Propellers

[bernard]

Here's a question I've been asked about propellers: where are you getting 20 cm diameter propellers if you are not making them yourself?

I've heard of people trimming last year's 24 cm diameter Ikara propellers. If you are doing this, are you cutting a swath around each blade off or just cutting off the tips of each blade?

[retired1]

Not knowing what the ES will decide, I trimmed last years blade to a complete radius which meant cutting both the front and the rear of the tip as required. One person that I talked to thought that it would only be at the spar position. IE a single measurement jig that only checked the total horizontal distance. My position was that the prop had to turn within the 20 cm jig.
This will be a good question once the FAQ's open up.

[DoctaDave]

At nationals we had to turn the prop inside a 24cm circle. I believe the normal ikara props would pass without issue, but the ikara flaring props had some trouble.

[Less_Incidence]

I thought Ikara manufactured a 20cm diameter prop with a slightly different blade shape than the 24cm. I'm making my own props this year so I personally won't have to worry about that, but as I recall LaserCutPlanes's Osprey came with a 20cm Ikara.

Edit: Sorted through my big pile of Ikaras from last year and found the one. It's 22.5cm. Personally I'd take the 24cm prop (or the flaring prop) and trim it down to 20cm, because the blade shape on the 22.5cm one is just strange (more area behind the spar than in front).

[jander14indoor]

This is one of those places you really have to ask yourself how much benefit you get vs challenging the judge and risking a DQ.
Its just not that hard to draw a radius from the center on the longer blades and cut a curve instead of a straight line.
Then consider rounding the corners as air really doesn't like sharp corners a lot.

Jeff Anderson
Livonia, MI

[Less_Incidence]

Another thought -

For anyone besides me who is making balsa props, how are you planning to build them tough enough to survive the high RPM of a 20cm prop and the hard landings of a (potentially) 14-gram model? I'm planning on building mine out of 1/16" thick wood at the spar sanded to 1/32" at the edge of the blade and epoxying a piece of carbon tow around the blade edge. This is going to take me bringing my little homemade hot box out of retirement and having quite a fine touch though (in my klutziness I'll probably go through 5 blades before I get a good one), so I'd be curious to see if anyone comes up with a simpler method.

[DoctaDave]

The issue with that is the wood will still crack along the grain with hard impacts, even if the edge has carbon reinforcements. I'm thinking of possibly laminating two sheets of thin balsa together to make a very stiff plywood structure, and then have any flaring I want coming from the spars.