Replies to This Discussion

Permalink Reply by james sowell on May 2, 2012 at 11:42pm

http://www.ecalc.ch/xcoptercalc_e.htm?ecalc

• ▶ Reply

Permalink Reply by James Dinh on May 3, 2012 at 11:30am

Thanks, I'll keep trying to understand all of those data inputs and what they mean.  One day. . . but for now, does anyone have a frame and motor size similar to mine and can suggest a prop make and size?

• ▶ Reply

Permalink Reply by Vernon Barry on May 3, 2012 at 12:25pm

Ok, this is really, really easy. You have the big 880kv motors so you need the most disk area you can get. Think of it like this, disk area is prop diameter times 3.14 (AKA area of a circle). Since you have hexa take one prop area time 6 and that's the total disk area. Now divide that area by the total heli weight. This gives you a disk loading number, meaning every inch of blade carries X amount of weight. It doesn't take a rocket scientist to figure out an inch more of prop (10 going to an 11, or 11, to a 12 inch prop) times 6 makes a huge difference here. So minimally we want the biggest prop that motor can swing and not hit the other props. Normally, those motors would use a 12x 4.5 prop. the 4.5 is how much pitch or how far the prop would travel forward in a single rotation (theoretical with no load).

The problem here is that you may need to make your arms of the frame longer to use 12 inch props if you aren't already. The next thing to do is increase the pitch but only until you hit the max amperage limit of the motor, which means you either need to use the online calculators, or actually measure one motor on the input side to the speed control full throttle. I believe those motors are maxed at 20amps and the 4.5 pitch at 12 inches will be the max you can push. According to the specs page and the testing, they say they got around 1300g of thrust using the 12x4.5 prop. In theory you could take that number x 6 and get a theoretical max thrust, but there is a loss not accounted for by the props being too close to each other (determined by how long the frame arms are). Also, massive word of caution is that your home built frame with the arms made of C channel aluminum is not up to the task. I can already see you made the motor mounts weak by cutting the lower half and just from the pic, it looks like one arm has the motor twisted off dead vertical axis. Those arms will flex in flight and then when the props hit each other, well, bad day is an understatement.

Sorry, but this is basically the poster child of why you don't cheap out and try to build your own frame. Second part here is that you likely built your own power distro setup. So let's do the math, 6 motors at 20amps each is 120 amps. Your giant batteries in parallel can probably meet that requirement (Battery capacity in Ah times the C rating=max amp draw). So a single 4000mAh battery rated at a concervative 20C can deliver 80amps easily. 30C pack could hit the magic load number 120Amps. But let's look at individual parts in that path. If you use red Dean's connectors,, they are rated at 50amps, or the new yellow connectors are rated at 60Amps. So a single battery, full load would smoke the connector. Luckily, if it all worked out and 2 batteries with yellow connectors is rated at 60+60Amps, we meet the 120 Amps margin.

But wait, then that means the power distro has to be able to pass 120Amps!!!!!, I don't know what you used or how you set it up, but if not done right, it will be the smoke point. Further, and heaven forbid, if it gets hot and allows a short (solder melting or copper peeling off a pcb) then those 2 batteries dump maybe as much as 1K amps until they catch fire and you could see the plasma from about 3 states away.

Second part here is forget about efficiency. You blew past that a long time ago just by choosing a heli which must beat the air to submition to stay up. Your batteries, motors and overall weight aren't even in the efficiency zone. Just be happy if you can get enough thrust to get it into the air.

From your original question though, the best you can hope for is the biggest props with a pitch the motor can get near maxing out amps is always going to be the best in this application. SO agian, looking at a chart for the motor, typical max prop is 12 inch, and then at 12 inch, 4.5 pitch gets you to the motor's max amps without burning up. That's how you calculate props.

 

• ▶ Reply

Permalink Reply by James Dinh on May 3, 2012 at 2:23pm

My PCB is not homebuilt.  It seems to be a decent quality, and my solder points are good.  I have been very careful every step of the way, even to solder all my bullet connectors from ESCs and motors.   The aluminum is 6061 aircraft grade,  and the frame was purchased as a kit from guys who have flown UAVs for over 10 years.  I have been able to hover at only 40% throttle with 9x5 cheapy props with a full payload of 2 heavy batteries.  My main concern though are the motor mounts (not really mounts at all, just 2 holes in the arms, where the motor screws go through)  Even with unbalanced props,  I detect very little vibration on the frame.  I did a video test with onboard camera, and there are no vibrations in the video which is super stable and clear.  The booms or arms are isolated from the main frame via anti-vibration bushings.  Everything seems stiff and very rock solid.  My 1 minute hover tests are very stable and it seems like a great platform for AP.  I don't think you'll see any plasma blasts from this area any time soon. lol. I don't feel much heat from the motors or the 30 amp ESCs or from the PCB (just a little warm).  After doing the computations that you sent me,  I will be going with 10" props with 4.7 pitch.  This will give me plenty of air space between props.  My arms are not long enough for 12" props.  So far,  I'm very happy with my hexa.  When I get better at the controls, I'll record some footage and post them.  Thanks for all your detailed advice.  You have helped this newbie quite a bit (after scaring me to death about plasma explosions and the like :).  I'll be sending you a friend request if you will have me.  Thanks again.

• ▶ Reply

Permalink Reply by wraith on May 3, 2012 at 3:35pm

hi, great post on the the working out what size props to use, , James be careful when using cable ties on Ur esc aye, if to tight u might damage the, love the go pro box for damage control

• ▶ Reply

Permalink Reply by James on May 4, 2012 at 4:22am

the short answer (at least for me) is: APC 12x3.8 Slow Flyer, theyre a huge improvement over the cheap 12x4.5 props.  APC also make 14x4.7 and have (quite recently it seems) released a 13x4.7, i dont know what the current draw would be on those motors with the 13 or 14" props.  the 12x3.8 will definitely work and in my experience could give you a 25% increase in flight time over the cheap 12x4.5s.

 

regards,

 

james

 

• ▶ Reply

Permalink Reply by Eddie Weeks on May 4, 2012 at 6:19am

I agree  APC 12x3.8 are the best I have tried... and I have tried a lot of different props..

Eddie

• ▶ Reply

DistributorPermalink Reply by Dany Thivierge on May 4, 2012 at 7:25am

That poor little APM2 needs some holes to get accurate barometric pressure... let it breath a bit! :) 

For props I am experimenting with these lately and found them a lot better. 

They are 10X4.5 carbon re-enforced nylon. yes as a result they are heavier (12grams each instead of 8 for cheap black ones) but they perform really well for me. 

http://www.canadadrones.com/Propeller-set-10X45-EPP-Style-carbon-fi...

Dany

www.canadadrones.com

• ▶ Reply

Permalink Reply by Geeteshkumar Shakyawar on June 13, 2012 at 6:47am

Dany..... m having really big problem with the propellers ...... today its second time my prop broke in mid-air..... the first time i was using this http://store.jdrones.com/product_p/ac1245bl.htm which were cheap one...........

after the first crash i simply dumped those prop.... and shifted to  http://store.jdrones.com/product_p/ac1245comp.htm and after few flights....... today even that broke in mid air........

so need some more reliable props.......  what are you reviews about http://www.canadadrones.com/Propeller-set-12X4-5-EPP-Style-carbon-f.......... are these reliable one......?

• ▶ Reply

Permalink Reply by Ellison Chan on May 4, 2012 at 8:00am

Your motors look like these RCTimer 880kv ones, here.

As everyone else mentioned, they were tested with 12x6 props for the 3s battery and 9x6 props for the 4s battery, according to the specs above.  But, seeing as how your hexa is short on prop space, you could go with these 10x4.5 props with a 4s battery.  Really nice props, and carbon reinforced.  Good prop for the price, and not cheepo plastic props, like from HK.  They're 10" instead of 9" for the reference props, but the pitch is lower, so should be similar loading to the 9x6.

• ▶ Reply

Permalink Reply by Vernon Barry on May 4, 2012 at 8:47am

My immediate concern is that the original question was about efficiency as well. The quad frame is wrong and further limits the choice to smaller props which combined with being close together limits efficiency and lift.

If there is nothing that I have learned from a really world experience is that prop diameter is the single biggest factor between flying and not. My first quad with 850kv stock motors and the suggested black 10x4.5 props would barely lift with my larger, heavy, outdated 3S3P 6600mah 11.1 volt battery. The problem was weight, but the fix was to run not suggested props being the 12x4.5 which is supposed to be all wrong for those motors, however, it flies like a dream and the motors, speed controls and everything else takes it fine. So some backyard engineering tells me that in the static thrust, partial throttle situation we need to hover, Bigger diameter is always the best choice and going to a smaller diameter, higher pitch is not the best choise.

Again, in a heavy lift situation, the larger props were the difference between flying and not flying. The 10 inch props could not easily get the machine out of ground effect, but once they got in clean air, then It would fly with probably 75% throttle. The 12 inch props take off with like maybe 35-40 percent throttle and none of the weird stuff coming into and out of ground effect.

Spinning a small prop faster is always a loss in efficiency as higher tip speed just equals more drag. Basically, that 4th cell and all it's capacity is just being burned off to give you the equivalent lift of a 3 cell which has enough prop diameter.

Being that he has already got the batteries, making longer arms and bigger props is the "correct" fix to get the max flight time from his setup.

 

Some info that backs me up:

http://aircraft-world.com/prod_datasheets/hp/emeter/hp-proptalk.htm

There are a number of established rules which apply to the use of propellers:

• The larger the diameter, the more efficiently it will perform - some use a rough rule of thumb that the efficiency is proportional to the diameter - for applications such as aerobatics which involve a lot of long vertical climbs or hovering  this is a good pointer. The ultimate example of the increase in efficiency with diameter is the helicopter rotor which, due to the large diameter is able to lift the helicopter with minimum power.
  
  So, diameter is the single biggest factor determining the required motor power and, in fact, has a quadruple law relationship to power. That is to say that to turn a prop which is double the diameter at the same speed will require about 16 times the power needed for the smaller prop, assuming the same pitch.
   
• Similarly, the power needed is proportional to the pitch for a given diameter.
   
• The power required from the motor is approximately a cube law relation to the rpm. So, if you want to double the rpm, then you will need about 8 times the motor power

Thus, my main points are valid, a larger prop is usually the best choice in hover, and using a smaller prop to spin faster takes a lot more power (thus the 4th cell). Being that the motors kv# determine the relationship also to rpm based on input voltage, this explains the "why" behind a 4S battery and a 9x6 prop as suggested. Again, the 4th cell is just being burned off rather than adding flight time capacity.

 

Other posters have pointed out the higher efficiency of the 12x3.8 is probably the single best choice. I'm using the cheap black props but totally understand in the article linked how the amount of pitch change across the prop could be better.

• ▶ Reply

Permalink Reply by Eddie Weeks on May 4, 2012 at 12:38pm

I agree with everything you said above about large prop dia is better for efficiency...

But active stability come from the ability to speed up and slow down the prop…

Too large props on low voltage and the craft gets sluggish.

Like everything… its better to a certain point.. then it’s a compromise.

I think 12" prop on 3 cell... is not pass that point, for most motors.

I like 12" props on 4 cell with 600 kv motors...

http://www.youtube.com/watch?v=ET1NbSOKS0U

Eddie Weeks

• ▶ Reply

• ‹ Previous
• 1
• 2
• 3
• Next ›
• Page