Need Assistance In Motor Selection Please!

[Murocflyer]

Funny stuff... WCL is 2.8. The 3D planes we're flying are all between 7.5 and 9.0.

Anyway... I think I can scale down the math, so here we go.

At 16 oz, we can start as low as a 50% thrust to weight ratio, and a 100% would be better, but more than your application requires. This means that for acceptable power you only need between 8 and 16 ounces of thrust. Since weight is an issue here, let’s start with a light 3S battery.

At a 55" wingspan and a 34" fuse length, we have enough prop options to start with from 9 to 13 inches. Larger props will develop more power at lower RPMs, but Amps and flight time are typically a trade off, so let's start in the middle and see what happens with a 6.6 gear ratio.

A 12x6 light application prop needs to spin 5100 RPMs in order to develop 0.75 pounds of thrust, which is 75% thrust to weight. This would also draw a current of about 7.33 Amps if the efficiency of the motor and ESC are in the average range. I use my PropEngine to figure this out.

So…

[1] at 7.33 Amps
[2] and anticipating using no more than 1760mAh of the 2200mAh capacity (80%)
[3] and flying around at an average of 50% throttle (if half-throttle actually draws half of the WOT Amps)

Then we should get a C rating of just over 4 (4.17C), which is 14.4 minutes at WOT, and 28.8 minutes at half throttle (or more accurately, half-power).

So we have a prop that is about 22% of the wingspan and 35% of the fuse length, will deliver about a 75% thrust to weight at about 7.33 Amps and use 80% of our 3S 2200mAh battery in 29 minutes.

I did the math, so now you need to find a motor to team up with a 6.6 gearbox that will reduce the RPMs to 5100 on 11.1 volts and handle 7.33A peak. Sounds to me like something around 3600 to 3800 Kv with the 6.6 gearbox.

If they don’t make one, try looking for a 550 to 575 Kv motor without a gearbox that weighs about the same as the Himaxx motors made for this type of thing, or even for foamies . It should produce the same results. I'm not sure what that motor weight is but just checking the Himaxx website should give you the info you need. I'll bet that places like Hobbyking, Hobbypartz, nitroplanes, etc, have motors like this for under $20, and a 10-15 Amp ESC is probably okay. I have very little experience with ESCs in this range so you're on your own as far as that goes.

For example, I have a 31 inch 200g foamie with a 10A ESC swinging a 9x4.7SF prop with a 2S 450mAh producing at least 0.5 pounds of thrust, and probably more, for about 6 minutes. Five times the battery capacity means 5 times the flight time... 30 minutes.

Wow, that is a ton of info! I appreciate the time to post that. Just so I am understanding this correctly, you mentioned all these figures at a base plane weight of 16 oz. My plane sans battery and gopro is 18 oz and 24 oz with battery sans gopro. Will your calculations be the same and should I go with the numbers you suggested?

Thanks again,

Frank

 

[Murocflyer]

Assuming your calculations will work at 18oz as with 16oz, here are a few motor options I found. And let me say, thank you again for the help. This question has stumped a lot of people and I appreciate you taking the time to work with me on this.

And the good news about the above is that you found that a 3s will work. That is a good thing since there are a number of people including myself having trouble locating a small motor with gearbox that can handle a 4s.

Direct Drive Choices on 3s:

HC2812-0650 (64g) Thrust ??? (I can't figure that one out)


Gear Box and Motor Choices on 3s:

HG2025-3266 (80g) Thrust 25.7 oz @ 7.2A Gear ratio Gear Ratio: 6.6:1 (but the rpms are lower than what you stated I needed above)

That is all I have found so far. Once again I appreciate the help and will continue to look more when I get the chance.

Frank

 

[Aeroplayin]

Although I've never tried it personally, I'm convinced that as long as you have a runway long enough to achieve takeoff speed, you can probably get an airplane in the air with a 1:3 thrust to weight.

The values in grams after the motor is probably the motor weight, and thrust will typically be labeled as such.

It also looks like the designation after the hyphen is the motor Kv, so 0650 is probably 650Kv, which will NOT need a gear if that's the case.

Also, since most motors list Watts, and are very conservative about it, the motor you are looking to fly an 18 oz plane will have plenty of power with anything approaching 112 Watts per pound. It looks like the 65g motor is listed at 160W, which would make sense. Another way to figure out power is with the current, and with 3S and 14A max, then the motor will peak at 160W at 3.8 volts per cell, so this all makes sense.

The only difference is at 14A, a 1760mAh capacity will be spent in 15 minutes at 50% power, and 22 minutes at about 35% power. You can change that by going to a smaller prop on the same motor and battery, though.

Find out what props are recommended for that motor and I'll run some numbers, unless you absolutely want a geared motor.

 

[Murocflyer]

The four props recommended for the HC2812-0650 are 12x6, 11x5, 10x5, and 9x5. I am guessing that the 9x5 would probably be closest to what I need? Since I am staying with the 3s, my AUW is going to be around 25 oz. It would be interesting to see how that compares to the HG2025-3266 geared combo.

I am still hunting motors. Not many in that low KV range out there.

Thanks again,

Frank

 

[3dNater]

Boy that low kv on 3s with a 9x5... we really are talking about a LOW power setup. Like Aeroplayin said, it is funny for us 3d guys to talk about it because it really gets us out of our normal range of thought. Thanks for the thread so far, I have had fun following it

 

[Aeroplayin]

Boy that low kv on 3s with a 9x5... we really are talking about a LOW power setup. Like Aeroplayin said, it is funny for us 3d guys to talk about it because it really gets us out of our normal range of thought. Thanks for the thread so far, I have had fun following it

Yep, and if we're not getting 250 Watts per pound and about 1.8:1 thrust to weight, we're not happy. But for scale planes and park flying, a lot less power is needed. If I remember correctly, a full scale plane, including the huge 747, operates at a 1:3 thrust to weight.

The four props recommended for the HC2812-0650 are 12x6, 11x5, 10x5, and 9x5. I am guessing that the 9x5 would probably be closest to what I need? Since I am staying with the 3s, my AUW is going to be around 25 oz. It would be interesting to see how that compares to the HG2025-3266 geared combo.

I am still hunting motors. Not many in that low KV range out there.

Thanks again,

Frank

So for a park flyer, we also need to keep in mind that if we are going to fly the plane for 30 minutes at 35% power, we also must have enough power to keep the plane in the air at 35% power, and not only at 100% power.

With a 650Kv motor on 3S, and a 12x6 prop, 100% power will be about 115W and 10.4A at 5770 RPMs. That’s about 1.04 pounds of thrust, and 35% of that is 0.37 pounds. So for a 1.125 pound plane, that means a 1:3 thrust to weight at 35%, so we should be good. This, BTW, is 115W at WOT for a 1.125 pound plane, which is also about 100 Watts per pound, which is what scale and warbird pilots look for.

More power will be less flight time and less power may mean you will have to lean on the throttle a little more, so you’re not going to save flight time going that way either. If you find you have more power than needed, you can either lay off the throttle more, or try a 12x5.25 prop and see how that affects the capacity usage.

As with all new setups, you should fly normally for 3 minutes, land, recharge, and record the capacity you put back into your battery. After a three minute flight time, you should only have to put back about 176mAh. This would mean that you can fly for a full 30 minutes…

2200mAh x 80% = 1760mAh
1760mAh / 30 minutes = 58.7mAh/min
58.7mAh/min x 3 minute test = 176mAh capacity used

If you don’t put back 176mAh, then take the restored capacity, divide by 3 minutes, then divide 1760mAh by that number for the ‘real’ flight time. For example, if you put back 210mAh after three minutes, then:

210mAh / 3 mins = 70mAh/min
2200mAh x 80% = 1760mAh
1760mAh / 70mAh/min = 25 minutes

Non-gear motors like this should be easy to find, and should be very inexpensive.

 

[gyro]

Would think a MOTROLFLY DM-2215 custom wound to whatever KV you want would work nicely.

Whatcha think [MENTION=81]Ken Man[/MENTION]?

 

[Aeroplayin]

That's the motor I would have suggested if I were going to start suggesting motors. Motrolfly is one of my two favorite motor brands because they are light, run cool, run hard, and run long. And of course, there's Ken's support. If Ken could wind a custom 650 for a reasonable price, it would be the way I'd go. The stock 850Kv with a 2S 2650 would probably be marginal power for 20-25 minutes at an 80% efficiency and 3.7v per cell.

With an 11x4.7, the 850 on 3S would need a 3S 2650mAh batt for 28 minutes... about 12.9A and 145W at 7550 RPMs. You may be able to put your camera back on there. It should handle up to 23 ounces.

 

[Murocflyer]

I'm still looking for the low KV motors and I have asked for help to find motors and gearbox combos. This has turned out to be more challenging that I had anticipated. Keeping the weight down and flying for a "long time" are two things that typically don't go hand in hand. Unless you are talking indoor plane weights.

I wonder what a custom wound motor would cost? That sounds like the way to go in this application. The motor will have a very boring job, but this motor needs to do its job and do it well.

Frank

PS saw this on the Himax motor spec sheet:

• Sport Airplanes – 50% thrust to weight ratio will give you good flight performance and duration. Really? Only 50%? Yes, the thrust
to weight ratio of Boeing 747 is around 35%. It is the lift – generated by forward speed – that suspends the airplane in the air.

 

[gyro]

I'm still looking for the low KV motors and I have asked for help to find motors and gearbox combos. This has turned out to be more challenging that I had anticipated. Keeping the weight down and flying for a "long time" are two things that typically don't go hand in hand. Unless you are talking indoor plane weights.

I wonder what a custom wound motor would cost? That sounds like the way to go in this application. The motor will have a very boring job, but this motor needs to do its job and do it well.

Frank

Frank, I'll be Frank with you (couldn't resist....) and say that the Motrolfly motors from Ken are about the BEST out there, and I've used just about all of them.

Usually, he charges an extra $30 to rewind a motor. I'd send him an email directly for a final price quote.