Joined Oct 2003
Flight Performance of 1400mm FMS BF-109
I finally got my 1400mm FMS BF109 into the air today. To make a long story short, I was somewhat disappointed in the flight performance. I flew it twice and on both flights I spent the whole flight at full throttle to get anything approaching enjoyable performance. I am not a speed demon, but I would like to fly and enjoy the speed at less than full throttle except for vertical maneuvers. Now I'm new to these electric foamies and maybe I am expecting too much, but here goes...
4c 2650/3000 mah batteries, the motor says it's a 520kv motor, the ESC instructions cover three different models (20A, 30A & 50A), I'm thinking it's the 50A versions. the plane is flying on a 13x9 3-blade prop that I know hurts a little.
So I'm not that up on the technical part of this electric stuff, not even sure what is interchangable. the plane is stuck with a 3-blade prop because of ground clearance.
Could someone help me out here with what I would have to do to increase performance a bit.
Here's the problem....the kv of the motor is too low for the pitch of the prop for the given voltage(4s)
A 520 kv motor using a 9 pitch prop on only a 4s lipo.
Here's the rundown....
4s lipo = 16.8 volts charged (minus a few tenths for battery condition....we'll use 16.5. A decent lipo will push out the amperage similar to a fully charged battery till about 3.85 volts per cell so you dont see much of a drop off in power output till then)
16.5 volts x 520kv = 8580 unloaded rpm
8580 x 9 pitch of prop = 77220
77220 x 60 minutes = 4633200
4633200 / 63360(inches in a mile) = 73 miles per hour(no drag or prop load, efficiency factored in)
So take 75% of 73 mph and you get 54mph. So..yeah...not exactly screaming fast.
if you change to the 600kv motor, you'll get 84mph unloaded, 63mph real world. A nice improvement.
If you get a spinner with a 2 blade cutout, use a 13x10 and you should see 60mph real world on the 520kv motor or 70mph real world with the 600kv motor.
In all honesty, a 520-600kv motor is suited for a 5cell setup, not a 4s.
I have a 500kv turnigy motor possibly going in my bash of this me-109(whatever...E/F/something) and Id be using a 5s lipo with it
5s = 20.5 volts
5535000 / 63360 = 87mph unloaded or 65mph real world with the stock prop
Or using an e-flite power 25 with a 870kv motor on 4s
7751700 / 63360 = 122mph unloaded or 91mph real world
I was using the power 25 with a 12x6 APC E prop on an E-flite Erratix 3D 55" wingspan balsa plane and had unlimited vertical.
The 3 blade 13x9x3 from FMS might be a bit much for that motor, but as long as you dont go full throttle often..it'd be fine
A very nice cheap upgrade is the monster power 32 from hobby partz(22.00) (out of stock) i put a hyperion 3025-990 in mine , on 4s were guessing 80 mph , was 39.99 at aircraft world , 2.50 shipping , just going to a 2 blade prop will help a bunch , 13x10 ish
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