SIMON CRAGG Posted May 1 Share Posted May 1 I have more or less finished converting a Robbie high wing trainer into a twin 70mm 4s (2842-3400kv), powered model. EDF's under the wing fairly close to the fus. same ESC'S and all settings etc. equal. During the balance process, I found that a 40-80c CNHL 2700 MAH 4s lipo balanced the model ok. However, when running up the motors, they sagged considerably after about ten seconds at maximum throttle. Obviously I am trying to take to much from the Lipo!. I am not an expert on the subject, is it the C rating, capacity? or something else? The packs are fairly new and charged up ok. Can anybody help please?. Quote Link to comment Share on other sites More sharing options...
Shaun Walsh Posted May 1 Share Posted May 1 (edited) 17 minutes ago, SIMON CRAGG said: I have more or less finished converting a Robbie high wing trainer into a twin 70mm 4s (2842-3400kv), powered model. EDF's under the wing fairly close to the fus. same ESC'S and all settings etc. equal. During the balance process, I found that a 40-80c CNHL 2700 MAH 4s lipo balanced the model ok. However, when running up the motors, they sagged considerably after about ten seconds at maximum throttle. Obviously I am trying to take to much from the Lipo!. I am not an expert on the subject, is it the C rating, capacity? or something else? The packs are fairly new and charged up ok. Can anybody help please?. You need to post a bit more information about the edf units and the speed controllers. If you are running 2 70mm fans off a single battery you are probably drawing too much current for the battery to maintain voltage. You need to connect a watt meter/ammeter just after the battery and monitor current draw and voltage. Edited May 1 by Shaun Walsh Quote Link to comment Share on other sites More sharing options...
SIMON CRAGG Posted May 1 Author Share Posted May 1 THE ESC's are Hobbywing 80amp. The EDF'S r the Powerfun version sold by 4Max Models. Quote Link to comment Share on other sites More sharing options...
PeterF Posted May 1 Share Posted May 1 Thst sounds like much to much fan for a 2700mAh battery, that size battery is typically running 55mm fans, it.might do 2 55mm fans but not 2 70mm fans. I have a model with twin 70mm fans on 4S, it pulls 105A and I use a 62500mAh battery. Your may not be that high powered but as said above, you should always have a Watt meter to check. Quote Link to comment Share on other sites More sharing options...
SIMON CRAGG Posted May 1 Author Share Posted May 1 Just checked on the watt meter 77A / 1145W. Looks like I might have to try a 3s! Quote Link to comment Share on other sites More sharing options...
Chris Walby Posted May 1 Share Posted May 1 I have a SU47 with twin 70mm edf's and it can kill a lipo in 2 min 15 sec on WOT with a 6S4500, don't ask me how I know 😞 You are simply asking way too much from the lipo, not forgetting the bigger the lipo capacity the greater the overall max current based on the same size C rating e.g. 2700mAh at 50C = 135A 4500mAh at 50C = 225A I have a Habu with single 70mm and 80A ESC and if I fly more that 10 seconds at WOT it will give me a low lipo voltage warning (not go into LVC), but if I hold it at that for 30 seconds it will go into LVC. I will fly at 30% throttle and I only use WOT for loops etc and get 5 minutes without knackering the lipo EDF's give lipos a hard time, but then again the Focke Stick does the same at close to 120A on a 4S4000, but it is very fast (what it was built for) Quote Link to comment Share on other sites More sharing options...
SIMON CRAGG Posted May 1 Author Share Posted May 1 Understood. I'll do some experiments with a 3s and see if that is any better! Quote Link to comment Share on other sites More sharing options...
Tom Gaskin 1 Posted May 2 Share Posted May 2 If you have the room why not connect two packs in parallel? Double the capacity should help your performance. Tom Quote Link to comment Share on other sites More sharing options...
Chris Walby Posted May 2 Share Posted May 2 13 hours ago, Tom Gaskin 1 said: If you have the room why not connect two packs in parallel? Double the capacity should help your performance. Tom Tom, In my experience with edf's weight and wing loading are critical as opposed to a more tolerant approach with propeller aircraft (probably due to the overall lower efficiency of edf's once you add inlet and thrust tubes). My point is that you need to almost build the model around the battery/edf fan size with low drag and minimum wing loading/low drag to get something that flies well (slow and fast). The old adage of just banging in a bigger power source does not work as the wing loading goes up so it has to fly faster and generally the stall becomes more abrupt. take off and landings are faster so it eats its way through UC or becomes marginal if hand launched. Generally a dog! Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted May 2 Share Posted May 2 I agree entirely with Chris W The question is do you actually need 1145W? You will certainly reduce the power (amps) by about 50% going to a 3s but it does then suggest that the 70mm EDFs are a bit bigger and heavier than you actually need. However if you can stand the extra weight there is a small advantage in reducing the voltage to an EDF, it becomes more efficient at converting the reduced Watts into thrust but this is only relevant if you have an adequate performance at the lower power levle. Ideally you try to size the EDF to match the needs of the plane as an EDF and its battery is heavy enough as it is. Quote Link to comment Share on other sites More sharing options...
SIMON CRAGG Posted May 3 Author Share Posted May 3 Even on 3s it was pulling 84a / 960w, so its time to bite the bullet and get two 50mm fans and run on 4s. Good learning exercise so far, and good ideas on here as well. Thanks. 1 Quote Link to comment Share on other sites More sharing options...
SIMON CRAGG Posted May 6 Author Share Posted May 6 Well, good news on the test flying front. The model flew perfectly, but not wanting to smoke the lipo I flew around at less than 1/4 throttle. As I suspected, it is way to overpowered on 2 x 70mm EDF, so two x 50mm on order. Lighter 60a ESC as well. However, a VERY rewarding model, which was given to me, and a great experiment (Robbie Cessna 150 - I think!). 1 Quote Link to comment Share on other sites More sharing options...
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