Author Topic: Twin Electric 1/4A (Read 3588 times)

john vlna · « **on:** February 07, 2011, 10:43:30 AM »

I thought this would be of more interest here than in the AMP’d thread. It is my latest Electric model, a flat foam P-38. This type of construction is popular in the RC community. I have made a couple others and they all fly really well.

This one is made of Dollar store foam board. I even left the paper covering on it , dry weight is 7.2oz, ready to fly it weights 11.4oz. Motors are about equivalent to an .02, although exact comparisons between glow and electric are hard to make. These will turn a 6x3 prop at 13000 RPM with a 2 cell Lipo.

I use throttle control with a U/Tronics system mounted in the handle, and insulated .008 or .012 line that I make myself.

kenneth cook · « **Reply #1 on:** February 07, 2011, 04:27:18 PM »

John, that looks real cool. Nice job if I do say so. Ken

Dennis Adamisin · « **Reply #2 on:** February 07, 2011, 04:47:46 PM »

WAY TO GO John! BTW APC makes their 6x4 electric props in both LH & RH roation - so you can do opposite rotation too!

jim gilmore · « **Reply #3 on:** February 07, 2011, 10:54:16 PM »

Sounds more like 1/2A if your talking (2) engines...

john vlna · « **Reply #4 on:** February 08, 2011, 07:10:51 AM »

each motor is equivalent to a Pee Wee .020, although electric are able to spin larger props.

CircuitFlyer · « **Reply #5 on:** February 09, 2011, 09:19:02 AM »

Awesome!! Great work, John. Could you please provide some more details. Did you add carbon fiber rods for reinforcement? Battery size and flight time? One ESC or two? Motor size or rating (watts)?

Sorry for all the questions. The whole reason I got back into the hobby a few years ago was to build that multi-engined plane I always dreamed about when I was a kid. With electric power it's so easy. As a matter of fact, yesterday, I just ordered a couple of motors and ESC's for an twin RC foamie I'm building. I've not worked with foam before, so it's all part of the big learning curve. I wish there were more plans available for aircraft just like yours.

john vlna · « **Reply #6 on:** February 09, 2011, 12:01:13 PM »

Silver Dart,
Don’t worry about questions, that is how we learn. There are tons of plans for flat foamies, they are RC but easy to convert to CL. You will find a lot of info at http://www.rcgroups.com/foamies-scratchbuilt-428/

I have used Carbon fiber (2 or 3mm) tubes for this size model, balsa, spruce and even bamboo skewers. The P-38 shown uses Bamboo Skewers. The motors are nominally 60 watt each, http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=5432

Some times it is hard to tell what a motor is rated at. A good rule of thumb is 3watts per gram. These are high Kv motors which will not spin a 5 or 6 inch prop on 3 cells without overheating so I am using a 1000mah 2s 20c battery. With 6X3 props I draw 7amps (static). Current draw will go down some it the air and since I use throttle once airborne I am using less power because I can throttle back. I have never run out the battery, but it will fly at least 7-8mins. Each motor needs its own esc. I use Exceed RC 10A units Control inputs are the same to each esc http://www.hobbypartz.com/vose10abresc.html

Attached is a wiring diagram. CL is the same as RC with these exceptions. CL requires more power. I use 50-100% more that I would on a RC plane for the same type of flying. Why? The weight and drag of lines and need to maintain line tension. Also the control signal must be sent up the lines. Check the vendors section for the U/Tronics system which does this nicely.

john vlna · « **Reply #7 on:** February 09, 2011, 05:33:03 PM »

The picture in my previous post doesn't clearly show the esc connection, so to clarify I've attached a picture of an esc. The ESC has Power leads(2ea), Motor leads(3ea) and a control lead(3ea). The control port is where the timer plugs in if you are using a fixed speed control such as a stunt plane would. The U/Tronics also plugs in here, but only the black and white leads are used. The red is disconnected.

CircuitFlyer · « **Reply #8 on:** February 09, 2011, 06:01:55 PM »

Thanks for the info John. Did you have any trouble matching the thrust? I know some lower cost motors can vary in power output.

Great link, a lot of great plans and inspiration for design, but only a few twins, they don't seem to be real popular. Your choice for motors seems real smart - efficient and robust for its weight. I like working with foam and think it should be usable for ECL. You could build some fun models quick, simple and cheap. With luck, I'll try to build a profile scale twin CL foamie this spring.

Paul

john vlna · « **Reply #9 on:** February 09, 2011, 06:38:22 PM »

Paul
Thrust, especially for these cheap motors does vary. I static test them and then mount the high reving motor inboard. In the P-38 case the difference is about 250 RPM.

You are corrrect that there are not a lot of twins. But if you look at the SEMFF II thread http://www.rcgroups.com/forums/showthread.php?t=1012551&page=119 You'll see that it is pretty easy to design your own twin.
John

Bill Little · « **Reply #10 on:** February 09, 2011, 07:57:59 PM »

Hi John,

I think it is really neat! AS I get more and more into electric, I can see projects like yours being a LOT of fun! And yours looks great.

Bill

john vlna · « **Reply #11 on:** February 09, 2011, 08:30:59 PM »

Here are two others that I currently have, a grumman TBF, and a Boeing b-390 (never built, just a mock up) The funny wringle in the TBF wing is caused by the KFM2 airfoil .

Bill Little · « **Reply #12 on:** February 09, 2011, 08:33:45 PM »

NEAT!

What do you have to do to finish the Boeing?

(the TBF is a real sentimental favorite of mine!)

Big Bear

john vlna · « **Reply #13 on:** February 09, 2011, 08:38:40 PM »

The Boeing is finished. It flys with either a Tower pro 2408 (130W) or a TP 2409 (230W). It is very tricky to fly. I use LH rotation to improve line tension. Props range from 7-9 inch and 2or 3s batteries depending on how fast I want to go.

john vlna · « **Reply #14 on:** February 09, 2011, 08:47:04 PM »

While I am at it, here is my AEG IV, it has flown, not quite finished yet, another advantage of electrics flying them doesn't make all greasy and hard to finish.

Bill Little · « **Reply #15 on:** February 09, 2011, 08:52:27 PM »

STOP!!!! You're killing me!

That is all just TOO NEAT! Nice stuff, there, John.

Bill

CircuitFlyer · « **Reply #16 on:** February 09, 2011, 09:00:11 PM »

Great stuff!!

Motors, span and weight of the AEG?

Paul

john vlna · « **Reply #17 on:** February 09, 2011, 09:26:48 PM »

AEG motors are ALPHA 300-1380, span 36 inch, about 13.5oz with two 500mah batteries. a little lighter with a single 1000mah This was designed originally as a glow model so I am a bit restricted on prop diameter. Currently I am using 6 x5.5APC's but would like to use 7X4's,

john vlna · « **Reply #18 on:** February 27, 2011, 11:17:10 AM »

The weather finally allowed me enough flight time for a report. I have been flying the p-38 on 6-3 props. The battery is a single 1000 mah two cell lipo. I get about six minutes of good :airtime. It will fly longer but the battery starts to reduce voltage. The model flies well nice touch and goes. About all you want for a scale plane. There is not enough power with the present setup to stunt however. I'll experiment with props but for stunting I think I will have to go to a 3cell battery.

minnesotamodeler · « **Reply #19 on:** February 27, 2011, 10:11:50 PM »

Not sure who judges these things, but if you're swinging a 6" prop at 13000 rpms you're way beyond .020 performance.

john vlna · « **Reply #20 on:** February 28, 2011, 10:16:42 AM »

Yes it is hard to compare glow and electric. You could never get a pee wee to swing that big a prop at 13K. But if you compare the power they are about the same. A good rule of thumb is 2000 times displacement. .02X2000= 40 watts. Some people use 3000 which would be 60 watts. My motors are in between the two numbers with this setup. My input is 100 watts (includes both motors) and my power at the shaft is 60 watts. By the way my simulation program predicts that the motors will turn a 4X3 at about 15K if that sounds more like a .02 to you. I have not tested that yet. Proping a motor correctly is a big part of tuning an electric motor. No needle valve.

jim gilmore · « **Reply #21 on:** March 06, 2011, 12:20:48 PM »

Are you saying that the thrust .. not power...will be the same

Seems another example where the watts and actual performance between gas and electric have no straight comparison.

john vlna · « **Reply #22 on:** March 06, 2011, 01:23:29 PM »

Jim
Yes the power is the same, not thrust. Thrust is a function of rpm and prop diameter/pitch. More diameter and pitch, more thrust. With electrics if you put on a bigger prop, say go from a 6x3 to a 7x3.5, the current draw goes up and the power goes up. The bigger prop should also produce more thrust. The comparison to glow is just a rule of thumb to gauge whether the glow and electric motor are in the same power range. They don't necessarily perform the same. My AX 1806's are equivalent in power to a Cox .02, but the Cox and Ax will make their max power using two very different props and rpm's. When I build an electric plane, the most important parameter I look at is power to weight. My experience indicates that CL needs at least 100 watts/pound for scale operation and more like 150-200 for stunt.
John
P.S. I have changed to 7x3.5 props, just about max for this motor, but not flow the plane yet. The motors turn 11K with this prop. I don't know where a Pee Wee is at max power but I'll bet it is more RPM with a smaller prop.

john vlna · « **Reply #23 on:** March 06, 2011, 01:39:16 PM »

Here is an interesting chart by Dr. Kiwi from RCgroups on Props, rpm amd thrust.

jim gilmore · « **Reply #24 on:** March 06, 2011, 02:11:40 PM »

Ok, so which is more accurate ?
If a Sta ndandard 1/2A model...Not a pathfinder halfA which must have a good 1/2A and is more akin to A powered models....flys on 30' lines such as a the original baby ringmaster with 24" built up wing flew on a 6x3 running at 13,000 .
Would you not say THAT AN ELECTRIC TURNING THE SAME IS PRETTY MUCH THE SAME ? Regardless of the power you put in or leave in the battery ?
Granted a larger prop can switch the amount of effeciency and give more thrust in some way but the effect is that a 1/4 A or .020 would never fly it. And such that your using 2 makes it double to begin with...

Even I think D. Adminsin (sorry about the spelling) admits the electric power system for the 1/2A pathfinder to be comprabl;e to an .10 not a .049...
So my point is that electric power needs a better way to compare....
Or the formular being used is wrong ?

john vlna · « **Reply #25 on:** March 06, 2011, 06:15:52 PM »

Jim,
It seems to me that two planes turning the same rpm on the same prop should fly the same. However, I think the power comparison is valid. Here is another real life example of a plane that has flown both ways similar to your Ringmaster example.

I have a 0.049 a typical all sheet balsa 1/2A model. A Cox Black widow with tank weights 2.2 oz. The plane weights 6 oz dry for a flying weight of 8.2oz. I converted it to electric. The all up weight is 8.5oz.

My rule of thumb says the .049 is nominally a 100 watt motor.(.049 x 2000) . The electric motor is running at about 100 watts. I have flown the model with both configurations and they fly about the same. Both use the same prop a 5x3. So if your question is can you make them the same, the answer is yes. However I could use a lot of different props on the electric, this motor will turn an 8x4 for example. I don’t think a glow .049 would be happy using an 8x4.

It is probably impossible to directly compare the two power systems. I have never flown a Pathfinder but Larry Renger said it is sized for a .061. I would use 3000 x displacement, or ~ 180 watts when looking for a motor.

john e. holliday · « **Reply #26 on:** March 07, 2011, 09:22:26 AM »

What gets me is all the heavy wiring for such a little motor. Which combination would you recommend for a .15 size carrier plane? I would be starting from scratch, so would need info on motor, ESC, battery and charger.

john vlna · « **Reply #27 on:** March 07, 2011, 06:01:18 PM »

Doc,

There is not a simple answer to your question. Let me go over a design I have in progress. My approach is a bit different than the way Pete Mazur, Bob Frogner and others have gone. I believe Frogner has outlined his 15 before in the carrier thread. But I am trying to develop lighter planes with smaller motors.
Since this is really a carrier topic, I'll start a new thread over there and hope it doesn't go into rules as usual. If that doesn't work I'll PM the info to you.
John

john e. holliday · « **Reply #28 on:** March 08, 2011, 09:14:12 AM »

Thanks John, have been over there and printed it off already.