Interesting! I would think that with that small margin between the pitch speed and airspeed you would get significant unloading in flight, since the angle of attack of the blades would be significantly less than static. Is it because of the inflow to the propeller? There is definitely something going on that I don’t understand.
I totally understand why a longer prop with a lower pitch speed is more efficient than a shorter prop with a higher pitch speed. No disagreement there! What I don’t understand is why the larger prop with a lower pitch speed doesn’t lose a significant amount of thrust between a static (zero airspeed) and in flight (trim airspeed) condition at full throttle. With a airspeed to pitch speed ratio of 1:1.5, theoretically at that airspeed you are reducing the angle of attack by 66% compared to zero airspeed, which should result in a corresponding reduction of power (drag) and thrust (lift).
One other question: Are you using that 1:1.5 airspeed-to-pitch-speed ratio for cruise RPM, or full power RPM?
the different is 7 kilogramms at: 49 kg km/h 0 and 42 kg km/h 36
So the 14% reduction in thrust won’t have a corresponding 14% reduction in power (watts)?
the power is the same. the “lost” is in the air. in principle, however, the values are not important. if the propeller on the stand reaches the thrust value that has a good g / watt balance and pitch speed, it fits. how efficient it is in flight can only be determined in flight with a vario and many comparative flights based on the increase depending on the wattage. therefore, the propeller design is 50% on the pc and 50% on the airfield. that is why hobbyists often have better propellers than large series manufacturers 
small changes based on experience can easily and quickly change quick due to different lengths of middle parts and other angel of the middle parts that change the pitch. series manufacturers don’t do that.
Got it. I guess ultimately I’m just going to have to fly this thing on some short hops to get data.
Quick update: No significant improvement on motor heating with timing change. I’ve tried 20, 15, and 10 degrees. Next I’m going do auto, 25, and 30, but conventional wisdom says the motor will make more heat and be less efficient as the timing is advanced. We’ll see. If I either current/throttle limit the system down to 8.5 kW or re-prop it down to 8.5 kW (both low/no cost changed) I should still have enough thrust to fly and the motor should run within safe temp limits.
As I gather more data I’m starting to realize that this motor is significantly less efficient than Neumotor claims. I’ve been using their motor constants in eCalc (KV, Rm, Io) and that was showing >90% efficiency anywhere the motor was producing useful power. Now that I’m getting numbers real numbers, the motor is using ~1Kw more than expected at full throttle, and it looks like the real efficiency numbers from level flight to full throttle are 85-81%. That has a pretty significant impact on flight time, motor heat… basically everything.
Since I already have everything here, and I’m confident it’ll make enough thrust to fly, I’m still proceeding. I am, however, seriously considering reconfiguring everything into a “Sunny Motor” type ultralight paramotor setup, that will fit in the back of my car without disassembly and should be good for ~15 minutes of flying with the batteries I have. Realistically I think the best use for this power setup is a quick, light, easy motor for 10-15 minutes of tooling around before or after work. Without the ability to climb 2000’ it seems unlikely I’ll be able to get up into the ridge lift/glassoff that we have around here, and if I can, I really won’t need the power-off performance that my pod harness will give me. The long, cantilevered frame design I’m using now is going to be awkward to launch for sure, and without the use of my pod there is really no point to having that style frame.
I am disappointed that things aren’t working out as well as I had hoped, but I still think I can make something flyable out of these components. When I started this project I knew it was a bit of moonshot, so this outcome isn’t totally unexpected.
@bratwurst can you recommend a propeller pitch/diameter combination that will maximize efficiency at 4000 RPM and 7kW of mechanical power/17Nm of torque? I know these are not ideal numbers, but realistically that is what I can safely produce with this motor. You seem to have a good handle on real world propeller performance at low advance ratios, which is not really reflected in traditional prop performance calculations.
Yeah, I read that, but I’m just testing all my options to see what works best. It’s not that I don’t believe you, I’m just testing things myself because it’s wintertime, I can’t fly, and I’m curious.
I totally understand that, but I want to tinker with what I have. Just like you, I like experimenting, even if I know that the outcome isn’t going to be the 100% most efficient solution. I do want to actually fly this motor, even if it is far from ideal. If I’m going to order a new prop to bring the power level down to something the motor can tolerate, I might as well optimize it. The prop I have now is optimized for advance ratio (low diameter, low RPM for power applied), so if I’m going to order another one I might as well try one that is optimized for low pitch speed.
Are you referring to the tip speed > .65 mach? What is the maximum tip speed you recommend?
if you work at 4000 rpm with a 34 inch propeller you are already at 648 km / h! as everyone knows, my view is that electric flight should be much quieter than a petrol engine. otherwise you disturb your fellow human beings and the animal world. I fly with systems that go below 550 km / h at the start and in the level below 400 km / h so that it is also efficient in the g / w area. i know many who have never designed porpellers themselves or made hundreds of measurement flights speak of high speeds for efficiency. yes that’s true with very fast planes. with ultra slow it is not like eppg. the world is different here. it is an older calculation by the godfather of propeller development, but it is still the basis today. page 21 : https://www.mh-aerotools.de/company/paper_15/Hepperle%20-%20Elektrisches%20Fliegen%20Stuttgart%202015.pdf
1 Like
I totally get that… It seems like traditional models of prop efficiency don’t work well at low advance ratios and low angles of attack. That is why I’m asking for your advice. I understand that you think this is the wrong motor, and that 4000 RPM is too high. I agree with you. However, it is the motor that I have, and I intend to test it and fly it before I get rid of it.
it is of course good that you want to make your own experiences. in all areas. This is the best way to learn it and one day you will surely say to other people who are just starting: “If you want, I will give you tips so that you can avoid mistakes that many have made before you, so that it does not cost unnecessary money and time” , all those who fly successfully at some point say that. all who give up because they are unsuccessful or change nothing do not say it of course. there are people who have never flown well with their project but are trying to get beginners to think that they should make the same mistakes. i don’t know why these people do it and they like it when someone fails. look in youtube there are many videos from eppg that never show a start or an entire flight. just always talk and say and write fake information at flight time. many believe it, recreate it and fail … again and again. hobbyking has certainly written to hundreds of people that the 120/70 is not an eppg engine but a model aircraft engine. nevertheless people buy every week and are angry with hobbyking because their project doesn’t work. that for almost 10 years …
I’m still fumbling around on where to go with this project. @bratwurst’s suggestion of a bigger motor and bigger prop absolutely make sense from an efficiency and thrust standpoint, but I’m not really interested in having a traditional PPG setup with a giant prop. If I was, I’d just buy a gas PPG and be done with it. I like the idea of having a small and light setup that I can leave assembled in the back of my car, but that is really pushing the limits of what is possible with small motors and small propellers.
One possible solution, and it’s not one that I’m convinced will make this setup viable, is to go to an ultra efficient, very expensive propeller. The XOAR PJP-T-L line, according to XOAR, has some really impressive efficiency numbers. The 38x10 in particular matches my setup well. XOAR lists the maximum thrust at 38.6kg at 4100 RPM, with only 14.9Nm of torque. I figured those numbers were total bullshit, until I found some independent test data on the XOAR PJP-T-L 40x10 prop on rcbenchmark.com, which very closely matches the listed performance data for the 40x10 prop. The 40x10 test data has similar efficiency numbers to the 38x10. Plugging all of that into eCalc, using the real numbers from my motor (which is drastically less efficient than neumotor’s specs would indicate), gives me thrust and electrical power numbers that should be enough for short (15-20 minute) flights and a .8 m/s climb rate at full throttle. The tip speed is high but still sub-transonic, so it will be loud but not screaming loud.
What I’m not totally sure about is if that brings the electrical power down to the point where I’m not pushing the motor too hard. eCalc says 7.6kW at a slightly higher efficiency than I have now, which is down from 10.5kW on my current setup. All of that sounds great, but it is going to cost me $700 to get one of those props, because they only make them in matched pairs and I have to buy the set. And if the prop doesn’t make the specified thrust, or uses more power than anticipated, it is not going to solve my problem. Before I commit to buying a prop set, I plan to throttle-limit my current setup to 7.6kW and do some temperature testing to see how the motor heat compares to where it’s at right now.
yes it is true a large propeller needs an unbelievable amount of space, also a 3.5 kg motor etc. now I have to laugh … I have these things including the wing that you see in a bag on vacation with the 80cm / 55 cm / 55 cm. it fits even in an old minicooper. assembly takes 2 minutes without tools. the total weight is around 18-19 kg if only one rescue parachute is included. this takes you up to 900 meters above ground. please do not tell me that this is a traditional paramotor just because it is efficient … i have magcad propellers from other projects myself. they are only worth your money for incredibly loud drones or other purposes. the physics and aerodynamic is clear. money? if you buy right you have something that works forever. like with tools in business or machines. you should never forget the most important point. this is the safety of the pilot. always remember there will be situations where the air will sink, not rise. if you don’t have a reserve to climb well, it will be dangerous. ask experienced ppg pilots what they say.
Again, no question that what you have is an elegant solution. The problem is that you have it, not me 
I do not have the money, technical ability, or drive to replicate what you’ve done.
You are building these:
I am trying to build this:
1 Like
the harness was bought, the used one was 400 Dollars (ebay). the fewer parts you have to make, the cheaper it is. in principle, a board on the back is enough for the motor to connect to the harness. a simple frame. the battery in the harness where the water ballast is otherwise. ready ... forget all the push rods and things typical for paramotors. you don't need those things. A 2-part e-prop costs around 250-300. I have almost always built everything with used cheap harnesses. that’s enough. Many parts made of wood, simply laminated with CFRP fabric. just because it looks nice doesn’t have to be expensive. if you use things that work on the market for a long time, you will be very successful. there are so many good examples that work. just look and build it as simple as possible.
So, if I did want to use the batteries and motor controller that I already have, and I was to switch to a hacker Q150 motor and an appropriate e-prop propeller, what would you recommend? Q150-45-4 on 12S? Q150-45-6 on 18S? Typical takeoff elevation is <300m, 25C
1 Like
this is impossible, the hacker industry motors may only be used with batteries and esc that are suitable for aviation. no motor manufacturer wants the brand to be badly done if it were an accident. the motor type you write is from the model building side. eppg has its own special Q 150 industrial types. with less speed. if you want to buy something write me with which hardware (esc and batteries ) you work, then i can send you prices. please understand that flying is only possible with maximum safety for the pilot. if someone wants to build something cheap he has to do it himself and be responsible for it. chief executives from hacker, mgm, rotex, geiger and others, and people like me, safety comes first.
1 Like
I was out of the country, but I am back to working on this project. I have been going back and forth about re-propping the system, but based on eCalc the performance numbers are almost identical for my existing 34x22 propeller throttle limited to 7.5kW, or a XOAR/Tarot 40x10 propeller (which is 7.5kW at full throttle). eCalc says slightly more static thrust with the 40x10, but slightly less thrust at trim speed because of the lower pitch. Overall efficiency in level flight are predicted to be about the same. Either way, I am pretty sure I am going to abandon my existing frame design, since it is just too unwieldy and has too much angular momentum to be safe. I experimented with strapping on a frame backpack over my Little Cloud Turtle seat harness, and it seems to fit well, so I think I’m going to build a support frame for the motor/controller/batteries/cage that can be used with that harness.
I should have some time next week to set up the throttle limiting on my motor controller chip, and do some test runs at 7.5kW to see how much the motor heating is reduced.
1 Like
New update: I re-programmed my throttle control chip to limit max throttle to 93% and re-ran my heating tests with good results. I was able to run for four minutes with winding temps leveling out around ~110C and the motor end housing just reaching Neumotor’s recommended max temp of 100C. External rotor temps maxed at 60C. Peak power was 7900W, dropping to 6900W after four minutes. I am happy with those numbers, at least for a level that I can use to get in the air to test everything in flight for some short hops, with a temp sensor on the motor case so I can keep an eye on the heating.
Also based on those power numbers, after much hand-wringing, I ordered a pair of 38x12 propellers (they only come in pairs). The diameter choice was based on the maximum cage size I can fit into my car without disassembly, and the pitch was the lowest I could go while still producing a flyable amount of thrust (~40kg at full charge, ~35kg at mid-charge). I still think that the XOAR 40x10 prop is really the ideal for this motor, but at $700/pair vs. $215/pair for the T3812 props, I decided to test the 38x12 first. The predicted power numbers are slightly higher than my 93% throttle test, but I expect to get slightly better cooling since I can actually run the motor in the correct direction for the centrifugal fan to draw air through the motor in the direction of flight. Also, based on some other data I found I expect the prop to unload slightly at trim speed, so the peak power should come down by a few percent. Long, full power climbs are going to be out of the question, but I expect that I’ll have enough thermal margin to get off the ground and climb to a safe altitude before I need to throttle back.
E-calc still insists that the smaller-diameter prop with larger pitch is more efficient, with more thrust available at trim speed using less power, but I guess I’ll be able to test that myself. The same data that I found that indicated the 38x12 prop should unload slightly also indicated that the 34x22 prop will actually load up MORE at trim speed compared to static, which I did not expect. APD is going to replace my motor controller so I can have something with accurate logging, so by the time I’m ready to fly this rig I should be able to capture good in-flight data for comparison.
I’m accumulating parts for my backpack style frame. The base is going to be this freighter frame, which I have already received and fits nicely over my lightweight seat harness. I would like to integrate it more into the harness at some point, maybe ditching the backpack-style straps, but it should work for now. I’m going to fabricate a cage out of 3/4" aluminum EMT, which is cheap, light, easy to work with, and locally available. All the cage parts will be coupled together using maker pipe connections.
2 Likes
My new frame is built. Everything went as expected. I made a net from .65mm dyneema, and it’s taped in place in the picture to check the coverage. I have all the parts to assemble my motor mount, but I’m still waiting on the new propeller. I’m going to test run it on the existing frame since that is convenient to secure and run at full power. After I’m satisfied everything is good, I’ll move the drive system over into this new frame. I haven’t decided how I want to mount the batteries yet, but it will likely be some sort of rack system on the back of the frame below the motor.
4 Likes