T-Rex 250 tail fix

 

OK I know a few have been struggling to get acceptable performance out of the T Rex 250 tail.

Here’s what I found.

I have the habit of running this sized model up carefully in my hand to feel for any vibrations, check tracking and tail function and when I did this with the 250 and observed the tail pitch slider movement everything was fine until a certain head speed was reached at which point only a very small movement happened but you could also clearly see the tail pitch bellcrank flexing as it tried to transfer the servo movement to the slider. (this explains why some guys are stating the bellcrank is too flexible!)

Fundamentally the bearings in the tail blade grips are not up to the job and lock up at high rpm, this is due partially to the size of the bearings but also the quality, I think you can get away with small cheap bearings on mixing levers where there is only a small load on them and they are been used in the right sense to take radial loads but they don’t work well on tail rotors when trying to cope with thrust loads which they were never designed to do.

The tail grip bearings  seem to only work one way and they need to fitted so the bearing cages are facing the tail centre hub, you can confirm this on your model by carefully pushing the bearings out of the grips and insert a cocktail stick in the centre of the bearing while holding the bearing between your fingers, spin the stick and feel how smooth the bearing is (or not) and then try again while gently  pushing on the stick, mostly likely the bearing will feel ok in one direction but flip it 180 degrees and do the same and it will feel bad!

With all the bearings fitted the correct way, any locktite washed out of them and lightly oiled, I found my tail to be better but still not good by any means.

Next up was to acquire some better quality wider shielded bearings ( the std ones are open so can get full of locktite easily) as these take up more space in the grip it does push the head of the retaining bolt up so that it hits the root of the tail blades but there is enough material on the tail blades to allow some to be filled away (just)

I ran the model up in my hands and confirmed visually that the grip bearings were no longer locking up at high RPM.

My tail was 100% better and I can now actually 3D the model although I think some fine tuning of the gyro/servo is still needed as it will still blow out if your not careful.

I suspect guys have been able to partially get around this problem both though luck if their tail bearings are some of the few good ones but mainly by strengthening the tail linkage and running a much more powerful tail servo so just forcing the grips to move, obviously if they did all this and had functioning grip bearings to their tail would be even better!

The standard grip bearings (item No.132 in the Manual) are 1.5 x 4 x 1.2mm, yep only 1.2mm wide, the wider ones I fitted were 2mm wide, other sizes stay the same otherwise modification of the grips would be required.

 

The Heli Stig