Thread: Discussion HK Radjet 420
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Old Sep 12, 2012, 05:16 AM
e_lm_70 is offline
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Originally Posted by Ben D Wing View Post
As an addition to my last post: With the CG set at 220mm back, the left elevon has approx +0.5mm reflex and the right +1mm, but even so I think the balance point could benefit with going slightly further forward as the RJ is slightly pitch sensitive and does not pull out of a dive and this of course indicates a neutral CG.
After balancing the prop many times, my RJ has always displayed a resonance at full throttle with the dome prop nut looking very blurred. After trying many different props, I decided to swap the prop nut with a standard hex alloy nut from my spares box, and guess what, the vibration issue was cured! Now the motor is smooth through the whole rev range and this can only be good for the motor, Rx and servos as in my experience long term vibration usually causes some sort of failure. It seems that the supplied dome nut is not very accurately machined, but then I've also found this phenomenon with larger dome nuts of varying makes.
Finally, having never bothered to measure the motor's output before, I dropped my Wattmeter across it and at WOT it's drawing 3.1A with a power level of 23.5W (in). This may not sound much, but it does have a PWR of about 173W/lb and gives the RJ excellent vertical performance. And talking of performance, it's uncanny that this little RJ has almost identical flying properties to it's larger cousin, the original Multiplex Funjet. Well done HK!
It is looking to me that the 5g 2000kv motor on 3S is a better choice ... I'm flying with a 3S 180mAh ... and I have lot of power.

If you get vibration from your motor ... you better need to check the mount ... I did originally not glue the motor mount on the fuselage ... and I was having bad vibrations ... now with one side glue, and a line of transparent tape for reinforce & tight the foam around the mount ... it is better.

Stock propeller, as well ... it is not as good as a Graupner Cam Speed 4x3 that I had in my RC box since ages ...



ps: I don't have the stock motor, I build my R420 from spare parts ... including the propeller and propeller adapter ... but possibly the propeller save adapter from my 5g motor is even better then stock adapter ... I will give a try with a normal nut if it will get better ...

pps: My 3S RadJet 420 , without removing the bottom decals (they really help me for orientation) ... in ready to fly configuration is 69g ... so ... still pretty light ... I get over 5min full power fly with my 180mAh 3S lipo. .... I can fly also with 2S 300mAh ... this is over 10min ... if I use the stock propeller , I can even fly longer ... but not sure I can have enough power with 2S set up

pps: Do you have an anemometer or a RPM meter ? ... I'm curios to know how many RPM or how much wind you can produce with stock power ... my guess it is that 3S 2000kv is more efficient then 2S and 3000kv ...
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