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May 02, 2008, 04:08 AM
Rotary~myPast,Present &Future!
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Discussion

A pair of Kings == Esky Tandem "Chinook"


Hi folks,

I was so fascinated at the sight and sound of a full-sized Chinook warping the air, just above sea level with the coastal waters splashing up, forming a 'fountain' around it. What a magnificent sight it was, on National Day in Singapore.

Little did I know that I will be building a "Chinook" with a pair of Esky King1 kits a few months after that. It's really simple forward thinking that since a King1 heli uses a torque-tube for driving the tail rotor, one could easily hook up another King1 body to that shaft driven boom unit, replacing the tail rotor entirely.

The King1 Tandem will have a single outrunner driving the 1st rotor directly and the 2nd rotor driven via the torque-tube.

Ideally, the King's 275mm blades will be replaced by the Esky CP2's 246mm blades. The plan would have been 'flawless' in a flybarless Tandem setup.

To prevent the blade of one rotor from clashing against the flybar paddle of the other rotorhead, the CP2 blades need to be trimmed down to 200mm.

Arh, the same length as the blades you see on a 200-size micro model! The GAUI EP200 is a good example! Unfortunately, the EP200's blades have a narrow airfoil width and they can't generate enough lift to carry the bulky mass of two King1 bodies.

The 200-size SkyArtec Wasp V3 FP-heli has blades which measured '200mm x 30mm'. They generate enough lift but unfortunately, they are semi-symmetrical because they are designed for fixed-pitch's application.
V3 later came out with a CCPM version but the CP woodies are a tad longer and will clash with the flybar paddle.

Eventually, I ended using a pair of trimmed-down CP2 blades for the C-CW rotor and another pair of the SkyArtec FP blades for the CW rotor.

Semi-symetrical blades generate more (upright) lift than their symetrical version of the same size. In terms of lifting power, this airfoil property allows the slimmer SkyArtec woodies to match my trimmed-down CP2 blades which are slightly more than 30mm-wide.

I might buy the new Wasp V3's ABS plastic blades in future and cut them to the desired lengths if I require more lift from these little rotor blades.

101 things can go wrong in a conventional heli, let alone tandem rotors, so do wish me luck!


Warm Regards,
Terence
Last edited by terencechan; May 02, 2008 at 04:13 AM.
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May 02, 2008, 04:26 AM
Rotary~myPast,Present &Future!
Thread OP

Counter-clockwise Rotor Mechanics


After I cut the CP2 woodies to exactly 200mm, it's time to disassemble the CP head on one of the Kings. The objective is to reverse every part of the head, "laterally-inverted" or symetrically-reversed so that the head mechanics are able to withstand the stress on it by blades that are swinging counter-clockwise at a very high headspeed (considering how short those blades are).

This modified head will be used for the front rotor of the Esky Tandem "Chinook" since a real Chinook has its front rotor going C-CW.

A short video clip will be coming up shortly which shows how the modified head looks like.

Esky King Tandem - Front Rotor Setup (2 min 4 sec)
Last edited by terencechan; May 02, 2008 at 11:25 PM.
May 02, 2008, 04:59 AM
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Almaz's Avatar
Looks good. I remember I saw a long time ago a huge thread doing the same and I can't find it anymore. Is there a video of a finished product?
May 02, 2008, 11:24 PM
Rotary~myPast,Present &Future!
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Quote:
Originally Posted by Almaz
Looks good. I remember I saw a long time ago a huge thread doing the same and I can't find it anymore. Is there a video of a finished product?
Thanks mate, it is still in works. I have combined the two kits into one, but it is only a bare skeleton, no electronics yet.
May 02, 2008, 11:35 PM
Rotary~myPast,Present &Future!
Thread OP

Battery Tray, how to check C.G???


The kit seems to be heavy for the 200mm wooden blades. So, I decide to use balsa to make the battery tray. CF board will be better but it costs 10 times more.

How do we check the COG of a tandem heli? Hmm, I can't tell exactly where the battery should be placed.
May 03, 2008, 01:29 AM
Rotary~myPast,Present &Future!
Thread OP

Cheap Tandem Mixer for Dual CCPM


The ms025 tandem mixer board looks like a worthy candidate to handle the yaw, cyclic and collective inputs for the tandem heli. Its price is affordable too.

However, I found out a critical problem. The mixer board is intended for use on the MS-composit CH46 tandem heli originally. And the layouts of the swash for both front and back rotors on the CH-46 are alike. So, applying forward cyclic input will tilt both swashplates forward when using the ms-025 board.

On my Esky King Tandem, the swashplates' layouts are symetrically-opposite to each other. Applying forward cyclic will tilt the front rotor's swash forward and the back rotor's swash backward, the cyclic forces in opposite directions will rip my heli apart!

Unfortunately, the ms-025 board is non-programmable.
A friend suggested using a Dionysus Servo Reverser to reverse each individual cyclic servo.

That won't work either as the Servo-Reverser will mess up the entire CCPM mixing altogether and we will end up having negative collective on the back rotor with throttle-up input!

Basically, all of the above pretty much concluded that the MS-025 tandem mixer won't work for my Esky Tandem.
Last edited by terencechan; May 03, 2008 at 01:51 AM.
May 03, 2008, 01:45 AM
Rotary~myPast,Present &Future!
Thread OP

Tech-MP tandem mixer board


Well, it looks like the Tech-MP tandem mixer board saves the day!
It allows me to re-program the swash plate layout of each rotor on the tandem heli, plus lots of other programmable settings that comes with it.
Impressive piece of work by Joe Orlando at Tech-MP but it is expensive though.

http://www.tech-mp.com/th-2_support.htm
May 03, 2008, 05:14 AM
Rotary~myPast,Present &Future!
Thread OP
Quote:
Originally Posted by spinup
Right on, Terence! That board should save you from some serious mixing headaches.

Another thing you might consider is a stronger driveshaft and gears. I think the TwinnRexx above uses a 5mm steel shaft. The 2mm graphite rod in a King1 tail is OK for a tail rotor, but it will twist and flex, and maybe even snap, under the load of a main rotor. Plus those unkeyed small gears seem likely to spin or strip. But maybe not. Try it and see.
I spooled up to 70% throttle and both rotors were at zero pitch.
The shaft-driven system seemed to be working fine but it might be a different story with +5degrees pitch in both rotors.

I figured that I could add another outrunner to drive the 2nd rotor without affecting the synchronisation of both rotors. The one-way bearing in each main gear allows for a difference in the motors' speed.

An analogy would be 2 persons riding on a double-bike, each pedalling at his own speed.
Both rotors will follow the speed of whichever outrunner that is faster. Adding a second outrunner to drive the main gear of the back rotor will significantly reduce the load put on the bevel gears and the torque-tube's shaft.


I need another Phx-35 ESC and another outrunner.
May 03, 2008, 05:21 AM
Registered User
Almaz's Avatar
Quote:
Originally Posted by terencechan
I spooled up to 70% throttle and both rotors were at zero pitch.
The shaft-driven system seemed to be working fine but it might be a different story with +5degrees pitch in both rotors.

I figured that I could add another outrunner to drive the 2nd rotor without affecting the synchronisation of both rotors. The one-way bearing in each main gear allows for a difference in the motors' speed.

An analogy would be 2 persons riding on a double-bike, each pedalling at his own speed.
Both rotors will follow the speed of whichever outrunner that is faster. Adding a second outrunner to drive the main gear of the back rotor will significantly reduce the load put on the bevel gears and the torque-tube's shaft.


I need another Phx-35 ESC and another outrunner.
Since you'll be running shorter blades you can get away just by using 2 Esky 3100kv motors and 2 1000mah-1300mah batteries or just one 2200mah lipo. You can buy 2 cheap 12-18amps ESCs and you'll save probably 100g.
May 03, 2008, 08:26 AM
=>] Smack 3D Head [<=
HeliSmith's Avatar
Terence you continue to amaze me. Yours is the first blog I have ever subscribed too!
May 04, 2008, 03:24 PM
hover happy
spinup's Avatar
Quote:
Originally Posted by terencechan
I figured that I could add another outrunner to drive the 2nd rotor without affecting the synchronisation of both rotors. The one-way bearing in each main gear allows for a difference in the motors' speed.
That's a really neat idea! Wow. I wonder if anyone else has used one-ways like this, to merge the output of 2 motors on one heli?

The nice thing about a stronger, fixed drivetrain is that you could synch the phase of the rotors and run longer (overlapping) blades. Long blades with lower headspeed would be much more efficient. Not to mention the 100+ grams saved on the power system.

But those frames don't leave much room to install a strong drivetrain, so maybe dual motors and tiny blades are the only way...

Great project! Keep it coming...

-Jeff
May 04, 2008, 10:06 PM
Rotary~myPast,Present &Future!
Thread OP
Quote:
Originally Posted by spinup
That's a really neat idea! Wow. I wonder if anyone else has used one-ways like this, to merge the output of 2 motors on one heli?

The nice thing about a stronger, fixed drivetrain is that you could synch the phase of the rotors and run longer (overlapping) blades. Long blades with lower headspeed would be much more efficient. Not to mention the 100+ grams saved on the power system.

But those frames don't leave much room to install a strong drivetrain, so maybe dual motors and tiny blades are the only way...

Great project! Keep it coming...

-Jeff
Hey Jeff, I can't thank you enough for warning me about the potential stress put on the weak shaft system. You have saved me from stripped gears, twisted shaft or worse, an airborne "disaster" resulting from a failing drivetrain. The idea to make use of the one-way bearings came across suddenly when I noticed that the other main gear was not moving when I spooled up the motor.
The King Cool has a longer boom/torque-tube. It's a pity I can't find that esky part anywhere. Nowadays, it's hard to find a King1 kit in stores, I guess the King Cool could be non-existent already.
Last edited by terencechan; May 04, 2008 at 10:48 PM.
May 04, 2008, 10:44 PM
Rotary~myPast,Present &Future!
Thread OP

Dual BL-Drive Tandem Video


Quote:
Originally Posted by Almaz
Since you'll be running shorter blades you can get away just by using 2 Esky 3100kv motors and 2 1000mah-1300mah batteries or just one 2200mah lipo. You can buy 2 cheap 12-18amps ESCs and you'll save probably 100g.
Thanks Almaz, I think you just solved my heli's weight and budget issues.

Following your suggestions, I have ordered these 18amps ESCs.
http://www.hobbycity.com/hobbycity/s...idProduct=4312

Also, I found these HDX300 3800kv outrunners which have greater torque than esky ones and each's a svelte 35g as compared to 55g of the current 3500kv outrunner.
http://www.helidirect.com/product_in...oducts_id=1524

I'm running a 10T pinion with the same HDX 3800kv outrunner in my FP heli which uses 238mm blades. In a 3S setup, Headspeed (in theory) is about 3000rpm but the drag of its belt-driven tail rotor will reduce it to about 2500rpm.

The tandem however uses smaller (200mm) blades. It should require a higher headspeed but it does not have a tail rotor. So, I'm guessing that having a 10T pinion on each outrunner should produce enough headspeed to handle the AUW that includes a bulky 2200mah 3S 25C Lipo.

Here's a video of the implementation's test ->

Esky King Tandem - Dual Motor Drive Test (1 min 51 sec)
Last edited by terencechan; Jul 04, 2008 at 12:55 AM. Reason: Adding Test Drive Video
Jul 14, 2008, 10:11 PM
Rotary~myPast,Present &Future!
Thread OP

Electronics Placement for the Esky Tandem


Here are some more pictures showing the development of this project.

Things are moving slowly, I'm getting lazy these days.
Aug 05, 2008, 10:17 PM
Rotary~myPast,Present &Future!
Thread OP

Success! Video of Maiden Flight


The TH2 mixer from Tech-MP is amazing. There are lots of "fine-tuning" required via USB programming on the computer to set the mixer right, but it's definitely worth all that trouble!

Specs of the Esky-DIY-Tandem

Tandem Heli Kit Parts
2 x Esky HB KingI or KingII Rotor Head (1 head is re-assembled for C-CW use)
2 x Esky HB KingI Main Shaft
4 x Shortened main blades from Esky HB CP2 or E-Flite CP2 (210mm)
4 x Align CF light-weight paddles

2 x Esky HB KingI MainFrame
2 x Esky HB KingI or KingII Main Gear (140T) with one-way bearing
2 x Esky HB KingI bevel gear
2 x Esky HB KingI crown gear
1 x Esky HB KingI Torque-Tube (boom unit)
4 x Boom Struts
1 x 6mm-Thin Balsa Plank (as battery tray)
2 x Horizontal Fin (as gyro mounts)
2 x Esky HB KingI or KingII Landing Gear

Electronics
2 x TurningG Plush 18amp BL ESCs (or HobbyWing Pentium)
2 x HDX 3800kv 35g BL Outrunner Motor
2 x 10T pinion
1 x AR6100 Receiver (or any 6-ch FM receiver)
1 x DX7 Transmitter (or any radio with min 6 channels and 1x program-mix)
1 x Tech-MP TH2-GP (tandem mixer board)
1 x mini-USB cable (for programming TH2-GP)
1 x Castle BEC 5V 10amp (or any BEC > 3amp with STRICT constant 5.0V)
2 x Futaba GY240 HH Gyro
6 x 9g-0.12sec Micro Servo
1 x ThunderPower Prolite 2100 (2200mah >15C recommended)

8 x 20cm-Servo Extension cable (2 extras are for gyros)
1 x 15cm-Servo Y-harness cable (to splice throttle channel for 2 ESCs)


Flying Review

There's quite a bit of ground resonance during takeoff. When this is picked up by the 2 heading-hold gyros, the resonance effect amplifies with gyroscopic correction, causing each end of the tandem helicopter to lean in opposite sides. The heli won't take off nice and vertical with all that shaking so rudder and elevator inputs are applied to level both rotor "discs" when throttle is applied. Not a big issue, just needs getting used to.

Hovering is extremely stable with almost zero aileron drift. In windy condition, there is a noticeable rudder drift and an annoying 'rocking horse' movement due to the gyro trying to level the 2 rotor to the same plane via DCP (differential collective pitch) whereby the DCP gyro changes the pitch in each rotor in order to level the heli elevator-wise. This was minimized by reducing the amount of DCP travel (reducing EPA values of the DCP mix) and increasing the DCP gyro's gain.

Elevator and aileron movements are quite aggressive and need to be tamed down by adding EXPO of about 25-30. Rudder movement is slow but provides adequate control in manuvering the heli around. Forward flight exhibits a noticeable amount of pitch-up effect. Suprisingly, this small tandem handles the wind quite well (not more than 8kph) but the pitch-up behaviour during forward flight increases tremendously and throttle management is a must.

I must say that this project turned out pretty well. Thanks guys for all your support.

Maiden Flight Video


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