More advice on Wot 4 motor?

[PatMc]

That's virtually the same performance as the motor you already have.

What performance are you wanting from the model & why do you want to use such a large dia prop ? Also what sort of flight duration due you want ?

[Clifford Stone]

Its specked better in watts, amps, it fits, I'm into large prop diamiters, better efficency. I supose its my old ic days creeping in . . . vertical performance and high pitch dont go together, and it takes time for high pitch props to accelerate, although the final top speed will theoreticaly be greater? The Wot 4 is not a ptlon racer.

I'm trying to get the best out of a 3 cell set up, 4 cell is a bit of a brute, that the2820 has not servived!!!

I'd rather not go down the hack it about rout to fit a bigger motor, trying to stay withing known paramiters to learn . . . then I can re think. Flight duration seemed to be 7 minuits on 3 cells and a 12x6, the recomended 11x5.5 was slightly less, although that was my reaction not timed.

I am probably making classic errors? I'm certainly relying on the wat meter, I did a few tests this morning, using the recomended 11x5.5 on 3 cells . . . as I say, it gave up the ghost when I swaped onto 4 cells, did not get to full throttle, my falt. The Turigy will take a third more amps and twice the watts. I'm not going to push it to its limit, in fact I suspect the SEC (ordered a Turigy 60amp) will need to be watched via the watt meter.

So the thinking is, I understand that alowing the reves to increase helps keep things under control in electric power. A 10x6 or 7 would be the way to go? I have tried the 10x6, not enough power, ineficient small prop disc? Hence the thinking, keep diamiter and unload with less pich . . . more efficient in manouvers, but less overall streight line speed.

Its me looking at the issues from a diferent perspective, the final combination of torque power, amps and watts to be within the prescribed envelope according to the watt metter and my apeciation of the performance as the pilot. I'm not sur if the same motor is used in the Wot 4 Foamy? plenty of power, the plane is half the weight. Four pounds 10ozs of 'woody' needs a different aproach.

I get the idea the 2820 is somthing in the 35/40 sports ic engine bracket? That would have flown a wood Wot 4, but not to much more than simple loops and rolls. We used to put powerfull 40's or 60 4 strockes in, I understand there are pilots using 75 and 90 4 strocks these days?

So, there it is, I'm 'sucking and see' a bit, I have learned from your input, 'watt meter', prop size etc. But I do pust the boundres, I alway have, its my nature, can get expensive and end in desaster some times . . . after all these years you would thing I'd have learned . . . I enjoy it, I listen to the advise, for which I am greatfull, but that urge to go that one step further or aproach from a different angle . . . its as strong now I have discovered, as when I was competing as a younger man, the only differance, the competativenes has gone, I simply want to 'try things out' . . . the enquiring mind . . . ?

Hope that makes sence, and that you will keep trying to point me in the right direction??? CJS

[PatMc]

Clifford, according to the specs the KMS motor can take up to 45A, the Turnigy motor up to 50A. That means that on 3s one can manage about 500W the other 560W. The 910W quoted for the Turnigy would only be achievable with 5s or more lipos.They are both about the same KV (the quoted KV is never exact) so they would probably take their max current on the the same or similar size props.

Any lower KV motor of the same size would have a lower current limit therefore lower power on the same cell count. To use a bigger prop & have the same or greater power you would need increase the cell count with a lower KV motor. This one for example.

I'm using a Turnigy 3548 900KV on a tired 4s driving an 12x6. It takes 30.5A for 464W.   If I increased the prop size to take 40A the power would be around 600W.

I don't know what capacity battery you are using but at 50A you will only get up to 4 minutes WOT on something like 3300mAH before LVC.

Edited By PatMc on 30/07/2012 16:06:03

Edited By PatMc on 30/07/2012 16:07:45

[Clifford Stone]

Penny droped PatMc . . . amps = airtime, less amps, more flying . . . I have had a look at the two low kv 3548 Turnigy motors, they would have been an option if I had understood the 'airtime; factor . . . doh

Sorry if I dont seem too take advise, I have this 'suck it and see gean', bourn of a dyslexic mind that has to get things lined up in its own way so that I can remember. So if I keep the amps low on the existing . . . new 3548/04 keeping an eye on the watts.

OK, you do it with calculation, I am forced to use the watt meter, making notes as I test. This means I might come up with some strange combinations. Pick out the best looking option (s), strap a ballance on the back of the airplane and measure static thrust and/or, a final flight test. I know, a lot of faffing about, but thats the only way my inadequate dyslexic mind works I have to turn calulation into practical action, a dyslexic version of 'a picture is worth a thousand words' . . .

Its not perfect, I am proving that! . . . but its my only option, I could go back to noisy, smelly ic engines . . . Alternativly, plan 'B' . . . get the fishing rods out for some piece and quiet . . .

Batteries, 4 Cell, 2680mah, 3 Cells 2,200mah.

CJS

PS, Wot 4 foam 'e' . . . better option? lighter, useing less power = more flying time?

Edited By Clifford Stone on 30/07/2012 17:55:02

[Peter Beeney]

Clifford, thanks very much for your reply @ 29/07/2012 11:48:05. With regard to the arithmetic, unfortunately electric stuff often seems to be about figuring out the values, such as volts, amps, resistances and watts, to name but a few. But I wouldn’t have thought it’s that difficult, once you’ve kicked it around a bit, I’m sure you’d soon get the hang of it.

But when we get down to the nitty-gritty I think motors are really all about magnetic fields and reactions. When current flows down a wire it becomes surrounded by a magnetic field and electric motors exploit this fact to the full. In our model aeroplane case, together they work in conjunction, or rather opposition, with the permanent magnets in the motors, on the basis that unlike poles attract, like poles repel. As it happens, the more current that flows, the stronger the magnetic field wrapped round the wire. This is a good state of affairs, but of course this current in turn flowing down the wire, which is a resistance, will create heat; and this is a bad state of affairs. We measure the current to get an idea of the magnetic field strength.

If we take this back to to a hypothetical motor, let us assume that it’s resistance is 0.5 ohm, or 500 milliohms; and let us assume that the current flowing is 50 amps. We also know from previous examples that the the calculation to obtain the watts, is 50A by 50A by 0.5R, which equals 1250 watts; this will dissipate in heat. If we reduce the resistance to 0.05 ohm the heat will be 125 watts, and if we can get the resistance down to 0.005 ohm the figure will be 12.5 watts. So we can soon see that what sorts the men from the boys in terms of electric motors will be it’s resistance. Or lack of it, rather. But like all good things, it might come at a bit of a cost.

I tried to check a few HK motors for their resistance levels, but they don’t seem to always list them. Checking some Hacker motors at random, they vary from 0.XXX, hundreds of milliohms, down to 0.00X, less than 10 milliohms. But this is now getting expensive, these motors cost hundreds of dollars.

Pat, as usual, I think we shall again just have to gently agree to disagree on this one. My figures were in general terms, and I’ve not measured anything, and unfortunately Eagle Tree have no plans to become compatible with a Mac at the moment, so that rules that out, but I think I’m happy with what I said. I’m fairly convinced that if 11.1 volts are applied to a resistance of 0.041 ohms, then provided the voltage source can hold up, 270 amps will flow. As you say, though, there is also the ESC, so for a start the total resistance is never going to be that low. I also remarked that the motor instantly starts to turn, and as soon as it does the current starts to fall, so this starting condition only lasts for a very short time indeed.

For a modern permanent magnet brushless motor, I would consider the torque to be at a max when the motor is stationary and a voltage applied, and the speed to be at it’s max when the torque is at it’s minimum, that’s when it’s unloaded, no prop on. I also think that it’s best rated speed might be very approximately at about three quarters of it’s unloaded speed; but this must be a bit flexible. Maximum torque is created by the maximum current flowing, a good example of this may sometimes be seen when a model goes in nose first, thus the motor is prevented from turning, the throttle is left fully open and soon a column of smoke can be seen emanating from the frontal area.

[Peter Beeney]

PART TWO

I’m not an electric motor man at heart by any means, what I’ve said is all supposition and reading in times past, but I think I have to be careful when talking about power when relating to motors. Firstly there is the mechanical power, the motor converting electrical power into rotary power, and driving the prop, the output level which can be given in watts. Then there is the electrical power, the input level into the motor which can also be given in watts. The mechanical power is given by the moment arm multiplied by the effort, or in the case of the motor, the rpm multiplied by the torque. Taking an extreme case, our stuffed model, as it prepares itself to become a burnt offering, we have a situation where there is maximum torque applied but no revolutions at all, so the (mechanical) power output will be zero. On the other hand, the output, from our power source, the battery, volts times amps, will be at a maximum. I think it’s possible that for a very brief period of time at startup the same can also be said. Once the motor is running these two power levels start to move closer to each other, and there must be just one ideal best ‘speed/load’ point. As I said, this is extreme and would not normally encountered, but occasionally the power train is overloaded and the pilot is often left wondering why. Also I think it’s why I sometimes notice some people manage to get electric models that perform very well, but there also some that do leave something to be desired.

Also you may also be right about the non cooling effect or otherwise, but again I seem to have read on occasions that by installing a very efficient cooling device various electrical equipments outputs can be uprated; in some cases to a considerable degree. So I would consider that if I ran a motor with it’s ESC in a model I would ensure, at least as much as possible, that there is at least a through draft of air. Perhaps not always that easy, though.

Very little here of any substance, I’m afraid, I’m not familiar with motors, but I’m rather surprised when I read the specs. and information on some of the motors how it often contradicts itself. It must be confusing for newcomers trying to get a handle on it……

PB

[PatMc]

Posted by Peter Beeney on 30/07/2012 18:04:26:

Pat, as usual, I think we shall again just have to gently agree to disagree on this one. My figures were in general terms, and I’ve not measured anything, and unfortunately Eagle Tree have no plans to become compatible with a Mac at the moment, so that rules that out,

Peter, no need for an Eagle Tree, just connect an ammeter with a voltmeter on the battery terminals.(or a Wattmeter) I can promise that there will be no drama. If the current rose anything like the magnitude that you're suggesting the battery voltage would drop & the BEC woud fail to power the Rx. As it is even if the motor is accidentaly stalled this doesn't happen.

The torque is proportional to the current so there's no massive torque at the start up. In fact once whilst flying my Easy Glider as a slope soarer I powered up forgetting that I'd left an elastic band holding the folding prop closed. The motor didn't manage to turn in the few seconds that it took before I realised my error, so not enough torque to throw off one of RM's famous red bands. In this model the motor takes 300W at 8600 rpm. Allowing for a motor efficiency of 80% the torque would be of circa 37 oz-ins. The band was holding the blades at a dia of 3" therefore there would be a force of around 24.5 oz trying to turn the prop. But if what you are suggesting is correct this would be greatly magnified so the band wouldn't have stood a chance..

IIRC I read that someone burnt actually out an ESC in similar circumstances.

Sorry but your completely wrong.

[PatMc]

Clifford, as a comparison you might find usefull - my Druine Akrobat is powered by one of these using a Flightmax 4s 2650 battery, 60A Hobbywing ESC driving a 10x7 APC prop. I tried several props & settled on this one because the set up gave a good aerobatic performance with vertical climb capability but held the current down to 32A at WOT. (Power is 470W) IMO the model is a little too fast with this prop & I would like to have tried an 11x4 if they had been available. I might get around to trying some of my now redundant ic props.

The model is 57" span & the AUW is 4.25lb. Ripmax's spec for it is 3.75lb using a KMS 2826/05, 3s 3200mAH & 11x5.5 prop. I did try mine with the 11x5.5 (on 4s) but it took 39A for 560W which I thought just a bit too much.

Last 2 flights were 10 mins & 11 mins. Battery recharges took 1740mAH & 2320mAH.

PS IMO the E-Power is a smoother motor than the Turnigy 3548 that I have & seems better all round quality.

 

Edited By PatMc on 30/07/2012 21:01:29

[Clifford Stone]

Posted by PatMc on 30/07/2012 20:56:59:

Clifford, as a comparison you might find usefull - my Druine Akrobat is powered by one of these using a Flightmax 4s 2650 battery, 60A Hobbywing ESC driving a 10x7 APC prop. I tried several props & settled on this one because the set up gave a good aerobatic performance with vertical climb capability but held the current down to 32A at WOT. (Power is 470W) IMO the model is a little too fast with this prop & I would like to have tried an 11x4 if they had been available. I might get around to trying some of my now redundant ic props.

The model is 57" span & the AUW is 4.25lb. Ripmax's spec for it is 3.75lb using a KMS 2826/05, 3s 3200mAH & 11x5.5 prop. I did try mine with the 11x5.5 (on 4s) but it took 39A for 560W which I thought just a bit too much.

Last 2 flights were 10 mins & 11 mins. Battery recharges took 1740mAH & 2320mAH.

PS IMO the E-Power is a smoother motor than the Turnigy 3548 that I have & seems better all round quality.

Edited By PatMc on 30/07/2012 21:01:29

Thanks for the reply Pat, I'm getting the message, low amps, less than 500 watt on 4 cells, a decent ESC . . . equivilent thinking on 3 cells . . . thats about how many cells I have working in my brain? More to this electric flight than meets the eye . . . but I do think I am begining to get the picture in my own way.

I am going to order a 900'ish kv motor in the morning. I had a Turnigy 3548, 900kv lined up . . . the E-power will save me a tenner . . . !

I can sleep a little more easy tonight, thank you . . . CJS

[Peter Beeney]

Pat, I’m afraid I’ve been ‘wrong’ more than a few times in the past, and I’m sure I will be again in the future, so I can live with that ok. I quoted those figures because they were topical, and they would be in line if the total impedance of the power train was that low, but would a typical value might be in the order of 150 - 200 milliohms, although I’ve no idea about this, but if so would certainly lower the current flowing considerably. Also as I remember, at one time ‘soft start’ was a big buzz word relating to ESCs. Perhaps they all now come with a soft-start facility as standard?, I believe this applied the voltage, and thus the current, in a slow but progressive manner, to prevent the motor accelerating too rapidly and possibly snatching. If this is the case, then I’m sure this will make a difference, too.

I suspect the reason that one of Royal Mail’s finest was able to defeat your motor with such aplomb was simply the fact that because the motor was stationary there was no mechanical power output. I’m not quite sure if it would be quite so successful at restraining the prop if it were turning at 8000 rpm. It might consider that was stretching it’s capabilities somewhat! But there are bags of electrical power being expended by this seemingly innocuous and quiet state though, a max amount in fact, but unfortunately this is all disappearing up the chimney as heat, and shortly after as smoke, witness your comment about the ESC popping it’s clogs under similar circumstances. As your ESC may well have done had you inadvertently left it for a while longer; and in fact, some folks don’t even have to go to these lengths, they just put a bigger prop on, increase the current flow and occasionally they get the same effect.

My friend? once said I was always wrong, too. In fact he said I was only ever right once… …and unfortunately that only happened to be right in somebody’s road……

PB

[Clifford Stone]

Ordrered the Tilingy 3548/04 motor yesterday at midday, plus a few other items, all from Ebay 'Nileinstall RC' based in my old stomping ground of Hertfordshire . . . 9am this morning the door bell rings, its Christmas Thats not bad, less than 24 hours.

Only problem, I've only just sat down from the chores of the day, I wont be installing or testing anything today and its just started to rain . . . retired, who said I wold have loads of time?

In the order was a Turnigy 60amp ESC, impressed by the solid weighty feel. The postman also brought a 'Mystery' 85amp ESC, orderd a week or so ago, via Ebay, Chinese company, UK stock, I've been getting a bit hacked off with the time stuff takes to get here from China and Hong Kong.

I'm even more impressed with the 85amp Mystery ESC. Built as a two PCB sandwich, a finned heat sink in the middle of the PCB sandwich, plus one more on the top, 12swg cable, its seems to be built for some serious work?

I'm going to sit back and stay with the 3548 as is using 3 cell batteries for a while, I'm jumping from one idea to another. Better to learn and get the best out of one set up first, then move on, if the Wot4 is still in one piece.

Dark nights, 'wet, cold weather' not far off . . . 1st of August tomorrow I dont venture out when its wet and cold, my club bas an indoor winter schedual.

CJS

[Clifford Stone]

OK, Wot 4 all back together, I'm getting the message . . . I think its; if I want to use 4 cell 14.8v, I must use a motor with a lower kv'. So, as I only have one 4 cell battery, for the time bing I will stay where I am, see the summer out and re group over winter???

I did a watt meter test this morning, see how the Turnigy 3548/4 compared to the KMS 2820/5 on 3 cells.

The two relevent props APC 12x6 and 11x5.5:

Turnigy: 12x6, 48a - 512w . . . 11x5.5, 41a - 480w

KMS: 12x6 , 41a - 452w . . . 11x5.5, 31a - 346w

I dont know how this stacks up to those of you with experiance? My next move might be to go from 2,200mah betteries to 3000mah to lenghthen the flying time, curently 6-7 mins using the KMS.

I'm waiting for a few more props to arrive . . . I will be very interested to see how a 13x4e shapes up on the watt meter? As Pat has mentioned, it may be worth looking at ic props, see how they perform, get the rev counter out as well, see if the 'e-thin' actualy works?

CJS

Edited By Clifford Stone on 01/08/2012 17:18:40

[Steve Hargreaves - Moderator]

My 2p for what it is worth Clifford is that you have a lot of power there.....plenty for a Wot 4e.....

You seem to be pushing those batteries darned hard too....over 20C....but, & this is the interesting bit, you aren't using anything like full power most of the time.....if you are getting 6-7 mins flying time this is less than 10C & equates to less than 22A average current which is around 220-230watts....

Certainly worth experimenting with a few props but remember watts on the wattmeter & static thrust are only part of the story....what really matters is how the model flies.....

I did a bit of subjective testing with a model & a few different props & found than an IC prop I had was the "best" in terms of watts & also how much static thrust the model appeared to have (by the very scientific method of holding the model at full power).....

The IC prop was completely rubbish in the air compared with the same size APE e prop....

[Clifford Stone]

You 2p's worth is valuable to me Steve as I'm on this steap learning curve . . . near vertical! Its true, I tend to fly at 2/3's power most of the time, just an extra squese on the stick in manouvers. Its still early days for me. I dont supose I have had more than a dozen and a half flights . . . then I did the Red Arrows 'smoke on' bit!

I'm still smoothing the rough edges after nearly 20 yeas lay off . . .

Interesting your coments on the ic prop, I have my rev counter and spring ballance all ready to go! The old 'pylon racer' in me creaping out . . . but as you say, the way she flies is the final test. I have not been happy with the vertical performance but I'm begining to put this down to me not the plane or its power train. The fingers or nearves are not as steady as they used to be, the eyes are not as good either . . . orientation in the evening can get the adrenalin running . . .

How do yo arive at the 'C' figure Steve, what does its definition?  Sorry another one of those dumb questions.

CJS

Edited By Clifford Stone on 01/08/2012 18:36:07

[Steve Hargreaves - Moderator]

No dumb questions Cliff....apart from the ones you don't ask....

C...basically the capacity of the battery........a quick example for you....

Before we begin though understand that there are 1000 milliamps in 1 amp so a 2200 mAh battery is the same as a 2.2 Ah battery...I plan to show the example in amps because the numbers are a bit easier

OK so capacity of a battery is a function of current & time thus is we take a 6Ah battery this will supply 6 amps for 1 hour....or 12 amps for half an hour (2C) or 24 amps for quarter of an hour (4C).

We often talk about 20C batteries...this means they are capable of supplying 20 x their capacity...in this case 20 x 6 amps equals 120A however they will only do this for a very short time.....one twentieth of an hour....which is 3 minutes so not a very useful flight time.....

The energy in a battery is finite...we can either take a lot out quickly or a little bit out slowly....the thing to remember is that pushing a battery to its limit (ie using a lot of current very quickly) will inevitably shorten its life...simply because you are pushing it very hard. Add to that the very short flight times that would result & it doesn't make any sense to use the battery like this.

So if we look at your situation.....if you have a 2200 mAh battery (2.2Ah) & are taking 48A out of it then this 48 divided by 2.2 which is 21.81818181818 C. Call it 22C. Your battery might be 50C rated & will handle this quite happily but you will flatten it in 60 divided by 22 which is 2.7 minutes....assuming you are flat out all the time....a bit of a short flight really.... That said you won't be flat out all the time so you would probably get around 4 & a bit minutes in the real world....

Doing the same sums with the 3000 mAh (3.0Ah) battery we get 48 divided by 3.0 which is 16C & this would give you 60 minutes divided by 16 equals 3.75 minutes.....probably over 6 mins in the real world....so you get a longer flight time AND you aren't pushing the battery so hard...

Personally speaking I try & keep the max current to around 10 to 12 times the battery capacity (10 to 12C) this gives me a flat out run time of one tenth to one twelfth of an hour...ie 6 or 5 mins & this generally equates to a flight time of around 8-10 minutes.

Hope that helps...

[Clifford Stone]

Thanks Steve . . . understood . . . I cant handle the numbers, I understand them but they will be forgoten by the morning However the concept is not a problem, get the amps as low as possible.

I'm wondering, would a 3548 motore with a lower kv help, compensating with a bit more pitch or my preferance, diamiter, 13x4e or trim the diamiter, 12" . . . on second thoughts, that might be a solution with the current set up? . . . I was looking at PatMc's sugestion for a 900 kv motor, however I was still wondering about staying on 3 cells? Any idea what the numbers might predict?

Beginning to think lighter air frame as well . . . the 'woody Wot 4e' has a CG issue, requires 5ozs in the nose to bring it to a sencible point, thats 3 cells + 5ozs or 4 cells + 2ozs. If I had my time over again I would build a new fin, rudder and tail/elevator from quater grain 1/4 x 3/8 and 1/2 strip, cut a few more holes in the unders side of the rear fusalarge. The other thing to do on a woody Wot 4e, is to cut a hatch in the top of the nose, and use the 'dead void' above the battery to house the ESC. If it survives to the winter, its jobs I might still do?

CJS

[Clifford Stone]

OK . . . the brain is in gear, not dificult to enguage a single 'little grey cell' . . .

I had a few more props arive this morning, amoung the much anticipated APC 13x4e. So this triggered some more static tests:

13x4 43a 470w

11x5.5 36a 370w

Then the racer in me took over . . . 'speed hound' a bit maingy these days but its still there:

12x4, (cut from 13" prop) 40a 426w

11.5x4, (cut from 13" prop) 35.5a 356w 2.8lbs static thrust @ 9270rpm

11x5.5, 36a 370w 2.6lb static thrust @ 9480rpm

10x7, (cut from an 11x7) 38a 389w 2.2lbs static thrust @ 9060rpm

By this time the 3 cell 2200mah battery was getting tierd showing 11.17v, so I put in a fresher 3 cell battery 11.88v and just did the 11.5"x4" cut APC:

11.5x4, 36a 370w 3.22lbs static thrust @ 9360rpm.

All APC 'E' props, batteries are 3cell, 2200mah

The 11.5x4 looks interesting . . . flying test will show it up one way or the other. The standard 11x5.5 looks the other alternative?

Ballancing the props was interesting, a standard APC is not nessicarily in balance, again, experiance in my previous life was very usefill.

I'd be very interested in any coments as to how you read and interperit these findings . . . my thinking is based only on my ic experiance.

CJS

 

 

 

Edited By Clifford Stone on 02/08/2012 15:32:07

[Peter Beeney]

Clifford, Apologies for chipping in again, but I’m wondering if it might be a case that you could decide on a motor perhaps by approaching from another angle. I’m certainly not sure how this would work out, it might need some physical checking out, but if you decide, say, a particular model requires a 12 inch propeller and it wants to be able to fly at 50 mph, flat out, and you want to use a pitch of 7 inches, then you can calculate that it requires to turn at 7550 rpm. So if we now take a hypothetical motor, with a kV of 1000 and a resistance of 0.05 ohm and we apply 10 volts to it, it will turn at 10000 rpm unloaded. If we assumed that it’s best rated speed was about three quarters of this, then that’s 7500 rpm. If we now do some checks find that if our propeller actually reduced the revolutions to 7500, we could do some arithmetic to find that 50 amps would be flow at this level and and 125 watts of heat would be produced by the motor. However, if we increased the prop size such the revs were reduced to 5000 to try and get more performance we would find that the current has increased to 100 amps, it’s doubled, but it’s now generating 500 watts of heat, this has risen by a factor of 4.

From this I think that I might glean a couple of points, I would need to keep the motor turning as fast as practically possible, to keep the current flow to a minimum, and also the the resistance wants to be as low as possible, to keep the heat generated as low as possible.

Of course, this may not be too far removed from what you would actually do, using a wattmeter, but tinkering with i/c engines I find a tacho invaluable, and for any electrical queries I often use a contact thermometer too. But that’s just my personal choice.

It may seem logical to double the prop size to halve the revs, but as I’m pretty sure that in general to double the speed of any fan or propeller you have to square the power applied, so then the prop size in practise might not have to actually increase that much. Going up just a couple of sizes might make quite a difference.

The ‘light is right‘ approach is also good, I think this generally applies to all models in the main, but perhaps particularly to electric types. Looking at a friends Sebart Angel yesterday, it certainly appeared that this is a case of paying more for less. Certainly it looks as though some considerable effort has been made to adhere to the old and simply straightforward adage - ‘For success, add lightness!’. Looking at the spec. this looks like a 50 inch span aerobat that weighs in at 3 lbs 1 oz ready to fly. And fly it certainly does!

With regard to the cooling again, it may well be the ESC is a bit of a higher resistance point in the chain, and thus vulnerable to higher currents. Looking a a colleagues small fast electric models, he brings the ESC to the outside of the model, so that’s it’s hanging out in the breeze, but with the help of a bit of Velcro maybe; this perhaps requires a bit of customising, but I think it’s probably something that is done fairly frequently. ESC’s do seem to be a bit suspect.

Good Luck.

PB

[Peter Beeney]

Cliff, - Sorry, I didn’t mean to post over you. Didn’t see your post in time. It looks as though you can ignore what I said, perhaps you’ve cracked it already!

PB

[Clifford Stone]

Posted by Peter Beeney on 02/08/2012 15:48:12:

Cliff, - Sorry, I didn’t mean to post over you. Didn’t see your post in time. It looks as though you can ignore what I said, perhaps you’ve cracked it already!

PB

Hi Peter, thanks for your time putting the post together . . . As I have said befor, my dyslexia make things like formuli dificult to cope with. I have to break things down in to something I can understand and work with, the technical explination is helpfull, as I break things down in my mind I keep refering back to the original explination, I supose I am translating it into my 'in brain, dyslexic language'. Your post seems to confirm that my thinking, in my off the wall way, is along the right lines? Certainly not normal, but the figures I have come up with seem right to me?

The lightness thing when related to the woody 'Wot 4' . . . I just cant see what they were thinking when they put it together it could easily have come in under 4lb instead of the 4.5lb that it is with a CG position issue.

Whilst I was doing the test this morning, I kept a very wary eye on the ESC, feeling how hot it was, it was out in the air earlier in the week when I did the first set, it got quite warm/hot. All is now installed, the ESC is in the line of the new large hole in front of the undercarriage, feeling after each run, it remained supisingly only warm to the touch.

Here you go, radio bay blackened from the previous b-b-q'ed unit. You can clearly see the new forward large hole and the proxcimity of the new ESC with its heatsink

[DH 82A]

11.5 x 4 @ 9270rpm = 35mph

11 x 5.5 @ 9480 == 49 mph

10 x 7 @ 9060 = 60 mph

11.5 x 4 @ 9360 = 35.45 mph

Are these airspeeds satisfactory ?

[Clifford Stone]

Posted by DH 82A on 02/08/2012 16:57:15:

11.5 x 4 @ 9270rpm = 35mph

11 x 5.5 @ 9480 == 49 mph

10 x 7 @ 9060 = 60 mph

11.5 x 4 @ 9360 = 35.45 mph

Are these airspeeds satisfactory ?

It depends DH on how they are maintained in manoeuvres. Speed is not the essence of a 'Wot 4' it is how it 'flies'. I have all ways preferred finer pitches/ larger diameters for fun machines in my previous life. Plus the fact that a high pitch often suffers from high slip when loaded in anything other than straight and level.>>

The proof of the pudding will be in the flying . . . I still think the 4" pitch will do well in manoeuvres, the power transfer on static thrust was very good. It will be a close thing between the 11x 5.5 and the 11.5x4, the 10x7 might be the dark horse, will that straight line theoretical speed, convert into vertical performance rather than simple inertia?

I also ask the question, how will 7" of pitch load the motor/batteries verticaly, compared with the 5.5 or 4 inch pitch? Again, a question for on the field practicality.

CJS

>>

[Peter Beeney]

Clifford, I have to say, my ramblings were perhaps getting toward conjecture anyway, I’ve not really done very much in the way of practical electric flying for a long time. Although I think my take on your model would be just a teensy weensy bit questionable, as I think DH 82A was also hinting at.

You have a Wot 4 weighing 4.5 lbs. If I were using an engine in this I think I’d want a least a lively 46, so that I would be able to obtain a fairly varied performance. This engine is going to have at least an output of 700 watts, close to one brake horse power, and bear in mind this is mechanical watts, at the crankshaft, whereas electric watts are measured at the output at the battery, the mechanical watts at the motor shaft are always going to be less than this.

However, as you say, the proof of the pudding… Fine pitch, large diameter props are popular on the ‘fun-fly’ type of model; but how you regard your your model’s performance on these very fine props, at what seems to be fairly low revs, will be very much a personal thing. Eventually it may come down to perhaps higher performance but lower duration, or an average performance and longer duration.

It’s all in the eye of the beholder anyway, as they say.. I watched an electric trainer flying, the pilot was a real electricity man, he also had an electric car. It really staggered around, I considered it was severely underpowered, but he seriously thought it was the bees knees. It had a totally duration of about 5 minutes, so he only flew it for 4 at the time. If there had been any wind at all, it might have soon been in trouble.

I’m sure you will come back here soon with your verdict on all of your trials and tribulations, lets hopes they’re favourable!

Whatever you do, don’t bend it…

PB

[Clifford Stone]

Yes Peter, they are 'trials and tribs' . . . I feel the woody Wot 4 is not a patch on the ic version or the Foamy . . . ! Neaver flown a foamy, but from what I have observed . . . One has owned a wood ic Wot 4, that was simply the dogs swingers.

I was going to buy a new, lower kv motor and a couple of extra 14.8's . . . but thinking about next year, spent a lot of time this afternoon wondering what to do . . . looking at all the goodies on the internet? My feeling curently, making the best of what I have for the rest of whats left of the summer.

I fancy one of two routs, an arepobatic biplane, Panic style, something I can draw a bulid myself, have you seen the new 'kits' avaliable @ £120 . . . thats a kit, there aint much in a Panic!!! . . . or a Pitts Special, but thats not relaxing? Also fancy the, Super 60 type route, I used to have a S60, I simply loved the relaxing nature of flying it, after tearing around a pylon course at the weekend, an evening with my 60 was a pleasure.

In the meantime, I think the 'wood Wot 4' is not what it is made out to be, living on a name, reputation and old nostalgic pilots like me to buy them Not the happiest of bunnies this evening, a personal view of course.

CJS

[Steve Hargreaves - Moderator]

Clifford I think you are getting a bit too hung up on theory here.....worrying about this kv & that kv & different props & pitches. All good fun for the determined tinkerer but really not necessary for a successfully flying model

The watts per lb rule is a tried & tested method.....what is the AUW of the model ready to fly? Multiply it by 100 for Sport performance, 150 for good vertical performance & 200 if you are a hooligan....this gives you the total power in watts you need. Use a prop around 6" pitch & a motor with a kv around 11-1200 for 3S & around 850-900 for 4S. Divide the watts by 10 (3s) or 13 (4S) & this will tell you how much current is likely to be drawn & hence the size of the ESC. Divide this current by the capacity of the battery in amps & this will tell you how hard you are pushing the battery...if its getting on for 20 consider fitting a larger battery.....

Check the results with a wattmeter........& maybe tweak the prop size slightly either way to get the performance you want.

Fly & enjoy....

Thats pretty much all there is to it....