Prop advice please.....

[Roger Dyke]

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 9]

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 Dyke]

Hi Dave,

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

[Allan Bennett]

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 Chaddock]

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 Dyke]

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 Watkins]

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

**LINK**

[Roger Dyke]

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 Walsh]

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 Beeney]

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 R]

"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 Dyke]

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 Walsh]

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 Walsh]

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 Dyke]

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 Dyke]

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

[Dad_flyer]

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 Dyke]

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 CRAGG]

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 Dyke]

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.

[Dad_flyer]

ECalc can be quite off - particularly if you want to match the limit of something as that is quite a tight target. However it should be pretty good on relative power / thrust / wasted power, as you change things.

[Nigel R]

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

If I'm honest I think you've approached this from the wrong way, you've tried to fit some electrics into an existing space, rather than making the space fit around the electrics that the airframe needs. 

The size and weight says compact 40 to me - so I think you're flogging the expired horse a bit with the current electrics. You need a complete change rather than noodling with props. At 5lb I'm thinking you need around 750W for spritely aerobatic performance - way too much for a small 3536 type motor on the lipo you have. I'd say 3548 minimum, possibly even a 42xx type, and a 4S4000 ish size lipo as starting point.

I have to say my thoughts would be to give up with the existing arrangement and get stuck in with a rebuild of the nose. The CAT designs look very short in the nose, you don't have the wiggle room you need if you're stuck with the original FW location.

I would -

Pull the firewall / saw off the front

Splice some fuselage side extensions on - or just use some ply doublers - anything to get the FW forward another inch or maybe even more.

make a new FW

go with a 3548 900kv, prop between 11x6 and 11x8

make a nice large hatch on top for easy lipo access

make a new cowl, if that's a thing on this model

while everything is opened up, arrange a useful location for the ESC, probably below the lipo, or go al fresco and rubber band it to the bottom of the fuselage between the U/C legs. If you make your new FW location in the right place you should be able to hide the ESC indoors though.

 

Edited By Nigel R on 30/06/2020 08:38:55

[Roger Dyke]

Oh Nigel,

I thank you so much for your your suggestions and I can feel your enthusiasm brimming over with ideas for my CAT 500. I think it was originally designed as a racer for a hot 36 or 40 IC motor. I always used it with a cooking 40 and was always happy with it. In fact, it could be converted back to an IC 40 very easily as everything is still in place. With it's conversion, all I have ever wanted to do with it is to lazily fly it around in straight and level with the odd rolls and loops. that's it. I have no desire to punch holes in the sky with it as it just represents a piece of nostalgia to me and is of sentimental value only as a tribute to my dear friends passing. With the current set up it flies to that end quite well although it does have to be 'flown' and I don't mind that. Perhaps being an ex private pilot might have helped me in that regard. My original question was simple, should I go for the 10x6 prop or not?

Thanks again for your very valued reply.

[Peter Beeney]

So to try and answer your original question, Roger, is that I at least would consider whilst the 10/6 won’t really do much harm it might not make that much of improvement either. Giving it a spin is perhaps the best way to find out; I might be completely wrong, it has happens more than once or three times; but your static figures do suggest that it won’t be quite as efficient. I’m pretty sure that the mechanical turning power at the prop shaft is just about as close as it can be to equalling the electrical power flowing from the battery using the smaller 9 x 6.

Also your post describing the battery capacity flying time and discharge calculations now makes it all much clearer. That’s much better! I’d had another look a it and it really didn’t add up very well at all (for me). I was beginning to think there must be something seriously wrong with your 9/6 prop!

Having now said all this it seems the model might well have been designed as a pylon racer. If so then it would have been the heavy hammer all the way and then some, no quarter given. Success was going as fast as possible… So with that in mind I now think that you may be flying just outside the border line of some sort of rather ‘unusual’ flight envelope; but you’re obviously very successful with it and it’s exactly what you are wishing to do as well. So all power to your elbow! Or perhaps that that should be stick finger!

Many more happy landings.

PB

[Roger Dyke]

Hi Peter,

Thank you so much for your interesting post. You are almost spot on with thinking about pylon racing. I have just looked out the old magazine where the plan came from. The author said that he based the design on the American AMA Quickee Class Rules for the AMA Racing 500 class (500 sq/in wing area) and was designed for hot two strokes or 45-50 four strokes and prop size 9" to 11" diameter with 6" to 8" pitch.

I tried the 10x6 prop today and you are right. It was not quite as good as flying with the 9x6. Also as expected it shortened the flying time due to the increased current. I agree with you that I am operating it way outside of it's design envelope and that is why it "needs flying". It in no way is it stable and it can easily catch you out. Especially as it's powered by electric as you cannot hear the throttle rpm. It was much easier to fly with the IC 40 I used to have in it as I could hear that. I think that I have explored all the avenues without carving up the model so I think that I have come to the end of the road with it now so I will just settle for what it is. After all, it has a lot of sentimental value to me so I am happy.

Roger

Edited By Roger Dyke on 01/07/2020 20:29:16