Thrust and motors

[CaptBill]

I'm doing some bench testing with the 4 motors and 5 size/pitch props I have on hand. It's very interesting to see the change in thrust, with the same motor, battery and throttle position. Using a home made thrust meter build over a postal scale. And recharging the 2200 3s battery between each motor swap.
So far I have run 2 of the 4 motors with the props I have. I'll report my results when I have completed the other 2 motors.

At this time I'm not able to check actual rpm. Just working off of 25, 50, 75 and 100% of the throttle stick throw. Actual rpm and amp draw would be useful but I don't have the tools to measure these, and unless I really fall down the rabbit hole with this hobby I probably won't really ever need to have that ability.

It brings up a question.

How much thrust in necessary to fly a given plane? Obviously this isn't an easy question because of all the variables but I'm wondering if there is a sort of baseline that would provide a safe starting point.

For example lets consider a Tiny Trainer at AH E of 300 g.

Would a motor producing 200 grams of thrust be parquet to take off from the ground and fly? How about 100 g of thrust. Thinking of full size planes it's only been a recipient development that plane power exceeds weight so I'm guessing that, in the case of the 300 g. TT anything above 300 g of thrust is overkill, and considering the wing style kill might be just the right word.

I know many keep talking about watts but, to me, watts is still to abstract. How much pushing or pulling power is being actually produced seems much more usefully.

Thanks

 

[ViperTech]

Wow that is a tough question to me! I have seen the " 100 watt to 1 pound" ratio just about everywhere and it is hard to wrap my head around this. I know that 100 watts is .135 ish mechanical horse power but that doesn't really help one darn bit! There are motor that are rated in watt and if you know the models weight you could roughly calculate the most effective motor based on watts. That still doesn't answer the given question "How much thrust is needed?". I don't know!
I just build a plane, be it my own design or some one else's and use the recommended size or large motor, if I design it, I take the size of that plane and by comparing it to like size planes, that gives me a jump off point for a motor. BUT here is where TLAR ( that looks about right) kicks in for me I put on the motor and prop and just hold the plane vertical and throttle up if it will pull the plane's weight up out my hand then she has enough power to fly.
Scientific, NO! Works effectively, YES

I do however have a watt/amp meter and check to be sure I'm not over loading my motors, I hate to blow a motor!
Basic watt meters are not too bad in price. https://hobbyking.com/en_us/hobbyking-hk-010-wattmeter-voltage-analyzer.html

I am interested in what your results are. I am currently working on a balsa plane that was given to me over the summer and I have a dilemma as I don't want to buy a new motor for the plane as I have a large enough motor for it but ground clearance is an issue. I am interested in how much prop pitch affects your thrust readings. My current motor will support 12x6 prop on 3s battery but that prop will hit the ground. I found a 10x10 prop but will have to check the amps it draws when it gets here.

 

[Hai-Lee]

I applaud your idea and your work. The results will be relevant to the motors you have chosen.

As you will know by now "Props ain't props". Unfortunately without a tachometer or wattmeter you will not be able to determine what loading is applied to the motors during the test except for the thrust generated and measured.

Some props load a prop at lower revs and others load the motor at high revs. A mismatch can be inefficient situation where either the motor is lightly loaded or even overloaded, (under or over prop'd).

As for the "Watts per Pound mentioned", it should be considered as the actual power used rather than the rating of the motor unless you have a proper propeller matched to the motor.

From your tests you should be able to determine which prop loads or matches the motor best when the thrust follows a nice increase curve with throttle setting and just a minor plateau at maximum. Under prop'd motors will show a very linear thrust curve though the thrust will be a little weak. Over prop'd motors will produce a curve which shows less thrust increase as the throttle is advanced to max and even possibly a true plateau at much approaching 75% throttle or more. (Plus the motor running temperature will be far higher. A.K.A. the motor will be running very hot).

With all that said you also need to understand that your bench thrust tests will only be applicable for slow speed performance. Some props are specifically designed to be most efficient, (develop highest thrust) when already travelling at speed through the air.

Looking forward to view your results and even your test setup if possible.

 

[CaptBill]

I completed testing 3 of my 4 motors then messed up my back. Have needed to take break for a bit.

I did find something that may seem obvious to some of you but that I had not thought about.
While testing a 2412 2000 motor I strapped on a 9x 4 prop. By the time I was up to 3/4 throttle its sound and the flexing off my test rid, and possibly the motors x mount itself was making me question my own safety in pushing it to 100% . But I did briefly. More on that in another post.
Later I was reflecting on the experience and decided to try to figure the speed of the prop tip movements. I have my math downstairs which I will post later for review. But assuming that the motor could ONLY reach 60% of its rating while under load my calls indicate that the tips of that $1, mass produced, sharp edges toy prop was moving well in excess of 200mph. Food for future caution while "playing" With model planes.

 

[CaptBill]

I ran my 4 motors With a verify of props and got some interesting results from one motor to the next. All in all I compiled a list of 20 differing thrust measurements. Then due to my noticing a sloping prop I went back and did some interesting. My defeats revealed that my home made thrust stand and my test parameters were to lax to develops enough accurate results to hope for anything meaningfull.
Therefore I have decided to stop playing scientists and to devote by available time to building better planes.

 

[ViperTech]

Well CaptBill too bad on your results but hey you never know till you try! Nothing wrong with experimenting and you gained knowledge of what doesn't work, so you still learned something.