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Prop advice please.....

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Dad_flyer29/06/2020 18:18:00
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311 forum posts
315 photos

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 R30/06/2020 08:32:10
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3987 forum posts
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"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 Dyke30/06/2020 10:52:29
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310 forum posts
14 photos

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 Beeney01/07/2020 19:26:28
1593 forum posts
59 photos

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 Dyke01/07/2020 20:28:27
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310 forum posts
14 photos

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

Peter Beeney03/07/2020 22:09:00
1593 forum posts
59 photos

Many thanks in turn for your reply Roger, much appreciated. And at the same time I’d also tend to think that your ability to be able to consistently fly a somewhat underpowered and at the same time rather portly model regularly and safely must be a honest and lasting tribute to your model flying skills too…

Being naturally a bit lazy I’ve always picked up the tacho first, at least I can very nearly understand this particular device! When tinkering with a new electric motor I alway check the unloaded revs first, then bolt on a likely looking prop and check again; then in a trice it’s possible to be able to predict to within a reasonable degree of accuracy as to how it’s going to perform and how close (or not!) I am to arriving at the ideal prop size. Of course an added look at the current flow with a clip on power meter and a temperature check for local hot spots with a contact thermometer is never a waste of time either and makes it all a little more interesting anyway; also it might help to find out how wrong, or perhaps not quite so wrong, my guesstimates were in the first instance. Although a slightly more difficult decision to make before you even begin this caper though, is maybe choosing what size of propeller to use for a particular model and also how fast it has to turn to get the result you want…

Anyway, all best wishes for a future long and unbroken life for your model…

PB

perttime04/07/2020 05:47:38
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164 forum posts
11 photos

How much does the current 3S pack weigh? Do you need the weight to balance?

Perhaps there's a 3S pack that fits and is lower capacity, lighter weight, higher current rating. The lighter weight might improve your power to weight ratio slightly.

Trevor Crook04/07/2020 07:30:58
990 forum posts
71 photos

Roger, there is one more thing you could try. In my experience, bigger props at a lower rpm are more effecient than smaller props at a higher rpm, so if you installed your lower kV motor you could try an 11" or 12" prop, with the pitch chosen to give the appropriate current. This depends, of course, on the ground clearance available.

Roger Dyke04/07/2020 10:00:37
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310 forum posts
14 photos

Hi Peter,

Thank you for your very kind words regarding my flying abilities. That was very generous of you, but I'm not that good really. Thank you also for your very detailed explanation of how you go about motor and prop selections for your own models which I found interesting. I intend to keep this particular model forever. Even it it has an unscheduled arrival which results in it being grounded as it holds a lot of sentimental value. I also have another one (also converted) which holds a lot of sentimental value which I built in 1973 (Mercury Galahad). It has been repaired so many times but still flies. All my other models don't really hold the same sentimental value, so if they went in big time, they would probably reluctantly be replaced.

With the one in question, my governing factor was the battery and had to work back from there. The biggest capacity battery I could get that would fit in the bay in all directions was a 3S 3000mA graphene type. Then in order to have any reasonable flying time (5 minutes) I had to calculate the current draw that I could afford (about 30A). This then set the scene for the motor and if I couldn't get one that was powerful enough to pull it around the sky the show was over. Anyway, I found a 3536 1050kv that at 30A would turn a 9x6 prop at 10900 rpm with a static torque of 2.9 lb (my own static tests). Not ideal but should work. I did test a few more motors too but found this suited my requirements rather better. So, in this shortened version of the conversion path, this is where I have ended up. I am now reasonably happy.

Roger

Roger Dyke04/07/2020 10:18:39
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310 forum posts
14 photos

Perttime,

Thank for your reply. My battery pack weighs 214 grams. There is also a lot of lead ballast in the nose of the model as the motor/battery combo weighs a lot less than the old engine/exhaust/fuel tank (which also had some lead ballast).

I'm not sure what you mean by a battery with a lower capacity and a higher current rating. The power from a battery is the same no-matter how we derive it (volts x amps). Higher voltage with lower amps or lower voltage with higher amps. Size wise and power wise they're the same.

A lower capacity battery would give me less flight time, which at the moment is about 5 minutes (leaving about 20-30% of capacity left).

Roger

Roger Dyke04/07/2020 10:38:35
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310 forum posts
14 photos

Hi Trevor,

I was considering doing that as I do have a 3542 800kv motor and which pulls about 30 amps with a 11x6 prop at 8500 rpm (my own tests). However when playing around with eCalc, it didn't show any improvement to performance to what I currently have. In fact it was a little worse so decided not to bother. maybe I might have another look on my static test set-up though to confirm my findings.

Many thanks.

Roger

Edited By Roger Dyke on 04/07/2020 10:39:45

perttime04/07/2020 12:27:46
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164 forum posts
11 photos

Apparently, you need the weight of the battery for balancing too, so a lighter battery wouldn't help.

You say that you are taking about 33 A from the battery. You can get the same 33 A, and more, from a smaller 3S pack. 30C packs are everywhere, now, and I've seen people mention 100C. Clearly, flight time becomes a limiting factor with a lower capacity pack, too.

Roger Dyke04/07/2020 13:22:03
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310 forum posts
14 photos

Yes, you are correct. The weight of my battery is included in the amount of weight I need in the nose to balance the plane.

Yes, I can certainly draw more current form my battery or even from smaller capacity batteries, but drawing around 30 amps or so from my 3000 mA battery is my maximum limit that I can draw without impeding my flying time of around 5 minutes.

Peter Beeney04/07/2020 15:50:34
1593 forum posts
59 photos

Yes indeed, you do have a little predicament, Roger, not really many easy answers here I guess. Changing from i/c to electric is not always that simple by the look of it.

Just a fag packet type of speculation but if you were starting with a bit more leeway here 2 x 2Ah 4S packs in parallel one behind the other in the fuz might have been an option. That in turn might now make it possible to find the right CoG without any additional weight so it might not be that much heaver anyway. Selecting a prop, to me something like a 10 x 7 looks like a likely first candidate for a fictitious 50in span speedster weighing in at something well under 5lb,(hopefully), that should be turning at least in the order of 10,00rpm. Such that a 40 i/c might do. But 11k would be a bit better, though. Such that a OS MAX-35AX might do. The extra watt minutes we have will allow us to bump up the current to around 35 amps and still have just about 5 mins duration down to 30% capacity. Using nominal figures, an 850 kV motor will spin unloaded at 12580rpm, ideally our prop needs to revolve at about 11000rpm to be within our preset limits, slightly faster would be better. So an 850kV 800 watt motor might be choice. 800 watts still gives us a bit of headroom on the max current flow with fully charged batteries.

First setback here maybe a bit of a lack of 850kV’s but a 900 pops up here straightaway which looks to be just about perfect all round for our fag packet…sorry, model. Job done…

But if it were only that simple…….

With the greatest possible respect all round Roger, I have to say I’m a bit suspicious of the 11/6 at 8500rpm though. As usual I may be completely wrong but that seems to me to be now lurking marginally beyond the borderline of a rather marginal place to be in the first instance!

Take care.

PB

Trevor Crook04/07/2020 15:55:20
990 forum posts
71 photos

Roger, I must admit I've never used ecalc, I tend to use a combination of experience, advice and trial and error!

A few years ago I wanted a bit more urge from a 3s model that was pulling about 40A, so fitted a 4s battery and came down in prop size until I was back to the 40A level. This involved going down from a 12x6 to a 10x6. The model (a Galaxy Magician) made more noise but didn't fly any better, despite absorbing 30% more power. I reverted to 3s and 12x6 and tried different makes of prop, and got an improvement from a different brand.

Roger Dyke04/07/2020 16:32:20
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310 forum posts
14 photos

Hi Peter,

It is what it is I'm afraid. The battery compartment is maxed out in width, height and length and I have trawled the websites for any shape and combination of battery that might fit. So that was my starting point. The fuselage is quite narrow and the distance from the firewall to the wing bay former is what it is. I fully understand what you suggest, but if I could get a bigger battery in the bay in any combination at all it would open up all sorts of new possibilities. But I can't I'm afraid. So there we are.

Just for interest. Regarding the 11x6 prop. On my static rig and with the 3542 800kv motor fitted, I tried a few different prop sizes with a 3S battery.

The results were:-

10x7 - 8300, 11x6 - 8500, 11x7 - 8300, 11x8 - 8100. The currents recorded between 28 and 33 amps.

Roger

Roger Dyke04/07/2020 16:41:38
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310 forum posts
14 photos

Hi Trevor,

Thanks for the useful info. and the benefit of your experience. I too tend to do my own experimenting, but as I have the iCalc tool I thought that I would give it a go. I must say that very often it doesn't come up with the same answers that I get. Still it's useful to do comparisons.

Regarding changing props. Unfortunately, I have already been through the prop experiment with all the thereabout sizes and makes. I've even tried the half sizes, but now think that I've found the optimum in the 9x6 thin APC. Some of the others come close. But none of them any better.

Roger

Edited By Roger Dyke on 04/07/2020 16:45:15

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