When is bigger NOT better?

[srfnmnk]

I already learned the hard way that you can't stick a park 480 on an FT Nutball...but...what about when the motors have the same dimensions and weight? The only reason I could come up with is because if you are going to fly a small prop and low speeds, (i.e. you don't need the power) you're probably missing the efficiency curve...but the up-side seems to outweigh that because when you need the extra power or you're ready to put that bigger prop on there or switch to a bigger airplane, you already have your equipment.

All that said, please help me understand which motor you think I should get. I am building several of the swappables and have been using the 2208/14 and it's working just fine. It may be a little underpowered for an 8x6 prop, but not quite sure. I accidentally bent a shaft and am needing to replace a motor...now that I've learned a little more about motors, props and power...I'm wondering why I wouldn't just go with a more powerful motor like this one...exact same footprint...just higher output (watts).

I like to fly slow at first with an 8x4.5 for a bit but then I think I would like to go to an 9x5 or even maybe a 9x6 (prob too big) to get the higher airspeeds. So do you agree that the stronger motor is right for me in this case?

Thanks for your help

 

[nerdnic]

I think you're going to get many different opinions about 'what is too big' but ultimately you want a motor that will provide the right amount of power. What is the right among of power? If you still have fly able flight characteristics then I'd say you're within the right range.

The small 3 planes need a BW style motor but the other swappables can handle a lot of muscle. I wouldn't get the motor you linked as the kv is pretty high so you'd need a small prop. Stick to the 8x6(1450kv) 9x6(12-1300kv) range for good overall performance.

Altitude hobbies has a great 70g 1400kv motor (same as their spitfire package) that will work well with all the normal sized swappables.

 

[srfnmnk]

Thanks. Do the normal sized swappables include everything starting in the intermediate range? The nutball, delta, and flyer being the three small ones? If so, then I'll just get one more of the smaller motors and hold off until I decide which one to do next. I am thinking about trying to use the park 480 i have laying around on a blunt nose versa with fpv and a 2200-3000 mah battery up front. That motor is pretty heavy though.

 

[nerdnic]

That's correct. The three small are the nutball, flyer and delta.

A park 480 will work on a versa, it's not too heavy but it will give you a lot more power than you're currently used to, so hold on tight

 

[Techno]

Sorry that this is irrelevant, but fires aren't better when they're bigger.
But yeah, 1400KV and 8x6 sounds right. I'm running 8x4 on my tri. I should get bigger props...

 

[Balu]

Sorry that this is irrelevant, but fires aren't better when they're bigger.

That depends on your point of view. =)

 

[FAI-F1D]

Sorry that this is irrelevant, but fires aren't better when they're bigger.

Nope...bigger is better with fires and all things that go boom.

Mmm...fire...

Ok, back on topic. One of the things to consider is that a lot of those motors, like the Suppo's, for example, have a rather boring thrust to weight ratio. Your motor limit is really going to come from three things: weight, battery capacity, and the stability limits of the plane. A GWS 2205/15 weighs considerably less than that thing and will produce considerably more thrust for the same power draw. Same for an Emax 2805. Or you could look at the HiModel 2208/12, which is the same size and cost, but produces enough thrust to drive your plane out of control.

Bottom line is that there's a lot of research to be done in selecting a good motor. There's also the issue of what you really want to do. Right now, I want something that will make my 2M sailplane go straight up, so I'm going after some hot motors, but on the other hand, the little pusher I'm working on to let my wife goof around is going to get a very small motor so she won't lose control of it and will have long flight potential. And that's why I'd suggest not buying Suppo motors, because you have to use more power to get the same thrust.

 

[srfnmnk]

Great add FAI, thank you. What do you use to determine thrust output capabilities and power consumption? Watts?

 

[FAI-F1D]

What do you use to determine thrust output capabilities and power consumption? Watts?

There are a couple sites out there with worthwhile data, like flybrushless.com, and bhabbott.net.nz, where you can get some independent test data. Ecalc has good stuff as well, but is practically worthless to me since they don't feature any of the motors that interest me. flybrushless also has a prop calculator (which naturally only features a handful of folding props) which you can use to get thrust data if you find someone who has presented rpm data for a given setup but not measured thrust (also useful if you have a tach but not a reliable thrust measurement device). I believe there's a program called propcalc which provides some of this information as well.

I don't like looking at watts other than for efficiency concerns because in my line, watts is worthless data. Watts only measures what you put in. Thrust and pitch speed is a measure of what you get out (and there's a simple equation for that item). Personally all I care about at the end of the day for anything other than my sailplanes is thrust and pitch speed. By the way, if anyone knows of a setup that can drag a 150 g, 36" span model up at 3600 ft/min on a small 2s, I'd really like to have information on it.

Your concern would likely be a multivariate measure weighting the thrust, power consumption, and motor weight. Don't forget that the more watts you draw, the heavier your esc is going to be, too.

It's a long list of tradeoffs. As soon as you start getting addicted to climb performance, it all becomes voodoo because there are so many factors that have to weigh in, and hotliner folks just walk around it by adding more voltage. Pretty worthless if the contest rules say "no more than two cells in series).