ESC 'headroom'

[Yorkman]

I've just bought two of these-

**LINK**

for my Mosquito, now I need new ESCs cos only have 30amp ones for the original motors. As you can see, Giantshark are recommending 70amp items-for a motor that supposedly draws a maximum 35.8 (very precise!) amps....

Just read a thread on here that recommends 20% headroom, which sounds good, but even being generous with one's calculations that only comes to 45amps.....

So-would 50amp controllers do the job? Going up to 70s really pushes the size, weight and cost up. Interested to hear what anyone thinks.

[Simon Chaddock]

Yorkman

The important bit is not the maximum current that the motors can take rather how much current will they take in your Mosquito. This of course wil depend on the prop they are coupled too and how many cells are driving it.

If it were me I would use one of your existing 30A ESC and a Watt meter to check what was really happening.

If say full power got really close to 30A then it would unwise to use that but a 50 would be fine.

As soon as you move away from stock bits a Watt meter really becomes vital piece of kit to avoid expensive smoke!

[Yorkman]

I'm sure I just replied to this, but the post just disappeared...

I have run both the motors on the 30amp ESCs. The first I saw a peak of 43 amps before it expired, the second one I kept below 30amps, which was about half throttle.

Does the fact I saw 43amps mean I'm going to need to re-prop it?

[Keith Simmons]

Hi Yorkman

You say 43amps before it expired? Were you able to go max throttle or did the ESC go before then?

you can drop the cell count or change the prop for a lower wattage output..

[Yorkman]

That was full throttle. I'm changing the motors for more performance, as was a bit slow on the original setup. Wondering now if I might have gone a little too high on the Kv on the motor. It was running master 10x7 three-bladers on 3-cell Lipo.. Master 10x5s are available, but will that put me back to where I started?

[Pete B]

That motor you linked to is 1700Kv, which I suspect is way too high for a 10x7x3 and I'm not surprised you saw 43A, IMHO.

I reckon something around the 1000Kv rating may be more suitable. A little detail about the model, AUW etc might help. Twins tend to not need so many W/lb as singles for a certain level of performance so if you were to aim for about 100W/lb that may give you the result you're looking for....

Pete

[Yorkman]

Model is 54" span, weighs just over 4lb. When I measured the original setup one motor was running 25amps, 250 watts, so 500 watts total.

What I am looking for is more airspeed-she leapt off the ground no trouble, but had no top speed.

And I needed to replace the motors as one got rather bent in the last 'too low and slow' incident...

[PatMc]

If you you maintain the same current draw by increasing the props' pitch at the same time reducing the dia this will increase the top speed but at the price of reduced rate of climb. It's all a matter of balancing the manner that the available power is used.

[Yorkman]

ok, seem to be drifting away from my original query!

I assume from the motor spec that the power train should be set up so that the motor is drawing no more than 36amps?

So, assuming I achieve this by reducing pitch or diameter of the props-is a 50amp ESC ok?

Incidentally, just out of interest-what does that 'max amps' figure actually mean?

[Pete B]

I'd say 250W per motor should be plenty. It's a matter of getting enough pitch to give you the speed you're looking for without cooking the motor, which has a 35A Max rating. One option would be to go to 2S packs. 250W/35A = 7.1V, which is close enough. That motor has a suggestion for a 10x6 prop on 2S here. You might get away with a 10x8 2-blade which would give you that extra bit of speed.

The other benefit of 2S is a lighter pack and if you can max the amps at 35, you'll not need any more than a 40A ESC, saving weight again.

TBH, I'm struggling for a low or zero cost option with what you have available at present.

Anyone else any ideas?

Pete

[PatMc]

A 50A esc is more than adequate if drawing 36A at WOT.

Max amps is the limiting factor for a motor, at higher currents the motor windings will overheat & burn out same as a fuse. Also be aware that max amps is often quoted with two figures, one for short bursts of power (typicaly 30secs which is fine for a powered glider) & a lower figure for constant running. The motor will get hot running at the higher current.

I don't think going to 2s is a good idea. It will mean less power for the same current draw. It won't allow a lower current rated esc to be used. The only reason that the battery will be lighter is because it will be giving less power per minute.

IMO a pair of cheap motors of similar weight to the originals & around 1000kv or less would be the best answer.

 

 

Edited By PatMc on 05/05/2014 21:57:44

[Yorkman]

I just bought these motors....seems I over-estimated the kv required!

Is it possible, if I for example, went down to 9x5 3 bladers and got the amps down nearer the mark, to then 'tune' the Tx using the endpoint setting to get the current draw within limits?

[Chris Bott - Moderator]

Posted by Yorkman on 05/05/2014 22:39:51:

I just bought these motors....seems I over-estimated the kv required!

Is it possible, if I for example, went down to 9x5 3 bladers and got the amps down nearer the mark, to then 'tune' the Tx using the endpoint setting to get the current draw within limits?

No.

Using Tx endpoints is not the way to do it. When the ESC is controlling speed it does so by switching between full current and no current, many times a second. It just varies the ratio of time between off and on. So every time it's in the on state, the full current is flowing through the ESC and the motor and is always likely to destroy something.

Sorry.

A 1700kv motor will be trying to spin at almost 18000 rpm on 3S, although the prop will slow this somewhat.

Trying them on 2S may just work. Then they would try to spin at 12900 rpm at 7V. If we guess that this would be slowed to 10000 rpm by the prop this gives us pitch speeds of 57, 66 and 76 mph on 6, 7 and 8" pitch.

However the only real way to tell is with a handful of props and a wattmeter, and only use combinations that keep the current in limits. Then use a tacho to measure rpm, and from that work out pitch speed to know if it will help the speed of the model.

[Biggles' Elder Brother - Moderator]

How fast do you want to go! 60mph is quite a lick with a 50" plus model of a Mosquito! Just to put it in perspective the model is about 12:1 scale - assuming you want to replicate fast low level flying, scale speed would be about 30mph. Now Ok, there are all sorts of reasons why that might not be practical - but you're heading towards twice that! And you have to be able to land it so it must be able to fly slower than that - or those landings are going to be very interesting!

BEB

[Yorkman]

**LINK**

It flew ok, but I have been caught out a couple of times with tip-stalling when landing, and a comment from an observer on its first flight was that it looked slow. I needed to fit new motors after the last crash, so decided to add a little more performance...it just seems that I'm trying to add too much more...

[PatMc]

The 1700KV motors & 3s lipo are going to take around 35A driving an 8x4 two blade prop at around 15000rpm.

I have some figures I recorded a while ago using a 1600KV motor on 3s, I'll post the details tomorrow.

[Biggles' Elder Brother - Moderator]

With those pointed wing tips there is always going to be a potential for tip stalling. But more speed isn't really going to protect you so much - especially on landing, where you can't really use the extra power anyway.

For its size the model is quite light - so the wing loading is relatively low (for a warbird). I certainly don't think you need a lot more power - a bit more is always handy!

Tip stalling happens because you are flying too "nose up" so the Angle of Attack is too high. It tends to happen on landings because folks try to regulate their rate of descent with the elevator rather than the throttle - so sooner or latter they end up trying to extend the approach by pulling back. This can happen in the final turn - when you think your rate of descent is too high in the turn so you add up elevator, or on finals if you feel you are going to touch down short.

However/whenever it happens the result is always the same - it flicks and because you're low in goes in.

The only solution is to go for a greaser everytime. Keep the nose down through the final turn and along the final approach. Its easier if you have flaps - that way you can keep some power on, nose down, hopefully without gaining too much speed. Level (level - not nose up!) out much lower and further out from the strip threshold than you normally would - then you just have to wait! Hold it level, don't attempt to flair - just leave the throttle at a lowish setting that will allow it to gradually sink. This can take a long distance - hence why you level out early, well short of the threshold. Its a very flat approach, Finally just let it settle on the mains - don't three point it - chop the throttle, keep it straight and let it sit down on its tail. Easy to say - more difficult to do.

BEB

[Yorkman]

yes, I know all that! It does take some landing, needs plenty of power all the time and has to be flown onto the ground. And I realise more performance isn't going to reduce the tendency to tip stall-tho it might get me away from danger a little more quickly!

As I've said a couple of times now, I needed to change motors as the originals have been bent and (almost) straightened too many times now, but I've over-estimated my requirements-working with no info about the original motors hasn't helped...

[PatMc]

The 1600KV motor I mentioned got the following on a mid-life 3s 2200 driving an APC 8x4 - 10.8v ; 34A ; 367W ; 14550rpm. That was plenty for good aerobatic perf & 60 - 70 degree climb in a 3.25lb model.

With a 3s 2200 on it's last legs & Graupner 8x4.5 folder - 10.6v ; 26A ; 275W ; 13500rpm. Used in a 60" span warm liner weighing 2.5lb & enough power for near vert climb & general aeros.

The pitch speed for both the above is around 55mph.

IMO the only way you will be able to use your motors with anything bigger than around 8" dia props will be with 2s but that would reduce the power by 33% at the same current & motor rpm to about 10000rpm. To get around 55mph pitch speed would need a 6" pitch prop. A 10x6 on 2s might work but personaly I'd still rather buy a couple of lower KV motors than some 2s lipos.

[Biggles' Elder Brother - Moderator]

Posted by Yorkman on 06/05/2014 00:59:41:

I realise more performance isn't going to reduce the tendency to tip stall-tho it might get me away from danger a little more quickly!

No it won't! All it will do is ensure that you hit the ground going faster!

BEB

[Yorkman]

It seems I vastly over-estimated the amount of motor I need. I have to say that all the numbers associated with brushless motors mean very little to me, so I've been going by 4 criteria-

weight, watts, amps and kv

and I have come up with this-

**LINK**

what does anyone think?

[PatMc]

Dunno where GC got the chart from but IMO it's wildly optimistic. The motor manufacturer give this chart.

[Yorkman]

what does that chart actually tell us?

[Yorkman]

just did a quick measure/calculation- 25.8 oz/sq ft

[PatMc]

Posted by Yorkman on 07/05/2014 12:43:38:

what does that chart actually tell us?

The important figures are the weight, the KV & the max current.

You can estimate the likely size prop using a particular battery voltage & aprox power.
Loading the motor to the max current draw gives max power at that voltage.
The rpm at max current can be reasonably estimated at 80% of nominal battery voltage times KV. This means that you can predict the aprox size prop for the power that the motor can produce using any voltage.

E.G. 1700kv motor, 35A max current :
2s battery : power = 35A x 7.4v = 260W ; at 80% x 1700kv x 7.4v = 10000 rpm.
3s battery : power = 35A x 11.1v = 390W ; at 80% x 1700kv x 11.1v = 15000 rpm.

The graph below is taken from an old Aeromodeller showing power against rpm. The original showed bhp which I converted to Watts. It used a range of ic props but I've found it's still a reasonable aproximation.

In the examples calculated the props would be a 10x6 & around 8x5 - 9x4, bearing in mind these are aproximations we at least have an idea of what power to expect with what props.