Fly RC

August 25, 2013

Did my Spektrum PowerSafe receiver just save my airplane?

Had a nice day flying at our Annual Toys for Tot’s charity fundraiser yesterday and today did the recharge on the flight packs on the plane I flew. This particular plane has a pair of A123 2300mah batteries that plug directly into my Spektrum PowerSafe 9110 receiver. This thing is a $200 item but it’s main feature is the ability to plug two batteries directly into the receiver via high current EC3 type connectors. With this method the power supplied to the servos is shared between the packs and does not have to go through any switches. A soft switch is used, which means if the switch fails the airplane is on and the current to run the servos does not pass through the switch so it is not a limiting factor in supplying power to the high torque digitals that are used on all flight surfaces of this aircraft.

When I went to recharge the batteries I started with the port side pack (just because) and charged them up to full… they took about 850mah. That seemed high as I flew maybe 3 or 4 times and none were much over 7-8 minutes. As I moved to the starboard pack something even more interesting occurred. The charger said no battery was present!! Double checking the leads quickly lead to the discovery that the negative lead ended in a nice solder ball that was no longer attached to the battery! At some point it had become disconnected… I don’t know if it was vibration, poor solder joints (from the manufacturer… I didn’t have a hand in this connection) or some combination of the two.

I have no way to know if this happened before the flight or during but I can tell by the fact that the second battery took only 100mah or so to charge that I made several flights without it! If the connection was dead before I took off the first time… then it really would have only meant I wouldn’t have flown that plane on that day if I hadn’t had this system. If it broke at some point during the first flight it likely means my airplane is only here today due to having the redundancy in the system. Glad I had this system.

Of course, this receiver is not the only way to get this level of redundancy and reliability. There are other power distribution systems out there but for this size and type of airplane I think its a very nice option and one I will probably continue to deploy. I will have to look at my pre-flight and assembly routine to see if there is a way I can check for this failure mode in the future. In the meantime its nice to know its there protecting me and my airplane from disasters.

August 22, 2013December 20, 2013

Toys for Tots is Saturday!

Looking forward to a great day Saturday 8/24 with a casual day of flying but with the added bonus of contributing to a great cause. I’ve heard pulled pork and tenderloins are on the menu and the list of prizes and some silent auction items sounds impressive. Weather says ~85 and light east wind with lots of sunshine… sounds like about the best you are going to get for August in Indiana!

I think official start time is 11 (maybe 10 so don’t quote me). Landing fee is $10 or a toy for the kids (unwrapped). We are looking to give more than ever to the Marines for the kids so if you can, stop by and support us!

Gene has done a great job setting this up and procuring prizes. This one is becoming a local favorite.

I plan on getting in some flights on the 50cc WH Slick, of course the T28 and maybe even get out and do a skydive or two!

August 13, 2013December 20, 2013

Updated throttle servo linkage on the P51

Spent a few minutes last night to make a few changes to the throttle linkage that caused me issues on the P51. Here is the updated version.

As you see I swapped arms to a fixed heavy duty Hitec arm as well as shortening the 2-56 rod and soldering on a coupler that takes me to a 2-56 threaded end. Then put a clevis with jam nut and retaining clip on to minimize chances of another disconnect. After getting the jam nut tight I coated it with a bit of clear finger nail polish to further eliminate any vibration induced loosening. I then coated the threads on the screw going into the servo (which are all metal) to help on that side as well. I could use some thread lock but need to make sure I can move it one more time when I adjust the throttle throws (if necessary) whenever I run it next.

I am open to suggestions but I think this will eliminate a re-occurrence of the issue that has me ordering a new wing! At least I hope so. The throttle end is held on with a 4-40 bolt through type ball link connector with a nylon insert nut so I don’t expect issues on that end. I cleaned up any other issues under the cowl and will reassemble that tonight and then the Mustang will get parked safely in a corner of the shop until the wing gets rebuilt. Probably will become a hangar queen for the winter and look forward to maiden day next year!

August 11, 2013August 11, 2013

P51 crash landing… investigation Part 2… Conclusion

Continued the investigation tonight. Set the body in my trusty PVC holders on workbench #1 and started examining from the front. Pulled the cowl and found a couple minor issues… One cowl screw insert coming loose, ignition box moving a bit, but no smoking gun under the cowl. Even pulled the plug and examined. It actually looks like I have heard it should. Sort of uniform dark grey. Gap looks fine and no carbon buildup obvious. Top of the piston still nice and shiny as well. All linkage and tubing plus the telemetry sensors and wiring looked OK. I then decided to skip an actual ignition spark test for the moment and moved on back into the wing saddle area. Again nothing obvious so I turned on the plane and started looking to see if (as I thought) the ignition voltage sensing is beyond the kill. The plane died quickly like that occurred but the telemetry didn’t show the ignition voltage loss. This all worked as expected but it brought to mind the idea that maybe I had a throttle curve that brought the throttle below idle or some sort of misconfiguration of a mix that would cause that. Looking at the radio yielded nothing but when I moved the throttle back and forth I saw some odd movement and looking more closely I found the culprit. Here is the problem:

With this screw backed out this far the adjustable arm can end up extending enough to cause the throttle to bottom out and shut down the engine. As you can guess, I will be coming up with a different type of connection. I know, some folks will say this type of connection in a big gasser is just stupid to start with, but with so little force required to move the throttle I was not worried about torque causing slippage or the arm bending or any of that. What I obviously should have been concerned about was vibration loosening the screw! I will probably replace this linkage with a standard threaded clevis (once I solder on a threaded coupler) and a standard servo arm… non-adjustable this time… A drop of something to keep the screw in place might be used as well. Perhaps nail polish or the like. I don’t think thread lock works on nylon!

Obviously its hard to be happy about a crash but the good news is the damage seems to be limited to my pride and the wing. My pride will recover… its a nasty bruise but they heal. The wing is not that difficult to replace, and not very expensive in the grand scheme of things. The better news is that I now know what caused the crash and it is easily rectified. I was hating the thought of fixing all the damage and not being able to figure out a cause. That would really make me paranoid and give me no confidence to fly the plane again. Now I’m back to looking forward to it!

August 10, 2013August 11, 2013

P51 crash landing… investigation part 1

Pulled the landing gear and servos out of the wing this evening. Port side gear bay and main spar and ribs took the brunt of the damage. Cracks and buckling apparent. Would be difficult to fix. Would need to re-manufacture at least one rib and remove and re-web the main spar. I imagine parts of it are cracked or broken as well. The other side the top sheeting and some of one rib is crushed. I am fairly sure I will just order a new wing this fall and reconstruct over the winter.

Both retracts had some flexing/bending of the mounting ears. The starboard side was bent worse as you can see here.

Some hammering against the anvil/vise seems to have straightened those out pretty well. I don’t expect the retracts are actually hurt at all. They seem to operate smoothly and the struts compress as before. Best I can tell everything is pretty straight again now.

I need to carefully inspect the fuselage but on first inspection it looks unhurt.

First look through the telemetry data has nothing significant to help pin down a cause. Both the pack voltage and ignition voltage stayed in range both before and after the engine quit. Overlaying the altitude and RPM graphs it is pretty easy to see when the engine plummeted in RPMs over a few seconds, ending at zero! I will have to carefully examine the wiring beyond the sensor, check the ignition itself as well as the plug, the hall sensor and of course the fuel system to see if I can determine a cause. The log showed no signs of problems and even if it had gone to failsafe the throttle would have simply gone to idle. I will confirm that too but don’t believe it would have quit if I took a hit and it doesn’t appear I did anyway. There was plenty of fuel in the tank when it quit so will be sure to check the plumbing as well. Still a mystery for today.

August 10, 2013August 10, 2013

P-51 crash landing today

Due to a flame out at a very bad time the P51 made a landing in the bean field 20 feet off the end of the runway. Analysis is underway to try to determine a cause and assess damages. Quick guess is the wing will need to be replaced due to landing gear attachment point damage. More to follow when I have time to work on it. Will be looking at telemetry data as well for info leading to reasons behind the failure.

August 9, 2013

Had to pretty much program the P51 from scratch so…

Had to pretty much program the P51 from scratch so tomorrow will be a second “first flight”. Will fly noon or so to 6 tomorrow.

August 9, 2013August 9, 2013

PVC… Great for plane holders, wing racks, etc…

I needed to create a wing rack for when I’m carrying a larger number of planes in my trailer such as when I go out for trainer night at the field when I usually displace my 30% Edge with some combination of my Telemaster 40, Sturdy Bird and a sport/fun fly plane along with my Radian glider. Usually 2 or 3 of these join the mix. It becomes a problem to find a place for all these 4-6′ one piece wings so I decided to build a small wing rack and of course to do that I reached for some trusty PVC pipe. This stuff is cheap, strong and there are a variety of “joint” pieces available to join two, three or 4 pieces together at 90, 135, and 180 degree angles, plus caps, etc… You can build all sorts of stuff useful in RC.

I will add a few example photos for you to give you some ideas about where this can be useful. Here is a pic of a stand we created to hold the Giant Scale P40 while we worked on it last winter on the bench. Also in the pic is the pipe cement and the special cutters that work kind of like ratcheting scissors to give you a quick and easy PVC pipe cutting ability. A wood or hack saw work OK too but they make a mess and take about 10 times as long to accomplish the task.

Here is a boat holder I created. The two posts sticking out were used to hold the surface radio I used to carry with it.

And this one is the new wing rack I just completed. It will get some padding added soon.

Sitting next to it and housing my P51 mustang is a combination structure using 2x8s along with PVC and velcro. I took one piece of PVC and drilled holes through it to match the wing mounting block in the body to give me a cross piece that drops into notches in the 2x8s for a very reliable and secure hold down method.

I have created wing racks in the garage that hung on the wall, plane stands for holding a couple of planes nose down and many other useful items. With the different sizes available and combining velcro, pipe insulation for padding and even combined with wood at times where some heft or additional strength is required, you can create a whole host of useful stands, racks, carrying aids and much more. Best of all you can assemble and do some “custom fitting” without using any glue and then go back and disassemble and reassemble as many times as necessary to get what you want before applying the cement. And the material is relatively inexpensive!

Let your imagination go wild. With a few sticks of PVC you can do a lot for less than you might think.

August 8, 2013

Spektrum Telemetry and Spektrum Telemetry Viewer (STV) – Part 2

The next display I’ll show you from my air show flight combines the speed recorded by the GPS and the speed recorded by the airspeed sensor (Pitot tube/pressure). This is interesting for a couple reasons. First, you would expect to be able to see the difference in ground speed measured by the GPS (at least when flying level??) versus the airspeed measured the airspeed sensor. Here’s what it looks like.

According to this my top ground speed (GPS.. in purple) was about 98.5 mph while the airspeed sensor shows a top speed of about 97 mph (in red). Trying to compare at any single point doesn’t appear to work very well as the discrepancy between the two sometimes makes no sense. It appears from looking at the numeric data that the sensors disagree on the time stamp associated with the sample so correlation is difficult. This is interesting data and will be very interesting to correlate with a flight after I get retracts installed to see how much the lessened drag changes things.

Next we can look at the G sensor output. This shows X, Y and Z axis which is Nose to Tail, Wing to wing and top to bottom respectively.

The most interesting thing seen here is the maximum G force (Z Axis in gray) is nearly 4.25! Using that handy custom graph again and you can correlate a climbing altitude a few seconds before then bottoming out a few seconds before this reading, along with increasing speed on the same basic curve… This would likely indicate I was performing a loop and this high G was just past the bottom of the loop as I pulled back up to gain altitude. Imagine the wing of the airplane combined with the engine thrust having to create 4.25*21lbs = ~90lbs of lift to arrest the dive and bring the plane back to a zero sink rate. Maybe a little larger loop and less throttle on the down side would be a good idea! I guess the glue joints and main wing spar are holding up.

For a final look at what we can see using STV and the data supplied by the Spektrum products I will do one more post soon showing the GPS data and logging information soon.

August 8, 2013August 8, 2013

Spektrum Telemetry and Spektrum Telemetry Viewer (STV) – Part 1

I have been working with some software from ROBO software called Spektrum Telemetry Viewer. STV as I’ll call it allows you to do several things based on the .tlm file that a Spektrum radio saves to the SD card when appropriately configured. Look for a link to them soon on my links page. Spoiler alert, I like the product and think it really makes the telemetry information gathered by the Spektrum TM1000 into something even more useful than what the radio can do on its own!

Using STV you can really dig in a bit more to the telemetry data that you have collected during flight. I love knowing what my battery voltage was at the high and low point in the flight, how fast, how high etc… Much of it is very cool and parts of it are excellent in determining just how well your radio link is being maintained, if your on board batteries are performing as expected and even some things related to engine performance!

As an example lets look at some data I gathered from my P51 during a demo flight at our recent air show. For reference, the P51 is running a DA-50 with a Xoar WWII style 22×10 prop. The ignition is fed through an Ultra IBEC which includes voltage regulation and is powered off of the same flight packs which power the rest of the aircraft. Those batteries are a pair of 2 cell A123 2300mah packs. These feed the Spektrum power safe 12 channel receiver. There are 9 servos plus and electronic ignition cutoff switch in the bird. All servos are Hitec brand, ranging from 5625/45 digitals, 645 analogs and a couple of standard 425s if memory serves. This is the giant scale Top Flite P51 and it weighs in around 21lbs.

For Telemetry I have the TM1000 which plugs directly into the data port on the receiver to receive flight log information as well as power. I have the head temp probe positioned a couple fins below the top of the head toward the back of the cylinder. The RPM probe is the magnetic type and it is glued into a wood stick and positioned to sense the magnet embedded in the hub that also “fires” the hall sensor. I also have the GPS module installed as well as the 3 axis 8G force sensor, the altimeter unit and air speed sensor. I have the voltage probe wired into the ignition side of the IBEC so I can monitor ignition voltage as well. Having covered all that lets look at some data.

This first graph is the RPM/Volts/Temp page:

First take a look at the blue line. That is the temperature line. Looking at some of the other data I can tell you that throttle up to takeoff was around 90 seconds into this recording. According to this graph (with a bit of zooming and scrolling which the application allows for) the temperature at that point is around 170 and it takes another 30 seconds or so before the engine gets up to that 220-240 range that it appears to be in for the majority of the flight. This tells me there is no worry with the engine getting to hot sitting and running on the ground as the temperature is only very slowly rising during the time it is idle or taxiing. I always wondered with a tightly cowled engine like the Mustang just how hot it might get if forced to sit for a couple minutes on the ground. Apparently just prop wash airflow is sufficient to keep the engine cool… at least for a couple minutes! Looking at the green (voltage) line tells me my ignition is seeing anywhere from 5.17-5.29V for the duration of the session. Apparently the regulator in the IBEC is getting a fairly constant voltage source from the receiver or is doing a great job regulating what it gets as this voltage is pretty solid and what variance there is does not track with RPM. You would think running the engine at high RPM would require the most current draw and therefore cause a voltage drop if there was any problem with the regulator supplying the needed current so this is reassuring as well. Lastly, the RPM graph shows RPM readings in the air peaking around 7600 RPM. This is a good 800 RPM above static revs on the ground which is a bit more than I would have expected but not alarming and well within the limits that DA recommends. So far all looks well and I’ve learned a bit about how the engine is performing and surviving in the air.

I’ll just call this part one and move onto some other graphs and information gleaned in the next post.