Prop Selection and battery life
 

Hey guys, just a bit of a query, I'm unfortunately commited to running my existing setup for the next F3a comp (next weekend), which is a 6S setup as stock in the MYTHOS 125e (hacker A60-6xs).

I'm finding that I'm running out of oomph, right when I need it, i.e. the knife edge half square loop on the corner.

I'm currently running a 19x12, and I see a lot of the 3d guys using 20x11's on the same motor.

The airframe is pretty draggy, with all its canalisers fences and fat trailing edges.

My question is should I expect a decrease in battery life, if I went for a 20x11? As it is I think I'm burning milliamps away by trying to fly too fast, but if I fly slow I'm in the air longer and run out of pep.

My thinking is that if I can fly on a prop that doesn't use as much wattage to fly at the same speed I could expect a little longer battery life.

Thoughts?

RE: Prop Selection and battery life
 

Have you tried this site yet?  I have found it helpful solving these kind of problems in the past:<div>
</div><div>http://www.ecalc.ch/motorcalc_e.asp?ecalc</div>

RE: Prop Selection and battery life
 

Hi,
What is the all up weight of your set up.

Brian

RE: Prop Selection and battery life
 

Challenging power management is what you pay for all the pros in electrical setup.

You may consider one or more of the following|:

- using larger prop by itself will increase the motor current and overall battery consumption
- Make sure you don't use extra power when you don't need it e.g on initials maneuvers.
- I would suggest not reaching final voltage of 3.8V per cell for pack longevity
- Consider flying at lower speed. Power required for flight is in 3rd power of the speed
- Consider using larger capacity lipo pack
- if neither of the tips is practical for the upcoming competition, you may consider not changing anything but accepting some damage to your packs for the sake of good scores.

yoav

RE: Prop Selection and battery life
 

I am no expert on this, but it sounds like the right move to me, theoretically.

The 19x12 would slip more at acceleration and lower speeds. The 20x11 would bite more, and give you power when you ask for it. I think it would help you to fly slower with more deliberate throttle usage. You might struggle a little to maintain the same constant speed (like feeling more touchy) - but we are not supposed to be judged on speed.

My only relevant experience is with my patternized 120 Yak. I had a 'pattern' setup with an 18x12 and a Hacker motor. It was definitely a consistent, quick speed and battery time was short. I also flew an 18x10 and it was considerably slower, but I found I was on the throttle much less. Speed was down but I can fly the plane forever on this setup. Maybe its a touch too slow for pattern, but it demonstrates the difference. With the lower pitch, I used throttle where I needed it, and backed off when I didn't. I had a harder time finding that line with the higher pitch prop. (and I think a lot of this has to do with pitch/diameter ratio, not just pitch - which is what the calc tools warn about 'prop stall thrust')

RE: Prop Selection and battery life
 

First things first... I have zero experience to offer, just thoughts about how I might consider resolving this issue...

In calculating the volume of air pulled per revolution between a 19x12 and a 20x11, the 20x11 actually pulls about 1.5% more volume. Take this and couple it with the additional mass and length of the prop, I would say if everything else remains a constant, I would anticipate it pulling more energy from the battery.

Going to a 20x10 would pull only 92.33% of the volume of air that a 19x12 would... Which I would believe will be a better solution than a 20x11.

However, all things will not remain a constant... The plane would fly slower and thus less energy required due to the drag co-efficients... Gets into physics I don't understand...

Experiment with a couple different prop combinations... I would think a lower pitch prop will provide for more torque...But that may result in pulling more energy from the battery for your flying style...Too many variables to deal with for me...I would recommendgetting a couple of different props and go with the one that fits the needs and your style of flying...

Food for thought...

RE: Prop Selection and battery life
 

SomethingI just thought of... When I was looking to determine what battery I should use with my new 2M Monolog, there was much discussion about the "C" rating for the battery.

It seems to me that a higher "C" rating would actually provide for better battery performance resulting in more available energy to the prop later in the flight. Lower internal resistance, thus the battery would maintain a higher voltageover a longer period of time...

What is the "C" rating for your battery?

If it is a 20 or 25C, perhaps going to a 30C or 40C battery solves your problem as well as less risk of damage to the battery... As somebody else mentioned, capacity seemed to be part of the recommendations to me as well...

Food for thought...

If you can post your battery specs, it might go a little further in helping you out...

RE: Prop Selection and battery life
 

Well the results are in. I took her out for a practice this morning, and with the 20x11, I had a slightly lower cruise speed (more time to think!) and had enough power to get through the schedule, even with a 15knot headwind, and putting them on the cellpro, the packs are reading 18% a little lower than I'd like but markedly better than before.

For reference the packs are 2x 6s Zippy Compact 3300's in parrallel, the packs are rated at 30c.

RE: Prop Selection and battery life
 

Something worth doing is comparing what your charger tells you when you start charging and how much it actually puts back in. I use the Hyperion DUO chargers. My packs will show 20% or so left when they go on charge. So for my 4400 packs that means I should put back at least 3520 mAh into the pack. But I almost never see that. Generally the packs only take 2800 to 2900 mAh to get back to 100% charge, meaning that the pack was really at about 35% capacity.

So take a look at how those numbers compare. You might be surprised.

RE: Prop Selection and battery life
 

I do that regularly, but I thought in this instance the numbers really only mean something to me, but I'm putting back about 1700-2000mah into each pack, so that's quite manegable.

RE: Prop Selection and battery life
 

Power is watts. Changing propeller sizes, pitch, and RPM can increase or decrease efficiency.
The task you have requires a certain amount of power.

Within reason, you can play with props, current and voltage limits using ECalc/prop calc.
 
The motor you are using with a 6s battery should turn ~9000 RPM at full throttle with the correct prop,
assuming 4v per cell. The current should be about the maximum rating for the motor for maximum power out.
Prop pitches above 8-10 tend to be less efficient.  A larger prop with a smaller pitch
is usually preferred to a smaller prop with a high pitch.