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Help! Anyone know of a good 12-15 kilowatt brushless inrunner motor?

#1
Hello,

Does anyone know of a good brushless inrunner capable of a continuous power rating of about 12-15 kW that has reasonable weight (under 5kg)?

For a bit of context, I am building a drone and I need two motors like this to connect to the back of two gas turbines in the drone to output a large amount of power for the drone. Also, I will be liquid cooling these motors so heat will not be an issue.

(kV and output voltage of motor doesn't matter too much, but the output shaft will be spinning at about 6200-6300 RPM)

I have looked around for quite some time now and can't seem to find exactly what I need. If all that exists are outrunners or heavy inrunners, that is fine but light inrunners are preferred for mounting and cooling reasons.

I thank anyone in advance for any help!
 

evranch

Active member
#3
This project is pretty far out of scope for an RC hobbyist forum! You are talking about a military scale UAV drone with 40HP+ of gas turbine and a motor with a power-to-weight ratio of 3kW/kg. Most people here are building airplanes out of foam from the dollar store.

I repaired, sold and wound industrial motors for years, including specialty motors, and you are just not going to find something that ticks all of the boxes off the shelf. 20HP brushless motor under 5kg? Hard already. And liquid cooled? And at twice the rated speed of a standard 2-pole, 3 phase motor?

To be fair, I never worked with electric car motors, and this is probably where you want to start looking for high power densities and liquid cooling. That 6000RPM requirement is going to be hard to meet, though, as high speeds tend to be avoided in most designs. Can you gear it down? Yes, gearboxes are heavy. But even using a belt drive to cut that down to 3000RPM would make a big difference in motor selection.

Note that liquid cooling is going to weigh your drone down a lot just with the mass of the liquid and radiator, there is a reason almost all light aviation engines are air cooled.
 

FDS

Well-known member
#4
I can’t really see why anybody would need a UAV that size? You are essentially making a guided missile, which I imagine is most interesting to your countries security services?
 
#5
This project is pretty far out of scope for an RC hobbyist forum! You are talking about a military scale UAV drone with 40HP+ of gas turbine and a motor with a power-to-weight ratio of 3kW/kg. Most people here are building airplanes out of foam from the dollar store.

I repaired, sold and wound industrial motors for years, including specialty motors, and you are just not going to find something that ticks all of the boxes off the shelf. 20HP brushless motor under 5kg? Hard already. And liquid cooled? And at twice the rated speed of a standard 2-pole, 3 phase motor?

To be fair, I never worked with electric car motors, and this is probably where you want to start looking for high power densities and liquid cooling. That 6000RPM requirement is going to be hard to meet, though, as high speeds tend to be avoided in most designs. Can you gear it down? Yes, gearboxes are heavy. But even using a belt drive to cut that down to 3000RPM would make a big difference in motor selection.

Note that liquid cooling is going to weigh your drone down a lot just with the mass of the liquid and radiator, there is a reason almost all light aviation engines are air cooled.
Thank you for your response!
What I meant by 5kg however is preferred mass, but it can go over if there is nothing else. As to the liquid cooling, I will be using coils to go around the motor so the motor does not need it to be integrated into the can. Keep in mind that these are not destined for the main rotors but instead for the main generators, which will output to the mains 100VAC voltage rail.

You made a good point about electric vehicle motors, and I will look into that. Also, I am able to use a gear reduction, which is actually what I had in mind, but I forgot to mention that part (my bad). I am not worries about the weight of the liquid cooling however, as it is negligible compared to the drones planned weight, which will be over 125 pounds as of now.
 
#6
I can’t really see why anybody would need a UAV that size? You are essentially making a guided missile, which I imagine is most interesting to your countries security services?
Fair point about security, but this drone is a prototype for now and will probably be flown indoors in a very controlled environment. As to its purpose, it is mainly for hobby and just because it is kind of cool (Yes, I know its a bad reason but it is true). The UAV is a 4 rotor serial hybrid with full scale custom telemetry which monitors everything from coolant temperatures to motor RPM all the way to the radar system surrounding it (which I plan to add via some TI AWR1642 sensors or some other radio frequency radar if those do not work).
 
#7
Fair point about security, but this drone is a prototype for now and will probably be flown indoors in a very controlled environment. As to its purpose, it is mainly for hobby and just because it is kind of cool (Yes, I know its a bad reason but it is true). The UAV is a 4 rotor serial hybrid with full scale custom telemetry which monitors everything from coolant temperatures to motor RPM all the way to the radar system surrounding it (which I plan to add via some TI AWR1642 sensors or some other radio frequency radar if those do not work).
125 lb uav inside? This whole project sounds cool, but can you please give us some background on why you are making it, because we don't know if we are aiding in making a weapon of terror. I know this sounds like I'm jumping to conclusions, and I probably am, but I just can't think of why someone would make a turbine generator drone when there are agricultural drones that are full electric and can lift that easily. The only thing going for combustion powered aircraft is flight time and range, which is illegal in most countries to go out of line of sight. You should check the laws of whatever country you live in before staring this expensive project.
 
#8
125 lb uav inside? This whole project sounds cool, but can you please give us some background on why you are making it, because we don't know if we are aiding in making a weapon of terror. I know this sounds like I'm jumping to conclusions, and I probably am, but I just can't think of why someone would make a turbine generator drone when there are agricultural drones that are full electric and can lift that easily. The only thing going for combustion powered aircraft is flight time and range, which is illegal in most countries to go out of line of sight. You should check the laws of whatever country you live in before staring this expensive project.
Well, to start, I have been working on this drone for about 3 years now and have put about 3000 USD into it and plan on spending a significant amount more. It started as a concept of a shield helicarrier that I wanted to make very large but it soon evolved into something more advanced and robust with less body work (hence its fairly simple aluminum and steel box style framework).

As to it being inside, that is only until I can prove it to be stable and functional. It still is not finished and wont be for a while. This drone's purpose is a variety of things, primarily for fun and hobby but also because it is a great learning experience to build something so advanced. It is not designed to be practical.
 
#9
Well, to start, I have been working on this drone for about 3 years now and have put about 3000 USD into it and plan on spending a significant amount more. It started as a concept of a shield helicarrier that I wanted to make very large but it soon evolved into something more advanced and robust with less body work (hence its fairly simple aluminum and steel box style framework).

As to it being inside, that is only until I can prove it to be stable and functional. It still is not finished and wont be for a while. This drone's purpose is a variety of things, primarily for fun and hobby but also because it is a great learning experience to build something so advanced. It is not designed to be practical.
Wow! That sounds really cool! Do you have a youtube channel or a separate build thread where we can follow your progress on this? I'd like to see what goes into such a big build like this.
 
#10
Wow! That sounds really cool! Do you have a youtube channel or a separate build thread where we can follow your progress on this? I'd like to see what goes into such a big build like this.
I'm glad you like it! I don't really do much advertising or documentation on it unfortunately but I plan on making a personal portfolio site in the future that will definitely include the drone.

Also, a little note along with safety and legality, I do plan on registering this drone as an experimental aircraft with the FAA. I haven't seen anything that says I can't so it's worth a shot. Plus, the category specifically says "to conduct aircraft operations as a matter of research or to determine if an idea warrants further development" and "to exhibit an aircraft’s flight capabilities, performance, or unusual characteristics for air shows, motion pictures, television, and similar productions, and for the maintenance of exhibition flight proficiency".
 

Tench745

Well-known member
#11
To be registered as an experimental aircraft I'm almost positive it has to be a manned aircraft. If not, you will still need to prove to a DAR (Designated Airworthiness Representative) that it meets airworthiness and safety requirements. If you're going to try and register as an Experimental/Amateur-Built (E-AB) you'll also need to prove that you did more than 51% of the construction of the aircraft. Documentation is key no matter what.

At 125 pounds, and if it were manned it may qualify as an Ultralight. See FAR part 103 for ultralight requirements.

Chances are this would qualify as a UAV. Being 125 pounds, you may need a waver from the FAA. I know the AMA (Academy of Model Aeronautics) requires special certifications for any model weighing over 55 pounds. I don't know if this is only their requirement, or mandated by the FAA.
 
#12
To be registered as an experimental aircraft I'm almost positive it has to be a manned aircraft. If not, you will still need to prove to a DAR (Designated Airworthiness Representative) that it meets airworthiness and safety requirements. If you're going to try and register as an Experimental/Amateur-Built (E-AB) you'll also need to prove that you did more than 51% of the construction of the aircraft. Documentation is key no matter what.

At 125 pounds, and if it were manned it may qualify as an Ultralight. See FAR part 103 for ultralight requirements.

Chances are this would qualify as a UAV. Being 125 pounds, you may need a waver from the FAA. I know the AMA (Academy of Model Aeronautics) requires special certifications for any model weighing over 55 pounds. I don't know if this is only their requirement, or mandated by the FAA.
Thank you for the info, and I will definitely look into a waiver when I finish this drone.
 
#13
This thing seems dangerous for a number of reasons, first of which it's weight is waaaay past the legal allowance. Secondly flying this indoors is much more dangerous than flying it outside, although a building might be able to contain the aircraft, flying tight formations of a plane of this scale will more than likely lead to a crash. This looks like a cool project but you will probably need to make adjustments to your plan.

edit: Do you think you could upload a picture or two? I know this sounds crazy but a project of this scale has to potential to be VERY dangerous and I would like to know that this is not the case.
 
#14
I understand your concerns. The framework is not completely done and most internals are not in it as of now but I do have the base framework which will show the physical dimensions of the aircraft. Currently I am not able to get a picture but I will probably be able to get one tomorrow or the next day.
 

evranch

Active member
#15
Cool, it will be good to see what you are working on! I'll admit I was leaning towards the "cruise missile" design as well - who flies gas turbines indoors and builds at this scale for a hobby? But then, I build tractors for fun. So I can't talk.

If tethered appropriately in a large space I don't see the problem with test flying indoors.

If you are not fixated on the gas turbines or haven't already purchased them, have you considered just going with piston engines? Their RPM is a better match for common generators, and they are definitely cheaper and likely more efficient, since you won't be running at full throttle to hover. They also can ramp up and down in output a lot more rapidly. There are lots of good, lightweight piston engines available these days at low cost. Unless you want the sound and aesthetics of turbines.

Note that cooling coils around a motor are far from ideal. Heat has to get from the windings, to the stator, to the housing, and then you will have a poor contact with the coils unless you braze them to the housing. You want a motor that was factory built for liquid cooling, or you may be better off with forced air.
 
#16
Cool, it will be good to see what you are working on! I'll admit I was leaning towards the "cruise missile" design as well - who flies gas turbines indoors and builds at this scale for a hobby? But then, I build tractors for fun. So I can't talk.

If tethered appropriately in a large space I don't see the problem with test flying indoors.

If you are not fixated on the gas turbines or haven't already purchased them, have you considered just going with piston engines? Their RPM is a better match for common generators, and they are definitely cheaper and likely more efficient, since you won't be running at full throttle to hover. They also can ramp up and down in output a lot more rapidly. There are lots of good, lightweight piston engines available these days at low cost. Unless you want the sound and aesthetics of turbines.

Note that cooling coils around a motor are far from ideal. Heat has to get from the windings, to the stator, to the housing, and then you will have a poor contact with the coils unless you braze them to the housing. You want a motor that was factory built for liquid cooling, or you may be better off with forced air.
I do plan on tethering it during testing as well or using some sort of securing method that will prevent an accident. Also keep in mind that the drone has a fair bit of progress left until completion, in fact more than I'd like. Things may change for the later components in the aircraft.

As to your suggestion of piston engines, I initially was in fact trying to decide between those and turbines, but I settled on turbines due to the fact that turbines generally vibrate less and are less prone to cause imbalances given they are mounted properly. Also, I am leaning on turbines because I do have some designs that I have made of a simple centrifugal engine that I made in Autodesk Inventor that I put some effort into. I will link an image of this specific model. Despite this, costs of manufacturing of a turbine like this are less than ideal and the model needs to be tweaked and run through dynamic simulation as well as stress testing, so I may not go this route.

The cooling of the generators specifically will consist of thermal paste sandwiched between the coil and generator casing to help dissipate heat better, along with some sort of epoxy or plastic based shield to prevent the paste from escaping. I do see your point however and I will also try to see if there are any motors like that out there.
 

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