# Why four motors, instead of one motor?

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 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.

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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!

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.

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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.

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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?

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 and have no efficiency at low airspeeds. They’re designed for high airspeed applications.

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

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.

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

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.

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

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
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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…

Ok, so @paul_oz you mentioned wings that are designed for use with traditional ICE paramotors to count p-factor. So are there wings we should stay away from if we are going to have an openPPG?

That’s a good point, I’ve not really thought about it. Off the top of my head I’d say use a regular free flight wing. Probably avoid a reflexed wing as they trade efficiency for stability, you probably don’t need trimmers. What do other pilots think.

This is what I did in the late 90’s but with gas. Boxer engine - the whole thing weighed 19lbs and was attached to a true paragliding harness equipped with a 20cm medium density back protector, hip protectors with medium density foam shaped in concave thin fiberglass shells, double stage speed-bar, foot-rest, true paragliding attachment connections, no bars extending from the frame because there was no frame, the motor was directly attached to the harness, a 6 flap dorsal reserve parachute container with a reserve handle that could be placed right or left of the harness - I could choose to fly with or without the motor while using the same harness.I could remove the entire motor from the harness in 30 seconds and put it back on in about 1 minute. The torque effect was barely felt.

We called it the APPS ( Assisted Paragliding Propulsion System), and the idea was exactly what you wrote, and it worked superbly. I could lift off the ground with a wing similar to today’s EN-B gliders, and catch thermals. Only had enough gas from a Dubro RC tank for about 10 to 15mn. I was nailing the throttle and never let go until I ran out of gas, searching for thermals, then kept going free-flying - Was usually doing it before Sunset when all the heat was released evenly throughout the area. Best of all, I could weight-shifty as much as I wanted, just as when paragliding because there wasn’t any frame. It was not a powerful machine but served its original purpose of thermal hunting or reaching dynamic lift on a ridge. The same thing could be done with an OpenPPG. The maximum magic number for the machine is 22lbs before the roman scale effect takes place.

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Love the old school flying suit, turquoise! Are you sure this wasn’t late eighties not ninetyls

Still have it. It was a Winter flight suit with fleece lining made by SUPAIR

Here’s my input on buying a paraglider.

#1 Work with a dealer/school that’s focused on selling their product. If they spend as much or more time bashing the competition, RUN don’t walk, RUN as fast as you can to another dealer. In the long term, this will be one of the best decisions you will ever make in your flying career.

#2 If the dealer starts spouting off about their own professionalism or engages in another form of confirmation basis or tells you they want you on “The Team”, see #1.

#3 Support your local instructor or school if it is a viable option. Having a local resource for advice and/or potential issues/repairs is worth more than saving a tiny bit by going out of the area.

#4 Avoid buying a glider on eBay, etc. I’ve never, ever heard of anyone actually getting a good deal. There is usually a reason someone is using one of these venues to sell their equipment; It’s junk they can’t unload anywhere else.

#5 Be realistic about your ability and goals. It’s much better to fly a glider 100% of the time rather than have the glider fly you the 1-2% of the time when conditions are lousy and/or downright dangerous.

#6 Stick with a glider that is either EN or DGAC certified. A few smaller manufacturers will only certify the more popular sizes while the XS or XL sizes for example are not. This may not be a problem but from experience, you don’t want to be the test pilot. If the glider is EN rated but not DGAC, double-check with the manufacturer whether it’s appropriate for PPG. I have seen a beginner level paraglider used for PPG that had issues with powered flight. As soon as the pilot went to the throttle, the glider would start to roll each time. No throttle, no roll. After several tries, he exchanged the glider and was much happier.

#7 If your flying an OpenPPG, skip the trimmers. Since there isn’t a torque issue, they aren’t needed. All paragliders are sold with a speed system and are much more efficient than trim tabs. Besides, if a pilot forgot the trim tabs were engaged and then added the speed bar, it’s going to be real ugly real quick.

#8 With regard to a reflex wing vs. a non-reflex paraglider, check out page 224 in Jeff Goin’s PPG Bible. Reflex airfoils have the benefit of increased stability at higher speeds but require a different steering technique/design.

Good luck. Have fun. Fly high. Fly far.

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@Critter Thanks so much for the info, very helpful. One more question, and sorry if it is a dumb one. Are trimmers on every wing and you’re saying just don’t use them? Or are there specific wings that have them and some that don’t?