1/2A electric planes

Heretic!!!!

I don't even know you...  

I also can't answer your question, bet Ian can though.

pkrankow wrote:Pop over to Stunthanger.com (stunthangar? they use both like a joke...)  There is a thread in the "getting amped up" section that might be of help.

Phil

I've looked through SH's "list your set up". Thanks. I thought I'd ask here also.

Cribbs74 wrote:Heretic!!!!

I don't even know you...  

I also can't answer your question, bet Ian can though.

Come on Ron! Let's be friends, when we shake hands, mine won't be covered in castor  .

JPvelo wrote:http://brodak.com/electric-power-package-for-1-2a-series-does-not-include-plane.html

http://brodak.com/electric-baby-clown-black.html

Thanks Jim.

My Cox Electro Charger P-51 Fireball I restored. Very carefully split the fuselage with a exacto,,careful to not break the molded alignment pins. Removed the old tech rechargable Nicad that had "greened" over. Cleaned up the solid brass wire that connected battery to motor. Pulled the motor apart and cleaned up the commutator and brush style contacts. Added a very small 2 cell 160mah lipo battery about the same weight of the Nicad. Peeled the old decals off very carefully with light heat and a exacto and had them reprinted locally. They got those spot on with the colors and everything. Added a Carbon fiber reinforce rod to the wing to the already molded-in trench in the wing. Flew it last summer finally. That little sucker leapt into the air and proceeded to cut some fast laps. Small battery gave it about the right amount of run time of just a few minutes for my waaay behind inputs,,,lol. It turned out pretty nice I think.

Very nicely done, Keith!

SuperDave (SD)

Hey, Duke!  I guess that little electro Ringmaster whetted your appetite.  I have a couple of motors and E-Flight timers that I bought with Christmas money ready for an electric 1/2A build.  I didn't sit down and calculate my power requirements and then order the motors and batteries.   I just bought the on-sale motors at Value Hobby, and I'll build to them.  I guess I'm just cheap that way.  The selection and variety on motors is still too confusing to me.  I just look at the motor specs, the recommended props, and then ignore it all.  I'll put a couple of 5" and 6" props on the motors on a test stand with a watt meter hooked up and tach in hand, and see what works best with what I have.  I can always set the speed using the timer and get the prop RPM/thrust I want.

The Cheap Conservative Mark

Mark
Look at this thread, http://stunthanger.com/smf/index.php?topic=32174.0, post #11 by Tim. Tim lives near you and flies at Delta Park sometimes. You wrote this up to help me understand electric set up sizing. I can even understand it, so you should take to it like a duck to water. Or a Canadian Goose to the soccer fields at Delta Park.  

I've been wanting to build a e-stunt plane for a long while, but the two e-trainers I built increased my interest.

[quote="duke.johnson"]Mark
Look at this thread, http://stunthanger.com/smf/index.php?topic=32174.0, post #11 by Tim. Tim lives near you and flies at Delta Park sometimes.  You wrote this up to help me understand electric set up sizing.  I can even understand it, so you should take to it like a duck to water. Or a Canadian Goose to the soccer fields at Delta Park.  
quote]

  

I read this post.  Some I agree with, some I don't.  His big GCE is that he describes battery capacity and battery voltage as linear.  It doesn't work that way.  The voltage drops off on a curve.  By his calculation, a 3S battery (11.1) discharged to 75% is ~8.3 volts.  That would be 2.76V/cell, which is bad.  The normal lowest voltage cutoff setting for most of the speed controllers I've used is 3.0 volts per cell.  Electric gurus usually recommend around a 3.2V/cell cutoff with 3.0V/cell as the absolute minimum.  I figure the best way to know good numbers for me is to hook up my watt meter, that shows watts drawn, batt voltage, and current expended, and then see what the battery voltage is at various discharge points when I run the packs I have (800 and 850mAH 3S LiPo's).  That will tell me what voltage to use when calculating:
(Voltage)(KV) = Motor RPM

His esc calculations I agree with.  For the battery, I don't know how he got the 0.6 or 0.7 watt-hours per flight. First he says you'll use 0.6 or 0.7 watt hours per flight, then says you'll need 14 watt hours for a 20 oz plane.  I assume he meant 0.7 watt-hours per ounce, which works out to 14 watt hours for a 20 oz plane.  But earlier, he said you need 11 watts per ounce.  I don't see how he correlates 11 watts/oz to 0.7 watt-hours per ounce.  So, I'll ignore that part.

The 11 watts per ounce number I've seen a couple of times reading about e-C/L planes.   The thumb rule the electric R/C guys use is 100 watts/lb for a sport model, or 6.35 watts/oz.  So, I guess 11 watts/oz is good for factoring in line drag.

The motors that I got are 140 watt motors, one 1600KV and one 1200KV.  Using the general thumbrule of 11 watts/oz, I can run up to a 12.7 ounce plane.   I'll test both motors with different props to see what numbers I get.  But, the motor specs say both motors have 15A max efficiency, and 18A max current.  So, I'll use an 18A speed controller, and set the low voltage cutoff to 3.3V/cell and see what I get for a run time.  It's easier to do an actual run to see how much current I use vs. general thumbrules.

The specifications that I really would like definitve numbers for are:
1) What size prop for a given airplane weight?  I guess I can default to the sizes usually used for IC engines for a given airplane weight.
2) What is optimum prop speed for given prop sizes?  Prop manufacturers don't seem to list the optimal speeds for their props.

Hopefully I've not side-tracked this thread or confused you more. I just figured I share what I was thinking on 1/2A e-C/L.

The Calculifying Mark

nice restoration on cox fireball maybe they will one day become popular i think most guys like the scream of those. 049s  

batjac wrote:
What is optimum prop speed for given prop sizes?  Prop manufacturers don't seem to list the optimal speeds for their props.

The Calculifying Mark

APC lists LOTS of technical info for their props... Here is a link to "maximum" rpm recommendations for different applications... which isn't necessarily "optimal" speed.. so this may/may not help with this discussion... but I was curious about the APC 4.75 x 4's that I plan to use on my Speed Contest model. According to their chart.. this prop has a max rpm of 44,750.. I doubt I'll even come close to 1/2 of that.. although "Jim" (JPvelo) might!

See also; the left hand "site navigation" column under "Engineering" and also "Downloads" for performance data. The "Online Store" tab has a propeller list.

(EDIT).. The "Downloads" tab/Performance Data... should be helpful if you're considering an APC prop for your electric model.

http://www.apcprop.com/v/html/rpm_limits.html

Roddie

Mark
Don't worry about the thread. I started it and don't mind the rabbit trail, it's all part of the topic to me. The guy who wrote that up for me is an electrial engineer and flies RC and CL. He wrote that up for me to have somewhere to start. From most guys I talk to say that they figure CL a little different than RC. This was just a starting point. You'll have to tweek it a little. I used it and it worked on two planes.

To start figuring props, I was told two ways. One, look at the motor spec you are using. And two, figure on the IC motor, prop, and RPM's you would use on that plane with an IC motor, which is what I used.

The owner of my local hobby shop rattles this stuff off the top of his head and thought it was a good a starting point also.

I like using this to get a guide to performance

http://www.drivecalc.de/

I have all the others - Motocalc, E calc and Scorpion - plus a few that won't install properly - but this one is quite nice as a propeller to motor matching guide.

Motocalc is the most comprehensive but the tables do not have any of my preferred motors. If I enter the details of a model and get it to make suggestions - the motors aren't available in the UK.

DO NOT RELY ON THESE GUIDES> ALWAYS TEST WITH A WATT METER. I have fried three motors - yes smoke and flames through not following this advice.

The last motor I fried was on the Wizard. It had a 7 x 5 grey APC clone prop. Worked fine but a didn't like the colour so I fitted a black EMP 7 x 5. The motor burned out in seconds while I was trying to video the "Ian pull test". Unfortunately, I missed recording it - just got a blank wall!

Under prop the motor (slightly). You get into danger of letting the smoke out when you start pushing the limits of the motor.

OTOH if you go way under on the prop the motor is not operating in a range that has good efficiency, meaning a lot of power is used to run the motor compared to the power used to provide thrust, but it will be heavy and not at risk of burning up.

Phil

Definitely get one of the watt's up watt meters when playing with the props and such.  Also a good way to check load on motor and controller as both are broke in/checked. You can actually watch the resistance drop as they wear in.




Was very helpful when I built my Phase 3 Fantom EDF.  The instructions were vauge to say the least,,so some battery and load testing was in order.  Matched the Thunderbird speed controller to the maximum output the EDF motor could sustain or was reccommended based on it's Kv rating.  Added in 1 extra amp,,for some emergency "military" power if needed on a flash basis,,definitely not sustained for very long.  Filled and silked this foam model with some thin Habotai silk and Minwax water based polyurethane, also added some carbon fiber strips,,for a little more integrity here and there.  Came in under 1 oz for max reccommended flying weight.  It flew very nice with impressive vertical under "extra" power. The open air scoop behind the cockpit directs air right onto the speed controller heat sink down in the fuse under a cut out hatch held on with mini magnets. Back of model was opened to allow scooped air/heat to escape.