I am still a little confutes on what motor to buy for a cretin airplane If I have a airplane that weights 5LB and I go with 150 watts per Lb do I buy a motor by wattage or KV ????
Watts more than kv... but it also depends n the style of airplane. If you don't have enough watts then you won't fly at all. If you have the wrong kv it won't fly well.
High kv for high speed style models and lower kv for slow flyers and 3D aerobatics. The high kv motors swing short props very fast which is great for going fast but bad for "static thrust"
thanks that helps alot
A lot of motor specification sheets spell out the intended performance range for that model#. That's what I usually go by.
As an example, the following spec.s were pulled from an E-Flite Power 32 web page:
Product Description\Key Features
That is just one example, A lot of manufacturers include info like that.
It usually entails a lot of reading, but a lot of times you can find on the forums (here, RCG, etc) where someone has built that same model and talks about their setups.
Kv is a determination of prop size and cell count.
a 600 watt motor with three cells might be 3000kv and have a 4.1x4.1 prop with would barely move a 6lb airframe. a 350 watt motor around 400kv running on three cells could have 180oz of thrust on three cells, but would require a 16" prop. there are quite a few variables, and they all matter.
you need to properly match all needs for the plane.
id reccomend using headsuprc at the very least as a guideline for what motors to look at. anything from a .25-.60 size motor may be appropreate for your plane. compare the cell counts of the motors to what you have and the size prop that would best fit the plane.
you moght want a 10" prop for ground clerance, or a 14" for lower watts and same thrust on the same motor, again depending on cell count.
i learned far more from reading on headsup site than almost anything else. another perk, send an email or call jeff at heads up, he will give you a recommendation, and you will not.be disapointed.with what you will end up with.
The Kv constant (motors have just one Kv, not 180Kv), is not a rating, not a figure of merit. More windings will give lower Kv, less windings will give higher Kv, that's all there is to it, no big deal, anyone can do that.
A motors Kv constant says nothing about a motors max.power, max.current and rpm.
Motorcurrent wants to go up with voltage squared and with Kv cubed.
About choosing Kv
brushless motors Kv? - RC Groups
Check Posting #5 in this thread. This is the size model you're looking at:
These models are slightly larger than 5 pound, but you might find something useful.
Thread on 70 size glow engine conversion to electric
Hacker 6S2P A123 powered Models
Hangar 9 Kantana Model
Hanger 9 Twist 40 Model (This model weighs exactly 5 pounds)
AEAJR's Site on Electric Power
What's a cretin? Not familiar with the airplane. Wing Span? High wing? Low wing? LG?
I have a few 5# + models . Using a 32 on the Escapade but a 46 on the HK Beaver and Phoenix EMB 312 Tucano and GP Piper Cherokee.
+1 on Headsup RC as a quick reference to Motors, lipo voltage, Watts generated, amp draw by prop size, etc.
Sometimes getting the perfect combo takes a little experimentation.
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