Twinn Rexx owners' thread

[Gra55h0pper]

Very cool tungym! Seems you've got it well dialed in. Also nice to see you have a 3s 1800 setup, vs. my 4s 2170 setup. Seems you've got plenty of power. What kind of flight times are you getting?

Also very cool that you have HH on rudder to work. Seems you're not getting the oscillation problems I was getting with the 401. Can't wait to fly mine and experiment a little more with the delay, limit and Tx ATV settings.

[tungym]

Grasshoper, did you connect the yellow signal wire for GY401?

I am using cheap Esky HH gyro 704A ~$30 USD.

I just wonder what else should I tune. I am experimenting the 70% ELE->CH5 mixing for DCP . On bench testing, it seems higher mixing ratio makes the DCP move faster without the "slow motion". No actual flight test with >70% DCP mixing , at least till I accustom to the flying skill.

1800mAh I got 5 mins flying around. 15T/500T/ 90% flat throttle curve. I am not sure whether the governor on the cheap ESC (Himodel 45A ~$18 USD) works.

I use Align 3A linear BEC with 7.4V tapped from the balancing plug of the 3s lipo. Clamp current takes 3.5A as Joe comment. I like the Align BEC as it has voltage light on it. When the red light glow, it is time to land.

[v22chap]

I think a little higher head speed will take care of the drop when you apply rudder ...as that should not happen .

Looks like you have it flying pretty good :thumbup:

[MadDogDan]

h0pper,

I am in the middle of my TwinRex build now. I should have it up in the air in a week or so. The only batteries I have now to use are 4S 2500 25C's. Is this overkill? The footprint of the batteries are rather samll and they ballance the heli quite well.

Any advice would be greatly appreciated,
MadDog

[chinookmark]

MadDog, if the battery fits, I think it would be fine. You're not going to push that battery at 25C lol!

[chinookmark]

I got the replacement 500T and new Scorpion 45A ESC installed. The ESC is pretty long, so I mounted it on the bottom, and moved the BEC up to were the ESC used to be. I had the whole thing disassembled to mount the wood/foam body supports.

I ended up doing strips on the body, instead of the tabs. I cut a cargo hole in the bottom and reinforced it with a plastic doubler. It's larger than scale, but not nearly large enough to remove the battery. I can plug it in, and there's enough room for a cargo hook, so that's all I'm concerned about. For glueing the body, Acetone works great. Faster, easier, and cleaner than CA. Just hold the two pieces together, and used a Q-tip to apply a drop at the edge. It will wick into the joint, and the parts are bonded in about 10 seconds.

I did a little experimenting with blade phase adjustments. With the blades roughly 45 deg apart, they are a lot quieter than at 90 deg. Unfortunately, the "brap" is gone, but quieter usually means more efficient. And there's room to use 335 or longer blades (Rotortech 350 anyone?) and a 220mm flybar.

[billyd]

Yes the acetone is alot more convenient. However I have found that if you sand the mating pieces together, the CA bond is a little stronger than the acetone bond. (But don't sand the mating parts when using acetone !!). Using continous tabs, there is no question the acetone method is more than adequate.

I ended up cutting the hole in the bottom too. I put a power switch in the side of the body as well, so scale is no longer an issue (the switch is partially removed after engaging it, so it is very low visibility). (It is a jones witch from vampower.com, excellent device). Accessing the battery connector to power on and off is just too inconvenient with the tandem! I even drilled a small hole in the side to access the dsp gyro avcs switch and gain (for adjusting pitch with the body on). Is it just me or did everything change when the body went on??? My goal is once the body is completed and installed, I hope never to have to remove it, except in crash repair.

After thinking about it for awhile, my modelling skills are such that worrying about scale issues is really a waste of time for me. In fact I am thinking about painting the body in some wild scheme, along the lines of a metallic purple with flames down the sides. If I can't make it look like a real CH-47 I might as well have some fun with it. :YeaBaby:

[chinookmark]

The body looks cool, but it sure does get annoying! I like the idea of a power switch, and maybe add a charging tap, too. I'm with you on the DCP peep hole. I had to turn down the gain when the body went on.

As for paint schemes, how about Coast Guard? High visibility, relatively simple .. call it "could be" scale. I'm starting to reconsider painting mine like my real chinook. I hate black fins and skids on my Trex. I can't imagine trying to fly a completly OD green helicopter.

[v22chap]

Do a search in http://www.airliners.net/search/photo.search?front=yes&s=1&keywords=chinook

There are some cool civilian paint jobs out there

[Gra55h0pper]

Grasshoper, did you connect the yellow signal wire for GY401?

I am using cheap Esky HH gyro 704A ~$30 USD.

I just wonder what else should I tune. I am experimenting the 70% ELE->CH5 mixing for DCP . On bench testing, it seems higher mixing ratio makes the DCP move faster without the "slow motion". No actual flight test with >70% DCP mixing , at least till I accustom to the flying skill.

1800mAh I got 5 mins flying around. 15T/500T/ 90% flat throttle curve. I am not sure whether the governor on the cheap ESC (Himodel 45A ~$18 USD) works.

I use Align 3A linear BEC with 7.4V tapped from the balancing plug of the 3s lipo. Clamp current takes 3.5A as Joe comment. I like the Align BEC as it has voltage light on it. When the red light glow, it is time to land.

tungym - Yes, I have the gyro signal wire (for setting the gain and for switching between HH and rate-mode) connected. I checked the documentation but it doesn't seem to say anything about a capability to leave it unconnected so I'm not sure what gain would really result.

[Gra55h0pper]

h0pper,

I am in the middle of my TwinRex build now. I should have it up in the air in a week or so. The only batteries I have now to use are 4S 2500 25C's. Is this overkill? The footprint of the batteries are rather samll and they ballance the heli quite well.

Any advice would be greatly appreciated,
MadDog

MadDog -- I'm with Chinookmark. You'll be fine with the 4S 2500. It's only 15% more than my 4S 2170.
What I'm not sure about is whether the additional capacity will lead to extra flighttime. I don't think anybody has really done any experiments in that respect yet.

[tungym]

Try adding the flybar metal weight (~1.8g). It makes the Tandem much more stable and real. I can off my fingers off the Tx for 10 seconds now.
You may need to reduce the gyro gain then.

The front rotor vortex is rather interesting. I intentionally reduce the pitch gyro gain <40% to see the effect. Funny enough, as Joe said, at certain FF speed, the rear rotor suddenly drops ~20cm, DCP try to compensate by reducing the front rotor collective as well, this results in the whole bird drops by >30cm in a sudden. First I though it is low voltage throttle cut. Then I remember it is what Joe mentioned. You must increase the throttle stick (as much as 2/5 throttle ) to regain altitude. This is real fun to see it happens!

I also observe another interesting phenomenon that does not happed in single rotor heli. When I do a fast piro, the whole bird seems obtunded after turning 180degree. I guess the front rotor turned into a position where the rear rotor was before. The turbulance will then counteract the coming rotor a bit.
Anyone can confirm this?

I will try to do some stall turn , just like those video in the Air Show.

[v22chap]

I finally got a test on the 401 without the remote gain wire ...It works ,,but there is no where to set the gain .. it just seems to be stuck somewhere around 50 % ,,, so I wouldn't recomend running the 401 without the remote wire hooked up .

[chinookmark]

Questions for you guys with the 500T:

1) What advance do you use?
2) What PWM control frequency should I use?

I've got a new Scorpion 45A ESC, and I'm just wondering what settings to use. Right now, I have advance and PWM both set to auto. Available setting for advance: 5, 15, 25 degrees. Available setting for PWM: 8, 16, 32.

[Gra55h0pper]

Chinookmark -- I'm not sure whether this really helps you, but for my 500T I'm just using the default settings in my Align 35A ESC. I'm not sure what they are though and have to say I haven't tried any other settings.

[v22chap]

I think I have found my glitch ... Well atleast I flew one short hover in the garage before I went to bed last night and it seemed to work .
I found an ail servo on the front that was acting funny . Traveling further than the rest was what I spotted first and after a quick up / down collective it would lag behind and have to go both ways past neutral to finally center . I unhooked the gyros to make sure it was still doing it and it was ,,,so that seems to have taken care of the nose boobing like it use to .
I replaced it and now I am putting all my wires back into cable protectors and will try it again .Just to check for sure :thumbup:

[tungym]

"Aerobatic" flying of Trex tandem. Quite windy today ~20km/hr.

Also checked with eagletree , hovering <10A @3s , 18A max. Max 180W only !!!

http://www.youtube.com/watch?v=uIXWUlWvVKY

[v22chap]

Tungym
Looks like you are having way to much fun with the tandem ... good job .
Thanks for sharing .

[billyd]

Well I ended up turning off governer mode in my esc. That combined with the th2 and all of that mixing was driving me nuts trying to get this bird tuned in.

It seems to fly best in fixed endpoints mode and a simple 0 25 50 75 100 curve on throttle.

What do your throttle and pitch curves look like? Right now I am running

0 25 50 75 100 for throttle and
36 43 50 67 84 on pitch (to get -4 at min stick, 0 at mid and +8 degrees at max)

Anybody playing with it? I had switched to governer and worked ok for awhile, but when I put the body kit on it got all wacky. Once I went back to fixed endpoints everything was fine.

[chinookmark]

pitch: -4 0 9 degrees
throttle: 0 75 80 85 90

I've never used governor before. On my other helis, the throttle curve looks more like 0 85 85 85 85, but the Twin seems to respond better to a rising curve. I like to get the rotors up to speed in the first 1/4 stick.

[v22chap]

billyd
I played with gov for a little and mine too went wacky ,,even in stock frame . , but then again I have a glitch thing going on that I can't find ,,probably related to vibration so higher RPM in gov mode for me would do that !

My setup is throttle : 0-69 -75- 95100 ,, I too like to get the rotors up quick and it seems to like it better ..

pitch : 0-24- 28-60 -67 to get -4 ,,, +8

Ch Mark ,, your throttle curve is almost gov mode anyhow !! only using the pitch curve to do it .

[tungym]

billyd
I played with gov for a little and mine too went wacky ,,even in stock frame . , but then again I have a glitch thing going on that I can't find ,,probably related to vibration so higher RPM in gov mode for me would do that !

My setup is throttle : 0-69 -75- 95100 ,, I too like to get the rotors up quick and it seems to like it better ..

pitch : 0-24- 28-60 -67 to get -4 ,,, +8

Ch Mark ,, your throttle curve is almost gov mode anyhow !! only using the pitch curve to do it .

Balancing the twin rotor is quite challenging as a single rotor can vibrate without much amplification. In twin rotor, any small vibration will "amplify" due to the front and rear rotor/ tower are binded together in a "stiff" frame.

In my own DIY frame, most of the time, vibration is more pronounced in the rear tower. I have to increase the frame stiffness by adding larger washer on the external side of the fiberglass frame. The torsional vibration is also minimize by adding a piece of 2x2cm fiberglass on top of the rear tower (just above the servos and front of the swashplate). This completely resolve my vibration issue.

I have been thinking of putting a piece of rubber sheet between the area of the fiber glass frame and the aluminium tube. Since my vibration issue is solved with frame strengthening, I drop this idea.

In my last video, I have to use a complete pitch range +/- 10 degree in windy condition ~20km/h. Otherwsie, the tandem will FF perfect headon wind, but will interestingly drop in altitude in down wind direction ( I don't know why ??) . I can say this tandem can fly great in windy condition, even when my 600N cannot hover steadily !

I use governor. Much more stable.
I am thinking of the V22 project now.

[billyd]

Are full scale tandem helis' rotors connected mechanically?

Has anyone ever played with dual motor tandems using a single esc to synchronize the motors instead of a mechanical connection?

[chinookmark]

HH gyro update:

I haven't flown it yet since a fresh set up, but I was noticing something about the HH gyro. (It's a RC-Tek G730 gyro, which supposedly used updated firmware from a GY401. As far as I can tell, it funtions and feels just like a 401, but half the weight.) I had noticed during previous flight tests that if I made a hard turn and released, the gyro would stop the turn buy almost violently overcorrecting. Turning up the delay smoothed it out, but didn't totally solve the problem. Reducing the rudder channel endpoints helped as well, but mostly by lowering the piro rate, thereby reducing my ability to make a hard turn. On thing I didn't experiment with before was the Limit adjustment.

We're all used to setting up Trex style helicopters, where you adjust the tail and gyro for maximum travel possible. But the Trex is a 3D helicopter with a rock solid tail. I had set the limit on the Twinn Rexx down just enough to stop the servos from buzzing and any visible binding in the links. The swashplate would tilt, say, about 40 degrees. Now, this is way farther than I would ever purposely control the swashplate, but this is the limit allowed to the gyro. When I turned the rudder endpoints down to +/-75, I could only tilt the swashplates about 20 degrees (these angles aren't accurate, only to give a picture of what's happening to the heli). BUT, when I turned the helicopter in my hands, the gyro would still correct all the way out to the original 40 degrees. This time, I turned down the Limit setting so that the gyro only has a little more travel available than me, about 25 degrees.

I haven't had the chance to fly it yet, due to weather and work scheduling, but I think this will reduce the violence of the gyro's corrections. With the delay turned up and the limit turned down, the gyro can only make smooth moderate corrections. This also has a side benefit -- with full swash travel allowed during yaw, there is none left for roll. Adding roll inputs to full yaw results in binding and servo buzzing. Reducing swashplate yaw travel to about half, leaves another half available for roll. Oh yeah, guess how much the swashplate moves during full roll inputs -- about 20 degrees. I think this makes more sense -- now the yaw control has about the same authority over the swashplate as cyclic control.

[chinookmark]

Are full scale tandem helis' rotors connected mechanically?

Has anyone ever played with dual motor tandems using a single esc to synchronize the motors instead of a mechanical connection?

Chinooks and Sea Knights have a driveshaft connecting the rotors. There's more to it than that, but that's what it boils down to. Both rotors turn 1:1, with no way of coming out of phase, short of catastrophic failure. Those are the only two tandems I'm familiar with, but I can't imaging any non-mechanical synchronization of the rotors, unless maybe if they're non-interference -- meaning the rotor discs don't intermesh.

If you were to build a non-interference RC tandem, you could possibly use separate motors and controls. But in an overlapping system such as the Twinn Rexx, there are just too many variables to get separate mechanics timed perfectly.

Interestingly, though, Expansion pack 4 for Realflight G3/4 has a CH-46, and if you dig into the controls, it's powered by two sets of mechanics and two motors. I haven't yet had a blade strike in that. :D