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Old Feb 01, 2015, 11:24 AM
Kimmo Kaukoranta is offline
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New Product
Balsa props for F3P

As some of you might know, I have been working on making lighter props for F3P, especially the contra systems. Since there are two props and the trend seems to be larger props every year, the weight of the props is starting to be significant portion of the total.

Last winter I made some transparent carbon mylar props for my dual motor contra system. While these were fairly light and many people have commented on loving their look, they proved very time consuming to make. Several small pieces of carbon need to be laid to the molds individually, while I’m on the clock with the epoxy setting. Finishing the molded part, balancing, covering and tightening the mylar. Also scaling them to different sizes was hard and new molds needed to be machined for each prop size. An easier and even lighter solution was needed. We also wanted larger props this season, instead of the 10” ones, making the weight of the props even more important.

In november I was attending InSkaala2014, an indoor flying event in Pietarsaari, north of Finland. There we have a lot of indoor scale and free flight planes, in addition to F3P and general RC. I was admiring the hand carved balsa props in the free flight models and the thought occurred to me, maybe they would be enough for F3P. The weight of the planes and power demanded has gone down each year. In a contra system the power is split to two props, which also reduced the strength required. Also why not make them the modern way, with a CNC mill, instead of carving them by hand.

So during the next couple of weeks, after little bit of magic with the computer, my router was machining the first ones from some scrap balsa. Testing proved the efficiency was good and the scale was showing very low numbers, even though this was definitely not the lightest wood.

Next step was to order a big pile of contest grade balsa. With good quality wood, the lightest I was getting was below one gram for a 10x3.8, 1.2g for 11x3.8 etc. Easily half the weight of comparable strength carbon props. A real breakthrough!

Carbon vs Balsa
If you look at the densities of the materials, it’s easy to see why balsa is a good material for the props. Typical carbon fiber - epoxy laminate is maybe somewhere around 1600kg/m3 (got the value from an R&G table, not actually measured). Good quality balsa can easily be less than 100kg/m3. So for the same weight prop from carbon, you need to make it very thin. A thin, long prop does not equate to a stiff one. It also does not use the advantage of carbon, it’s huge tensile strength, to full advantage. With low density balsa the prop can be thicker and thus stiffer, while still remaining lighter.

Where carbon is clearly better is durability. If you crash with balsa props, you probably need new ones. Then again competition pilots do not crash their models very often, but every gram counts. For someone who crashes often, the carbon props are clearly a better option.

Details, finish
One thing I added after the first couple prototypes was a small carbon disk to the back of the hub, where the prop contacts the prop saver. A little more strength is needed in this area to prevent it wearing out so the prop stays straight and true on the shaft, even after being removed several times.

The props have just one coat of 50/50 thinned nitrocellulose lacquer. This is hopefully enough to keep them from absorbing moisture, yet keeping them as light as possible. More layers would improve the finish, but getting a real shine would add way too much weight.

The size and other markings are made using some custom rubber stamps I machined on the same mill as the props. I did not originally have any proper name for my dual motor contra system, so many have started to call it a “Kimmo system”. I thought that was pretty funny and I could not come up with anything better, so these now have “KimmoProp” stamped on one of the blades ;-)

Applications
These props were originally destined to my new prototype single motor contra system (more details on that one later). We have tested several prop sizes on those with good results and now have hundreds of flights on them. The next logical step was to try them on some popular F3P motor systems already on the market.
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Last edited by Kimmo Kaukoranta; Feb 01, 2015 at 11:48 AM.
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Old Feb 01, 2015, 11:28 AM
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Balsa props for my dual motor contra system

The 10x3.8 balsa prop works great as the front prop with my dual motor contra system. I have one in my last season GB Models Trivia, together with a carbon-mylar rear prop. I did not really see much difference in performance, only a bit less weight. The front prop is the one that usually breaks first, so if there is anyone with a broken front prop you might want to give the balsa prop a try and save a couple of grams at the same time.

The rear prop is a bit more difficult. The problem is that it would require a 7mm hole, which is no problem in a carbon prop but it would make the balsa prop too weak. I may investigate this further later on, maybe a little bit carbon in the hub or something. But no balsa rear props for that system right now, sorry!
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Last edited by Kimmo Kaukoranta; Feb 01, 2015 at 11:38 AM.
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Old Feb 01, 2015, 11:33 AM
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Balsa props for the Lantsov contra system

Since Risto (the BEC guy) had one of the very popular and elegantly simple Lantsov contra systems in his Elanor, I made a set of 10x3.8’s for him to try. He seemed to like them, since he’s still using them. The downline braking was improved and obviously the weight was reduced. On the downside the flight time was a bit shorter, but after removing some now unnecessary fixed airbrakes, returned to almost the same.

I did some static testing on another Lantsov contra system to compare. The testing proved the balsa props were a little more load than the original carbon ones, but also provided a little more static thrust.

For the Lantsov system the rear prop has a 3mm trough hole with an additional recess for the 5mm bearing located in the front part of the prop saver. The front face near the hub is shaped to keep the rubber band holding the prop from touching the plastic tube. Just like the original carbon props. The front prop just has a 2mm trough hole.
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Last edited by Kimmo Kaukoranta; Feb 01, 2015 at 12:09 PM.
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Old Feb 01, 2015, 11:36 AM
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Balsa props for the Glavak contra system

After the balsa props were found to work great on the Lantsov system, it made sense to try them on the also very popular and excellent Glavak contra system as well. So I made some 11x3.8 props since that was a size that seems to work on most of the different variations of the system.

The props were tested on a Elanor and the Glavak CR02 system with the smallest v9 motor. The weight with Glavak 11x3.8 carbon props was 63g and with the balsa props 2g less.

In summary the top speed was lower and the flight time was lower compared to similar size carbon props. However the downline braking was improved and was closer to a 12” Glavak than a 11” one. Also the 2g lighter plane seemed to somehow fly a little better and more stable. Whether this was due to just the reduction of weight or some other effects remains unclear.

In my view the thickness of the balsa props explain part of the lost flight time, but since the top speed was also lower, the balsa props could just have a little lower average pitch. That results in a higher throttle setting used throughout the flight and thus higher current consumption. Probably both. I may need to make something like 11x4.3 to test in the future.

The balsa props for the Glavak system have a similar cone shape on the back of the prop as the original carbon props. The front prop has a 2mm hole which is really only needed for balancing. The rear prop has a 4mm hole, which is a good size for fitting it in a balancer but it may need to be enlarged by ~0.1-0.2mm using a small round file to make them fit the shaft.
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Last edited by Kimmo Kaukoranta; Feb 03, 2015 at 04:25 AM. Reason: Mistake on the motor system versions, the testing was done on the smallest CR02 v9 motor, NOT the stronger v6
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Old Feb 01, 2015, 11:38 AM
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Very cool Kimmo
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Old Feb 01, 2015, 12:29 PM
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for those of use with CNC machines this is very exciting.

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Old Feb 01, 2015, 01:03 PM
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Do it AGAIN! This time BETTER!
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Oh this is very cool! Thanks for sharing.
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Old Feb 01, 2015, 01:09 PM
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Do it AGAIN! This time BETTER!
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Another option for finishing is spray can clear.
Apply a couple light almost dry coats then wait a few hours and splash a small small amount of water on them. If it soaks in apply another couple more light coats. Eventually the water will sit on the surface with very little weight gain.
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Old Feb 01, 2015, 03:16 PM
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Thanks guys! I may try the light mist of spray later to see the results, but I'm quite happy with the traditional nitrocellulose finish as well.

If anyone is interested in giving these a try, I have a some ready to be shipped and a pile of light balsa waiting to be made into props. The 10" ones are priced at 25€/pcs and 11" ones 30€/pcs. Please contact me privately at kimmo.kaukoranta@gmail.com
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Old Feb 01, 2015, 04:15 PM
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You don't need a CNC machine. Hand carving of balsa props has been the just fine in free flight since the 1930's.
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Old Feb 01, 2015, 06:18 PM
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WOW Kimmo!!! These are amazing!

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Old Feb 02, 2015, 04:23 AM
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Hand carving is just fine, but a bit too slow and labor intensive for my taste. But sure one could make his own light props that way and after a little bit of practice they could be just as good and precise as CNC machined ones. The sanding and finishing is still done by hand, even though the basic shape is carved out by a machine.
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Old Feb 02, 2015, 04:28 AM
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Shipping

About shipping. The props are thin enough to be shipped in a letter size carton (350x250x20mm). One carton will fit 4pcs 10” or 11” props. Attached is a photo how I intend to package them. Some obechi veneer strips on bottom and top, in the thickest part of the prop. One strip is glued to the cover and other to the bottom for the carton. Balsa longerons cut from scrap balsa left over from machining the props between them. One between each prop, glued to the bottom strips.This should hopefully prevent the box from being crushed during transit. Props are attached to the base with some nice tape that easily peels of and does not stick too much.

The props don’t weigh much, but just the box is over 80g and the reinforcements about the same. The weight goes up to the 100-250g price range. The shipping cost for me is 3.40€ for europe and 8.40€ outside europe. I was thinking of rounding that up to 5€ for europe and 10€ euros outside europe to allow a little for the shipping materials and to have a nice round number. Shipping time is listed as 2-5 days to scandinavia and central europe, 3-6 to other european destinations and 4-10 days bigger cities outside europe. No tracking unfortunately.

For larger shipments I can quote the shipping price separately or it can be divided to several cartons. Also tracked or express shipping is possible if needed for extra cost, I will quote them separately. There is also an economy option for outside europe, but it’s slow.

For payment PayPal is fine outside europe. Inside the eurozone I would prefer a bank transfer if it’s not too inconvenient for you (it should be free of any bank service charges) but PayPal is OK too.
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Old Feb 03, 2015, 08:37 AM
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If we're not worried about mid-air collisions, balsa stick airframes are the next logical step, and probably a lot cheaper and easier to work with than carbon tubes.

As TRuss said, balsa has served the free flight community just fine since the 1930's. As far as I know, the hardcore F1D guys have not yet switched to carbon.
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Old Feb 03, 2015, 08:43 AM
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A friend of mine (who does not fly F3P) keeps telling me balsa would be a better choice for the airframe. He flys FAC No-Cal scale profile rubber models.
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