I have run into several occasions of Jesus bolt breakage when the traditional heli landing in auto mode. I am using an external governor. The motor interlock was disable by the system when land in auto mode, and the throttle servo’s arm will change from the full rpm to idle in very short time, this caused the sudden stress on the Jesus bolt and broke it. I have changed the new main shaft, new feather shaft, new main blade grips, new main shaft bearings, and balanced the main rotor blades, the Jesus bolt kept breaking occasionally (and the main rotor blades with the main rotor head block flew out of the main shaft) upon auto mode landing when the motor interlock was disabled by the system.
The transition from full rpm to idle is very sharp when motor interlock is disabled (Unlike the transition from idle to full rpm, the throttle servo’s arm move slowly when the motor interlock is enabled).
Does anyone know how to slow down the transition from full rpm to idle when the motor interlock is disabled? If I have to modify the source code, which code is safe to be modified?
I don’t have a solution for you but I would think that issue is handled mechanically with a one way bearing on 99% of helis. Even full size manned aircraft use a one way bearing system to allow the rotor to spin faster than engine drive.
There are one way bearings on the main gear of this heli. I think there might be something wrong with yours or I am misunderstanding your post. A sudden engine slowdown should not be sheering the jesus bolts.
Can you free wheel the main rotor on the ground with the engine off in the flight rotation direction?
Yes, I can turn the main rotor in the clockwise direction (look from top to bottom) when the engine is off. The Tail rotor also turns when I rotate the main rotor in that clockwise direction. It did not sheer off the Jesus bolt each time, but occasionally. The drop of the rpm is very suddenly as I can see the throttle servo arm move from full rpm to idle immediately when I disable the motor interlock (I did tested the engine in a bench and saw the engine went to idle or quit when the motor interlock disabled) I use an external governor.
That should not be able to cause a jesus bolt to fail ever. It sounds like there is something else mechanically wrong with the heli. Maybe the oneway bearings need to be lubricated or gear mesh isnt right. The motor should always be able to stop in flight with out causing damage though. There are helis designed as an acceptation but looking at the manual this isn’t one of them.
I dont think arduheli has anything built in to slow decelerate the motor. You could us RC passthrough to directly control throttle I guess but that has its own issues.
Your description of the failure sounds wild. That would get my heart going. Just to make sure im on the same page though you are saying the bolt that holds the head on is being cut off because the main rotor momentum and the abrupt stop of the main shaft right?
you are saying the bolt that holds the head on is being cut off because the main rotor momentum and the abrupt stop of the main shaft right?
Yes, the Jesus bolt holds the head to the main shaft got sheared occasionally, I guess it is caused of the sudden drop of the engine rpm and the momentum of the main blades using H_RSC_Mode = external governor.
When I enable the motor interlock (in external governor mode), I can see the throttle servo’s arm move slowly, not instantaneously from idle to full rpm, the source code which handle these should be able to be modified for motor interlock disable.
That doesn’t necessarily mean that the one-way bearing is working correctly because there is also the centrifugal clutch. When the engine is not running, the clutch is disconnected, so you can still turn the main rotor even if the one-way bearing is locked. I’ve seen this kind of problem in helis where the oil on the bearing was old and started to resinify. It’s also possible, that the one-way bearing doesn’t let go after it was enabled under load once. So you should definitely carefully check or better replace that bearing. Also check the rotor shaft, if it has the correct diameter and is still unscratched in the position of the bearing.
I agree with @spova, that there has to be something mechanically wrong with your heli. It has to be possible to shut the engine down very fast in case of an emergency without damaging things or even losing the main rotor. As long as the freewheeling mechanism is working correctly, that shouldn’t cause any damage.
One other thing: Normally (and also in your case), the lower part of the rotor head assembly is split in two halves so that it can be clamped on the main rotor shaft with high force. That means, that the load on the actual Jesus bolt (which is running through the shaft) should be fairly low. As far as my understanding goes, it just helps to prevent the assembly from slowly wandering away, while the load is transferred by friction.
That means, you should definitely also check the two screws that are providing the clamping force and you should remove any grease or oil that is in between those parts.
If the rotor head is not clamped down to the shaft correctly, that would cause the head assembly to repeatedly bump against the limits of the Jesus bolt and eventually cause a fatigue fracture. Maybe that is what happened in your case.
@steveross1234 I think that @LupusTheCanine is correct that the H_RSC_SLEWRATE should slow the movement of the servo.
But I strongly agree with the others in that I don’t believe that the fast movement of the throttle servo for engine idle cut off is causing your problem. I also think that it has to be something mechanical.
I didn’t tighten the two bolts (clamp on the two halves of the lower rotor head) very
tightly because I need to remove and re-install the rotor head for using swashplate
leveling tool from time to time due to possibilities of minor crashes which caused the
breakage of the cyclic servo arms. I did put some grease between the rotor head and
the main shaft to help the removal and re-installation of the rotor head. I tighten the two
bolts which just allow the removal and re-installation of rotor head with some frictions but
not making the the alignment of the Jesus bolt holes (main shaft and rotor head) very
difficult.
What should I do on the two bolts to make the removal and re-installation of rotor head
possible but not causing the shearing of Jesus bolt?
Thanks for clarifying that. I’m afraid, there is no workaround. You have to tighten the two bolts before flight, to make sure there is enough friction between the main rotor shaft and head to transfer the load. In flight there are many sudden load changes (in torque and also in vertical direction), that would make the head constantly bump against the limits and finally break the Jesus bolt.
I have to admit, that in your heli the problem isn’t solved in the best way. The manufacturer tried to reduce the part count by giving those screws two tasks: clamping the rotor head to the shaft and simultaneously holding the swashplate follower arms. I don’t really like that, as I would want to tighten the clamping screws with high torque, while on the other hand I wouldn’t want to press against the bearings of the follower arms too hard.
But as you are having problems with the Jesus bolt breaking, you should definitely tighten those bolts harder. You can check afterwards if the arms are still moving freely.
When I understand you correctly, you don’t loosen those clamping bolts to remove the rotor head? When you can pull off the head by hand that means, you have way too low friction and all the flight loads go through the Jesus bolt. It’s probably not designed for that.
I think, you should carefully remove all the grease between the rotor shaft and head (you can use a paper towel and some IPA for example) and tighten those two clamping bolts properly. I understand, that you are looking for an easier solution for faster removal, but I’m afraid, you have to unscrew the clamping bolts every time aswell. Also make sure to use threadlocker.
If you are looking for an easier way to level your swash plate, you could use a magnet and a nail instead of the swashplate levelling tool. That way you wouldn’t have to remove the rotor head every time you want to relevel. Just snap the magnet to the rotor shaft and attach the nail diagonally to it to find the highest point. The method is described in this video: https://www.youtube.com/watch?v=oA-9FFYC6Ek&t=423s
Thank you very much for the precious info. I just replaced the main gear hub (it has two one-way-bearings), as it has not been replaced for 2 years. The bearings on the main gear hub cannot be removed even after I used torch to heat the hub and press the bearings with Arbor press. It must has used some retaining compound when the factory installed them, so I have to replace the whole main gear hub. I did put some automatic transmission fluid (ATF) on the one-way-bearings. The MA Interceptor’s manual suggested to use synthetic grease, I did that in the past, but this time, I used thinner ATF. I also tightened the two clamping bolts on the lower part of the rotor head much tightly than before, Previously, I can remove the rotor head without loosing the clamping bolts by pulling the head after I removed the Jesus bolt. I think I will have difficulty to remove the rotor head from the main shaft now. What should I do if the head stuck on the main shaft after loosening the clamping bolts? It happened to me before, I tried to rotate the head on the main shaft, but it seemed to be weld on the main shaft, the two halves of the head clamped on the main shaft very tightly even after removing the clamping bolts. I hope all these steps will help to stop the shearing of Jesus bolt.
That all sounds very good. I’m confident that the Jesus blot won’t be sheared off again.
I also normally grease those bearings. But since you had problems with it, it’s probably a good idea, to use thinner ATF.
Actually, that shouldn’t happen. I’m wondering, if either the shaft or the head is a bit off dimensions. If it’s the shaft, that would explain, why you’re also having problems with the one-way bearing.
The good thing is: If the rotor head holds to the shaft so tightly, you’re definitely not going to shear off the Jesus bolt again. If you have to remove the rotor head and you can’t pull it off, just use a bit of WD40. That should creep between the parts and free them up again. When you’re done, you can use some IPA and a paper towel to remove the WD40 before putting everything back together. Just make sure that you don’t get the WD40 on the bearings as this could wash out the grease.