Small motor, over-volt battery?

[kiwibrit]

Recently I needed a new Cardinal.  The kits had become temporarily unavailable in the UK - so, though I was unfamiliar with electric power, I took the plunge and bought an already built one- complete with all its electrics installed form one of our regular UK Stunt fliers.  The model flies brilliantly on the 11x6 three bladed wooden prop it was set up for  - with good line tension at all times.

It has a Will Hubin Timer with Castle Phoenix ESC. But here's the first unusual bit - it has an OS OMA 3825 750 motor.  Reading around, that motor is generally seen as a   .30 IC equivalent, but it has easily enough power in the Cardinal.  Here's the second unusual bit it uses 5S batteries.  Again reading around, most folk use 4S batteries with this motor.  Moreover, - the factory’s performance figures for the motor are shown in this page: http://www.osengines.com/motors/motors/motor-specifications.pdf - and they list a 4S battery in use only.

I asked the seller why he had used this motor/battery choice - and he told me that he had taken advice in stunt hangar - and suggested I do likewise!  So here I am, asking why a smallish motor is being used with an over-volt battery?  It does work really well, but most of my other 'electric' friends are flying with 4S batteries and motors such as the EMax BL2820/07 in similar size models (and using the Renecle system).  I'm guessing that it gives a good power to weight ratio, but would like to know, as I am converting another stunt model to electric power.

[Tim Wescott]

The .30 that motor is equivalent to is a screaming full-throttle leaned-out high RPM helicopter 30.  Not a 30 that's been richened up, repropped, and otherwise tweaked to deliver a good stunt run.  We run our stunt engines at an operating point that delivers far less power than they're capable of, because that's what delivers the sort of controlled power we need for good behavior in the air.  This is why an OS LA 46 will easily power that Cardinal at 90 miles an hour for ten minutes on four ounces of fuel during a "run away", yet is usually run to fly the thing at 55 miles per hour for six minutes on the same fuel and prop.

The 5 cells comes about for one of two reasons: the running speed is above 8300 RPM (which is about the maximum you could get away with using a 4 cell pack) or the 5 cell pack is in a convenient form factor.  8300 RPM sounds too low for a 6-pitch prop, so that's probably your reason.

You can use a higher cell-count pack for CL than might be wise with RC because our motors are regulated in speed -- they never run at full throttle, nor do we want them to.  There's an insignificant difference in performance between your motor running at 80% throttle with five cells and full throttle with four (the motor and ESC run slightly hotter, and the power from the battery is oh-so-slightly higher: that's about it).  So you size your battery voltage to insure that at the end of the flight, your motor is still running part throttle, so that there's always some extra 'oopmh' available to keep the motor speed up.

Your EMax BL2820/07 example would work, too, and work with four cells.  The difference between it and the OS motor is the kV -- the OS has a kV of 750 RPM/V, while the EMax has a kV of 950 RPM/V.  That means that with four cells, the EMax can turn at 14000 RPM at full throttle and no prop, while the OS can only turn 11100 RPM with the same battery.

[kiwibrit]

Thanks.  I think the revs are in the region of 9,500 (not sure since the setting seems spot on, and I've not put the programmer on it yet). I guess the final choice of motor was personal choice / what's to hand / fits the model elegantly.