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Ripmax Nova 40 conversion - Help Needed

Old 07-27-2022, 05:49 AM
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rc_fly
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Default Ripmax Nova 40 conversion - Help Needed

Hi folks. A while ago I built a Nova 40 fitted with a 46 glow 2 stroke. I also have an Arising Star that I've just learned to solo on.

My son wants to learn, but isn't interested in the noisy stuff, so I decided to convert the Nova to electric for him.

Using the 4-Max setups, I took the Precedent Hi-Boy as a baseline, as it's a similar sized trainer.

So I fitted an Overlander Thumper 3548 900kv, and a 60A ESC.

I ran some tests, using a 4S Lipo and an 11x7 glow prop. I have an Etronix power analyser.

The battery was about 80% charged.

At 70% throttle, the current was around 27A, and at 100% it was around 58A. The power on the meter was reading about 820W at 100%.

The spec sheet for the motor shows Power 710W. What is this for? Is this the max power I should be aiming for, so I'm already over it?

The spec sheet also shows the minimum prop should be 11x8 for a 4S. I'm running an 11x7, and already at limits. Going to an 11x8 would put me over the limits more. If I use an electric prop, would this help?


I'm hoping to maiden it soon, as the conversion is complete, but I don't want to turn it into a smoke generator
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Old 07-27-2022, 08:07 PM
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pmullen503
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I certainly wouldn't go up in pitch or diameter, you are close to the limits of your ESC You really don't want to fry your ESC if you are using it's BEC to power your rec.. Try it with a 10x7 or 11x6 depending on what flies better; more pitch speed or more thrust. You don't have to fly at full throttle all the time!
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Old 07-27-2022, 09:52 PM
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ron_van_sommeren
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Originally Posted by rc_fly View Post
.... the spec sheet for the motor shows Power 710W. What is this for? Is this the max power I should be aiming for, so I'm already over it? ...
That 710watt is the maximum continuous power rating, you should keep power drawn below that maximum. Otherwise the motor will overheat. See my next post about derating.
You can go lower, no problem, motor will run slightly more efficient, operating closer to point of max. efficiency.

Originally Posted by rc_fly View Post
... The spec sheet also shows the minimum prop should be 11x8 for a 4S. ...
That is merely a suggestion, e-motors are way more flexible than ic-engines.

IC engines try to keep constant torque, e-motors try to keep same rpm.
You cannot overrev an e-motor by underloading it, rpm can not get higher than rpm_noload, not even in a dive.
.
rpm_max = rpm_noload = voltage velocity_konstant_Kv
.
Whenever a prop- and a voltage-range are suggested, you should always combine biggest prop and lowest voltage to keep current and power drawn in bounds.
Combining the lightest prop in the list with lowest voltage poses no problem, it will give you the lowest current and power of all combinations.

Originally Posted by rc_fly View Post
... I'm running an 11x7, and already at limits. Going to an 11x8 would put me over the limits more. ...
Both current and power drawn are proportional to pitch and proportional to diameter⁴
Overal massively disproportional and expensive effects.

In your case current and power would want to increase by factor (8/7)=1.14.
Worst case, ignoring extra voltage losses in battery, wiring, controller and motor itself due to higher current.


Some expensive realistic and some extremely silly examples, hopefully instructive as well.

Originally Posted by rc_fly View Post
... If I use an electric prop, would this help?
Yes, but don't use a slow-fly prop. Those are for low rpms, not strong enough.
Electric props will help, they have thinner blades, less air resistance, lower current, more effective that way.
Current and torque are proportional, in the same way that rpm and voltage are proportional.
Never use e-props on ic engines though, they cannot handle the constant hammering.

Last edited by ron_van_sommeren; 07-28-2022 at 12:04 PM.
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Old 07-27-2022, 10:10 PM
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ron_van_sommeren
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Power system derating
It is a good practice to have some headroom built in. Motor runs cooler, slightly more efficient too, and you have some leeway should things go south a bit.
About derating motors, controllers, batteries, electronics in general:
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Old 07-27-2022, 10:38 PM
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rc_fly
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Thanks for all the info. Thatís plenty for me to keep in mind.

After I posted earlier, I actually found a 10x7 and an 11x5 prop and tested with those.

Both were well within limits, so I opted for the 11x5 for the thrust, and slower flying as itís a trainer.

I maidened the Nova earlier, and it flies really well. Cruises around at 55-60-% throttle, and only needs a burst of full time get off the ground. Iíll get some electric props for the extra efficiency.

Thereís a video on my Instagram of the flight.
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