• This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn more.

Model RC math

MrGravey

Senior Member
#1
I need to learn some more of the math behind electic flying. Any information I need to better understand this hobby helps. I'm willing to read if you are willing to answer or link to some site with the information I need. There are a lot of numbers on batteries and motors on the internet. I don't understand them well enough to move forward yet. Warning, the following may be very stupid, be gentle, I ask because I don't know and I want to.

When I'm looking at a motor and it says it is a 250 watt motor what does that mean to me? I'm assuming I need to be sure my battery is able to put that many watts out without damage to the battery. From what I have gathered this means I need to be sure the battery has the needed C rating for discharge. Again I assume the C rating for discharge is calculated the same as the rating for charging. So a 500mah 3S battery with a 20c constant discharge rate puts out 111 watts ( (amps x discharge)volts = watts) based on the many assumptions I have made already ( :D ) the 250 watt motor I mentioned at the start would destroy this battery. Please let me know how far off these assumptions are. The lack of this knowledge is officially stunting my growth in the hobby at this point.

Is there a way to calculate the amount of time a given battery will run a given motor under a given amount of stress? The point here is to get a rough idea of how much time I can spend in the air. Every time I get a new plane I have to set everything up and run the battery for a short period of time and check the battery voltage. Doing has so far given me the information I need in time, but if I can narrow the time frame down a little more with math before I hit the flying field my time spent there would be much more productive.
 

Foam Addict

Squirrel member
#2
A 250 watt motor equals Amps x volts = Watts
a C rating on a battery is; C rating x (MAH/1000)= safe amp draw.
A prop pitch speed is; Kv x volts x pitch of prop / 12 / 5280 x 60 = Pitch speed in MPH.
I know a lot more, but I am on break from Calculus right now, so if you want to know more, just ask!
 
Last edited:

MrGravey

Senior Member
#3
A 250 watt motor equals Amps x volts = Watts
a C rating on a battery is; C rating x (MAH/1000)= safe amp draw.
A prop pitch speed is; Kv x volts x pitch of prop / 12 / 5280= Pitch speed in MPH.
I know a lot more, but I am on break from Calculus right now, so if you want to know more, just ask!
Thanks for the information. Looks like I had part of this right. Its always nice to not be completely crazy.

I am a bit lost on the pitch speed in MPH however. What is that really giving me. I'm running the numbers off my delta through that equation there and getting 1.541666... Maybe I'm doing that wrong somehow or else I'm misunderstanding what this pitch speed actually represents. Please expand on that for me. (2200Kv motor running on 3S 11.1v batter with a 6x4 prop if you need to recreate my math to see what I did wrong).

Thanks a ton for the information. This kind of stuff isn't easy to dig out and is very important for those building there own planes.
 

Foam Addict

Squirrel member
#5
I forgot, multiply that by 60 for mph, I gave you Miles per Minute!
Your pitch speed will be 92.5 MpH.
What motor are you running?
 
Last edited:

pgerts

Old age member
Mentor
#6
The KV is rarely what you get in actual rpm - it is more common that the actaul rpm is 50% than 100%.
2200KV *11 volts give 24000 rpm but more likely you will get about 15000 more or less with load.
There are actually motors with KV close to the actual rpm but KV is the rpm witout any load or friction at all to the motor axle.
There are true rpm meters you can put in fron of the propeller to get the actual value. There is also online rpm meters you can put on a plane and read on your transmitter during flight with telemetry.

15000 * 4 /12/5280 = ??? That does not seem right?

15000 rpm * 4 inch gives 5000 ft/min = 57 mph - more right?
Pitch speed is not the same as plane speed unless you have a plane with no friction to the air at all...
Other parameters are that the 11 volt battery give 12.6 volts fully charged and close to 9 volts when empty.

Other fun math is the wing loading.
1.1 oz. /sq. ft. is really light (Parkzone Vapor)
38 oz/sqft for a 15 lbs heavy P47 Thunderbolt
 

MrGravey

Senior Member
#7
I believe it is a 2212/6 2200Kv Suppo. I know the Kv rating and make are correct but off the top of my head the size might be a bit off. I picked it up at SEFF this year. Looking at the stats for the motor on altitudehobbies.com it looks like that is the right motor. I haven't run the plane with this motor yet, if I get lucky with the weather that run will happen Thursday. IMHO the FT Delta is the best of the swappables I have flown yet. The Bloody Wonder is next. I hope to have that ready for first flight Thursday as well.

Oh, that delta will be running on 3S 850 mah with 25c constant discharge rate. Looks to me like the battery should match up with this motor pretty well. Might not last long but it should fly for a little while. Any way to figure out what kind of time I should get out of this battery with this motor running on it?
 

MrGravey

Senior Member
#8
The KV is rarely what you get in actual rpm - it is more common that the actaul rpm is 50% than 100%.
2200KV *11 volts give 24000 rpm but more likely you will get about 15000 more or less with load.
There are actually motors with KV close to the actual rpm but KV is the rpm witout any load or friction at all to the motor axle.
There are true rpm meters you can put in fron of the propeller to get the actual value. There is also online rpm meters you can put on a plane and read on your transmitter during flight with telemetry.

15000 * 4 /12/5280 = ??? That does not seem right?

15000 rpm * 4 inch gives 5000 ft/min = 57 mph - more right?
Pitch speed is not the same as plane speed unless you have a plane with no friction to the air at all...
Other parameters are that the 11 volt battery give 12.6 volts fully charged and close to 9 volts when empty.

Other fun math is the wing loading.
1.1 oz. /sq. ft. is really light (Parkzone Vapor)
38 oz/sqft for a 15 lbs heavy P47 Thunderbolt
Wouldn't running a 3S battery to 9v pretty much kill it? Clearly I'm just learning a lot of the things the numbers really mean, but I was under the impression that running the battery bellow 3.7v per cell was a bad idea.

Any math out there to help you pick your prop? I see that the wattage changes with the prop so this could start to matter pretty quickly.
 

Foam Addict

Squirrel member
#9
I apologize. It is theory, not constant.
Reduce those numbers by about 50% on the ground and 70% for the air and they are more applicable.
 

pgerts

Old age member
Mentor
#10
Wouldn't running a 3S battery to 9v pretty much kill it? Clearly I'm just learning a lot of the things the numbers really mean, but I was under the impression that running the battery bellow 3.7v per cell was a bad idea.
.
3.7 volts are the nominal voltage for standard LiPo.
Never below 3.0 !! Never above 4.2
When you fly the speed controller shuts of the power to the motor in time and it is often possible to set the limits to 3.0 3.1 3.2 or 3.3 volts but when you measure on ground you will find a much higher voltage as the battery "recovers" as soon as you shut the draw.
It is important to disconnect the battery as soon as you have landed, not to drain the reamaining energy with your radio, servos or by taxi your plane. You will find that you might not put in more than 80% of the stated capacity (about 1800 mAh in a 2200 battery) after running to the ESC warns you by shutting of the motor.

Your flight time depends a lot on your flight style.
I am only getting hardly half the flying time when it is windy (20-30 mph wind) as i get on a calm day as i need a lot more speed to be safe.
 

MrGravey

Senior Member
#11
I am just trying to be careful here. I can run my batteries down to 3v per cell without damaging them? I was told to stop at or near the 3.7v per cell nominal voltage as going below that too often would cause damage to the pack. I can get a ton more flight time out of my packs if this is correct. It seems wrong to me, but a man can hope. (The voltage numbers mentioned are of coarse on the ground post flight)

I understand that you shouldn't run the mAh rating too low. The 80% mark you mentioned is the magic number I keep seeing. If there a way to check this with a standard volt meter? I don't have my watt meter to tell me that in the field right now so being able to do it with the tools I have would be great. What is the real risk of draining the mAh too low? How will I know I have done it? What are the signs that a battery has been damaged in this way? So many questions come to mind. Getting the answers seems to be a little difficult when just searching the internet. Please just throw the information you think I might need at my brain and I'll do what I can to make it stick.

Again, thanks guys. I'm learning so much more now that I'm sticking my neck out to ask. You guys have been great. You are keeping 4 guys flying with your answers and believe me, we are grateful!
 

pgerts

Old age member
Mentor
#12
Batteries are like many other things. "You get what you pay for".
The voltage is not the only thing to look for.
Your flying style, how fast you are discharging, the temperature/cooling of the battery and other parameters influence.
I know that some serious persons make really good tests cykling batteries on the bench for 100s of times with close to the rated C-value, making statistics on how many cycles you get with the specified capacity of the battery.
There are everything from below 10 cycles on low quality, up to 100s on the premium brands.

There is a reason that the ESC normally have the 3.0 - up to 3.3 volts as limit for the motor output.
Still you can fly a reasonable time using the battery for the rx and servos.
And the battery voltage will normally recover quite a bit after use if you land and disconnect direcly after you get the esc to shut down/reduce power.

In my opinion the hardeast for the battery is if you are pulling to low from a big battery wher i will choose the highest cut of voltage on the esc.
For "more normal" flying the current drawn will decrease both in the battery but also in the cables and connectors and in some way protect the voltage inside the battery to go to low.

The only way you can be certain is to make your own tests with your brand of batteries and your plane.
 

MrGravey

Senior Member
#13
Batteries are like many other things. "You get what you pay for".
The voltage is not the only thing to look for.
Your flying style, how fast you are discharging, the temperature/cooling of the battery and other parameters influence.
I know that some serious persons make really good tests cykling batteries on the bench for 100s of times with close to the rated C-value, making statistics on how many cycles you get with the specified capacity of the battery.
There are everything from below 10 cycles on low quality, up to 100s on the premium brands.

There is a reason that the ESC normally have the 3.0 - up to 3.3 volts as limit for the motor output.
Still you can fly a reasonable time using the battery for the rx and servos.
And the battery voltage will normally recover quite a bit after use if you land and disconnect direcly after you get the esc to shut down/reduce power.

In my opinion the hardeast for the battery is if you are pulling to low from a big battery wher i will choose the highest cut of voltage on the esc.
For "more normal" flying the current drawn will decrease both in the battery but also in the cables and connectors and in some way protect the voltage inside the battery to go to low.

The only way you can be certain is to make your own tests with your brand of batteries and your plane.

So the only way to know is pick a battery and be willing to destroy it to find out?

I trust your answer and at the same time I have to hate it. Not becasue I feel the information isn't correct, but becasue it points to a very clear like of consistancy in the batteries out therer. I understand higher quality, but there has to be a safe minimum. Ther reason I started this thread is I knew that math and electric current go hand in hand. We know how to figure out much more than not in the world of electricity at this pont. There has to be a safe answer to this question regardless of the battery. That said I'll kill a few if I have to do so to figure out what the deal is.
 

pgerts

Old age member
Mentor
#14
..There has to be a safe answer to this question regardless of the battery. That said I'll kill a few if I have to do so to figure out what the deal is...
I hope that you are right.
I am flying almost every weekend regardless of weather.
A few winters ago i used the light blue Turnigy and had just a few minutes and one takeoff.
Then i got the "new" NanoTech and was so impressed it was almost "religious". 3 times the flight time, several flights and more power.
I got a new light blue turningy last autumn and expected it to be a "summer battery" but it proved to be at least as good as the nano..
I normally fly to the esc tells med to land. I have had batteries working for 6 years and others just half a season.

So - Start killing some batteries to find out. If you find any calculations then "I will follow you master".
 

MrGravey

Senior Member
#15
"I will follow you master".
While I'm flattered I must insist that you not go so far. I'm just very hopeful that I can find a better way. I know a few very smart people. My father-in-law has been working construction for a very long time so maybe I can get to a few good electricians and start there. They wont know much about RC, but they should know more about electricity in general and maybe that will be a place to start. My mind naturally questions things so as long as I know how to get the information putting in the effort to learn isn't a problem for me if I have the time. This has become something I have to know more about to shut my brain up, maybe I can learn something handy for us all in this. It is however for than likely I'll run into something that requires more thought process than I'm able to put out. I have a limit on my voltage as well you see.

Thank you again for your help in this thread. If nothing else I have learned those Nano-Tech batteries get the job done. If I can't find the standard safe answer I'm looking for at least I have that. I'd love to stay and chat, but I have batteries to kill. I'm going to throw something in the air.
 

pgerts

Old age member
Mentor
#16
..I have a limit on my voltage as well you see.
You are young and happy!. Yesterday i was flying an oldtimer (SAM in the USA).
When i started with RC one of the 3 batteries in the 1 channel transmitter was a 90 volt something for the tube.
You could put the battery poles on other batteries to your tounge to feel the power - but not on the high voltage one - more than one time.
130519-04.JPG

I was flying a Radio Queen designed 1950 - first RC model to cross "the channel"but i did not manage so well in the competition ;-) The "ceiling" was low and the plane disappeared at a hight of about 70 meters (200 ft). The competition was to glide without motor 2 minutes and land in a cirkle with a diameter of 30m (100 ft). I managed to glide about 50 seconds. The plane is old and heavy.

As for high voltage, i am working with up to 50 kV at the job and for batteries we have an UPS capable of 250 kW in +20 minutes.