22-08-2013, 03:44 PM
I’ve been doing some work on getting the new fuse ready.
These NTM 28-30-3000 motors are pretty grunty little units, The specs say they are rated at 755W and I reckon that, seeing how hard these planes fly, this is pretty close to true.
Dynamite is designed to go flat chat from seconds after launching until the 2 minuet timer alarm sounds, by which time the Lipos are pretty much at 3.75V per cell. That means there’s going to be a substantial amount of heat generated.
I wanted to force as much air coming in through the front of the plane as possible to go through the motor in an attempt to take this heat away. Air poring through a fuselage creates drag so I wanted the best cooling value for this drag. In a search for such value I read a few articles where people have cut holes in the pointy ends of spinners to let the air flow through the spinner and hopefully into the motor. It might seem like the go but looking closely at this there are problems. The spinner is rotating and any air entering it will also begin to rotate helped in this rotation by the hub of the propeller and the roots of the blades. With the Dynamites motor running at close to 40,000 rpm that rotating air will, due to centripetal force, want to fly out sideways as soon as it exits the back plate of the spinner. The spinner will act like a pump throwing air out at some high angle to the slipstream and this will, not only, steal power from the motor, but may add heaps of unwanted drag by upsetting the air-flow around the front of the fuse.
I thought the best way to try and get air to cool the motor was to have the front section of the motor surrounded by a reasonably close fitting duct where for a distance of about 20mm aft of the nose, there is only about 1mm clearance between the duct and the rotating bell of the motor. This 1mm annulus presents less projected area than vents in the motor so I would hope that more than half the air coming into the fuse would go right through the motors windings and help to keep it cool. You can see form the photo the duct is just made from a piece or PVC water pipe thinned down a little on the lathe.
One thing I found a bit tricky with the original Balsa fuse was getting the motor mount bulkhead at just the right angle.
I wanted Zero down thrust and zero side thrust. But it’s just as hard to get that, as any particular combination of angles. There is nowhere to measure it from, to get a reference angle, particularly with regards to down thrust. The plan is to use a long piece of rod with holes drilled and tapped in one end at the same spacing as the holes in the back of the moter. The rod should be long enough to stick out the through the nose and allow me to measure at what angle it is, in reference to the nice flat table top. I'll set the fuse up so the table top is parrellel to the cord line of the wing. That way I should be able to set up the thrust angle nicely. Well that’s the plan
You can see the aluminium rod in the picture I’ll just bolt that to the "motor mount bulkhead" line up the rod and glue the bulkhead in place then I can unbolt the rod and my bulkhead will be in the the right place and set at the right angle as well.........hopefully
These NTM 28-30-3000 motors are pretty grunty little units, The specs say they are rated at 755W and I reckon that, seeing how hard these planes fly, this is pretty close to true.
Dynamite is designed to go flat chat from seconds after launching until the 2 minuet timer alarm sounds, by which time the Lipos are pretty much at 3.75V per cell. That means there’s going to be a substantial amount of heat generated.
I wanted to force as much air coming in through the front of the plane as possible to go through the motor in an attempt to take this heat away. Air poring through a fuselage creates drag so I wanted the best cooling value for this drag. In a search for such value I read a few articles where people have cut holes in the pointy ends of spinners to let the air flow through the spinner and hopefully into the motor. It might seem like the go but looking closely at this there are problems. The spinner is rotating and any air entering it will also begin to rotate helped in this rotation by the hub of the propeller and the roots of the blades. With the Dynamites motor running at close to 40,000 rpm that rotating air will, due to centripetal force, want to fly out sideways as soon as it exits the back plate of the spinner. The spinner will act like a pump throwing air out at some high angle to the slipstream and this will, not only, steal power from the motor, but may add heaps of unwanted drag by upsetting the air-flow around the front of the fuse.
I thought the best way to try and get air to cool the motor was to have the front section of the motor surrounded by a reasonably close fitting duct where for a distance of about 20mm aft of the nose, there is only about 1mm clearance between the duct and the rotating bell of the motor. This 1mm annulus presents less projected area than vents in the motor so I would hope that more than half the air coming into the fuse would go right through the motors windings and help to keep it cool. You can see form the photo the duct is just made from a piece or PVC water pipe thinned down a little on the lathe.
One thing I found a bit tricky with the original Balsa fuse was getting the motor mount bulkhead at just the right angle.
I wanted Zero down thrust and zero side thrust. But it’s just as hard to get that, as any particular combination of angles. There is nowhere to measure it from, to get a reference angle, particularly with regards to down thrust. The plan is to use a long piece of rod with holes drilled and tapped in one end at the same spacing as the holes in the back of the moter. The rod should be long enough to stick out the through the nose and allow me to measure at what angle it is, in reference to the nice flat table top. I'll set the fuse up so the table top is parrellel to the cord line of the wing. That way I should be able to set up the thrust angle nicely. Well that’s the plan
You can see the aluminium rod in the picture I’ll just bolt that to the "motor mount bulkhead" line up the rod and glue the bulkhead in place then I can unbolt the rod and my bulkhead will be in the the right place and set at the right angle as well.........hopefully “The knack of flying is learning how to throw your machine at the ground and miss.”
"When once you have tasted flight, you will forever walk the earth with your thoughts turned skyward, for there you have been, and there you will always long to return." ~Leonardo Da Vinci