Went up to the Palomar R/C Fliers Heli Fun Fly Friday and Saturday. As a park flyer, I flew at their heli field, which was reserved mostly for scale (just as well since I wouldn't have coexisted well with all the big nitro action at the main field.) Met a couple of scale pilots and was very impressed both by their machines and their slow, precise flying skill.

Can't say the same about my my own performance, but I pulled off some loops and only crashed once (an embarrassing tipover just after a nose-in landing which didn't damage anything.)

The Palomar club has a nice field and seemed like a good bunch. Unfortunately the drive is a little on the long side.

So I shipped the AR6100 back to Horizon to get the firmware updated. After they got it I received a cryptic email that said "REORD AR6110" which I interpreted as meaning they were going to send me a new AR6110, which would have been nice. After a week waiting for Fedex Ground, I got my original AR6100 back with no indication they had done anything. I called and they had no record of what they had done (it had the velcro I put on, didn't behave as though the firmware had been changed and was still bound), so after a couple of days they told me they were sending me a new AR6100. OK. I waited another week for Fedex Ground (I hope it's cheap because it sure isn't fast) and what they sent me was an AR6100E, which doesn't fit in my Trex without major wire rerouting. Sigh. I guess they tried but the company could be a bit better organized.

In the meantime I've been flying the heck out of my mSR and find myself wondering if they are working on a CP version, and if so, how flyable it would be. The mSR is remarkably crashproof but I doubt a CP could be made that way.

My primary Trex's receiver failed a range check last weekend, so I figured it was time to send it back to Horizon to get the quickconnect firmware update. I flew my backup Trex a little bit but noticed that one of the linkages to the flybar cage had loosened up again, so I pulled it apart to replace it and either broke the flybar cage or it was broken already (this heli has had trouble holding blade track for a while anyway.) So both 450s are down until I get some spare parts in the mail.

In the meantime I'll have to satisfy myself with some time on the Radian or the mSR. Naturally it was dead calm perfect heli weather all weekend.

I spent a little time thinking about buying a nitro heli, like a Raptor 30. (RC Heli magazine has been 75% nitro lately.) Finally came to my senses -- I'm not that interested in cleaning up nitro slime.

I haven't posted much on RCGroups for two reasons: first, I got tired of arguing with people about the best way to set up heading-hold gyros , and second, I got sucked into high-power rocketry.

But I continue to fly the Trex pretty regularly, though my aerobatic progress stalled somewhat after a couple of major crashes. For Christmas 2008 I got a Parkzone Vapor, which sat unused with a broken rudder after a few attempts to fly it in the house, but a couple of months ago my employer moved to a new office with a large unused conference room, so I got a chance to fly the Vapor a fair amount and ended up getting a Parkzone Radian as well. I also got a Blade mSR, which is really pretty impressive for indoor flying.

I've been flying loops with the Trex for about a month with mixed success (had a near-crash once flying through a bush when pulling out a bit too low) so they're still a little stressful. My collective management needs some work; when I try to reduce pitch going over the top I think I'm putting in some rudder accidentally, so I'm still flying mostly 9-shaped loops and losing too much altitude. But I tried some rolls to the right this morning and found them more confortable than the loops; the potential for losing altitude seems less, so as long as you start high enough, crashing doesn't seem too likely.

After over 300 flights with the Trex and many tens of stall turns, got up the nerve to try some loops. (I know, slow learner.) They went about as well as I could have hoped -- fairly smooth and I never felt like it was out of control or in danger of crashing. Of course I still had to land after the first one because my hands were shaking so bad.

The blade noise when I pulled out was just awesome! I've been flying so conservatively most of the time that I rarely hear the blades. Maybe that'll change.

I wish I had some photos; the heli was against a clear blue SoCal sky and the waning 3/4 moon. Very pretty!

The almost universal opinion is that HS-65s are the only way to go for cyclic. Now, the HS-65 is a fine servo (I've got them on my Trex) but they're fairly expensive and use a lot of current, forcing the use of a pretty hefty BEC if you want to be conservative. So I found myself wondering if all that torque is really needed.

There's a nice servo torque calculator at http://www.csd.net/~cgadd/eflight/calcs_servo.htm but it's only for fixed-wing. But just to get an idea I assumed that the rotor blades were just a big control surface travelling at the tip speed (which is not right since the root of the blade is going much more slowly than the tip, but it gives us some margin.) For typical Trex blades, 2*325 mm long with a chord of 32 mm, 2400 RPM so about 320 kph, and 10 degrees full pitch at 30 degrees servo deflection, that gives about 12 in-oz of torque, which presumably would be split across all three servos.

Compare that to the HS-65 max torque of 25 in-oz (31 in-oz at 6V) and it seems that the 65s are overkill for the actual loads.

Now this ignores friction in the system and any effects of the lever arms in the linkages (which probably only help) as well as the fairly small additional load of the flybar paddles, but it seems that a typical mini could get by with no more than maybe 10 in-oz of torque on its cyclic servos. This may explain why people were happy with HS-56s, and suggests that even people using HS-55s or other small servos are not crazy (though the HS-55's speed is poor, and that's another factor.)

If you run at 3000 RPM and +12 pitch, then the torque load goes up to 27 in-oz, so people flying in that range may really need HS-65s.

What I'm not sure of is what the transient load of a fast cyclic input is. That's probably when you want your servo to not stall.

I've now got 25 flights on the Trex. It's remarkable how much more controllable it is compared to my King 2. While I suspect the K2 could be improved with better setup (the tracking has never been quite right after my last crash), the Trex just feels a lot more solid in the air.

Now, if only I could fix the yaw shudder/twitch I've been getting to some extent on the last 10 or so flights. Static and gyro problems have been pretty much ruled out. Hopefully it's not the belt going: it seems fine.

After some cogitation, decided that I'd put a Dimension Engineering SportBEC on the Trex. I had an unused ParkBEC, but I figured that its 1.5A limit was a little too tight (after all, if I ever using a lot of servo current, it'll probably be in a situation where I don't need the additional complication of a Spektrum lockout ). The thing to note about a switching regulator is that it'll shut down based only on the current, not as part of thermal shutdown like a linear (see, for example, the National LM25575, rated for 1.5A and will limit at 1.8A). There have been a few reports that the ParkBEC can't handle HS-65 loads.

A CCBEC would have been a little cheaper and is rated for twice as much current (7A with <12V input, but they assume 5 mph airflow), but there have been a few complaints of QC problems with this converter, and I didn't need the hassle of needing a Castlelink to set the output voltage.

I put my Trex together for a budget of $300, not counting the Telebee gyro and AR6100 receiver that I had left over from a previous project. So I got a 450S kit with the Align lipo, motor, and 35X ESC with 2A 6V BEC. I figured that the 6V gave me some margin against undervolts on the AR6100.

Doing a simple WCA didn't look too good, though. I have HS-65HBs on the swash (yeah, I know, karbonite gears ) and an HS-81 on the tail. Suppose, per Servormances data, that each HS-65 has a current of 300 mA and the HS-81 is 300 mA at 6V. Then if everything was running at the same time that'd be 1.2A total. Each of the two regulators in the 35X would be sourcing 0.6A; suppose the battery voltage was 11.5V, so each would be dissipating (11.5-6)*0.6 = 3.3W. Suppose the regulators are something like TO-252 LM317s with a thetaJC of 12 C/W and a thetaJA of 103C/W. Suppose through copper board area they get the thetaJA down to 50 C/W (could be even better, but I don't want to guess at the cooling from forced convection coming off the main gear fan). Then each regulator would be 165C above ambient, and we'd be SOL, since they shut down at a junction temp of 150C if not lower.

So I figured some bench testing was in order. Running the receiver and servos off a lab supply, I measured about 60 mA idle, about 300 mA peak with one servo moving, and about 1.1A stirring the collective as fast as I could so all four servos were moving. With a 1A current limit I was flashing the...Continue Reading

I'm going through the usual helicrack progression: Havoc Heli (Apr 07), CX2 (Jun 07), HBFP (Jul 07), King 2 (Aug 07), Compy FP (Nov 07) and Trex 450S (Jan 08).

I'm an avionics engineer by trade, and I've become interested in the various behavior and limitations of things like ESCs, HH gyros and Spektrum radios. At one point I was interested in autonomous helicopters, but got sidetracked by how much fun helis are to fly manually.