250 Blade balancing

[zenmetsu]

You can see that it hops around a bit at start up. I am beginning to believe that it is just due to the blades not being pulled outwards in the grips at low speed. I'll just have to learn to be a little quicker on the throttle while keeping the spool up smooth. The plus side... zero tail rotation in this video... yay gyro.

 

[Tony]

Yeah, that wobble in the beginning was caused by the blades off tracking. Not an issue there. I did see a little vibration when the heli spooled down though, but again, not an issue. There are a couple issues I saw though. One, it spooled down way too fast. I'm thinking either you have a bearing in the main blocks that is not spinning free, or the one way bearing is not spinning free when it's free spinning. As long as the bearings are good, don't worry about the one way, just thought I would mention. The other issue is your throttle curve. 85% until you get lift? Wow. I think it's time to take a couple hours and set up that head. You should have 0º pitch at mid stick, and about 10º positive and negative pitch at high and low stick in idle up. This will let the heli come off the ground at about 65% throttle in normal mode.

Yes, it's a royal pain to setup the head, but it will be much better once you do. Bird looks great bro, now lets see it flying

 

[zenmetsu]

It probably is the one-way bearing. I probably could clean it up a bit... maybe even put some lube in there and run the motor backwards to break it in... (just kidding). Is there anything that would cause the bearing to bind, or should I go look for a new one?

The head is set up just fine mechanically. As for my pitch/throttle curves, I have mine set up this way in the DX6i (obviously not idle-up settings):
Throttle (L,1,2,3,4,H) : 0, 40, 85, 90, 90
Pitch (L,1,2,3,4,H) : 50%, 47%, 50%, 75%, 100%

The reasoning behind this is that flying at low head speed is going to result in stability problems. I want to rely on rotor pitch for lift and not head speed, so keeping a nearly flat head speed like they do with real helicopters is desired. I can easily get the thing into the air at 50% throttle, but head speed will drop once the rotor pitch is increased. Once I build a decent tachometer to measure head speed, I can tweak my throttle curve more by measuring head speed at 100% throttle, 100% pitch, and then working backwards to find the throttle setting that gives the same head speed at zero pitch when there is minimal loading on the motor. (if that makes sense) I suspect that this isn't needed though since the ESC should be controlling speed and not power input. Definitely a different story on fuel powered units though.

I also did not want negative pitch at lowest stick because, well, parking the blades into the blade retainers is MUCH easier when there is zero rotor pitch. The little bit of negative while I am spooling up is just there as a precaution to help pin the helicopter to the ground in case a gust of wind comes along... I'm not even sure that it would help in the end.

 

[Tony]

Yeah, you don't want to oil the one way with just normal oil, but it sounds like you already know that lmao. If you want to replace it, Align all the way. As for the main block bearings, they should spin free with no grit.

When you said 85%, I thought you meant stick position, now I see you are talking about throttle position. It's all good. Video Time!!!

 

[zenmetsu]

main block bearing is smooth as silk. It has to be the one-way. That may be a bit of a b***h to find locally. I might just order a new main gear that includes the bearing just so that I have more spare parts.

 

[Tony]

If it's just the one way bearing, then dont' worry about it. This heli can't auto rotate since the main blades and tail blades are not locked together. As long as it's spinning under power and not slipping, and the main gear is straight, I would just fly the crap out of it.

 

[zenmetsu]

Thanks. I didn't even think of that. I eventually want to get into auto rotation. I didn't think that tail authority would be critical during R/C autorotation since you aren't applying torque to the shaft via anything fixed to the airframe. Hell, maybe I'll try to auto this thing anyways once I get better.

 

[zenmetsu]

You nailed it Tony... the bearing above the main shaft block was not so happy. I loosened it, cleaned the shaft underneath, and then locked it back down. The rotor now spins much more freely. Now the wobble during deceleration is more pronounced since the rotor spins longer. Dunno what that is about since I have the two blades balanced down to 0.01g and the centers of gravity are exactly even... must be the head itself that is off balance.

 

[Tony]

There is a natural harmonic on ALL helicopters (including full scale like the Blackhawk and Chinook) during spin up and spin down. It's just the way it goes with helicopters. As long as there is no vibration when you are flying, you will be fine.

And we totally thread jacked this thread. lmao. My bad for not catching that sooner.

 

[Irishrover]

Hello from Austria.
i have a question that maybe some of you blokes can answer for me.

I had a heavy landing with my TREX 250 and the training gear snapped flicking up into the main blades causing all kinds of crap..... I have rebuilt it and have noticed the Feathering shaft is a little off...

Question is, will the vibration form this be enough to make the gyro act funny. Its just that before the Heavy Landing it was fine then its as twitchy as a mad thing...!
ts the only thing I can think off...
Thanks

 

[zenmetsu]

A bent feathering shaft will cause the blades to track incorrectly. This will, in turn, increase the torque required to drive the rotor. This increased torque will cause a shift in the left/right bias (for instance, in clockwise rotating main rotor, you will need more right rudder to compensate for the added torque). Since you probably adjusted your tail rotor center position to the lower torque configuration, the heli is now fighting a leftward tendency. You will likely need to replace the feathering shaft, move your tail center position (and thus the end travel) to produce more right-rotation, or reduce gyro gain to prevent tail-hunting (which is what mine does when the tail is off center by a fair amount).

 

[Tony]

The feathering shaft, if it's bent, will cause all kinds of problems. From blades not tracking correctly, to vibrations, it's never good. I would pull the head apart, check it and see if it is in fact bent. If it is, it's time to replace it. No getting around that one. And yes, the vibrations can cause abnormal behavior with the gyro.