View Full Version : Twinnrexx power options

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09-17-2008, 06:00 PM
Hi guys I've owned the Twinnrexx for awhile now, and I have it flying really well. My biggest gripe with the heli is it seems underpowered to me. I am using the as spec'd motor and esc (JGF 500T and 35amp controller).

I was thinking about upgrading the motor to something with more pop like the 1600kv Scorpion and a 14 tooth pinion with a 60 or even 80 amp CC controller. My biggest concern with that setup would be flight times off of the 4s 2200 mah battery would be probably very short so I would have to go to a larger battery and lose power to the heavier setup.

Long story short has anyone out there experimented with other power setups for the twinnrexx? Joe have you experimented with other setups?

The issue I am running into is on windy days sometimes my twinnrexx has barely enough lift to pull out of the occasional sudden lift loss that can occur when changing directions in strong winds or when the wind changes directions.

09-21-2008, 06:26 PM
Anyone want to try out a JGF550T ? We need someone to test this new motor and report results. We want a comparison to the JGF500T. We need the results produced by a FDR (flight data recorder).

email me as support@tech-mp.com



09-22-2008, 08:22 AM

eMail sent.

10-02-2008, 12:19 PM

I have purchased a scorpion 1600 kv motor and CC 80 amp speed control. Also have a 4s 3200mah kongpower lipo. I am currently machining a new motor mount (the supplied one is too small for the scorpion). Once I get it all hooked up and flight tested I will post a report.

The only concern now is whether the existing fuse will interfere with the motor. Eyeballing it it looks like it's going to be a close fit. If it hits I guess I will use a heat gun and reshape the fuse as necessary. Better modelers are welcome to pipe in here with suggestions for reshaping the fuse if necessary.

I also have upgraded the main drive gear with a microheli cnc gear (aftermarket) and I will be adding align carbon fiber blades front and rear.

I think this new setup is going to have gobs of power and I look forward to the flight test.

10-02-2008, 03:28 PM
Will be interested to see how the scorpion does ...been eye balling that one my self .. as it seems like during the hot weather and the fuse on it could use just a bit more power ,,,,. but I haven't seen any store with them yet ??? were did you find yours ???

10-04-2008, 12:54 AM
I bought it here:


But it looks like they're out of stock again. I'm sure they will get more soon.

If you don't want to wait the 1400 kv is in stock and you could use it with a mikado 16 tooth pinion.



1400 kv is slightly heavier but can pul 80 amps continuous (vs 70 for the 1600). It might even be a better choice to tell the truth. But I don't think they had the 1400's in stock so I bought the 1600 lol now it's the opposite.

Just beware that the supplied motor mount won't work with the scorpion, you have to make your own. I have a drawing that I made of the one I am making and will post it when I get the chance. Perhaps Joe would consider getting these laser cut and supplying them assuming the scorpion motor power upgrade is successful.

10-04-2008, 08:10 AM
O.K. that explains it ,,, you are using the HK 3026 series ...
I am looking and waiting for the newer light weight 1630 KV series that they are bringing out called the HK 2221 -10y ... it weighs 2.79 oz and has a 3.17 shaft size .. instead of the having the 5 mm shaft and 6.9 oz weigh of the 3026 series .

And this is still 100 watts more than my current JGF 500 T

Joe .
They list the kv of the jgf 550 T at 2000 and the 500 T at 1900 ... I thought we would want to be going lower to get more power ???
I am guessing the extra length of the bell ,less poles ??and the timing are the key ,, what do you think???
Do you have anyone testing the 550 T motor yet ??

10-04-2008, 08:28 AM
The way I see it, the twinrexx is really more like a 500 than a 450 due to having to drive two rotorheads and main blade sets. So why not use a 500 motor?

The hk 3026 may be somewhat overkill, but I like it that way :lol:

I was able to find .5 mod pinions for a 5mm shaft (by Mikado) that work with the 450 main gear, so I figured why not?. If the power is too crazy I can always tone down the pitch curve.

10-04-2008, 08:35 AM
You got a point there ..but having to change the fuse looks and motor mount ... just don't sound appealing :roll:
Hope that it works though :thumbup:

10-04-2008, 08:47 AM
Regarding the mount, fortunately I have access to an extensive machine shop (I am a partner in a steel manufacturing business). I know it's a huge issue for most, since making a one off mount is going to be expensive. It's really a waterjet job, (or laser) and I don't have that capability so I am making it on a cnc mill. I could job it out, but I'd have to get many 100's made at a minimum to make it cost effective.

Any mod to the fuse will be very minor if at all, the interference (if there is any) will be very small, perhaps a 1/16 to an 1/8 of an inch or so. I don't think it will be very noticeable.

Certainly this is an extensive and expensive mod to the rexx and it may not work out. The 450 components may not accept all that power, or perhaps the additional power may cause issues with stability of the mixer/computer. But I can always use the scorpion and 80 amp CC esc as backups for my 500 heli if it doesn't work out.

10-09-2008, 08:16 AM
Ok good news the motor mount is complete, and it works perfectly. The motor fits without any interference (no mod to the fuse required but it is CLOSE). The bad news is I went with the wrong pinion I really need a smaller one (12 tooth and they don't make them) and oversped the heli (I had 100% throttle curve at zero pitch) and bent both main shafts. lol. Oh well.

Moral of the story here is the 1600 kv is too fast for the 4s setup and the better choice would be the 1400, 1210 or even 1000 KV scorpions.

I am changing over to a 1400 kv and a 13 tooth or 14 tooth pinion to make sure the rpm stays below 1900 rpm to prevent bending main shafts again. That will be plenty of speed for the kind of power this motor puts out.

Stay posted. I will post the Scorpion motor mount drawing today. Downside is it's not easy to make at all without a press brake and a waterjet or laser cutting machine.

10-09-2008, 08:33 AM
Ok here is the motor mount design for the HK 3026 scorpion motors and the twinrexx

10-14-2008, 09:05 PM
Success!! The original motor and pinion are fine. The reason I bent the main shafts was the plastic swash guide on the front was broken and I didn't notice. When I spooled up the swash twisted and created a situation where side load went into the front shaft causing it to bend. The rear followed suit from the subsequent vibration.

So the set up I am running is the Scorpion HK 3026 1600kv with a 14 tooth pinion. I am still playing with the pitch and throttle curve (100 straight across looks to be the ticket on the throttle curve). But there is so much power to spare here, I am going to get as much pitch in the blades as I can. I will play more over the coming days and let you know.

I am using a Kong power 4s 3200 battery which is heavy but nicely counters the additional weight of the scorpion. I still have a good cg, where Joe recommends it, so no problems there. Maybe slightly forward but not enough to cause a problem. Flight times right now are over 10 minutes, but that will drop when I get the swash jacked up to use all that potential power.

Another consideration would be to pump up the head speed to 2300 rpm or more with a bigger pinion. I think with the additional weight we won't get the vibration issues Joe had with the original Twinrexx setup.

This power setup will provide alot of benefits. Being able to fly in wind a big plus, and the additional weight makes the heli fly alot better. And with the additional power, there is a substantial speed increase even with the extra weight.

Right now I am only drawing about 17 amps peak in a basic hover and light to medium pull out, so there is alot of power not being used. It will take some experimentation before I figure out how much pitch angle and head speed I can get and still have a stable system.

10-15-2008, 04:31 AM
Hey ,, that is good to hear ... well not that the crash was caused by a cracked part ,,but that it flies and flies good with the extra power and weight ... as I was worried that would be an issue and is why I was waiting for the HK 2221 10 Y scoripion .
And I thought I was using heavy batteries with the 4 s 2200 DN batteries :noteworthy

Don't think I would want to bump up the headspeed ..just because I hate the whine of the 3-D headspeeds ... the chinnook just isn't right with screaming me me's for head speed :Bang

10-15-2008, 09:05 AM

That's the other thing I forgot to mention, the sound. It is way better with the bigger motor. I guess because of the lower pitch of the bigger motor (and maybe the CF blades), the heli has a deep throaty sound when flying with the fuse on. A happy accident so to speak.

As far as bumping up the headspeed, don't worry with all that extra weight, there is categorically no chance of this ever being a whiny 3d machine! LOL

10-15-2008, 01:02 PM
there is categorically no chance of this ever being a whiny 3d machine! LOL


10-15-2008, 02:22 PM
By the way what would happen to the TH2 if you did flip the Twinrexx upside down and went negative pitch on the blades? Would it go insane?

10-15-2008, 03:50 PM
It would go insane after the blades hit the backbone ..:arggg::thumbdown:
Walt Farer inverted Joes hornet size one and it was actally upside down for awhile until the blades hit .:dontknow:cheers
but the TH-2 would be o.k. with that,, it don't care whither you are right side up or not .

10-20-2008, 03:32 PM
OK another update. I put a 16 tooth pinion on the setup and it flew great. Problem is, if you spool up too fast at that rpm, it's very easy to bend the main shafts.

I upgraded the rear shaft to hi strength align, but the forward main shaft is not available in alloy steel. So I will probably make my own.

That being said the heli flies fantastic with this setup. Amazing power and speed. You just have to keep your Tx curve (fixed endpoint esc mode) something like 0-50-80-90-100 and it's fine. When I went to 0-80-90-95-100 I bent the (regular steel) shafts. The key is not to have too high a percentage when the blades aren't under load so it doesn't overspeed and bend the shafts. With alloy shafts I think you could run 100 flat no problem.

The safest bet would be to stay with the 14 t or even 15 t pinion, and that will protect you from bent mains. But the heli performance takes a big hit. I am unwilling to go that route, so I will continue to try and get the most speed I can. If I have to dynamically balance the rotor heads so be it ! lol. The performance at 2300 rpm was just too good to ignore.

My amp draw is still pretty low, around 50 amps peak. So you could definately run with a 45 or 60 amp esc and not have a problem. My current hi power setup would be something like this:

Motor: Scorpion HK 3036 1600 kv (requires custom made motor mount)
Pinion: 16 t (mikado 5mm .5 mod)
ESC: CC 45 or 60 amp
Battery: Kongpower 4s 3200 mah (you have to move the yaw gyro forward a little to fit this battery)
Main blades: 325 mm cf align 3k
Main shafts: Align hi strength for rear. Custom made alloy forward shaft.

Tx curve: 0-50-80-90-100 (spool up 1/4 stick and move up slowly)
Pitch curve: 40-45-53-76.5-100

Flight times are about 8 minutes.

10-20-2008, 06:31 PM
Hey Billyd,

Your set up sounds great! I love heavy power houses!

I too want to try out your power setup but I don't have access to the necessary machinary! But one thing came into my mind - do you think I could use a motor adapter plate to convert the 25mm screw spacing to 19mm? Or is the diameter of the motor too wide?

10-20-2008, 09:20 PM
The biggest problem with the stock mount (other than the obvious hole pattern difference) is that it places the motor too far toward the rear of the heli on the gear. In order to fit the larger diameter scorpion, the center of the pinion has to move about 3/16" to 1/4" towards the front of the heli. The reason is because this movement, following around the diameter of the main gear, also moves the motor about that same distance away from the side of the front frame. This provides the necessary clearance.

If the hole pattern was the only problem, the existing mount could be modified to work. I originally thought about slotting the holes on the existing mount to slide it forward about 3/16" but this solution is not a great one because there wouldn't be much meat left on the mount around the holes and would probably lead to cracking.

There might be other motor choices that will work with the existing mount. As far as I can tell so far, the scorpion I am using is way over what is really needed to get the same results I am seeing. If you can find a motor of around 800 watts that would be about perfect and still target the same head speed of 2200-2300 rpm. There is still the problem of the main shafts bending however. But at least you can get the rear shaft in alloy steel. Align sells that. So if you're careful and keep a fair supply of forward shafts on hand you could get away with it.

If enough interest is generated I could get a quantity of mounts made for the scorpion mod. But it would still probably be expensive, even if we found 100 people to do it.

The forward main shaft in alloy steel would be pretty cheap for me to make, so that one would be more feasible in low quantity. The problem is my shop is just not set up to do the kind of work the motor mount requires (you need a press brake and a waterjet machine, I have neither). The main shafts I can spit out all day on one of the cnc lathes we have. And they wouldn't be much more than the material costs in even a small quantity.

For example I could get the mount made one off for you for about $200 (yikes) but the main shafts I could do for about 1/3 of that in small quantity. Obviously in quantity those numbers drop off dramatically. But no matter what the motor mount made by me isn't going to ever be cheap because I don't have the right tools to do it cheaply. And the only way I could farm out the job would be with quantity, and I doubt the demand is there to even make 100 pieces. It would be tough to get anyone to take the job without quantity.

10-26-2008, 02:32 PM
I've been looking at potential motors after your post - the best I could find was the NeuMotors 1115/3D which is 1900KV rated at 700watts but it will cost over $200 with shipping over the to UK and import tax on top of that OR there is the medusa 28-56-1400 rated at 500 watts Looks like I'll just stick to what I've got.

Anyway how's your power setup going? Have you tried it with alloy front main shaft?

10-28-2008, 03:15 PM
The setup is great. I am currently building the alloy main shafts. But until they are complete I am just being careful at spool up and keeping my throttle percentage at 0-40-80-90-100.

Flies like a dream.

I did find an 800 watt scorpion but it's an airplane motor. Don't know if it would mount on the standard mount or not.

10-29-2008, 12:49 PM
Joe sent me a JustGoFly 550T motor to test in the Twinn Rexx. For those not familiar with this motor, the specifications can be found here:


I installed the motor with a 12T pinion and powered it with a ThunderPower 2200 mAh eXtreme V2 4S pack. Also onboard was an EagleTree data recorder with a brushless motor RPM sensor. The model was flown with the body installed. Blades were Align Carbon Fibers.

As you can see from the attached performance graph, average current draw was about 14 amps with an average head speed of 2100 RPM. Vertical performance was not stellar, but the motor did move this machine around pretty good. Overall efficiency was excellent.

10-29-2008, 04:12 PM
That is about the same results I have found with my JGF 500T ,,, not sellar but moves it o.k. ,,, but not enough if it is a little extra windy and you oops it .:o :arggg: