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-   -   P-38: How to do this? (http://www.Wattflyer.com/forums/showthread.php?t=73485)

Textanker 04-02-2014 09:24 PM

P-38: How to do this?
 
I've searched this forum but I can't find a definitive answer.

I have a Richland models P-38. I have 2 motors, 2 escs and 7 servos. I've got everything wired up, but I want someone to check my work.

Each motor has its own ESC and battery; the throttle wires are on a Y to a single channel. There are 7 mini servos. No other power. I haven't disconnected any of the esc throttle wires. It seems to run fine on the ground.

Am I missing anything?

Thanks in advance.

hayofstacks 04-02-2014 09:40 PM

Y your batteries together so both motors cut out at the same time. you also need to disconnect on of the power leads from one esc. is it a switch mode bec on the esc's? With 7 servo's id want 6 amps or more. possibly in a seperate ubec.

crxmanpat 04-02-2014 09:52 PM

Pull the red wires from each of the ESCs, and use a separate RX battery for power to the RX and servos (be it Nicad, NiMH or LifePo). Otherwise, you are asking for trouble relying on a BEC to handle the load of 7 servos.

And agreed with doing a Y on the batteries. You don't want one battery to go flat and give you adverse yaw. That will put you in a death spiral so fast you won't believe it.

Wildflyer 04-02-2014 10:53 PM

On My P-38, I am very happy I waited until I could get counter rotating props. The kit came with normal props.

I decided I will not fly any twin without counter rotating props, there is just no pull to the left.

I may build a 3D twin just to see if it would work ok.

gramps2361 04-03-2014 12:22 AM

I had this plane loved it, but you have no choice but to use 2 separate battery's I Y mine like others suggested. I hope you used a UBEC to power the servos those 9 gram ones draw some amps. Not a lot of room in this plane sure you have found out with all the wiring things start to get tight quick.
This plane is a nice flying one just keep some power on in the turns and she lands on the hot side. i used the contra rotating props on mine too.

fhhuber 04-03-2014 03:24 AM

Full scale used "reverse" contra-rotating (top away from the center) due to a strange issue with normal contra-rotating (top toward the center) creating an instability. SOME scale models will exhibit the same issue and it might be dependant on the setup.

Just something to be aware of if it acts weird and you can't find the cause.

If others successfully used the "normal" contra-rotation with this same model then start that way.

**********

Disable one (to use the other to power the RX) or both (to use a separate RX power supply... BEC/regulator or separate battery) BEC(s).

OR use a schottky diode between each ESC and the RX in the + lead. The issue you are working to prevent is one ESC seeing the other as a load (and both can see each other as loads...) The diodes prevent the BECs from "backfeeding" each other.

There are some ESCs where you can leave both BECs active without the added diodes... but its always safer to put the diodes in when there is any doubt.

hayofstacks 04-03-2014 04:33 AM

http://www.headsuphobby.com/Sky-Powe...-BEC-H-105.htm

id pick two of these up and unload the other esc's if yours aren't up to it.

6 amp ubec, then throw i diode in like was suggested. each one should power 9 standard or 12 micro servo's.

kyleservicetech 04-03-2014 05:27 AM

Quote:

Originally Posted by fhhuber (Post 944420)

OR use a schottky diode between each ESC and the RX in the + lead. The issue you are working to prevent is one ESC seeing the other as a load (and both can see each other as loads...) The diodes prevent the BECs from "backfeeding" each other.

There are some ESCs where you can leave both BECs active without the added diodes... but its always safer to put the diodes in when there is any doubt.


Which ESC are you using? Are they "Linear" regulator type, or "Switching" regulator type? Switching power supply types of regulators require isolation from each unit with Shottky Diodes.

As for those Shottky diodes, this will show how that is done.

http://www.wattflyer.com/forums/showthread.php?t=67094

If your ESC's use linear type regulators, IMHO, they are not safe to use with any model with over two LiPo cells for motor power, especially with more than a couple of servos. If this is your situation, one of those Castle Creations 10 Amp uBEC regulators are smaller than a 4 cell battery pack, and can provide far more current to your servos with no voltage sag than a typical 4 cell Nickel Hydride battery pack.

In fact, I'm using that CC uBEC on two of my giant scale models with seven high current servos. Along with a two cell A123 backup battery. This combination works very well for the larger models.

Textanker 04-03-2014 04:22 PM

It's two of these:

http://www.hobbyking.com/hobbyking/s...C_4A_UBEC.html

I am going to use CR props. I think I am going to go ahead and use a separate battery pack for the servos. Now I have to pull the whole thing apart again!

rcers 04-03-2014 06:14 PM

Smart - twins deserve a good strong power supply. No ESC BEC's for me - except on my very small twin stuff with 3 servos.

Mike

crxmanpat 04-03-2014 08:49 PM

Better to pull it apart now to install an Rx-battery, than to have to collect it in a garbage bag later. ;)

pmullen503 04-03-2014 10:00 PM

One way to use both BECs and reduce the current loads is to use two receivers. Wire each receiver separately to one BEC.

I've used one receiver in the fuselage and one in the wing to facilitate breakdown and assembly for transport and storage (only one power connection to make/break.) In the case of a P38, just wire half of the servos to each receiver.

Another benefit is usually less and/or simpler servo wiring.

road king 97 04-03-2014 11:03 PM

Good advice flying with a extra battery for the reciever and servos ,i do it on all my big scale planes . You will get more flight time on your motors and wont worry about it shutting down your servos /reciever . I had my big seaplane motors shut down once on me at 100 ft high and was able to still have control to glide it down. joe

fhhuber 04-03-2014 11:58 PM

Depends on what you are doing...

A good example is the Dynam Me-262. It ships with little "diode-board" extensions for between the 2 escs and the RX. 110 mph straight out of the box. 1500 mm span, Twin 79mm EDF model using frm 4S4000 to 4S 5000 mah (some have put in 6S and reported good results, but I don't see the need). 5 servos (3 X 9 gram and 2 "mini" size) + the trike "electronic" retracts.
No issues at all with getting enough power to the RX from the ESCs.

If the system used provides the power needed for the RX and servos its fine.

Then there was the Polks DC-3 I attempted using older version Dynam ESCs and "22" series motors... and you couldn't keep the BEC's from overheating no matter what you tried (including disabling the BECs)
If its bad its very bad...

I used to hate Dynam ESCs ("Dynam" label). The new ones ("Detrum" label) are pretty good.

road king 97 04-04-2014 02:39 PM

1 Attachment(s)
Quote:

Originally Posted by fhhuber (Post 944477)
Depends on what you are doing...

A good example is the Dynam Me-262. It ships with little "diode-board" extensions for between the 2 escs and the RX. 110 mph straight out of the box. 1500 mm span, Twin 79mm EDF model using frm 4S4000 to 4S 5000 mah (some have put in 6S and reported good results, but I don't see the need). 5 servos (3 X 9 gram and 2 "mini" size) + the trike "electronic" retracts.
No issues at all with getting enough power to the RX from the ESCs.

If the system used provides the power needed for the RX and servos its fine.

Then there was the Polks DC-3 I attempted using older version Dynam ESCs and "22" series motors... and you couldn't keep the BEC's from overheating no matter what you tried (including disabling the BECs)
If its bad its very bad...

I used to hate Dynam ESCs ("Dynam" label). The new ones ("Detrum" label) are pretty good.

Hmmmmm for the price of a 20 dollar nimph pack to run my servos and reciever and not throw the magic dart that my esc will work all the time ,i think i will always go with a nimph or nicad pack for my scale planes . I use to use 2 recievers in my big gas planes in case one went out and back up batterys . Kinda sucks building big planes for over a year and not using every method at hand to help the last for a while. :D joe

dahawk 04-04-2014 03:02 PM

I know on the Deuces Wild, I'm using Jeti opto's with UBEC. Might also be able to add a Shottky diode.http://www.rcgroups.com/forums/showthread.php?t=12210
Put one in my Tucano single. If the linear BEC fails, the UBEC takes over immediately.

On my FW Mossy, I flew it right out of the box as is. No disconnecting anything. Has retracts, sequence doors, lights. No problem.

The P-38 is the only plane I know of that has outwardly CR props. Very cool.

Best of luck !

-Hawk

TriStar_drvr 04-05-2014 07:45 AM

I have the Rich Models P-38. I use two Sky power ESCs from HeadsUpRC. This ESC has a 4 amp, 5 volt switchmode BEC on the 30 Amp motor ESC. HeadsUpRC says it will power 8 micro (9 gram) servos. I run counter rotating props powered by 450 size motors. I cut the power (red) lead from one of the ESCs and have had no problems with the airplane. Here's a link to the post on my airplane. BTW the weak point is the main gear mounting blocks. They will come loose if your landings aren't perfect (mine aren't, that's how I found out).

http://www.wattflyer.com/forums/show...9&postcount=12

Here's a link to video of my maiden flight.

https://www.youtube.com/watch?v=2Hq8...ature=youtu.be

Good luck. It's a fun airplane, but it lands fast.


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