Archive for June, 2011

Pinion Gears and Calculating Head Speed

Calculating head speed for a heli requires a few pieces of information.

  • Pinion Gear Teeth
  • Main  Gear Teeth
  • Motor kv,  rpm per volt of input
  • Battery Voltage
  • ESC head room to maintain speed

head speed = (battery voltage) * (head room multiplier) * ( motor kv ) * (pinion teeth) / ( main gear teeth)

For a 6 cell pack the voltage will vary from:

  • fully charged 4.2V x 6 cells =   25.2V
  • down to 3.4C x 6 cells = 20.4V ( recommended max discharge )

Stock TREX 550e calculations

Motor:  600M/600MX = 1220 Kv
Pinion: 16T
blades: 520mm

21V * .90 * 1220 * 16 /170 = 2170   rpm

My calculations

Motor kv Neu 1515 2.5DF =  1650 kv
Pinion: 13T
blades: 550mm/553mm

21V x .90 ( esc headroom) * 1650 kv * 13T pinion/ 170T main  gear = 2384
21V x .90 ( esc headroom) * 1650 kv * 14T pinion/ 170T main  gear = 2568


550e Upgraded Electronics Wiring


The servo’s are all benched with end points set and sub zero’s in place using the SK-720, their Windows software connected by USB port and configuring the DX8. I started using a receiver battery, but once the battery and ESC had end connectors soldered, it made sense to use the BEC circuit instead.

The castle 4mm bullet connectors are soldered to the motor and ESC along with the Spectrum brushless rpm sensor on wires 1 and 2. I’m looking for a good dielectric grease to help with this connection.  I’m currently looking at going from a 16T to a 13T pinion based on the KV that this battery can pull, the 170 code main  gear etc…  The gear ratio and number of poles needs to be programmed into the DX8 for an accurate head speed calculation from the telemetry module.

The Deans connectors are soldered to the Hyperion battery and ESC. Female side on the battery to avoid shorting the battery. I used a Deans connector jig to hold the wires and connector in place. It worked very well. The flight pack voltage sensor is soldered to the ESC side. The shorter leads are also soldered together on the Hyperion battery because they split the battery as a 2S + 4S configuration.

The ESC has been set to mode 4 for helicopter use. Programming done while connected to the IO(T) connection. I’ve put the programming jumper somewhere safe.  

I used the following video for reference when setting up my ESC

Out of the box the BEC circuit is giving me 5.5V, and the Hyperion batteries are reading 25V as delivered without needing a charge. Also there is a note that the ESC may have issues regulating the speed of the motor without a load on it and what I saw was the speed run away. Hopefully once I have the blades attached it will work properly.

I’m not particularly worried about  the Neu 1515 2.5DF having overheating problems, so for the time being I have not connected the temperature sensor. I’m considering finding a way to mount it to the battery tray where the hole in the tray is located so it can  be raised to touch the battery, but I’m going to back burner that for now.

When benching the servos, it was interesting to see how easily a single 8717 could pull the receiver voltage down well over a volt. Once my Power Bus gets here and I have both BEC circuits in place, it will be interesting to see how stable the BEC voltage is. This BEC circuit is rated at 5A continuous and 15A peak.

550e Mechanicals done, starting on electronics

Modifications from 550e instructions.

  1. Extra shims added to L & R cyclic servos so they would fit in the frame.
  2. Rubber skids not installed on landing gear. This is to allow it to slide and not catch on concrete.

Downloaded the Skookum firmware update  to version 3.0.2 and Installed my new alloy servo arms and Lynx Tail fin.

I had an experienced heli guy look over my build and we bench checked all the cyclic servos connected directly to the AR8000 to find their centers and then I learned how to sub trim them on my DX8. The BLS251 is supported by the SK-720 but not the AR8000 so it is not connected as shown below.


Then I moved a DSMX satellite to the SK-720 and bound to it, removing the AR8000 from the equation completely.

Build list additions

  1. Second DSMX satellite
  2. SK-PB1  Power bus
  3. Spectrum Telemetry Brushless RPM sensor
  4. Additional  wiring and end plugs.

Below is  my current planned wiring for this except the SK-PW7 is an SK-PB1

SK-720 Wiring

Next up soldering Deans and bullet connectors and configuring the Kontroniks Jive 100+LV for mode 4 operation. When I have it all wired up on the bench outside the heli, I’ll create a picture of that.

TREX 550e Build Progress, Build list continued…

When you think you have a build list complete, think again.

I’ve read a number of complaints about the 550e 1.6mm thick tail fin being too flimsy.  Many have upgraded to 2mm thick  600 tail fins to reduce resonant vibrating at certain rotor speeds and to help protect the tail blades. A number of people also reported replacing the stock DS650 tail servo with a BLS251 which took care of the vibration problem as well.

Lynx has a 2mm thick tail fin designed for the TREX 550/600  that is supposed to cause less turbulence and make the tail more efficient. They claim the tail gyro gain can be dropped by 5% because of this improved efficiency.

Lynx Heli T-Rex 550-600/Raptor 50 2mm Carbon Fiber Vertical Tail Fin.

In addition since my upgraded kit didn’t have the servo arms meant for the original DS610 servos, I’ve ordered a set of alloy servo arms that will fit my JR 8717 cyclic servos. This is a mixed bag. On the one hand they promise improved response, especially in hard 3D use.  On the other hand the factory plastic arms can be considered a sacrificial part that could save other more expensive parts from failing.

TX55C081-BK Quick UK T-Rex 550 Alloy Servo Arm Set – JR – Black

The JR 8717 servo’s are also a bit deeper than the DS610’s and require an extra shim on each side to fit the frame properly, Otherwise their backs will touch and tightening the bolts will push the frame apart.

I’m also finding that the NEU 1515 is a very tight fit in this frame. However, the BLS251 fits very well.

A local friend is bringing over a jig for soldering Deans connectors. Female ends on the batteries so they don’t short circuit, and male ends on the charger and the ESC/BEC.

I also found out that my soldering iron is not nearly powerful enough to solder the new CC bullet connectors, or deans connectors with some of the heavy gauge wire I’m using.

The build will continue  once the soldering iron gets here.

Next to be added to my build list is a power bus of some kind and possibly additional Rx cables.