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Anything wrong with doing this?

Old 06-07-2011, 05:34 AM
  #1  
flyyy
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Default Anything wrong with doing this?

The way I think that I understand this is a longer prop with more pitch gives more low end torque. Shorter props with less bite is better for high end speed. So, the bigger the batteries and the bigger the ESC the more they cost. If you think you have enough power and you want more prop wash can you run a bigger prop and limit the throttle stick. In other words if the max of the motor and esc is say 25 amp and you reach that amp reading at say 60% throttle can you put some sort of stop on the transmitter so you won't exceed that [like a Popsicle stick taped across there] and fly like that? I'll bet someone will say the motor is not reaching it's peak efficiency or something like that, but will this work without burning something up prematurely?
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Old 06-07-2011, 05:52 AM
  #2  
Larry3215
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Originally Posted by flyyy View Post
The way I think that I understand this is a longer prop with more pitch gives more low end torque. Shorter props with less bite is better for high end speed.

Not quite. There are 3 things you need to ballance when working with props. Those are:
1)diameter
2)Pitch
3)RPM

Those things together combine to give you the total power load on the motor AND they determine how the plane will fly. Each is important in different ways. You need to take all three into account. You also need to take into account a 4th item - motor kV

1)The diameter of a prop is related to the thrust generated. Larger diameter props give more thrust. Smaller diameter props generate less thrust. That assumes the rpm is the same. In order to generate more thrust, your motor has to work harder - which means it will draw more amps.

2) Pitch on the prop determines the speed the prop want to move through the air. More pitch means more speed. More pitch ALSO means your motor needs to draw more amps to generate more power.

3)RPM is the next one. More RPM means more thrust AND more speed which also means more power draw. So RPM is also very critical. The RPM is determined by the kV of the motor and the battery voltage. A motor with a kV of 1000 running on a battery with a voltage of 10 volts will want to spin at 1000 x 10 = 10,000 rpm

So if you want more low end torque, you need a motor with a lower kV and a prop with a larger diameter


So, the bigger the batteries and the bigger the ESC the more they cost. If you think you have enough power and you want more prop wash can you run a bigger prop and limit the throttle stick. In other words if the max of the motor and esc is say 25 amp and you reach that amp reading at say 60% throttle can you put some sort of stop on the transmitter so you won't exceed that [like a Popsicle stick taped across there] and fly like that? I'll bet someone will say the motor is not reaching it's peak efficiency or something like that, but will this work without burning something up prematurely?
See my answers above in RED

Sorry - Ive been interupted.... more later
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Old 06-07-2011, 09:39 AM
  #3  
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Originally Posted by flyyy View Post
The way I think that I understand this is a longer prop with more pitch gives more low end torque. Shorter props with less bite is better for high end speed.
Not quite. For the same rpms:

1. More diameter (longer prop) gives more THRUST or acceleration (e.g. better climb/take off). Lower diameter (short prop) gives less thrust/acceleration.

2. More pitch gives more speed, lower pitch gives less speed.

Both diameter and pitch add to the load on the motor and so the rpms that it's getting. So if you want lots of thrust but aren't bothered about speed you need more diameter and LESS pitch. For higher top speed you need less diameter and MORE pitch but then it won't accelerate as well.

If you limit the throttle, as well as potentially damaging something, you're reducing the rpms so you just get less of everything. It's normally better to use a prop that works with the motor you've got.

If for some reason you really want to use a prop that's too big and lower the current you could always try a battery with less voltage e.g. 2S instead of 3S....that has the advantage that a 2S battery is cheaper .

Steve
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Old 06-07-2011, 11:33 AM
  #4  
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Hey now,
All that's true. Also as I understand it limiting your ESC by not going to full throttle doesn't exactly help. Basically the controler is either switched on or off. When on it passes full power and off is zero power. Sixty percent throttle means it's on sixty percent of the time. So you will eventually fry the unit.
RobII
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Old 06-07-2011, 01:01 PM
  #5  
Figure.N9ne
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Default Anything wrong with doing this?

I know I'll be told I'm wrong like I always when I say this. But the whole esc still seeing the full amperage when at part throttle doesn't make sense to me. What makes the amps go up is the amount of power it takes to turn the bigger prop and defeat the resistance of it spinning in the air. If you're at part throttle, your rpm are lower so the resistance is lower so the amps are lower. I understand that the esc is just a switch that pulses to 100% but the rpm doesn't reach 100% every time it pulses so the resistance shouldn't.
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Old 06-07-2011, 01:21 PM
  #6  
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Yes, but remember that it is still accelerating the prop with each pulse, against the momentum of the prop. Much like how the amps peak the instant that a prop is started, each of these pulses is going to be seeing resistance from both the prop's momentum and air resistance.

At full throttle, where the prop is at a truly steady speed, it is only countering the aerodynamic resistance, not the prop's momentum.

However, I agree that in practice the amps going through the power system are probably different at full throttle than during a powered pulse at low throttle. Nonetheless, it's still good practice to use an ESC rated for the maximum your motor could draw.
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Old 06-07-2011, 03:10 PM
  #7  
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Another factor not mentioned is the manufacturer and type of the prop. All 12x6 props are not created equal. When calculating whether a certain motor/ESC/battery/prop combination will work it is important to have the brand and manufacturer of each component as part of the equation.

Also, just plugging in the advertised specs for motors, ESCs and batteries won't get you what you want because advertising lies. Only actual measurements using that specific piece of equipment have any validity at all. Taking the advertised numbers from a Whatsit 2220-13 motor and plugging them into a web calculator is just plain useless, except for very general, huge ballpark numbers.

Then, as you may end up using real world numbers in theoretical combinations, you need some way to weight the accuracy of your now calculated result. If you have data on the Turnigy Super Brain 40A ESC but it is with another motor than the one you are testing, And you have data for the motor you're interested it, but it was attached to another ESC, there is a certain uncertainty in your result when you calculate the result of combining these units. You need to know how much uncertainty results from that situation.

All that is covered in a free program called Drive Calculator: a large and continuously expanding database including brand and model numbers of components, numbers based on actual testing, not advertised numbers, and the ability to pick a prop, battery and ESC, then ask for a list of suitable motors. It also tells you if the result for your combination is 100% reliable, being based on real world testing or a calculated result with a rated confidence number. I believe this is unique among both free and paid programs.

MotoCalc gets a lot of promotion here at Wattflyer and it is undoubtedly a decent program. I just don't see any reason to spend the money with an excellent and improving program like Drive Calculator around. That's most likely a fatal character defect, I know.

On your part throttle question, did you know that your ESC runs at a higher temp and works harder at part throttle than full? Just looking at the data dump from my Super Brain ESC proves this, with a marked temperature spike whenever I'm working part throttle, and I'm at part throttle just about all the time. Therefore, if you're overpropped, it will have MORE effect at part throttle than at full throttle. Overpropped is overpropped at any speed. If you're looking for less amperage draw you would be better served by going with a smaller motor and propping it correctly. If you're looking to save money, properly matched components are the way to do it.
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Old 06-07-2011, 03:37 PM
  #8  
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Originally Posted by Figure.N9ne View Post
I know I'll be told I'm wrong like I always when I say this. But the whole esc still seeing the full amperage when at part throttle doesn't make sense to me. What makes the amps go up is the amount of power it takes to turn the bigger prop and defeat the resistance of it spinning in the air. If you're at part throttle, your rpm are lower so the resistance is lower so the amps are lower. I understand that the esc is just a switch that pulses to 100% but the rpm doesn't reach 100% every time it pulses so the resistance shouldn't.
I don't know if this picture explains it.

Digital switching, (as in an ESC), is either on or off. It's how long it's on or off for that gives variable control.

Full on is fairly obvious.

Half on, also equals half off. So the current drops during the off and meters tend to show the average of the two.

Low throttle, is similar to half, just off for longer.

But in all of those throttle positions, the 'on' is full voltage, full current. Hence although the meter may only show a lower current at half throttle, because it has averaged out the 'on' times and 'off' times, the ESC transistor switching, (FET's), are still seeing those full voltage 'spikes'.
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Old 06-07-2011, 05:07 PM
  #9  
sparky1963
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Default Limiting throttle range

Answers given, regarding prop selection are important.
The prop size and pitch need to be suited to the max rpm and current draw for your particular motor/ESC/battery.
Manufacturer's info for motor, should tell you an optimum range of suitable propellers.
It should also tell you the predicted current draw, for a given size/pitch .
(If you don't have that information on paper, then the internet should tell you what you need, via maker's websites etc).
Don't select a prop which will draw more current than your ESC can handle.
Propeller dimensions provide a 'load' for the motor - increased load will draw more current, so it is important not to overload it by using an oversized or overpitched prop.

Originally Posted by flyyy View Post
...can you put some sort of stop on the transmitter so you won't exceed that [like a Popsicle stick taped across there] and fly like that?...
On a nitro engine, the throttle operates a mechanical linkage from servo to carburettor. The amount of throttle opening (and therefore engine speed) can be limited by reducing servo throw.
With electric motors, the ESC takes the place of that function.
Think of your ESC as just another servo - the range of 'movement' can be reduced from 100%, the same as any other. - Nothing as crude as a popsicle stick though
Throttle 'travel' or 'throw' (terms usually related to servo horn movement) can easily be adjusted with a basic computer radio, even though there isnt any actual physical movement to see.
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Old 06-07-2011, 05:08 PM
  #10  
flyyy
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Wow..... This is more complicated than I thought. The small amount of flying that I have been able to do vs. building and tinkering I thought that I was really protecting the equipment by flying around coasting and throttling back ever chance that I get. I even try to takeoff at less than WOT unless something starts coming at me. Slipstick said I should try like a smaller battery. Now that really confuses me because I was taught that the lower the voltage the amps would increase by the same multiplier. You know the old E over I times R. Also on the KV of the motor. Seems like the lowest numbers I can find is 1000 or 850 something like that. I don't like being wasteful but I think the thing for me to do is stay with cheap stuff and if it burns it burns. I've been back in this about a year now and my time in the air vs. testing and tinkering is less than say the man hours it takes to keep a B five two going. Thanks all. One more thing. These suggested props are listed on a motor from HK. 2 cell ...8/4 3 cell...7/4 Is this not backward? I thought a 8/4 was harder to pull than a 7/4.

Last edited by flyyy; 06-07-2011 at 05:26 PM. Reason: I forgot
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Old 06-07-2011, 05:53 PM
  #11  
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Well you're partially right. If the equipment is well matched you really aren't doing anything to save the equipment by throttling back. However, you ARE getting longer flights that way. And isn't it amazing that we've gone from electric planes barely being able to fly to having just about every plane ridiculously overpowered. (Ridiculously overpowered? Just right!!!)

My ridiculously overpowered Slow Stick has unlimited vertical capability and I fly it just about all the time at 60% throttle or less determined digitally, not by throttle position on the transmitter. Like you, I don't take off with 100% power and still have a two or three foot roll before I'm in the air.

But when you lollygag by in a close pass at 5' just above stall speed and hit the jets when it's right in front of you climbing vertically to 500' it definitely gets attention!

Once I get the pointing problems licked on my hat cam maybe I can get some video of that. It's been tougher than I thought it would be.
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Old 06-07-2011, 06:22 PM
  #12  
flyyy
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I don't understand the controlled digitally. I'm still on 72 and will be until its outlawed. I could probably scratch up enough to get a computer radio; However, I'm already at the top of my learning curve if you know what I mean.
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Old 06-07-2011, 08:32 PM
  #13  
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Oh, sorry. I meant that my throttle position was determined by my data logging ESC's numbers, not my throttle stick's position on the radio, which I would have estimated as below half throttle. Apparently there is some curve in the throttle response on the DX5e radio. I would suspect that without actually measuring by data logger just about all radios would cause you to misstate your throttle position. I was really surprised when I saw the numbers. The ESC doesn't care whether it's hooked up to a digital or analog radio.

Although the $50 DX5e radio is 2.4 digital it is not technically a computer radio. I love the thing!
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Old 06-07-2011, 09:10 PM
  #14  
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Originally Posted by flyyy View Post
Wow..... This is more complicated than I thought. The small amount of flying that I have been able to do vs. building and tinkering I thought that I was really protecting the equipment by flying around coasting and throttling back ever chance that I get. I even try to takeoff at less than WOT unless something starts coming at me. Slipstick said I should try like a smaller battery. Now that really confuses me because I was taught that the lower the voltage the amps would increase by the same multiplier. You know the old E over I times R. Also on the KV of the motor. Seems like the lowest numbers I can find is 1000 or 850 something like that. I don't like being wasteful but I think the thing for me to do is stay with cheap stuff and if it burns it burns. I've been back in this about a year now and my time in the air vs. testing and tinkering is less than say the man hours it takes to keep a B five two going. Thanks all. One more thing. These suggested props are listed on a motor from HK. 2 cell ...8/4 3 cell...7/4 Is this not backward? I thought a 8/4 was harder to pull than a 7/4.

If the work remains the same E= IR.
Problem is add more cells to the pack and the motor tries to spin faster, thus doing more work, more amps.

If you go from a three cell to a two cell pack, the voltage drops down and the motor doesn't try to do as much work, amps drop.

Sometimes prop suggestions from Asian companies are flat out wrong.
I like trial and error, but I do try a lot of props and use a wattmeter to
make sure I don't draw too many amps.
There are a few prop and motor calculators that come very close for choosing a prop.
But in the end, the prop has to be matched to the motor and the airplane.

paul
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Old 06-07-2011, 09:32 PM
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kyleservicetech
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Originally Posted by flyyy View Post
The way I think that I understand this is a longer prop with more pitch gives more low end torque. Shorter props with less bite is better for high end speed. So, the bigger the batteries and the bigger the ESC the more they cost. If you think you have enough power and you want more prop wash can you run a bigger prop and limit the throttle stick. In other words if the max of the motor and esc is say 25 amp and you reach that amp reading at say 60% throttle can you put some sort of stop on the transmitter so you won't exceed that [like a Popsicle stick taped across there] and fly like that? I'll bet someone will say the motor is not reaching it's peak efficiency or something like that, but will this work without burning something up prematurely?
Just think of that ESC as a toggle switch (Actually three SPDT or Three - Single Pole, Double Throw switches.), only it's switching on and off at perhaps 5 or 10 thousand times per second. Since the motor is not turning at full RPM at reduced throttle, its "Instantaneous" DC resistance is a lot lower than it would be at full throttle. So you start getting higher peak currents. It's not an issue with properly sized ESC and batteries. But those peak currents has to do with the motors Back EMF voltages. This Back EMF voltage is what causes the current pulled by the motor to drop way down when it's run without a propeller.

So, if you're running your motor at 60% power, with a popsicle stick on your transmitter to prevent going any higher, you are likely to be running your motor/esc/battery at lower than optimum efficiency.

At any rate, when playing with different propellers and different batteries on your equipment, it is wise to pick up one of those wattmeters, such as Astroflights Whattmeters. Just going from two Lipo cells to three Lipo cells with the same motor and same propeller can easily increase the current and watts pulled by the motor by two or three times. And let the magic smoke out. If your setup has an issue, those wattmeters will quickly show you've got a problem. Before burning up something.
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Old 06-08-2011, 06:36 PM
  #16  
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Like pd1 said: If you go from a three cell to a two cell pack, the voltage drops down and the motor doesn't try to do as much work, so amps drop.

Suppose you have a motor whose Kv (revs/volt) is 1000. If you apply 7.4V to it (like a 2-cell lipo) it will try to turn 7.4 x 1000 or 7400 rpm. Consider that the motor's "target" speed. If you put a prop on it, it won't quite be able to turn 7400 rpm. Maybe it will turn 7200 rpm. It will keep working, trying to get to 7400. Now change to a 3-cell battery, at 11.1 volts. Now the motor will try to turn at 11.1 x 1000 or 11,100 rpm. It takes a lot more work to turn the prop that much faster. Maybe now the motor can manage 10,000 rpm. The motor is turning the prop a lot faster with 11.1 volts pushing it, but the target speed increased more than the actual achieved speed. That's why the motor pulls more amps at higher volts - it's working harder, turning that bigger prop and trying to close the increasing rpm gap.
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Old 06-08-2011, 11:54 PM
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[QUOTE=tom1968;814409]Like pd1 said: If you go from a three cell to a two cell pack, the voltage drops down and the motor doesn't try to do as much work, so amps drop./QUOTE]

As it works out, going from a two cell to a three cell battery will come to slightly more than doubling the watts input to the motor, using the same propeller.


Without a wattmeter, you'd never know.
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Old 06-09-2011, 09:12 AM
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Originally Posted by kyleservicetech View Post
As it works out, going from a two cell to a three cell battery will come to slightly more than doubling the watts input to the motor, using the same propeller.

Without a wattmeter, you'd never know.
You could have a good guess though

The new voltage is 1.5 times what it was, so the current will also be about 1.5 times what it was. It follows that total power (watts) will be somewhere around 1.5 x 1.5 = 2.25 times (or slightly more than double ).

Steve
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Old 06-09-2011, 12:07 PM
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Originally Posted by Elfwreck View Post
Hey now,
All that's true. Also as I understand it limiting your ESC by not going to full throttle doesn't exactly help. Basically the controler is either switched on or off. When on it passes full power and off is zero power. Sixty percent throttle means it's on sixty percent of the time. So you will eventually fry the unit.
RobII
Someone better tell Heli flyers then to not program throttle curves if that's true .......

Most Heli's if flown on non Idle-Up are at 60 - 80% throttle when taking off ... hovering etc. You don't see Heli guys changing ESC's ...
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Old 06-09-2011, 03:17 PM
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Originally Posted by solentlife View Post
Someone better tell Heli flyers then to not program throttle curves if that's true .......

Most Heli's if flown on non Idle-Up are at 60 - 80% throttle when taking off ... hovering etc. You don't see Heli guys changing ESC's ...
I think he meant that will happen if the ESC isn't matched up to the amp requirements. Most heli guys fly with an ESC that will support the spikes given by 100% throttle curve and hard pitch maneuvers so it's not a problem of not having a large enough ESC.
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Old 06-09-2011, 05:12 PM
  #21  
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Originally Posted by slipstick View Post
You could have a good guess though

The new voltage is 1.5 times what it was, so the current will also be about 1.5 times what it was. It follows that total power (watts) will be somewhere around 1.5 x 1.5 = 2.25 times (or slightly more than double ).

Steve
Kind of true. I ran the numbers through www.motocalc.com. Problem is, motocalc also allows for the increased loading on the battery pack, resulting in lower battery voltage than expected. And, more losses in the motor.

If you have a very "Stiff" battery source, and very efficient motor, the increased watts will be around 300%, going from two to three cells. (RPM ratio raised to the third power)

Has to do with the propeller load. Doubling the prop RPM results in requiring 2X2X2 or 8 times more horsepower at the motor shaft.
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