Skymaster F18D Hornet twin build
#178
My Feedback: (24)
I think as Marty said, the issue is with the engines interfering with each other's RPM sensor readings - leading to faulty operation, as in one or both engines failing to respond to throttle inputs, or responding incorrectly to throttle inputs.
I believe that the engines use Hall effect sensors to read RPM and the magnet is in the rotating part (i.e. the compressor, nut, or shaft, I'm not sure which). The two spinning magnets next to each other create magnetic field interference that leads to faulty RPM readings.
BTW, I think that JetCats used to use optical RPM sensors...
Bob
#179
My Feedback: (10)
JetCats used optical RPM sensing a long time ago but also switched to magnetic. I have no idea if this magnetic cross talk is a problem with JetCat or not. The only twin I have has turbines mounted much farther apart (MiG-29).
I will say I have used (in another life) a material commercially referred to as Mu metal for magnetic shielding of certain test enclosures, it might work in this case, who knows.
I will say I have used (in another life) a material commercially referred to as Mu metal for magnetic shielding of certain test enclosures, it might work in this case, who knows.
#184
#185
This issue is not a concern when using Jet Central engines, you will be fine
#186
My Feedback: (24)
JetCats used optical RPM sensing a long time ago but also switched to magnetic. I have no idea if this magnetic cross talk is a problem with JetCat or not. The only twin I have has turbines mounted much farther apart (MiG-29).
I will say I have used (in another life) a material commercially referred to as Mu metal for magnetic shielding of certain test enclosures, it might work in this case, who knows.
I will say I have used (in another life) a material commercially referred to as Mu metal for magnetic shielding of certain test enclosures, it might work in this case, who knows.
That does seem like a long time ago...
Bob
#192
Thread Starter
My Feedback: (9)
After lots of thinking about the F18 and why it was so unstable on one engine even though they are close to the center line. What I have come up with is with the engines mounted in the tail so far from the aircraft CG the yaw has a much greater effect. Same way adding or removing weight far away from the CG has a large effect. After learning what I have learned on this plane I would not use tail mounted engines on twins.
On a few flights the plane flew fantastic and was really loving it .But if one engine fails you will be in deep trouble. If I had mid mounted engines I am sure my plane would still be here. Its a hard lesson learned and I hope others can learn before loosing a plane. In my opinion tail mounted engines should not even be an option.
Also it is very important to run your engines 1 at a time and double check for any issues between them. We already discussed the RPM issues with the close mounted engines. Also make sure your ECU wires are ran separately.
On a few flights the plane flew fantastic and was really loving it .But if one engine fails you will be in deep trouble. If I had mid mounted engines I am sure my plane would still be here. Its a hard lesson learned and I hope others can learn before loosing a plane. In my opinion tail mounted engines should not even be an option.
Also it is very important to run your engines 1 at a time and double check for any issues between them. We already discussed the RPM issues with the close mounted engines. Also make sure your ECU wires are ran separately.
#193
My Feedback: (7)
After lots of thinking about the F18 and why it was so unstable on one engine even though they are close to the center line. What I have come up with is with the engines mounted in the tail so far from the aircraft CG the yaw has a much greater effect. Same way adding or removing weight far away from the CG has a large effect. After learning what I have learned on this plane I would not use tail mounted engines on twins.
On a few flights the plane flew fantastic and was really loving it .But if one engine fails you will be in deep trouble. If I had mid mounted engines I am sure my plane would still be here. Its a hard lesson learned and I hope others can learn before loosing a plane. In my opinion tail mounted engines should not even be an option.
Also it is very important to run your engines 1 at a time and double check for any issues between them. We already discussed the RPM issues with the close mounted engines. Also make sure your ECU wires are ran separately.
On a few flights the plane flew fantastic and was really loving it .But if one engine fails you will be in deep trouble. If I had mid mounted engines I am sure my plane would still be here. Its a hard lesson learned and I hope others can learn before loosing a plane. In my opinion tail mounted engines should not even be an option.
Also it is very important to run your engines 1 at a time and double check for any issues between them. We already discussed the RPM issues with the close mounted engines. Also make sure your ECU wires are ran separately.
Do the planes call for tail mounted engines or is that an option you chose? Is it an either or deal for the kit out of the box ??
Dan
#195
After lots of thinking about the F18 and why it was so unstable on one engine even though they are close to the center line. What I have come up with is with the engines mounted in the tail so far from the aircraft CG the yaw has a much greater effect. Same way adding or removing weight far away from the CG has a large effect. After learning what I have learned on this plane I would not use tail mounted engines on twins.
On a few flights the plane flew fantastic and was really loving it .But if one engine fails you will be in deep trouble. If I had mid mounted engines I am sure my plane would still be here. Its a hard lesson learned and I hope others can learn before loosing a plane. In my opinion tail mounted engines should not even be an option.
Also it is very important to run your engines 1 at a time and double check for any issues between them. We already discussed the RPM issues with the close mounted engines. Also make sure your ECU wires are ran separately.
On a few flights the plane flew fantastic and was really loving it .But if one engine fails you will be in deep trouble. If I had mid mounted engines I am sure my plane would still be here. Its a hard lesson learned and I hope others can learn before loosing a plane. In my opinion tail mounted engines should not even be an option.
Also it is very important to run your engines 1 at a time and double check for any issues between them. We already discussed the RPM issues with the close mounted engines. Also make sure your ECU wires are ran separately.
Maybe mounting them in the tail makes it harder to evaluate the angle: a long pipe just a little off is very noticeable, whereas a short engine nozzle off by the same amount would seem pretty much straight?
Looking at this picture it seems like the engines were quite convergent:
Last edited by mauryr; 03-27-2017 at 04:40 AM.
#198
My Feedback: (57)
Jay
#199
My Feedback: (18)
I wonder if zero degrees offset for tail mounted would be better, and possibly adding some level of thrust vectoring. Not something crazy like 3D vectoring, but something mild. This way the model does not end up with a static setting. If you lose a turbine you or a gyro would still have some control of the thrust angle. Just spit-balling here. These concepts are complicated, I believe the full scale jet has a million lines of code to help compensate for a lot of these contingencies.
#200
I think some testing should be done, SM supposedly test these things single engines but unlike the f-14 they did not video prove their testing. Jim has had both one with tail mounted and now mid mounted, not sure if he went mid for thrust vectoring or something else. Maybe he could chime in on this, I think he lost an engine last year on his but that could have been the f-14.