Swissflyer

Well here is a slightly different slant. As an electronics engineer I am very conscious that the movements of my thumbs & fingers are translated into electronic signals by potentiometers. Over time, and depending on the quality of the potentiometers, the tracks in those potentiometers wear and can create spurious noise that does not help flying at all (maybe you have heard a noisy volume control on a radio to understand the point?) Let’s just say that I trust more the quality of the Japanese potentiometers in my two Graupner/JR transmitters over the more modern production of a neighbouring nation. So when 2.4GHz came along I just replaced the 41MHz RF links with the “cleanest” 2.4GHz modules I could find. That let me keep those high quality Japanese potentiometers, all the settings in my two transmitters AND enjoy seven years of glitch free flying - wonderful I do have one model on 41MHz, it does suffer glitches where my 2.4GHz models notice nothing. So I can happily focus on improving my flying ...

Getting a buzz? Well, being an impoverished teenager back in February 1969, I did build a home brew two channel version of the RCM&E digital proportional radio system that was featured back then (see “The Way We Were February 1969 article). It worked and for a teenager that was truly heady stuff. Yes, it was only two channels as the “impoverished” budget would only stretch to two servos! Many of the electronic components were donated by the local electronic repair workshop where I made holiday money by repairing six transistor radios. Remember them? These days radio gear is so cheap that doing a DIY job would make no sense. I have used HiTech, Futaba and Graupner/JR, the two Graupner transmitters upgraded beautifully with drop in 2.4GHz modules. That saved a lot of re-programming. I have to say that I have not had a radio glitch since moving to 2.4Ghz. My next step is to test out my new, nicely engineered, Chinese transmitter that offers downlink telemetry and promises to buzz/vibrate in my hands when the flight batteries are flagging. Now who would have imagined that back in the ‘60s? Happy flying Mark

My original Sky Scooter had a Mabuchi 380 (Speed 400) motor driving a 9 x 6ish folder through a 3:1 (I think) gearbox. The useful point is that Multiplex used the same drive train for their Easy Glider and there is plenty advice on how to do brushless conversions on Easy Gliders (I have one of those too & did do a brushless conversion) If you can find a 2'000-2’400 kV brushless motor 20-28mm diameter with a 2,3mm shaft you can think about moving the pinion from the original motor onto the new one. That would be OK for a 3S configuration, a 3’600 kV motor should be OK on 2S. Although the Easy Glider did need more power, I have no plans to modify the Sky Scooter as it flew just fine with the original brushed motor. Good luck with the conversion.

I noticed that HK is installing software to try and protect themselves from DDOS attacks (as per the attack using video cameras etc that blocked access to major US Websites last week). I would draw a couple of conclusions from that: 1) HK is suffering from DDOS attacks. 2) Maybe the people instigating the DDOS attacks are trying to blackmail HK “We will stop flooding your servers if you pay us a lot of money every day”? For anyone who does not know it, please be aware that Web criminality is a huge, growing and very well organized global business. So if you are having issues with the HK Website, the chances are that HK have bigger ones!

Posted by Tony Smith 7 on 13/02/2016 13:45:27: I've done nothing with my models for quite some time, and today trying to get one of them airworthy again I find I have a dodgy LIPO. It's a bit puffed up, and squishier than the other one, and one cell reads almost flat compared to the others. At the moment, after an abortive flight resulting in loss of power, the cells read 4.08/4.08/2.92. Hi Tony, May I point out that the recommended storage voltage for LiPo’s is 3.80V? I discovered that some years ago after an expensive 3S LiPo pack had one cell exhibit high internal resistance after storage at 4.08V (yes it did that in flight). Since then I always store LiPos cool at 3.80V and my LiPo attrition rate has dropped dramatically. Pity to throw away your pack when it is 2/3 good! I do own 2 x 2S packs that started as 3S packs Good luck Mark PS Before I start flying again this year (too cold right now) I will be checking the internal resistance of my LiPos under moderate load. That way I could have avoided the failed flight.

Posted by Alan Gorham_ on 12/02/2016 15:16:43: Mark Well, a 60" very scale MC-72 would get my vote! I built a pseudo MC-72 a few years ago and it was a good flyer: Hello Alan, Glad you would vote for a MC-72. Also congratulations on yours, your photo captures exactly what I have in mind, a model that looks great to start with and can be a platform for as much detail as one wants to add. Yes, I have seen Tony’s S6B plan somewhere that shall not be named My post did have an element of competitive mystery behind it... Years ago Tony did a twin engine, airliner project for RCM&E and was happy that it was considered to fly better than a Twinstar. I think he even admitted in the article that this had been his objective. Now my favourite flying spectacle is slow speed aerobatics by a beautiful scale model. Since we all know that Tony can design beautiful scale models, maybe I was hoping to prod his competitive instinct into a design that would both be beautiful and at least equal Seb’s model in performance? Mark

When I wanted insurance here in Switzerland, the insurance company told me that I had to be a member of a registered model flying club. The club shares the airfield with full size, light aircraft and has a strict procedure. They asked me to put in six appearances and fly with the supervision of a senior club member, there was strictly no flying alone at the club site during that phase. Once that was done & they were sure that I knew the procedures for giving priority to full size aircraft, they gave me the forms to apply for insurance and club membership. So is this a “Win-Win” procedure? To my mind “Yes”. The insurance company knows that insurance is being given to “checked out” pilots. The club knows the capabilities of the new member well and the airfield knows that the club members know the local procedures. The new member meets experienced local pilots, they are nice guys who really help. It also means that most of the modelling community is in a club; however, the insurance is world wide so I can also fly with insurance cover in the field outside my front door. This also means that any “apparently irresponsible” flying by uninsured, unqualified people can be checked as the friendly policeman will check your insurance card to be sure that you have been checked out. And of course he can take away your model on the grounds that you have no insurance. So yes, there are rogues, but they don't seem to last long...

So how about a beautiful, 60 inch wingspan, racing seaplane? You can see one original plan here: **LINK** It is definitely for balsa bashers & shows fuselage construction as 3mm sheet balsa on 3mm ply formers, I would let RCM&E sell me those formers together with the wing ribs & main spar laser cut. The wing is parallel chord with symmetrical ribs dropped onto a slotted main spar so we are well away from tip stalling issues and the aerobatic performance may startle you, watch the video to check that out. If I am providing some translation, you might be guessing that we are talking about a foreign design. If I am talking about beauty, you can probably guess which nation (apart from the UK) made beautiful racing seaplanes? So why not? Surely Tony could breathe some British genius into his own version; a 40-48inch span would suit well from the power train budget but still be big enough to have real presence in the air. Or got to YouTube & search for watch?v=rvacC694QZo to see a great pilot flying his version of this aircraft. At minute 3:30 on that video, you can see how the model would work at your local grass patch… Hope you enjoy it Mark

Hi David, Thanks to Toni for his support and compliments to you on your tenacity. Of course there are no dumb questions, only dumb answers. So as to your question: “Is it that power is fully on for the cells chosen and the current is the result drawn for a particular prop?” The answer is yes, and that is a very useful conclusion as you can experiment with different props and voltages (2S, 3S, 4S LiPo packs etc) to find a good solution for your application. A very practical way for you to move forward would be to download and try DriveCalc (it is free of charge). Then you could look up the Hyperion motor we have been discussing, set the voltage to a “constant voltage” 14 Volts and play. **LINK** Hopefully you will then discover that the curves are always the same, however, different props will load the same motor in different ways. If you have questions, please ask. Mark

Hi David, Great, you are spot on with your understanding of the green efficiency curve & the red power curve (I guess that you noticed the scales are on the right hand side of the graph). On the left hand side of the graph you can see a blue scale called n (rpm) and that is what the blue line is. It says that with no prop, the motor will turn close to 12’500 rpm, the bigger the prop you put on the motor, the more the rpm will fall. In the case that Drive Calculator is showing you, an 11 x7 APC E is “slowing” the motor down to 10’120rpm. Put on a bigger prop and you will go further down the blue curve to the right (lower rpm). Do let me know if all this is clear to you? If you would like to, we could also look at how to avoid the “max power” trap that some unscrupulous vendors use to relieve the unwary of their hard earned cash? Mark

Posted by Erfolg on 26/01/2016 15:47:27: As for the DH Rapide a total waste of time, wings far to pointed, then there is the issue of rigging, if a thin aerofoil section were to be used. Beautiful to look at. Hey Erfolg, washout & low wing loading can manage tip stalls. **LINK** Once the current Stearman PT17 is finished, my Dumas Rapide is next. If its got two wings, let’s have two engines as well and put one on Tony’s list for 2017

Christian Persson, the main author of DriveCalc knows about these deficiencies and designed DriveCalc to “choose between several motor models for the computation, depending on what measurement data are available, to achieve the best possible accuracy”. I do confirm (from practical experience) that DriveCalc adapts its accuracy according to the quantity & quality of data you put in and its accuracy, with good data, suggests that it goes one or more steps beyond the 3 constant model. Better still, Christian Persson built tools into DrivCalc for estimating the motor (& prop) calculation reliability. If you look at the extended version of the Hyperion HS3026 data (below) you will notice Motor Calculation & Prop Data Reliability at the bottom right. The motor calculation reliability is derived from the “Measured Data” on the left. You can see that DriveCalc indicates -13.57% and +17.09% swings in the measured data and says expect only low to medium reliability from this data. Whoever measured the data for the Turnigy 540S did a much better job and is told to expect medium reliability, I guess that DriveCalc had enough good data to choose a better algorithm. I used exactly this motor to power my Ripmax Spitfire some years ago, with the weight gain from moving from NiCads to LiPos and the power gain of the brushless motor it flew like a dream. If you look carefully, DriveCalc says you can be quite confident on the motor results but not with the prop. DriveCalc relies on us (the modelling community) to supply that prop data and no one has said much about a Graupner 9x6 folder on a 45mm hub. I switched to some well known 10x6 props APC & GWS HD. Then DriveCalc told me to expect 22.6 Amps current draw at 11.25 Volts, my Wattmeter measured 22 Amps at 11.25Volts so I was comfortable with that. Christian Persson says if you find an error of more than 3% relative to the DriveCalc prediction, look carefully for the reasons. I have followed that advice over the years and found a 12 turn motor missing a winding i.e. 12:12:11 turns, LiPo battery packs with a rogue cell hidden in the middle, faulty ESCs and motors with Kv labelling that is just wrong etc. In summary, yes, there are traps and many variables with electric motors. However, well measured motors in the DriveCalc database let you check what you are seeing against a calculation reliability scale & home in on anything that looks odd. If it looks odd, it probably is and be sure that you understand the cause. Happy flying Mark

Posted by David Hall 9 on 27/01/2016 09:38:02: Thanks all for such detailed replies. There's a lot of technical info here for me to work on. The performance graph is really interesting. Hello David, Good to see that this thread is useful to you, thanks for the comment on the efficiency curve; yes those curves have helped me enormously. Ready for another step? Since this thread is about understanding electric motors, I think BEB’s point on “Why is my fag packet so good?” needs to be addressed. On the one hand an electric motor, especially a brushless one, looks like a simple piece of kit but why do so many calculators fall down and fag packets work? Please allow my to tell you a little about me and radio controlled models as part of the answer comes from there. Basically I grew up in the 50’s & 60’s with a wartime flight engineer and pilot for a father. He gave me his Mills 1.3 and later bought me a Mills 0.75, I scaled up (doubled) a Keil Kraft Playboy plan and spent my life savings to buy a McGregor single channel RC with a rubber sequential escapement. That became a “galloping ghost” and soon I learnt how to strip components off computer boards to make my own RC gear. Grown ups in those days could afford 10 channel reed outfits! At university, our professors instructed us in the 3 constant model for electric motors, that is where I met our good friends : Rm (Motor Winding Resistance) Io (Idle current assumed independent of voltage applied to the motor) Kv (The motor internally generated back emf when rotating) All was well under I returned to modelling at the end of the 90’s and jumped into the electric end of the spectrum. In those days the batteries were very heavy and the Speed 400/600 motors were not so efficient. I quickly found those old equations from university posted on RC Groups by Joachim Bergmeyer. All seemed well; they said increase the operating voltage, set ground rpm about 20% to the right of the peak of the efficiency curve (see my first post) and things will improve. They didn’t and although Io is supposed to be constant and independent of applied voltage it wasn’t and the promised efficiency gains didn’t come! So was it back to fag packets? Or did the 3 constant model need improving? Various experts dug into this and realized that the 3 constant model takes no account of the eddy current losses between the laminations (and other secondary effects). Worse, eddy current losses increase with the square of the current. In concrete terms, increase your operating current from 10 Amps to 20 Amps and your eddy current losses multiply by 4. So called 4 constant (and maybe more) mathematical models were developed that incorporated the eddy current effects and gave an excellent fit between theory and practice. Peak efficiency in a 4 constant model happens at a lower current (less eddy current losses) than in the 3 constant model. However, if you choose propellers to maximize the in-flight efficiency, you will be helping to minimize the eddy current loss effect. If you look at the efficiency curves you will see that a motor close to peak efficiency in flight will only draw about half the maximum recommended current so will be creating 4x less magnetic eddy losses (remember to square the electric currents). However, and here is where the fag packets win, most of the calculators (freeware or paid for) that are out there use the 3 constant model so beware if you trust them too much. continued in next post