|Aug 13, 2003, 09:34 AM|
Central Lake, Mi.
Joined Nov 2002
.40 size Piper Cub conversion
To start with this is a Great Planes model that has been flown with 4 stroke engines and is a great flying plane. However I am tired of butchering expensive glass cowls and cleaning the mess from glow fuel , and since I seem to be flying more electric than glow nowdays I want to convert it to electric.
Using E-calc , I tried a Mega 2245-3 and 2:1 ratio with 18 3000HV cells , 13X10 prop calcilated 89 watts / lb. Should be more than enough to fly this plane in realistic Cub manner.
The question is: Is this motor too physically small? It is designed to handle more than the 40+ amps calculated.It just seems that after seeing people using the larger cobalt 40 motors these brushless motors look too small to be able to produce that kind of power.
|Aug 13, 2003, 10:03 AM|
The more volts the faster you rev, the more power. The more amps, the more power. In the end what limits you is the mechanical integrity of the motor at high RPM, the ablilty to commutate at hgh RPM (ESC) and the efficiency of the motor - i.e. how much of the power you are pouring in comes out as heat, and how good it is at getting RID of that heat before it destroys the motor.
For eample, as power is proportinal to torque, which is proprtional to current, and is also proportional to RPM. which is proprortional to voltage, going from a motor which will do 30k RPM at an efficiency of 70% to one which will do 40k at an efficiency of 85% means pretty much that you have halved the heat for a given current, which meanas you can get twice the power for the same heat, and in addition you have 33% more RPM available at the same current, so that motor which doesn't at first sight seem to be THAT much better - just 15% on efficiency and 30% on RPM - is capable of delivering 2.6 times the power for the same heating effect.
Makes you think....
Race car engines of 3 liters are currently delivering around 800bhp using revs in the 18k+ bracket.
My 4 liter V8 road engine only delivers 360 and that's with a charger on it..at 6000 rpm...
If we could build an astro 010 that would rev to a few hundred k rpm, and a controller that would commutate it, it might have all the power of a good 61
As it is most brushless and ESC are good for 40k and no more. But with 85%+ efficiencies you can pour the juice in.
Batteries are where the real problems are. Still too heavy to really give HUGE durations, tho the power is certainly good enough for 10 minutes. I calculate that about 4 hours level flight is the limit for LiPoly technology at the moment. Maynard Hill has just done 40 hours on a 4-stroke.
|Aug 13, 2003, 11:12 AM|
United States, VA, Arlington
Joined Dec 1996
We had a GP 40 sized Cub in our group - built straight out the box, with a huge sheet of ply in the floor to take the 16 nicad. It used an Astro 25G, which was way overkill.
My slightly larger Sig 1/5th has presently got a MaxCim 13Y on 20 cells - spends most of time acting like a Cub with around 30% power setting, so is way overpowered, she gets tried on 16 cells when we stop having thunderstorms / rain / miserable humidity around here. Idle calculation of my sort (guesstimations ) suggests mine is flying around on something like 200W and gets airborne well before I get to half throttle stick in far too short a run for a Cub.
Suggest you look to propping / gearing for around 30 - 35A current draw, should be possible and doesn't open the poor motor to being flogged too much. In terms of physical size, my MaxCim is idling at 600W and isn't too much bigger.
One of my realworld magazine column readers recently told me how he swapped the brushed motor in his 14 cell Cub for a BL and found the model to be unpleasant to fly on the new motor. Sensorless BL have very high and abrupt idle speeds compared to brushed motors and sensored BLs, and this meant he couldn't use a low enough power setting to happily fly his Cub like a Cub should.
Easy answer - as long as you stay in maker's specs re V, A and motor RPM, your BL choice isn't 'too small' because of physical dimensions.
Batteries - don't get suckered by lipo if they leave your wallet in shock. My Cub indicates she's capable of ten minutes on 2000 mA nimh, which I had handy. As I get bored rigid of cruising around after six minutes, and the cells are cool enough to start recharge on landing - what's the problem, despite my cells not being all that sexy ?
Free advice, worth every penny (as is most internet advice ) - go do it! You know the model flies well, you have a handle on the drivetrain, so clean off the old castor oil and give your old faithful a new lease on life.
|Aug 13, 2003, 03:10 PM|
Joined Feb 2002
I have flown the Kyosho Tiger Moth with the Mega 22/45/03 at 2:1, 12 2400mah cells and a 11x14 prop. This is a ".40" size model at about 7 lbs, so I think it would be similar to your cub.
It flies great, very scale. I started at 18 cells and was happiest at 12 cells. I have found that with this setup, 40 watts per lb is good for scale perfomance.
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