Current ratings on motors

[orange_rc_pilot 0]

Hi all,

I’m coming to the end of my next scratch-built project, the Sphinx VTOL (photos on my Instagram page orange_rc_pilot), and I was wondering to what extent I can run my motors beyond their max current rating. I’m using 3 of these: https://hobbyking.com/en_us/turnigy-d2836-9-950kv-brushless-outrunner-motor.html on 4s and 1147 props, and I got 27.4A max current, whereas they’re rated for 23.2A - is this all right?? (My ESC is a plush 30A one so I’m not worried about damaging that)

I’d only ever be at max throttle for a few seconds on takeoff/landing, or perhaps not at all if I can get away with just 80% throttle or thereabouts. So it’s not like I’m constantly going to be pulling 27A - but I don’t know how long these motors can handle being pushed beyond their limits. 

Any ideas at all would be greatly appreciated - I’m still learning! I’ve spent over 150 hours on this project and I’d hate for something as stupid as a burned-out motor to cause a crash. 

Thanks,

Sevan. 

[Bravedan 106]

Don't know the model and cannot get to see your pics.

On 4S (nominal 14.8v) a running current of 23.2A gives in excess of 343 watts.

Your figures of 27.4A means the motor on 4S is dissipating over 400 watts.

Either way that is a long way past their max stated, as HK states their max power being 243 watts.

You would need to be sure that the steady state running current would be below that by a margin, then (maybe!) a brief surge above might be OK

In the words of Clint Eastwood  "Are you feelin' lucky, Punk?  Well, are ya??"

 

I know the Suppo 2300kV 2212-6 motor well. That can be pushed well beyond that stated for it. On quads I have 2S motors running on 4S and 3S motors running on 4S, and there the load is fairly constant. I do not however know what reserve your chosen motors have, but as they are only 950kV, I doubt the conductor size would cope well with large overloading

Edited February 11, 2018 by Bravedan

[orange_rc_pilot 0]

Hi Dave,

Thanks very much for your detailed reply. Your calculations make it look much worse than what I had hoped for! And sorry Mr. Eastwood, I ain’t feeling particularly lucky about this!

In that case, I think I’ll up the size of the motors, maybe another few mm wider and in the 1200kv range. I’ll do some HobbyKing hunting. 

Thanks once again!

[Bravedan 106]

If you can get photos on here or a solid link, and an indication on weight, we may be able to help more thoroughly, but the "we" won't be me though as I'm about to go offline for a few days.

[Pilot Ben 28]

Going anywhere/ doing anything nice Dave?

[orange_rc_pilot 0]

Update on this topic:

 

I know it's been a while since I posted this, I've been busy recently. Long story short, I bought some new motors with higher power rating. I'll fiddle around with different props to see which gives the best power/thrust ratio (I imagine 11" props on 4s will draw more than 400W, which is what the motors are rated for - we'll just have to see ). I'll post a full chart of my results if/when I come to testing. Note that this may not happen for another few weeks.

I've included a couple of pictures* and a SketchUp rendering* if that helps at all? Essentially I'm running 3 identical motors, with 11x4.7 props on the front two motors and an 11x4.7 chopped down into a 9x4.7-bullnose on the rear motor. Each motor has a 30A (Burst 40) ESC. AUW is 3.1kg, wingspan is 1480mm. It's a VTOL tilt-wing, so I imagine I'll be using about 80-90% throttle on takeoff/landing (probably 100% on landing, since my batteries will have dropped significantly), then probably 40% throttle in flight. Therefore, if my full throttle current draw is getting into the burst current range, that's ok because I plan to use full throttle very sparsely.I did some calculations and used some airflow software and calculated that its terminal velocity will be about 90-100mph, and stall speed around 35mph, so I can definitely bring the throttle right down. I haven't tested it yet, but I imagine thrust will be around 3.5kg max.

I know that's probably a lot of useless info, but I'm just putting it all in there in case someone has an ingenious idea, or spots a fatal mistake which I haven't yet noticed. You guys are much more reliable than my Instagram followers at giving reliable feedback! (Don't worry Ben, I'm not having a go at you  it's the other 95% of Instagrammers who don't know what they're talking about).

 

Thanks in advance!

 

Sevan.

 

*the eager-eyed amongst you may notice that the nose is much longer in the picture than in the rendering - it was so tail-heavy once I had built it that I had to extend the nose so I could put the batteries further forward without having to use any dead weight.

[Bravedan 106]

Hi Sevan,

That is looking very good, I like the Fus style. 

I don't disagree with your figures, the only thing I'd say is that you need to make sure that your cell packs can deliver that current in ACTUALITY for as long as you need without going over weight in pack choice.  Sellers are pretty adept at enhancing the truth!

Mechanically it also looks OK with one reservation. I really do not like the apparently limited flex room given for the wiring from the rotating rear prop bar. I suggest a "loop" of cable to allow greater freedom. It's not only loading for the servos, its also mechanical damage to the inner cores of the wiring flexing back and fore that tightly.

There have been a few Tx makers that have come unstuck (or their buyers have) by not taking into proper account the flex of the wiring going to the gimbals on hall effect sensing assemblies.

Also undercarriage/wheels. These are far too small for grass running, and while its purpose is not to conventional ground run, for easy VTOL a wider spread on the rear gear would be useful i think?  Watch any VTOL or Helicopter and the final put down is rarely level or smooth.

But then both the Spitfire and ME109 survived (though not always their crew!!) with narrow spaced gear. 

Edited March 9, 2018 by Bravedan

[Pilot Ben 28]

Surely if you want to takeoff/land with VTOL, and you want to save as much weight as possible, you could do away with the gear entirely? I don't see much practical use for them and if you really wanted to land it properly surely a pair of heli style skids would work just as well?

And if you wanted to stick with the fixed gear you could make it wider by mounting each leg from the front of the tail boom?

 

[orange_rc_pilot 0]

Thank you both for your replies!

Dave: In terms of battery power, I've got two 4s 3000mAh 40C (50C burst) running in parallel to create 6000mAh, so my maximum burst current is 300A! Hopefully I won't be going anywhere near this 

As for the tight wiring, the picture is  quite misleading. Where the wiring enters the foam booms on the sides, there is some hot glue there but that hot glue is not actually attached to the wires! It's just there to make the hole in the boom wall slightly smaller, so the wires are not secured in that seemingly tight position. Essentially, when the prop bar rotates back, the wires are free to slide out of the foam booms (I have about 10cm of slack within the booms). The zip-ties around the prop bar just make the wires slightly springy, so that when the prop bar rotates forwards, the wires slide back into the booms and are tucked away. I thought about creating a loop as you suggested, but since the prop is a pusher-style in this setup, it might suck any wires into it and rip them to shreds. Also the motor itself tilts independently from the prop bar (for yaw control in hover mode), so the tips would be scratching any wire loops at the intersection of the bar and boom.

The landing gear is pretty narrow, I know  it was just to save weight and limit how much aluminium I had strapped to the bottom. To make the wheel base wider, I initially planned to have the gear protruding from the two booms just as you suggested Ben (as you can see in the digital rendering), but when I built it, the sideways stress on the gear was so great that it bent the two booms outwards a bit, as well as the landing gear wire itself. Whenever I set it down on the table, it would just slump to one side as if comically drunk! I can't put the gear on the wings either, as this would put far too much pressure on the wing tilt servos. My only other option would be to make the gear wider and attach it to the fuselage as I've done now, but that's extra weight and a lot of leverage on a very thin aluminium joint (especially with 3kg+ of plane sitting on it).

Ben: I would have loved to eliminate landing gear altogether, and just land it on the nice flat undercarriage, but the wing chord is too fat - it sticks out below the bottom of the fuselage by about 50mm when in hover mode. I could have designed the wing to have a higher aspect ratio, and then be skinny enough to fit above the fuselage, but I would have needed a wingspan of more than 2m and this would produce too much stress at the joint between the wings and the booms.

I do like your suggestion of heli skids though - I'll do some internet hunting and see if I can find any with the right sort of dimensions which aren't crazy expensive. Thanks!

 

After I fly this one for a bit and check that it all works without any odd tendencies (and then rectify all the problems I'm bound to encounter!), I plan to do a revised version with sleeker curves and a more efficient setup. It'll also be lighter, so I'll have leeway to extend the landing gear, or reduce the wing chord since I'll need less lift. But that's for later; this is just a prototype with big old clunky equipment 

Edited March 9, 2018 by orange_rc_pilot

[Bravedan 106]

23 hours ago, orange_rc_pilot said:

I'll do some internet hunting and see if I can find any with the right sort of dimensions which aren't crazy expensive.

Have a look for secondhand Heli stuff , something like Hirobo Sceadu, TRex 700, etc

 

https://www.ebay.co.uk/itm/TREX-700-ORANGE-LANDING-STRUTS-BLACK-SKIDS/132476002775?hash=item1ed82f61d7:g:9E8AAOSwnsRaYgRs

 

two 4s 3000mAh ....................   two 4s 3000mAh??  Are you going to sit on it to fly it??  

[orange_rc_pilot 0]

Those skids on the link look good, but I worry that they (along with lots of the other ones I found online) are too short - vertically, I mean. I need to put the base of the fuselage a good 13cm up off the ground, and looking at pictures of some of the biggest helis, they only look to be about 7 or 8cm. If I have them too short, then any slight wobble when I come to land will smack the wingtips into the ground and likely strip the wing tilt servos.

I think I've got to stick to the tall and narrow landing gear for now. If the narrow wheelbase becomes a real problem, I'll make them a bit wider, but then use thicker aluminium to make it strong. We'll cross that bridge when we come to it, though.

 

As for the batteries.... yeahhhh TBH I couldn't really be bothered to install an RX and bind it to my TX and create a new model and all that jazz, so I'm just going to bolt a yoke and pedals on top of the fuselage and straddle the booms....

 

I realise I'm going a tad overboard... initially I wanted two 2200mAh 3s in parallel, but 3s wasn't enough for the motors so I decided to use 4s. The only 4s batteries I have are 3000mAh, but 3000 isn't quite enough for a proper flight - I'll probably only get 3-4 minutes out of it even if I run it down completely (which I don't want to do). I can't spend that much more money on this project, so I'm having to work with what I've got and just use two 3000mAh 4s...

[orange_rc_pilot 0]

Good news!

Just did a quick power/thrust test on 4s and found the following (note, I only tested the front 2 motors and then inferred the data for the rear motor, as it was difficult to test all 3 motors simultaneously):

- The front 2 motors drew 402W each at full throttle, which is perfect (barely above the 400W limit on the motors, and well under the 40A burst limit on the ESCs - I imagine the slightly smaller prop on the rear motor will draw about 300-350W)
- The front 2 motors produced about 1.3kg of thrust each at full throttle, which is also perfect (I imagine the rear motor will produce 900g-1kg, so that's about 3.5kg of total thrust, which is more than its weight (3.1kg) so I can do VTOL, yay!)

All in all, that means I've got about 1.15kW of total power (I've never had to work in kW before, this is excitingly powerful ), which is a whopping 1.5hp, and 3.5kg of thrust, which puts my maximum forward airspeed at 93.7mph!

I did a few rough calculations and found that I'd probably be running my motors at just under half throttle (1.5kg thrust) during forward flight, which gives me 55mph airspeed (well above the 35mph stall speed), and at 1.5kg thrust, I'm only pulling 24A across all three motors put together! That means that I'll have 10-minute flight times if I use 70% of my 4s 6000mAh-equivalent battery, which is great. Of course I'll be using quite a lot of mAh on takeoff/landing, so in reality it'll be like 8-9 minutes, but still good enough 

I know there are a lot of exclamation marks in there, I'm just excited that everything works as it should  my only concerns now are the small wheelbase if I'm landing in turbulent conditions, and the possibility of the wings folding if I pull too many Gs in a turn...

[Pilot Ben 28]

Surely 94 mph is a bit high in the real world for a foam and hot glue model? I know foam planes have reached those speeds but they’re all EPO/EPP carbon reinforced jobs. At those speeds with a model of that weight you’ll need some nice size servos!

How are you going to “trim” the motors so that in the hover it stays where it is? I know a multirotor model setup on tx will have this but a fixed wing? Lots of advanced mixing and programming ahead!

Any idea when it’ll be complete?

B  

[orange_rc_pilot 0]

1 hour ago, Pilot Ben said:

Surely 94 mph is a bit high in the real world for a foam and hot glue model? I know foam planes have reached those speeds but they’re all EPO/EPP carbon reinforced jobs. At those speeds with a model of that weight you’ll need some nice size servos!

How are you going to “trim” the motors so that in the hover it stays where it is? I know a multirotor model setup on tx will have this but a fixed wing? Lots of advanced mixing and programming ahead!

Any idea when it’ll be complete?

B  

Yeah I never actually plan on going that fast. Maybe one low pass, and then slow down and pull out of it reeeeally slowly! I've only got 9g servos, so I shouldn't do anything too wacky.

I don't actually need to do anything within the TX. I've got a KK2.1 hard-case board flashed with OpenAeroVTOL (open source software: love it ) which acts just like any other FC firmware, except it's designed specifically for VTOL so it has 3 modes: hover, slow forward flight (SFF, where the wings are tilted forward a little bit) and fast forward flight (FFF, just like a fixed wing). You just designate a particular  channel from your receiver to be the mode channel (i.e. gear, aux etc) and it does all the fiddly stuff inside. It took a good 10 or so hours to programme it, but once it's done it's done. It has auto-level and all. I could even enable stabilisation in FFF mode but I'll just leave it manual for now.

I think I am done? IDK, it seems like there should be more to do, but I think the next step is just to take it to my local park and hover it a few ft above the ground just to test everything, maybe putz around in SFF for a bit. Possibly this weekend, if not then definitely soon.

[orange_rc_pilot 0]

Hi all, I know this is an old thread, but I’m just letting you know that the Sphinx VTOL is now complete, and she will see a maiden flight tomorrow (Saturday) at the field! Apologies in advance for any tears if she fails to meet my expectations; we’ve been through a lot together...

 

I’ll post some pics and vids shortly afterwards 😉

 

Sevan

 

 

[Pilot Ben 28]

Hope all goes well!