Prototype Tandem TRex - Result... Not good.

[Super-Hornet]

Hi all

Just finish fixing and trying my very first Tandem (also known as Chinook) TRex. This is very first prototype.

Result:
Not good because notice that even both side (Front and Rear) are using same TRex main gear and tail pulley, somehow the ratio division is not equal. With that, after a while, the both frontal and back rotor HIT EACH Other. If both front and rear rotor spin in equal speed (Which should be because it is driven by a single motor), it should not hit each other.

Alot of mixing is require to make this fellow works. Might require gyro for Elevator movement. Currently in the picture, there is a GY240 but it is turn off and it just act as wire extension.

Wasted one pair of Wooden 315mm blades due to blade hitting. Will try to make them spin in totally equal speed before can try again.

SH.

[Super-Hornet]

BTW... also accidentally burn out my Home Made BEC in the process due to somehow can shorted in reverse polarity even when using DEAN connector.

SH

[Helicopter]

looks like 2 trexs having a tug of war.

[HZM]

U are the man

Mike

[Brandon Chng]

nice job...

i think the rotors hit because the position of the one way auto gear is in the main gear, so when flying, one is sure to speed up and collide. maybe you could remove the one way from the main gears and transfer it to where the motor is.

[Daniel Wee]

Might it be possible to use a longer main shaft on one of the rotors so that they're not on the same level, kinda like the real Chinooks I believe. Should not be too hard to do I think. May need longer control rods for the swash but other than that, it should be straight forward. It's a good idea though.

Daniel

[Super-Hornet]

I don't think it is the "One Way Bearing" because:
Both rotor blades are directly connected to the secondary main gear (lower gear). And both secondary main gear is directly drive by tail pully. With that, Assuming the all tail pulleys and secondary gear are equal in size, it should be equal speed....regardless whether it is in autorotate or not.

To the blades hitting problem, either:
1 - Find/Replace equal balance pulleys and secondary main gear.
2 - Rear rotor is higher than frontal rotor... just like in real Chinook.


BTW... Daniel... I was typing this till I realise you said the same thing on some of thing I said.

SH

[koon]

some math calculation on LCM might tell you that if your gear is in even number of teeth your belt must be in even number aswell. Sound "chim" hor.. I've read this from other forum.

If belt and pinion come in odd and even number of teeth respectively, each pinion tooth will have a chance to come into contact with every tooth on the belt,and that will even out the wear on each tooth..

That is also the reason why even number teeth pinion( like 10,12 or 14) are not produced by Align,and they only supply 9,11,13 and 15 for matching their 150 teeth main gear.

However, for your case. you would need both belt and pinion come in even numer, else after each turn 1 of the blade would start to advance 1 tooth space and eventually hit each other.

[Super-Hornet]

Update:

Try to make belt very tight due to suspect maybe belt slipping on loading that causing frontal-rear rotor out of timing (that causing blades hit). Try to hover for one pack of battery (even with chip off blades). So far no blades hitting and all still in equal distance.

Flight characteristic:
Difficult to fly!!!!! Tend to See-Saw up and down and sometime turning (yaw). Now that maybe due to alignment out. I use GY240 on the Elevator and so far so good on elevator but yawning might need a bit of gyro as well.

Can really feel the lifting power is there.

SH

[Daniel Wee]

I think tracking may present a challenge here since you have to match the lift of the front and rear rotor blade sets. This might contribute to the see-sawing effect you're experiencing. Maybe you want to consider using smaller rotor blades. I've got a hunch that will lead to better matching of lift and slightly more stability. Something like 220-260mm?

Daniel

[Super-Hornet]

Hi Daniel

More tracking/alignment is expected because now I have more rotor blades...just like any multiblades heli.

I do not want to go anything smaller than 315mm blades. In fact, if I can use 335mm blades it will be better. The reason is that I want to create a heavy lifter heli. Going smaller blades defeat the purpose.

As I said, using GY240 for stability for Elevator does help cut down the "See Saw" effect. Compare to my previous flybarless TRex using Gyro as stability, it looks like really GY240 (or higher) is much much much better than using GWS PG-03.

I manage to do a very short hover in my living room. Not easy to fly. Just like learning new heli or flying flybarless heli without added gyro stability. I need further tweaking on this fellow.

SH

[HZM]

The more I look at this thread, the more I felt its an interesting project. Keep us posted w more pictures and information

Cheers

Mike

[skyblueblue]

Maybe can use 1 belt to connect the 2 main gear together, juz like the tail. U save 1 motor

But i wonder if the motor can handle the load

[HZM]

wat abt making some bracket so that one side of the "trex" will be higher then the other. I love the idea of using trex

Mike

[Super-Hornet]

Skyblueblue.... It is using only one motor. It is my old reconditioned 400DH motor. This motor is driving the frontal rotor just like normal TRex. Then instead of belt going to drive tail rotor, it is connected to another tail pully of the rear TRex.

HZM...It would be nice to make tail rotor section higher than main to make it more Chinook scale. That mean more design...which I don't have CNC machine leh... unless I can find some substitution from "off the selve" parts.

I think I may have solve the "blades hitting each other" problem by tightening the belt as much as I could because i suspect the belt slipping that causing out of timing.

I almost give up on this project due to limitation of Futaba 9CHP TX in the programmable mixing. I finally found ways to program all the necessary mixing before I started this project.

Now is waiting for me to get new pairs of 315mm wooden blades to further testing.

SH