telnar1236
Elite member
Not crazy, but it was right at the edge of what might have been flyable. I was considering trying to fly it anyway but decided against risking it.
3D Printed 50mm EDF Trainer Jet
- Thread starter telnar1236
- Start date
Captain J
Well-known member
man i run into the same issue 😑
i tend to build “longer nose” or more delta style now, which gives me a lot of room…then i end up nose heavy 😑
telnar1236
Elite member
Definitely a challenge with 3D printing given the weight of the material.
telnar1236
Elite member
The plane is now mostly rebuilt with the new fuselage designed to give it a more forward CG and reduce weight. It now actually weighs in closer to the intended 900 g (this version is 881 g with a 1600 mAh pack) and can hit CG with between a 1600 mAh pack and a 2200 mAh pack without added nose weight.
I wasn't as much of a fan of the gray and green scheme as I expected, so I switched to black and green, using black ABS so that it will hold up to the sun. I've also improved the ease of printing of a couple of the more annoying parts to make it easier to build.
I wasn't as much of a fan of the gray and green scheme as I expected, so I switched to black and green, using black ABS so that it will hold up to the sun. I've also improved the ease of printing of a couple of the more annoying parts to make it easier to build.
telnar1236
Elite member
I tried flying it today, and unfortunately it has problems that I don't think I can fix. Specifically, even with the CG as far forwards as it will go (at the red line), it still feels tail heavy in flight.
I can't do much to move the CG further forwards since the EDF, battery, and servos are all as far forwards as they can go already and I've cut all the weight out of the tail I can without making it too weak. My best guess as to the problem is that the big inlet to the duct is acting a bit like an annular wing and that the fuselage is contributing to the pitching moment. I'm going to try making the inlet smaller to see if that improves things and add some length to the nose to try and move the CG even more forwards, but if that doesn't work, I'm out of ideas.
I can't do much to move the CG further forwards since the EDF, battery, and servos are all as far forwards as they can go already and I've cut all the weight out of the tail I can without making it too weak. My best guess as to the problem is that the big inlet to the duct is acting a bit like an annular wing and that the fuselage is contributing to the pitching moment. I'm going to try making the inlet smaller to see if that improves things and add some length to the nose to try and move the CG even more forwards, but if that doesn't work, I'm out of ideas.
L Edge
Master member
One more suggestion for you to try. Looking at it from a vector approach, try shimming either the LE or the trailing edge of the wing to the fuse and it may change the tail heavy condition. Since I don't know the wing shape, position of CG, location of battery etc, see if you can increase the lift component (vertical force) of the wing to counteract the tail heaviness. Use scotch tape to cover gaps between fuse and wing. I've explored up to + and minus 5 degrees and won.
telnar1236
Elite member
Worth a try
telnar1236
Elite member
This is the other change I'm making to try and fix things. The nose is a bit longer and the intake smaller.
I left the old inlet geometry in since I'm trying to add nose weight anyways, so in this cross-section you can see the old vs. new geometry. It's smaller and further forwards.
I left the old inlet geometry in since I'm trying to add nose weight anyways, so in this cross-section you can see the old vs. new geometry. It's smaller and further forwards.
telnar1236
Elite member
At this point I think I've pretty much tried everything - and it did help some, but not enough. The best I've gotten it so far was with the new nose and using tape like L Edge suggested to shift the COP further back. I roughly doubled the area of the horizontal stabilizer and added as much to wing as I could, but it still wasn't enough. I suspect something is going on with EDF thrust line that I don't understand, possibly resulting in the nose inlet producing an unexpected amount of lift for some reason when the throttle is up, or that maybe the canopy is interacting with the horizontal stabilizer in a weird way. Foam board models glide great where I have the CG so whatever is going on either only happens with power or requires the curvature of the fuselage, and I suspect the former. I tried CFD and it did predict instability between 5 and 10-degrees AOA so but showed stability below and above that, but for a simple 50mm design, I don't think it's worth spending hours simulating it to understand why it's behaving weirdly.
At this point, I think I need to accept that there is something fundamentally wrong with the design for reasons I don't fully understand and move on to a new project.
At this point, I think I need to accept that there is something fundamentally wrong with the design for reasons I don't fully understand and move on to a new project.
telnar1236
Elite member
Since the fuselage is completely undamaged from the previous less than successful flight tests I've decided to try and increase the sweep of the wings either until the plane flies well or the fuselage is destroyed beyond usability. I'm starting with 25 degrees of sweep and may add more depending on how this goes. At this point, the plane is slowly turning into a combination of a MiG 15 and an F-86.
I probably won't bother to model the internals of the wings and will just use infill for the sake of ease till I can get the plane flying well.
I probably won't bother to model the internals of the wings and will just use infill for the sake of ease till I can get the plane flying well.
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