Why four motors, instead of one motor?


#1

Ok guys, here is a fundamental issue.
Why this design has four motors?
Is this design more efficient than one bigger and more powerful motor?

I have recently heard of the Duyunov’s new asynchronous motor-wheel which is based on the innovative technology of combined windings. Due to the combined windings, the parameters of the motors are significantly improved and energy efficiency is increased. The motor uses a special winding technology - partially “star”, partially “triangle”.

On electric bikes the async motor provides the following characteristics:
maximum speed is 101 km/h.
torque - 200 Nm
maximum power 20 kW.
(48 V 16 mA*h - 67 km, at average speed of 35 km/h)

The motor-wheel was designed by a Russian inventor. Instead of a belt of strong rare-earth magnets, as in classical electric motors, the motor has a wide ring of copper winding.

photo-1


#2

Motors themselves can have the same efficiencies even if they’re smaller. There are many factors that go into motor efficiency stators size, amount of poles, types and strength of magnets, winding and enameled wired size, winding types, also the ESC driving the motors play a big role in efficiency/heat. Props are another big factor, there are just so many different factors that determined efficiency. The bigger prop/motor is a general rule of thumb but a well-designed motor/prop/ESC combo can perform just as well.

Now that’s not the only reason for choosing the four motors, just to list a few of the bigger reasons.

  • Truly zero torque
  • Transport ( smaller props and motor allow the whole unit to fold up nice and compact in seconds)
  • Cost ( Smaller props, motors, ESC are more cost-effective, along with easy and inexpensive to replace)

Electric bike motors are designed differently as they address a certain uses case. Kinda like saying let’s put a Lamborghini engine in the generator that way we can generate a lot of electricity. Sure it might generate a lot of energy but for just a little bit because will run out of gas quickly and overheat. Each motor should be designed for it area of expertise

Thanks for asking the question I’m sure other people have thought about the same thing.


#3

To add to @Pdwhite point,

  • Redundancy & Safety - Electric motors are very reliable but like anything, they can fail. Having 4 motors and 4 ESCs greatly reduces the risk that one failure will impact your ability to continue flying to a safe spot to land.

This is a great question. We should add this one to the FAQ page.


#4

The biggest and best reason…

Paul @PDWhite listed ‘truely zero torque’ as the first reason for using multiple motors, but didn’t really elaborate on it. A lot of pilots, especially those with previous experience of paramotors, will get that straight away but it occurs to me that many will of not noticed the ramifications. So here’s a VERY simplistic explanation of why zero torque is such a big deal, aimed not so much at most of the people here but at those passing through or only just starting to look into becoming flyers.

There are two ways to turn a paraglider. Pulling on a brake line, those are the lines attached to the trailing edge of the wing and held in the pilots hands, or weight shifting. In weight shift the pilot leans over increasing the wing loading on that side. (Though a paraglider looks like one skinny parachute it’s actually two inflatable wings connected by a bellows section. All the lines on say the right wing collect together through some straps called risers and are fixed to the right side of the harness slightly higher than the pilots hip. The left ones all go to the left side. The brakes are lines connected to a wings trailing edge, the back, again left hand to left wing only). When the wing loading is increased that wing slows thus if the pilot leans hard to the right the right wing slows a little and the left wing speeds up by a slightly greater amount resulting in the paraglider turning direction to the right. Note that no brake has been imputed which means that the wing has not been distorted resulting in a more efficient turn.

Now add a motor. In the diagram below the black arrow represents the turning propeller. Its beating the air with around 12hp. Remember back in school when they said every action results in an equal and opposite reaction? The prop is not just pushing air behind it resulting in a reaction forwards (which we want) but is also spinning against resistance which causes the motor and its mounting to the harness trying to twist the opposite way, red arrow (which we don’t want). This force translates into a pull through the risers on that side, green arrow. Effectively it’s a weight shift input resulting in a turn, yellow arrow.

Historically, (and I’m an old fart who started paragliding in 2000) there have been several systems to try and negate the effect including: Variable hang points (the point at which the risers are fixed to the harness). Reflexed wings that respond poorly to weigh shift. Motors mounted to produce offset thrust. Trimmers on the risers to cause one wing to be a bit faster than the other or to effectively raise or lower the relative hang point. All of these methods are ways of causing the craft to turn by about the same amount, but to the other side, that the motor is causing a turn thus negating each other. Aside from the loss in efficiency this produces another problem. Generally a paraglider flies at one speed, it’s trimspeed (yes I know about speed bar etc but this is a Janet & John article :slight_smile: when the throttle is opened a paramotor does not fly faster, it climbs. When the throttle is closed it does not fly slower, it descends. So if the torque compensation is set to fly straight and level at a certain power setting then it follows that when you want to take off it’ll try to turn one way and when you come into land it’ll turn the other!

On Paul’s design two of the propellers are turning clockwise and two anticlockwise so any resultant torque effects are balanced within the motor framework and transmitted no further. This also means that the harness does not need to be so rigid and can be tuned to allow the pilots (and only the pilots) weightshift input to cause more control (lower and wider set hangpoints). It used to be very difficult to fly a paramotor as a paraglider, gaining altitude on the motor then thermalling XC, but with a torqueless setup it should be possible.

Wow. This got a lot wordier than I’d intended! Apologies to anyone who feels I’ve been stating the bleeding obvious and missing out a few important bits, but as I said previously there will be people hitting this forum who share the dreams but don’t fly… Yet.


#5

I was a glider pilot for 5 years before I stopped due to university, and I always wondered how well paramotors are suited for flying thermals. Thanks for clearing this up, and thanks for your post!


#6

In addition, we could create a variant of the OpenPPG frame that is a take off assist for paragliders - maybe just 2 motors? Use it just for a bit of help to take off, or a bit of extra lift in the air, and then turn it off and thermal. We could base it around a paraglider harness instead. Also means you could use a smaller battery pack, since you’d get most of your run-time by thermalling.


#7

I would think the advantage of 4 motors is price and availability. One large specialist motor is expensive, large single prop is probably more efficient, that large single motor requires large capacity heavy wiring, big fuses, large esc, all specialist and more expensive.

4 x smaller motors are cheaper to buy off the shelf, choice is wider, esc are smaller, wiring smaller, props are cheaper, redundancy built in, cheaper to replace or repair any failure during ownership, using more proven technology. Small props can stay on when folding. Load dispersal in thrust and electrical loading.

Thats just my take on it.


#8

I really like the idea of 4 smaller motors and props and the way they fold for portability. Was wondering though if there is any merit in putting ducts around the props - from what I’ve read, smaller props are less efficient but using ‘ducted fans’ improves overall efficiency. Does anyone have experience with this?


#9

Yeah, I’ve got experience with ducts. They won’t make sense for this application. In order for ducts to add efficiency, you need tight tolerances on the separation between the prop and the duct. The shape of the duct also matters a lot. So it’s pretty hard to do.

Now, let’s say you managed to deal with the above, using a duct wrecks your portability, and also adds a bit of cross sectional drag.

So, overall, I don’t think they’re appropriate for this design.

Now if you’re talking EDFs - like ones that spin at ~100,000 RPM, they’re even LESS appropriate for this purpose :slight_smile: and have no efficiency at low airspeeds. They’re designed for high airspeed applications.


Ducted propellors
#10

+1 on what Dave is saying, ducts make sense when either hovering or high-speed EDF/Turbine applications.


#11

Hi… We have been flying electrics for three years and the one thing no one has ever talked about is the torque. There just isn’t much and what there is can be overcome in the harness setup. Counter rotation is great for eliminating torque but you introduce complexity. I LIKE the idea of four motors but wonder about the complexity AND the sound… or more precisely, the noise. One prop at 2500 RPM driven by electrics is much quieter and a different pitch to petrol. I’ve heard Glauco’s REVOLT and thought one of the downsides is the sound four high RPM props make. Where we fly, the public does not like noisy props so we are a bit sensitive to the issue.


#12

Has anyone done the efficiency comparison between 4 and 1 prop? Obviously 4 has the portability benefit, and I’m guessing price as well. Might be smart to have a pros/cons in the FAQ


#13

Very interesting empirical observation, thank you. Torque and power of electric motor should be similar to the two-stroke engine it replaces, the smooth and flat electric torque/power curve versus the “power band” of two-strokes must be the reason of the reduced torque effect.


#14

But I don’t want to overcome it in the harness setup! I want to be able to use any harness, including my Thin Red Line model and I want to switch off my motor at an appropriate altitude, wether it be into the thermal or just climbing from the beach to the ridge lift band, and fly predominantly by weight shift :sunglasses:


#15

davek79 said “we could create a variant of the OpenPPG frame that is a take off assist for paragliders - maybe just 2 motors? Use it just for a bit of help to take off, or a bit of extra lift in the air, and then turn it off and thermal. We could base it ar”

Just two motors was what I started on…


But I love the way this 4motor system folds into itself without having to remove the props. Looks neater than my proof of concept build too

#16

Hey @paul_oz, did you ever get that system flying? It looks like it had plenty of thrust.

I figure if we did a paraglider variant we’d actually find a way to put it on a paraglider harness - something more reclined than the standard motor harness. That would be a bit harder since there aren’t really hard mount points on a glider harness howver…