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replacement for 10.5x9 prop

Old 08-03-2020, 07:04 PM
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AirmanAirhead's Avatar
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Default replacement for 10.5x9 prop

I have an old foamy, 300 Extra, and I'm trying to find a replacement prop. It came with a 10.5x9 and it appears those are no longer produced. What would be a good alternative?

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Old 08-03-2020, 08:01 PM
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Not first time a factory has supplied a daft size !!

First you should look at the type of prop it was .. many of these strange sizes are Slow Fly or as I like to call them : Floppy Props ! I HATE THEM !!

But anyway .. IF it is a Slow Fly ... then a lot of thrust is lost when it flexes ... you can hear the prop WOMP as you throttle up .. This means that often a change to a stiffer E prop can either give same or better stable thrust especially when punching out.

A 10.5 x 9 Slow Fly could be easily replaced by a 11x8 ... or my favourite 11x7.5 ....

You can check suitability by using eCalc ... one of the best tools to give you performance values ... enter the values and it will tell you all you need to know to make informed prop decision. Then next is to fit a prop and run a Wattmeter test to determine if you are within motor load range etc.

There is often an advantage gained by sizing the prop to suit YOUR flying instead of just taking factory spec.

Maybe you can post here the brand of model, model size, motor size ... KV .... weight of model etc. as much details as possible so we can possibly advise a prop .. a photo would be nice of the model as well ..
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Old 08-03-2020, 09:39 PM
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Thanks I will add more details a bit later. It is not a "slow fly" prop. I will pull the cowling off and check out the motor, as well as send you a pic of my trusty 300 Extra, a bit battle weary, but still fun to fly.
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Old 08-04-2020, 02:08 AM
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And don't forget to list battery voltage as well.

To give you a rough idea, current drawn is proportional to pitch, #blades and diameter⁴
Current is also proportional to voltage≤ , Kv≥ and gearing≥ .

Silly scenarios:
  • Doubling voltage would would result in a 2≤=4fold increase in current.
  • Doubling Kv would result in a 2≥=8fold increase.
  • Doubling diameter would result in a2⁴=16fold increase.
Realistic smoking scenarios:
More e-calculators, motor- and prop-data in this sticky (note: garbage in, garbage out):
e-flight calculators (compilation) - RCG

Last edited by ron_van_sommeren; 08-04-2020 at 02:59 AM.
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