Choosing Power - "The power struggle"

A question was brought up when I was choosing a helicopter for the job.  Turbine, nitro, gas or electric?  I asked around for other people's advice and what they were using.  I found most were using electric for AP. 

The main concern people had was noise.  People didn't want to attract attention in certain areas.  They wanted to be discreet. 

With electric it's cleaner.  You simply plug-n-fly.  You don't have to deal with the mess of liquid fuel.

The disadvantage of electric is shorter flight time compared to a gas helicopter.

Quite honestly, I think both gas and electric has its place in AP.  

Most aerial photographers use Lithium Polymer (LiPo) for more power.  But due to the weight of their camera mount, camera body, lens and various other electronics the flight time is considerably reduced.  Even with these disadvantages electric still fulfills the requirements for most AP work.  For a seasoned AP pilot the average flight time ranges between 5 to 8 minutes to acquire the images they need.

How much power? 

Initially I wanted the most power I can carry so I wanted LiPos.  Then I realized they came in different configurations.  Then someone asked what speed controller will you be using?  Then another, "What is your target head speed?".  and, "What's the Kv rating of your motor?"  Things just got very complicated.  @#$%^&*!.  I really didn't expect any of this.  LOL  I thought everything was plug-n-fly.  NOT!

Main battery above (2 x 6s per flight) and receiver battery (2s) below.

ESC (Electronic Speed Controller...  some call it Engine Speed Controller)

ESC controls the motor. 

I needed an ESC that can handle the high power of LiPos.  For high power capacity the Kontronic Power Jazz and Schulze Future 40.160 speed controllers are recommended.

UPDATE (2010) :  There are newer models from Kontronic and Schulze.

Schulze Future 40.160

Head Speed

Head speed is the RPM of the main rotor.  The necessary head speed to fly ranges 1200 to 2000 RPM.  The head speed you choose depends on the size of blade you will be using.

Since this is for AP work a lower head speed between 1250 - 1450 is all you need for an 800 (800, 810, 830) size blade.

For a 700 (690, 700, 710) size blade you could run between 1600 to 1700 head speed.

For a 600 (600, 610, 620) between 1800 to 2000 head speed.

Motor Pinion

The motor pinion partly determines your head speed.  

The motor pinion is the gear that attaches directly to your motor.  This comes in various number of teeth.  This can be changed to achieve the necessary head speed.  More teeth the higher the head speed.

16T Pinion

12T and 13T Pinions available at Minicopter.

Kv

Kv is the no-load RPM of the motor. (per volt)

The Kv rating for the Pletti motor is 510.

What does this all mean?

So the number of cells the LiPo batteries has, the number of teeth on the motor pinion and main gear and the Kv rating of the motor will determine your head speed (HS).

There is a math formula to calculate your head speed.  I'm not a math mathematician but it goes something like this:

For Voltage: Lipo Voltage per cell x Number of cells.  ie: 3.8 x 12 = 45.6 Volts

NOTE:  A Lipo's max voltage is 4.2v.  But during load the average voltage drops to 3.8v.  That's why I use 3.8v to calculate with.

For Motor speed:  (Voltage x Motor Kv) x Efficiency.   ie: (45.6 x 400) x 90% = 16,416 (motor speed)

(NOTE:  You could change the Efficiency value to figure out how your heli is truly performing.)

For Gear Ratio: This depends if your heli has a one stage or two stage gear reduction system.  Let's go with the Maxi Joker's two stage.

(2nd gear / Motor Pinion) x ( Main gear / 3rd gear).  ie: (34 / 13) x (67 / 14) = 12.52 Gear Ratio

For Head Speed:  Motor Speed / Gear Ratio.   ie:  16416 / 12.52 = 1311.55 Head Speed

This is a rough calculation to get you close to your target head speed.  There are many other variables that can affect your exact head speed.

Power Management