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Tachometer or RPM Sensor

Old 01-06-2010, 04:18 AM
  #26  
alaursen
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Originally Posted by kyleservicetech View Post
That Rimfire 42-50-800 motor weighs in at 7 ounces, so running it at 450 watts is about 450/7 or about 65 watts per ounce of motor weight. IMHO, that's fairly conservative.
Hmm, I take it you mean underpowered. If so, I wonder why HL has this recommendation posted on the box for the Telemaster. Unless my power calcs are way off. Even if I use the best case 3S voltage of 11.1 (which we know the motor doesn't "see") times my measured 40 A, I still get around 440 W. It is a trainer, so maybe its OK for that purpose; what do you think?

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Old 01-06-2010, 04:30 AM
  #27  
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Originally Posted by alaursen View Post
It may seem low, but if the manufacturer says the KV=800 and my measured RPM = 7300, doesn't that mean the motor must be "seeing" 9.1 volts? Maybe I don't understand KV correctly, however, one thing I know for sure is that battery voltage into the ESC is certainly not what the motor will get from the ESC output even at full throttle; right?

Oh I meant to ask; where is your wattmeter connected?
Those wattmeters are connected in between your battery and the ESC.

Back in the brush type motor days, you could easily measure the voltage at the battery pack, and also at the brushes of the motor. In these brushless motors, that is a whole different story.

Very little voltage drop exists inside the ESC itself. (If the ESC did have even one volt drop at rated current, the mosfets in the ESC would quickly melt from the heat.) These Electronic Speed Controls are in effect a device that converts the batteries Direct Current (DC) power to three phase AC power. And, you get to deal with RMS (Root Mean Square) voltage, average voltage, effective voltage, and the output of these ESC controls, which is a three phase square wave.

So, if you tried to measure the voltage on two of the motors three input wires, you'd be measuring a square wave voltage at very high frequency. Since these digital multimeters have been designed to assume the user is measuring a sine wave voltage, the voltage the meter reads will likely have serious errors. And, most digital multmeters have a real problem in trying to measure frequencies over a few thousand hertz (cycles per second) A typical brushless ESC switching frequency is 5 or 10 times that.

The only way to measure this voltage is to use an oscilloscope, something I've done with my Tektronix 2236 scope. Believe me, that voltage wave shape is a real mess, especially when you are running that ESC at less than full throttle, where the pulse width modulation of the ESC is used to reduce full power to the motor.

(Before retiring, I measured the input to one of my brushless motors with our departments $6000.00 Tektronix four channel 400 Mhz oscilloscope. Using three channels of that scope allowed me to see those three phases in three different colors. I could also watch how the pulse width modulation operated to vary the speed of the motor, while using the throttle on the transmitter.)
Hope that helps.
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Old 01-06-2010, 04:36 AM
  #28  
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I understand your point about 3 phase versus DC and large voltage drops (power or heat = V X A). However, what do you think about the KV versus RPM? Assuming I am measuring accurately with the tach 7300 RPM and it is an 800 KV motor so isn't that 9.1 V? What am I doing wrong?
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Old 01-06-2010, 04:44 AM
  #29  
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The battery won't always deliver its nominal voltage when it's under load, but that's the only place you'll see a voltage drop. Losses in the ESC and in all the wiring from the battery to the motor shouldn't even be measurable with consumer multimeters - if you were really dropping from 11.1 to 9.1 across the ESC, that would mean the ESC would have to dissipate (2V * 40A) 80 watts of waste power - that would require an enormous heatsink (think about trying to keep an 80 watt incandescent light bulb cool enough to touch).

The actual reason your estimate is so low is that Kv is only a valid measurement on an unloaded motor - no prop attached. Once you add a load, the motor will slow down even if it's still getting the same voltage.

I'm a bit concerned about your power system, though. You definitely have enough speed - 7300 RPM with an 8-inch pitch is a 55 MPH pitch speed (7300*8/1056), which is just about perfect for a plane like the Telemaster. You're definitely underpowered, though. The normal rules of thumb for power into the motor are as follows:

<50 W/lb: Unflyable
50-75 W/lb: Able to sustain flight from a hand launch. Climbs will be very gentle and full power will be required for much of the flight.
75-100 W/lb: Able to perform ground takeoffs and perform basic aerobatics. May need to dive for speed before some maneuvers.
100-150 W/lb: Capable of nearly any maneuver from level flight.
150-200 W/lb: Capable of sustained vertical climbs and "3D" aerobatics.
200+ W/lb: Hahaha!

Even for a big, lazy Telemaster, I would aim for at least 100 W/lb. Even if you just want to float around at minimum speed, having that extra power in reserve means you can get out of trouble easily - takeoff is simpler, stall recovery is simpler, and aborted landing approaches are stress-free. The extra power also means you can try big, graceful loops and hammerhead turns, both of which are great fun in that kind of plane.

Many kit manufacturers still haven't gotten caught up with lithium batteries and brushless motors, and many still suggest very inadequate power systems for their planes. Don't listen to Hobby Lobby's suggestion. Just remember - if you add power by going to a larger prop, keep an eye on your pitch speed. If your pitch speed drops below about 40-45 MPH, you'll probably have issues, even if the total power is high enough.
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Old 01-06-2010, 04:44 AM
  #30  
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Originally Posted by alaursen View Post
I understand your point about 3 phase versus DC and large voltage drops (power or heat = V X A). However, what do you think about the KV versus RPM? Assuming I am measuring accurately with the tach 7300 RPM and it is an 800 KV motor so isn't that 9.1 V? What am I doing wrong?

Uh Oh
I've never done that, comparing the KV rating of the motor with the battery voltage, and the RPM the motor turns over.

I believe the KV rating is determined by turning the motor over at a specific RPM, and measuring the voltage generated at the motors terminals. Comments everyone???

Also, you must measure the motors RPM with no propeller, since that RPM will be less when a propeller is installed.
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Old 01-06-2010, 04:53 AM
  #31  
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OK due to the excellent input from this thread, I will assume the power is close to battery voltage (ESC input) times the measured current. The RPM cannot predict motor voltage when loaded with a prop. However, it can determine speed (RPM X Pitch /1056).

In this case I am getting about 425 W to the motor; apparently still underpowered at 7 lbs weight. Sorry for the dual subject on this thread but the whole reason I bought the tach today was to try and get an idea of these parameters before I flew it.
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Old 01-06-2010, 04:58 AM
  #32  
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Originally Posted by alaursen View Post
OK due to the excellent input from this thread, I will assume the power is close to battery voltage (ESC input) times the measured current. The RPM cannot predict motor voltage when loaded with a prop. However, it can determine speed (RPM X Pitch /1056).

In this case I am getting about 425 W to the motor; apparently still underpowered at 7 lbs weight. Sorry for the dual subject on this thread but the whole reason I bought the tach today was to try and get an idea of these parameters before I flew it.
Seven pounds at 425 watts? I'd be reluctant to fly it off of grass.
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Old 01-06-2010, 05:14 AM
  #33  
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It is a paved field (SCCMAS.ORG). However, the motor is rated for more than twice that. Again, what gives with the Hobby-Lobby reco? Seems like 3S LiPo is at the very bottom of the performance range.
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Old 01-06-2010, 05:17 AM
  #34  
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I think your post is great and I liked your table on watts/lb
My father and I are working on converting an old 1/4 scale cub and I used just the 50w/lb rule and ended up going to finding at 75w/lb

We will be choosing roughly a 1100watt output with an 18x10 prop on a ~340kv motor from Scorpion thanks to Lucien.


Originally Posted by aramid View Post
The battery won't always deliver its nominal voltage when it's under load, but that's the only place you'll see a voltage drop. Losses in the ESC and in all the wiring from the battery to the motor shouldn't even be measurable with consumer multimeters - if you were really dropping from 11.1 to 9.1 across the ESC, that would mean the ESC would have to dissipate (2V * 40A) 80 watts of waste power - that would require an enormous heatsink (think about trying to keep an 80 watt incandescent light bulb cool enough to touch).

The actual reason your estimate is so low is that Kv is only a valid measurement on an unloaded motor - no prop attached. Once you add a load, the motor will slow down even if it's still getting the same voltage.

I'm a bit concerned about your power system, though. You definitely have enough speed - 7300 RPM with an 8-inch pitch is a 55 MPH pitch speed (7300*8/1056), which is just about perfect for a plane like the Telemaster. You're definitely underpowered, though. The normal rules of thumb for power into the motor are as follows:

<50 W/lb: Unflyable
50-75 W/lb: Able to sustain flight from a hand launch. Climbs will be very gentle and full power will be required for much of the flight.
75-100 W/lb: Able to perform ground takeoffs and perform basic aerobatics. May need to dive for speed before some maneuvers.
100-150 W/lb: Capable of nearly any maneuver from level flight.
150-200 W/lb: Capable of sustained vertical climbs and "3D" aerobatics.
200+ W/lb: Hahaha!

Even for a big, lazy Telemaster, I would aim for at least 100 W/lb. Even if you just want to float around at minimum speed, having that extra power in reserve means you can get out of trouble easily - takeoff is simpler, stall recovery is simpler, and aborted landing approaches are stress-free. The extra power also means you can try big, graceful loops and hammerhead turns, both of which are great fun in that kind of plane.

Many kit manufacturers still haven't gotten caught up with lithium batteries and brushless motors, and many still suggest very inadequate power systems for their planes. Don't listen to Hobby Lobby's suggestion. Just remember - if you add power by going to a larger prop, keep an eye on your pitch speed. If your pitch speed drops below about 40-45 MPH, you'll probably have issues, even if the total power is high enough.
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Old 01-06-2010, 05:23 AM
  #35  
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have you tried to put it on an actual watt meter?
You also got to realize your flying a telemaster great high wing airplane that I used to fly that 40 size motor at near idle most the time
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Old 01-06-2010, 05:31 AM
  #36  
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I measure volts and current (clamp-on) separately so can easily compute watts. I'm new to powered flight - thanks for the input on the Tele. I just put at least 80 hours building it over the last 4 weeks and I'm covering it tonight. Don't want any mishaps, at least not in the parameters. Now the flying part is a different story but I will have an instructor. Thanks everyone for the advice - I also like the table!

Art
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Old 01-06-2010, 05:38 AM
  #37  
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I think you will be fine..

You could always do what i've seen guys do at the field and just hold the tail and say "feels strong" and then go fly
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Old 01-12-2010, 12:17 AM
  #38  
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Ok so I got my Tach and i was running some tests...

I am getting like 6k RPM on my one airplane on a 2 cell lipo and a 1500KV motor.

getting 11k RPM on another motor which is 2200KV on a 2 cell lipo.

not sure what RPM is going to help me with
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Old 01-12-2010, 01:55 AM
  #39  
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Originally Posted by ShoGinn View Post
...not sure what RPM is going to help me with
What props do you have on each setup, and what type of plane is each meant to power?

Think about skimming through this thread and re-reading a few of the longer posts. RPM, by itself, is a totally useless value. It has as much impact on the way a plane flies as its color does. When you also include the propeller size, though, you can determine pitch speed, which can be extremely important.

When I'm looking at how a plane will fly, I think there are three things which are far, far more important than any other information - wing loading, watts per pound, and pitch speed. I am not particularly comfortable putting a new plane in the air at all unless I know those values - I think most people know the importance of the wing loading and power loading, but I have also lost a plane due to an inappropriate pitch speed. There are plenty of other things you can measure about a plane or motor, but in most situations all the important information is covered by those three.
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Old 01-12-2010, 02:05 AM
  #40  
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good info, i'll check some of the posts, these are good flying planes I just wanted to see how these values impact anything really.

the 1500kv motor is a smaller motor with a 9x4.7 GWS slow fly on a foamie airplane
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Old 01-12-2010, 06:15 AM
  #41  
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That means your pitch speed on that system is right about 30 MPH - totally appropriate for a slow flier. I assume you've already flown the plane and determined that the system worked well, but in the future you can learn a bit more about how a plane should behave before you ever put it in the air.
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Old 01-13-2010, 07:55 AM
  #42  
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Where the rpm will help is when adjusting ESC's or comparing two similar setups to see which is "best".

Watts out is only part of the story.

Lets say for example you start out with a motor/battery/prop combo thats drawing 100 watts. Then you make changes to the timing or PWM rate or what ever on your esc and now your drawing 125 watts.

At first you might think - this is great! Now I have a more powerful setup.

But wait - what happened to the RPM when you made those changes? If the rpm didnt go up then all your extra 25 watts is doing is heating up your motor and reducing your flight time.

If those extra watts are not making extra RPM then they are wasted watts and in fact causing harm rather than good.

Lets say instead that the watts went down instead of up. You might think thats bad - but what if the rpm stayed the same or close to the same? That would actually be a good thing.

So, it important to know the rpm as well as the amps and volts.

The power required to spin a prop changes as the 3rd power of the change in rpm.

So to spin a prop at twice the rpm takes 2x2x2 = 8 times as much power. If you cut the rpm in half, it takes 1/8 as much power - all other details remaining unchanged.

You can use that formula to tell if the extra power your drawing is doing its job of spinning the prop or just making heat.
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Old 01-13-2010, 04:56 PM
  #43  
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Originally Posted by Larry3215 View Post
Where the rpm will help is when adjusting ESC's or comparing two similar setups to see which is "best".

Watts out is only part of the story.

Lets say for example you start out with a motor/battery/prop combo thats drawing 100 watts. Then you make changes to the timing or PWM rate or what ever on your esc and now your drawing 125 watts.

At first you might think - this is great! Now I have a more powerful setup.

But wait - what happened to the RPM when you made those changes? If the rpm didnt go up then all your extra 25 watts is doing is heating up your motor and reducing your flight time.

If those extra watts are not making extra RPM then they are wasted watts and in fact causing harm rather than good.

Lets say instead that the watts went down instead of up. You might think thats bad - but what if the rpm stayed the same or close to the same? That would actually be a good thing.

So, it important to know the rpm as well as the amps and volts.

The power required to spin a prop changes as the 3rd power of the change in rpm.

So to spin a prop at twice the rpm takes 2x2x2 = 8 times as much power. If you cut the rpm in half, it takes 1/8 as much power - all other details remaining unchanged.

You can use that formula to tell if the extra power your drawing is doing its job of spinning the prop or just making heat.
These are all good points! That's why increasing your batteries cell count by one cell can go from an underpowered dog, to a situation where the motor/batteries/ESC can burn up.

One thing that helps in all of this is a program such as www.motocalc.com that gives an indication on just how hard you are pushing your power equipment.

Something I've noticed on those "slow flyer" props is you don't want to run them at to high of power levels. I tried running an 8X4 "Slow Flyer prop" at 150 watts, that prop sounded like a paddle wheel at more than 70% power.
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Old 01-13-2010, 07:41 PM
  #44  
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Originally Posted by kyleservicetech View Post
These are all good points! That's why increasing your batteries cell count by one cell can go from an underpowered dog, to a situation where the motor/batteries/ESC can burn up.

One thing that helps in all of this is a program such as www.motocalc.com that gives an indication on just how hard you are pushing your power equipment.

Something I've noticed on those "slow flyer" props is you don't want to run them at to high of power levels. I tried running an 8X4 "Slow Flyer prop" at 150 watts, that prop sounded like a paddle wheel at more than 70% power.
Yes - slow flyer props are designed to run SLOWly They have a much lower max RPM safety rating than the regular E props - a fact which many ignore at their risk. A buddy of mine almost lost an eye when a slow flyer prop he was over reving shot a blade into his eye scratching it seriosly. He had eye problems for over a year after wards.

Check out APC's web site:

http://www.apcprop.com/v/html/rpm_limits.html

Regular E props have a max RPM of 190,000/diameter

Slow props have a max rpm of 65,000/diameter

So a 10x6 E prop has a max rpm of 19,000 rpm.

A slow fly 10x6 has a max rpm of only 6500 rpm.

Thats a big difference.

Just to compare - it will take roughly 100 watts to spin a slowfly 10x6 at 6500 rpm - the max rpm it can safely spin.

A regular E 10x6 can spin at 19500 rpm. That will take well over 2000 watts.

So you can run a regular E prop at much higher power levels than a slow fly prop.
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