New to the forum and just wanted to firstly say how exciting it is to finally see a viable electric paramotor, I can’t wait to order one from batch 4!
I wanted to ask about prop choice, and open a discussion about potential alternatives.
I have been flying r/c and drones for the last 20 years, and my knowledge comes from practical trials rather than academic theory, so am not claiming to know everything!
My observation is that I understand the maximum diameter is set by the frame, but 10” pitch seems suited to an airspeed much higher than the speed of a paramotor? This would lead to the prop stalling and not being completely efficient. In my experience, dropping to something in the 6” pitch range should produce similar thrust for a lower current draw.
I think you bring up a good question. I would love to collect test data from stuff like that but I don’t want to spend over 100 dollars per set for multiple sets of props just to collect data. Specially when the machine already works beautifully the way it is. Although fewer amps would be nice.
If we use a smaller pitch we would need to increase the RPMs to get the same thrust. Unless the props are currently stalling like you are suggesting which I don’t think is the case. The higher RPMs could be achieved with higher volts like 14S. But what would the increase in RPMs do to the noise and vibrations. Specially if the tips are approaching mach 1?
I plan to convert to higher volts (14S) and use a lower Kv motor with the same props. In the end my props should spin at the same RPM but with higher volts and lower amps. A new set of props would certainly be cheaper than a new set of motors though and potentially give the same results I’m looking for (except for maybe the noise).
I remember someone else on this forum commenting that drone props wouldn’t be as efficient because they are designed for static thrust since most of what a drone does is hover. That comment was made in a conversation about finding matching counter rotating props which is easy to find for drones. But perhaps with our slow flying speed they might be ideal.
Unless you go with a variable pitch prop the pitch should be selected for the desired goal. For instance if you were to always mountain launch and never need power to get in the air then the pitch should be selected for the most common airspeed say 20mph (maybe 12-18 pitch). However if you wanted to only take off you would want prop pitch to be ideal at static to 10-15mph or 4-8 pitch. So unless you go variable pitch there is always a compromise and you’ll end up somewhere in the middle. Obvoiulsly rpm, amp draw, kv, motor rating etc need to be considered when changes are made. While static testing is nice and you can determine force and power draw for a given setup and translate that into efficiency i.e. grams / watt it wont give you data on force and efficiency at speed. Based on models you should expect to lose 30-50% thrust going from static to 25mph, but the best way to be sure would be a windtunnel or a test stand mounted to a vehicle with airspeed sensor. I’ve built a test stand that can be vehicle mounted to do prop testing. I can share some pics later. Here is a youtube video https://youtu.be/BL2mR6wcnDE of the test stand i made 2 years ago, i’ve completly overhauled everything since then. Also this was designed for a powered Hang Glider setup, not for the PPG.
Even though they look like they have a lower aspect ratio than the wood ones we use now, the “more curved airfoil camber” could potentially push more air at a slower RPM and reduce noise while increasing efficiency. I’d love to test them but don’t want to pay that much for them. Specially if it requires also switching to a lower KV motor. Although, the 22 inch version only has a 6.8 inch pitch so the current KV might work fine (assuming those numbers are inches).
Those are unfortunately “static thrust” props, designed for hovering, like a drone. We need “dynamic thrust” props, with a much higher pitch to get efficiency through moving air. That’s where we need to be up in the 22x10+ range
It’s all (mostly) about the pitch. At 37 km/h cruise speed, you’re definitely in dynamic thrust range for a 22" prop. Think of it as the blade needing to “grip” the air to generate thrust. At a slow incident airspeed (hover) you need a lower pitch to grip the air. At higher airspeed, the air is already moving and you need a steeper pitch. This all really just drives the equivalent angle of attack (AoA) of the airfoil segment.
You can “catch up” with the incident air stream by spinning a lower pitch prop faster, but it becomes increasingly inefficient.
Thanks! I understand the concept just as you explained it (good explanation by the way) but how do we determine the optimal pitch for the speed we are going? Of course the AOA has to be positive relative to the incident airstream but how much? Too much would stall.
So what if we used a higher pitch with a lower RPM and a lower KV and perhaps increased the camber too? Would that gain efficiency as well as decrease noise?
Nobody on this forum has ever reported results from anything other than the standard prop. It would be interesting to see though.
For any of this to have reasonable value to promote one prop configuration over others-- testing would need to be done in a controlled environment on a static test rig - IMHO
Maybe a number of us could pool our resources to buy some test props and fabricate a test rig.
A test rig would be needed that could produce a free stream velocity similar to our cruise. Maybe a test rig could be mounted in front of or on top of a vehicle. It would need to be mounted far enough away to minimize the aerodynamic effects from the vehicle.
Does anyone have access to a wind tunnel?
I would be willing to donate some money to someone that has some hardware to test.
I have a test rig for this exact purpose, though, it was designed for an electric propulsion hang glider rig. Over 4 years of time spent designing and building it. Very interested in discussing pooling resources and test data. Will post description, pictures and video later today when at computer.
Problem is that to do this right you actually need dynamic thrust measurements, which means you need a wind tunnel or an ability to measure thrust while airborne. Wonder if there is a clever way to use strain gauges to do this while airborne…
100% agree you need to be at airspeed and more specifically in laminar flow at constant airspeed to get any useful data. To that end I built a dynamic test stand video here. Below are some of the details.
Sensors:
Airspeed
Thrust
Current
Voltage
RPM
Temperature
SPECS:
Bluetooth connectivity to receiver so it can be controlled inside car.
Data logging of all data.
Telemetry is sent back to receiver so you can drive car based on the airspeed feedback from the pitot tube.
*Adafruit Feather ESP32 used in receiver and test stand, test stand also has another Feather processor to handle the thrust sensor.
Here is a link with a lot of data on how airspeed effects thrust. Below is a plot of the theoretical thrust to airspeed for a 22 x 10 prop at 6,000 RPM. As you can see there is almost a 50% drop from static to 25mph.
If there’s interest in this type of testing and research would be great to pool resources.
It might be an idea to post your whereabouts, I’m pretty sure there’s a few members who would like to get on board if located near enough. I’m in Brisbane Australia and have a few props available to compare.
