ensure all of the back face of the prop, the recently cast Al filled epoxy, plus all other exposed faces shall be treated with release agent, and as before, polished accordingly. The aim now is to totally fill the second channel and the cavity which is to form the back face of the prop. Complete as above, again carefully filling and on completion hold everything together with gee clamps- place in the heat box at 60 degrees and walk away for about 4 days- the longer the better, but we are all impatient beasts when it comes to admiring our own work. Remove from the heat box with gloves as everything will be hot then let the casting cool down- I normally leave it to cool for about 6 hours. Remove all the clamps and very carefully tap the casting with a soft piece of wood- it will come apart with patience, just DO NOT rush. I have a 50mm wide chisel for the worst case which I used once or twice as a lever but be very gentle. Once the mould has parted the only remaining work is to remove the pattern- again not difficult but I have found the best tool for doing this is a broken Plastic prop.

Here is a male portion of the mould showing hub and blade shapes, and the mould run off at the end of the blade. This allows the resin soaked carbon rovings to extend beyond the mould, and when cured allow easy removal of teh propeller from the mould. Note that the mould has been keyed into an Aluminum Bar.

• Once the pattern has been removed you can then look at the perfect impression left on the two mould parts- if there are minor imperfections I then fill and repair, or sand with 1200 w&d sandpaper used wet.

• Finish off the moulds by filing gently a channel at the end of each prop blade- this should go right to the end of the mould at each end. This will allow you to run the Carbon rovings past the end of the mould and with epoxy will allow for easy removal of your prop.

•   File a big location chamfer across one corner of both moulds. This will allow you to easily recognize the correct assembly of the components when closing the mould after a prop lay up....be warned because I was once caught , and ended up with a reject prop.

MOULDING THE ACTUAL PROP:

After you have made your mould you can now make your props.

Items/Materials Required

Heat box     This can be a Portable Cooler with a 60 watt light bulb installed in the side, running

                   through a thermostat. Surprisingly the 60 w bulb will give a temperature of 60 c.

Epoxy         No secrets here but I have used a variety        ADR 240, ADH 341 (Heat Cured)

                                                                                         Ciba Geigy PY 303

                                                                                         ARALDITE F (Cable Joiners Resin)

Syringe                                                                              Say 12ml                                                                                        

Carbon                                                                               (160k)

Chopped Strand Carbon                                                    Carbon about 10 mm long                                                          

Release Agent                                                                   Ciba Geigy QZ5111 or

                                                                                         Freekote

Mixing containers

Surgical Gloves

Variety of tools              Gee clamps, tin snips, chisel, hacksaw, newspaper, rags.

 Prop balancer, pitch gauge

 PROCEDURE. (Please follow diagram at the end of this article)

•  The first step is to treat the mould very thoroughly with the QZ 5111 release agent and I use 3- 4 coats. Any shortcut here could see the unthinkable happening.......yes, there is nothing worse than a locked mould.

•  Next a batch of epoxy to the required quantity is mixed- say for your first efforts use 15 ml.

• Next is to cut the carbon to a length approximately 25 mm longer than required- the carbon I use allows for stripping in width and can get 4 widths per length. The amount of carbon required is based on trial and error but as a guide the propellers on display have used only 2 full lengths of carbon.

•  The first process is to mix half a teaspoon of chopped carbon with a very small amount of resin and spread the carbon around the vertical face of the hub- this helps form the hub and in my mind if not done correct tends to ruin the cosmetic/ aesthetics of a propeller.

•  The general principle is to use the least amount of resin to wet out the strands of carbon and I dribble some resin onto the newspaper, lay the carbon into the resin and with a balsa squeegee - apply extra resin and squeegee any surplus resin from the carbon, turn the carbon over and repeat the process then carefully lay this into the mould, around the central pin and at the mould ends fanned out to the full width of the blade. Repeat this procedure but on the opposite side of the pin, again fanning at the mould tips. Carefully spread the carbon the best you can but do not be too concerned about the little triangles formed on either side of the pin. You will notice the carbon hanging over the ends of the mould- with your tin snips cut about 10 mm off each end of the overhanging carbon and place in the triangles at the pin. At this stage you will/ should have all parts of the prop portion of the mould with carbon.

• The next stage is to repeat the first process however you now cut in half lengthwise the carbon

• and starting at three quarter radius you position the wet carbon heading back to, and around the pin positioning the carbon tail back towards the start point. Repeat this process to the other blade and carry on this process starting at half radius, around the pin, tail towards start point, and carry on until the hub appears to be over full. Finally repeat the stage one and two process.

•  You will need to overfill the hub and the final fill should be done with more of the chopped carbon. Carefully spread in a systematic way the chopped carbon around the central pin ensuring an equal amount of carbon is evident to all sided of the pin. Whilst the mould will do some pushing of the carbon when the mould is closed you should really aim to do a very good job of placement of the carbon- it will certainly help the balance of the hub.

• Close the mould and with three Gee- clamps locked tight on the mould then place in the heat box. If I am using the ADH/ADR resin I give 6 hours for the cure, and when using the Araldite F then 3 hours seems to be more than adequate.

• When the resin has cured remove the mould from the heat box, remove the three Gee clamps and then drive the central pin out form the mould, then very carefully with the chisel you can then part the mould. If you have done things correctly then you should have carbon appearing from the ends of the mould.......this is very important as this will help you remove the prop from the mould. Simply with the edge of the chisel whack the ends of the overhanging prop adjacent to the end of the mould- this will break the bond from the mould and with and old APC prop you should be able to lever out the prop. With a sanding block or curved tin snips you can then trim the carbon flashing from the blades and admire your piece of work.

• Epoxy still requires further curing and I have purchased a little deLONGHI bench oven which I use for POST CURING of the epoxy. It has been known for a long time that epoxy gets stronger with POST CURING and this is what I do. Turn the oven onto 60C and place the prop( generally a dozen at a time) in there for an hour, increase then to 70C for another hour, and continue raising 10c per hour until I reach 100c. Let cool down naturally and you will find a rock solid product.

• Check the Pitch Distribution and modify as required, check the balance and alter accordingly.

One of my mates made me a sort of Electrical Dynometer that will allow me to spin the propellers at 9,000 RPM and record Wattage/ Grams/ RPM. Run from a 12 volt battery I am able to make all my props as close as practical to each other by comparing the results – I found it amazing how finishing affects the performance of propellers - I have included a photo of this appliance for your interest.

Typical (actual) results are as follows: 

 Prop one                            Prop two                       Prop Three

 Watts   Grams     RPM      Watts   Grams   RPM     Watts   Grams RPM

162       324        9,000     159       322       9,000     160      322     9,000

 One could of course say that the 9,000 RPM is not our operating 30,000 RPM however it proves to me that if I can make/ modify propellers to this close tolerance then they must - within reason- perform similarly at the upper level.

 FINALLY.

 When the props are finally tested finish off with 2-3 coats of thin CA, rubbing lightly between coats and finish off with 1200 wet and dry paper used wet. Recheck the balance and then finish with car polish.

 Generally I can lay-up/ make a propeller in 30- 40 minutes, balance and check pitches in 15 minutes, and CA and polish in 10 minutes.......say an hours work.

THANKS TREVOR FOR PASSING ON YOUR KNOWLEDGE: