Twin rexx build tips

[billyd]

Hi guys I thought we might start a thread of Twin Rexx build tips and ideas. I have started building mine and have a couple of suggestions to start out.

1. Be sure to install the servos as a first step of the building process. Attach them to the frame plates prior to assembling the front and rear frames. This will make life alot easier as installing them afterwards is alot more difficult.

2. When applying CA to the miter gear and sleeve assembly, be sure to remove the m2x20 screw before the CA sets, otherwise it could get stuck and you could break off the head of the screw trying to remove it. (Yes this happened to me). Better yet just stick your hex drive through the hole and don't even use the screw when applying CA.

3. I had to open up the slots for the landing gear pin support plates as they were so tight I couldn't assemble them. This is a good idea anyway as later on you will want to be able to adjust the landing gear legs so that they are perpendicular to the ground. If the slots are too tight, you will have no adjustment. With the slots opened up you can rotate the gear into position and them use goop glue to set them in place.

4. I couldn't get less than .03 backlash in the drive shaft because the slots in the rear frame weren't long enough on my model. I would suggest lengthing the slots by another .03 to .06 inches prior to assembly such that the overall length of the heli increases. This will allow you more room to be certain to achieve the necessary backlash of .02 or less.

That's all I have for now. Please feel free to add to the list or comment. This is a great model and an excellent design. I am very impressed with the overall quality of the frame. Looks like it could survive a 20 foot drop without damage. Which with my pilot "skills" is a good thing. :lol:

[Gra55h0pper]

Good idea! Here's one from me. The Twinn Rexx assembly manual figures .pdf document as posted on the tech-mp web site actually contains very hi-res pictures. When printing it, the pictures come out fairly small. Displaying it on your computer however allows you to zoom in (easily to 400%) showing all the detail you might need. It helped me figure out how the "flybar limit kit" exactly needs to be installed (only needed in case you plan to fly with the fuselage).
See attached zoomed-in jpeg as taken from the online manual.

[billyd]

I've progressed a little further and wanted add a few more points...

5. When setting up the links, there are two special links for the model which attach to the lowest servos on each rotor assembly. The threaded bar supplied by tech-mp is a larger diameter than the standard Align link bar. I found it helpful to pre drill the links with a 1/16 drill about quarter to half way into the link to make it easier to get the thread started. Otherwise it's real easy to strip out the link. (Yeah I did that too :o).

6. Regarding the rearmost servo on the rear rotor... I installed mine in the lower portion of the frame to allow use of the body (I assume most will do this.) However the figures show that servo mounted with the actuator closer to the center of the assembly. I found that if you mount the servo with the actuator closer to the rear of the helicopter, the link was more vertical which is better for keeping the swash level throughout the range of the collective. The only caveat is that if you do this you will need to shorten the supplied threaded bar about a 1/4" to make it work.

That's all for now.
ps. I still haven't quite figured out how to use the flybar limiter Gra55h0pper, so please feel free to elaborate on that. Does the rubber tubing go through the rotor head above the seesaw? I can't see it. (i skipped over that until I can figure it out, but I do have the 1 mm hole drilled.)

[joe@tech-mp]

Some comments on your notes....

4. There should be plenty of adjustment. This dosen't sound right.

5. The links we supply for the P and E links are already drilled 1/16.

6. The stock setup for the rear E-link is perfectly vertical. There is no better way to do it.



Joe

www.techmodelproducts.com

[billyd]

Hi Joe. Thanks for your post. I would love it if you have the time to describe the flybar limiter installation. I have guessed how it goes in, through the 1mm hole with the wire and the double tube passes through the rotor and above the flybar, but that leaves me with alot of extra tube and cable, plus the picture is only from one angle so I am unsure I have it right

Item 4. It's pretty simple because the rear frame has slotted holes and the aluminum tube has slotted holes (in the rear sides), and that is the only adjustment available that I can see. Am I missing something? On my model with the rear frame pulled as far back as it can in the associated slots, there is still .03 slop between the miter gears and their nearest bearings.

Item 5, is strange, since the links supplied for P and E did not have .06 holes in them out of the bag in my kit. They were standard Align links, same as the bagged Align links supplied for the standard rotor links. I would guess the the holes were about .04 or so.

Item 6. I have the ball mounted on the servo arm 13 mm from the center and the rotating part of the servo oriented toward the rear and the link is vertical right to the swash, with all rotor links and rotor installed and adjusted perfectly.

[joe@tech-mp]

For the limiter your description is correct. The extra materrials are just that.

Call me to discuss the other issues further. Too much to type.


Joe

www.techmodelproducts.com

[billyd]

Thanks Joe.

Regarding item 6. I found the discrepency. I never bent the rear link bar. I missed that step in the instructions. There seems to be enough clearance with the body kit as is (to the worst case servo arm position), so I will leave it alone.

[chinookmark]

-The kit includes three spacers to customize the motor mount spacing. I suggest using all three. There's still plenty of pinion engagement for smaller motors, and you need all three for the JGF 500.

-The instructions say to use Deans micro connectors on the BEC so you can set up the electronics without involving the ESC and motor. That's great and all, but that's the only thing I have with Deans micro's. If I were to do it over, I would just just Deans Ultra connectors (or what ever you use on your batteries).

-The ESC is mounted near the nose, and the battery will probably end up at the rear for balancing. Keep this in mind when wiring the aircraft. On my setup, the battery wires just barely made it to the ESC wires. Not a problem during initial flights, but I had to add 3 inches of wire to the ESC to connect with the fuselage installed.

[billyd]

Good post. I ended up using all three spacers as well. It's hard to imagine a motor setup that couldn't use all the spacers. The motor mount has ALOT of adjustment and even with all three spacers my motor (500T) still had plenty of adjustment to the inside of the mount.

Another issue I ran into was during the reversal of the rotor parts on the front rotor. Be careful when pressing out the bushing on the washout base (metal rotor parts only). After I pressed mine out and flipped it around, I had to ream it out a little to get it to run smoothly on the main shaft. I guess it got a little out of round during the process. I used the align tool for removing autorotation sleeves and it seemed to work pretty well, except it wasn't a perfect fit on the washout base. Just be sure to check it for smooth and free movement on the main shaft prior to getting everything assembled.

[chinookmark]

Tip: Use a good soldering iron, good solder, flux, and take your time to do a good soldiering job.

I rushed and used a crap iron and got some cold solder joints on the ESC. Results: smoked motor and fried ESC.

[billyd]

In addition to mounting the servos prior to assembling the front and rear frames, it is also a good idea to install the body spacer mounts early if you intend to use the body. Alot easier to install them onto the plates before the rest of the assembly.

Also, you should mount the laser cut balsa body support frames onto the main spine before final positioning of electronics, wires and battery. I will have to go back and relocate and move wires in order to fit the balsa frames onto the spine.

Regarding servo postioning, the default constants file supplied does not default with all servo constants at zero. I would suggest when you are ready to program and load the TH2, remove all servo arms from their servos. Load the constants into the TH2 and then set all servo centering constants to zero. Then, turn on your electronics, set your throttle pitch to zero, and mount all your servo arms into their desired (perpindicular) positions as close as possible to ideal. Then just some minor adjustments to the servo centering constants gets you perfect.

[billyd]

Hi guys I thought we might start a thread of Twin Rexx build tips and ideas. I have started building mine and have a couple of suggestions to start out.

1. Be sure to install the servos as a first step of the building process. Attach them to the frame plates prior to assembling the front and rear frames. This will make life alot easier as installing them afterwards is alot more difficult.

2. When applying CA to the miter gear and sleeve assembly, be sure to remove the m2x20 screw before the CA sets, otherwise it could get stuck and you could break off the head of the screw trying to remove it. (Yes this happened to me). Better yet just stick your hex drive through the hole and don't even use the screw when applying CA.

3. I had to open up the slots for the landing gear pin support plates as they were so tight I couldn't assemble them. This is a good idea anyway as later on you will want to be able to adjust the landing gear legs so that they are perpendicular to the ground. If the slots are too tight, you will have no adjustment. With the slots opened up you can rotate the gear into position and them use goop glue to set them in place.

4. I couldn't get less than .03 backlash in the drive shaft because the slots in the rear frame weren't long enough on my model. I would suggest lengthing the slots by another .03 to .06 inches prior to assembly such that the overall length of the heli increases. This will allow you more room to be certain to achieve the necessary backlash of .02 or less.

That's all I have for now. Please feel free to add to the list or comment. This is a great model and an excellent design. I am very impressed with the overall quality of the frame. Looks like it could survive a 20 foot drop without damage. Which with my pilot "skills" is a good thing. :lol:


Please ignore item 4 and do not increase your slot lengths. I was way off base there. Unfortunately I can't edit the original post. The backlash is taken up by the gears, not the bearings. No excuse, just plain stupid on my part. Joe straightened me out, but it took awhile.

[Gra55h0pper]

Tip: Spend your time getting the mechanical setup right before fiddling too much with eletronics settings, mixes, trims, auto-pilots (:D) etc. It makes all the difference in the flight characteristics of the heli!

E.g. 1: if the heli doesn't stay level in the elevator plane during hover (both rotors should fly in the same plane) check the swash-plates to see what's really going on. (When test-flying and checking, it's very important to have the DCP gyro in rate mode as otherwise it can "hide" what's really going on).
- If the CP of the rear rotor is different from the front rotor (with the CG in the middle), then that's what needs to be fixed on the linkages. If you try to fix it by adding elevator trim on the transmitter, you may to be improving the helicopters handling, but you are not fixing the problem at the source which means it will never reach its full potential!
- If both swashplates slightly tilt forward or backward with the elevator stick centered (make sure the driveshaft is level with the horizon as you check this), also then it should be fixed on the linkages. If you try to fix it by adding elevator trim on the transmitter, then don't forget that you are changing the DCP as well (as DCP is mixed in from the elevator). Also here, the problem should be fixed at the source.
- If one swashplate slightly tilts forward when the other tilts backward, then make sure to fix that first, try to hover again and then find out whether to mechanically change the DCP or elevator.

E.g. 2: if the heli doesn't stay pointed in the same heading during hover, don't try to correct this by using ruddder trim on the transmitter (unless perhaps you only need tiny amounts which cannot be fixed mechanically anymore). As the torque of the (counter-rotating) rotors is cancelled out on a tandem when in hover, the real problem lies in the mechanical setup. Examine the swashplates and find out which one slightly tilts right and which one slightly tilts left (thereby inducing yaw), then fix it by adjusing one of the linkages. A very small amount can make all the difference.

I think this is one of the main differences between a regular heli and a tandem.
- On a regular heli you can effectively fix roll and elevator centering issues using transmitter trim (even though also there it's always best to have a look at the mechanical setup first). Most yaw issues are handled by our trusted heading-hold gyros...
- On a tandem, using transmitter trim is really only effective for fixing roll centering issues. For elevator/DCP and yaw centering, you really should try very hard to fix it as much as possible by changing the mechanical setup -> Try not to have to touch the elevator trim and rudder trim on the transmitter.

Bottom line: always optimize your mechanical setup first (easiest to do on a day without wind), then fine-tune the electronics (transmitter trims, mixes, heading-hold modes on gyros, increased gyro gain, auto-pilots (:D), elevator-yaw mixing, etc.). I recently setup my Twinn-Rexx from scratch (after I re-positioned the rear elevator servo) and it became all too apparent to me again how important having a good mechanical setup really is...

My $0.02.

H0pper.

[joe@tech-mp]

H0pper,

Well said ;)


Joe

www.techmodelproducts.com

[tungym]

Agree.

It also makes life easier if 2 pairs of blades with similar weight are used.

My bench test tells me that I was wrong to put front pair of carbon and rear pair of glassfiber blades. The different momentum makes rudder control difficult.

Anyone expereince ground resonance as well. I did ! The rear head is shaking violently like in the famous youtube movie. Turn out to be my mistake of not having tighten up the rotor hub and the bearing blocks.

[tungym]

I spend hours to figure out where the vibration originate in high rpm. Blade balanced, miter gears clearnace, rotor hubs tight, feathering shaft checked,......

Later I find out the vibration comes from the too high headspeed >2000. I also strengthen the junction of the aluminium and the fiber frame with CA.

Then, it hover quite stable but my GY 401( on yaw control, without remote signal wire connected) keeps hunting(shaking the swashplate too much) a lot. Since I only spare a 6Ch spektrum receiver, I will try to buy another cheap HH gyro (with on board gain) for yaw control ( As the cheap Aing 500X locks the DCP quite well).

[Gra55h0pper]

Then, it hover quite stable but my GY 401( on yaw control, without remote signal wire connected) keeps hunting(shaking the swashplate too much) a lot. Since I only spare a 6Ch spektrum receiver, I will try to buy another cheap HH gyro (with on board gain) for yaw control ( As the cheap Aing 500X locks the DCP quite well).

tungym - I'm pretty sure leaving the 401 signal input unconnected will indeed cause this. The electronics need a fixed voltage input or "erratic" behavior will result.

I'd suggest going with a 240, even though I find the 401 to work better on rudder, the 240 certainly works fine also and doesn't need a gain signal input. Note that I'm not convinced yet from my side anyway that a HH gyro can be made to work in HH mode (on rudder, i.e.). If it can, I doubt it will be a cheap(er) gyro. Consensus seems to be that the 240 doesn't work in HH mode (rate mode only). Results with the 401 in HH mode are mixed so far.

[v22chap]

I first used my 401 on rudder without the remote and it worked fine ,,It probably depends on the setup of the machine ..
And Grasshopper the setup you talk about is gospel to tandems ... I think this may be the glitch like deal I have going on now ... This next weekend(baring no more grand babies and son moving into a different house and needing carpenter and plumbing done ) I hope to start from scratch again on the setup of my tandem .:thumbup:

[chinookmark]

If you use a 401 (or similar) gyro without the remote gain plugged in, what gain setting will it run at?

[v22chap]

It doesn't say in the manual ,,but other gyro's do ... it goes to rate mode (no heading hold ) and you use the regular ATV of the TX rudder to set the sensitivity . I have never setup one this way ...but know from what I did on the 401 test that it will work with out the remote . Will have to test it more and see what happens with it unhooked and what controls the sensitivity for sure on the 401 .
Good question .:thumbup: