T-Rex 450 FAQ

Executive summary: “Everything you ever wanted to know about setting up the T-Rex 450 electric helicopter”

Reference: You'll find most of the info on this page in the manual, if you read between the lines: Page 17, “Equipment illustration”, “Assembly standard position illustration”.

Main rotor setup

Q:Why is it so difficult to set up the main rotor mechanics?
A:It's not! The setup procedure is logical and straighforward, if one only knows that...

... as a 3D heli, the T-Rex is designed symmetrical. That means:
At 0 degrees blade pitch, all “moving parts” are in center position.

Main rotor (swash) setup, step 1

Both when setting up a new heli from scratch, or repairing after a crash:
The first step is to temporarily change the transmitter settings so that it forces all swash servos to zero pitch position:

Go to the CCPM mixing (swash) menu and set "pitch" to 0 percent. Now the servos are forced to center position, and don't respond to the collective stick
Alternatively, move the collective stick all the way down, and set the lowest point of the pitch curve to 50 percent

This is only temporarily, for setting up.

Main rotor (swash) setup, step 3: Swash servo arms

With the servos forced to zero pitch position - as in the previous step - attach the servo arms, so that they form a 90 degree angle with their pushrod, as accurately as possible.
Note that there are two possibilities to attach a two-sided servo horn, and four for one with four sides:

Fine-adjustment of a four-sided servo arm by turning it 90 / 180 / 270 degrees

Pick the one that comes closest to a 90 degree angle before cutting off the other sides.
The length of the servo arm should be about 15 mm for HS-65 servos. At this length, it should point straight upwards, when the servo horn is exactly horizontal.
The pictures below shows how to assemble the horns and ball links (note, two on the inside, one of the outside of the servo arm). My servo arms are one hole longer than recommended, about 16.5 mm. If so, they should still be set to 90 degree angle at this stage (which is not the case in the pictures).
If the servos have metal wheels, remember to use threadlock also for the servo horn screws.

left swash servo

right swash servo

back swash servo

Main rotor (swash) setup, step 4: Swash servo subtrim

In practise, it is impossible to achieve exactly 90 degrees by mechanical adjustment alone.
Use the subtrim function of the transmitter to fine-tune each servo to a 90 degree angle with its pushrod.

Main rotor (swash) setup, step 5: Linkage rods at default length

The manual gives default lengths for the linkage rods. Reset (A), (B), (D) now, according to the manual or the table below:

(A) blade holder to control arm: 20 mm

(B) flybar to washout: 23.5 mm
(D) control arm to swash: 42 mm
(M) swash to left servo: 31.3 mm
(N) swash to right servo: 45 mm
(O) swash to back servo 40.3 mm

Please adjust (A) very carefully for equal length. A mismatch of one turn may result in a finger's width of blade tracking error.
The (B) links are usually not a problem - set them once and forget them.
But the (D) links tend to “creep”, after several blade tracking adjustments (for example, when changing blades). It is often a good idea to reset them to default length before adjusting blade tracking.
(M), (N), (O) don't really matter, we'll set them in the next step anyway:

Main rotor (swash) setup, step 6: Swash plate

First, change the two push rods to the front side CCPM servos so that the swash plate is level. A swash plate leveling tool could be used, but is not required.
Next, set the height of the swash plate so that blade pitch is zero degrees: Look at the heli from the side, the blades should “look as one”, as in the picture below. Don't rely on a pitch gauge, it is too inaccurate.

At zero pitch, blades are parallel (overlap)

You may gently (!) push the swashplate down, against the resistance of the servos: Observe, which way the blades move - if the angle between both blades gets bigger, the swash plate needs to move up.
Although this process is tedious - one has to repeatedly remove and reattach the backside ball from the swashplate, in order to change the length of the elevator push rod - please do this adjustment as accurately as possible (+/- one turn).
When finished, the blade pitch should be exactly 0 degrees and the swashplate is level.

Main rotor (swash) setup, step 7: Setting pitch range

Now enable the collective stick on the transmitter: Set the CCPM/swash “pitch” mixing back to for example 50 percent.
Further, set a linear pitch curve.
Program a linear pitch curve, 0-25-50-75-100 on most radios.
I'd actually recommend to use this linear pitch curve in all flight modes: it's logical for a 3D heli, and makes maintenance easy. But that is a matter of personal preference. In any case, we need the linear pitch curve at least temporarily for the setup procedure.
Then, move the collective stick to either end, and compare the blade pitch. When the setup has been done correctly, they should be equal.

Full positive pitch

Full negative pitch

If maximum and minimum pitch are not equal, check the lengths of the linkage rods against the values given in the manual. At zero pitch, the washout and control levers should be horizontal:

Washout levers should be horizontal at zero pitch

Control levers should be horizontal at zero pitch

Blade tracking, part 1: Basics

Blade tracking means, that both blades run “on the same circle”.
If for example one blade has more pitch than the other, it will run high. Blade tracking causes vibration problems and makes the heli difficult to fly.
As a rule of thumb, blade tracking error should be less than one blade thickness. In other words, if a gap is visible between blades, tracking should be adjusted.
The pictures show how to identify which blade is high / low.

Blade tracking: one blade marked on the front edge

Blade tracking. Here, the marked blade runs low

The tracking error in the second picture was caused by one full turn on the ball link connected to the blade holder.

Put a piece of dark adhesive tape over the front edge of one blade, to show which one is which.
Spool up the heli, and watch from a safe distance (!). Maximum blade separation appears around zero pitch.
Maximum separation appears around zero pitch. Find out, whether the marked blade is running low or high
Stop the heli, fold back the blades and check your full positive and negative pitch setting. They should be equal, in reality they aren't. Which end has more pitch?
If the heli has too much positive pitch, lower the higher blade.
Alternatively, if the heli has too much negative pitch, raise the lower blade.

This procedure avoids a common beginner mistake, where repeated tracking adjustments add up and affect total pitch.
Blade tracking is best adjusted through the “long” linkage rods between swashplate and control levers. One turn changes tracking by about one blade thickness. Increasing the length raises the blade.
In severe cases, one may adjust the “short” linkage rods between control levers and blade holders. Half a turn offsets blade tracking by 7..10 mm. Adjusting those should be avoided, check the length instead with a caliper rule. Increasing the length lowers the blade.

Blade tracking, part 2: The Voodoo

Q:Help! I can't get rid of the blade tracking problem. I have tried everything, this does not make any sense!
A:There are cases where it is simply impossible to get reliable blade tracking:

Is a feathering bolt loose? WARNING this is dangerous.
Is the main shaft bent?
Are the bearings in the blade holders worn, damaged or not glued to the bladeholder?
Is the feathering shaft bent?
Are the damper rubbers worn?
Are the blades bolted down too tight?
Are the blades Align glassfiber type? (get some woodies or carbon fiber instead)

Blade tracking, part 3: Bits and pieces

Blade tracking may change, if blades are swapped (solution: Mark one blade and one blade holder, and keep them together).

Blade tracking may vary randomly, if the blades are mounted too tightly. Either loosen the bolts, or run it briefly at full throttle or do some flips etc to straighten out the blades before checking.

Align glassfiber blades are notorious for tracking problems. Just avoid them.

Q:Should I worry about blade tracking for the tail rotor?
A:Usually not. If a problem is visible, some part is bent and needs to be replaced.

Checking for a bent mainshaft

Q:Maybe the main shaft is bent, but I'm not sure. How do I check it?
A:Remove the main rotor (i.e. upper Jesus bolt and links to swashplate).
Spin up the heli to moderate speed. Hold the fingernail of your thumb to the main shaft, just above the upper bearing.
Now hold the fingernail close to the tip (just below the hole for the screw). If the vibrations are worse, your main shaft is bent. If in doubt, replace.
If the main shaft is bent, it causes vibration problems. Also, it affects blade tracking (because the whole rotor runs now at an angle).

Check feathering shaft

Q: I'm not sure, maybe my feathering shaft (the steel rod through the rotorhead that connects both blade holders) is bent. How do I check it?
A: Remove the blades.
Use a hex key to turn either of the feathering bolts (the screws that attach the blade holder to the feathering shaft) clockwise.
The shaft will rotate inside the rubber dampers.
Feel for any “wobble” of the blade holders. If there is even a small wobble, replace it.
A bent feathering shaft will cause random blade tracking problems.

Checking the rotor head dampers

Q:Why should I worry about the dampers? My flying isn't that wild yet...
A: Unfortunately, the dampers may wear out rather quickly. When that happens, the whole rotor gets pulled by centrifugal force to either side, and blade tracking problems result.
Try to pull either bladeholder away from the rotorhead. If a gap shows, the dampers are worn and should be replaced.

Ball links

Q:Are those ball links symmetrical?
A:No, they are not. The opening is wider to one side than the other, that's where the ball should be inserted. For most adjustments, it's sufficiently accurate to change length by a full turn.
An exception are the links to the blade holders: already half a turn will make a noticeable difference.
When lenghening a push rod, there is a tendency that all the thread remains on one side, and too little is left on the other. Hold the steel rod with pliers, and turn both ends by the same amount.

T-Rex The Frog

Q:When I hover, the heli doesn't stay at one altitude. Suddenly it jumps up or down. What's wrong?
A: One likely reason is that the main shaft has slop in up/down direction. This is easily checked by holding the rotorhead in one hand, the body in the other, and pulling / pushing.
If needed, move the main shaft collar to get rid of the slop.

Belt problems

Q:My T-Rex 450 SE V2 is chewing up belts! After a few flights, the belt runs to the side on the tail end and frays

A: Apparently, there is a design problem with the T-Rex 450 SE V2 tail.
Removing the tail idler pulley (the silver aluminium wheel that runs on top of the belt) usually fixes the problem.
The Mavrikk belt is highly recommended instead (397 teeth). Or get it from Readyheli.com.

Setting up the gyro

Set the gyro to rate mode via the gyro channel of the transmitter. This moves the rudder servo to center position.
Next, attach the servo wheel so that the rudder pushrod forms a 90 degree angle (picture).

Set a 90 degree angle in rate mode

Then, hover the helicopter still in rate mode. Observe, which direction the heli drifts (pirouettes):

If the nose turns to the left, move the rudder servo slightly towards the nose
Or, if the nose turns to the right, move the rudder servo slightly towards the tail

This adjustment should be done repeatedly, and as carefully as possible. The servo needs to be placed with an accuracy better than one mm.

Next, set the servo endpoints or limit: With the rudder stick at either end, the tail pitch slider should use the maximum possible travel but not cause any binding.

Finally, switch to heading-hold mode. Increase the gain until the tail starts to twitch or wag. Reduce the gain slightly.

Note: The above instructions follow those in the manuals of most modern gyros. However, one important exception is the Futaba 401.
The manual apparently suggests to move the servo to give equal travel of the tail pitch slider in positive and negative direction.
The above procedure has been tried with Futaba 401 and gives good results, but it may lead to a limited negative pitch range.

Components, parts, upgrades

Last but not least, this is what I'd buy for my own T-Rex 450:

Radio

For example Futaba 7C or Spektrum DX7. The 6-channel models work fine, too.
Before buying any radio, check this thread regarding radio latency. When I buy my next radio, a measured low latency will be one of the deciding factors.

Swash servos

Hitec HS-65 MG. Those can take a beating.

Rudder servo

Futaba 9257

Speed controller

The Align 35X (i.e. 6 V BEC) because it's cheap, or Kontronik Jazz.

Motor

Neu 1107/2Y/3.2H on 13 tooth pinion (3s), 11 tooth pinion (4s). This is a huge improvement over the Align 430XL motor.

LiPos

Outrage 2200 25 C 4s
Outrage 220 25 C 3s (maybe even 30 C)
No el-cheapo-LiPos like ZippyMax, they don't last.

Gyro

Spartan ds-760
Logictech 2100

Landing gear

The Align nylon upgrade part (chrome plated). The chrome plating has come off, but my autos haven't broken this one yet.

Flybar paddles

3 g carbon fiber from helidirect.com

Washout assembly/levers

PFMOD from helidirect.com. More accurate and improves roll / flip rate quite a bit.

Blades

Align CF or SAB 0390.

Tail pitch slider

Align HS 1175 (two-sided version). Less slop than in the one-sided tail pitch slider.

Belt

Mavrikk belt.

Pinion

Rhino hardened steel pinion. The Align brass pinions are too soft and don't last long.

CNC tail support clamp

I got one mid-air boomstrike when the plastic part that attaches the boom supports to the boom (under the horizontal stabilizer) broke in flight.

Rotorhead dampers

Trueblood (note, different version V1/V2)

© Markus Nentwig 2007-2008
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Please send me a mail! mnentwig@elisanet.fi