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I have a Turnigy G46, 420kv brushless motor, that I plan to use on 4s, 4000 mah. How well will it fly a Sig Kadet Senior kit that I plan to build ? I'll be using a 15 x 8 E prop, and what kind of flight times can I expect ?.

I have a Turnigy G46, 420kv brushless motor, that I plan to use on 4s, 4000 mah. How well will it fly a Sig Kadet Senior kit that I plan to build ? I'll be using a 15 x 8 E prop, and what kind of flight times can I expect ?.

I just ran the numbers of your setup through www.motocalc.com (free for 30 days, then $39).

Motocalc indicates that your power system will turn the 15X8 prop at around 5500 RPM, pulling around 30 Amps, and 400 Watts. Assuming you keep the model weight down to 96 Ounces total, that would be around 70 Watts per pound of airplane. The model should fly OK, but power would not be very high.

Going to a 5 cell LiPo battery on the same motor and prop will increase the RPM to 6700, while increasing the motor current to 41 Amps and power to 700 Watts. This provides 120 Watts per pound of airplane, a quite reasonable value for a model of this type and size.

DennyV
Retired and the days are just too short, busier than ever!

[QUOTE=kyleservicetech;966199]I just ran the numbers of your setup through www.motocalc.com (free for 30 days, then $39).

Motocalc indicates that your power system will turn the 15X8 prop at around 5500 RPM, pulling around 30 Amps, and 400 Watts. Assuming you keep the model weight down to 96 Ounces total, that would be around 70 Watts per pound of airplane. The model should fly OK, but power would not be very high.

QUOTE]

I am not looking for high power, just good flight times. Does Motocalc tell you anything about my possible flight time ? ? ?

I just ran the numbers of your setup through www.motocalc.com (free for 30 days, then $39).

Motocalc indicates that your power system will turn the 15X8 prop at around 5500 RPM, pulling around 30 Amps, and 400 Watts. Assuming you keep the model weight down to 96 Ounces total, that would be around 70 Watts per pound of airplane. The model should fly OK, but power would not be very high.

QUOTE]

I am not looking for high power, just good flight times. Does Motocalc tell you anything about my possible flight time ? ? ?

Thank you

Eddy

With your 4000 Mah battery pack, that is 4.0 Ampere Hour battery pack. Which converts to 4 Amp Hours times 60 minutes per hour, or 240 Ampere minutes.

At 30 Amps full throttle for the entire flight, you will get 240 Amp Minutes/30 Amps, or 8 minutes to run the battery completely flat. So, at continuous full throttle, you can safely fly 5 or 6 minutes. If your motor will continue to fly your model at half power, you can easily double your flying time to 10 minutes or more.

As usual, you need to verify the current pulled by your setup with a wattmeter. Some motor specs are not to accurate.

With your 4000 Mah battery pack, that is 4.0 Ampere Hour battery pack. Which converts to 4 Amp Hours times 60 minutes per hour, or 240 Ampere minutes.

At 30 Amps full throttle for the entire flight, you will get 240 Amp Minutes/30 Amps, or 8 minutes to run the battery completely flat. So, at continuous full throttle, you can safely fly 5 or 6 minutes. If your motor will continue to fly your model at half power, you can easily double your flying time to 10 minutes or more.

As usual, you need to verify the current pulled by your setup with a wattmeter. Some motor specs are not to accurate.

Dennis that is the best and most comprehensive answer that I have received yet, thank you very much. I will use that knowledge to fly with.

Since I have now found an expert on the subject, I have just one more question. If a motor is offered in different KV ratings, such as 400KV, or 800KV at the same price, why would anyone select the lower 400KV motor ? Wouldn't the higher 800KV motor be superior ? I just don't understand why they bother making the lower KV motor ? Does a lower KV motor offer some special advantage for some type of flying, or some type of plane, or some type of prop ?

Thank you Sir, you have been very helpful to me.
Eddy

Cheeky b****** .... I seem to remember this thread topic being asked elsewhere ...

OK onto kv :

On the question of why different KV ratings ...

They give alternative prop size / battery voltage combinations. Low KV gives larger prop and or higher voltage battery capability.

Higher the voltage ... lower the KV

I tend to use a factor system I devised myself for models. KV x Cell count .. ie : 1400 KV x 4S = 5600
Lets say as example : a factor to fly your model is 5000

So you can use a 4S ... which gives : 5000 / 4 = 1250kv or 3S ... which gives 1666 kv .... assuming similar prop ...

With Wattmeter you then work to find the ideal prop. Usually finding that the higher kv will take a smaller prop.

Above numbers are purely as example and NOT my actual table numbers.

Cheeky b****** .... I seem to remember this thread topic being asked elsewhere ...

OK onto kv :

On the question of why different KV ratings ...

They give alternative prop size / battery voltage combinations. Low KV gives larger prop and or higher voltage battery capability.

Higher the voltage ... lower the KV

I tend to use a factor system I devised myself for models. KV x Cell count .. ie : 1400 KV x 4S = 5600
Lets say as example : a factor to fly your model is 5000

So you can use a 4S ... which gives : 5000 / 4 = 1250kv or 3S ... which gives 1666 kv .... assuming similar prop ...

With Wattmeter you then work to find the ideal prop. Usually finding that the higher kv will take a smaller prop.

Above numbers are purely as example and NOT my actual table numbers.

Nigel

Thank you Nigel, you are a true gentleman. As for the others ? ? ?

Dennis that is the best and most comprehensive answer that I have received yet, thank you very much. I will use that knowledge to fly with.

Since I have now found an expert on the subject, I have just one more question. If a motor is offered in different KV ratings, such as 400KV, or 800KV at the same price, why would anyone select the lower 400KV motor ? Wouldn't the higher 800KV motor be superior ? I just don't understand why they bother making the lower KV motor ? Does a lower KV motor offer some special advantage for some type of flying, or some type of plane, or some type of prop ?

Thank you Sir, you have been very helpful to me.
Eddy

Hi Eddy
Thanks!

As for the 800KV motor vs 400 KV for the same motor physical size, the 800 KV motor is going to turn at roughly twice the RPM when pulling the same watts.

That said, if you replace an existing 400 KV motor with an 800 KV motor, keeping the same prop and battery pack, that could be a good lesson on how to burn up a motor.

Kind of like a car engine, versus a similar horsepower farm tractor engine. Both put out the same horsepower, but at vastly different RPM's. (Of course, running a car engine at maximum horsepower, it won't last long. Different with a farm tractor engine)

Now, which KV motor you select really depends on the model type. In general, a low KV number suggests a slow flying model like a Piper Cub. You'd select a big diameter slow turning prop for the low KV motor. Compared to a high KV number with a fast acrobatic model with a small diameter high RPM prop. If you've seen those small ducted fan motors in some of the foamie jets, those units use a real small diameter fan, but turn them over 20,000 RPM. Some of those two foot wingspan foamies can approach 100 MPH.

Here is where it is important to use one of those wattmeters during setup of a model. What would appear to be a small change, like going from 3 LiPo cells to 4 LiPo cells can nearly double the watts input to your motor, using the same prop and battery pack. If the motor/ESC/battery is pushed beyond their ratings, you can wind up with an expensive pile of charcoal!

One thing that helps in this stuff is one of those electric computer programs like www.motocalc.com. Free for 30 days, then $39. Just input your motor/ESC/battery pack, along with the models wingspan, wing area and weight, and motocalc will give a clue on how your model will fly. Just check the "Opinions" feature.

DennyV
Retired and the days are just too short, busier than ever!

Note: to pull the same watts with the 800kV and 400kV... you must use different propellers. The one on the faster turning motor being considerably lower diameter and probably lower pitch.

There's no way that they'd pull the same watts at the same voltage with the same props.

**********

And... You'll often find that 2 different winds of the same motor for different kV will have the motors rated at different watts.

Cmpare the Cheetah motors ... 2208 series. 4 different kV. Same exact size and appearance motors (except the laser etched numbers on them) The difference in them is the windings for different kV and different watt ratings (and sometimes different cell count ratings)

Note: to pull the same watts with the 800kV and 400kV... you must use different propellers. The one on the faster turning motor being considerably lower diameter and probably lower pitch.

Yup, from posting #12 above.

"As for the 800KV motor vs 400 KV for the same motor physical size, the 800 KV motor is going to turn at roughly twice the RPM when pulling the same watts.

That said, if you replace an existing 400 KV motor with an 800 KV motor, keeping the same prop and battery pack, that could be a good lesson on how to burn up a motor."

DennyV
Retired and the days are just too short, busier than ever!

So let me be clear on this. When I am selecting the best prop using a watt meter, I am looking for the prop that pulls the Most Current, is this right ? Without exceeding the limit of the motor or course ?

No.... you are looking for the most efficient use of the power for your model.

By selection of the best motor, ESC and prop for your airplane you can get better results with half the current (or less) vs a poorly selected power system.
Best endurance, speed and better overall performance is the goal.

So let me be clear on this. When I am selecting the best prop using a watt meter, I am looking for the prop that pulls the Most Current, is this right ? Without exceeding the limit of the motor or course ?

This is where those programs like www.motocalc.com can be of benefit. This program allows you to select a range of propellers on one motor and battery pack.

The program will provide "Opinions" on how each of these props will perform. As an example, selecting a Hacker A50-12L motor and 6S LiPo, using a 13X12 prop, you will get 62 Amps, 1400 Watts, 7100 RPM, 170 ounces of thrust, and a "Blade Pitch speed" of 81 MPH. That would be for a fairly fast model.

Or, using the very wrong prop like a 17X4 unit, you would get 62 Amps, 1370 Watts, 7100 RPM, 280 Ounces of thrust, but only 27 MPH blade pitch speed. At the 27 MPH, if your model stalls at 27 MPH, it won't get off the ground. If you use that 13X12 prop on a slow draggy giant Piper Cub that only flies at perhaps 40 MPH, your motor is wasting a lot of performance. Selecting the proper propeller diameter and blade pitch to match the flying speed of your model is the way to go. In doing so, your propeller will unload in flight, dropping the watts input to your motor. Some of the Castle Creations ESC's have a data recording feature that records the Amps/Volts/Watts to your motor for the entire flight. That can be downloaded to your computer to display in graphic mode. The CC software has all these capabilities.

Needless to say, it's not likely you can find a 17x4 prop, but motocalc shows what would happen if you did use one.

These electric systems are very different from a glow or gasser. These glow/gassers are kind of like a constant torque unit, where the engine will wind up until its peak torque capability is reached. Put on to large diameter of a prop, and you'll lug it down. Two small, and it can over-rev. An electric motor is kind of a constant RPM unit. Do the propeller stuff to an electric motor, to small, nothing happens, your model won't fly. To large, and your motor will pull your model straight up out of sight, while leaving a trail of blue smoke from its windings.

These electric motors are DUMB. They will happily put out a lot of power, for a short while. That is why a wattmeter is so important. You will easily be able to find out if you're over rating your motor/ESC/battery pack before damaging anything.

DennyV
Retired and the days are just too short, busier than ever!