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which prop to go for ?

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ericrw09/05/2017 18:00:17
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My FMS Mustang flies with a four bladed 16x6 prop and a wingspan of 1600mm. However, there is a huge drain on the 4 cell 3700 battery. To overcome this problem, I have been advised to use a two bladed or a three bladed prop. Could anyone on the forum, advise me of what size to go for with either a two or three bladed prop ? Appreciate any advise !!

flight109/05/2017 18:12:51
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The problem you might have is you are using a 4 cell battery pack and the plane requires a 6s battery, you could always use two 3s packs in parallel to get the 6 s as its far more cost effective

Steve Hargreaves - Moderator09/05/2017 19:30:06
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What are you trying to achieve here Eric? Is the current draw too large for the motor/battery/ESC to handle?

A lower current will mean lower watts & less power to fly the model.

Denis Watkins10/05/2017 08:41:27
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This is already a big prop Eric, but rule of thumb is to add an inch for each blade reduction.

This leads to a huge prop that may hit the ground on turning, and the power consumption needs renegotiating.

A 3 blade would go to 17 inch, or increase pitch to 7 inch.

Power calculations take precedence here

ericrw10/05/2017 09:16:12
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I`ll try two 3cell batteries; but is it possible that faulty ESC could have an effect on the power drain ?

MattyB10/05/2017 10:35:34
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Posted by ericrw on 09/05/2017 18:00:17:

My FMS Mustang flies with a four bladed 16x6 prop and a wingspan of 1600mm. However, there is a huge drain on the 4 cell 3700 battery. To overcome this problem, I have been advised to use a two bladed or a three bladed prop. Could anyone on the forum, advise me of what size to go for with either a two or three bladed prop ? Appreciate any advise !!

FMS do not make a 1600mm Mustang to my knowledge - lots of 1400s and definitely some 1700s (though I am not sure they are still available), but no 1600s. That means it may be a Starmax, or you may have the span wrong in your OP.

Either way we need more precise details of model and motor specs (especially the Kv) to make a recommendation. However it is now pretty standard to re-motor and change the prop on the 1400mm foamie warbirds for 6S operation; they are all a bit undercooked on 4S. If you go to RCGroups and search there on "6S [manufacturer name] mustang conversion" you should get lots of hits recommending powertrain components.

Posted by ericrw on 10/05/2017 09:16:12:

I`ll try two 3cell batteries; but is it possible that faulty ESC could have an effect on the power drain ?

It is highly unlikely the ESC has anything to do with your problem - remember it is the motor and prop specs that define the current draw, not the ESC.

Do not go 6S (2x 3S in series) without doing some calculations and understanding what the current draw will be first; if you do your current will increase dramatically (probably around 2x given the dramatic increase in voltage) and it is almost certain you will cook the ESC and probably the motor too. You have to apply science and maths not guesswork if you don't want to fry components - time to do some reading on how electric powertrains work.

Edited By MattyB on 10/05/2017 10:54:14

MattyB10/05/2017 10:50:45
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Posted by flight1 on 09/05/2017 18:12:51:

The problem you might have is you are using a 4 cell battery pack and the plane requires a 6s battery, you could always use two 3s packs in parallel to get the 6 s as its far more cost effective

Errr no, that's wrong on two counts....

  1. There is no way of knowing from the OP if the model is intended for 6S operation - he has not included enough detail. Throwing a 6S battery at it without understanding the specs of the powertrain is likely to end in tears. If his current drain is already high it is only going to get (dramatically) higher on 6S with the same prop.
  2. 2x 3S packs in parallel is still a 3S pack, but with twice the capacity i.e. 2x 3S 2200 in parallel = 1x 3s 4400. To get a 6S pack you would need to connect them in series, but they then still have the same capacity as labelled i.e. 2x 3S 2200s in series = 1x 6S 2200.
Chris Walby10/05/2017 12:11:49
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Hi,

What size model is it as the FMS ones appear to be 1700 mm or 1440 mm wingspan not as OP?

One recommends 6S the other 4S.....

ChrisB10/05/2017 15:21:56
1226 forum posts
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It's a starmax P51 Eric. http://www.modelflying.co.uk/forums/postings.asp?th=107673
MattyB10/05/2017 15:56:54
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OK, based on 400Kv and the fact he already has a large 4 blade prop fitted there are essentially two options:

  • Fit a higher Kv motor more optimised for 4S operation on a 2 blade prop;
  • If your motor can take it, go to 6S on a two blade prop of similar diameter to avoid ground strikes, maybe with a tad more pitch (eCalc or similar can probably suggest some props).

Ultimately I am still not sure we understand the problem the OP is trying to solve. He needs a certain level of power to get authoritative performance, and that needs to be generated whether the model runs on 4 or 6S. What is "a huge drain on the 4 cell 3700 battery" in real terms i.e. current drain in A? What is his desired duration of flight? Without answers to these questions we are just guessing

Edited By MattyB on 10/05/2017 15:59:32

ericrw10/05/2017 18:55:22
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First I must apologise as the model is a Starmax 1600. My frustration is I`ve had this model two years and I`ve never completed a full circuit, due to loss of battery power. This is my only low winged `plane and after construction, my friend had, including the maiden, successful flights. In between times my efforts to fly it were not successful and copious amounts of Gorilla glue was used; not that much to upset the balance though. I`ve achieved the flying weight of 2700g This loss of battery power is only a recent problem; therefore, I cannot accept that it is the fault of the 4 bladed prop. The model flies (well did) with the makers standard equipment and recommended 4 cell battery. On it`s last fight I took off, it flew nice and level in to wind after about a minute flying ,my experienced friend, shouted that my battery was running out of power, as it came down in a Rape field. After retrieving it, the battery was reading 77% , it had started with 92%. As it took approx half an hour to retrieve the model, I assumed the battery had regained power.

Dave Hopkin10/05/2017 19:48:14
3672 forum posts
294 photos

There are two measurements you need to get to find out whats happening...

1) Is a battery tester that give you not only a %age of charge but also the voltage on each of the cells (from the description above I suspect you may have 1 of your 4 cells failing under load - or you are not balance charging the LiPo

2) You need to measure the current draw with the prop on (take care) most watt meters also display the current draw

Steve Hargreaves - Moderator10/05/2017 20:11:47
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Dave (& others) is right Eric...we need some numbers to establish were we (you!!) are...otherwise it's all just guess work.

Ideally kv of the motor plus current & voltage at full throttle.

I wonder if the motor is way over propped & you're pulling so much current from the battery, causing the voltage of the battery to collapse & the ESC is shutting down...?

MattyB10/05/2017 20:19:01
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Yep, this is either an overpropped powertrain, a time expired battery or both. The numbers from the watt meter will give us a much better idea, especially if tested over a min or so.

Dave Hopkin10/05/2017 20:58:17
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The C rating of your LiPo would help too... perhaps you are pulling too much current for it....

Chris Walby10/05/2017 22:13:23
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...just a couple of "non" scientific points

1 - Would the manufacturer not sized the motor & prop correctly, so we can assume its not overloading the motor or ESC, thus if its a low C rating or sick lipo likely to be the problem area

2 - Low KV motors (from experience) and low lipo cell counts (4S) puts you in an awkward area of ending up with big diameter props with large pitch !

I have a 370KV motor that can pull a 16 x 10 three blade and pull 26A (giving 2.2kg of thrust), big but here as some motors are not rated above 4S however I can take mine to 6S and get 48A (4.6kg thrust).

Lastly my money is on the lipo...I had one go low voltage on one cell....It was 5S and read 97 % ,,,,, but look carefully and the lipo checker only displays 4 cells.... the other trick the lipo has one cell fails under load so the ESC detects say 4S when arming...then after a large drawn (take off) it drops a cell...the ESC detects low battery voltage and cuts/backs off the motor (although you should still have control) but without the big fan at the front working the only way is down!

My conclusion...you won't get a big enough two blade prop without it compromising ground clearance + it will have a very large pitch....unless you up the cell count (if you can) or fit a higher KV motor

Chris Bott - Moderator10/05/2017 22:36:12
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My money is on the lipo too. But my conclusion then would be that the lipo needs replacing.

Ericw is this a model that has its original factory fitted motor and prop? If so then I'd try to find a way to test with a good known battery.
ericrw11/05/2017 09:49:55
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IYes! it is a factory fitted motor ,esc and prop. I`ll get a friend to check it with a Watt meter. I`ve just checked the the battery that I used with a Battery checker. the lipo is 18 months old with a 25c rating and the cell readings are (1) 4.012, (2) 4.023 (3) 4.030 (4) 3.994. Is the latter reading the culprit ?

All my batteries are Turnigy from HK but this year I purchased a more expensive one from a well-known UK supplier in Feb this year. In May, I had to get rid ,as it became bloated.

MattyB11/05/2017 09:59:32
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Don't just get your friend to check, buy a wattmeter - it is essential kit if you want to avoid frying components and will pay for itself quickly for that very reason!

Re: the battery voltages, that is slightly out of balance but not necessarily unrecoverable. I would put it on a slow (0.5-1C) charge and see if it evens up at once it has reached full charge, then run the wattmeter test and recheck the voltages afterwards. If you can check cell IR that would be good too, but your charger may not have that functionality.

PS - What voltage have you been storing your batteries at? That can have a significant effect on their longevity and IR.

Edited By MattyB on 11/05/2017 09:59:57

ericrw11/05/2017 12:36:41
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I have one but it looks very technical; rather off-putting ! I have just balance charged the questionable battery and the readings are now (1) 4.200 (2) 4183 (3) 4.210 (4) 4.201 99%. I do not store batteries as they are used quiet often, even in winter. I appreciate your responses to my problem and being of a certain senior age they are helpful !!!

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