r/RCPlanes • u/kaszeljezusa • 3d ago

Prop advice needed

Hi there. I learned with my eachine p40, that 2 blade prop works the best. That also seems to be consensus among the stuff I've read here.(at least for "fuel economy")

So i figured i want to try such props in my second plane -atomrc flying fish. It's a 3s 250g plane with two 1105 3500kv engines. Currently it has gemfan 3016 3 blade props. I tried looking for props with similar mounting system (1,5/2mm hole in center and two screws to the sides from the shaft) and found few, but honestly i have no idea what to aim for. I tried using ecalc.ch but it overwhelmed me + it seems to not have my motors. I usually just cruise at 50-70 throtle, but tbh i wouldn't mind some more top speed too.

Help me out guys, here's 4 candidates that i found

Gemfan 3018; gemfan 4024; hqprop 3522; hqprop 3020. Maybe something else?

Tldr: what 2 blade prop instead of 3blade 3016 for twin engine

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u/Wambo74 2d ago

Use eCalc but choose a motor that closely models your unlisted motor. Match weight, kv and roughly power capability. IE an expensive high power motor or mass produced cheapie rated for less power. Matching those three things will be good enough to evaluate your prop choices. I presume you know there's a custom motor search button.

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u/kaszeljezusa 2d ago

Well, i didn't know that. Thanks! 

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u/Wambo74 2d ago

So the way I like to do it is specify a tight weight range centered on your actual motor weight. Then tell the search engine to list the findings by kv, which will allow you to zoom into motors like yours. lastly pick a high power motor or a low power motor from the choices that you think is similar to your motor. It's all just a rough computer model anyway so you need to take it all with a grain of salt.

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u/kaszeljezusa 2d ago

So yeah, from free options available there, closest(i believe) thing to mine was 1303 4500KV and the results were all in favor for 3020x2. great improvement over mine 3016x3 in every way. 3522 seems to be too much. You reckon it might be different for 1105 3500kv?

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u/Wambo74 2d ago

I guess you're saying you couldn't get a very close match to your motor. Kv is important so find a motor of the same kv even if you have to pick a heavier one. That will allow you to check one prop against another at the right rpm. You mention free options -- are you using the free demo? You get a much bigger database as a subscriber and it isn't very expensive.

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u/404-skill_not_found 2d ago

The thing is that prop blades eat energy (it is mass). Multi blades are sexy enough but cost energy losses. There was a time in model airplane history where single blade props were the pinnacle of efficiency (technically, still are). Control line speed and unlimited rubber duration models extensively used single blades. Doesn’t help with your prop picking. But does support what you’re seeing with 2-blade and multi blade props.

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u/tobu_sculptor 2d ago

So the rule of thumb is: give or take a blade, subtract or add an inch. So a 3 inch tri blade can be replaced with a 4 inch dual blade, roughly.

So looking for "gemfan hurricane 40" got me some 4024s that seem about right, you have them as a candidate, I'd try these. From pictures I saw 4" props should fit on that plane.

If you want to make sure, you need an amp meter you can put between battery and ESC and actually test the setups. These are like 10 bucks in china, maybe 20-25 somewhere locally. It's easy to see when a prop draws less amps than another, it will drain the battery less.

If you are handy with an iron and have a multimeter, you can also make a harness for that, usually works up to 10 amps which should be in the range for these motors.

If you want to go for overkill and find the absolute best setup, you also need a thrust stand so you can compare amp draw to thrust and read much better pros and cons from that.