sfaust

My Feedback: (11)

I agree with Kevin about getting the wiring right from the start. No reason not to. The post I referenced earlier lists all kinds of things one should do in the wiring and electronics setup to minimize voltage loss, and maximize the overall potential of the system. Using heavy duty 22ga wire, minimizing long leads, minimal connectors, eliminating Y's, heavy duty switches, etc. These should all be done first, regardless if you are running 4.8v or 6v. I should have mentioned that, as my eariler posts seems to convey that the band-aid approach is acceptable. Its not! It was just an example to show the differences between 4.8v and 6v in a practical example.

However, I tend to disagree with the matchbox scenario with separate power systems. Most modelers agree that batteries and switches are the most failure prone electrical devices we put in our airplane. The last thing I want to do is add more of them unless they are redundant. If I add another switch and battery, and the system is now dependent on both of them for flight control, I have doubled the risk of a failure over using 1 battery and switch. If you put them in a redundant configuration, where you have two, but only one is needed for control, you have greatly increased your reliability. So, if you are going to put in two batteries and switches, make them redundant if at all possible. Why take a reliability hit if you can increase it instead.

I would much rather just use a regulator than use two batteries and switch of which either are capable of taking down my airplane. If the plane could support the weight, I'd opt for two batteries, two regulators, and two switches. Full redundancy for each regulator, battery, and switch. A failure of any of those won't cause a crash. Further, it also helps with voltage drop, current capacity, etc.

As Kevin suggests, reduced range exists at the higher 7.2v level.
But the only time this reduced range will be an issue when running unregulated, is in the first few minutes after you take the airplane off a fast charge. The 7.2 volts is residual voltage from the fast charge. Once the pack settles down for a couple minutes, the voltage drops below 7v, and everything is back to normal. If you pull a plane off fast charge, then immediately try to fly it, you may experience reduced range. So, take the airplane off the fast charge, then pre-flight it, fill it with gas, etc, then check the batteries with an ESV meter. The voltage should be be below 7v at that time, and you would be fine.

But to be safe, its worth running a quick range check directly off charge. It might not even be an issue at all in your airplane. To be extra safe give it 5 or 10 minutes of ground time to let the voltage fall, and don't fly if your ESV check is above 7v.

Kevin Greene

My Feedback: (85)

Steven,

Good points!!! I understand what you are saying concerning the extra batteries and the Matchbox. I was offering those ideas to those guys running the extreme servo scenarios. You know the ones---the guys that have servos ganged on servos. In my jets I have never felt the need to run separate packs. Although the servo load can get high it is not that high. I try to plan accordingly and choose the right servo for the job. For example on my new Super Bandit I will run digital servos on the elevators and ailerons. The flaps I will use coreless, high torque, non digital servos. Why? Due to the extreme area of the flaps and the high drag they will impose I'm concerned that the two digital servos would cause a very high amperage load. The load on the two servos will be very high for a long duration. The duration would last from the time I made a gear/flap pass until landing. This could be for a few minutes if flying at a jet rally while waiting your turn to land flying in the pattern.

I simply use the highest capacity battery pack that I can find given space and C/G requirements. As far as voltage is concerned---Voltage is like pressure. The smaller wire restricts pressure (voltage) so for long runs you should run larger wire. Also, wire strand count plays an important role. The higher the strand count the better as the electrons flow on the outer skin of the wire.

We have noticed a mesurable decrease in range in the voltages over 6 volts. Most of the guys that I know that are running voltage reglators are keeping the voltage below 6 volts.

Call me lucky but I have only had one battery failure in the past 30 years of flying. The one failure happened to a fairly new pack during charging and was attributed to a defective cell which fried the pack. Actually I don't think it was luck at all but preventative maintenance. I keep my packs conditioned and replace them after two years of use. Some of the packs that get more use I will change every year. It is cheap insurance. What is a $30 battery compared to a $7000 jet?!? Even a $500-$1000 investment is worth changing your packs often. It kills me when people can be penny wise but dollar foolish. I've seen many times where modelers will try to save a few dollars here and there----Cheap servos, thin wires, cheap switches, etc.... Your time and money investment, not to mention yours and others' safety hinges on the weakest link!!!

Kevin

BBriBro

My Feedback: (2)

Wonder why all the servo extentions are stranded wires? maybe they should make a solid conductor for the longer lengths.

Kevin Greene

My Feedback: (85)

BBriBro,

Solid conductor doesn't allow electron flow as easily stranded wire given both are the same guage. Stranded wire that is the same guage as a given solid core wire actually has more surface area for electron flow. The higher the strand count the more electrons can flow. So, for a given guage, a stranded wire of 40 strands will offer less resistance than a 20 count stranded wire. You just read my post backwards, that's all. Also, I should have said that MOST of the electrons flow on the skin of the wire.

Kevin

BBriBro

My Feedback: (2)

Kinda getting off topic, but then what is the advantage of solid wire, such as household wiring? Just the stiffness?, for feeding through walls and such? I knew many strands makes wire nice and flexible, but I didn't realize it actually made it "better" for electron flow too.

Lynx

Stranded wire doesn't provide that much less resistance than solid core. The skin effect you're talking about only happen on a noticeable level at high frequency and high voltage levels. In R/C I'd imagine it's more of a flexibility issue, solid core conductor wire is much much more liable to breaking, especially if it's high purity copper. There are other issues involved I know the electromagnetic flux travels slightly differently in stranded versus solid core, but these are issues for audio technicians not R/C freaks => We just don't deal with voltages currents or frequencies worthy of specialty cables.

sfaust

My Feedback: (11)

I'm with you on the Jets and dual batteries. I have a Bobcat turbine that I was considering putting dual batteries in it. I needed a tad more nose weight when I put in the P-120, so I was contemplating going with two smaller capacity packs with a higher weight, or just a single higher capacity pack with more weight. Either way, I will get an increase in capacity just to offset the rearward CG.

While I never fly my larger aerobatic planes without dual redundant batteries, even though they cost the same as a jet, I've stopped and pondered on the Bobcat. Mainly since I don't want to add more weight to it, and its a really full interior to begin with, so space is at a premium. I have space for the battery, but not the switch! I'll probably shoe horn in redundant packs, but I'm still undecided at the moment.

I personally never use the 6v as a way to get more out of a servo, and feel the same as you. Buy the right servo for the job from the beginning. Whatever increased benefit I get from running 6v is purely secondary. Usually this the increase in speed that I enjoy, and not the added torque. But I use the 6v for the increased headroom in voltage.

The longer duration higher current drain with flaps is an interesting point that I hadn't thought of. I really wish someone would develop a very small and light weight gadget that we can temporarily install in an airplane to measure the current and voltage levels in flight. There are some small devices that I could put in a giant scale aerobatic airplane, they are too large and heavy for a jet. Being the kind of person that likes to know and test for this kind of stuff, it would be neat to be able to measure it in flight. Maybe someday