Prop size?

[SIMON CRAGG]

I may need to change the 13x7 3 bladed prop for a two bladed equivalent. Am I right in thinking a two bladed 14x7 would be about right. Dynam FW190.

Edited By SIMON CRAGG on 11/02/2019 12:42:14

[Denis Watkins]

That's it Simon, 1" diameter increase is in the right direction

Test with the wattmeter for changes

Edited By Denis Watkins on 11/02/2019 13:54:37

[SIMON CRAGG]

I was hoping to find an on-line conversion chart of some kind, but no joy so far!

[Martin Harris - Moderator]

You will find that two props from different makers with the same nominal dimensions can give very different results so conversion tables might be misleading unless comparing particular makes. At "normal" sizes, the 1 inch correction seems to be a very good starting point but as Denis says, you need to confirm any changes by measurement - and testing.

[SIMON CRAGG]

Posted by Martin Harris on 11/02/2019 17:31:44:

You will find that two props from different makers with the same nominal dimensions can give very different results so conversion tables might be misleading unless comparing particular makes. At "normal" sizes, the 1 inch correction seems to be a very good starting point but as Denis says, you need to confirm any changes by measurement - and testing.

Thanks for the info. I'm a bit challenged when it comes to working out what the Watt meter is telling me. Is there an idiots guide anywhere, which explains how to use it, and what its telling me? As a long time ic flyer, I have moved with the times, but my electric models all come set up, so when it comes to changing props etc. I am wary of burning something (expensive) out!".

[Martin Harris - Moderator]

The Wattmeter used for our applications displays voltage (V), current (A), power (W) and current consumed (Ah).

The problem is often in identifying the specifications of components used. The basics of using a Wattmeter are as follows:

You need to know how many watts (W) the motor can handle safely for a start. If there's no info, the safest thing is to note the wattage on the standard set-up and keep within this. Otherwise, you could monitor the motor's temperature and avoid any set-up that gets it more than warm to the touch.

Your ESC will have a maximum current (A) capability marked on it - you don't want to exceed this and it's best to aim for a maximum of 3/4 of its value.

Your batteries will limit how much current you can let them supply. You multiply the C rating by the capacity of the battery to determine the maximum current you can draw - a 2000mAh (2Ah) 20C battery will supply a maximum of 40 amps (2 x 20)...if you believe the makers claims!

The current draw will allow you to calculate duration at max power - a 2Ah 15C battery delivering a (comfortable) 20A (10C) will last for 2/20 hours = 1/10 of an hour or 6 minutes. A 30C version of the same capacity can supply twice the current but giving 40A (max would be 60A...2Ah x 30) only lasts 2/40 hours or 3 minutes in real money.

Using the Wattmeter allows you to experiment with different combinations and stay within these maximum parameters...

Edited By Martin Harris on 11/02/2019 22:11:49

[Peter Beeney]

Just having a little ponder on this set up, it all seems to me to be a mite confusing; I think I might want to look carefully at the three blade for a start. I’m sure it must be ok otherwise it would soon be flagged up, but could it be improved?

May I ask, Simon, why you would want to change the propeller in the first instance, please?

PB

[PatMc]

Actually the only reading you really need to look for from the wattmeter is the current (Amps). So long the max continuous current limit for the motor isn't exceeded & about 80% stated current of the ESC isn't exceeded there should be no problems.
But if you are running close to the current limit it's also best to check that the motor isn't over hot, as the claimed max current may be optimistic.

Assuming there's no overheating problem with the 3 bladed prop you only need to ensure that the current remains the same, or less, with the 2 bladed one.

[SIMON CRAGG]

Thats very useful info chaps, many thanks. I am just being lazy, and need to play around with the watt meter a bit more. I know all the figures for the ESC etc. The reason I need to change from a 3 to 2 bladed are several. Our patch tends to be a bit unven / longer grass at this time of year. The model has very small wheels and a forward c/g. With gear down, (even with a LOT of up elevator), the model will nose over. Probably break the prop on a belly landing.Anyway, I have sent of the total of £1.85 to HK for a 14x7e prop, and am looking forward to experimenting!.

[Chili Pants]

I have a Dynam FW190 it is fitted with a Detrum BM3720A3-KV500, the motor specs can be found here :- http://www.dynam-rc.cn/Product/view/id/280.html

I have run this motor through Ecalc with a 4s setup and this shows the motor could handle a larger 2 bladed prop quite easily but it is always best to use a watt meter.

[SIMON CRAGG]

Posted by Chili Pants on 12/02/2019 02:14:44:

I have a Dynam FW190 it is fitted with a Detrum BM3720A3-KV500, the motor specs can be found here :- http://www.dynam-rc.cn/Product/view/id/280.html

I have run this motor through Ecalc with a 4s setup and this shows the motor could handle a larger 2 bladed prop quite easily but it is always best to use a watt meter.

Thats interesting. I had a quick look at e.calc. (GULP)!.

Please could you let me know what prop size it suggested?.

I am using 4s, and as I have sent off for a 14x7E, hopefully that is the one they suggest!.

To get a benchmark, I am going to check and record the tree bladed set up today.

[Peter Beeney]

When I checked the motor specs., Simon, it gave a watts figure of up to 580, a continuous current of 35A, max burst of 49A for 10 secs, cells 3 - 4S lipo and recommended props, 2 blade, of 9 x 7.5 - 12 x 7. May I respectfully suggest (in my opinion) that the smaller size, 9 x 7.5, appertains generally to the 4S pack and the 12 x 7 to the 3S.

Hence my confusion…

PB

PS  All driven by a 40A ESC…

Edited By Peter Beeney on 12/02/2019 09:06:07

[Chili Pants]

Simon, here are a couple of screen shots from Ecalc based on that Detrum motor,(it looks daunting but is very simple to use) both stock prop and larger 2 bladed ones. I have to say that I have in the past found Ecalc a very useful tool and simple to use, well worth the couple of quid subs.

I have found Ecalc is slightly conservative with its results, however on this occasion I think the results are extremely conservative, from the way my Fw190 flies with the stock setup, I believe that it must be producing at least 400 watts, (at least) that is why I did not put the screen shots up initially, to me the 14x7 should be a safe bet for you. If you notice according to Ecalc this prop gives slightly less watts than the three bladed job.

There is quite a large discussion on this model and various mods done on another forum (RCG) one of the most common mods done to help with the nose over issue is to put some spacers under the rear of the retract mounts (lolipop sticks) to angle the retract slightly forward, I have done this mod and it does help but does not eliminate the nose over problem.

Hope this is of some help.

Greg.

[SIMON CRAGG]

Greg, many thx for taking the time to post the above.

I tested my stock set up on a 4s: 31.3A 16.73V 470W 133AH

So I am within the spec. of OE as expected.

Be interesting to see how the 14X7e performs.

I take it Watts = power?

Thanks again for your help!.

Simon

[Chili Pants]

No problem Simon, glad to be of some help.

I`m glad you posted your test results, they will surely be of help to other people, its also good to see that the motor and esc are running well within specifications on the stock setup.

From what I understand, yes watts = power, if you could after testing the new prop, would you post the results?

Greg

[SIMON CRAGG]

Posted by SIMON CRAGG on 12/02/2019 17:06:58:

Greg, many thx for taking the time to post the above.

I tested my stock set up on a 4s: 31.3A 16.73V 470W 133AH

So I am within the spec. of OE as expected.

Be interesting to see how the 14X7e performs.

I take it Watts = power?

Thanks again for your help!.

Simon

Well, this is not going to plan at all!.

Tried a 14x7:

Amps down from 31.3 to 22.6

Watts down from 470 to 363

Ah down from 133 to 67

Completely confused now. Can anybody help?.

[Steve Hargreaves - Moderator]

I think a 1" increase in diameter to compensate for the "missing" blade is a little conservative. I seem to recall a rule of thumb something like going down 2" in diameter & up 1" in pitch when converting from a two blade to a three bladed prop. If we reverse that logic a 15x6 would be the appropriate size two blade prop to replace a 13x7 three blade.

Simon's figures would certainly indicate that a 14x7 is too small as his power has dropped away significantly. To get the power back Simon you need to increase the load on the motor...which means a larger prop......I'd try a 15x6 or a 15x7 & see how that works out.....do you have the ground clearance for these sizes?

Ironically the main reason for full size multi-blade props is to allow a smaller diameter prop to be used so that ground clearance can be maintained. Imagine the diameter required for a two bladed prop fitted to a full size Corsair....

[SIMON CRAGG]

Posted by Steve Hargreaves - Moderator on 15/02/2019 16:34:20:

I think a 1" increase in diameter to compensate for the "missing" blade is a little conservative. I seem to recall a rule of thumb something like going down 2" in diameter & up 1" in pitch when converting from a two blade to a three bladed prop. If we reverse that logic a 15x6 would be the appropriate size two blade prop to replace a 13x7 three blade.

Simon's figures would certainly indicate that a 14x7 is too small as his power has dropped away significantly. To get the power back Simon you need to increase the load on the motor...which means a larger prop......I'd try a 15x6 or a 15x7 & see how that works out.....do you have the ground clearance for these sizes?

Ironically the main reason for full size multi-blade props is to allow a smaller diameter prop to be used so that ground clearance can be maintained. Imagine the diameter required for a two bladed prop fitted to a full size Corsair....

Thx Steve, I will go and try it. Does it HAVE to be an "E" prop, or will it be ok to use a std ASP jobbie?

[Steve Hargreaves - Moderator]

I'd use what you can lay your hands on Simon....if only to guide you in the right direction.

Eprops possibly a tiny bit more efficient & certainly lighter than an equivalent IC type prop so probably worth fitting at some stage.....

[SIMON CRAGG]

Posted by Steve Hargreaves - Moderator on 15/02/2019 16:44:32:

I'd use what you can lay your hands on Simon....if only to guide you in the right direction.

Eprops possibly a tiny bit more efficient & certainly lighter than an equivalent IC type prop so probably worth fitting at some stage.....

Steve-sounds good to me. Just tried a 15x6 APC which as you suggest is better:

22.6A to 25.6A 363W TO 393W

Maybe I am getting a bit anal about it!

The 15x6 seems to give a hell of a lot of thrust!

I suppose its a case of go and fly it and see what happens, or is it best to try get at least what the three bladed was producing?

[Steve Hargreaves - Moderator]

I would say go & try it....yes you are a handful of watts down but as you say there is lots of thrust & I'm sure there will be enough to fly it....after all these are full power figures & you rarely fly on full power for long. The two blade prop will be slightly more efficient than the 3 blade & at 15" diameter will be moving a larger column of air so you may even find it flies just as well on less input power which will equate to longer flight times....

The weekend weather looks very good so as a wise man once said "Don't just stand there...get one up!!"

Just watch the ground clearance as that prop tip is now a whole inch closer to the ground.....

[SIMON CRAGG]

Posted by Steve Hargreaves - Moderator on 15/02/2019 19:43:20:

I would say go & try it....yes you are a handful of watts down but as you say there is lots of thrust & I'm sure there will be enough to fly it....after all these are full power figures & you rarely fly on full power for long. The two blade prop will be slightly more efficient than the 3 blade & at 15" diameter will be moving a larger column of air so you may even find it flies just as well on less input power which will equate to longer flight times....

The weekend weather looks very good so as a wise man once said "Don't just stand there...get one up!!"

Just watch the ground clearance as that prop tip is now a whole inch closer to the ground.....

Thanks again Steve. One thing I find very confusing is that the spec. of the motor (BM 3720A-KV500) suggests a maximum of a 12x7 prop which was gutless and way down on power. Am I missing something here, as there is a big difference between a 12x7 and a 15x6!.

[Martin Harris - Moderator]

Is that a typo, Simon? Even at the theoretical max revs of that motor/battery combination would only give (14.4v x 500kV) 7200 RPM and a 12 x 7 at those revs would hardly set the world alight! There is a massive difference between those propellers...

[Denis Watkins]

Unlike a Motor Car Simon, we don't have a gearbox, or gear change on the plane.

Imagine going down the motorway in 1st gear !

On full size aircraft, we have variable pitch, to take off, cruise, and land

On landing the full size changes down to low thrust.

With a model we compromise with a fixed pitch propeller and sometimes we notice

The model is fast on the ground, slower in the air

Or a longer time on the ground and quicker than the 1st prop, in the air

We compromise, and use Watts as a easily read guideline of efficiency, with low reading being best when compared to the spec of the drive train

[SIMON CRAGG]

 

https://www.aliexpress.com/item/BM3720A-KV500-580W-40A-Brushless-Motor-For-Dynam-FW190-BF-109-F4U-Corsair-P-47D-Thunderbolt/32492599272.html

Gets more confusing by the minute.

Bring back the OS.40 & 10x6 prop!

Edited By SIMON CRAGG on 16/02/2019 11:06:56