So today I started working on the best layout for platypus, and come across some design problems. Nothing I can't fix in the next prototype though...
Ok, step 1.
PDB, SBEC and ESC's are in. The PDB is a Gryphon Octo, minus the BEC lid (it will fit, but I need to get some smaller risers to get it to fit, so a plain ol' 5.5V SBEC will have to do). The new hobbyking "big amp" PDB will go in here too.
ESC's are hobbyking multistars. And gawd, what a pain in the **** they are...they got the right idea for solderless builds - bullets at both ends - but both ends bullets are completely the wrong size, so need cut off and replaced anyway! I may fling them and throw in some 40A simonK's instead. They'll be lighter for a start, and that becomes important later on....
Anyway, this frame is designed so that the ESC's only really fit in one place. There are pro's and con's to this approach, but in the end, it makes a tider bay. The main disadvantage is the length of the power wires for 2 ESC's (or 4, of you go XY8). Noisy? Maybe...time will tell.
The next prototype has the standard PDB mounts, and the holes are closer to the ESC's, so hopefully keeping the cables sort and equal. One advantage of standard/longer lengths is the ability to twist them, and this frame was designed with that in mind.
Step 2:
Simple step - put the ESC plate on. This helps secure the ESC's, and their wires, while also providing a platform to add 4 more ESC's (for an XY8). FPV camera mounted as well.
Step 3:
The biggie. The main electronics desk. Wow, where to start...
- A solid plate of CF here, to help cut down on "noise" from the PDB and ESC's below.
- APM (and so to be pixhawk) mounted slightly to the side. Effects on flight should be minimal
- rx place beside. The concern is that with so much CF around, the signal will be weak. I will be placing the satellite on one of the landing legs in "diversity" mode. I will also be testing the orangerx r800 with two "carbon fiber" (long wire)satellites attached.
- spektrum telemetry tx moved as far away as my cable will allow.
- spektrum alt meter located...well...anywhere frankly. It's "rx". It's mainly the "tx's" you need to worry about.
- GPS/compass located right next to APM, with ideal being that elevation will be easier with the DJI GPS pole. It can certainly go higher than the standard 5cm now, even with the *stupid* 5/6 cable 3DR provide (seriously guys....why??? why not a 5/5 or a 6/6???)
- immersionrc OSD located....poorly, frankly. But there are some reasons to put it here - the main reason being easy access to the buttons.
- fatshark 250mw 5.8 tx located *way* out the back. BUT....I'm going to change that, and place it *underneath* the bottom plate for better signal, in the same way as the 3DR telem radio (see next slide).
With this layout, only 3 cables need disconnected to remove the top plate, and all the electronics can stay in place. This was by design - nothing worse than having to completely disassemble your airframe just to get to the internals...
Step 4:
Underneath. Ok, not happy with this.
- a 490mAh 3S is running the gimbal and the FPV system, but I'm not happy with this cable running. What a mess. I should really find a way to run the lot from the 6S.
- the 3DR telem is here
- the optiflow and sonar are in place too. Cables route straight up the APM deck in front the battery. (note: next prototype will have fitments for px4flow and - hopefully! - the lightware SF02 (once the dimensions of the bracket are in). These will be on the side though, to ensure battery cable routing is kept clear.
- I propose to add the immersion tx here on the other side; as well as upgrade it to 600mW.
And that's it! Just need to hook up the motors and maiden it.
Good news: This looks like it'll take 17" if required...and if you can find the motors to turn them! (I'll be running 15", as seen here).
Bad news: Holy **** it's heavy. Too heavy. With all the FPV stuff onboard, and the 2nd battery circuit, it's coming to 3.5kg. And that's WITHOUT motors. 4 x MT3515 on there and this will weight 4.5kg - 300g over my planned budget, and still 150g over budget if I remove the FPV stuff. (though in that case I would be able to power the gimbal with a 12V bec, saving more weight).
Also, I've noticed a bit of flex nose to aft, if you lift it from the gimbal plate and the rear. Which means I'll need to rethink moving some parts to 1.5mm CF, and even add a strengthening spar!
PS - does anyone know where I can get *good* m2.5 x 37mm screws? Mine are garbage.
Comments
Ok, tonight sees it rebuilt with MN3508-29's for a reasonable weight comparison to the 3510's (360kv) which I will order shortly.
And the weight is promising! AUW - with the 10,900A 6S, gimbal, MN motors, NEX5, FPV gear, 1555 CF props, all electronics is....4.35kg
I recon there's around 100-150g saving in the frame to be made. So that bring it down to 4.15kg.
Then...use the t-motor 30A pro OPTO ESC's to shave another 80g rather than the heavy 45A multistars. A lighter, lower amp PDB - another 80g. Remove the Optiflow and sonar sensors - 30g.
Which brings the AUW to....drum roll please...a very French friendly 3.96kg! Even with the 10.9A 6S!
That adds up to 11.2mins mixed flight time and 21.4 in hover. Hover throttle still above 60% (61%), but damn...it's looking promising!
Rig now just needs all the calibrations and some flight testing.
You should be able to loose more weight since my VulcanUAV based X8 with gimbal and 1100g battery weighs 4800 g ( having eight motors, MN3110-17). So at equal number of motors, i weigh the same as your frame with gimbal and NEX5 camera included ( even less if i rempve four ESCs). -> diet.
Interestingly, the "French legal" version (8000mAh, 4000g AUW) comes out at 8.6mins mixed, 15.6mins hover, and just above the maximum acceptable hover throttle (62%). It should fly better, but as expected will only have average flight times (around 12-14 minutes in the real world I recon).
re: motors...I'm still "in the red" on ecalc.
4300g AUW (inc drive), with 6S, using 3510-25's + 1555 props = max watts of 369.9w (motor is rated for max of 330w). It does meet the flight criteria though - with a 10900mAh it gives over 10 mins on mixed flight, and almost meets the 20 mins hover criteria!
67% hover throttle though...was hoping for < 60% for best flight characteristics.
Woops, forgot to mention - the landing legs are are 45'. This is great for video and general platform stability, but puts a torsional stress on the mount when doing the "6 inch drop" test. Ie the legs splay outward. A move to 30' will certainly reduce this, plus allow me to extend the height easily.
But can I find one off-the-shelf on the market? No way...and the rest of the stuff from china is just crap or won't fit.:-( I might have to look to getting one 3D printed, or - ideally - CNC'ed from aluminium, but this will be beyond my CAD skills. And there's the evil weight demon lurking in the corner down that route...:-)
Excellent suggestions Hughes.
In fact, the motor on the last photo is a 3508-29 (underpowered) purely because of the weight issues, and because it's what I've got lying around at the moment :-). Will certainly give the 3510's a look now, give my criteria is very close to the scenario you describe. My mileage may vary since I'm limited to 4 motors.
Re: Weight - if only Platypus would do the same as me...I've lost 12kg since moving to Switzerland (83.5kg to 71.5!) :-) But yes - 4kg would be perfect, and was very close to my original goal. At the end of the day, final weights obtained by others who use the frame are outside my control, but you're absolutely right - being able to demonstrate getting under the 4kg legal limit with a NEX5 onboard would be real bonus. I propbably could quite easily right now with a 8000mAh 6S pack, rather than 10900, but the flight times will suffer, and drop to "merely average", when I wanted to push the boundary a bit with what a 4 motor can do.
Re: The batteries - yes, there are two slits available for the batteries (you might be able to see them in the pics, along with my botched-together straps). You are correct to be concerned about the ESC heat, and the plates being cut right now are slitted in the same style as the top plate. Will save weight (a few grams!), but hopefully aid with ventilation of the lower ESC's.
Other weight savings in the pipeline (finalising diagrams this week) are: Battery plate reduced to 1.5mm, slats to rear of front arms on bottom and top plates, slats in the gimbal plate and gaps around the PDB (lower plate). Should save around 100-150g without affecting strength in the important areas.
Re: The landing gear - I had similar thoughts too. There is definitely space for 4 main legs, and 2 longer horizontal spars. I even had that in at one point on the CAD (where the 3DR telem is right now, and the current ones pushed forward), but decided to try the single legs first. I will most likely resort to the original idea, and accept the weight penalty!
Euan,
-For your weight issue (not referring to an eventual diet for yourself), you really need to think about the MN series rather than the MT series. The motor you have chosen is 188g and the motor I would recommend you is only 97gr : MN3510-25. I have done all sorts of simulations to try get off the ground a 5000 gr craft with 8 motors. Since you may fit up to 17 inches props, know that for example the 3510-25 with 15inches props can largely lift 5000gr (with 8 motors) and with the best flight duration than any other tiger motor (I have simulated them all on eClac with 6S).
To confirm the simulation, Frantz Forrest is testing the 3510-25 right now this week. I will post you the real measured thrust values so you can judge for yourself if these motors would fit Platypus or not.
Weight is indeed a critical parameter you need to optimize, not only for motor/prop sizing but also for certifications : in France for example where the law is already in place there is a weight limit of 4Kg (4000g) under which you can get very flexible certifications and autorisation to use drones for commercial activities. Above 4Kg it becomes much more difficult (although not impossible). So if you have a craft able to do professionnal photography under 4Kg you have a strong commercial lead over competition (in France and probably in the close future for the whole of Europe because France is the most advanced on this kind of legislation and they are paving the way for the european commission!)
-Another remark from your photos : I see batteries being apparently just laid down on CF plates. Did you plan to cut in your CF plates some 1 inch slits that would allow to use a velcro band to secure the batteries ? You could plan to cut out parallel slits at different widths to accomodate different battery sizes ?
-About your ESC plates (to fit 4 additional ESCs in case of a 8 motors configuration), it would be useful to cut slits on these plates to let maximum air flow cool the ESCs. Eventually Aluminium would be better than CF for these plates to act as radiators (you could have black light aluminium plates).
-How do your plastic endings of the landing gear under the bottom plate resist to such weight ? Needs to be tested too. For information, I have seen big mikrokopter crafts doubling the vertical landing boom on each side because of the force of impact (a single boom being not stable/strong enough for rough landings)
that's it for now...(I have to get back to work)
@ Eric: For the main body itself, yes that's about right! With the gimbal plate in place, it's 44cm long (around 17").
Re: M3's...Yes, I contemplated this. I'd prefer M3's myself, especially with the 25mm arms, but they're rarer than hens teeth in the weight and colour I want, and they are expensive, even wholesale! The mounts themselves would need re-designed as well, as M3-using clamps are wider.
I can get the clamps I want in M2.5 that are light and in the colour I wanted. But the hex bolt heads will round off at the slightest torque. I think they're from China - do they use some funny allen key sizes over there? Anyone who dealt with the TBS Disco bolts of 2012 will understand where I'm coming from!
This only affects the motor clamps; everything else is M3.
re: The small flex - turns out I'm actually worrying about a small problem. By lifting it from the tail and the nose with my hands, I'm actually exerting forces on it which it would not experience in flight because the main forces would be seperate. The motors would stress the main body, while the gimbal plate will try to flex around it's front risers as a secondary effect. Stress the airframe like this, and there is barely any flex at all!
Very nice, lots of room!
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