I think you’re bang on! There are a few imperfections to this solution, however. The biggest of which is that you won’t be very aerodynamic (compared to your typical XC cocoon). That aside, you’ll have all the benefits of flat field launches and can get yourself out of a lot of dodgy situations! This is one of the ways I really plan to use the system as well!
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Or forget thermals altogether and fly in the morning and evening when the winds are the lowest then your risk factor will be much lower. That is the beauty of motoring.
Yes. But that might be like trying to stick to sport climbing when you actually prefer trad.
Good point. What if we had a unit that had just two caged triple props (like these KDE-CF155-TP Propeller Blades, 15.5" x 5.3, Triple-Edition CW/CCW Set | KDE Direct) that mounted on two arms so that when you reached a thermal, at the push of a button, small servos motors on each arm could fold back the arms together like a clam for better aerodynamics . Press the button again to quickly power back whenever you need it. The two 15.5-inch diameter triple props might provide double the thrust of the four 9-inch double props and be a lighter overall unit.
Thoughts?
I’ve always liked the idea of just two props. I started to make a prototype, you can see it over at e-ppg.com (look for the parapush project) but there is a big advantage with this OpenPPG design. Four props mean the props are smaller which is how the wonderful foldable design is possible 
Interesting. But I think two props could be more foldable and compact than four. and easier. see attached sketch.
Also if two bigger triple props produce twice the thrust of four smaller double props, then at half power they can produce the same thrust, and, since there are now only half the number of motors, the combined effect would be 4x longer battery duration. ie 80 minutes with the light setup instead of 20.
Am I way off?
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oh I just watched Paul’s latest video and it looks like those 22x10 props are bigger than I expected (ie not metric or if so just measuring one side perhaps). But, no matter, would like to hear some feedback on the concept.
I think I prefer this idea! Looks like you could avoid disassembly of the huge cage each time. And skip the footer in favor of just an airbag. The whole unit could be way smaller. 10lbs??
Servos in the folding arms would be the cat’s ass! Perhaps not that difficult to add either.
@Pdwhite could you make me one of these?
Hi @mateo I’ve a couple of comments.
The props I used were the size recommended by Hobby King for the 50cc outrigger motors I got from them. The 22x12 sizing is in inches, the 22 refers to the diameter of the prop and the 12 to the blade pitch (the way it’s measured is that if the prop were a screw thread winding through the air then each revolution would result in it going forward 12 inches. It’s a lot easier to visualise with boat screws which is where the system was adapted from a century ago)
I don’t think halving the number of props would double your battery run time. At its most basic what we are doing is taking electrical energy from the batteries and converting it to kinetic energy of thrust. There is still the same amount of electric fuel regardless of how we spend it. Think two cylinder motorbikes and four cylinder motorbikes. You’ll get a very similar mileage out of each litre of fuel.
I do like your twin prop design. I’m assuming you’re thinking a left and right motor rather than the upper and lower system I had. I’ve still got the parts, motors, ESC’s etc. I’m going to have to have a good think on this. As you say, folding it up in flight once you’ve reached a good thermal hunting altitude would be way cool!
Paul in Oz
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Hi @paul_oz, thanks for the clarification. yes left and right is the idea. I see the openppg thrust figures are listed as:
Thrust: 150lbs (22×10 props), 165 lbs (22×14)
Which makes me wonder why the same length KDE carbon props are only 21.5" X 7.3.
I don’t know how to apply pitch to a thrust calculation so I just ran some numbers based on total prop area. Looks like to replace four 22" props we’d need two 30" props. Which is way too big. KDE states that the triple props improve thrust by >70%. So going by that (and ignoring their pitch numbers) looks like two 24" triple props could provide the same thrust as the four 22s.
Two 24" triple props is about the most I’d want on my back. Something more compact would be better. Like those two 15.5" triple props that I originally linked to. By my area calc looks like to achieve the same thrust with those the motor sizes would have to be increased by 1.5x. Probably less battery life though.
Or to increase thrust on those smaller props would you just increase pitch?
you can not just increase pitch of the propellors, as the motor that are spinning the props, also need higher torque (more force).
In general, you can say that less RPM (rotations per minute) with higher torque of the motor and higher pitch of the prop is more effective than faster/more RPM with less pitch and less torque.
this video sums it all up pretty well:
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Here’s a complete electric unit with a folding prop:
http://razeebuss.com/en/radicall/
Interesting thrust angle compensation demonstrated at Razeebuss Radicall v4 presentation - YouTube
The “virtual cage” in action. Seems to work alright, bit sketchy though. Test Cage Virtuelle Razeebuss Radicall - YouTube
Now I’m wondering if a single-cage coaxial contra-rotating setup would be a simpler solution. as per
According to the performance data on this KDE motor, it can spin up a 30.5x9.7 triple prop to 48lbs of thrust at 3800W (12S).
Two of them nested together in contra should provide ~100lbs/45kg of thrust and no torque, right?
The Razeebuss Razmott produces 45kg of thrust with 130cm prop, apparently this is sufficient to get up to the thermals.
To simplify construction, a small 31"cage and frame could be 3d printed in one piece with carbon fiber polycarbonate composite. The whole setup with cage, motors and props might only weigh ~5kg.
What do you guys think?
This topic came up in another thread, its over my head I’m just posting the link.
Also the e-glider has folding prop.
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The E glider looks very interesting. Personally I love the idea of thermalling and “gliding” as apposed to 100% powered flight. I wonder if we can get a similar idea together that focuses on these goals?
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What would you like to see different? Something more like an XC paragliding harness? Because if you’ve got a universal wing, like the Dudek Universal, you can trim between flying styles.
I’d like it to be smaller and have less drag.
I wrote KDE asking if they build coaxial motors and if they had performance data available. They replied that they will build a test rig and gather performance data.
Referring to a previous build, the KDE engineer said “the thrust capability increased >180% from a single motor – with 100% thrust-capability being recognized on the upper coax edition, and approximately 80% thrust-capability being generated on the lower-coax edition (losses due to pre-accelerated airflow of the upper plane). Overall, the losses in thrust versus two-independent motor ranged from 10% up to 14%, depending on the throttle position and air-conditions (weather conditions).”
So it sounds like a 14S coax 10218 with their 30.5" triple props could supply at least 100lbs of thrust which would make for a good compact no-torque e-ppg sufficient for reaching thermals.
Wondering if 2-6" larger props with greater pitch could push that value much higher while keeping efficient g/W performance. Looking forward to seeing their data.
I also contacted e-glider to see if they could design contra-rotating folding props, they’re thinking on the solution.
Perhaps no servos are even needed if a simple spring mechanism holds props folded together. Upon providing power, the thrust would open the props into flying position.
I really like the two prop side by side design but have a major safety concern. One motor failure during flight or even take off could cause pilot/paramotor twist consequently causing riser twist and of course loss of glider wing control. The speed (rpm) of each propeller must always remain equal to each other even during failure of one of them. If accomplished mechanically, the motors could be pulled closer together with propellers even operating in the same air space, allowing use of larger props for increased thrust.
How does the Hobby King 50cc motors compare to the custom wound ones?