Folding Prop Hub for Electric Paramotor
Dec 12, 2019 22:34:59 GMT
Derek the Admin, Bruce, and 2 more like this
Post by rayrv9a on Dec 12, 2019 22:34:59 GMT
Its been a while since I posted an update on my electric paramotor project; all the machining for it has been done on my Power Route and somewhere in Project Talk there's a thread if you're interested. Today's update is to show the results of my project to adapt a design for a folding propeller hub for use on my paramotor. Folding props for a paramotor are beneficial for two reasons. When you're not using the motor to gain altitude you want to reduce drag as much as possible to maximize glide efficiency. A folding prop hub allows the blades to fold back on themselves and align themselves with the airflow thereby reducing drag. When you start the motor, its torque unfolds the blades back to the normal, thrust producing position. This folding action can also be very helpful during take off and landing. It keeps the prop tips out of the way of the wing lines and only allows the prop to rotate when the wing is fully overhead and the wing lines are vertical and out of the arc of the rotating tips. At any other time, with lines potentially slack, stopping the motor allows the prop to fold back on itself out of the way of the lines.
The disadvantage of a folding prop hub is that its elegance comes at the price of added mechanical complexity and cost. Its alternative, used on gas powered paramotors, is to mechanically attach a hoop to the paramotor frame. The hoop provides support for netting which effectively puts a hood around the arc of the prop and prevents lines from entangling with the prop.
I decided to adapt a public domain design of a folding prop hub for self-launch gliders to my paramotor. The pictures below show the results. At this point I have not spun this hub and prop on the test stand but I have spun it, on the paramotor, without the prop blades attached to ensure the hub is mass balanced around its center axis so it does not cause vibration and shaft bearing wear. The modifications I made allowed some critical parts to be machined on a 3 axis machine rather than a 5 axis one. This required replicating the design in Fusion 360 (its only available on line as a set of 8 engineering drawings) and then cutting the most complex parts using construction planes in such a way the resulting pieces could be cut on the Power Route and then bolted together to create the equivalent part. This added the complexity of choosing screw sizes, screw placement, etc to provide the part with the same or higher strength as the one piece version of the original design.
It was fabricated from 2017A aluminum. To given you an idea of size, the base plate is 10mm thick, 100 x 113mm and the sides are about 90mm high. The design was originally sized to accommodate engines up to 50hp spinning prop diameters up to 2m at 2000RPM . My application uses a 13hp motor spinning a 1m diameter prop at a maximum RPM of 1750.
The first two pictures show the completed folding hub with the propeller blades mounted. The base plate mounts directly to the motor. These pictures show the hub with the blades unfolded; i.e., in their normal thrust producing position.
If you look at the top-down view of the hub, you can see that the two "pockets" for the root ends of the prop blades are geared together. This cogs them so that the blades synchronously sweep through their folding arc even in potential wind conditions that might put more force on one blade than the other. The original design had this piece as just that; one piece. But you can see how I "cut it" in Fusion360 in to three pieces, each of which could be machined in three axes, even the gears.
Here's a picture looking down as the blades fold up:
Here's a picture with the blades fully folded. When mounted on the paramotor and it's strapped to your back like a back pack, the folded blades would point backwards, perpendicular to your back, in line with the airflow and producing the least amount of drag:
The disadvantage of a folding prop hub is that its elegance comes at the price of added mechanical complexity and cost. Its alternative, used on gas powered paramotors, is to mechanically attach a hoop to the paramotor frame. The hoop provides support for netting which effectively puts a hood around the arc of the prop and prevents lines from entangling with the prop.
I decided to adapt a public domain design of a folding prop hub for self-launch gliders to my paramotor. The pictures below show the results. At this point I have not spun this hub and prop on the test stand but I have spun it, on the paramotor, without the prop blades attached to ensure the hub is mass balanced around its center axis so it does not cause vibration and shaft bearing wear. The modifications I made allowed some critical parts to be machined on a 3 axis machine rather than a 5 axis one. This required replicating the design in Fusion 360 (its only available on line as a set of 8 engineering drawings) and then cutting the most complex parts using construction planes in such a way the resulting pieces could be cut on the Power Route and then bolted together to create the equivalent part. This added the complexity of choosing screw sizes, screw placement, etc to provide the part with the same or higher strength as the one piece version of the original design.
It was fabricated from 2017A aluminum. To given you an idea of size, the base plate is 10mm thick, 100 x 113mm and the sides are about 90mm high. The design was originally sized to accommodate engines up to 50hp spinning prop diameters up to 2m at 2000RPM . My application uses a 13hp motor spinning a 1m diameter prop at a maximum RPM of 1750.
The first two pictures show the completed folding hub with the propeller blades mounted. The base plate mounts directly to the motor. These pictures show the hub with the blades unfolded; i.e., in their normal thrust producing position.
If you look at the top-down view of the hub, you can see that the two "pockets" for the root ends of the prop blades are geared together. This cogs them so that the blades synchronously sweep through their folding arc even in potential wind conditions that might put more force on one blade than the other. The original design had this piece as just that; one piece. But you can see how I "cut it" in Fusion360 in to three pieces, each of which could be machined in three axes, even the gears.
Here's a picture looking down as the blades fold up:
Here's a picture with the blades fully folded. When mounted on the paramotor and it's strapped to your back like a back pack, the folded blades would point backwards, perpendicular to your back, in line with the airflow and producing the least amount of drag:
