Pre-production Twinn-Rexx build pictures

[Gra55h0pper]

Hi Leapfrog,

Cool! Nice to see your experiences so far are as good as mine! That's great! :D

Grasshopper.

[melsman]

Good post, Gra55h0pper. Keep it coming.

As for retractable landing gear, the CH-46 gear is fixed. In fact, we used to mock the fixed-wing guys in the landing pattern on the boat when they reported that their landing gear was "down and locked". We would sometimes report that our gear was "down and welded".

In the latter 1980's, when the MV-22 project was still in doubt, Boeing produced the Model 360 - a more modern composite tandem technology demonstrator that was a less-expensive follow-on to the CH-46 that had retractable gear. There's a "History of Choppers" series that runs on the Military Channel and on the History Channel that has a brief clip of this demonstrator flying during the intro to the show.

I've not heard of a NASA CH-46. I'd be interested in reading any information you can find on that, rotorblade.

Ashley

[rotorblade]

Thats it in the picture, 360 was the name. I always heard that NASA was involved in it some way, though never heard how or WHY!!!! I always figured they let them use the wind tunnels they had. One picture I saw of it years ago looked like it had a very slick skin, not many rivets and very streamlined. Everyone I have ever talked to about it back in the day understood when I said the nasa Chinook.

BUT is it for sure a CH46? sure looks like one.

[hommer75]

Gra55h0pper, what did you pay for the kit?

[Gra55h0pper]

Installed most of the electronics today! Only the high voltage stuff still needs to be connected. Though there's plenty of room on the frame, things did get a little crowded as I couldn't resist and installed a Spartan-RC infrared flight stabilizer with FMA infrared sensor... :mrgreen: Check www.spartan-rc.com for more information. I've had great experiences with this unit, as I use it for an "emergency bailout switch". Whenever the heli enters an attitude you have trouble recovering from (i.e. you get the oh #&*# feeling...), flip a transmitter switch, the heli levels itself, add some collective and you're flying again! Couldn't resist; have to find out whether it works as well on a tandem... :D

[Gra55h0pper]

Some more pics...

[kgfly]

Fascinating.

Some of those photos show the gyro mounted with it's labelled face not horizontal. The sense axis for the GY240/401 gyro is perpendicular to the labelled face so that face must be horizontal for the sense axis to be parallel to the main shaft(s). Maybe this is just an artefact of the incomplete build ?

[kgfly]

Ah ha, more photos => there are two gyros. Are both of these connected to the Spartan ? On for yaw, one for pitch, what about roll ? Are you aiming for a flybarless setup ?

[Gra55h0pper]

Kgfly,

Indeed two gyros are common for a tandem heli. The 2nd gyro stabilizes the Collective Pitch (CP) between the rotors, i.e. it will adjust the Differential Collective Pitch (DCP) for the heli to maintain its attitude in the "elevator plane" (this is why one of the gyros is mounted to the side of the frame).

Note that neither of these gyros are connected to the Spartan. Only the Aileron, Elevator and Pitch channels are Spartan controlled using the infrared sensor.

Summarizing:

Aileron channel -> stabilized through Spartan-RC infrared sensor to level the heli with the horizon
Elevator channel -> stabilized through Spartan-RC infrared sensor to level the heli with the horizon
Pitch channel (CP) -> stabilized through Spartan-RC infrared sensor to level the heli with the horizon
Rudder channel -> stabilized through gyro1 to fight unwanted forces causing a heading change
Differential collective pitch (DCP) channel -> stabilized through gyro2 to fight unwanted forces causing the heli to tilt

Note that in the transmitter, the DCP channel is mixed into the Elevator channel to make a change on the Elevator channel cause ~70% of that change on the DCP channel.

Also note that the DCP gyro will be operating in AVCS mode (i.e. "heading hold"; even though that's not a good term now) whereas the Rudder gyro will operate in regular "rate" mode. From what I understand, trying to operate the Rudder gyro in AVCS mode doesn't work so well.

Is that :glasses2: or what?

[kgfly]

Certainly :glasses2: :)

[Gra55h0pper]

Actually, thinking some more about my post above, I'm not sure whether this will work :?: . The potential problem I see is that with the Spartan in the Elevator channel and a Gyro in the DCP channel, the Spartan and the Gyro may actually start "fighting" eachother, leading to flight instabilization instead of stabilization... :arggg: Problem is that the Gyro doesn't "see" the attempts of the Spartan to level the heli by applying elevator and will therefore try to maintain its attitude by applying DCP... :( This could lead to excessive Elevator input one way and excessive DCP input the other way... :bomb:

I see two possible ways to resolve this:

1) Ask Angelos at Spartan-RC whether he could make the Servo-4 output always equal a percentage (~70%) of the Elevator function. Then the mixing of Elevator to DCP is done by the Spartan-RC unit and the DCP gyro can simply be put behind the Servo-4 output. In a way, this approach would be the nicest as the Spartan-RC will then control both the Elevator and DCP functions, just like the pilot does through the right stick on the transmitter.

2) Let the Spartan-RC unit level the helicopter through the DCP channel, instead of the Elevator channel. The Gyro can then be put behind the Spartan-RC unit and thus "sees" the inputs the stabilizer and/or the pilot are giving on this channel. With this approach, the Elevator channel runs from the receiver straight to the TH2 tandem CCPM mixer. This approach doesn't require a software change in the Spartan-RC unit, but has the downside that the infrared horizon sensing will only "steer" through the DCP channel. This approach may also be the most feasible, as I guess Angelos may have better things to do than to change his software for some idiot trying to put stabilization on a tandem heli... :mrgreen:

Anybody else have experience using an infrared-based stabilizer on a tandem helicopter? Care to share experiences and give feedback on my analysis?

[kgfly]

Not sure if it will help but take a look in the AP forum on the threads about flybarless setups. They are using 3 gyros + Spartan to give them flybarless operation. Some of their discussion might give you extra insight into your problem.

[Gra55h0pper]

Made some more progress today. Finshed the high voltage electronics (will be running a 4S setup). Programmed the speed control and the TH-2 for servo centering. Also did a quick test-run of the installed motor and everything runs smooth! Having lots of fun here... :D

BTW: I decided to run the DCP channel through the Spartan-RC (ap2000i) before running it through the DCP gyro. This way, the Spartan and the gyro will not be "competing for control" as the gyro will see the inputs the Spartan gives as if it were human inputs. Only on the DCP channel though, which means that the spartan will (can) only level the heli using DCP, without any elevator mixed in. Whereas the pilot inputs both DCP and elevator at the same time. Should be fine for an "emergency levelling switch" though, and perhaps also for flying around with some stabilization gain. We'll see... :mrgreen:

Attached some more pics here. Note that the Aileron and Pitch servos for the rear head needed to be swapped around which was kindly pointed out to me by leapfrog. Would have discovered this when installing the head of course... Must have been running late :arggg:

BTW2: if you'd like to see it for real, I plan to bring it to the circumgyration event at the Bayside RC club in Fremont this weekend! I won't be flying it yet though, but you can have a look at it if you like! :mrgreen:
See www.circumgyration.com for more information.

[Gra55h0pper]

Note that Leapfrog has a thread with his experiences too, with very nice detailed pics! Check it out on: http://www.thetrexforums.com/index.php?topic=1465.0 and then come straight back to helifreak of course... :roll:

[good2win22]

Looking great Gra55h0pper!!

One thought, the full scale 47 has one wheel and tire on each side of the rear of the aircraft. Is there anyway to recreate this detail?

[Gra55h0pper]

One thought, the full scale 47 has one wheel and tire on each side of the rear of the aircraft. Is there anyway to recreate this detail?


Sure! I'm more into fun-scale myself (which means I don't mind so much), but you could just shorten the axis, omit the inside tire and put something like a retainer collar on or something like that.

[ShooShoo]

It's nice to see someone else building their Twinn Rexx. I'm hoping to get mine airborne this weekend.

Shawn

[Gra55h0pper]

Hi ShooShoo!

Nice to hear that! Mine should be airborne this weekend as well. I'm finishing the CCW head and then it's just blade balancing and go-fly! :)

GrassHopper.

[melsman]

OK... so I have a question.

Will the IR sensor work within the fuselage (when you fit it) or do you intend to either a.) remove it at that time and fly without it, or b.)mount it outside of the fuse?

I haven't played much with them, but my spidey sense tells me that the sensor needs a clear view of the horizon.

Ashley

[Gra55h0pper]

Haven't posted in a while... :? Time to catch-up!

Here's pictures of the finished heads. Note that the front head is set up for counter clockwise rotation, which means the head assembly is mirrored compared to the rear head assembly (which rotates in the regular T-Rex clockwise direction).

Some more details can be found here:
http://www.thetrexforums.com/index.php?topic=1465.0.
A very nice build description of a fellow Twinn-Rexx pilot... :mrgreen:

[Gra55h0pper]

We're airborne! :mrgreen:
Ran through about 4 batteries earlier today. After a minor mishap last week, she flies great now!

When I first spooled-up last week, she lifted up without any problem, but then started to rotate quickly around the yaw axis. I countered (of course) by applying opposite rudder and by reducing the throttle as I was only 1 feet or so up. With a heli with skids the landing while rotating would have been no problem, however I was unlucky in that one of the wheels "caught" in the grass causing the heli to tip over... :( Yaw axis gyro set for the wrong direction... :oops: And I thought I doublechecked...
Luckily, very minor damage: one bent front main shaft. Had to order a new one from Joe though. His service is excellent! After only a few days I had a new shaft waiting to be installed.

Today, I tried again... After reversing the gyro direction of course... :mrgreen:
No problem whatsoever this time! She took off, I hovered for a few seconds. Landed to add some subtrim. Took off again and flies great!!! Though not neccessarily more difficult or less stable then a regular heli, it definitely is somewhat different. You have to be on top of the rudder as the DCP causes torque differences to occur between the front and rear rotors causing the heli to want to yaw. Need to experiment with the "yaw compensation" function in the TH-2. If you've ever flown a cheap helicopter with a cheap rate-only gyro, then you'll have no problems "staying on top of the rudder".

After a couple of hovers, I switched the elevator gyro to AVCS-mode ("heading hold" is not a good term here). When attempting take-off, the heli wanted to tilt forward badly so I had to reduce power to prevent it from tipping over. I figured I probably needed to make sure the gyro was "reset" so I powered off and on again. On second take-off, she went straight up. Definitely more stable in the elevator axis! Though without AVCS, she was perfectly flyable, the difference is like a regular helicopter with heading-hold and without heading-hold. Flying without AVCS (rate mode only) is certainly no problem, but with AVCS on things are much nicer.

The Twinn-Rexx rocks! Time for Bob "Finless" to do a build video!!! :mrgreen:

[Gra55h0pper]

Sorry, no pictures of my airborne Twinn-Rexx yet...

Still practicing holding my transmitter in my left hand and my camera in my right... :roll:

[Gra55h0pper]

Will the IR sensor work within the fuselage (when you fit it) or do you intend to either a.) remove it at that time and fly without it, or b.)mount it outside of the fuse?


The IR sensor will not work within the fuselage as it needs to be able to "see" the horizon.
Not sure yet what I'll do. Either I'll mount it under the bottom of the fuse (needs to be level with the horizon though) or I'll take it off and put it on one of my other helicopters...

My first flights have defenitely shown that this heli is more stable and flies easier (or at least: "hovers easier", as I haven't attempted forward flight yet) than I had expected. In that sense, the sabilizer (which I haven't turned on yet as it won't do any good in my backyard), is certainly not a neccessity.

Only thing that worries me a bit is that it may be hard to distinguish the front from the rear when it's at a distance from you. If you lose orientation, then having an "emergency levelling switch" is always nice... :)

[andrew03957118]

Hi everyone, i'm fresh new in r/c field but was my dream since childhood, and am searching for a web page containing videos teaching how to assemble a helicopter and program it, to be encouraged to buy my first heli. Thanks for your help in advance and best regards to all. Andrew.

[good2win22]

andrew03957118,

You should check out the finless heli tech room. His room can be found on the home page, I think it's like the third one from the top. He has numerous build vids as well as basic setup and programming vids. They are extremely helpful to the hobby newcomer.