|Jun 16, 2012, 04:23 PM|
Hi Conrad, are you planning to use 5" props or do you have space for larger props?
On mine, I used 5" 3-blade props on motors I re-wound (BP2408-21 motors of about 1800kv) to high kv in heavy gage wire.
If you plan to ROW you need a lot of power to get air over the elevator (held full 'up' at take-off) to stop the nose digging into the water.
For land based take-offs (sliding off grass) this is less necessary.
I think mine ended up around 2500kv swinging a 5x3 3blade prop (Testors i.c. engine props which are not very efficient either) on a 3S 2200 LiPo
They were capable of ~20amps each and I used a single ESC (not ideal though).
Lots of thrust from this setup, lots of spare power.
It cruises on about 40% throttle
|Jun 16, 2012, 11:08 PM|
Hi Phil, I will be using the Master Airscrew 5x3 3 blade props. I remember you writing in your build thread that you had rewound your motors. I have never attempted rewinding motors, but would be interested in what you think about me using these off the shelf Turnigy motors.
Since this is my first rc plane as well as my first electric plane, choosing a motor is something I do need help with. My wing covering is not on yet, I could move the motors outboard still by one wingrib bay if needed to run a pair of 6 in props as another possibility.
I should be back at home finishing this plane this week, but the boss held me over at the job for an extra week, which means by the time I fly home, I will need to turn around and return to work in a couple of days. Needless to say, I will unfortunately miss the contest deadline. I will finish this plane though, just be a few weeks late.
|Jun 16, 2012, 11:13 PM|
|Jun 17, 2012, 12:41 AM|
I am far from an expert, and have not yet attempted to set up a twin. But, I have successfully set up a number of good performing scratch-built aerobatic biplanes.
I believe that thrust is what makes airplanes fly. Besides, I lack an Electrical Engineering degree. So rather than try to understand the relationships between amps, watts, and weight, I pay attention to the thrust output that results from the motor, prop, and battery combination.
I always wait as long as possible to select the power system, so that my airframe weight is as close to final as I dare, which useually means that cover is applied. Then I factor in estimates of the airframe remaining to go. Armed with this weight value, I start looking for power systems that can offer between 0.75:1 to 1:1 thrust to weight ratios. Of course you have to factor in the weight of the power system. And I always add in some cushion, being a little over-powered helps ensure that you are not under-powered, and will provide extended run time and cooler temps if you can cruise at low power settings.
Your Catalina would probably fly nicely, and rather scale like with 0.75:1 thrust to weight (the real airplane has nowhere near that good of a power loading). So if your power system falls within the .75:1 and 1:1 window, I'd say your close enough.
The trick with this method, is finding a provider that offers thrust performance data for power systems. My favorite is HeadsUp RC (http://www.headsuprc.com/servlet/StoreFront). HeadsUp offers good prices; good service, and provides performance data on their website. Feel free to ask questions (I have), the owner is very knowledgeable.
I start with the scale propeller diameter and blade count and work from there. Look for the power system that can safely provide the thrust required using the prop diameter and blade count you desire. Everything else will follow.
|Jun 17, 2012, 07:32 AM|
Ha, I'm in the same boat with my SM-81. Doubt I'll meet the deadline but I'm not going to rush it.
Participation is the part I like & the reason I play with everyone else in the sandpit.
I doubt I'd win, but I enjoy the game.
OK, I know the props you speak of as I have some myself, though I couldn't get them to balance so went to 2-blade on my B-24.
If I were to use those motors at 1080kv, I'd go for a 4S battery.
If you read the comments on that motor, no-one uses them on a 5" prop. they all quote 7" & above.
Don't worry about going to 14.8Volts, it current that kills motors.
To give you a very basic understanding of power, imagine your garden hose.
Voltage - is the PRESSURE of the water
Current - is the VOLUME of water it can deliver per second.
A 1/2" garden hose at x pressure delivers x gallons of water per second.
The only way to increase its power is to increase PRESSURE as the size of the hose is fixed (= size of the winding wires in the motor)
The enamel insulation on the wires is quite capable of withstanding the pressure but the diameter of the wires can only cope with 9 amps.
Typically motor winding wire will withstand 32+ volts of pressure.
Now comes the tech bit which works in your favor here.
When it comes to power, voltage and current are inversely proportional.
That means as the applied voltage goes up, the amount of current required to make the same amount of power goes down
As an example: (Power = Volts x Amps)
11.1Volts x 10 amps = 110 Watts
14.8Volts x 7.43 amps = 110 Watts
So you see you are being (to an extent) kinder to your motors by this setup.
The downside is the output rpm is higher so bearing life is shorter (not that this is really an issue until you've logged a few hours airtime).
The other requirement is that your ESC's need to be capable of this setup.
Generally they are as we're only talking about a shift of 1 cell count.
Only the real 'el Cheapo ESC's will struggle (misfire)
This ought to give you a truckload of torque and high thrust.
Your props - make sure they all run true and are balanced as now you are entering true 'performance' rpm figures and anything short of perfect prop trim will result in reduced efficiency and differential thrust (= difficult fly-ability).
Don't be afraid to ditch them for a 2 blade, 3B's look pretty but that's about all they're really good for. You want this bird to fly and no-one sees a 3B prop at 100yds doing 8000 rpm
But don't get bogged down worrying about all the technical stuff, just make sure (as with any mult-motor) you do a rpm check at WOT on the bench to make sure the rpm's are less than 5% difference.
(10,000rpm x 5% = 500rpm)
Anyway - enough technical stuff but I'll add the obligatory disclaimer that the above is subject to dozens of quirks, conditions and circumstances but as a basic guide to electricity it is a rule-of-thumb.
|Jun 17, 2012, 07:32 AM|
The way you explain thrust in relation to power/weight makes sense to me. I can understand now why many builders test run their motor prop combo's with setups to measure the thrust. Seems so obvious the way you explain it, guess I just never wrapped my dense skull around the concept. I appreciate the explanation.
|Jun 17, 2012, 07:48 AM|
Thanks much for the explanation. Believe it or not, and I am embarrassed to say it, I work in a power plant! I am not an electrician, but understand a lot of the concepts you mention. For some reason though, picking a motor/prop combo for an electric airplane system has had me stumped. Power plants rely on the concept of boosting voltage to reduce current for transmission purposes, or else it would take huge diameter lines. It is making much more sense now with everyone's help.
Balance is something I am good at, so I will give this a high priority, and, as you say, go the two blade route if it is a problem I cannot dial in. The more I learn as I go, the more I am getting hooked on this hobby!
|Jun 17, 2012, 10:25 PM|
|Jun 18, 2012, 12:11 AM|
Rick, actually you did say thrust to weight in your reply, and I miss quoted you, even though in my mind I was thinking thrust to weight! Gotta keep an eye on me!
|Jun 18, 2012, 06:28 AM|
I am going to move my motor mounts enough to use 6 in. props I have decided. This will give me more options for off the shelf motors. I have already ordered the Turnigy Park 300, 1080 kV motors from Hobby King, so I will make a test setup, to see how they perform, and build a thrust tester to measure this also.
I am also looking at ordering a pair of these motors, keeping 6 in. props in mind for this build.
Firepower 300 Speed Specifications:
Weight = 1.2 ounces (34 grams)
Diameter = 1.1 (28 mm)
Motor length = 0.9 inch (23 mm)
Shaft = 1/8 inch x 0.5 inch (3.17mm x 13mm)
Voltage = 6.0 - 13.0
Current = maximum of 12 amps for 30 seconds (130 watts)
KV = 1600
THe prop data that applies to this motor looks promising for my plane.
Propeller data for the Firepower 300 Speed using a 3-cell Lipo Battery:
GWS 7035: 15 oz of thrust at 9 amps
EMP 6x3E: 16 oz of thrust at 9.5 amps
EMP 6x4E: 15 oz of thrust at 10 amps
APC 6x4E: 13 oz of thrust at 9.5 amps
I wish they had 3-blade prop data, but the two blade specs look as though it would be a good system, I think.
The links everyone provide are very helpful!
|Jun 18, 2012, 07:50 AM|
Yep - looks about right to me on the EMP 6x4E.
This plane doesn't take much to keep it in the air once it's up. that wing is enormous and does it's job well.
|Jun 18, 2012, 09:22 AM|
Joined Jan 2005
The MAS 3 blade 6x4 counter rotaters are a better choice for the Turnigy motors. The two blades don't load the motor enough to get the best performance. (Electrical devices only work as hard as we allow them to.)
Also, the LH and RH props will remove all the torque roll from the aircraft. This is critical on a seaplane like the PBY, as two motors going in the same direction will cause one float to dig in, and it will ground loop before you get enough airspeed to get aileron to lift it out.
For the motor wiring you'll need some Deans "wet noodle" to make a three wire extension for each motor from it's ESC. (regular automotive type wire is way too stiff) And you must use two ESCs. Don't believe any nonsense about using one for both motors. That only works with brushed applications. I find that motor wiring is best done with 4mm bullet connectors. 3 male on each motor; 3 female for the motor end of each extension; 3 male for the ESC end of each extension, and 3 female on each ESC.
I use Dean's connectors for my pack to ESC connection, and on the charger, but that's a personal choice. They can be tough to deal with. You can find those at Tower, along with parallel connectors.
Here's the towerhobbies.com catalog numbers on all this stuff:
LXFTE7 MAS 6x4 3 blade tractor
LXNGV1 ditto, pusher
LXKX26 Deans wet noodle. Two feet of red and two feet of black. You might need two sets of this. Depends on the distance from your ESCs to each motor. Remember: DO NOT lengthen the battery wires on the ESC or the pack. If you can, shorten them. Long battery wires result in blown ESC capacitors.
LXJPY4 4mm male bullets (you'll need 4 sets)
LXJPY6 4mm female bullets (ditto)
If the prop adapter that comes with motors won't fit the MAS props, you can get one of those from Tower also. I use the collet types, and they have one for any diameter metric shaft. You can get them with pretty prop nuts that aren't too far off what the PBY had.
I can't stress enough that you should not fly this thing yourself on it's first flight. Find an experienced seaplane pilot who's flown a lot of different types. New airplanes are always out of trim, and this will result in an immediate crash if you do it yourself. Once it's all trimmed up, then see if the experienced pilot can get you a buddy setup with it so you can learn to fly RC. (You'll need the same variety of transmitter he has, so don't buy one until you know.)
If you can't get someone to buddy with then you need to get a simulator and some kind of small electric ARF or RTF to learn on. The Multiplex Easy Star is a good choice: LXFXV0 It's big, easy to fly, and easy to fix, and is ready to go pretty much out of the box. RC flying is a lot harder (physically) then flying a full size airplane, because the orientation changes so often.
|Jun 19, 2012, 09:06 AM|
Does the 6X4 pitched prop have better pull out of the hole, than the 6X3 pitched prop? Also, does a 3 bladed 6 in prop of the same pitch drag the motor down resulting in less thrust than a 2 bladed version of the same pitch?
|Jun 19, 2012, 09:17 AM|
Actually I did buy a simulater, I bought the Phoenix simulator since it came with a usable transmitter. I have logged a couple hundred hours on this thing, and am feeling pretty comfortable with using the transmitter controls. I also bought a Hobby Zone foam cub, 45" WS I think. I was advised by others here to learn this way, good advice as I have found out! The cub flies great, not a crash yet.
This false sense of security will not let me try to maiden my PBY myself though, I agree with you on the need to let an experienced pilot do this.
I bought some Electrify PBY hubs for the props that are alumiinum and look like the real hubs. Depending on the motor shaft, the only mod to these might be some very minor reaming.
Thanks for the tips on the wiring fine points.
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