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Old Oct 11, 2013, 04:13 AM
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Prop/battery/ESC size advice please!

Hey there, just acquired a delta wing, funjet/radjet style jet, its quite large and has a motor already on it, the motor is this one http://www.airpacmodels.com/twe/prod...roducts_id=507 could anyone suggest a prop/esc/battery set up for this motor? it is a ducted fan motor but isn't being used as such at the moment, its just on as a pusher.

Many thanks
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Old Oct 11, 2013, 04:20 AM
Wake up, feel pulse, be happy!
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How big is "quite large"? It's impossible to recommend anything without knowing exactly what plane we're dealing with here.
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Old Oct 11, 2013, 04:43 AM
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Sorry about that, not new to flying, but new to setting planes up from scratch, it is just shy of 1 meter from prop adapter to tip of nose, and around 1.2 meters from wing tip to wing tip, pusher jet style.
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Old Oct 11, 2013, 04:47 AM
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Old Oct 11, 2013, 07:16 AM
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The motor is rated for up to 1500watts, and 75 amps (80 burst).

You didn't mention the weight (important for better, more accurate calculations), but here's a few numbers to start with - based soley on the 1500 watts, should you actually want/get that.
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Old Oct 11, 2013, 08:08 AM
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Weight is around 2lb 9 ozs according to kitchen scales.

And I have to admit to being fairly clueless with that graph you linked, I think i kind of get it, but everything I have flown up until now i have either built with manufacturers recommended set up, has a well known set up already, or has come with motor/prop/esc etc, so with this I am fairly lost.

Was going to just put 2 2200 3 cell lipos in it making it 6 cell, 60 amp ESC and take a random guess at prop lol.
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Old Oct 11, 2013, 08:08 AM
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That weight includes motor and servos.
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Old Oct 11, 2013, 08:13 AM
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the motor linked is fairly similar in spec to the Mega 22/20/4 inrunner I use in my Funjet.

6S 2200 and a 6x5.5 prop is good for about 800W ~ 38A peak. Sporty and fast enough for me in an 820g model, but apparently wimpy for the serious speed freaks.
7x5 on 6s would be around 1000W. Or a 6x4 if you are happy with about 550W

If you're willing to accept higher current, 4S and an 8x8 should be around 50A/650W, and as long as your battery and ESC can take it you should have scope to push up to say 9x9 ~ 800W for speed, or 10x6 for a lot of thrust but not so fast.

I doubt 1500W is realistic for a 200g motor and in any case you're not going to go close to that unless you're on 6s because the current limit will be the main factor, but even so, you should get very good performance.

I see you're looking at 6S 2200. I wouldn't go above 6" - your packs will likely struggle with the current.
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Last edited by scirocco; Oct 11, 2013 at 08:15 AM. Reason: pack size
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Old Oct 11, 2013, 08:59 AM
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Looking closer at the motor specs, I don’t see a voltage rating. Am I missing it? Will this motor handle 22.2 volts (6S)?
I also noted the listed weight as 185grams. There is a rule of thumb of 3 watts/gram of motor weight, which would be 555 watts – only 1/3 of the stated 1500 watt max rating.
I’ll leave it to you to confirm/verify those numbers, but here’s some other numbers, just as examples or things to consider.

Giving yourself a little room for battery weight, let’s round up and say your plane will end up weighing 3 pounds.
For general purpose flying (some hot rodding) I usually aim for 100watts/pound, but for jet-like performance we usually go up to 150-200 (or higher) watts per pound. The watts/pound equation is applicable even up to full-scale planes, so it’s a fairly reliable way to determine power systems, based on your desired flying style.

3 lbs x 175 watts = 525 total watts
3 lbs x 200 watts = 600 total watts
3 lbs x 225 watts = 675 total watts
3 lbs x 250 watts = 750 total watts

Again look at the rating on the motor and determine if it’s able to perform in the desired power range.

Now let’s look at amperage
At 525 watts:
-a 3S battery will pull @47.3 amps with a 4.2 to 1 voltage/current ratio (acceptable numbers)
-a 6S battery will pull @23.6 amps with a 1.0 to 1 voltage/current ratio (better efficiency)

At 600 watts:
-a 3S battery will pull @54 amps with a 4.8:1 v/c ratio (a tad high, want to stay below 5:1)
-a 6S battery will pull @27 amps with a 1.2:1 v/c ratio (again quite efficient)

At 675 watts:
-a 3S battery will pull @60 amps with a 5.47 v/c ratio
-a 6S battery will pull @30 amps with a 1.36 v/c ratio

At 750 watts:
-a 3S battery will pull @68 amps with a 6:1 v/c ratio
-a 6S battery will pull @34 amps with a 1.5 v/c ratio

More power = more speed potential. And now we can see that if you want more power, going with the 6S power system would be more efficient. Now you can take a guess as to home much power you want, and then ensure that both the motor and the ESC are rated for that voltage and currant (amperage).

Now let’s look at potential flight times, given your stated idea to use two 3S 2200mAh batteries in series.

At 525 watts, the 6S 2200mAh pack, drawing @24 amps (round up just to be safe), will discharge at 10.9C and run for 5.5 minutes at wide open throttle (WOT). BUT this will destroy the battery so we apply the 80% rule (5.5 x 0.8) and see that you could fly full throttle for 4.4 minutes without exceeding 80% capacity of the battery. Flight times would be increased with any use of lower throttle setting, but you can be confident that your flights will last at least 4.4 minutes without “stressing” the battery or other components.

600 watts @27 amps (12C) = 4.8 minutes WOT or 3.9 minutes on the 80% rule
675 watts @30 amps (13C) = 4.4 minutes WOT or 3.5 minutes on the 80% rule
750 watts @33 amps (15C) = 4 minutes WOT or 3.2 minutes on the 80% rule.

One other option to look at, if you’re not looking for SUPER power & speed is to stay with a 3S (11.1 volt) system and shooting for something like 150-175 watts/pound or @450-500 total watts, and then run two of your 3S batteries in parallel to double your capacity to 4,400mAh.

At 450 watts, 3S will pull @40 amps with a 2:1 v/c ratio. The 4400mAh pack would be at 5C and would fly for 13.2 minutes WOT or 10.56 minutes on the 80% rule.

Hope this helps a little. I’m happy to run other numbers, or can email the calculator I use to you (it’s an Excel document) so you can play with and compare the numbers.
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Old Oct 11, 2013, 08:59 AM
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Looking to go with speed over thrust with this one I think, though having a prop option for both would be good, thanks for the advice.
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Old Oct 11, 2013, 09:02 AM
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Wow thanks a lot for all that info, much appreciated.

Was hoping for good speed, but 4min run time is a little low, though having said that my radjet only runs for 4-5mins.
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Old Oct 11, 2013, 09:12 AM
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I've been flying 3.5lb Gremlins in combat @500 watts using 3S 4000mAh packs and getting up to 6 minuets of WOT time.

A simple bump up in mAh will boost your flight times, or if you only spend 1/2 the flight at full power, but completely understand wanting to use the packs you have, or which are common for use in other aircraft.
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Old Oct 11, 2013, 09:46 AM
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With the radjet, I don't really go on anything other than WOT to be honest, so unless this is a lot faster than radjet at WOT, I would imagine I will fly it pretty much the same lol.
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Old Oct 11, 2013, 09:48 AM
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Oh and the only info I have on the motor (it only has model numbers on it) is from the web site I linked, I seem unable to find this motor anywhere else but on that site,

ED Fan that can be used : AP-900DF 90mm Fan In Stock !!
Kv (rpm/V) : 1430/V
Max Power (W) : 1500W
Max Amp : 75A - (80A/20sec)
Motor Diameter : 36mm
Motor Length : 49mm
Motor Weight : 185g
Shaft Diameter : 5.0mm
Built in Fan : 90mm Fan (AP-900DF)
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Old Oct 11, 2013, 10:30 AM
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1500 watts / 22.2 volts (6S) = 67.5 amps so the 75 amp ESC suggested seems in line.

1500 watts / 4 lbs = 375 watts/pound (I know one guy who has a wing with 500 watts/pound)

I'm going to try a 1450 kv motor on my Gremlins, and had to go down to a 8" prop to keep the amps and flight time where I needed it.

Numbers are looking OK, now it's up to real world testing (watt meter) and experimenting to fine-tune the system to your particular preference.
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