bobbart

What size Electric motor would I need to run a 6 x 7 prop with four blades? I 'm building a C-130 with four motors, I don't want use
brushless motors..I thinking a 470 can do the job?
Thanks
Bob

jakjr

My Feedback: (18)

I dont think its so much of a question of "what" motor, its more a question of how much thrust you need.

One you figure that out you can find cross reference things or even calculater programs that will get you going in the right direction.

bobbart

Can you please help me figure out out the trust I would need so I can purchase the correct size motors? Just tell me what
information you need from me.
Thanks
Bob

jakjr

My Feedback: (18)

Somone else will have to help you with that as I have no clue.

However, I beleive you first need to figure out the airplanes total weight then take it from their.

I think the amount of thrust you need has to be equal to or more than the airplanes total weight, but like I said I really have no clue.

To find out how must thrust youll get from a cerain configuration you can use a online calculator, I dont have the link for it but I think its call ezcalc or pcalc or something lke that. Itll ask you for a number of things, battery type, niumber of cells, what motor, what prop, etc.

Ive tried it before but I didnt know what to put on some of the questions so I dont think I got accurate results from it.

smokingcrater

it doesn't even have to be close, unless you plan on hovering the thing! a general rule of thumb is to calculate watts/pound that you need. (I don't remember what is recommended, maybe someone more experienced in eflight can help 'ya...) anyway, once you have watts needed, you work backwards from there, figuring out how much each motor will contribute to that budget with a given prop.

Matt Kirsch

My Feedback: (21)

Where did you get "6x7" four-blade props? Being "over square," that is, having a higher pitch than diameter, they're going to be horribly inefficient if my understanding of propellers is correct.

Let's take a step back here. Right now you don't really understand what you need. You're just asking random questions, getting even more confused, and getting further and further from your goal. I understand that you want four-bladed props on your C130 so it looks real, but as I've said many times in the past, it may not be practical.

First, the size of the motor does not matter. What matters is how much power you put to the propellers, and that power comes from the battery. EAM specs out a 7-cell battery for this plane, and 280 motors draw about four Amps each in typical applications. A little quick math here: 7 cells * 1.2 Volts/cell * 4 Amps/motor * 4 Motors = 134 Watts. That's a little anemic for a 2-1/2 pound airplane, only about 55 Watts per pound.

Let's shoot for 75 Watts per pound, or a total of around 185 Watts. Split that into four, and we get around 47 Watts per motor. Notice that there's a little "fuzzy math" going on, estimation. Don't worry about it, as long as you round up. We could run 7 cells as recommended, but 8 cells would be better. Adding cells is like adding cubic inches to an engine, boring out the cylinders

47 Watts on 8 cells is about 5 Amps. That's a little much for the stock 280 motors, but well within the capacity of a 300-class (i.e. Speed 300, "370," or GWS EPS300C/EPS350C) motor. You want to run a 6" diameter 4-blade prop, which translates into an 8" diameter 2-blade prop. It'll have to be geared. A 300-class motor would draw about two zillion Amps trying to turn a prop any larger than about 3" on 8 cells... It doesn't have the torque.

GWS gearboxes are ideal. They're inexpensive, compact, and easy to mount. GWS also provides a set of application charts for each gearbox, utilizing several props and batteries. If we could convince you that your best bet is 2-bladed props, a GWS EPS350C-2S with a 6x3 prop would draw 5.9 Amps on 8 cells, and give you well over 200 Watts to fly the airplane. Extrapolating, from a 6" 4-blade prop to its 8" 2-blade equivalent, it appears that an EPS350C-CS with an 8x4 prop would draw a similar amount of current. If you can find a 6x4 4-bladed prop, you will be right in the ballpark.

bobbart

Matt, below is the link, this is the propeller I purchase ( four of them) I cut the propeller down to 6 inch's in dia.
The propellers are now to scale , all four propellers are cut down to 6'', they really look great for the C-130......... will this work?
As of now I did not purchase any motors, waiting to hear from you guys first before I make the purchase.
Thanks
Bob

http://www.gws-online.com/products.p...d=EP1080-X4-2P

Matt Kirsch

My Feedback: (21)

Ah... Like I said, the 7" pitch isn't going to be very efficient. They might work okay, or they might be completely stalled and provide absolutely no thrust.

So let's look at the tables and see what combo will spin an 8x7 2-blade prop on 8 cells, and see if we can't get down around 5 to 6 Amps...

Looks like GWS doesn't have an 8x7. Closest they come is an 8x6 or a 9x5. A GWS EPS350C-DS drive on 8 cells will draw around 6 Amps on 8 cells with a 9x5 prop. Using our rules of thumb, that's a 7x5 4-blader. Knock an inch off the diameter and add a couple to the pitch, and you should come out about even. In theory it will still be well within the capacity of the motor.