Need Assistance In Motor Selection Please!

[Murocflyer]

Mike told me this is the place to go and I am a little embarrassed to even ask this question as I should be able to figure this out on my own, but I don't have a motor calculator to assist me.

Here is what I need. I am working on this special project and need a motor that will allow me to fly a 25 oz airplane for 30 minutes on a 2200 mAH battery with room to spare so I don't ruin the battery.

I have heard that a 4s battery, since it is higher voltage over a 3s will give me better efficiency. So let's assume that is true for now. I am looking for a Cobra or Scorpion motor, preferably a Cobra since they are cheaper but if I have to, I'll spring for the extra cash. What motor and prop combo will I need that will allow me to fly at least 40 mph and be able to fly for 30 minutes at a time?

Oh, I should mention, if a 3s will do the trick, all the better, since I have two of them already.

Thanks for the help!

Frank

 

[SnowDog]

Don't forget to consider a Motrolfly motor as well.

 

[Murocflyer]

I have been told that a geared setup would be better than a direct drive 1:1. I remember Himax offered that at one time.

Here is the link to the geared combos: http://www.maxxprod.com/mpi/mpi-26.html

At first glance it looks like the HA2025-3200 may use the least amps but provide good thrust.

Thoughts on that selection?

Frank

 

[gyro]

Frank, [MENTION=95]Aeroplayin[/MENTION] has some really nice efficiency calculators, maybe he can help.

 

[Aeroplayin]

Okay, so you want to fly a 25 oz plane for 30 minutes on 2200mAh.

A safe capacity of 80% means 1760mAh
30 minutes means 58.67mAh per minute

At half throttle, a system using only 3.5A and a C rating of 2 will last 30 minutes, so you are asking a lot.

3.5 Amps times 14.8 volts is only 51.8 Watts, or 33 Watts per pound on your 25 ounce airplane.

So as you can see, this is a very tough request from an electrical system.

 

[Aeroplayin]

But more volts means less Amps and a better discharge rate, but it also adds weight at the same capacity so you will need to calculate the power to weight trade-off. The first question to answer is how much power do you need? Are you hand launching the plane, do you only need to keep it in the air for as long as possible? For aerobatic airplanes, we always start with the Watts per pound requirement. Everything else will fall into place from there.

 

[Murocflyer]

Thanks for the info. We are definitely getting somewhere.

AUW with the 3s battery is about 25 oz. I'm at 24 oz now but need to add TM to the plane.

(I'm removing the GoPro because I heard you can't take AP in TX legally now so that helps with 3 oz or I would be at 27 oz)

To answer your questions, I do not know how much power I need. I would like to be able to fly my plane, a Stevens Aero SoarStik at least 40 mph for 30 minutes at a time. Without the battery she weighs 18 oz. I could shorten the landing gear, and that may take off .5 oz, but I doubt that means much.

I really don't intend on hand launching the plane, but may have to a time or two depending on where I have to land at the end of the 30 minutes. Based on these requirements, how much capacity do I have to go up to on the battery and will the geared HiMaxx motors make the difference that we need to make this possible?

Thanks again for the assistance!

Frank

 

[Aeroplayin]

Okay, then the next step is to determine the prop diameter and pitch capacity on 3 and 4 cells in order to determine the pitch speed for something approaching 40 mph. We can then also calculate the Watts-out using the RPMs and the prop specs to see if you it will also deliver enough power for take-off. Does Himaxx publish the expected RPMs of their motor Kv using their gear box? It's not as simple as calculating the gear ratio because prop size will dictate how much voltage sag will occur, and now much of a drop in RPMs too.

Her's what we're shooting for....

If we know the wingspan, we can start with an average sized prop, like 25% of the wingspan.
We can then determine the RPMs that a few motors will produce with the prop and the gear ratios available.
When we know how fast we can spin the prop, we can then determine the pitch speed, Watts, and Amps.
We will then know if you can achieve your desired airspeed, and if you can produce enough power-to-weight to take off and fly the plane.
Once we know the power, we can calculate the expected current in Amps.
From the Amps, we can determine Amp-Hours and Discharge Rates.
From the discharge rate, we can determine the mAh per minute you will use.
From the mAh per minute, we can determine how much capacity is needed for 30 minutes.
Using 80% as a safe discharge capacity, we can determine the total capacity size you will need for 30 minutes.
From the battery size, we can estimate an AUW and check one more time that we have enough power-to-weigh.

So to start, we need the [1] wingspan, [2] the Kv of all intended motors, [3] any gearbox info you have like the true ratio, and [4] any anticipated RPMs for props and gearboxes that Himaxx can provide. If you can't get [4], we can estimate that as long as we can start with a prop size, Kv, and gearbox ratio.

 

[Murocflyer]

Awesome. Thanks for the help!

The wingspan is 55 in and the wing area is 465. More info if needed can be found here: http://www.stevensaero.com/StevensAero-SOARstik-TM-GWS-Slow-Stick-Soaring-Wing-Kit

Himax puts of the spec sheets for their motors which is located here: http://www.maxxprod.com/pdf/HG20xx-xx66.pdf The problem is that it looks these motors are only rated at 3 cells and not 4. Can the goal be reached using only a 3s?

I'll do some digging, but I am not sure we can find a motor that runs on 4s and is also geared that can be purchased at a reasonable price. Seems like once you get into that realm, it is no longer a "hobby" motor.

Thanks again for the help and I'll report back if I can find a geared brushless setup that won't cost me a car payment.

Frank

PS it looks like Axi makes gearboxes. Now I need to find a lightweight motor (around 60 grams) that can handle 4s batteries. http://www.modelmotors.cz/index.php?page=106

I'll keep looking and thanks again for the assist. Does anyone else know of any 4s capable motors that I can put a gearbox on?

 

[Aeroplayin]

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.