By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by CML

Prop size?

All Topics | Latest Posts

Search for:  in Thread Title in  
SIMON CRAGG11/02/2019 12:41:43
305 forum posts
4 photos

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 Watkins11/02/2019 13:53:33
3493 forum posts
165 photos

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 CRAGG11/02/2019 17:26:32
305 forum posts
4 photos

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

Martin Harris11/02/2019 17:31:44
avatar
8293 forum posts
210 photos

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 CRAGG11/02/2019 21:19:41
305 forum posts
4 photos
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 Harris11/02/2019 21:48:32
avatar
8293 forum posts
210 photos

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 Beeney11/02/2019 21:49:24
1550 forum posts
59 photos

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

PatMc11/02/2019 23:20:53
avatar
4063 forum posts
504 photos

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 CRAGG12/02/2019 02:10:57
305 forum posts
4 photos

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 Pants12/02/2019 02:14:44
16 forum posts
13 photos

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 CRAGG12/02/2019 07:57:13
305 forum posts
4 photos
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 Beeney12/02/2019 09:03:27
1550 forum posts
59 photos

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 Pants12/02/2019 13:43:08
16 forum posts
13 photos

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.

dynam fw 13x7 x3.jpgdynam fw15x7 2blade.jpgdynam fw14x7 2blade.jpg

SIMON CRAGG12/02/2019 17:06:58
305 forum posts
4 photos

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 Pants13/02/2019 12:54:30
16 forum posts
13 photos

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 CRAGG15/02/2019 16:08:57
305 forum posts
4 photos
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 - Moderator15/02/2019 16:34:20
avatar
Moderator
6695 forum posts
185 photos

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....smile o

SIMON CRAGG15/02/2019 16:41:02
305 forum posts
4 photos
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....smile o

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 - Moderator15/02/2019 16:44:32
avatar
Moderator
6695 forum posts
185 photos

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 CRAGG15/02/2019 17:15:35
305 forum posts
4 photos
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?

All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of RCM&E? Use our magazine locator link to find your nearest stockist!

Find RCM&E! 

Latest Forum Posts
Support Our Partners
Gliders Distribution
electricwingman 2017
Pepe Aircraft
Slec
Revoc
Wings & Wheels 2018
CML
Motion RC
Advertise With Us
Sarik
Latest "For Sale" Ads
Does your club have a safety officer?
Q: Does your club have a safety officer, or is the emphasis on individual members to each be their own safety officer?

 Yes we have a SO
 No, it's down to everyone

Latest Reviews
Digital Back Issues

RCM&E Digital Back Issues

Contact us

Contact us