motors for GWS P-38 ?

[john haz]

Hi Peeps,

 

Just found this forum after a little search around for info on the GWS lightning.

 

The natives seem friendly & helpful, so i was wondering if anybody might offer opinions on a good motor setup for my (soon to arrive) P-38 ?

 

Favourite supplier for all my heli stuff has been GC for quite a while & ideally i would like to grab a couple of motors from them (I have two matched 25A esc's) 

 

All ideas welcomed, but I *think* these might be ok ???

http://www.giantcod.co.uk/a220817-1100kv-outrunner-brushless-motor-p-402708.html 

 

 

Much appreciate any help (& if I have posted in the wrong section, please tell me (off) )

 

  

[Pete B]

Hi John,

Welcome from me! I had the E-Flite Lightning which is about the same size as the GWS and mine was vastly over-powered on these motors. With your header, I would imagine Gemma Jane will be along in due course - she did a great build of a GWS P-38 over on RC Groups. She'll put you right on the motors.

Loved my P-38 ( the twin motors sounded great) but it went in hard twice and is still in the bag after the second..........one of my earlier threads documents the first rebuild.

Good luck!

 

Pete

[john haz]

Hi Pete,

 

Thanks for the quick reply.   Your motors appear to be double the max of the ones I thought of, so maybe mine are not quite up to it !

 

I have read your build and GJ's, lots of pix missing now which is a real shame (I have to build mine soon & not placing big bets on the manual being much use).

 

All such a steep learning curve & so hard to get "cast in stone" answers with so many variables.

 

 Being quite new to planes, I don't want this one to be too overpowered, just something that will cruise comfortably at 50-75% throttle & have a bit in resefve for those dumb moments.

 

Looking forward to using this forum a lot  - !

 

thanks again,

 

john 

[Gemma Jane]

I'm not sure I would recommend the 18T bell motors I put in my 'beast' GWS 38! They develop four times the power recommended lol.

 

It is fun, but the trouble is if it's a belly lander it's really easy to bend the motor spindles as they are not properly hardened on these motors. Not too bad to replace as spares are available but it's a bit annoying as spindle replacement is a workshop job. 

 

For reference the RC groups build thread which has the piccies intact is at:

 

http://www.rcgroups.com/forums/showthread.php?t=1083990&highlight=gws+38 

 

I can't help posting a piccy of Porky II at her best

 

 

I'm sure Tim will be along soon with a motor suggestion for you and I'll be looking forward to your build thread 

[Tim Mackey]

Assuming your within the AUW of 25oz or so, then 150 -200 watts total should be fine.

The motors you linked to are 1000kv, so on a 3s Lipo, and say 9 x 5 or 9 x 6 props, should pull around 8+ Amps each so thats about 80 Watts.

If you want to keep the weight down I dare say these on the larger prop would likely work on 2s packs. The current limit is stated as only 7A maximum though, so you may be running a tad close to the magic smoke releasing.

This will take higher current and on the smaller prop should provide more than enough power on 3s.

Edited By Tim Mackey - Administrator on 26/07/2010 22:09:04

[john haz]

Gemma,

That looks great, have you still got it ?  I have bought the silver GWS kit, just liked the idea of the silver with a few decals on it.   I'm not a "must look 100% scale" flyer, maybe because i don't know so much, I am happy with anything that is easy on the eye

 

Shame i couldn't get the pix of your build, but I have read everything I cna find  

 

Tim:  Now you've opened a can of worms LOL.     All the motors I looked at were 100kv (ish) because I assumed that's what it had to be.   The ones you suggest are 1800.  But i like the idea of smaller props for more ground clearance on landing

 

I would prefer to run it on 2200 3s batteries as I have a dozen or so good ones for my 450 heli's, other "in stock" options are 3s 1300 / 2s 1300 / 2s 1000, so if i can use any of these it makes sense.

When you said "the smaller props" did you mean the 9x5 you mentioned, or something even smaller ?        I'm completely lost with prop / motor calculations, although watts & amps don't confuse me too much - if only this was as easy as heli's !!!

 

If it helps somebody to help me    Ideally i would like it to have reasonable power to hand launch but I don't need to tear around the sky.     Ami right in thinking a larger pitch prop will give me better "off the mark" acceleration from a hand launch & the larger prop would need to be nearer 1000kv ??

 

Thanks all for the help so far, I'm slightly wiser than i was yesterday  

  

[Tim Mackey]

John. If you want more thrust ( for hand launching and "climb" ) then you need diameter NOT pitch. Smaller higher pitched props will give you more top end speed as it were, but little actual "pull" from a standing start.

Think of it like top gear in car - whereas the larger diameter prop is like 1st gear.

 

 I suggested the higher kv motors because 

1) I thought you might want small diameter props

2) They can carry larger current  than the rather weedy 7A ones you mentioned.

 

This is why it is so important to give as much information as possible about just what it is you want from the powertrain.  As a minimum, we really like to know.....

 

a) AUW of the model ( estimate around 25% of bare airframe weight for powertrain )

b) preferred prop size

c) flying style of model ( fast and zippy, glider, or slow and scale moderate speed etc )

d) likely battery that will be used ( cell count or voltage )

 

PS have you read this article - it may hopefully help to demystify things a little?

Edited By Tim Mackey - Administrator on 27/07/2010 09:05:05

[john haz]

Hi TIm,

 

Ah! so I can't have both (fast climb and small props), that makes sense now.     In answer to the questions you asked, .....

 

a)  It is shown as a 720gram flying weight, but I guess it may be a bit heavier.

b) prefer smaller props for ground clearance / but want to hand launch  (difficult compromise i guess)

c) I am fairly new to planes & wanted something to fly scale / steady / slow.  I can zip around the sky pretty well in my little cessna, that doesn't bother me. 

d) Either 2200 3s or 1300 3s   I have plenty of both

 

If it has to be overpowered to give me an easy hand launch, that's fine, I can learn to land faster & always go around if need ne, launching only gives me one shot !!

 

artticle:  can't read that yet as it's subscription only (just off now to see what I need to do)

 

TO be honest, if you (or anybody here) said:  buy these motors, run this lipo, do xyz, I would do it because you all know bettr than me

thanks again for all the advice - ? MUCH appreciated

[Tim Mackey]

1)  25 oz in old money then. So lets play safe and reckon on 30 oz ....almost 2lb.

2) Whats the maximum diameter you could get away with dya reckon?

3) Around 80 watts per pound should do you then ( approx 80 watts each then )

4) I think the 2200 may be best for duration - and they will hold better voltage.

 

You need to subscribe

 

Ah but then you wouldnt learn anything

So, we are probably looking at 8" props maximum,  and each motor needs to handle around 80 - 100 watts maximum. On 3s lightly loaded  lipo ( 11V ) thats only approx 8A, so to help keep things light, 10A ESCs should just about be OK ( one for each motor required )

How about these budget motors? ( the 1050 Kv version )

[john haz]

1) makes sense

2) 8" would be fine

3) makes sense

4) my preferred choice

 

Subscribe - yep will run that one past the credit card

 

Learn -   trust me Tim, I'm not the sort that will leach information & accept everything I am told (for very long).  I DO like to understand but am having trouble with the motor calcs.    Heli's seemed so easy, I bought one RTF & then started building them from kits, read thousands of posts, watched video's & asked a few friends.

 

I know newbies can get a bit tiresome asking the same questions as the last guy, but I did search the forums, read GJ's posts etc, it's just these motor loads that bug me.   Tried some online calculators but they need information I don't understand yet.

 

Looking this morning, I saw those Keda motors & wondered, but couldn't get my head around if they were enough or too much.   With the keda being 170w & the others I looked at being 7A, there's no direct comparison  .......... or is it as simple at 170w / 11.1v =  15.3A ??

 

Looking forward to the kit arriving this week, I noticed they upped the price today from £57 to £67, i bought just right I think !

 

BTW  - of all the forums I have looked at recently, this seems to be where the members seem most tolerant, lookslike you're stuck with another resident newbie  

 

 

 

[Tim Mackey]

We do like to think we are the most "tolerant" John - I once nicknamed us "the friendly forum" some years back - and its stuck.

Glad to have you onboard, and ask away mate.

[john haz]

Well, i took your advice & went for the keda motors, the others can be for another day project

 

Can't wait to start building the P38, should be a bit of fun & no doubt i will have questions for you and Gemma  

[Tim Mackey]

I cant help with specific Q's about the model John - I don't own one.

I would suggest that if you dont already own one - you purchase a wattmeter as this is the best way to match your props to any given motor. Several things can affect the current draw even with supposedly same props - different makes, "E" versions etc etc can all produce different results, so to be sure you are not overdoing things.... meter it all up before commiting to aviation

There is a thread here about using the wattmeter etc.

[john haz]

Cheers Tim.

 

I do own a wattmeter (Emeter) & had a twiddle with it tonight on my mustang, just to get a feel for the results etc.  Different props do make a big difference don't they !!

 

Metering it &  reading the results i think are pretty simple now (after your help), the only issues I have now are if a given prop will create enough thrust to get it airborne quickly (hand launch).  Something I need to delve into more to understand......  or am I missing something here ?

 

 

 

[Tim Mackey]

Static thrust testing can help here - either using a scale or simply and very crudely just holding the model aloft and feeling how it pulls

[john haz]

Ah!  Light dawns in my empty little head.

 

Would this work ok ?? 

 

meter the setup to ensure i'm within the limits of the motor / esc / lipo's

 

and then use something like a lightweight spring scale to measure thrust ?    If my model weighed in at 600g, would i expect to see 400g of thrust to get reasonable power (plucking figures out of the air from memory here, but trying to find a way to do this without pestering the forum every time I get a new plane).

 

I have a feeling I read about using a scale once before, something I had forgotten until now.

 

Assuming this works  - would it be best to hang the model or run it on a smooth horizontal surface.

 

I suppose after this, the choice of prop is narrowed to pitch vs width for speed or torque ?

 

Am I barking up the wrong tree ?? 

Edited By john hazeltine on 28/07/2010 19:48:26

[Tim Mackey]

Yeh sounds about do-able. Best if the model has no friction to overcome - there were some setups published a while back - try a google for ideas.

Pitch = speed

Diameter = thrust

Yes.

One can get a bit more involved with things like pitch speed calculations and stall speed etc, but generally speaking your up the right tree

[Gemma Jane]

Hi John, 

 

Porky II was a sort of test bed model, lots or experimenting with finishing foam, twin engines etc. It does still fly but looks pretty awful now! Truth of it is, the model is too heavy. I knew it was getting that way during the build and it just proved that heavy models crash more. It is fast though!

 

I'm interested in your build as I've also got a silver GWS 38 kit. I'll be following your example to make one that is just fun to fly.

[john haz]

Shame your P38 ended up so heavy Gemma, it certainly looked the part.   

 

No idea when my build might start, I ordered it monday & it's still not arrived yet  -

 

It's the first time i have used Avicraft & they don't seem the quickest !!    4 days from payment to dispatch

 

 

26/07/2010	Paid - Processing
26/07/2010	Paid - Processing
30/07/2010	Dispatched

 

 

Edited By john hazeltine on 31/07/2010 06:40:49

[john haz]

i got my kit from  Avicraft last saturday & although it took them 3 days to depatch it, I got it 24 hours later, so 4 days total as shown on their site.  

 

All put together, not a brilliant job at all but it's my first & I will do the next one a lot better. just gotta hope she flies ! 

 

[Gemma Jane]

Best of luck with it John.

[john haz]

Thanks Gemma, probably need it