90mm Fan Motor Selection

[toothbuzz@hotmail.com]

Got a question for some of you gurus out there. I have several edf's (my favorite is my 90mm Stinger) and I want to upgrade my power system. I'm trying to decide between the 6s 1680kv inrunner and the 8s 1250kv inrunner - the Freewing fans appear to be the same for both power systems. "Tim 'the tool man' Taylor" logic says 8s all the way, BUT if you do the math, 6s at 1680kv yields more rpms at any given voltage than 8s at 1250. It seems that everyone on these forums almost always opts for the higher cell count.....but why?

[AceMigKiller]

8s vs 6s (or several other options) is mainly personal preference and can be debated as vigorously as 9mm vs .45acp :Silly:

Technically speaking, one can achieve the exact same performance and power levels regardless of cell count - you simply trade out voltage and amperage.

Power (Watts) = Volts x Amps

I'm moving about right now... more on this later...

Vic

[RCjetdude]

I am no expert but "usually" you are getting higher RPM with a higher cell count but that depends on the kV of the motor. Higher RPM "usually" means higher thrust so higher top end/better performance. I do know from experience that I put a JP 1400 kV 8S fan in my Stinger 90 and it improved the performance considerably. It would out run stock Avantis all day long. Huge loops, 45 degree climb outs from take off, etc. I now have that same fan in an F-15 that I am waiting to maiden. I originally had it in a 90mm F-16 but I was not really impressed with it's performance. Lots of factors at play. I look forward to hearing what Vic has to say on it.

[AceMigKiller]

Alright, back to why some people choose 8s vs 6s.

Another fundamental fact to keep in mind is that heat is a natural enemy of electronics. From batteries, to motors, to speed controllers - it makes them work harder, run less efficient than peak and is ultimately responsible for killing them.

Heat is simply wasted energy. For our hobby purposes, read this as high and/or excessive amperage.

A higher voltage system tends to run more efficiently than a lower volt system of comparable power; AKA 8s vs 6s. Let's put some fictitious, yet reasonably plausible math to it now... We'll use some generic numbers, since there's no way to account for every possible variable. (Uh oh, here we go!)

Let's say a stock 90mm EDF, at wide open throttle running on 6s, is pulling 100Amps (as measured by your wattmeter). We can reasonably calculate then that your power system is delivering about 2,220Watts.

Volts x Amps = Power
22.2V x 100A = 2,220W

If you want to swap over your Stinger 90 to 8s and maintain the same level of power, you would have to choose an 8s motor that is rated to deliver about 2,220W. Since it's a higher cell count motor, you could expect the Amperage to drop a little.

29.6V x 75A = 2,220W

The advantage here (running 8s) is that your system is sucking down significantly lower amps, so all your electronics are running happier. You could even run slightly smaller batteries to make up the weight of carrying 2 extra cells. Even though an 8s motor might calculate to a slightly lower RPM on paper, it might spin up higher in the real world due to the fact that it doesn't have to work as hard as a 6s system.


HOWEVER!!! :Cool:
As modelers, we *usually* aren't "upgrading for efficiency... WE WANT MORE POWA!!!
So inherently, the recommended upgrades we often see aren't designed to give you equal power (compared to stock), but higher wattage.


All that being said, the are tons of reasons why some choose to run stock setups and others choose to upgrade or buy an ARF Plus and drop in a custom Fan/Motor combo.

- Maybe you already have a ton of 6s batteries.

- Maybe you already have a bunch of 4s and want to double them up for 8s.

- Perhaps you fly in a hotter climate and the temps of your electronics are a real concern... opting for a higher voltage system can help mitigate that.

- Or more common, you just wanna take your plane to the Danger Zone!:)


In summary, the upgrades you find readily available for a given model are usually designed to be drop-in upgrades that give you more juice. Once you go off the beaten path, be sure that your Speed Control and Batteries are up to the task of supporting all this extra power.

Clear as mud??

Vic

[OV10]

Very Clear Ace ;) Nice breakdown of the tech into everyday average working man lingo.
I appreciate having another besides myself and a few others on this forum helping to bring an understanding of the Tron science to everyone. :Cool:

[AZFlyer]

Thanks Vic for the explanation. I'm not an expert by ANY stretch. But, I don't understand how an 8S running at full power can deliver less RPM than a 6S system at full throttle.

Can you explain that please so that a simple-mind like mine can understand?

Thanks!

[OV10]

RPM is Voltage x kV of the motor

Ex............650kV on the 1400 P-40B with 4S nominal power equates to............650kV x 14.8v (3.7v a cell x 4) = 9620 RPM

The first post 6S example is @ 37296 and the 8S is @ 37000

[fhhuber]

kV * V = RPM is assuming no load and not even the friction of the motor's bearings.

Real world expected under load is appx 80% to 90% of kV* V
When pulling closer to rated watts you'll be closer to 80%.
If you are getting more than 90% you are "under-propped" and can get better performance from the model for a fairly modest increase in current demand by increasing pitch or blade dia

You can do tach tests (or look at the graphs from a data logging ESC) and see that this is reality.

[OV10]

Correct.......I was just correlating the pure math theory. ;)

[toothbuzz@hotmail.com]

Thanks for all the replies! I’ve always been a “keep it stock” kind of guy, but since our club lost our field my jets have been pretty much sidelined. We went from a gorgeous 500ft x 75ft paved runway to a r-o-u-g-h grass field at a city park. I’ve flown the jets there, but it’s no fun HOPING to reach rotation speed and finally take off. So, I’m retrofitting some larger wheels with trailing link gear on mains and nose, and updating the power system. I have lots of large capacity 6 cell batteries, so was just wondering if there was something I was missing in the equation. I understand that if one motor is an out runner and the other is an inrunner then that’s yet another variable to consider. Since the two I’m looking at are both inrunner I just wondered if there was something else I should be considering. I have 130 and 155 amp speed controls so amperage shouldn’t be a problem. Temperature is always a consideration. And, I remember from my college chemistry days that theoritical yield is sometimes far different from actual yield, so like AceMig stated above, possibly one might perform better than the other for reasons that don’t show up in the calculations. So for that reason, I don’t usually like free-lancing when it comes to my equipment. Don’t think there’s anything wrong with it, I just prefer to use something that’s been proven to work in my jets. That’s why Motion RC has pretty much become my sole source of jet gear. If you get it from them and they say it will work, then it works! If you’ve been in the hobby for any length of time, then you know what a breath of fresh air that is. I’ve been in this hobby since the 80’s, and Motion has re-written the book on RC as far as I’m concerned. Thanks again, everyone!

[AceMigKiller]

I can sure try!

These concepts tend to be a little foreign to alot of people in the hobby, and there's nothing wrong with that. Unless you've got a mechanical, aeronautical, electrical, etc. background, these are not everyday terms and skills that get thrown around. Even then, applying stuff to the hobby isn't always a 'direct translation'.

No shame at all in asking for assistance!

Remember that "kv" stands for "revolutions per volt".
Also, this is specifically an unloaded motor - no propeller, fan, or even adaptor connected.

For this comparison, let's assume we're talking about two motors of the same size, one rated at 1000kv for 8s and the other at 1400kv for 6s.

Easy math tells us:
The 1000kv motor on 8s should spin at ~29,600rpm.
The 1400kv motor on 6s should spin at ~31,080rpm.

That's a pretty small difference.

Now let's drop those motors into an electric ducted fan. Once that rotor gets spinning, all sorts of aerodynamic, mechanical and electrical variables start coming into play...
Basically, we can just refer to the accumulation of these variables as 'resistance'.

We've discussed the basics of efficiency and how the 8s system in this case will be 'happier' than the 6s - AKA less amps required to deliver power.

Since the 6s system is working harder to keep that fan spinning (by consuming more amps), your motor, speed control and battery will start heating up.

Make no mistake - the 8s system will also begin to heat up, but at a slower and less intense rate than the other.

As your equipment gets hotter, you start losing peak performance. Internal resistance starts affecting your electrical equipment, friction losses start compounding, etc.

SoOoOo.... back to your original question - even though the math says the 6s system spins faster on paper, physics says that it won't likely be able to hold that for very long. Your 8s system will be able to sustain max power/rpm's longer before heat losses start taking thier toll.

If we ran the faster spinning 1400kv motor (rated for 6s) on 8s, of course we will see some really fast rpm's! But you're also likely to fry the motor in short time... this is why we have to back down on kv as we increase cell count.

It all boils down to how efficient your power system performs.

Vic

[toothbuzz@hotmail.com]

Thanks, Vic. I’m going to give the 8s system a go. I like a fast smooth jet, but it’s not (all) about speed for me. What I like is a nice fast pass on my first circuit right after after takeoff while the battery is fresh, then a routine of large graceful aerobatics followed by a max performance vertical climb into a glide back down to set up for landing. I like to maximize my flight time without risking damage to my battery, so I’m thinking my flying style can take advantage of the lower power setting an 8s setup will allow while still keeping rpm’s up and temps down.