Help needed for manned VTOL

I am building a one manned personal VTOL vehicle. It will only be for my own personal use, and have a 20 minute flight time. It will weigh approximately 300-350Lbs, with me in it. I was considering using 6-8 Turnigy 150cc, or 6-8 EP120900/120100 motors, with 30"-36" props, and then l came across these: https://neumotors.com/80xx-series-electric-motors/. I have seen this type of vehicle that l am building, all over YouTube, that actually do fly, and some of them use the Turnigy motors. On that note, would the Neu motors be a better option? As far as lipos, l can run any voltage from 48-96. I will figure out the ESC's, etc, after l get the motor options figured out. If the Nue motors will work for my application, what particular motor from the 80 series would be the best choice, and how many of them would it require? I would like to keep my propeller size to 30" (or less), and will calculate the optimal Kv, accordingly. I figure l would need at least 450-525Lbs of thrust at 75% throttle, to 600-700Lbs thrust at 100% throttle. Other than thinking that l am just nuts, any help or suggestions are greatly appreciated.

Regarding prop size, larger props are more efficient and will give you a better fly time.

In regards to power pack, I would suggest Samsung 21700 50E cells (no liabilities accepted). They are about 15% lighter than lipos for the same energy, smaller than lipos, and far safer than lipos. If you are at 50m altitude and one of the lipos bursts or catches fire -- you will be out of luck, unless the software knows how to handle this exception. On the other hand the 21700 may not have the C rate that you need.

As for voltage, I would go for 2x14S (104V). As a rough rule, the higher the better, if your inverter/motor can take it. Again, caveat emptor. Anything over 42V is considered a hazard.

600-700 lbs max thrust for a 300-350 lbs MTOW seems a bit excessive? What is the rush to get up there?

Regarding motors, I would go for 3P AC PMSM motors, but realize they are a can of worms. Brushless DC is a lot easier and cheaper. I suggest buying one motor of each kind and testing them for thrust, efficiency, heat generation, and compatibility with the ESC. I roughly estimate you need about 40-50kW of power. That is about 8 to 10 kW per motor.

If you do need 40-50 kW of power,...that would suggest you will need 15-20kWh of battery (3C continuous also), for a 20 min filght ( with no safety margin ?).....
......which in itself means a pack weight of around 100kg (220 lbs)
So, i hope you are on a very strict diet and your flight rig is super light,...if you are going to make that 350 lb weight target . :wink:

Thank you for the responses....they do help. As far as doubling the thrust needed for the weight, l always thought that you should only be flying at 50-75% throttle, and not at 100%, so l was doubling the nedeed thrust accordingly. Is that not correct?

Bgt2u said:

Thank you for the responses....they do help. As far as doubling the thrust needed for the weight, l always thought that you should only be flying at 50-75% throttle, and not at 100%, so l was doubling the nedeed thrust accordingly. Is that not correct?

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Electric motors can be driven over 50% of their continuous rating for short periods before they overheat. Of course the ESC and the battery pack should be able to deliver that power. So even if you are flying at 80%, you can go up to 150% in case of emergency. I don't think you need to go over 20-25% above your MTOW, to get what you want -- unless you are in a rush to get up and down.

It is unclear how much power you would need. I think Hillhater implies it would be less than 40-50kWh. That may be the case. But it depends on the efficiency of the ESC, motor, and propellers. BLDC motors are not very efficient, and propellers designed for ICE engines are not very efficient either.

Okay, got it. Thank you for clearing that up for me. I also am not a big guy, l weigh 130Lbs, and am 5'5"....so that helps. Since l wrote my original post, l came across ten KDE 8218-120 motors, really cheap. Do you think that they are a good choice for my application? From my calculations, they should work fine. They are more efficient powering a 24"-27" prop, vs a 30" prop. I figure a 24.5"-2 blade prop would actually be most efficient (for this particular motor) as with 10 of them, they would produce 302.3 pounds of thrust, while drawing only 439 amps. Using six 30.5" propellers would give me 309.12 pounds of thrust, while requiring 558.6 amps. As far as the propellers go, l have been looking for the light carbon fiber ones, that preferably are made for uav/vtol flight as apposed to fixed wing flight, and for electric not I.C.E. So farKDE, and Xoar, are looking promising.

Safety factor ~=0?
With a massive 10 motor bi-plane you might live through this pipe dream flight.

Yes, the safety factor is just about zero, however, l will be using redundancy, such as, even if TWO motors fail, l would still be able to land. I probably never will go more than 10ft or so off of the ground. I am not saying never, but most flights will be flying around the open fields by my house. I am not building this for any serious transportation, l just want to create something that is different, yet interesting at the same time.

Stealth_Chopper said:

Safety factor ~=0?
With a massive 10 motor bi-plane you might live through this pipe dream flight.

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He is not building a plane - but a drone to hover. Besides, you don't need 10 rotors for a plane to survive.

Bgt2u said:

l came across ten KDE 8218-120 motors, really cheap. Do you think that they are a good choice for my application? From my calculations, they should work fine. They are more efficient powering a 24"-27" prop, vs a 30" prop.

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Please always specify the voltage in addition to the amps. What is the wattage and voltage for these motors? And why do you think they are better with smaller props?

Bgt2u said:

I figure a 24.5"-2 blade prop would actually be most efficient (for this particular motor) as with 10 of them, they would produce 302.3 pounds of thrust, while drawing only 439 amps. Using six 30.5" propellers would give me 309.12 pounds of thrust, while requiring 558.6 amps.

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How did you arrive at these amp numbers? Because the 30" prop should require less power.

I agree the 30.5" prop should require less power. I got the information of of the KDE site: https://cdn.shopify.com/s/files/1/0496/8205/products/KDE_Direct_XF_CF_Brushless_Performance_Testing_KDE8218XF-120.png?v=1566595147 . From what l can see, at 53.9v, the 24", and 27" props are actually more efficient in all but two cases. (The 24.5x8.1 hex, and 27.5x8.9 hex are less efficient than the 30.5 dual). Otherwise all of the other 24-27 props are listed as more efficient, at the same applied voltage. I obviously want to choose a prop that gives me the most thrust per amp, so am l reading this wrong?

Okay, here's an update. I am going with eight 30" or 30.5" props. I do have a couple of questions though. Is there much, if any difference in thrust, between a 3010 prop, and a 30.5"x9.7? Also do noise reducuction props with angled ends have as much thrust as non noise reduction props, given both are the same...3010 for example?

I would not rely on what the manufacturer or reseller tells you. Best is to order one of each and measure the thrust. Measuring the thrust is not so difficult, and I can show you how.

Bgt2u said:

Okay, here's an update. I am going with eight 30" or 30.5" props. I do have a couple of questions though. Is there much, if any difference in thrust, between a 3010 prop, and a 30.5"x9.7? Also do noise reducuction props with angled ends have as much thrust as non noise reduction props, given both are the same...3010 for example?

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I don't know the answer to noise reduction.

Regarding prop pitch, note that these pitches are for flying, i.e. moving at some speed. With a drone, you are hovering, thus your vertical speed is essentially zero at all times. Thus you want the least pitch you can find. In my VTOL project, I am using a variable pitch prop so I can adjust it optimally. I am building a thrust and torque tester, and trying out different props, different sizes and pitches, number of blades, and different motors. I suppose I could also test noise reduction. You may find out that a 3008 or 3007 is the best.

Bgt2u said:

Okay, here's an update. I am going with eight 30" or 30.5" props. I do have a couple of questions though. Is there much, if any difference in thrust, between a 3010 prop, and a 30.5"x9.7? Also do noise reducuction props with angled ends have as much thrust as non noise reduction props, given both are the same...3010 for example?

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By the way this is one of the formulas used to estimate thrust. I don't know how accurate it is, and will soon find out once my thrust test stand is built. M is the thrust, P the power, R radius of prop in meters. M(kg) = cuberoot(square(0.04515 * P(W) * R(m)))

As can be seen, the thrust can increase at the rate of R to the power of 2/3, even if the power remains constant. Thus a 30" prop will generate 16% more thrust compared to a 24" prop.

This assumes ideal conditions such as ideal pitch and ideal prop. I have been told by some that this formula is not entirely correct and the result should be multiplied by 1.26. I'll have to test to find the answer.

Thank you for the detailed answers. I have constructed a thrust stand, with rpm, and amperage monitoring capabilities. I just haven't purchased any props yet. I have a KDE prop adapter, so l can use 2, 3, or 6 blade configurations on it. As you know, props aren't cheap, so l needed a 'ballpark' starting point. Do they make variable pitch props in both cw/ccw rotation? From what l have seen the 2 blade props always seem out perform 3,4,5,6 blade props, when it comes to thrust vs amperage draw. From what l can figure, motor efficiency, and amp draw, seem to be the two most important factors to consider for flight time. Since l already have the motors, l now have to focus on thrust vs amps.....My next obsticle to overcome is battery capacity. For that, l will be using 14s, for a max of 58.8v, which are what my motors would require. For some safety, l will power each motor, with a separate battery pack.....does this sound correct?

Great -- You are ahead of what I had thought. In theory 2 blades are the most efficient, but then they would require a higher RPM and generate more noise. If the BLDC efficiency drops at higher RPM, you will find out that a 3 blade or 4 blade is more efficient. Remember your DC amperage is a combination of prop efficiency, motor efficiency, and ESC efficiency. To figure out the prop efficiency only, you need to measure the torque. With larger props, 2 blade props can approach the speed of sound and thus loose efficiency. That is another reason to go to 3 or 4 blades, in order to reduce RPM.

Well, it is good that you are getting better results with 2 blades. This may be because the BLDC prefers a higher speed (RPM) and thus doesn't like 3 or 4 blades. I think by KDE you mean that you can easily have 2, 4, 6 blades. So test them out. With PMSM (AC) motors, lower speed is preferred by both the motor and the inverter (ESC). I have not seen variable pitch carbon fiber props in the 30" range. But I am not in that market. My props are 100"! Otherwise the thing would never take off. It will have 40kWh of batteries. And that is heavy!

Yes, 14S is a good number. You can easily find a BMS on eBay. ENNOID on this forum is also building a very good digital BMS for that. Yes that is a good idea to separate the packs. But with a digital BMS, I believe it becomes unnecessary to partition the packs. In your case you want opposite rotors to be on the same pack. If your pack is 10 kWh, then it should weigh about 44kg.

If your motor is 60V, you may want to go to 18S or 20S. Because the range for 14S is 42V to 58.8V, and most of the time, your motor is being driven at an average of 51V, which is not the max. With 20S, the range is 60V to 84V, and average 73V. I believe and I could be wrong, that you can tell the ESC to limit output voltage to 60V, while you can't tell it to raise the voltage from 42V to 60V. In my case, 14S works for me because I am putting five 14S modules in series to reach about 300V.

BTW, to measure current, use a hall effect sensor. Such as

https://www.ebay.com/itm/ACS758LCB-050B-PFF-T-Hall-Current-Sensor-Current-Module-NEW/223687370502

Wow, thanks again for the detailed information. 300 volts, what the heck are you building...a flying city...lol? Okay, my next question is: Given these motor specs, how much run time do you think one motor would run on a 60v 50Ah lipo pack?
Motor: KDE8128-120
Constant motor velocity: KV120
Constant Torque: .0796 Nm/A
Constant Km: .4137 Nm/W
Max continuous current: 110A (3 min)
Max continuous power: 5695W (3 min)
Voltage range: 22.2v - 60.9v
Stator poles: 24
Magnetic poles: 28
Prop size: 30.5"-TP (27.5"-TP max on 14s)

Bgt2u said:

Wow, thanks again for the detailed information. 300 volts, what the heck are you building...a flying city...lol?

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Well, I am trying to lift 1,200 lbs. That is half a ton. Each motor is 30kW and rated to 550V. If I had a 30kW 60V motor, it would need 500A !! The battery pack would have to use busbars as thick as a quarter inch. Imagine the fuse and contactor and cables and lugs that I would need for just one motor. With 4 motors that would be 2000A. The inverter (ESC) weighs more than the motor! I may even have to go to 400V.

One question comes to mind.......Why are you trying g to lift 1200lbs?

Bgt2u said:

iven these motor specs, how much run time do you think one motor would run on a 60v 50Ah lipo pack?
Motor: KDE1828-120

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I think you mean the KDE8218-120?

I am impressed by their website and the fact that they specialize in heavy lift drones. And they offer ESC and props. However, they do not differentiate between hover props and fly props which is worrisome. I think you can build your own variable pitch hub that would let you experiment with different pitch settings for their props. Get the 2-blade props and you can always use them in a 3 or 4 blade setting if necessary.

Their specs are interesting. As you can see the thrust ratio (g/W) is higher for lower RPM. In practice a ratio of 5 is considered very good and 7 is considered unattainable. The ratio for a Cessna and other general aviation piston aircraft with the best propellers is around 4. The reason you are seeing very high ratios like 10 or 12 is because the thrust (and the power) per rotor is very small, and unless you have 12 or 16 rotors, you will never be able to have such low thrusts per rotor. With 8 rotors, you may be able to get a ratio of 7 or even 8, which would be terrific. That is why you see some people go to 16 or more rotors.

As you can see, the larger props have higher ratios for the same RPM. The difference should have been greater. I don't know why they have not shown 1500 RPM for 14S, which would give a huge ratio. Surely their ESC is capable of lowering the RPM. Maybe the ESC efficiency drops considerably at low RPM, so they decided not to show it. It is unclear if the power measurement is before or after the ESC.

You may note from my formula that M is proportional to P^(2/3). Thus if you double the power to a rotor, you will NOT get double thrust. You will only get 2^(2/3) = 1.59 or only 59% more thrust. Thus the less weight per rotor, the better ratio g/W you will get, and thus more rotors is more efficient.

Oops, yes, that was a typo...l corrected it. I started another post, as this topic no longer pertains to the Nue motor 8000 series. I was posting it, while you were replying to me. Yes, l am going to use the two blade props, that way l can experiment with 3 or 6 blades, on the 6 blade hub. The motors 8218....got it right that time, are $595.00 each, but l actually went with those, because l figured 8-10 of them would work great for my application, and found them on ebay for only $200.00 each. I was considering the T-motors, or maybe EP120100's, but they are all expensive, and ebay was cheaper, and the KDE's should work fine.

Bgt2u said:

One question comes to mind.......Why are you trying g to lift 1200lbs?

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Because I want to put a huge battery in it and have a proper cockpit and floats.

Let's say you want 350lbs MTOW and thus 440lbs thrust. That is 200kg. With 8 30" rotors that is 25 kg/rotor. According to the KDE table, this is out of range (like 6,000 RPM and only for 30 seconds or so). And then you get a ratio of only 4. That means you need almost 50kW. Or 7 kW per rotor. The 8218 seems to be rated about 4.5kW continuous because their 5.7kW rating is only for 180s -- so why do they call that "continuous"? This will not work.

You need to go for more rotors.

Let's look at the KDE8218-120 again. It is rated continuous about 5kW. Run it at 10S (again, this makes no sense). The only thing I can think is that BLDC efficiency drops with higher voltage, unlike PMSM. From the table via interpolation you find ratio of 6.15 at 4100 RPM, lifting 14.2kg and consuming 2.2kW with 30.5" dual prop. The RPM is far better than 5000. You would need 200/14.2 = 14 rotors, consuming 31kW. An 10kWh pack will give you almost 20 minutes. Note that you are at 1/2 of the motor rating, so it will not get hot.

14 rotors is too many. Your only salvation is to go to larger props. Let's say 42". This will give you 24% more thrust at the same power and probably lower your RPM down to 3600. So your ratio goes up to 7.6, and thrust is 17.6 kg, and you need 12 rotors. If you reduce MTOW to 300 lb, you will get away with 10 rotors and even maybe 8.

With less rotors, you are consuming less power. 8 rotors at 2.2kW each gives you 34 minitues.

This is all theoretic. The only way to tell is to try a KDE8218 with a 30.5" and a 42" and vary the pitch. Also try 14S vs. 12S and 10S.

Thank you. You obviously took some time to figure all of that out. I guess l could go with 12 motors, three motors on each side, with the motors stacked on top of each other back to back with one prop on top, and one on the bottom contra rotating.....but will that set up lose thrust?

No problemo.

Counter rotating top and bottom props may work, but the props would need different pitch because the lower one is running in moving air.

In this case, get two motors and two props and test them. Again I suggest making a hub that would let you change the blade pitch.

In my calculations, it turns out that the 8218 is running at less than half the capacity. If you are going to 12 rotors, you may want to consider a less powerful and lighter motor (and smaller ESC), designed for lower RPM. Thus very low Kv. Let me know of your results.