Microheli CNC T-Rex 450 Tail rotor

Written by Ashley Davis Monday, 21 February 2005 00:00

Tail Rotor

The following is a review of the MicroHeli Precision Tail Rotor

The first part of this review will center around the build quality of the Precision Tail Rotor.
Initially some good close up photos of the Tail Rotor to give you a feel for the CNC quality.

Now, anybody who has paid any attention to the media (including this site) will be aware that the early Precision Tail Rotors were shipped with incorrect washers retaining the blade grips, this was an assembly error at manufacture, steel washers were specified in the design. There may still be some of these faulty tail rotors in circulation. Before installing and using the tail rotor check that the washers holding the blade grips to the central hub are either brass or steel, if they are aluminium they are not capable of sustaining the centrifugal forces acting on the tail rotor and you will get a blade grip failure. Please contact This e-mail address is being protected from spambots. You need JavaScript enabled to view it to obtain new washers (if you find your washers to be made of aluminium). An easy check is to use a magnet, aluminium is unaffected by magnetic forces.

If in any doubt please see the announcement on the front page of the MicroHeli website.

OK, public warning service out of the way, on with the review.

The tail rotor is nicely engineered and has a quality feel both in the finish and the smoothness in the bearings. The CNC machining is very nice indeed and the whole unit has a solid feel about it. There is very little slop or play in the unit as a whole. Those are the positives in terms of build. The tail rotor also comes with a new shaft, however, to use this the tail belt gear would need to be glued/loctited to the shaft in a similar fashion to a motor pinion as the supplied shaft has no splines.

On the negative side, the tail blades are held on using an 'M2' bolt and nut, whilst this is perfectly adequate I would have preferred to have seen a nyloc nut, in this case some loctite is a necessity. In most cases the forces acting on the screw will try to keep it in place rather than throw it out but I wouldn't want the nut coming off at high speed!

Secondly the blade grips themselves are held on with an #0 bolt and washer, personally I would have preferred an 'M2' in this position. To be clear the #0 can quite adequately deal with the forces placed upon it in this application but an 'M2' would be much more conservatively within it's loading limits. Before installation of the new tail rotor I would recommend just checking the #0 blade grip bolts for loctite, it is good practice to check this kind of thing on ready assembled goods.

Addendum: Having discussed this with MicroHeli they have changed the blade grip bolts on new Precision Tail Rotors to be an 'M2'. The reason was that if a washer were to fail the cap head on an 'M2' would not pass through the bearing like the #0, so it is an extra safety precaution. I have a new tail rotor to review but I can't review this change currently as my review model has the #0 bolts.

I removed my old tail rotor hub and directly replaced it with the MicroHeli unit. This is a one for one swap and the MicroHeli hub is correctly sized to fit onto the stock tail shaft. So no need to use the free supplied one, although the supplied one is slightly longer (which would allow for more pitch throw on the tail).

Below is the initial mounting picture ....

Having attached the unit to the hub the retaining grub screw can be loctited and tightened and the ball links popped onto the blade grip balls. In terms of build that is all there is to it.

Initial Flight Testing

My inital hover tests showed that this tail is certainly behaving differently to the stock Align unit. The gyro settings were immediately noticeable as not being correctly set for this tail. The weather wasn't being particularly kind and trying to setup the gyro in 10mph winds isn't something I would advise to the feint hearted. I settled for setting up non heading hold mode as the Align gyro is known to be a little bit more difficult to get going properly on HH mode.

Having set the gain for non heading hold things settled down quite a bit and I was able to get a feel for the new tail response. Initial thoughts were that the overall power of the tail was exactly the same as the stock unit, which is unsuprising given that the pitch change mechanism is unchanged by this upgrade. The ability of the tail to respond to very minor stick movements was more aparent and the tail had an overall more refined feel to it. I had noticed on the ground that even tiny stick movements were equating to an adjustment in the tail blade pitch which is normally lost in the slop on the stock unit.

My initial thoughts are that this upgrade has brought a more refined and precise tail response but given the weather conditions I would want for calmer weather to experiment with some more advanced moves than just hovering.

I'll be doing some more advanced flight testing and then subjecting the tail to some more extreme forces on my bench test, hopefully the tail will stand up to this type of testing now that it has the brass washers fitted.

Hovering and Circuits

Since my inital flight tests I have had some problems with getting HH mode to work without tail wag. This turned out to be the fact that the MicroHeli balls on the tail blade grips are slightly larger than Align balls. This means the ball links are a little stiff and introduces tail wag. I remedied this by giving each ball link a firm squeeze with some pliers whilst it was still on the ball. A few minutes later and all was freed up and tail wag was gone.

I also changed to a CSM SL420 micro gyro to get a better gain setting and locked on hold. This I certainly achieved. Having ironed out all the issues I reduced the yaw rates and set about having a proper feel of the tail response. Certainly my inital observations of a more precise response for very small stick movements is in evidence. For larger movements the tail feels exactly like the stock unit.

To get a true comparison I will switch between tail hubs on my next flights to see how the tail response changes. For the moment I'm happy in that the tail is performing perfectly with rock solid heading hold and perfect stops with no bounce on short sharp stick movements. Of course this could just be the gyro, only continually swaping the stock and upgrade hub will give me the information I need to draw final conclusions.

More on the hub swapping later, now for some stress testing.

Stress Testing

In order to properly test the overall stress that this tail rotor can take I needed to make it's job that much more difficult. The easy answer to this is headspeed. So armed with a totally inappropriate pinion and a nice powerful motor I set about upping the headspeed to really place some stress on the tail rotor system. I topped out at 2850 RPM on the head, any faster was going to cause some vibration problems plus sitting next to the thing going at that speed wasn't all that comfortable.

Once at full throttle I tried large tail rotor pitch changes to load it up. The tail coped perfectly well with this abuse. The only permanent damage being to my nerves. A close inspection of the tail post stress test revealed no evidence of imminent failure or fatigue. Based on this I think it can cope adequately with the most agressive flyers 3D moves.

Final Thoughts

I've taken a long time to come to the conclusions on this test. The tail on the T-Rex is an interesting piece of engineering. In stock form it appears to be sloppy but when run up to speed and matched to a good servo and gyro this is hidden by those same components. Unfortunately, matched to a poor servo or gyro the dreaded tail wag appears. This meant having to spend some time working out what was actually performance being facilitated by the MH tail rotor and what was just a good gyro and servo making up for the inadequacies of the stock tail. After various flights trying different flying styles it was quite by accident that the conclusion of this test came about. I was trying to iron out the problems of running governor mode on the 400DH and suffering tail wag due to the motor surging. It was during this problem that I did a hub change to the MH tail and suddenly the difference was quite apparent. The gyro and servo matched to the stock tail was realy struggling to keep the tail together through the oscillating motor RPM, making the tail wag and kick with each oscillation. With the MH tail fitted this problem reduced, it was still there but you could see the difference, the servo and gyro were coping much better. This might seem like an odd way to end the test but for me it proved the point. There must have been a performance improvement as the MH tail is such a solid and slop free system, finding the level of that increased performance just required a slightly unorthodox approach.

Before you decide to buy the MH tail rotor in order to fix that tail wag problem I would caution you to stop and think. A large number of the tail wag issues are down to the mechanical setup of the tail, a cheap gyro or a slow servo. If you are confident that these issues are not a part of your problem then changing to the MH tail will give you a more locked in tail response and 'possibly' get rid of that wag. A far better reason to buy this tail is to get a better and more precise responding tail rotor as that is the focus of this component. It is entirely possible that if your tail wag issues are mechanical then installing the MH tail could make it worse as all those nasty imperfections in your linkage system will be translated directly into tail pitch changes rather than being lost in the slop of the stock unit.

---

Add this page to your favorite Social Bookmarking websites