Power system comparison between Trainer and Acrowot Electric

[Mitchell Howard]

Hi all. I am trying to wrap my head around the differences in the power systems between my trainer and the recommended setup for an Acrowot I'm shopping for parts for.

 

Seagull Boomerang trainer ~2kg AUW; APC thin electric 12x8E > Ripmax Quantum .40 800KV > ZTW Beatles 60A > LiPo 4S 3700mAh 60C.

 

Acrowot ARTF; APC 12x8 > Ripmax Quantum .55 700KV > ZTW Beatles 80A > Lipo 5S 4500mAh 30C.

 

I'm confused over the Acrowot recommendations; it has a larger motor albeit with lower revs, with a 5S LiPo, but the same prop - assuming both models are a similar weight, the Acrowot has only an extra 1300RPM. Is the trainer simply overpowered? Is the Acrowot recommendation underpowered?.

 

My understanding is that the prop diameter and pitch creates the electrical load. If the 4S pack of the trainer can deliver enough power using a smaller motor on the same prop as the Acrowot recommendation, then in (my own) theory, that means the Acrowot should also run well on that system?

 

Also, it seems my trainer (recommended by local model shop) exceeds the max revs recommendation of the thin electric prop by ~1000RPM. How much, if at all, of an issue is this?

 

What am I missing? I've seen a recent electric Acrowot on YouTube with an inch larger diameter;

Motor Turnigy Air L5055, 700kv

Prop 13 x 8 APC

Power set up 5s 4500mah nanotech batteries 35 -70C

Receiver Futaba R3006SB T-FHSS, battery 6.6v LIFE 1600mah.

Servos all Hitec HS430BH HV

 

 

 

[Dad_flyer]

One thing that is harder to take into account is that the bigger motor will have a lower internal resistance. This gives less voltage loss to the internal resistance, leaving more for the prop revs.

 

As an example, I had a model with a 2830 1350kv which gave 9900rpm on a particular prop on a full 3s. Changing to a bigger 3536 1100kv motor on the same prop gave 9800rpm with a partially discharged 3s battery (0.5V lower, which 'should' be another 500rpm)

 

I expect you would find in practice that the Acrowot power train delivers a lot more thrust even though it looks to be similar.

 

 

[Frank Skilbeck]

Running the numbers through Ecalc the Boomerang numbers seem a bit OTT, on a 4s it's 840watts, 61Amps and 3.45kg static thrust, it will be a rocket, a 3s would seem more appropriate, 410W, 39A. 2.15kg, or a smaller prop on the 4s.

 

For the Acrowot on the Quantum 55 with 12 x 8 prop with a 5s then it's around 1200w, 70A and 4.6 kg

[Dickw]

1 hour ago, Mitchell Howard said:

...............................................

Also, it seems my trainer (recommended by local model shop) exceeds the max revs recommendation of the thin electric prop by ~1000RPM. How much, if at all, of an issue is this?...................................

 

As far as I can see the APC-E electric prop rpm limits are stated at 150,000/prop diameter - which for a 12” prop gives 12,500 rpm. Don’t forget that you motor will not be running at volts x kV but at about 80% of that figure, and volts on load will be lower than 4.2volts anyway.

Looks like it should be OK to me, or have you seen different max rpm figures for the APC prop?

 

Dick

[Richard Ashworth]

It’s not the fastest Acrowot but I fly an electric one with the Quantum 55,  13x8 APC electric prop on a 4S 4250 lipo on a 80Amp ESC. It pulls on the ground at full throttle 45amps on a fresh battery and takes off fine and will quite happily do moderately large loops.  If you already have 4S batteries you may want to start off with this combo more cheaply than buying 5S batteries, if it is too tame then you can easily go to 5S.

Edited May 11, 2021 by Richard Ashworth
Missed info

[Mitchell Howard]

40 minutes ago, Dad_flyer said:

One thing that is harder to take into account is that the bigger motor will have a lower internal resistance. This gives less voltage loss to the internal resistance, leaving more for the prop revs.

 

As an example, I had a model with a 2830 1350kv which gave 9900rpm on a particular prop on a full 3s. Changing to a bigger 3536 1100kv motor on the same prop gave 9800rpm with a partially discharged 3s battery (0.5V lower, which 'should' be another 500rpm)

 

I expect you would find in practice that the Acrowot power train delivers a lot more thrust even though it looks to be similar.

 

 

 

Thanks, that's interesting - wouldn't have expected so much of a change.

 

 

18 minutes ago, Frank Skilbeck said:

Running the numbers through Ecalc the Boomerang numbers seem a bit OTT, on a 4s it's 840watts, 61Amps and 3.45kg static thrust, it will be a rocket, a 3s would seem more appropriate, 410W, 39A. 2.15kg, or a smaller prop on the 4s.

 

For the Acrowot on the Quantum 55 with 12 x 8 prop with a 5s then it's around 1200w, 70A and 4.6 kg

 

I think I'm overdue buying a Watt meter and getting it checked. The club member who test flew my model did literally describe it as a rocket ship... The Boomerang default suggestion from the manual is "0.35-0.45 size, motor 35mm 835RPM/Volt, Prop 10x7, ESC 50A, LiPo 4S 3200mAh, so seems a lot less power than my LHS's suggestion.

 

If I had a peak amp draw of 61A on my 60A ESC according to Ecalc, I'd expect the ESC to come off a flight excessively hot, which it doesn't. Does Ecalc suggest a run time expectation? I get a good 7mins.

 

Reasonable then to assume the 5S setup doesn't experience the same losses as it tries to push the 12x8 prop, and it can push it harder at different revs.

 

 

9 minutes ago, Dickw said:

 

As far as I can see the APC-E electric prop rpm limits are stated at 150,000/prop diameter - which for a 12” prop gives 12,500 rpm. Don’t forget that you motor will not be running at volts x kV but at about 80% of that figure, and volts on load will be lower than 4.2volts anyway.

Looks like it should be OK to me, or have you seen different max rpm figures for the APC prop?

 

Dick

 

I found the same numbers Dick which made me panic - but the 80% figure must change based on sizing of other powertrain parts?

 

[Mitchell Howard]

10 minutes ago, Richard Ashworth said:

It’s not the fastest Acrowot but I fly an electric one with the Quantum 55,  13x8 APC electric prop on a 4S 4250 lipo on a 80Amp ESC. It pulls on the ground at full throttle 45amps on a fresh battery and takes off fine and will quite happily do moderately large loops.  If you already have 4S batteries you may want to start off with this combo more cheaply than buying 5S batteries, if it is too tame then you can easily go to 5S.

 

See this confuses me even more - when I contrast this with what Frank said, you should be able to go to a 13x9, 13x10 prop and pull more amps on 4S and get the performance livened up? On paper, my 3700 60Cs can knock out over 200A. I guess flight time must suffer, but it seems very low to be pulling only 45A. How long a flight time do you get?

 

Have you tried fitting a 5S pack at all? I've been told it's awkward and you have to buy a certain size pack. I wonder if that's old news and went out with the formal EP/GP mk2 ARTF.

 

I guess technically I could fit the Quantum .55, the 80A ESC and the standard 12x8 prop, unless that would yield too little thrust? how much thrust is needed? I'm fully metric where I can be - how many Watts per kg is the right target for the Acrowot?

 

[Richard Ashworth]

I don’t fly hooligan and get about 10mins out of the 4S 4250 with about 25% left in the battery. (They very rarely get warm as I fly happily at 60% throttle)

The Acrobat comes in at about 6lbs. I am led to believe that 100w per lb (general sport flying) gives reasonable performance so at 45amps on a 15V battery it calculates as 675Watts which is 112Watts per lb.

You are right about trying to get a 5S battery in. I have opened up the mid battery former to take a battery the width of the battery sled and 44mm high. I don’t think you would get anything higher than 44mm in easily.

Edited May 11, 2021 by Richard Ashworth
Correction

[Frank Skilbeck]

1 hour ago, Mitchell Howard said:

 

If I had a peak amp draw of 61A on my 60A ESC according to Ecalc, I'd expect the ESC to come off a flight excessively hot, which it doesn't. Does Ecalc suggest a run time expectation? I get a good 7mins.

 

 

It does, but I didn't note that down, but bear in mind that's the amps pulled when the model is static, you'll see quite a drop off when the model is in forward flight and the motor unloads, also if you were at 50% throttle you are pulling way less than 50% amps, power absorbed is basically the square of the motor speed. I had an SFM Fokker D8 which recommended a certain size motor, prop and 4s battery, it was way over powered and actual flew a lot nicer on a 3s battery. I am just flying a 54" span model that weighs 2.2kg on a 13 x 8, 700kv motor and 3s battery, max static amps are 33a (measured via telemetry), less than 80 watts per lb, it flies really nicely.

[Dickw]

2 hours ago, Mitchell Howard said:

 

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I found the same numbers Dick which made me panic - but the 80% figure must change based on sizing of other powertrain parts?

 

Not really. The efficiency and power output of any motor have a close relationship with the rpm range and the best and most useable efficiency and power is usually within the range 70% to 90% of the no-load rpm (i.e kV x volts rpm).

If you prop a motor for below 70% of no-load speed you will be wasting energy generating a lot of heat and even risking burning out the motor if you go much lower. If you prop for above 90% of no-load speed the power output will be very low and you might be better off with a smaller lighter motor.

In general, choosing a prop that will cause the motor to peak at around 80% of no-load speed is the sensible option to aim for and therefore very common.

 

Dick