An Electric Blejzyk Mefisto

[Devon Slopes]

The back story. I built a Blejzyk Mefisto a couple years ago as a holiday model. It's a 1.9m glider with a four servo wing for which I have both slope and electric noses, the idea being that it could be flown in a wide range of circumstances. As a model for that holiday it was a failure because I didn’t get it completed until just before I left! The lesson is that if you want a holiday model, get it completed well before the holiday. You will be flying it in unfamiliar conditions and so its good to have a model you are comfortable with.

On the other hand in the time since it has become a firm favourite. Its light (under 900 grams in electric mode), which means I’m happy to fly it in a large area of nearby parkland. Coming home from work in the summer and spending an hour thermal hunting is good relaxation. Its equally nice on the slope, when its even lighter, but also has thin wings which means it can hold its own in strong winds.

So, why is it back in the workshop? Well, I never managed to get the proper electric nose, so made my own by hacking a spare slope nose. The design I came up with was not very good, and so the electric version is a bit of a pest to operate. For example, to change from electric to slope mode you have to take off the prop, undo the screws which hold the motor in, slide off the nosecone, disconnect the ESC and other electronics… the list just goes on and on. Also, the battery is changed by taking the wings off. Oh, and its underpowered. All-in-all not one of my best design moments. Though see the attached pictures of the fuselage in the two modes.

So, my argument went, I fly this model often, but my design failures make it time consuming and difficult to operate, so surely upgrading it would be a good use of time and money. The problem is that the UK suppliers for Blejzyk have gone through various changes, which is one of the reasons I’ve never managed to source an electric nose. But there is now a new UK supplier on the scene called MyHobby who had an electric fuselage in stock, and so I snapped it up. You will instantly spot the creeping cost envelope of this project, electric nose has become electric fuselage.

Anyway, before I start detailing the assembly over the next few weeks, I should mention there are a couple of other Mefisto builds on the forum, one from FWAL and another from Phil May. The only electric build I can find is on rcgroups.

[Devon Slopes]

So, this is the collection of new bits that I have either made or bought for the electric fuselage. Not shown are the things I have from the old version, an ESC, spinner, prop (though I’ll probably need a different one), and receiver (I use FrSky gear).

Captive bolts. Above the fuselage are the structures which hold the captive nuts for the wings and tailplane. I found that the nuts originally supplied were very tight, so on the old version I simply glued metal nuts onto the wood, countersinking them slightly into the wood for the wing bolts, and putting a small piece of wood around the ones for the tailplane, in the hope of holding them in. Problem is this did not work well, and I have occasionally had the nuts come off. Not wanting this to happen on the new fuselage, I have used proper captive nuts for the wings, though they had to be cut down a little for the rear wing bolts.

The snakes are by Kavan, their 0016 snake outer, which takes 0.8mm piano wire as an inner. I was really pleased to find these, as the snakes that come with the Mefisto also have 0.8mm piano wire as the inner, which pokes into holes in the tailplane horns. So, I can simply move the tailplane between the two models.

The servos are a pair of Corona DS843MGs, which are a close match for the Hyperion DS09AMDs I originally used. The latter seem to be becoming difficult to source, and the Coronas are considerably cheaper. Lets wait and see if they are robust.

The motor is the thing which has given me the most pause for thought. As I said in the previous post one of the problems with the old version was a lack of power. I got about 90W out of the Overlander T2826/18 from a 3S battery, so my search was for something more powerful, preferably with a similar low kV (1000) as the Overlander, as (I’m told) slow props are more efficient. But, the problem is that the new fuselage is clearly designed for a 28mm diameter inrunner. Inrunners tend to have fewer magnets and so are faster. In the end the best compromise I could find was the 200W 2100kV Overlander BM400. Should have enough power, though a somewhat less efficient prop, but the potential problem is the weight. The T2826/18 was 50g, the BM400 is 114g, so that’s an extra 64 grams in the nose. How will I make this thing balance? The only way to find out is to start assembling it, and see where the Cof G comes out, and then move things around.

[Devon Slopes]

Here you can see the electronics laid out alongside the fuselage in roughly their intended positions. Starting at the front is the prop and motor, then come the servos placed in the tray I’ve made for them, and the battery which will go behind them. Next is the ESC, hidden beneath which is the battery voltage sensor. This is followed by the mass of wiring for the multiplex connectors which go to the wing. Finally comes the receiver, and variometer. The two small black rectangular bits, one amongst the wiring and one beyond the variometer are the aerials. The whole layout is about getting as much weight aft as possible, though I believe I am still going to end up with tail weights.

An unexpected time sink was making up the cables. Its the first time I've crimped up lots of cables for a model, but I think it has really paid off in terms neatness and compactness because all the cables are the right length, no excess lengths being pushed into spare space.

[Devon Slopes]

(Does anyone know how to rotate a picture?)

And here is the completed model. You can see the hatch, secured in the traditional way using wire epoxied to the inside. The nose area looks empty, as all the gear is as far back as possible, and the battery slides over the servos to sit well behind them under the leading edge of the wing. I’m fairly sure it is nose heavy, but have not yet started seeing how I can improve the performance by adding weight to the tail.

The all-up weight is about 830g and will probably go to 850g when its trimmed. That’s a bit of a disappointment, as the old version was 750g. On the other hand its really nice to operate, not having to take the wings off to change the battery (and worry about flexing solder joints on plugs so they break). Furthermore, its easy to change between electric and pure glider, just take the wings and tail off one fuselage, and screw them onto another. The old method of changing the nosecone was difficult, and stressful on a Saturday morning when I’d decided to go flying, but then had to spend 20 minutes changing the Mefisto into the appropriate mode.

I’m still fiddling to obtain the right prop, but more about that in a later post. But it has answered the problem about power. I well remember the first time Slopes Junior saw the old version of this model working. I opened the throttle and he said “Is that it?” in a derogatory reference to the power output. Those days are gone.

[Devon Slopes]

Props.

The biggest issue at the moment is the prop. My local model shop didn’t have the recommended prop, a 6”x4”, so I ended up with a 7”x4” Aeronaut prop. When I run with this on the ground the motor starts off by pulling 16A, but rapidly (seconds) fades to 12A. Of course the prop will be less highly loaded in flight, but I don’t have an ammeter on the telemetry (the FrSky one is too large and hefty). However I do monitor the voltage, and it drops pretty precipitately as you power up. In fact it gets too close for comfort to pulling the battery below 3V per cell. Once you’ve been flying for a bit, the drop is not as severe, so my conclusion is that it’s the 3S 450mAhr battery that’s the limiting factor, and it performed better when a little warm. That was easily tested on the ground by connecting a 2200mAhr battery up instead. Then, as one of my friends would describe it, we were cooking with gas – 18 or 19A draw, no obvious fading with time. Oh, and it pulled like a horse.

So, what to do as there is no way that huge battery is fitting in the Mefisto fuselage? The battery is marked 30C continuous, 50C burst, which translates to 13.5A and 22.5A. I’m not sure how long a “burst” is meant to be (does anyone out there know), but I’d hoped it was a typical e-glider motor run (maybe 10 seconds), but I guess this is just the well-known lesson of not pushing batteries to their nominal spec.

So, I bought some cheap props to try different sizes, maybe I could limit the current draw by having a smaller or finer pitched prop. So far I’ve only done 6”x4” (the recommended size remember), but that was even worse. It started at 16A and faded rapidly to 10A. When I tried flying it the pull was so poor that I was worried about clearing some low bushes as I took off! That to me is a victory for the efficiency of the Aeronaut prop, its bigger than the cheapo prop, has a similar current draw, but more pull.

So, for the moment I’m sticking with the Aeronaut prop and limiting the throttle to about 75%. Now, I recall reading somewhere that ESCs are most efficient at that level, is that right? The summer may improve things – I know my old the old version suffered from large voltage drop at winter temperatures.

[David Hall 9]

Nice to read of your experiences with the Mefisto. I gather that this design has been around for a while.

I bought one to fill in the gap between my 2M thermal ship and my foam Phoenix. I had hopes that it might serve as a back-up for my thermal e-glider in F5J comps.

I made the wing as a one piece part, making assembly at the field a little easier, but had to cut out the flaps so that they could be remounted in order to allow for 70-80 degree deployment as brakes. Works very well.

Mine came out at 760g w/o battery. My 850mAh 3s battery tipped off the balance as too nose heavy. I had to invest 3 one pence coins on the tail to get to the CoG that I liked. As wanted it to act as a F5J backup model, I only need to get to around 175m in 30 seconds, so a 7m/s climb is about right. The Propdrive 2826 1000Kv outrunner motor and 10x6 prop gives the performance I need, drawing around 150w initially, falling off quickly to nearer 120w static (it's a tight fit!).

I have bought a few 450mAh 3s 65C Nanotech batteries that might help out (they're tiny). At 40g, this loses 40g of battery and two pence in the tail. The power output falls off more quickly and only two or three 30 second power climbs will be possible on each charge....I'm awaiting better weather to try it out on this battery. For sport flying, it's back in with the 850 cells plus two pence.

So far (in its heavier config) it flies and thermals well, although it has to be "flown" all the time, it being less inclined to self stabilise. Looking forward to more opportunity to get out with it...

 

 

 

Edited By David Hall 9 on 04/03/2019 17:01:07

[David Hall 9]

Oops.... I should have written that it comes out at 760g inc lighter battery..

The motor was a "challenge" to mount. It's tight and it's easy to imagine the motor wires being rubbed bare by the rotor. In an effort to avoid this, I wrapped the motor in a layer of thin foam and then in a plastic bag. The wires were left outside of the wrapping. When the motor was installed in the nose (before the nose was epoxied to the fus), I gave the wires a good coat of UHU POR. After it was set, I removed the packing. I hoped that the packing would hold the wires against the fus side as the glue set, keeping them away from the rotor.... It seems to be working, time will tell...

Edited By David Hall 9 on 04/03/2019 18:39:42

[David Hall 9]

I'm interested in how you arrange the contents of the fus in order to balance it,,

Edited By David Hall 9 on 05/03/2019 09:16:34

[Devon Slopes]

Hi David,

750g including a 40g battery, that's very impressive, much better than my 850g, which also has only 40g of battery. How did you do it? Most of it will be the motor, my motor is nearly 70g heavier than the 2826, which sounds similar to the one I used to have, but I only managed to get 90W out of compared with your 150W. One of the reasons I wanted rid of the out runner was the tight fit, problems which you seem to have solved. Did you use the Blejzyk supplied electric nose, or make your own in some way. I'd be surprised if you could get the 2826 in the standard electric nose, which may be a warning for others thinking of this conversion. Do you have a picture?

And to answer your question, if you peer closely at the picture 30/11/2018 I laid out the contents of the fuselage in the order I put them in. Things were driven by my need to change the battery through using the cover hatch, and of course that you want to put the battery as far back as you can. This means it has to slide backwards from the hatch, and clear the servos, which dictates how far back you can put the servos. So the battery ends up under the leading edge. The ESC sits almost directly under the hole in the fuselage the wings fit on top of, and then behind the wing are the receiver and finally (pushed far down the fuselage) is a barometric altitude sensor. And after all that, its still nose heavy!

[David Hall 9]

My 2826 1000Kv motor is rather heavy at ~60g. It is my "standard" motor for 2m gliders on 3s. If they made a 2822 motor, I might try one to get lighter. I did mention that it is a tight fit, but it's not just the motor, it is a matter of routing the motor wires. I used the "electric" nose and mounted the motor before I glued the nose on. I'm not sure if the motor will come out without a struggle!. Before I tried it, I was ready to exit the wires at the very front of the nose and re-enter them around an inch back, on the top.

The 20A ESc and the Rx are just standard jobs, there is an image in my last post, you have to click on the icon.... not having much success in posting my images here.

My concern for a layout like yours is that you are likely to need lots of weight in the tail... mine is only a little heavier than normal (they recommend a 44g motor) and it has ended up with around 20g in the tail (with the 850 battery). Mind you, it flies very well....

Edited By David Hall 9 on 05/03/2019 20:49:12

[David Hall 9]

I was suggesting that a bit might be cut off the nose, back to where it's wider, but it's not round then....  may look odd.     

I had a rather similar problem on my Hobbyking Sigma, a 2M thermal esoarer with a tight fus.      In the end I mounted  a new bulkhead much further back in the nose and fitted a rear mount motor so that the spinner butted onto the fus, as it would normally.    That works very well as all the space in the fus is useable but, it turned out that that is even more nose heavy...  it had 7 pence in the tail!     

 

Edited By David Hall 9 on 05/03/2019 20:46:57

[PatMc]

To use an outrunner in a really tight nose you could drill 2 holes (about 5mm - 6mm) in the fus. The first just level or slightly behind where the wires emerge from the motor, the second just behind the motor. Thread the wires out & back into the fus to the ESC. Lay the wires neatly along between the holes then make & fit a small blister cover to streamline & tidy things up.

[Devon Slopes]

My layout IS still nose heavy, David, despite my best endeavours. I'm still trimming but expect I will end up with 20 grams in the tail. The previous electric version I built was done by cutting the front off a slope nose, and that was indeed not circular, which as you say looks slightly peculiar, but only slightly so. Given PatMc's suggestion of routing the wires outside the fuselage, the clever way for anyone else thinking of doing this may well be to shorten the nose as David suggests, and route the wires outside the fuselage if needed.

[Devon Slopes]

New Batteries. I was happy with my revised electric Mefisto, it climbed well, and allowed me 10-15 minute flights in a large nearby open area. But I suspect I was abusing the batteries, as they began to fade - clearly taking 40C out of them even for short periods was not good in the long term, even if they were rated 50C burst. So I invested in some Overlander 450mAhr 50C continuous batteries. A further transformation in power ensued, and I now think I have a power system which works, though arguably pushes various things to the limit.

I was just starting to enjoy this enormously last autumn, when during one session the Mefisto was coming in nicely, though slightly long. It glided past me and as I looked round to follow it, I saw a small tree that had moved whilst my back was turned. The wing slammed into the trunk, badly damaging it. Artur Blejzyk provided a new pair of wings, but it took me perhaps a month to get it flying again. All very frustrating, until it was fixed.

And what's the lesson, other than not to turn your back on those untrustworthy trees? Its that when you are pushing things to the limit, i.e. using small LiPos at high C, you can get good performance increases by using higher C batteries.

Edited By Devon Slopes on 19/03/2020 22:31:17

Edited By Devon Slopes on 19/03/2020 22:32:02