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

Prop advice please.....

All Topics | Latest Posts

Search for:  in Thread Title in  
Roger Dyke28/06/2020 18:30:30
avatar
310 forum posts
14 photos

Hi All,

I have recently converted an IC plane to electric. As is usual with these conversions, the model is a little on the heavy side for the powertrain available. Due to the fuselage size my maximum battery that I can fit is a 3S 3000mA. This limits me to about 33A to achieve a 4-5 minute flight. The motor I have fitted (1050kv) has a max current loading rating of 34Amps. I have been using a a 9x6 APC thin prop very successfully with a max current of 28-30A @ 10900 RPM.

I have been thinking of trying a 10x6 APC thin prop to try and squeeze out a little more performance. In a static test with the 10x6 and a freshly charged battery the current is 36A @ 10400 RPM for about the first half minute falling back to about 32-33A as the battery voltage drops. Motor temperature was monitored during tests and found to be okay (hot but not too hot to touch).

Would the prop change be worth the risk to the motor, or should I leave things as they are?

Roger

David Hall 928/06/2020 18:48:09
275 forum posts
16 photos

I think it could be a good size to use. Don't forget that the motor will unload once it's moving, it can reduce the power consumption by a significant factor.

Edited By David Hall 9 on 28/06/2020 18:48:48

Roger Dyke28/06/2020 19:20:24
avatar
310 forum posts
14 photos

Hi Dave,

Thanks for your reply. Yes, I hadn't considered the unloading aspect.

Allan Bennett28/06/2020 20:17:32
1700 forum posts
49 photos

Adding to what Roger has said, you surely won't be using full throttle for normal flying anyway and, since you've tested it for 30 seconds that should cover your initial take-off and climb-out, during which you may be using full throttle.

Simon Chaddock28/06/2020 22:32:28
avatar
5729 forum posts
3034 photos

I should ask the questions "What plane is it?" and "How much does it weigh?".

With electric flight most motors and props have about the same efficiency so comparing electrical input to plane weight give a good indication of what sort of performance can be expected.

If the Watts/lb are much below 100 then you wont get much extra performance without straining the 3000 mAh 3s.

Roger Dyke29/06/2020 08:23:39
avatar
310 forum posts
14 photos

Allan: Thanks for your useful info.

Simon: The plane is a CAT 500 (the one in my avatar photo). At it's current weight and power it is sitting at about 70 watts/lb which I know is quite low. The plane does have to be "flown" though in the true sense of the word, not like a lot of the 'overpowered' foamies that seem to be around now. But I don't mind that. The plane holds a lot of sentiment for me as when I was building it back in the 90's, my flying pal suddenly passed away and he was always asking me if I had finished it yet. Following his passing I shelved it for years and put it 1/2 built in storage. A couple of years ago I decided to finish it off as a tribute to my friend. So it's more than a plane, it's a memory so it has to fly.

Perhaps the bigger prop will be a good thing to try. Then if it doesn't feel any better, I can always revert back to the previous one.

Denis Watkins29/06/2020 09:36:38
4524 forum posts
121 photos

Just an alternate Roger, check out this 4 max page as 3S and some 4S, come out the same size.

**LINK**

Roger Dyke29/06/2020 10:28:53
avatar
310 forum posts
14 photos

Hi Dennis,

Thanks for the link. Unfortunately my battery length limit is 107mm and if I go to the 4S then the current capacity drops from my 3S 3000mA to 4S 2200mA so will be very little improvement as I will have to reduce the current taken considerably to keep to the approx 5 minute flying time. I think that I have visited nearly all of the battery suppliers and for the space available I think have the best compromise I can get. Thanks for your suggestion though.

Shaun Walsh29/06/2020 11:45:08
347 forum posts
50 photos
Posted by Roger Dyke on 29/06/2020 10:28:53:

Hi Dennis,

Thanks for the link. Unfortunately my battery length limit is 107mm and if I go to the 4S then the current capacity drops from my 3S 3000mA to 4S 2200mA so will be very little improvement as I will have to reduce the current taken considerably to keep to the approx 5 minute flying time. I think that I have visited nearly all of the battery suppliers and for the space available I think have the best compromise I can get. Thanks for your suggestion though.

If length is the problem, have you got room for two batteries side by side or one above the other wired to the ESC in parallel?

Peter Beeney29/06/2020 12:38:39
1593 forum posts
59 photos

Hi Roger,

From the motor performance figures you’ve quoted it seems (to me, anyway!) that the 9 x 6 is just about the optimum size you can use. It would appear to be turning at about the best possible point on the output curve. Certainly worth a try with the 10/6 but your figures for that are already suggesting a slight reduction in performance. The increased current flow immediately points to a reduced prop speed which you point out anyway, so slightly less forward speed (performance?) and it will also slightly reduce the battery duration too. So slightly less flying time per charge; and to say nothing of the slightly increased heating effect as well. Another point to make here, but again OIMHO, your 5 minute flying time points to a constant 33A discharge with a 3Ah capacity. Or in other words, more or less constant full throttle use. Might just take a bit of careful thought as how you can improve this, a different, i.e. more powerful motor perhaps, or as suggested, 4 cells. Here you would now definitely have to go down in prop size but if by some magical coincidence you found that trying say an 8 x 5 resulted in around a 26A current flow using a 2.2Ah 4 cell the increased performance might well give you the ability to use less full throttle, (max current flow), so even more flying time. So 27 - 28 amps might be a suitable starting point. Some experimentation might be called for…

Although to be quite honest and weighting it all up perhaps the more powerful motor option might really be a better bet. To improve the performance somewhat I’d guess you just need a bit more of the right sort of poke!

Re the prop unloading, I think this must happen to all props anyway. It occurs at S&L; and a bit more going downhill but it’s mostly disappeared when you’re climbing; just when you could do with a little help, maybe.

Please don’t consider this as any sort of criticism, or indeed any form or instruction on how to do it. It’s just interesting stuff and it’s only just the way I think about and consider it all.

I can really understand your motive about flying the model too, I had something of a very vaguely similar experience once, but I’ve not managed to finish the model off yet…

Very good luck with it all…

PB

Nigel R29/06/2020 12:53:23
avatar
3969 forum posts
714 photos

"The plane is a CAT 500 (the one in my avatar photo). At it's current weight and power it is sitting at about 70 watts/lb which I know is quite low"

Can we assume it has about 500 square inches of wing area?

What is its current weight including the 3S3000 lipo?

My guess - based purely no that 500 squares assumption - is you may accept a little surgery is necessary on the front end to get a larger lipo inside, somethine around 4s3700, to get a decent performance from the airframe. And a power of around 600W?

You may find a large hole in the firewall works rather nicely, to allow a longer lipo to project forward into the cowling space.

My own experience is that the airframe weight is not usually the problem - you can always set the power level appropriately - it is more that you must fit in electric gear which is not the same shape and size as the original IC clobber.

Roger Dyke29/06/2020 13:22:36
avatar
310 forum posts
14 photos

Shaun: Absolutely no room whatsoever for a bigger battery than I currently have. When I converted this plane, I researched the market on battery power verses size and the battery I am using was the very best I could come up with which is 105 x 35 x 30 mm. Any longer, higher, or wider will not fit. But thanks for the suggestion.

Peter: Thank you for your comprehensive reply. I have taken on board your thoughts and your interesting suggestions. At the moment using the 9x6 prop, 5 minutes flying time leaves about 40-50% in the battery. I was sort of thinking that with the 10x6 prop I could probably still maybe achieve about 5 minutes leaving about 25-30%. By the way. In carrying out my battery discharge time calculations, I am taking the figure of 80% of battery capacity use. I do have a more powerful motor but I don't know how that really helps me as I am controlled by the battery power. Either high voltage, low current or low voltage, high current. The power remains the same

Shaun Walsh29/06/2020 13:24:31
347 forum posts
50 photos

How about this?

**LINK**

107mm long, 3S, 5.2Ah.

It's 15 C which means you could pull 78 Amps.

I have the 3Ah version and it copes fine with 37 Amp draw.

Shaun Walsh29/06/2020 13:26:00
347 forum posts
50 photos
Posted by Shaun Walsh on 29/06/2020 13:24:31:

How about this?

**LINK**

107mm long, 3S, 5.2Ah.

It's 15 C which means you could pull 78 Amps.

I have the 3Ah version and it copes fine with 37 Amp draw.

Just read your previous post, this is too big too!

Roger Dyke29/06/2020 13:44:34
avatar
310 forum posts
14 photos

Hi Nigel,

The wing area is indeed 500 square inches. The gross weight all up is 5.0 lb.

Many thanks for your reply. The plane has already had extensive surgery to get to the stage where I am now and no further surgery is possible. It also has quite a slim fuselage. I have really come to the end of the road regarding the airframe so I was really just asking for any advice on the proposed prop change.

Roger Dyke29/06/2020 13:47:36
avatar
310 forum posts
14 photos

Sorry Shaun, you are right, It's too big. Thanks for trying though.

Dad_flyer29/06/2020 15:22:43
avatar
311 forum posts
315 photos

To squeeze a little more performance with your fixed input power, you could try a physically bigger motor, with higher maximum power, but prop it to the same power you are using now.

Bigger motors have less internal resistance, so less lost power at a given current. For example on one of my models I was using a 57g 2830 1350 KV with a 9x6 prop. 300W at 9500 rpm for a full 3s battery.

Same prop on a 109g 3536 1100kv gave 9400rpm with a partially discharged 3s (11.7v) and 276W.

Roger Dyke29/06/2020 16:35:24
avatar
310 forum posts
14 photos

Hi Dad_flyer,

Thanks for that. At the moment the plane uses a 3536 1050kv, but I do have two 3542's. One a 1000kv and the other an 800kv. It may be something to think about if my prop swap to the 10x6 doesn't work (or I might just leave it the way it is).

SIMON CRAGG29/06/2020 17:33:52
576 forum posts
5 photos

Have you tried punching the figures into e.calc?

You can try different makes / sizes of props

Different battery capacities and C ratings.

Worth giving it a go before you go much further!

Roger Dyke29/06/2020 18:06:09
avatar
310 forum posts
14 photos

Hi Simon,

Thanks for your suggestion. Yes I have. I more or less lived on eCalc for a while. I found it very useful to try out the effect of different set-up's. But the figures never matched what I actually got for real. Usually with the eCalc current was lot less than what I observed in real time. That was with all figures entered as accurately as possible. Couldn't find a reason for that. I know that they claim that their results are to be taken +/- 10% but the difference I found was generally much larger.

All Topics | Latest Posts

Please login to post a reply.

Latest Forum Posts
Support Our Partners
Slec
Sussex Model Centre
electricwingman 2017
CML
Advertise With Us
Sarik
Latest "For Sale" Ads
Has home isolation prompted you to start trad' building?
Q: The effects of Coronavirus

 Yes - for the first time
 Yes - but Ive bashed balsa before
 No - Ive existing projects on the bench
 No - Im strictly an ARTF person

Latest Reviews
Digital Back Issues

RCM&E Digital Back Issues

Contact us

Contact us

Magazine Locator

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

Find RCM&E!