What a difference the right motor makes!
Been flying a 2m sailplane I converted to electric. Been flying it for roughly two months. I'm kinda new to the hobby and really new to electrics. Started flying glider on a 1650KV motor. After accumulating a few planes and motors I removed the 1650KV for installation on a smaller plane and installed a 1000KV on the sailplane. The 1650 flew it ok but not knowing how it should react I really had no basis for comparison. Wow--what a difference the 1000KV made! Like flying an altogether new airplane.
It climbs with authority instead of the straining it did before. Runs quieter and cooler. For whatever reason flights seem smoother--not having to continually change throttle settings. Left wattmeter at home so couldn't make power comparisons.
I'm really impressed.
Maybe now I won't crash so often. :)
how do you get to carnegie hall.... practice,practice,practice
Did you just change the motor or other things as well?
With all things equal a 1000Kv motor will turn slower than a 1650 one so it should have given a lower performance (and take less current) unless your previous motor was seriously over loaded.
Each motor has a maximum rated current. The prop diam and pitch has to be selected to keep within that figure.
My 880kv power up 450 slow fly has less power, but much more torque and thrust at lower rpm's. When you hit full throttle, no real differance between half and full throttle.
My 1100kv power up sport actually takes less amprage untill about 3/4 throttle, but has less torque and thrust at lower rpm's.
They both hit within about 10 watts of each other (230 for the sport and 240 for the slow fly), but because I do most flying at 1/2 or less, the sport motor is just as, if not more efficient and hits about the same flight times if I horse it around, and longer if I don't. It also spins a 10X5 where the slow fly throws a 11X8.5 for peak performance.
On a "sleek" or symetrical wing though, I think it would be the other way around. I am amazed at how big of a differance you can make just with a prop change and watt meter. If I flew my slow fly motor with a smaller 10X5 or 10X7 it might last longer and pull less amps, but it would have less thrust. You need to find a good ballance between thrust and amprage. I like to have a lot more thrust then I need to fly so I can knife edge, hang off the prop and have plenty of extra power to get out of trouble.
When I first flew this sailplane with the higher KV motor I tried a few different prop sizes till my instructor was happy. I don't remember what we started with but I know we ended up with an 8X4. I was using a 20 amp esc and using his wattmeter I think I recall a current draw of around 17-18 amps WOT. But on launches with a fresh battery I had to refrain from WOT till after a few minutes of flight time otherwise the esc would squeal and shut down. I eventually swapped out the esc to a 30 amp. If I remember my ohm's law and power pie at those current levels, I was at around 200 watts +/-. Ya'll correct me if I'm looking at something incorrectly here. The plane flew reasonably well but again I had nothing to compare it with.
I then read more about motors, props and batteries and thought maybe I should try a lower KV motor (I'm more acquainted with glow engines) so I ordered this 1000KV. I'm currently using the same 8X4 prop. I'm anxious to again try some different props and my new wattmeter. But the way it performed today was a big improvement from the other motor. btw--both motors were inexpensive Chinese motors bought off Ebay.
Oh, and thanks for all the responses. I've sure learned a bunch from all you folks!
With a 1000kv motor, it would likely be happier with a 9X6 or 10X5.5 prop. Where its a sail plane, a lower kv motor will be slightly more efficient, but to get the same or maximum thrust for it, you generally want a slower, or at least larger prop for peak performance.
Leaving a 8X4 prop on it will not degrade performance if your happy with the way it flies. Throwing over propping an electric motor can make a lot of differance on the amprage and can burn up the motor if your not careful. Throwing a smaller prop on shouldn't hurt it at all, and will lower your amprage and draw on the motor, equalling less power used (generally) and longer flight times.
Being as it is a larger sail plane, you might be a lot happier with a larger slower prop to get it up in the air so you can thermal quickly. If your going to use the motor to fly the plane constantly, i'd probably keep it as is.
My slowfly motor recommended at 11X7 for best performance, but there was a noticeable differance running a 11X8.5, and put me right at the maximum ratings, with more thrust.
On my dad's advance, his motor was only pulling 230 watts with what we thought was the recommended prop. Turns out we wrote down the wrong specs. Just by going from a 11X5 to a 12X8 made all the differance in the world on 3 cells, and took it from a very under powered plane to being reasonable performance with limited verticle andpulling 340 or so watts running two 1800's in parelell. Throwing a 4 cell 2200mah in it gave much better performance on the same prop, and still being well under ratings. It pulled about 425 watts on the same 12X8 prop, and minus shorter flight times, was a much better performer, but still way short of the 550w it was capable of.
We decided that we couldn't run the proper 13 to 14" props to get better performance out of the 3 cell because of prop clearance. We figured if e-flight recommended a 12X6 on their motor for this plane, we should be able to get away with the 13" prop on this plane. Instead we had well then a fingertips clearance, and decided not to try it.
Right now we are thinking of going to a slightly larger 1100kv motor for it, to try for longer flight times and better performance on 3 cells so we will be able to run a smaller prop and not have to re-buy batteries. With this other motor we should be able to get into the 700w range on his 4lb airframe, which should give awesome verticle when we want it, and hopefully better flight times when we don't.
Right now we are getting a 6 minute flight or so on the 4 cell 2200 and 7-10 minutes on two 1800 mah 3 cells, giving us the equivilent of a 3600 mah battery. I think we are working the motor too hard with 3 cells, hence the similar flight times out of the much smaller 4 cell. I think we could get better thrust and similar to lower amprage if we could fit the larger prop. Right now the plane flies steady at 3/4 throttle with a slight climb, and needs a bit over 1/2 to stay in the air.
On my slowfly and sport motor, I can fly either on less then 1/4 throttle and gain altitude.
Make sure your motor doesn't get hot to the touch after a couple of power runs until you can get a watt meter setup. However, I feell you should be safe running such a small prop on that motor.
Almost forgot, 75-100watts per pound is the general minimum target to shoot for per pound as a starting point. On a glider, you should have very high lift and really shouldn't need much speed or thrust. It is a sail plane after all. If you want to get it high quickly and then thermal it out, nothing wrong with having good verticle performance to accomplish that.
I like to way over power my planes though. I am sitting abit over 125 watts per pound on my trainer and nearly 200watts per pound on my slow stick. My slow stick weighs 19.5 ounces as is and has 240 watts or so.
Here's a couple links that will take all the "maybe this - maybe that" guessing out of the equation...both examples below were suggested to me quite some time ago by some very experienced pilots at one of the clubs I fly at......neither examples have led me astray and take a laymen's approach to the subject....
Hope this helps those in need.....as well as those who still rely on weight as the key ingredient for all set-ups.....!
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