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PostPosted: Fri Mar 29, 2013 9:16 am 
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Some motor vendors have it all messed up. You just can't tell what you are going to get from the description.

Lets look at this motor for an example:

http://hobbyking.com/hobbyking/store/__ ... Motor.html

So they claim a kv of 500 and a max voltage of 19v. Ok, so that is a 5s battery. 18.5v x 500 gives you an rpm of 9250rpm. That is pretty stinking low. This KV should really be for a 6s motor as 22.2v x 500kv = 11100rpm. Yea, that's more like it.

Anyway, to get the power you need out of 9250 you are going to need a really long prop, but wait, the current rating is 57 amps (side note, I would bet money it would cook at 57 amps). Huh, wait a second, something doesn't add up. Lets' see, max voltage (19v) x max current (57a) = 1083 watts. There it is, yup, they claim 1350watts, but the max voltage and current would only be 1083watts. Not to mention, even at 1083 watts, that's 4 watts a gram, yea, I doubt that.

In the comments you see people complaining that it's not rated for 6s packs and that the old model it replaced is rated to use 6s packs. What is funny is that HK didn't really base the pack rating on any real data, this motor can run on 6s packs just fine, the problem is that you would need to keep the over all power down to 800-900 watts (which is much closer to what this motor can actually do). I suspect limiting this motor to 45amps is much more in line and will work on 4s 5s or 6s packs.

So there you go, you have a motor that is wound at a KV that really requires 6s, but because it's small, can only do 800-900 watts. HK gets around this by saying it's a 5s motor so people almost certainly stay under 900 watts, but then they only get 9250 rpm.

If we compare to a better quality motor of similar kv and weight:

http://www.innov8tivedesigns.com/produc ... cts_id=892

You can see that it is rated to do 1000 watts on 6s and 830 watts on 5s. This motor says the max continuous current is 45 amps which on 5s is 832.5watts and on 6s is 1000 watts. So the specs match.

Keep in mind the cobra motor almost certainly has better wire, magnets, and bearings.

So what is the point of this post? I'm sure many are wondering at this point.

The point is that cheap motors not only are made of cheaper materials, but the specs don't add up, so you need to make some assumptions in order to figure out what setups will work. The higher quality motors tell you what they can do from the onset are thus are much easier to work with.

I'll make one more point: Electric aircraft aren't like glow where you bolt the biggest prop the engine will turn and you end up with whatever performance that gives. Electrics allow you to dramatically increase performance with a simple battery or prop change, so it's more difficult to figure out what you can get away with when you are looking at maximum performance and minimal weight such as the case with heli's or 3d airplanes. Either motor will run all day long at 800 watts on 5-6s batteries, but things get more tricky when you are really after 1000 watts on the least weight possible.

schu

One more thing, as I mentioned, earlier, I don't have any 5s or 6s packs that are size/weight appropriate for a 5lb airplane, so if I was going to get a motor to work on 4s I would go this route:

http://www.innov8tivedesigns.com/produc ... cts_id=890

The kv puts the motor at 11544rpm on 4s, and running 60 amps puts the power at 888 watts. If that came down cool enough, I would prop it up to 70 amps for 1000 watts.


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PostPosted: Sun Mar 31, 2013 12:08 pm 
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I agree with most everything you said Schu,

But kind of funny I run that very (silly) motor.
http://www.hobbyking.com/hobbyking/stor ... duct=18123
On 6S and prop'd for 940 watts. So Yes you must look through what a lot of manufactures say in their spec's (not just HK).

Is HK Turnigy stuff inferior?? (Magnets, bearings, wiring) Once again I agree, probably yes.

But look at the prices.

$35
http://www.hobbyking.com/hobbyking/stor ... duct=18123

$120
http://www.innov8tivedesigns.com/produc ... cts_id=892

I've heard plenty of guys say without HK pricing they couldn't afford much hobby stuff.

All that being said I understand the point your making. Just adding that HK stuff is a good fit for a lot people.

Dean


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PostPosted: Mon Apr 01, 2013 2:16 pm 
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Of all of the people that actually have the silly motor, it had to be Dean!!! :wink:

What's up Dean? How are things going with you? Hope to see you at the field this summer.


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PostPosted: Mon Apr 01, 2013 4:07 pm 
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I meant no offense Schu. I have DA-120's that cost 180% of what a DLE-111 costs. And that DLE will do 95% of what the DA does. But the extra cost for the DA is still totally worth it to me. So I am unfairly on both sides of the costs vs performance issue.

Been busy with my new work schedule. I just work too much. But trying to get in some R/C flying when ever, where ever possible. Its been over a decade now and I am still firmly addicted to the R/C thing. To much fun.

Hope see you at the field,
Dean


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PostPosted: Mon Apr 01, 2013 7:01 pm 
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Interesting you say that, I've been thinking about a gasser, and was wondering of the DA-120 was worth the extra money....

schu


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PostPosted: Tue Apr 09, 2013 7:02 am 
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Huh, did hell just freeze over? Schu, I could have sworn up and down I heard you say you'd NEVER go back to piston powered models :)

I'm thinking about a gasser too. I'd like to see how well a DA starts and runs.

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PostPosted: Tue Apr 09, 2013 9:58 am 
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Oh, no, I never said that, my issue is specifically with glow. They are just too finicky, messy, and expensive for my tastes.

I've run OS, Saito, magnum, 2 stroke, and 4 stroke, and the only engines I really liked were the OS 2 stroke ringed engines. The rest never really made the power I wanted for 3D, needed valve adjustments, or were too heavy.

And this wasn't because I didn't know how to tune an engine, I completely understand the role that fuel/plug plays in timing, how to setup the low speed needle, and the need for good quality bearings.

Here are some of my engines running:

http://www.youtube.com/watch?v=oCXmEeRPJ30
http://www.youtube.com/watch?v=8mhKk6whs_8

Ultimately I didn't like the mess (my electric airplanes looked like new 100's of flights later), and I really didn't like the consistency.

If you have a good quality engine, a good installation, and really consistent fuel, then they aren't terrible, but not like electric. My electric stuff flies exactly the same week in and week out except for perhaps loosing a little bit of power after the first 100 cycles on the batteries, which really doesn't make that much difference in how they fly. I could count on my electric stuff to not do something weird when I'm 3' off the deck with a rather expensive airplane while tumbling.

While I don't think gas is as consistent as electric, it's a big step in the right direction because they have a real fuel pump, and the timing is electric so the fuel quality has much less to do with it.

This is why I really want a DA-120. I high quality engine with a fuel pump and 2 cyls to reduce the vibration.

Dean, if you find a good deal on something locally please let me know, that may be the best way to get into a larger gas airplane without as much cost.

schu


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PostPosted: Tue Apr 09, 2013 11:17 am 
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OK, I will.

You just missed two great deals. PAU 36% Edge DA100 RTF $1800 And a WildHare 35% Extra Crashed but repairable BME110 $600.

Dean


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PostPosted: Tue Apr 09, 2013 10:18 pm 
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Schu do you have that magnum 70 for sale? I will buy it if you do.


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PostPosted: Tue Apr 09, 2013 11:04 pm 
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Nope, all my glow stuff is long gone.


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