BB_DF

DF SAVS Fans,
My DF started to act oddly this afternoon. Even with a fresh battery, it would take about 80% throttle to hover, and was responding in strange ways. I brought it down close to the ground, thinking maybe I had put in an uncharged battery, when it suddenly dove into the grass. I put a fresh battery in it, but it didn't want to lift off, and took a lot of back stick to stay level. Then the front motor quit completely (which explained the previous nose-dive). I've been keeping track of the time on the motors, since they're supposed to be replaced at about 80 hrs. Mine have about 8 hours on them.

I was glumly looking at the $75 HD motor set on the DFI web site (acceptable after 80 hours, but not 8 hours), when I thought I'd better make sure it was the motor and not the controller. I held the DF firmly in my left hand, set the Tx on about 1/3 throttle, and holding the DVM probes like a pair of chopsticks in my right hand, I started probing the motor voltages at their base plates. Most were around 5V, but the nose motor (which was turning intermittently) read about 6.5V. While I was probing around the brush springs of the bad motor to verify the reading, suddenly it sprang to life and started running strongly. I followed with a test flight, and it now seems to be running better than ever.

So there's an important lesson here if you think one of your HD motors is failing. They've got a really different setup with the springs and braided wire on the bottom. You're supposed to be able to change the brushes on them. I noticed on a couple of the motors you could clearly see arcing going on, and on the others you couldn't. They're new to DFI, so there's probably not a lot of history to fall back on there. When my DF had less than an hour on the motors, it started popping fuses each time I tried to lift off. I noticed that 3 of the motors would start to turn, but one of them never did (not the same motor as this time). I worked the shaft back and forth to see if it was binding, but couldn't detect anything. However, sure enough the problem went away. I think we need to know a lot more about the care and feeding of these little black beauties.

Ciao,
Bruce

Old Man Mike

Bruce,

If those small braided wires get mashed in a side landing, you will have just the sort of problems you describe. Since they are so fragile, I'm surprised that DFI did not come up with some sort of protection. The bottom of those motors take a direct hit in anything less than a perfect landing. That is why I started using the motor protectors discussed in this previous[link]http://www.rcuniverse.com/forum/m_5844050/tm.htm[/link] link). With the color tape and holes punched, the total weight of all 4 is about 0.5 oz. They also slide on and off easily.

Mike

BB_DF

I think I figured out what was going on with the motor. The first picture is a "healthy" situation, where the braided wires are free and flexible, allowing the spring to push the brush into its channel as it wears. The second picture is the motor that had the problem. As you can see, the braided wire is bent into a "U" shape, and would thus resist the weak spring from sliding the brush into the channel. I have now lifted the wires away from the casing to allow freer movement, like in picture 1. Notice also that there are two versions of brush plate - one silver and one gold. The braided wires on the silver are smaller and more flexible, and the ones on the gold are thicker and will be more apt to cause this kind of problem. My DF had two of each type on it, with the motor pairs reverse wired to run clockwise and counter-clockwise. I don't know if the color means anything, or whether they just had two different manufacturing batches.

Sky High

Oh, the colors do mean something. I have mentioned this several times in the forum. According to DFI, the gold bottom ones should be used on the front and back and the silver on the left and right. They can be interchanged and mixed and matched but will not function as efficiently. All of my HD motors' wires look like picture two. I had some issues with my new SAVS early on such as a power failure while using those motors and some unexplained behavior like you did. Then when performing repairs from one those incidents, I accidently wired one of them backwards and after connecting it correctly it has never spun up to flight speed since then. So I took them off and went back to the standard motors which I like better anyway. Were all of your HD motors' wires together or seperated out of the box? I will seperate those wires on mine and see if that makes a difference.

BB_DF

Sky High,
Thanks for the info on the gold and silver. They're optimized to run cw/ccw I guess. The brushes might be made of different materials for positive and negative. The DFI says they can't tell a huge amount of performance difference between the HD and regular motors, but there's a big difference in price. Do they both fit the same motor mount? Do you use heat sinks on the regular motors?

From the factory, both silver motors looked like the one in the picture, and both gold motors had the wires bent in a "U" shape, like the one in the picture. The motor that I see arcing a lot is probably getting ready to have the same kind of problem.

- Bruce

Old Man Mike

I believe the color of brush plate corresponds to the clockwise and counter-clockwise motors. I've been looking at these motors under a microscope to understand the contruction better. Are you sure that the braided wires on the silver are smaller than the gold? There is quite a variation in the tightness of the braided bundles. I really can't say for sure that one set of braids are any heavier than the other. One thing I can tell under the microscope is that after repairing several mashes from crashes, many of the small wires in the braid get broken and the remaining wires appear charred from carrying the extra current. I have replaced all four motors and now plan to always fly with the protectors to help prolong the life.

Mike

Sky High

Yes, they are interchangeable with the same mounts. I did use heat sinks with those motors but quit a while back because they just didn't seem necessary and it reduces weight, however minimal. You should order a set of the standard motors. You wouldn't believe the difference in noise reduction and how they can spin up and down real slow without jerking like the HD motors do and the connections are as simple as they could be.

Sky High

Yes, I think I recall something like that having to do with the internal windings of the HD motor's components for each set. The standard motors' windings are universal therefore can be place in an mount on the airframe. Lacey seems to know alot about that stuff.

BB_DF

Mike,
The motor rotation is controlled by the wiring polarity. The black wire is connected to the red dot side on two of the motors. As Sky said, they will spin in either direction, but are somehow optimized for rotational direction. It would make sense that they are wired differently.

The braided wires are definitely different on the two types. The wires on the gold endplate look thicker and are stiffer, less tightly braided and shiny, while the silver motor's wires are more tightly braided, appear thinner, much more flexible, and tarnished-looking. You can see it in the pictures. It could be that the thickness is due to the shiny wires not being braided as tightly. If they were, they would be more springy and less flexible, which may be why they are allowed to remain flatter.

Sky,
I'll definitely give those standard motors a try next time!

- Bruce

Old Man Mike

Here's a disassembled view of a motor that stopped working. As you can see, the cause was a broken brush.

Mike

Sky High

So if a brush is broken, will the motor still run but not as well or just not work at all?

Old Man Mike

It first started to vary in speed probably because of the intermittent contact at the break point. Then it blew the fuse probably because the broken piece wedged against the armature. I think it has been broken for some time since I've had problems maintaining trim in yaw and that cleared up after replacing the motors. Thank goodness it did not fail when I was high flying with the camera.

Mike

Sky High

The HD motors seem to be a little bit too fragile. The only time I've replaced standard motors is when they were damaged from a crash unrelated to the motors. Those things just go forever.

laceycopter

- Bruce
As you said rotation is controlled by wiring polarity to the brushes. But performance is optimized by timing. CW motors are set with the timing advanced, CCW with the timing retarded(which is really advanced because of rotation).
I went over it all once for Sky here: http://www.rcuniverse.com/forum/m_4693225/tm.htm
Post #14.
I'm not typin it all again....
Happy Flyin,
Lacey
PS...The HD motors from DF already have the timing set, that is why they are in sets of CW & CCW. the standard motors are the ones I time.

BB_DF

Lacey,
Thanks for that info. You're da bomb. BTW I know you fly the T-Rex 600 a lot and could use your help. I'm considering getting some experienced guys to build me a 600 specially configured to just lift stuff and hover for 5/10 minutes (longer weighted flybar, radio setup, etc). It would be an alternative platform to the SAVS for police work, surveillance, etc., though the DF will still remain our major focus. However, we need to address some familiar shortcomings: more lift weight, better in wind, more rain resistent, and perhaps most critical, a Spread Spectrum link to absolutely guarantee no hiccups. Sure wish we could get those features on the DF.

I haven't committed yet, and would like to know your opinion on the idea. The gasser would also be good, but we don't want the noise. How hard would it be to hold the 600 in a high hover without any self-leveling equipment, etc? Overall how would you compare the capabilities of the custom 600 to the SAVS for this kind of work? None of that fancy flyin' like you, though!

Cheers,
Bruce

laceycopter

The T-rex 600 is Awesome...and very stable.(the new 600 kits come with the longer 440mm flybars) It could carry a camera no problem.
But I get about 7-8 mins doing 3D and the batteries cost about $300 each.
I don't think you would get anymore time with the extra weight. And I would put a better ESC on it then the one that comes with it. And the GY 611 gyro with the S9256 servo.
Best Big Heli for AP (for the money) is the Predator Condor Gasser...can lift 15lbs. and flies for about 30mins.
you can see it here: http://www.centuryheli.com/products/...?currentid=335
But none are as easy to fly as the DF....It is closer to flying the DF with the TI off....Bigger paddles and weights will help...But it still will fly like a conventional heli, not like the DF.
And Yes...Spectrum is the way to go...everything I have (except the DF) is on it... no glitches.
Happy Flyin,
Lacey
P.S. If you decide to get one and need help with the TX setup PM me and I'll tell you how to set it up.

BB_DF

Lacey,
Thanks, as always.

Have you checked on eBay for T-Rex batteries? When something gets to be that popular, pretty soon it becomes a commodity. The Align 4200 mah batteries are going for around $100. I'm also going to try and find a set of semi-symmetrical blades, which produce more lift with less drag (if you don't fly inverted!).

As far as Spread Spectrum goes, I know the DX7 is a great radio in terms of quality and features, but it is not really spread spectrum. I worked on SS systems for a defense contractor, and the Futaba FASST is nearly identical to the systems that were developed for the military back in the early 80's. Spektrum is really just a glorified primary/redundant scheme that could be jammed in 2 seconds with a scanner and two transmitters. The Futaba uses true frequency hopping technology based on industrial RC applications where if the link fails there is threat to life and limb. So for us the Futaba provides better underlying security, even though Spektrum offers better features.

BTW - did you know that the frequency-hopping spread spectrum technology was invented and patented in 1941 by the film star Hedy Lamarr? Her husband was a big-shot defense contractor and she got the idea by listening to the boys grousing about military communications at a cocktail party!

Cheers,
Bruce

Old Man Mike

I hope they are using data interleaving over the frequency hops. Frequency hopping without interleaving is like spaghetti without the pasta sauce.

(you might have heard of SINCGARS. I started my engineering career designing the FH for that system.)

Mike

BB_DF

Yes, I know SINCGARS - small world. I worked in that field for a couple of years early on, too.

I would surely like to see a technical document on how they accomplish the synchronization and timing of FASST. Seems like things could go terribly wrong if they got out of synch. More like spaghetti in the fan at that point. I doubt any of these companies would open their kimonos, though. I have considered that perhaps Spektrum is better because it is simpler. Using one primary and one backup frequency decreases the probability of accidental interference tremendously, but it's still possible. Freq hopping has been around long enough now, so maybe Futaba has come up with a solid approach. FASST is a double-edged sword: more security - with a catastrophic failure mode. Also there's a major difference in philosophy on the receiver end. Spektrum basically uses two receivers for redundancy and polarity.

Well at least they've made a huge step in the right direction. Having a choice - that's a problem we should have!

Cheers,
Bruce

Nicoleto

One of my HD motors died several weeks ago and as I didn't have any spare motor I had only two choices. First was to rewind the bad motor and although I’ve done this before, it takes lots of time and patience. Second was to replace the bad winding “all the inner parts of the bad motor” with some good parts of the stock motors. It sounds crazy I know, but guess what, it works!!!!! I used the same timing the motors had when new. Of course I had to change two motors just to make sure two of them had the same performance (matched pairs). The motors I repaired where the ones with silver end cup.

You can see my DF flying with this modification in the following video “which I already posted in another thread”.

http://www.vimeo.com/clip:192036

Any comments?

BB_DF

Nicoleto,
I love the way you fly that thing. I can see why you are attracted to those German hot-rods!

Fixing your motors - necessity is the mother of invention.

Ciao,
Bruce