Sunday, April 15, 2007

Well It Flies . . . . . Kind Of

It was indeed a beautiful morning for flying. The Sky Fly, the CAP232, and of course the Mugi Evo bored numerous holes in the sky much to the childlike delight of their pilot. I can't wait until the Mugi Mobile is up and running so I'll have enough room to take the Cub along a little more often.

Dick even brought out his latest Mugi, rendered in stunning blue and yellow coroplast. Power comes from an Align 430L 3550kv brushless motor (the same motor that's in my helicopter) twisting an APC 4.1X4.1 prop. This power combo has been well proven on several local Mugis already, so no one was surprised that the thing honked along at a pretty good clip. The best part was the visibility. Turns out that blue and yellow are an extremely easy to see color combination, which is a good thing when you're piloting a 30 inch wide wedge across the sky at over 100 MPH. I've got a fair supply of colored coroplast myself now thanks to Dick, but I haven't got anything built out of it yet.

Keyword: Yet. Some time in the near future I intend on doing a full build post on a Mugi Evo, complete with pictures and a plethora of hints and tips that I've come up with over the course of building several of these things. Stay tuned.

Shown alongside Dick's newest Mugi, is of course my battered beast. I still haven't had anyone tell me otherwise, so I'm still claiming that it was the first Mugi in Billings, possibly even the first Mugi in the state of Montana. It's still powered by the well worn Multiplex Permax 400 that I originally installed on it, and it's amazingly still kicking in spite of the 3 cell lithium polymer battery that I power it with. This is a 6 volt brushed motor, being powered by a 11.1 volt battery. Brushed Speed 400's, especially 6 volt brushed Speed 400's, aren't supposed to run on 3 cell li-po's, but this one is still kicking ass although I expect it to make a really cool smoke trail any day now. The brushes are getting mighty worn. The new servos and linkages performed flawlessly by the way. Rock solid control, better than it's ever been, and no control surface flutter even at the highest speeds I was able to attain by doing some mad high altitude dives to near ground level. For the first time since I built it, it was actually capable of sustained inverted flight as well. I finally got to make a few relatively low altitude inverted passes without stuffing it in the dirt. A welcome change indeed.

Now for the skinny on the heli.

As I mentioned before, the first flights were mostly for tuning purposes. When you build an RC heli from a kit, actually putting it together is the easy part. After it's assembled, then the real fun begins. First thing is to set up the head. All of those nifty little linkages need to be synchronized to each other for everything to work properly, and all of the links need to move freely but without any slop. First you level the swashplate, then level each set of arms all the way up the head, finishing by adjusting the blades to neutral, or 0 degrees pitch in other words, when the throttle/collective stick is centered on a linear pitch curve.

After everything is centered and leveled and moving like it's supposed to, then it's time to program the radio.

The first step in this daunting task is to get all of the servos moving in the right direction. If you move the cyclic forward, you don't want your heli to pitch backwards, and if you move the rudder stick to the left, you don't want your heli to yaw to the right. There's several parameters that need to be adjusted in order to make that happen, and all of them take time and trial and more time and more trial. A modern computer controlled radio transmitter is a complicated little bit of technology, and while programming one is usually pretty straightforward after a little experience, the first few times you do it it's a real head scratcher from time to time. I've programmed several for airplanes, but this was my first venture into the world of rotorwing aircraft.

Not only does everything need to move in the right directions, it all needs to move the proper amount. Too little travel and you may not have enough control authority to keep the heli under control if a gust of wind catches it or you need to turn suddenly to avoid an obstacle. Too much and the heli might be too touchy and near impossible to hold steady, or worse, something could bottom out in its travel and hit something else or bind. This situation can result in a torched servo, or a bent or broken linkage, and likely a crashed heli. Obviously, none of these are a desirable situation so all of the throws need to be tried and tested prior to powering up the rotor.

Now it's time to learn about curves. No, not those kind of curves pervert, although I really like those kind too. I try not to think about them when I'm flying though, it's too distracting.

On a fixed wing aircraft, moving the left stick on the transmitter up and down controls only the throttle, and it does so in a completely linear fashion. Down is low throttle or idle on a glow or gas engine/motor off on an electric plane, and up is wide open, with an infinite number of points in between. Pretty much just like the gas pedal on your car. On a heli however, the left stick also controls the pitch of the main rotor blades, and the throttle is not linear since it needs to ramp up faster as rotor pitch is added in order to keep the rotor at a nearly constant speed. Too much throttle and not enough pitch and you can overspeed the head and send parts flying, then send even more parts flying when what's left of your heli hits terra firma. Too little throttle and too much pitch and you'll bog down the motor and lose your head speed making your heli fly about as good as a meteor. Although head speed can be recovered if you've got enough altitude and quit ham fisting the rotor pitch, do it at the wrong time and it's curtains for your helicopter. It's pretty much always the wrong time to lose head speed, so it's a good idea to do some testing and make sure that your pitch and throttle curves are compatible with each other before doing any serious flying.

Most people program at least two sets of curves into their radio, one called normal or hover mode, and another called stunt mode or idle up which is used for aerobatics. In hover mode, the throttle starts at 0 so that the rotor can be spooled up gently without spinning the heli on the ground, and then ramps up sharply at the point in the stick's travel where the heli starts to lift off the ground so that the motor will be spinning plenty fast enough when the pitch starts to really come on. A typical hover mode pitch curve starts out with a nearly flat rotor, I run 2 degrees of negative pitch at the bottom on mine, then it ramps up slowly to positive 10 degrees or so by the time full throttle is reached. Full pitch/throttle is rarely used in hover mode, but it's nice to know it's there in case a rapid getaway is needed.

In stunt mode, the pitch is absolutely linear. It starts out at negative 10 or 11 degrees, and gradually moves up to positive 10 or 11 degrees at the top of the stick's travel. The throttle curve in idle up is a little different though. Since 0 pitch is at the center of the stick's travel, and full pitch is at the ends in both directions, we run full power at low stick, full power at high stick, with a slight dip in the middle to 80 or 90 percent so that the throttle curve looks like a "V". That way no matter which side of the pitch scale the heli is on, it'll have at or near full power at all times for aerobatics, but won't overspeed the head when the pitch flattens out in the middle and the rotor unloads. The lower half of the stick is used for inverted flight, the upper half is used when the shiny side is up.

This brings us to the other reason for the rapid ramp up of the throttle in normal mode. When flying an RC helicopter, you start with everything stopped, then gradually add throttle to slowly spool up the rotor. At some point after lift off, the pilot can then switch to idle up, but never before the heli is airborne. Switch to idle up on the ground or at too low of a head speed and your heli will spin like a top, likely tipping over and sending chunks flying. For that reason, we ramp up the throttle pretty quickly in normal mode so that the point where the heli is hovering on the throttle stick in normal, will be about the same in both throttle and pitch as it will be in idle up. That way the heli isn't as likely to pivot or jump when the flight mode is switched.

Simple right? Actually it is, but it's still damn time consuming programming all of those curves into a radio and taking measurements and reprogramming all of those curves into the radio, only to take more measurements and start all over once again until it's right. OK fine, it really isn't all that easy, but for people like me that like to tinker with things, it's a blast.

Oh yeah, I forgot to mention the gyro.

As most of you probably already know, a helicopter has a tail rotor to counteract the torque of the main rotor. Without the tail rotor, the heli would spin like mad in the opposite direction of the main rotor as soon as it left the ground. Since the torque of the main rotor is constantly changing whenever the throttle or pitch changes, we use a device called a gyro to make the heli more controllable by automatically controlling the tail rotor pitch in order to hold the heli on a constant heading. Moving the rudder stick will override the gyro and cause the heli to pivot when desired, but leaving the stick centered will once again let the gyro take over and hold the tail steady.

Before gyros, the pilot had to manually counteract main rotor torque with the rudder stick just like the pilot of a full scale helicopter would do with the yaw pedals. Early RC helis were difficult at best, and downright impossible for all but the most skilled RC flyers to manage. Counteracting the tiniest nuances of torque and yaw while sitting in a pilot seat looking forward and flying right side up is one thing. Doing it while standing on the ground and attempting to perform aerobatics is another altogether.

Did I mention that gyros don't adjust themselves either? Neither do the linkages that connect the tail rotor itself to the servo that controls it, and all of them take their orders from the almighty gyro, which has plenty of little settings and adjustments of its own.

Today was the day to find out if all of those things were going to come together and do what they were supposed to.

They did . . . . kind of.

The first couple of times that I lifted the heli off of the ground, it immediately wanted to pivot left. Obviously, a gyro issue. I began making adjustments and all was getting better, but about the time I got things to start to really steady out, there was a loud pop and chunks of the tail rotor servo went sailing across the yard. Luckily the heli was close to the ground or things could've gotten really ugly. Turns out that contrary to the advice given to me by someone that I figured should know a lot more about helis than myself, a Hitec HS50 isn't enough servo for the tail of a T-Rex. Lesson learned.

After recharging the battery and replacing the tail servo with a larger one, still not the optimum but at least big enough to handle the task, I started over trying to set up the gyro and actually managed get the heli hovering about 5 feet off of the ground for several minutes.


The tail was still pretty lively, but at least I could hold it steady with a little rudder input. I'll have to get someone with a little more experience to help me iron out the finer details after I secure a proper tail servo lest I invest a whole lot of time and still never get it right. Someone that knows their way around gyros can likely get it set up in a jiffy. I could tweak around on it for months and likely still never get it right, quite likely damaging the heli seriously in the process.

Then it seems that my good luck ran out. Right about the time the battery started to die, I thought I heard another pop, only this time from the area of the motor/gear train but immediately afterward the speed control went into throttle cut and I was forced to land. I went inside, replaced the battery, and went outside to fly again only to discover the source of the pop that I thought I'd heard.

I started to spool up, all seemed to be well, but a few seconds after the heli lifted off I heard the pop again, followed by the sound of the motor winding up as if it had no load and the rotor blades were gradually losing speed. Just as the copter settled back to earth, there was a LOUD pop, the whole heli jumped to the side, and the rotor began to spool up again only with a loud clicking noise every time that it rotated. Turns out that the one way bearing must've started slipping, allowing the main gear to spin up with the motor since it was disconnected from the rotor shaft. When the one way finally grabbed and forcefully hooked everything back together, the rotor had lost a good bit of speed, the motor had gained a good bit of the same, and the whole thing went BANG, effectively shaving a good section of teeth off of my main gear. Sort of like sitting at a stoplight, revving your engine to the max, and then jamming your car into gear without ever pushing in the clutch. Ouch.

Seems that I need some more pieces before I can do any more whirly bird flying, like a new main gear and a new one way bearing.

So the short answer to "i am cheap" aka "Closet Flyer's" question left in the comments of my previous post is both yes, and no. Yes, the T-Rex flew, for the better part of a battery pack actually. Yes, it's broken. And no, I didn't crash it and it won't be hard to fix. I even did my first autorotation today, two of them actually, even if they were only from two feet off of the ground.

Sorry I didn't get the video that I wanted, but about the time I got stuff ironed out and got the heli off of the ground, the battery went dead in the camera. If it weren't for bad luck, I'd have no luck at all it seems. There wasn't much to see anyway, just a pathetic helicopter noob doing a not too terrible but not all that good job of hovering a helicopter and thanking all things kind and good in the world that he had the good sense to install training gear (sticks with ping pong balls on the ends attached to the landing skids to help keep the heli from tipping over and sending chunks flying - My wife calls them training balls and thinks it's really funny) before attempting the task.

Special thanks to "finless" Bob White over at for his outrageously thorough and well made, and absolutely free for the downloading instructional video series on the T-Rex 450. I'd never have made it this far without some serious help from my heli flying friends without all of the knowledge that I've gained from his videos. He has loads of instructional videos on the building, tuning, and flying of several other RC helis posted as well, and all can be found in the Helifreak forums.

As far as the vehicle situation? Do you actually think that I had time to fix the car with all that helicopter tinkering going on? It's OK, I took her out to dinner just to make sure that she won't mind driving the Ford for another week or accidentally set the refrigerator on top of my T-Rex.

As for the rest of my evening? I don't know, maybe I'll work on this.

It started out as a toy free flight plane, but it'll be a full blown micro scale RC kinda sorta Piper Cub when I'm done with it. It's about half way there now, with only a few more minor details to iron out and parts to install before flight testing. The control surfaces have been hinged, the motor and speed control installed and taxi tested, and check out my made from scratch ultra strong and uber lightweight custom carbon fiber landing gear. This one's only a three channel, with rudder, elevator, and throttle control, but if it works the next one's going to have full four channel control, or at least ailerons instead of rudder. Maybe I'll convert one of these for the ultimate in micro scale cool factor.

Projects like this give me something to do while I'm waiting for helicopter parts. Projects like this also serve to confirm everyone's strong suspicions that I'm certifiably bat shit crazy.

You wouldn't want me any other way.


Morgan said...

Hi, nice looking Mugis as usual. I'm in the doldrums this week as my new FlyCamOne arrived on the same day my Transmitter gave up the ghost! So now I'm looking for a Spektrum - probably DX6 (for cost reasons). Range on these reads pretty well - I see you have the big brother long range DX7.

Available from your local model shop?

Justin said...

Yep, my good friends at Abell Hobby hooked me up with the DX7. My DX7 actually replaced the DX6 that I've been flying with for about a year now, and although the 6 is an excellent radio, the 7 is worlds better for not a lot more cash. They're both a heck of a lot of radio for the money, especially when you consider the servos/accessories that are included with them.

I still have my DX6, my son will likely inherit it, but in the meantime it'll make a nice trainer box to use with my 7 to hopefully teach him how to fly. My daughter is likewise itching to take her turn on the sticks, and both have been practicing from time to time on the simulator. Now if I could just get my wife a little more interested, we could spend quality family time together flying combat with our Mugi Evos, LOL. She has however, been spending a fair bit of time helping me build the last batch of Evos that I'm putting together, so maybe after she builds one all by herself she'll get interested in actually flying it.

As far as the range is concerned, I've had no range issues whatsoever, even with my DX6. Although it's billed as a "Park Flyer Only" radio, I flew my big Piper Cub with it several times out almost to the limits of my vision with no glitches whatsoever. When I got my DX7, I went ahead and put the long range receiver in the Cub since I had it, but I doubt that I'd ever have had a problem even with the park flyer receiver. I think the only reason the range is restricted is mostly for liability reasons, although I'm sure that they must've ran into at least some kind of problems during testing to limit their sales options the way they did.

Perhaps when the moon is full, it's raining bananas, the tulips are blooming in Holland, and you fly your plane more than two miles away while holding your transmitter behind your back, you're likely to lose signal but in normal conditions I've never had a problem. Unless you've got far, far better eyesight than I do, the chances of ever flying a small plane like a Mugi Evo far enough away to lose signal are slim to none. I've never managed to do it anyway, and I've flown my Evos up to altitudes where they become mere specks.

The best thing about the 2.4 gig radios that I've found, is the fact that they're nearly impervious to model generated noise. I don't even have capacitors on the Speed 400 on my first Evo, and I've still never had a glitch. It's great not having to pay too much attention to where I route all my wires, and the zero chance of some moron turning on his transmitter without checking channels and shooting me down is great as well. Basically, I love both of my Spektrum radios, and I'd recommend them to anybody that asked. Hell, I'd even recommend them to anybody that DIDN'T ask, LOL.

I highly recommend the DX6, but if you can spare the few extra bones I'd pony up and get a 7. Besides having an extra channel (seldom used but still nice to have the option), there's loads more programming options, especially for helis, and the 20 model memory is great as well. The biggest difference I've found though is the feel. My DX6 felt far better than the el cheapo radios that I was used to, but the DX7 makes the DX6 feel like a toy.

The 7 is a little more like what I'd expect from a JR product, everything on it from the gimbals to the switches to the programming keys seem to be of much higher quality. Far smoother, absolutely zero slop, and the programming is far more intuitive with the larger LCD readout and more detailed programming screens. The whole "balance" of the thing just feels a whole lot better in my hands, the difference was just amazingly obvious the first time that I flew with it. Supposedly the resolution is far and away faster on the 7 than on the 6 as well, but I'm not nearly a good enough pilot to notice such minutia. My only complaint is the lack of a proportional 5th and possibly even 6th channel. There's no knobs or sliders on either of these radios, but at least the 7 has the option of a three position switch.

So what I'm basically saying is, if you get a DX6, you'll love it, but if you can manage to get ahold of a DX7, you'll love it even more. I don't know how I ever lived without these radios to be honest, and if a person flies anywhere but a sanctioned field, they're worth their weight in gold.

So I'm assuming that a FlyCamOne is an airborne camera system of some sort? Details, I must have details! Oh yeah, and aerial video from an Evo skimming the ground at breakneck speed . . . . . with a few rolls thrown in for good measure. ;)

morgan said...

Aw now I can't get on their website thanks to the firewall here at school - their site redirects to an usual port. Thanks for the extra info. I think it will have to be the DX6 for me. Am I right in thinking the '7 comes with larger servos and a heavier and what looks to be more clunky receiver?
The DX6 looks to be more tailored to lightweight and simplicity.

I 'could' stretch for the 7 but would hate to have to buy another Rx for it. Need a new Tx asap as I'm itching to try this cam.

AS seen on a mugi ilmed by

Justin said...

Yeah, the 7 does come with the larger, full range receiver and standard sized servos, but they're digital servos, which one does not normally get included with a radio system. The servos that come with the DX6 are regular micro servos, similar to a Hitec HS55, and they seem to be pretty good. I had them in my Evo for a little while until control surface flutter took them out, but had I not had so much slop in my linkages they'd likely still be going strong. They seem to center a lot better than some of the 55's I've had as well. I bought a spare pack of them on ebay shortly after I got my radio, so I still have 6 of them - planning on using them in a built up balsa plane in the future.

Just one more added bonus with the 7 and then I'll quit harping about it. The 7 can use the AR6100 receiver, which is quite a good bit smaller and lighter than the AR6000 that's used with the DX6. You can use either micro reciever, as well as the full sized one with the DX7, but the DX6 only works with the AR6000. Just something you might want to think about.

PS - I tried to get on Abell's site last night and it was down. It could've been that causing your problem and not the firewall. I'm sure Clay will have it up again shortly, I'll probably talk to him today so I'll make sure he knows about it.

Cricket said...

So...what's the update with the T-Rex?? Heard you were doing some tweaking and testing this pics or developments?

Chuck said...

I haven't been on the blogs for quite a while and I just had to stop over and say HI to my bud Justin!

Hope all is well with you & yours!!!