Twinn Rexx owners' thread

[chinookmark]

No offense taken! LOL! I didn't intend this to be "my" thread. I figured Gra55h0pper had the pre-production thread, so I'd start a "production" thread for all of us present and future Twinn owners to share information.

For battery I'm running a Thunder Power Extreme V2 4s 2200. Well, until I killed my motor and ESC. The last few flights have been with a FP Evo25 3S 2170, along with a 3550kV motor. This is only temporary until I get a new motor from JustGoFly, then back to 4S.

For blades I'm using Align 325 Pro blades, red/white. I have all black 325 woods from Tech Model Products, but for now I'm staying with the Aligns for visibility. I'd considered 335's, but there isn't a whole lot of clearance between the blades and the opposing flybars. I'd like to know if anyone has tried flying with the blades out of phase -- that is, at any angle other than 90 degrees between heads. Do you think that would cause severe vibrations? If not, there would be clearance for longer blades. Of course, there would be plenty of clearance with 3 blade heads! :grin:

[Gra55h0pper]

Chinookmark - I wouldn't expect having the two heads a bit out of phase (compared to 90-degrees, that is) would cause vibration. After all, each individual head is well balanced and also with the blades perfectly at 90-degrees, you already have the CG shifting back and forth a little bit (at a very high rate). I'm guessing a bit though but don't really want to try it as then I'd have to adjust my rear miter gear again... :roll:

Looking at my bird, I have about 2cm clearance between the rotor-blade and the flybar-paddle. As the 335 blades are 1 cm longer (0.5cm on each side), that still leaves 1.5cm which should work OK, shouldn't it? Theoretically anyway... :D

[michaelfsB4MHX]

What are your thoughts on using a 4S 3000mAh the dimensions are 137 x 44 x 22 and it weighs in at 300g. do you think this would be to heavy or to large in size for the Twinn Rexx?

Regards.......Mike

[chinookmark]

Well, it's never a good idea to add extra weight if you don't need to. Especially since I get about 10 minutes on a 2200. However, the Twinn Rexx seems to have an abundance of lifting power, expecially when you crank up the head speed. I don't have RPM numbers, but when I turn up the headspeed from "a lazy hum" to just below "uncomfortably buzzing", the helicopter is very eager to climb. And that's at only 8 deg pitch. Unless your goal is to build light, I don't see a problem with a 3000 mah battery.

[Gra55h0pper]

The big question is of course whether adding additional battery weight/capacity will indeed result in longer flight times without affecting the heli's handling. On my Rexx, weight is not currently a problem (I have the additional weight of the Spartan-RC stabilizer), however I haven't flown with the additional weight of the body yet.

If you don't plan to fly with the fuselage, the Rexx has plenty of room for adding a bigger battery (depending a bit on your choices for electronics placement). If you plan to fly with the body however, there's a bit less room. I'm sure it'll fit with my Flightpower 4S 2170, but I'm not sure it will fit with your battery. Note that you'd like the battery not *too* far away from the CG. See the attached pic (borrowed from the tech-mp website) indicating the approximate battery position.

Bottom line: try it and please share your experiences... :D

[michaelfsB4MHX]

Thank you both for the reply. I will probably try that pack as it is available immediately and it's a good price. Will let you know how it goes.

Mike

[chinookmark]

Any of you guys epoxy the landing gear struts into the frame, as per the instructions? I'm a little leery about gluing them ... how would you remove them if damaged in a crash? Or how would you dissasemble the rear frame?

[Gra55h0pper]

Chinookmark - That's intreresting! I did indeed do that but wasn't aware the instructions now call for it (I built a pre-prodcution version). The reason I did so is to prevent the gear from havin play causing it to "wiggle" during flight. I tried to put the epoxy on in such a way that it's just enough to prevent the play but that I can easily break it loose if I need to reassemble things. That's the theory anyway :D as I haven't yet have had to do so...

[v22chap]

Mark
I used a silcone glue stuff to do mine ... (like the goop stuff ) and have had to pull it out and replace a rear axle and it worked fine .

[billyd]

As v22chap says, goop is best there. To my recollection I believe the instructions say to use goop type glue for this, not epoxy.

Anyway my slots were so tight, in my case it wasn't needed.


How are you guys doing with the body? I've got mine installed, but I had a really tough time with it. I had to buy the body halfs twice to get it right. And even on the second attempt I had undercuts where the two towers meet the top seam. Could not get the sanding right. I guess I would prefer a more rigid type of plastic, although that would crack easier.

I covered the undercuts with plastic lips on the inside of the body halfs, ca'd to one side. Now I want to fill it in with something. Plastic puddy (I never have much luck with that stuff) or maybe caulking prior to paint. Anyone have any ideas? I want to be able to remove the body without destroying the fill, if possible. So the fill job would have to adhere to one body half only.

The only alternative would be to buy the body halves a third time. This stuff is hardest on me. No talent in the artistic side of the hobby. :o

[v22chap]

The last fuse I had out of PETG I put the under surface 1/2" seam glued to one side that you talk about and just reinforced the certain points where I put small 002 brass screws in to screw the halves together with 1/8 " X 1/4 " square pieces of lite ply ...It don't look as good close up but you can remove 14 screws or so and pull the fuse off the mechanics to do service without removing the heads .

[chinookmark]

Landing gear: I didn't glue mine initially, as they fit pretty snug. But after a few flights and a whole lot of hangar abuse, the gear is getting pretty sloppy. Honesly, I haven't looked at the instructions in a while. I just remembered something about glue. I will try a silicon or goop glue.

Body: billyd, what do you mean by undercuts? Do you mean when most of the body is matched up, there are a few spots with space? When I first cut the body, I left a tiny bit of "frame" around the edges. After intstalling the four mount screws, there was a little overlap between the fuselage halves. Very slowly, I cut and sanded so they matched up. When I say "cut", I shaved slivers of plastic with an X-acto knife. I sanded with 220 grit wrapped around a sanding block made of a folded piece of cardboard. I have it fitting pretty good now. It's actually a tiny bit wide, compared to the four mounting screws, just so there is a little bit of pressure on the plastic holding it together. This makes it easy to see the high spots that need trimming.

I don't think I'm going to use joining tabs as described in the intructions. Instead of the tiny rectangles, I'm going to make strips of plastic that span larger lengths of the body -- basically one piece to span each straight line on the body. A friend of mine built a Darthdrk Jetranger this way and it came out really nice.

[billyd]

Chinook, the method of building you are describing sounds quite good. I am afraid I didn't spend enough time and was not not meticulous enough in the matching of the body halves. In the past I have always used a flat sanding table that I made for such things. I cut out the flashing with scissors leaving about 1/8" all around. And then sanded the two halves until the 1/8" trim either fell off or was very easily peeled off. But while relatively quick, it does not work for this material and canopy. The canopy is so flexible it was impossible to avoid deforming it during the sanding process. As a result I am left with two holes (the rest of the body halves line up well) right where the inside part of the two towers meets the top seam.

I have added lips to one body half to plug these holes, but of course, their is still a notch between the seems. I need to fill that in with something. That's where I am stuck.

I may have to reload and try the body halves again using the technique you describe. It sounds like that is the only way to get it right. I really like your idea of going with long continuous lips in lieu of the short tabs.

ps. I found that if you use CA to attach the tabs to the body as suggested in the instructions that it is best to sand both the local areas inside the body half and the tab prior to applying CA. I have also found that you SHOULDN'T sand the plastic when using acetone to join them.

[v22chap]

billyd
If it is just small areas ..why don't you put a square piece of scrap material over the area and make it look like an access hatch or something like that .


I have acutally put the extra trim piece on the outside in long strips to fasten the halfs together ...especially in the windshield areas ..but you could do that on all the joints if needed to clean them up ... at least you have straight line joints again ( two joints ,,but atleast straight ) .:thumbup:

[chinookmark]

billyd
Depending on how wide these gaps are, you could continue sanding the high spots until they match, and just have a slightly narrower body. This would probably work for holes up to 1/8". Where are the gaps? Are you talking about the inside corner where the tunnel covers meet the pylons? Even hand sanding, the corners dissappear quick. Try overlapping the body just enough so the gaps touch -- maybe you can taper the gaps away. I had to do that in a few spots.

[chinookmark]

In other news ... I got the prototype rotor heads back from my machinist buddy today. I asked for one because I was pretty sure it wouldn't work on the first shot, but he made me two just in case it did. Here's how it looks test fitted:

http://www.helifreak.com/attachment.php?attachmentid=36660&stc=1&d=1201305863

It's a little bit bigger than it was supposed to be. When I made the sketches, I drew them 2X size. On the sketch, I accidentally wrote down the drawing radius rather than the inteded radius. It's probably better this way, since if it came out the inteded size, there would've been clearance issues between the Jesus bolt and one of the droop stops. I'm just concerned about PC links binding on the swashplate. I still have some work to do. I need to mount the balls on the grips -- coming out perpendicular to the feathering shaft. The droops stops need to be improved. And I need to find a suitable length PC link. I think the Trex swash-flybar links will work. Oh, and I need to cut the feathering shafts.

[tungym]

Nice piece of work. I 've never seen any small rc heli with articulated rotor head.

[chinookmark]

I didn't have anything else to go on, so I just copied a real Chinook rotor head. It's really cool in theory, but I'm wondering if it will work with such lightweight blades. We shall see ....

[joe@tech-mp]

Our models require damping on the flapping axis (o-rings etc.). This is due to the scale factor. Full scale birds have much more centrifugal force acting on the blades due to the size and weight.

They sure look nice ;)


Joe

www.techmodelproducts.com

[chinookmark]

With blades attached:

http://www.helifreak.com/attachment.php?attachmentid=36667&stc=1&d=1201319857

[tungym]

My maiden video. Not too great as it is pretty windy >20km/hr with gust >30km/h.

I must say the collective pitch of the front and rear rotor must accurately adjusted. In the first flight, the front rotor pitch is 1degree (2 turns on uppermost linkage) higher than the rear, and it almost tipped over in strong head on wind. The tandem tipped 45 degree backwards despite I push elevator to the upper most. Then the DCP gyro (50% HH gain) takes over (though slower that I expect) and it recovered to forward nose-down position.

It is too much fun to hover around with this bird.

I still have to solve some vibration issue of the rear rotor.

http://www.youtube.com/watch?v=SetGU-bKgZE

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[Gra55h0pper]

tungym - Cool! Not bad for a maiden!!! I wish I was able to post a video of mine, but I don't have a camera.
It's tough to say as you had to cope with a lot of wind, but mine seems more stable around the elevator axis though. Do you have your DCP gyro in HH? Also, in my experience (although that's personal) the 70% mix from elevator to DCP with the DCP ATV set at 50% (effectively that's a 35% mix from elevator to DCP) is on the low side. You may want to try a little higher setting there and see how you like it.
- H0pper.

[Gra55h0pper]

Finally... :) Was able to make a video also. I've posted it in my Twinn-Rexx build thread... :D

[tungym]

Finally I fixed the vibration issue. This bird is rock solid. I can set my hands free from the transmitter for almost 5 seconds or more.

1) BTW, anyone try putting back the weight collar on the flybar to increase the stability further so that it looks more real in cruising around.

2) When I give rudder input , the bird tends to drop in altitude. Is is normal? Should I add RUDD-->throttle mix or pitch mixing ? My 500T with 13T/3s lipo has slightly under headspeed.

3) Yaw gyro in HH mode do the job perfectly.

Anyway, I must say even cheap (comparing to 240/ 401) HH gyro like Align 500X and Esky 704A works perferct ! !!!

Since it is rainy, hope I can post another video later.

[tungym]

rainy days....

cannot wait to test hovering after resolving the vibration...

It looks real in nose down on forward flight.

With flat throttle curve (governor), it flies much stable.

I set the yaw/ pitch gyro to 50% headlock mode.

Enjoy and kindly comment

http://www.youtube.com/watch?v=zbrLm1y0kYs