SAEairplane

Hi!

I'm looking to get some advice to size a propeller. I'm using the Turnigy SK3-3542-800kv Brushless Outrunner Motor, and am unsure whice size propeller to choose. Any help is greatly appreciated!

Thanks!

FallDownGoBoom

My Feedback: (1)

Very difficult question to answer without more information: what battery will you run, ESC specs, what aircraft, how much weight, how much performance is desired? Hobbycity says this is a 625W motor, with 4 or 5 cells (meaning 14.8-18.5V) input. If you go with a high voltage battery, you may be able to spin a small prop faster to achieve necessary thrust. You might want to visit the Wattflyer forums and ask over there... lot's of good experience and advice available.

Hobbycity posted some numbers with folks running 11 or 12" props. Depending on your set up, you might end up running anything from an 11" all the way up to a 14" prop. I'd probably start with something like a 12x8 and see what kind of performance you get. You also need to use a wattmeter to check the amperage draw to make sure it stays within specs for both the motor and the speed control.

Hope this helps you get started.

Gray Beard

My Feedback: (-1)

Just went through this yesterday with my electric Extra. The only way to do it is with a Watt Meter to make sure you aren't doing harm to your ESC and motor. There may be a good chart as Boom mentioned but as of yet I haven't seen one. Prop selection on electric motors can be a pain!! None of the recommended props I bought worked on my motor and ESC correctly, seems I had the best one on the motor when I started testing. Some motors can have a lot better selection then mine too.

SAEairplane

The battery to be used is a 2200 mAh, 3-cell 11.1V lipo. The airplane is being designed for 1-4lb, with a wingspan of around 48 inches. It is being designed for the Micro Class, and speed is not a factor (it can go fast, slow, in between!)

Thanks for the other forum suggestion, I'll give that a try too!

FallDownGoBoom

My Feedback: (1)

My initial reaction is that's waaaaay more motor than you need for the aircraft described, from both a power and weight point of view. The motor is spec'd for either a 4 or 5 cell LiPo, drawing somewhere between 30 and 40 amps. If you use a 3cell, theoretically you could draw around 56A, higher than the 42A motor rating. That's not to say I entirely trust the published specifications; but my gut feel is your SK motor is about the equivalent of a .32 sized glow motor.

I'm not sure what "Micro Class" means, but if the airplane does come out around 4lbs, then this motor might work, although I'd be looking at a 4S battery set up. If the airframe comes out around 2lbs, then your power-to-weight ratio would tend towards insanity, probably something like 250W/lb. Plus you'll have lot's of weight sitting up in the front of the plane, so you'll have to deal with getting the CG correct.

I'd think a more appropriate motor would be something in the Power 10/Power 15 class. For example, I'm flying a 32 0z miniShowtime with an e-Flite Power 10 on a 11.1V-2200 battery which take it vertical as far as I want it to go. And I've got a 55" T-28 Trojan that I flew for the first time last weekend. It weighs about 4.5-5lbs, and I powered it with a Rocket MC304-800 motor that has similar specs to your SK3-3542. I flew it on a 5S/5000mah battery I had sitting around, and it felt like it had gobs of power. I put a wattmeter on it last night and found out why... at half-throttle it was pulling about 25amps which was just fine. At full power I was drawing close to 60 amps, and showing 1200watts of power. Way more than than the motor is spec'd for (or the airplane needs), and why I'm looking for a 4S battery today!

scooterinvegas

My Feedback: (1)

Threads like this make me want to pull my hair out. Your ALL over the map with your specs. 1lb is VERY different from 4lb. The two would take completely different motor/battery combos. You need to come back with better numbers so you can get the correct help.

The motor you picked says right on the web page, "Voltage: 4~5S Lipoly". This is not an 3 cell motor, the Kv is too low. For a 3 cell, you need to stick to something in the 1050-1250 Kv range. You need about 100w per pound of weight for an average flying plane. Remember, the watt per pound is "propped" watt, NOT motor watt. You can have a 150W motor propped to only pull 80 or 100 watts. So lets say you want 300w for a 3lb plane, you wouldn't buy a 300w motor because you would be running the motor maxed out. This means a HOT motor. You want to prop a motor to pull a MIN of 10% under, but preferably 20-25% under. The motor will last longer that way. ESC are the same way. If your going to pull a load of, say 20A max, you want a ESC that is a MIN of 10% over. But again, preferably 20-25% over.

So if your plane is 1lb, your going to want a motor to be in the 125-150 watt range (propped @ 100w).
If your plane is 4lb, your going to be looking around 500w-600w range (propped @ 400w).

Remember, 100w propped, is ballpark range for an avrage flying sport plane. If you want a hot rod, or 3D type power, something closer to 150-175w per pound.

What ever the max AMP is for the motor you pick, get an ESC that is 20% bigger. That way your ESC will run cooler and last longer.

Got it? Good ...

Gray Beard

My Feedback: (-1)

Or go to the wattflyer forum and do some reading. Scooter gave a lot of the basics but if you just go to the bottom of the header bar and click on the wattflyer you will go to a site that is nothing but electric. They have all of this stuff posted on a sticky but you will need to do some reading all by yourself. If you can't figure it out then you should go into the beginner forum and ask questions. I had a number of questions myself for a couple of new planes I'm building but my WF pass word isn't working so I just used the basics Scooter posted. It works pretty well.

Jetdesign

My Feedback: (8)

what I would do: go to castlecreations.com and use their flight calculator.

the motor you are looking at is very similar to Hacker A30-14L (same diameter, same kV) - the hacker motor is longer and much more robust, so it can handle a lot more power. so the temperature ratings you get will be too conservative. but, look at different props with the batteries you want to use, and it will tell you how much power is going to the motor. keep that under the limits and you should be fine.

the comment about that motor needing 5-6 cells is not accurate. that is what is suggested by Hobby King, but it is very common to use a lower kV motor with a bigger prop for different performance gains.

my guess is, with 2 posts on RCU, the OP is probably in a class or working on a project to design a plane, and the goal is a combination of flight performance and payload, which is why the range of 1-4lbs.

Start with a 3S battery and a 14x8 prop, or if you want more thrust and less speed, a 15x6.
http://www.aero-model.com/Hacker-Brushless-A30-14L.aspx

SAEairplane

You're correct; we are given all of the electronics and need to design a plane around it and get it to fly straight! No turning, just building the plane.

Thanks for your help! Everyone's comments have driven us in the direction we need to go!