Morphocoupe Mk II and the brownout blues.

Moving-on from my last article, Morphin' Monocoupe, I got the plane back in the air, and it flew pretty well, but then I had a brownout and had to watch it do a good hard nose-hit, followed by a strangely mesmerising series of wingtip cartwheels. Deep breath – think calm thoughts – start the walk of shame.

Anyway, the tail got wrecked, but everything else survived.

Back to the batcave!

This time I replaced the whole tail section - grafting a new section onto the existing fuselage. Now all that’s left of the original monocoupe skin is the arrow-shaped bit at the top here – great stuff foamboard.

This version has larger vertical and horizontal stabilisers.

The previous tailfeathers were a scale version of the 1927 Monocoupe’s, which looked good but meant they didn’t have much authority.

I also added tail steering.

Now, before we move on to the next test flight, an aside about ‘brownouts’.

I’m using a second-hand Spektrum DX6i transmitter. It flies most of my models without problems – but for some reason I’ve had a lot of problems with the Monocoupe. I’ve done a fair bit of investigating and here’s a list of possible causes;

Too many servos for the BEC (battery eliminator circuit). BECs are an integral part of the ESCs I use. BECs drop voltage from the 11.5V supplied by the battery to the ESC, down to an output level (5-ish volts) that the receiver and servos can handle. It does this by wasting energy and producing heat! Apparently, if I got this right, the more components it’s servicing – the more energy it has to waste and the more heat it produces. If it produces too much heat the BEC circuit can fry, or, more often, the ESC senses the overheat, protects itself and shuts down (cutting power to the receiver). Another down side is that this disproportianate power use can create a high demand when all the servos are in use (+ high throttle) and drop the effective voltage - possibly causing the reciever to drop below it's operating voltage!

Or – it’s a faulty ESC.

Or – it’s a faulty receiver.

Or – it’s a faulty motor.

Or – apparently some receivers don’t like signal reversers on the servo leads – who would have thought it!

Or – it’s a timing problem with the ESC/motor combination.

Or – it’s a dry solder join, or a dodgy connector somewhere.

There’s probably a few more items that could go on that list, but before putting my Morphocoupe MK II back in the air I thought I had eliminated, overcome, tested, investigated and replaced etc. all these possible causes. And yes, it flew OK (in fact very nicely), for a while, but then it did it again. Brownout and nosedive!

Luckily, this time, it was mostly damaged pride, a bit more nose deformation and a broken prop.

Frustrated, I went back to the internet.

Further research took me to another possible cause, and I discovered this quite by chance, the transmitter.

There’s a service recall on a production series of my model and a few others – an issue with backward compatibility with some DSM2 receivers. (Guess what I use…) Described as occurring in “some rare instances” it produces exactly the kind of problem I’ve been experiencing. If you use DSMX receivers you’d never know there was a problem and the unit will work fine. This is not a new issue, but neither is my transmitter.

I checked my factory code and yes my transmitter was on the recall list. I filled-in the on line form and immediately Horizon Hobby arranged for a courier to uplift the transmitter at their expense. These guys have the right attitude! They know that bad press and rumours of unreliability can kill a company’s reputation. Instead of sticking their heads in the sand they’re meeting this problem head-on and sorting it out.

Apparently the unit tested OK but …"for your peace of mind"… they replaced the RF board at no charge to me, and again the unit was shipped back to me at no charge. That’s service!

Here’s a link to the UK page;

http://www.horizonhobby.co.uk/aeroonline/knews/karticles/k48_spektrum_update/k48_dsmx_bulletin.html

There’s bound to be other national pages for those outside the UK.

For many people this will be old news, but for us newbies it’s not. I imagine there are still a fair number of older Spektrum transmitters in circulation that have not been serviced. Some will be working fine with DSMX receivers, some will be paired with DSM2 receivers that don’t have the problem, some will be sitting on a shelf branded as faulty or unreliable. Some will inevitably get sold-on after years of reliable service, but then get paired with the ‘wrong’ receiver and start acting up. If you’ve been getting unexpected and puzzling brownouts with a Spektrum transmitter, it’s worth checking this as a possible cause - at the very least you can eliminate it from the list.

Well, back to the Morphocoupe. Keen to get the plane back in the air I decided to use my original Hobby King HK6S transmitter while the DX6i was off getting looked at. I’ve never had signal problems with the HK6S and its matching receivers, but there’s no dual rates and the only control over the throws is by adjusting the mechanical linkages. After setting it all up I went for it.

It was a bit of a tense experience - amazing what a few crashes with a particular plane can do to your confidence. The good news, no brownouts – the not SO bad news, the aileron and rudder were a bit fierce for me, but the elevator was fine. A few slow, nervous circuits for the usual trimming-out and I finally started to relax a little. However the plane, which had flown reasonably happily ‘bank-and-yank’ as the monocoupe, is now so stable it needs a little rudder input to help it into quicker  turns - so that was a new dimension. I flew the plane until the low battery alarm sounded and then managed a reasonably controlled landing - ending in the inevitable last second nose-over in the grass. Catch the video at the end of the article :)

My most recent mod is to change the wheels - bigger and fatter. Maybe I can stop the landing nose-overs?

UPDATE - In the most recent incarnation I installed a UBEC, which uses a much more benign and energy saving method to drop the voltage down for the reciever and servos. For multiple servo situations, this works better than the built-in BEC in most speed controllers, which waste energy in the form of heat to reduce the voltage. I notice that some speed controllers are being made with SBEC's built-in. Likely this will be an ongoing trend. Above I use the term UBEC, but then I mention an SBEC. The 'S' stands for 'switching' - the cicuitry grabs pulses of power from the battery, avoiding the need to 'waste' energy. My initial understanding was that UBEC's (U standing for ultimate) and SBEC's, both use switching to provide power, but looking at loads of adverts for ESC's I see the term UBEC used quite loosely for non-switching (linear and power wasting) BEC functions. All the external, stand alone, UBEC's I've seen DO use switching. But only ESC's with an on-board 'SBEC' definitely use power saving switching. This is confusing. Loads of times I've looked at an ESC that says it has a UBEC and thought - great, no need for an external 'UBEC', but when I check the small print, it's just a plain old 'linear' BEC that's going to heat the inside of my plane. Though, and this is the really frustrating bit, some of them with the title UBEC are actually using a switching BEC (SBEC). Please, can we get some standardised descriptions! On any plane that uses more than 2 servos I now fit either an external UBEC or use an ESC with a built-in SBEC - those are the ones with the little ferrite doughnut on the signal wire. This gives me two main benefits; at times of high servo demand (with a marginally depleted battery) I don't drop the voltage below the cut-off value for the reciever, secondly, I'm not constantly wasting battery power just warming the air!