Biggles' Elder Brother - Moderator Posted May 8, 2014 Author Share Posted May 8, 2014 6s are the largest batteries I use - and to be honest I don't use a spark arrestor, instead I tend to rely on a quick deft contact! As young engineer I worked for a short time for what was then the CEGB - based at Fawley power station near Southampton. Fawley is a 2000MW station and once you've seen arcing in the switch room there during change overs - well let's just say a 6s battery holds few surprises! Managed to get a useful hour in the shed tonight and basically sorted out how I'm going to hold this battery in place. I've fitted out the chute with some foam pads at the sides, a foam padded "backstop" to be sure there is no way this battery is going to slide backwards, a Velcro strap at the front and fitted the front third of the chute floor with Velcro,... The idea of the Velcro only going part way with the floor is that its only function is to stop the battery sliding forwards. The battery can't go backwards, because of the backstop; it cant go side-to-side, because of the rails and the side padding. Up and down at the front is stopped by the Velcro strap (I'll talk about up and down at the back later!) So that only leaves sliding forwards. By putting the Velcro only at the front I can slide the back of the battery in along the bare floor, then lower the battery (which only has Velcro on the front as well of course - and naturally its fluffy Velcro) onto the bristly Velcro on the front of the chute floor - works well. Here the battery in place as it will sit in the chute - snug as a bug in the proverbial rug,... And here it is in place in the model,... And from the back,... Ok - so what stops the back of the battery from moving off the chute floor? Remember, the fuselage is inverted here. Well another bit of good fortune comes to our aid. If we lay a straight-edge from one wing saddle to the other, we get this,... There is just a fraction of a millimetre between that straight-edge and the top of the battery! So a strategically placed thin foam pad glued to the top surface of the wing centre will give the battery all the support it needs at the rear. Neat eh? BEB Quote Link to comment Share on other sites More sharing options...
Phil Winks Posted May 8, 2014 Share Posted May 8, 2014 Thats neat building Dave, interesting to hear you worked at fawley power station I had a cousin worked there in the late 60's/early 70's, Small world eh Quote Link to comment Share on other sites More sharing options...
Peter Jenkins Posted May 9, 2014 Share Posted May 9, 2014 Guess I'm just averse to a flash and crack when I connect up! Nice job by the way BEB. Looking forward to the flight trials - which shouldn't be any different from before if weight and CG are the same. Might have a bit more oomph though! Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted May 11, 2014 Author Share Posted May 11, 2014 Small world indeed Phil! Yes Peter I'm expecting it to have a bit more umph! It was fine before but if we do get the expected 1.2kW, given the model weighs just under 8lb, the performance should be impressive! OK - what is it they say - "The best laid plans of mice and men" - eh? the "half Velcro carpet" idea is a failure. Oh, it does the job fine, I can get the battery in easily and it holds really well. And there's the problem, it holds too damn well - I can't get a good enough grip on it, accessing through the hatch with with the cowl in place, to break the bond of the Velcro and get it out! Bums! So I needed another solution. Here's the problem,... The battery is all snug; left and right, up and down and backwards. But as it is here, there is absolutely nothing to stop it sliding forwards; and slide forwards it definitely will as once the model is put right way up the battery is facing downhill! Now the last thing I need with this model is 0.8 Kg of battery sliding around inside! The reserve plan was a lip at the front - but as you can see there really isn't enough chute floor to fix it to in front. So, back to head scratching. What I felt I needed was a "front stop" - like the back stop. But of course it would have to be removable otherwise I wouldn't be able to get the battery in. So here's what I've come up with. First I made a pocket from hard balsa and ply that sits against the firewall just in front of the chute,... Then I made a plate that would sit in that pocket. Here's the plate from one side,... As you can see its fitted with a wedge of foam on this side. On the other side it looks like this,... With strips of fluffy Velcro fitted to the back and top surfaces. So we plonk the battery in and then wedge this plate into the pocket, as shown,... We then do up the strap that goes around the battery. Finally I've added two Velcro strips (fixed to the pocket) which go over the plate (attaching to the Velcro on that) an terminate by attaching to the strap around the battery like this,... And that's it - that battery ain't going anywhere! The longitudinal Velcro strips are attached to the pocket, as stated above. This is done by 4 small self-tappers through the Velcro and into the wood. This Velcro has quite a tough plastic backing on the bristly side so the screws hold well, but just as "belt and braces" I have soaked a bit of CA into the Velcro locally. Well I'm well happy with that. I've taken a good grip on the model and given it a damn good shaking and nothing moves at all! But this does show how when doing this sort of job, like any building, sometimes you just have to accept something isn't working and be prepared to take the one step back in order to make two steps forward! (But, TBH, that's not what I was saying when I discovered the problem mind you!) BEB Quote Link to comment Share on other sites More sharing options...
Pete B Posted May 11, 2014 Share Posted May 11, 2014 I don't want to sound like a partypooper, BEB, but I reckon you've made a superb job of insulating that pack..... Personally, I'd say an inch square patch of Velcro towards the front of the tray, with the existing retaining strap, would be quite sufficient. I do like to give all the leccy components a bit of air. Or course, if you 'arrive' rather than 'land' as a matter of habit, I take it all back.... Pete Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted May 11, 2014 Author Share Posted May 11, 2014 I did wonder about that Pete. But the air flow in there will be pretty high thanks to a whopping scale hole at the front of the cowl (plus a non-scale partner on the other side!) and a number of strategically placed exit holes - three inside the cowl itself and one aft. Also remember there was a 90FS in there - completely enclosed - and it never showed any sign of overheating. Add in that at full throtle it still wont reach 10C discharge rate on a 25C battery (and I expect to be at far less than full throttle 80% of the time) and I'm not too worried about the battery getting too hot - I'm far more worried about being absolutely sure it can't move. I also considered the small square of Velcro. The problem with that is that, right way up, the Velcro is on the top! Now a square inch of Velcro under a battery - no problem I agree. But a square inch alone on top of a battery, which is facing "downhill", secured by a single Velcro strap? I'm not so confident! BEB Edited By Biggles' Elder Brother - Moderator on 11/05/2014 22:52:59 Quote Link to comment Share on other sites More sharing options...
Tony F Posted May 11, 2014 Share Posted May 11, 2014 BEB may I ask for your full specs including prop size...I'd like to see if eCalc is working with info put in.......can't work out if my specs for my Hammerhead are correct ? Tony Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted May 11, 2014 Author Share Posted May 11, 2014 Motor is an OS OMA-5020-490. kV is 490. Battery is 6s 5000mAh. Prop is APC14x7E. Predicted current max is 47A, predicted max rpm is 10,500. Predicted max power is 1.18kW. I think that's everything - if you need anything else just let me know. BEB Quote Link to comment Share on other sites More sharing options...
Tony F Posted May 11, 2014 Share Posted May 11, 2014 Weight about 3500g mate ? And your esc ? Tony Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted May 11, 2014 Author Share Posted May 11, 2014 Weight 7.8lbs so that's 3.54Kg. ESC is now a Thrust (Turnigy) 70A job. It was going to be an Eflite 90A - but that seemed like overkill. BEB Quote Link to comment Share on other sites More sharing options...
Tony F Posted May 12, 2014 Share Posted May 12, 2014 Cheers BEB...ok mate this is what i get = current 46a , voltage 21.05 , revs 9540 rpm , electric power 966.4w , mech power 873.2w and efficiency 90.2%. Sound about right ? Tony Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted May 12, 2014 Author Share Posted May 12, 2014 Yes I would say so - all those figures are in the same ball park. 46A versus 47A - pretty good. 9,500rpm versus 10,500 - I'd be more inclined to trust your figure there as mine is too close to the full kV value I think. The figure here also depends on the voltage sag - which you are predicting to be 21.05. See comments on voltage sag in the next bit. Your electrical power figure is significantly lower than mine - 966 versus 1200. But its very hard to be definitive here. It really depends on the voltage sag we will get from the battery under full load. I would guess my figure might be more accurate when the battery is fresh but yours might be more typical at "mid run" sort of thing. Either way its plenty of power for the Chippie! All these calcs of course can never really be more than a guide. I'm aiming to get a wattmeter on it soon - probably back end of the week hopefully - then we'll have some real data to compare! Even then it will be ground based data not airborne - so still has an element of an approximation about it! BEB Quote Link to comment Share on other sites More sharing options...
Tony F Posted May 12, 2014 Share Posted May 12, 2014 Thanks BEB...looking forward to the Watt results to see how it compares with eCalc....By the by...eCalc says over 10,000rpm is outside the limits of that particular motor....not sure if thats the case ? Tony Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted May 12, 2014 Author Share Posted May 12, 2014 I'm getting my predicted figures mainly from OS's data sheets so if that's the case they don't know it! I doubt in practice that we will exceed 10k rpm but we'll certainly be close! BEB Quote Link to comment Share on other sites More sharing options...
John Timmis Posted May 14, 2014 Share Posted May 14, 2014 Hi re your battery fitting. The way I did it on my D/F Vampire was to insert the battery into the end of its holder with a steel rule between the two pieces of velcro. Once its in withdraw the rule. Re inserting the rule easily separates the velcro for removal of the battery. Can post a photo if it helps. Cheers John Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted May 14, 2014 Author Share Posted May 14, 2014 Now that's an interesting idea! I wonder if I can get a rule in there? I don't have pure horizontal access - only access at an angle, so it will depend if the angle is shallow enough. But I'll have a go. Thanks John BEB Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted May 16, 2014 Author Share Posted May 16, 2014 Well we have the answer to the $64,000 question - I put the wattmeter on her tonight. These OS motors may be pricey - but boy do they perform! APC 14x7E: 56A and 1280W!! Staggering! BEB Quote Link to comment Share on other sites More sharing options...
Tony F Posted May 16, 2014 Share Posted May 16, 2014 Holy (hope not) Smoke !!!! Thats Impressive BEB....You know , that doesn't give me great confidence in eCalc numbers.....RPM ? Tony Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted May 16, 2014 Author Share Posted May 16, 2014 I was too gobsmacked to record the rpm Tony! Besides which I had my daughter holding it whilst I ran it up - and one can trespass on the time of 18 year old young ladies only so far I find. Add in the fact that she was most unimpressed by the noise and the howling gale she was standing in and I didn't think producing the rev counter would have gone down very well! Re smoke - holy or otherwise - well fingers crossed as always of course, but the motor is rated at over 70amps and the ESC is a 70 job so it should be OK! BEB Quote Link to comment Share on other sites More sharing options...
Tony F Posted May 16, 2014 Share Posted May 16, 2014 Yeah you should be good to go...unless you have a spare 100a esc handy twenty notes on HK...lol Looking forward to the first flight BEB..!!! Tony Quote Link to comment Share on other sites More sharing options...
Peter Jenkins Posted May 16, 2014 Share Posted May 16, 2014 Bet you never got 1.7 HP out of your ic engine BEB. Why do you think so many folks are flying electric F3A aircraft? You get over 3 Kw out of most electric motors in the 2 mtr airframes. That's just on 4 HP pulling an 11 lb airframe around the sky! Enjoy your new ballistic Chippie! Think of those huge loops you can pull! OK - not very scale like I grant you! Quote Link to comment Share on other sites More sharing options...
Concorde Speedbird Posted May 16, 2014 Share Posted May 16, 2014 Posted by Peter Jenkins on 16/05/2014 23:07:33: Bet you never got 1.7 HP out of your ic engine BEB. Why do you think so many folks are flying electric F3A aircraft? You get over 3 Kw out of most electric motors in the 2 mtr airframes. That's just on 4 HP pulling an 11 lb airframe around the sky! Enjoy your new ballistic Chippie! Think of those huge loops you can pull! OK - not very scale like I grant you! OS 91 four stroke output: 1.6 BHP. So about the same, more than enough for large loops and climbs. CS Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted May 17, 2014 Author Share Posted May 17, 2014 Interesting point CSB. So - if all the figures are correct - I've got a 7% increase in power for a 4% increase in weight. Hardly any difference at all. Also, with that much power and only drawing around 10C on full throttle, scale-like flight times of 10mins plus should be no problem. A practical demonstration that there really seems to be very little to pick between the two power sources performance/endurance wise. BEB Edited By Biggles' Elder Brother - Moderator on 17/05/2014 00:37:14 Quote Link to comment Share on other sites More sharing options...
Peter Jenkins Posted May 17, 2014 Share Posted May 17, 2014 I have an OS 91 FS so I was surprised by CSBs comment. When I looked up the OS power figures they state "1.6 bhp at 11,000 rpm". Now, I don't know about BEB's engine prop setting but my OS is in a scale biplane and the prop is a 14x6 and it pulls around 8,500 rpm on the ground. Allow a further 500 in the air and we get 9,000 rpm. That's a long way short of the 11,000 that the 1.6 bhp refers to. My Irvine 53 is quoted at a cracking bhp but at 17,000 rpm whereas it normally turns a 12x6 at 10,500 on the ground. So, beware of comparing bhp figures quoted by engine manufacturers with actual power figures from an electric set up. BEB, what's the KV quoted for your OS motor? Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted May 17, 2014 Author Share Posted May 17, 2014 Its 490kV Peter. BEB Quote Link to comment Share on other sites More sharing options...
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