Help! Power Plant Questions (at least the ones I have so far)

[lpburke86]

So I've gone and done it. I've jumped down the rabbit hole and apparently it was flooded with information that doesn't even agree with itself.

A bit about me... I'm a tinkerer, a ham radio operator, a mechanic, a carpenter, and firefighter. So I have at least a basic understanding of electricity and circuits...

Ive also been told that I have obsessive tendencies when I decide I am going to do something, especially when it costs money. (It took me 3 months to pick out my last pair of sunglasses.)

I've always wanted to get into RC planes. I don't, however, have any interest in buying a ready to fly or bind and fly. If I'm going to do that, I'll just stick with real flight 9.

In my digging on planes to build, I'm left with several questions about power plants.

If you can answer any of these questions, it would be greatly appreciated.... whether you can answer just one, or answer all of them...

All these questions are disregarding physical things like size, weight, space, and balance issues. Most of the numbers i am using ar exaggerated just get the point across. You can build a motorcycle with a 357 chevy engine in it, but no sane person would ride it. I realize that, say, you cant put a 20 cell lipo battery in a tiny trainer. I realize that adding weight in the wrong spot changes balance, efficiency and COG. They also assume in most cases that whatever change got made, the rest of the power plant was balanced to match.

This is mostly a discussion to help me understand the theory.

1. If a plan calls for a 1100kv motor, what would be the problem with putting a 2200kv motor in it? Could you get the same performance and efficiency by just throttling back?

2. If a motor calls for a 2 cell lipo, what could you damage or what adverse effects would happen if you wired 2 2 cell lipos in parallel, increasing battery capacity without increasing voltage, to feed it?

3. What is the problem with using to many cells in general?

4. How do you calculate flight time, before going out in the field and finding out by trial and error?

5. If the power plant calls for a 20A ESC, is there a problem with using and way oversized ESC, say a 60A?

6. How does one decide on prop size, if the manufacturer doesn't provide a recommendation

7. How do you figure the current draw from the numbers provided when you buy a motor, like 1400kv?

I've been Googling these questions for days, and I find myself more confused now than when I started.

Thanks in advance for any help you can give

 

[lpburke86]

Anybody? This hole I've found myself in trying to understand is driving me batty.

 

[Pieliker96]

1. Possibly, although a 2200kv motor would pull a lot more current than a 1100kv motor at full throttle given the same voltage and same prop, requiring more motor cooling and more beefy ESCs. Barring propeller efficieny: If you want the same performance, put the same amount of power into each motor. This means having the same mass flow of air, the same propeller RPM, and therefore different voltages for each setup: a higher voltage for the lower kv, and a lower voltage for the higher kv. Lower voltage is less efficient (see my response to #3). If you're running the motors on the same voltage, the higher kv motor will generally have a smaller and/or finer pitched prop, which may be more or less efficient depending on your setup.

2. There aren't any adverse effects to doing this if both batteries are charged to around the same voltage. If there is a large disparity between the two, it could cause battery damage from exceeding their c rating. This is due to the large current that would flow to equalize different voltages through a resistor that is close to zero ohms. One of my designs used four batteries in parallel. I was a bit paranoid about this and made a 4-way equilizer plug which had 20 ohm resistors to a common positive, which was used on the ground before flight to slowly equalize the voltage between the batteries. It probably doesn't make any difference in practice. If you do this, don't include the resistors on the plane, obviously.

3. Higher cell counts generally mean better efficiency. The power dissipated by the switching FETs in the ESC is largely conduction losses, where Power = Current^2 * Resistance; Increasing the voltage decreases the current for a given power, which decreases the power lost. However, more cells means more cells to balance (more cells that can become out of balance), a higher effective internal resistance as they are in series, lower capacity for a given weight, and more.

4. I generally guesstimate the average amp draw from motor data sheets (keep in mind that static thrust tests pull more power than it will in flight) and use that with the battery's capacity in amp-hours to find the maximum flight time. This gives me a good estimate to go off for the first couple flights. If I am to be honest, I fly often enough to where I've acquired a bit of a spidey sense for when the batteries are as far as I discharge them (I shoot for 3.7-3.8v/cell to improve pack lifespan, feel free to go down to 3.3v if you'd like). It's a combination of hearing the tone of the motor at full throttle, feeling the throttle response, and an internal timer of sorts.

5. No, it usually just means extra weight and cost.

6. If you've got the manufacturer's recommended maximum power and a current measuring device (shunt, clamp, multimeter), you can find a prop that gets you close to that maximum power. Realistically, I would say to look at similar motors with data and go off them, or the good ol method of trial and error. Start with the smallest prop you've got and work your way up until the motor gets "ouch" hot, then dial it back a bit.

7. There are so many other factors that influence current draw than just kv: the average resistance of the motor, the voltage being used, and the prop being used, to name a few. Motors can have the same kv but be in entirely different power classes. Again, I would say to look at similar motors with data and go off them.

 

[skayes]

I will try to anwer a max a question so :

1.the kv are only there to specify a rotation speed according to the voltage, but this does not necessarily reflect its power, two motors can have the same Kv but one can tow a bigger plane than the other because it will be able to train a bigger propeller. so yes you can use an engine with more Kv without worries (as long as it remains correctly proportioned for the plane)

2. you can do it, generally we do this when there is not enough room for a single large battery ... but if you don't need it, you might as well take a large battery with the right capacity directly

3. Bigger voltage and tension, so it heats up more, the electronics must be bigger and heavier. The batteries are getting too big for too little capacity so you have to find the right balance

4. use the capacity of your battery and the consumption of your engine to determine the flight time and verify by test

5. no you can use a bigger esc, it's just that if it's oversized, you take on unnecessary weight and it consumes more (and it's more expensive)

6. I dont know, buy motors where it is specified

7. you cannot determine the current draw from the kv, normally, if the description is well done, it is specified, or it specifies directly which esc to use

I hope I answered at best, I could be wrong so ... dont hesitate to say if I am wrong

 

[Merv]

The questions you asked have been answered, I’m going to fill in a few of the questions you didn’t ask.

All of the amp ratings, motor, ESC & battery are do not exceed numbers. It perfectly fine to use less than the maximum. If you exceed the maximums, the weakest link, motor, ESC or battery will burn out first.

Everything begins with the prop. Larger diameter, higher pitch & higher RPM’s all increase a props amp draw. They make the motor pull harder. Kv is rpm per volt. If you increase the Kv or increase the cell count (voltage), the motor will spin faster. You will need to decrease the diameter and/or the pitch to prevent exceeding the motor’s amp rating.

It’s the motors amp rating not the Kv that determines the current draw.

Calculating flight time is very involved, most use trial & error. Flight time is greatly influenced by how you fly. Flying fast will decrease flight time, flying slowly will increase time. If I go full throttle all the time, I get about 6 minutes, if I back off, 12 minutes.

A large prop spinning slowly will always be more efficient than a small prop spinning fast. Efficiency is expressed as gram of thrust per watt of power, would be similar to mpg in cars. Racing quads, small props spinning very fast, get 1-3 g/w, most airplanes are 5-7 g/w, with a little effort you can get 10-12, I’ve seen a few setups approaching 20 g/w.

 

[lpburke86]

1. Possibly, although a 2200kv motor would pull..... depending on your setup.

In my remote, I have the ability to limit the endpoint of the throttle range, and change the rates for said limited range, if I used a 2000 kv motor, limited to 50%, could I, in practice or theory, use the original larger propeller to increase that efficiency again, with the added ability of more speed when I want it? Or would it leave the decreased efficiency, along with adding too much load on the motor from the higher kv?

2. There aren't any adverse effects to doing .... resistors on the plane, obviously.

The biggest reason for this question is location of batteries... If I can wire them with small cells, I can located them in different places for load balancing ideas without sacrificing amp capacity.

3. Higher cell counts generally ...... for a given weight, and more.

3. Bigger voltage and tension, s.....have to find the right balance

So basically, the answer on this one is truly "Yes and No".... that it has to be a balanced solution and i shouldn't just Tim Taylor it for the hell of it.

4. I generally guesstimate .......timer of sorts.

4. use the capacity of your ..... and verify by test

So a plane with a 20 amp draw and 3 aH battery would fly for (theoretically).... 9 minutes?

I shoot for 3.7-3.8v/cell to improve pack lifespan, feel free to go down to 3.3v if you'd like

Hows the less discharge extend the life? Don't they have to be drained every once in a while, or am I mixing up my battery chemistries?

5. No, it usually just means extra weight and cost.

5. no you can use a bigger esc, it's just that if it's oversized, you take on unnecessary weight and it consumes more (and it's more expensive)

I ask this because I have 40 amp ESC from another project that never went anywhere... How much bigger than a 20 is that usually? it's literally the only one I've ever seen up close.
Why would it consume more power than one properly sized?

6. If you've got the manufacturer's recommended maximum power and a current measuring device (shunt, clamp, multimeter), you can find a prop that gets you close to that maximum power.

With those two numbers and without comparing to other manufacturer recommendations, how do you find what gets close? is there a formula that gets you there?

7. There are so many other factors that influence current draw than just kv: the average resistance of the motor, the voltage being used, and the prop being used, to name a few. Motors can have the same kv but be in entirely different power classes. Again, I would say to look at similar motors with data and go off them.

So best practice on this is some time on the workbench after gathering some different sized parts of all kinds?

A large prop spinning slowly will always be more efficient than a small prop spinning fast. Efficiency is expressed as gram of thrust per watt of power, would be similar to mpg in cars. Racing quads, small props spinning very fast, get 1-3 g/w, most airplanes are 5-7 g/w, with a little effort you can get 10-12, I’ve seen a few setups approaching 20 g/w.

How does one go about measuring gram of thrust per watt, or is that something I can expect to find in documentation from a reputable manufacturer?

 

[lpburke86]

Also, basically what im getting from yall, (yall aren't saying it, but my head is after reading your answers) is that i need to quit overthinking and trying to find cheaper and/or bigger(more powerful) parts, build the damn plane to spec, then start playing with making changes....

 

[skayes]

An airplane can be mounted in several ways, some want power, others want speed ... depending on the engine assembly, esc, batteries you can have a very good or very poor performance, an excess of power and sometimes useless compared to the weight that it adds. A lot of people think that a plane flies well because it is powerful, no, an airplane flies well because it is light.
we can run a small engine with a big esc, we can run a big engine with a small battery and a small propeller but the plane that will fly the best will be the one that has an engine adapted to the flight style of the plane and with the battery and the esc that matches

 

[lpburke86]

You made a comment earlier that an oversized ESC would draw more power... How so? From the weight, or does the larger ESC inherently consume more juice? If it does, does that run a risk of burning the motor out if it isnt big enough for the ESC?

 

[skayes]

if the esc is too small, when the motor is going to ask for power, it will give all it has, and send the maximum power. so it will heat up, and will burn.
An esc, too large could send the maximum power that it can deliver and the engine could not support it ... in both cases it is not good.
And indeed a larger esc, because of his more complex design, will require more energy to operate but this will not have a huge impact

 

[Merv]

Grams per watt. With a scale measure the thrust a prop is producing, measure the watts with a watt meter, then do the math. Some vendors will provide the data.

You might be better off asking one or two questions at a time. It’s hard to respond effectively to multiple questions. Don’t scare up more rabbits than you can shoot.

Regarding using a larger prop on a high Kv motor running at half throttle. I’ve never tried it but I don’t think it will work very well. Here is my thinking. A prop spinning at 10,000 rpms will absorb a given amount of power. So we take a motor that can deliver that power at 20,000 rpms but now run it at 10,000 rpms. I just doubt the motor can deliver the same power at half the rpms.

You would be far better off using a motor made to deliver the power needed at the correct rpms.

 

[Tench745]

You can build a motorcycle with a 357 chevy engine in it, but no sane person would ride it.

Off topic, but apparently you've never heard of the Boss Hog?

Okay, now for something more on topic...
You've gotten some good answers so far. I'll chime in and try to answer some others and/or clarify to the best of my knowledge.

So a plane with a 20 amp draw and 3 aH battery would fly for (theoretically).... 9 minutes?

Yes, assuming you fly at full throttle all the time. You'll get more time at lesser throttle settings. I would only add that in the RC hobby batteries are usually speced in miliamp-hours, so that would be referred to as a 3000maH battery.

Hows the less discharge extend the life? Don't they have to be drained every once in a while, or am I mixing up my battery chemistries?

You may be thinking of NiCad batteries which can develop a "memory" if not fully discharged every so often. LiPo batteries, like many others, have a number of recharge cycles they're rated for. If you don't fully discharge them every time it's not as hard on the battery and they will last longer. The other thing you may be thinking about is that LiPo batteries need to be discharged to a storage charge if left for any length of time. If memory serves it's about 3.8 volts, or at 40% of the usable capacity. If you discharge them to less than this and leave them, it is hard on the battery and if you leave them at a higher charge the isolator sheets within the battery tend to break down faster.

I ask this because I have 40 amp ESC from another project that never went anywhere... How much bigger than a 20 is that usually? it's literally the only one I've ever seen up close.
Why would it consume more power than one properly sized?

You made a comment earlier that an oversized ESC would draw more power... How so? From the weight, or does the larger ESC inherently consume more juice? If it does, does that run a risk of burning the motor out if it isn't big enough for the ESC?

Changing nothing else about the power system, changing to a larger ESC should change nothing. You may have slightly higher draws from the larger ESC simply because the components have a higher internal resistance, but I doubt it would be noticeable or even measurable unless you went WAAAY oversize.
I think what Merv was referring to in his response is that if you're over-propped and you have a small ESC you're probably not likely to hurt anything because you'll hit a cutoff in the ESC and have a brownout. If you're over-propped and go over-sized on the ESC you might not know anything is amiss until you burn up a motor or puff a battery.

Size of ESCs varies somewhat from maker to maker. I have a 60A ESC at is about twice the size of my 30A ESC from the same manufacturer. I have a Plush 30A ESC that is about half the size again of my other ESCs. My favorite ESC, and by far the most expensive one I have is a Castle 15A Talon ESC. It is miniscule at 7.2grams, is rated to 15A and has a 3A (8A peak) internal BEC. It's about two-thirds to half the physical size of my Plush 30A ESCs.

With those two numbers and without comparing to other manufacturer recommendations, how do you find what gets close? is there a formula that gets you there?

So best practice on this is some time on the workbench after gathering some different sized parts of all kinds?

For sizing props and power systems I like to use two online calculators, eCalc's PropCalc and the now-defunct
Web-O-Calc which you can still access through the internet archive. If you play with those a little you may be able to answer some of your own questions, and generate a few more.

How does one go about measuring gram of thrust per watt, or is that something I can expect to find in documentation from a reputable manufacturer?

I would not expect to find this from most manufacturers, as it depends on what prop you're using. If you want to experiment to dial things in perfectly, a thrust test-stand will give you numbers you can use for comparisons like Merv suggests below

Grams per watt. With a scale measure the thrust a prop is producing, measure the watts with a watt meter, then do the math. Some vendors will provide the data.

The one caveat I would add to this is that on higher pitched propellers and/or at high RPM you'll actually be measuring "stall thust" because the propeller can't move through the air fast enough to get an aerodynamically clean bite every rotation, so your amp draws will read high and your thrust numbers will read low. If you size based off of these numbers you'll never burn anything up because this is the worse-case scenario and the prop/motor unload as they gain speed.

With that all said, it's worth noting that most people starting out don't bother building a matched power system beyond the very basic "Will this burn anything up?"

Hopefully I haven't been too confusing; I'm having a hard time focusing today.

 

[Merv]

No I was referring to the torque curve of the motor vs the torque curve of he chosen prop.
If you put a large prop on a high Kv motor, don’t expect it to preform well at half throttle.
I’m far from an expert, but here is an entry way down the rabbit hole, take the path as far as want.

 

[lpburke86]

No I was referring to the torque curve of the motor vs the torque curve of he chosen prop.
If you put a large prop on a high Kv motor, don’t expect it to preform well at half throttle.
I’m far from an expert, but here is an entry way down the rabbit hole, take the path as far as want.

Lmao. That video is actually already about 3 videos away in my watch later YouTube playlist... I had added that one the day I posted this topic. Good to see I was at least going in a decent direction.