Twinn Rexx owners' thread

[chinookmark]

I couldn't fly all weekend, but between bouts of helping my son with his Pinewood Derby car, I finished mockup #1 of my three-blade rotor head. Don't ask how it flies -- it's made out of balsa. I did learn a little about what won't work, what won't fit, and how small of a part you can't make with balsa. I've got drawings for the prototype that I'm going to give to a machinist friend tomorrow. It'll probably only cost me a six-pack, but it'll probably also take 2 months to make. :P

[v22chap]

It'll probably only cost me a six-pack, but it'll probably also take 2 months to make

and be a 6 bladed instead of 3 and egg shape:thumbs-up2:by the time he is done with the 6 pack :thumbup::noteworthy

[billyd]

This post is meant for Joe but anyone can pipe in.

I've been staring at the frame for awhile tonight, specifically the adjustment in the rear frame. As I've said earlier I was unable to remove enough backlash and I still have .03 slop in the drive shaft. Also, I am not happy with the engagement in the teeth of the miter gears at the rear end of the heli (I'm not in love with the front engagement either but it is the much better of the two). This is because I have the rear frame extended as far back as it will go to remove the backlash.

Here's what I want to do and you tell me if it's a bad idea. I want to bring the rear frame forward to improve the mesh in the miter gears, and then slot the holes that retain the rear driveshaft bearing block to remove the backlash (I would use star type lockwashers to keep the block from sliding in the slots). Do you see a problem with that? I think you should consider slotting the rear drive shaft bearing block holes in the kit (as well as the rear frame slots) to remove the backlash. This allows you to control the miter gear engagement as well, which in my mind is at least as important, as a poor mesh will lead to premature gear failure. Also since the drive shaft itself is not adjustable (obviously difficult) just moving the rear frame throws that relationship out the window.

Please let me know your thoughts. I know you asked that I call and discuss it, but you're busy and I'm busy and this way we can respond when we have the time. Also I think it's good info for others building the rexx.

[tungym]

Some questions:
1) Any reason why the front rotor in CC? I think Joe has some reason behind, o/w it isnot make sense to do additional modification to the 1-way bearing.
2) How much negative collective pitch is needed?

I hope I can finish it this weekend. I am building a flybarless Chinook, after some success with a Flybarless Trex450 of 2 gyros (without aileron/roll gyros). I think the DCP gyro (elevator) is a must. Do you think roll or yaw gyro is more important?

[joe@tech-mp]

BilyD,

Again, something sounds wrong. The rear frame screws should be near center in the slots.


Joe

www.techmodelproducts.com

[billyd]

I will take photos tonight and post them. It's very possible I made a mistake. But it's a mystery to me what it might be. It just doesn't seem that complicated and the instructions were very clear.

[billyd]

Ok Joe here's my Twinn rexx. I took a bunch of angles and from different directions.

Let me know if you see anything amiss. I know I have to rearrange a few things to fit the canopy.

[joe@tech-mp]

Looks to me like the rear frames need to slide forward a bit ( 2 to 3mm).


Joe

www techmodelproducts.com

[billyd]

It finally sunk in to my thick skull. I was worried about the bearings supporting the drive shaft axially, when you are using the gears to do that. Sometimes I am so stupid. Sorry for wasting your time. I have it now. :oops:

[chinookmark]

Some questions:
1) Any reason why the front rotor in CC? I think Joe has some reason behind, o/w it isnot make sense to do additional modification to the 1-way bearing.
2) How much negative collective pitch is needed?

I hope I can finish it this weekend. I am building a flybarless Chinook, after some success with a Flybarless Trex450 of 2 gyros (without aileron/roll gyros). I think the DCP gyro (elevator) is a must. Do you think roll or yaw gyro is more important?

1) Interesting point. I suppose for ease of use of readily available parts, it might make more sense to reverse the aft rotor. Real CH-47's forward rotor rotates CC, and since it's a scale bird, might as well follow suit.

2) -3 to -4 should be fine. Probably at least -8 if you're planning on flying inverted. :D

3) I have no experience with flybarless or multi-blade heads. With the flybar heads, a roll gyro is completely unnecessary. If it wasn't for the DCP, you probably wouldn't need one on the pitch or yaw, either. Because of the DCP, both DCP and yaw gyros are needed.

[chinookmark]

GYRO TEST

I swapped out the yaw gyro with a RCT-G730 gyro. For those unfamiliar, it is very similar to a 401, but smaller, lighter, and less expensive.

http://www.rc-tek.com/us_store/rc-tek-rct-g730-digital-gyro-p1798.html

Results: WOW! I thought the helicopter was stable before. Now it stays pointed in one direction. It hovered for close to 30 seconds with no input. I have to say, it takes a little bit of the fun out of it. On the plus side, I'm not afraid to nose-in hover the Twinn anymore. It also makes forward flight more predictable.

I didn't mess with it too much. Gain seemed happy at about 35%. At 30, it drifted a touch, and at 40, it wagged. It didn't wag the tail like a Trex will with the gain set too high, but you could see the rotor disks flapping back and forth. It was much calmer at 35. I kept the gain at 50% in rate mode.

With no tail slider to look at, judging Limit setting was a little bit of a guess. In rate mode, I set the limit just inside where the swashplate would bind, so the servos wouldn't buzz.

I tried the Delay at various settings, but didn't go below 50. I didn't notice any major difference in response speed between 50 and 100. At lower settings, it did seem to "bounce" recovering from abrupt yaw inputs. That is, if I gave a sharp right yaw, the gryo would overcontrol and send it back to the left, farther than when I released the input, then correct itself to the right. This might sound like a gain problem, but I don't believe so. The gryo isn't overly sensitive, but the helicopter builds a fair amount of yaw momentum, and can't quickly stop itself. I believe the delay helps here, because it gives the helicopter some time to slow itself while the gyro is correcting the movement, if that makes any sense. It did seem to "bounce" less at 100, than at 50.

Maybe it was just my imagination, but the helicopter seemed to have a lot more yaw authority, or at least yaw speed, than with the GY240. I can't say for sure, as I didn't do back to back tests. Since this is a scale bird and I'm not planning on doing any piro-flips, I turned the rudder channel enpoints down to 75 in the TX. This gyro functions just like the GY401, so the endpoints control piro speed. Recovery from a fast turn is pretty ugly, anyway.

CONS:
You need a separate channel on your reciever for the gain. That was my last open one on my AR7000 -- the one I was planning on using for steerable tail wheels, an opening ramp, or cargo hook release. Now I'm wondering if I should get a X9303 2.4 or wait for the 12X 2.4. LOL

[joe@tech-mp]

The rotors can at your choice rotate as desired, we chose the front CCW and rear CC to keep the rear rotor stock as a frame of reference since the rear shaft is stock. Just flip them as desired without consequence. Of course the motor and one-way bearing need to follow.

Inverted flying is an unresolved issue at this point. Yaw is done by differential roll which does not change when inverted but yaw function does. Think about it...... ;)



Joe

www.techmodelproducts.com

[chinookmark]

BODY MOUNTING

I had the body mounted with just the four screws, but it's time to do it right.

I had to pull of a signifigant amount of electronics to mount the wood formers. Now I'm going to have to rearrange some things, and hopefully I can still get a good CG. The aft former limits the amount of room for the battery, which I had almost all the way back. I've been using a 3S for the last week, and I'm pretty sure the 4S was mounted farther forward.

I originally drilled the four mount holes at the marked location on the body. The real chinook sits noticably nose-high on the ground. My fuse was pretty level with the ground. To get more of that reverse rake, I could've shortened the aft landing gear, but I didn't want to reduce the amound for forward tilt of the rotors. If anything, I wanted to exaggerate that tilt.

In order to tilt the body more nose-high on the frame, I had to shape the tops of the aft landing gear plastic struts. Half an hour with some #120 sandpaper had a nice slant on the plastic, sloping outboard. That allowed me to push the body farther down on the rear of the frame. I drilled four new holes for the mounting screws. The aft holes about 1/8" - 3/16" higher than stock, and the front holes 1/8" lower than stock. The nose of the body still has a 1/4" of clearance to the frame, but any higher and the forward body post wouldn't have any plastic to mount to.

It's not too drastic, but it does add a more noticable nose-high stance to the bird.

[chinookmark]

Inverted flying is an unresolved issue at this point. Yaw is done by differential roll which does not change when inverted but yaw function does. Think about it...... ;)



Oooohh ..... hadn't thought about that. That means in addition to whatever you have to train your brain and fingers to do upside down, you also have a gyro that's working in the the wrong direction. Can we say uncontrollable spin?

[v22chap]

Mark

Maybe it was just my imagination, but the helicopter seemed to have a lot more yaw authority, or at least yaw speed, than with the GY240. I can't say for sure, as I didn't do back to back tests

I felt this too on the 401 I used ,, but didn't work with it enough to be sure ...
I now have a front up and down bob that seems to be a servo glitch thing going on and can't find what is doing it . It is getting bad enough that I don't dare even lift off to hover ... and it seems to be only the front pitch servos that are doing it .:confused::thumbdown:

[tungym]

Any idea on the model weight , with and without fuselage?

Hovering and cruising current?

Need to plan the power plant, Lipo....

I get the 500T from techproduct and it is pretty light and muscle. You can see how tight the copper wires are packed inside.......much more than a 450T/TH.

500T 1900kv / 4S 2200mAh / 10T/ HS 1750 estimated
500T/ 3S 2200mAh / 14T / HS 1800

450T 2950kv / 3S 2200mAh/ 9T / HS 1890

[billyd]

BODY MOUNTING

I had the body mounted with just the four screws, but it's time to do it right.

I had to pull of a signifigant amount of electronics to mount the wood formers. Now I'm going to have to rearrange some things, and hopefully I can still get a good CG. The aft former limits the amount of room for the battery, which I had almost all the way back. I've been using a 3S for the last week, and I'm pretty sure the 4S was mounted farther forward.

I originally drilled the four mount holes at the marked location on the body. The real chinook sits noticably nose-high on the ground. My fuse was pretty level with the ground. To get more of that reverse rake, I could've shortened the aft landing gear, but I didn't want to reduce the amound for forward tilt of the rotors. If anything, I wanted to exaggerate that tilt.

In order to tilt the body more nose-high on the frame, I had to shape the tops of the aft landing gear plastic struts. Half an hour with some #120 sandpaper had a nice slant on the plastic, sloping outboard. That allowed me to push the body farther down on the rear of the frame. I drilled four new holes for the mounting screws. The aft holes about 1/8" - 3/16" higher than stock, and the front holes 1/8" lower than stock. The nose of the body still has a 1/4" of clearance to the frame, but any higher and the forward body post wouldn't have any plastic to mount to.

It's not too drastic, but it does add a more noticable nose-high stance to the bird.

I have to move alot of stuff around too, to fit the wooden formers to the spine. I am thinking of putting some abs plastic beams spanning between the front and rear landing gear and laying some of the electronics and wiring on those rather than using only the spine. This allows for an easier (roomier) arrangement, which will all be hidden by the canopy. I figure I will be able to use less tie wraps cancelling out most of any weight increase.

[chinookmark]

I was thinking about adding a sort of shelf to the backbone. I like your frame idea. Maybe CF instead of plastic?

[billyd]

I've completed the assembly portion of the body and now to finishing. But I decided against creating a large hole for the battery compartment and instead will provide access with an actual hatch. This will require a light frame and magnets. I will post pics when completed. (could be awhile).

Also in flying with the body on, even with a hatch or a hole it is impossible to connect the battery and get the heli still and level prior to the gyros initializing. I like the heli to be sitting perfectly still when I arm it. Perhaps this is not important on the 240 gyros? Anyway for convenience I have added a Vampower pro jones witch : http://vampowerpro.com/store/pid_125.htm

This will allow leaving the battery connected and simply arm the switch and fly. The hatch will allow access to the gyros and removal of the battery. I am thinking that perhaps the hatch should go in the side of the heli (perhaps make a scale access) instead of the bottom. Still haven't decided. I have the switch connected electrically and am in the process of mounting it mechanically to the rear frame such that the arming portion of the switch will protrude from the body.

[Gra55h0pper]

billyd - I think you have a good point about initializing the gyro's with the heli sitting horizontal possibly being somewhat of a problem. I guess I'm going to have to raise the heli by putting a box (or something) under each landing gear and then connecting the battery from below as the heli is sitting level. Hmmm, need to think about this...

[v22chap]

I just lay mine upside down on the rotors and change the batteries . the rotors let it set without falling over even ,,let it initialize and it is o.k. ,,,, It don't know if it is right side up or what ,,, just still and level ,,,,,, now that I think about it maybe that is the glitch I am gettting ;):roll::thumbdown:

[billyd]

billyd - I think you have a good point about initializing the gyro's with the heli sitting horizontal possibly being somewhat of a problem. I guess I'm going to have to raise the heli by putting a box (or something) under each landing gear and then connecting the battery from below as the heli is sitting level. Hmmm, need to think about this...


Yeah I had put the body on (no paint) and went outside to test fly it, and it was like .... duh how do I connect the battery now? lol. To make it funnier I hadn't even put a hole in the bottom. How stupid can one person be? Anyway, I have a few of these jones witch's laying around so I decided the Twinn Rexx needs a power switch. Much better now. I use these in several of my large planks and they are great. Especially in models where it's a huge pain to remove the hatch or wings to turn on the model.

Unfortunately I see they (vampower) are out of stock, I have an email to them now... wonder what's up with the stock on those. It's a very simple little device just uses a standard snap fit connector mounted to a plastic bracket. You could make one without too much trouble.

[michaelfsB4MHX]

Hi Grashopper, it's been a very informative thread. My Twinn Rexx should arrive in the next few days, and I'm in the process of gathering up all the bits I need. Could you tell me what 4S battery packs you are using and what blades.

Thanks...Mike

[Gra55h0pper]

Michael - You're addressing me, but please note that actually this thread was started by Chinookmark (I've started another one here on the forum, also very informative... :D).

I'm running a Flightpower 4S 2170 Lipo with the CF Align 325 blades (Align part# HS1231). It works great for me (I figured you can't go too far wrong with these blades) but please note that I haven't done any comparisons with other blades so there could be better choices out there. From my side, I'd be curious whether anybody has tried 335 blades?

Chinookmark: what are you running? :D

[michaelfsB4MHX]

Thanks Grasshopper, sorry if I have offended anyone, I've been reading so many threads on the Twinn Rexx lately that I must have lost direction. Sorry Chinookmark.