Written by Ashley Davis Sunday, 25 February 2007 00:00
The following is a review of the Align T-Rex 600 upgrade tail rotor hub. These are a direct replacement for the radial bearing blade grips and hub provided with the kit. There is no information provided with this upgrade as to what performance enhancement can be expected having fitted this item to your machine. However, with the new thrust races in this upgrade the tail should operate more smoothly. Additonally the grips are longer which will provide a larger tail rotor disc.
First some pictures of the upgrade grips:
Whilst this upgrade doesn't come with any instructions the align website does include a diagram showing the correct assembly of this upgrade part. I have included this below.
The main differences between this tail rotor hub and the original kit rotor hub are that this rotor hub has thrust races and the blade grips are slightly longer. These two items allow for a smoother operating tail (due to the better load handling provided by thrust races) as well as a larger rotor disk due to the longer blade grips. Obviously a larger rotor disk means more power for the tail rotor. The central hub is made of stainless steel and therefore should be more than adequate for the blade forces acting upon it. The blade grip root also contains a radial bearing as is normal for a thrust raced tail blade grip. I dismantled the tail rotor hub to check that it had been assembled correctly. I did find a problem in the build, the outer thrust race diameter on both races is identical. This will cause problems as the blade grip does not widen at all for the outer race. Effectively the thrust race has no room to work as a thrust race and the outer race will bind on the blade grip. In order to remedy this I ground the outer race outer diameter down by 0.5mm which then provided clearance for the thrust race to work properly.
I have a GY611 gyro on my T-Rex 600 and I have always run it in F3C mode as this provides the best tail rotor stops at the sacrifice of pirouetting consistency. Having just changed the tail rotor hub and nothing else my initial takeoff into the hover revealed a high-frequency tail wag. After numerous adjustments I was unable to remove this wag and instead decided to try 3-D mode. Having switched it 3-D mode the wagging reduced but didn't disappear. I dropped gain down to 34 which again reduced the effect but still the wag was present. I may experiment further by reducing the tail blades to Radix 92mm but for the moment I'm living with this wag problem.
I run Radix 95 mm blades and the longer grips do bring these quite close to the ground. Some care has to be taken to place the T-Rex on even ground otherwise the tail rotor can hit the ground. However, the overall length of the blades is not greater than the depth of the vertical tail fin. Therefore running 95 mm blades is still perfectly acceptable it just requires a little care. However, the machine doesn't seem to like it and tail wag has become a constant problem.
These issues aside I think the tail is both more powerful and can hold better due to that power. This improvement is overshadowed by the wag, whilst the tail holds nicely the wag does not inspire confidence. As soon as I can source some 92mm blades I'll be trying those as a potential remedy.
Flight review Update - 7th March 2007
After a lot of messing around I gave up trying to fix the hub and ordered a new one. Fitted it and straight out of the packet it works fine, no wag and running on 95mm tail blades. Go figure !!!!
Larger tail rotor disc
Better tail hold
More powerful tail
Error in thrust race design on the first one purchased, second one is OK.
Persistent wag with 95mm tail blades (only on the first unit, again second unit is wag free)
This new hub is clearly a better design (putting aside the thrust race error). The improvements being thrust raced and designed to provide a larger tail rotor disk. However, it's performance in flight whilst more powerful and providing a better hold is overshadowed by persistent wagging which cannot be removed through configuration or setup. I will try 92mm blades as a last resort but overall I'm not impressed. A 50 size machine ought to be able to run 95mm blades without inherent wagging, in this case this seem to be unachievable. Dropping to 92mm will reduce the overall power and hold once again and I'm reluctant to sacrifice those just to get a wag free tail. Overall I'm unhappy with this as an upgrade and in many respects the stock hub, whilst not as powerful, provided a better hold overall as it did not wag.
Conclusion Update - 7th March 2007
The new hub purchased suffers none of the wag problems, holds great and has plenty of power. This leaves me with a dilemma. How many are good hubs and how many will wag and be problematical? My advice is this, if you fit it and you get wag when you didn't have wag before then just take it back and exchange it. If you get a good one then it's a vast improvement on the original design, tail hold is much better and overall tail power is much better.
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