Please help an old sheetrocker understand Watt meter info!

[Rokcrln]

Hey guys I'm new back into the sport of flying. Waaaay back in the day it was a short period with gas planes out at the TooRock naval station and then several years with thermal gliders off of Sonoma mountain. But I got too into it and was spending way too much money (for what I was making at the time) and got out of the sport about the same time I got married! Well now jump forward 25+ years and I have some money to spend and a 9yr old of my own to do this with (helps justify with the wife )

So I'm wanting to learn about electric motors and prop selection and don't want to always ask what set up for what plane. So I picked up a hobby king watt meter but it is really lacking in directions. Can someone help point me in the right direction to learn more about what to look for and what it all means?

Here is a test I just did with GT 2215/09 1150kv motor, 30A ESC, Gens aco 3S1p 2200 25C battery.
Results,
12x3.8 SF prop (way to big) 35A, 360W
10x4.7 SF prop 25A, 295W
9x4.7 SF prop 17.9A, 212W

This is on my tweaked version of an Edge 540 and yesterday I maidened it on a 7-1/2min flight 1/2-3/4 throttle most of the time with a few full bursts. Motor and ESC felt warm but not hot and the battery still hade 11.2V after and this was with the 10x4.7 prop. This has an 810g fly weight.

So what does all this mean and for 3D flying is their a better set up for this plain? I was very happy with the results but also I don't know any different!

Thank you in advance for any help or pointers you might have.

 

[makattack]

Nice job! I think you're well on your way to figuring it out yourself!

As you surmised, a Watt meter is very useful for seeing if you go beyond the limits of your equipment. It sounds like now you're looking to perfect your setup, but I'm afraid the Watt meter is just one part of the equation. The rest of the variables have to be measured by any number of ways.

As I see it (and I'm probably over simplifying things), the other variables are weight, thrust, and flight time. All towards either optimizing efficiency and power, towards an extreme or finding a balance in between.

Some things are easy to measure, like weight, and flight time. Thrust can be measured with some equipment (thrust stands) or just sort of by "eye balling" it with seat of the pants flying, or carefully holding the model down and running it up while it's in your hands to see how strong the thrust is.

I'm sure there are many other variables, but someone can go down a seriously deep hole of trying to measure and tweak everything. I personally use my Watt meter to get in the ball park, and adjust if needed based on how it feels in the air.

As you're someone with prior service (thanks for serving!) I see it as analogous to this:

I was Army, but I know the Navy also used the M2 .50 Cal. machine gun. If you ever had to field strip the weapon, you would know that before you can function check it, you needed to check headspace and timing with the go/no-go gauge. That's what the watt meter is to me, but there are folks out there who swear they can put a M2 back without a gauge and know it by "feel" -- I personally never liked firing a weapon they put back together!

 

[Rokcrln]

Thanks for the response, yes I'm trying to dial it in but not got to the tenth degree with it. The information I supplied was just what the watt meter told me but I'm not totally clear what I'm seeing.
Sorry to miss lead but we just flew at the navel base I was 14 or 15 at the time but thank you for your service and I still get the analogy!

 

[Hai-Lee]

By the sounds of your setup you have an excess thrust to weight ration which is required for 3D flying though you do not quote any thrust figures with your setup.

For my 3D bird I have 2 modes setup on my Tx and they are extreme sport and 3D. Depending upon my mood I can with the change of a prop fly a sport pattern or a full on 3D depending upon the prop selected.

If you want to measure the static thrust of your setup just check the forum for posts on how to make your own test bed.

have fun!

 

[makattack]

Ah, I think I understand, you're wondering how those values relate, esp. as you go down in prop size?

So, the watt meter measures power, which is measured in Watts, and is a relationship between the voltage provided and the current drawn. You have a fairly constant voltage with the 3S (three cell in series) battery of, let's say an average of 11 volts.

Your watt meter measures both the voltage supplied and the current drawn by your load (the motor and prop). The final value is the two numbers (V=volt, I=current) multiplied together (W=V*I).

As you increase the prop size, the same motor will have to work harder to spin the larger prop through the air. Since your voltage is constant (mostly) it pulls more current (measured in amps).

You know you don't want to exceed the ESC's and motors rated amp/current limits, so that's what the watt meter can tell you.

Hope that helps!

 

[JimCR120]

One thing I do is try to have my LiPo C rating always meet or exceed that of the ESC. If the demand for power isn't there then it doesn't matter but if you set yourself up needing more power, then the ESC could be drawing more than the LiPo is equipped to give resulting in hot and puffy LiPo's or worse (when the magic smoke that make them work leaks out).

A watt meter should take the guess work out so there are no smokey surprises.