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Old Mar 23, 2011, 03:16 AM
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How to estimate current drawn "in the air" based upon current "on the ground"

Hi Guys,

How should I estimate the current drawn by a model "in the air", from measurement of current drawn "on the ground"?

I am trying to reassure myself that the current drawn by my motor/gearbox/prop setup, will be less than the max current rating of my ESC. I can measure the current drawn at full throttle while holding the model, but I understand that when the model is flying it can draw a higher current. I vaguely recall a "rough rule of thumb" that the current in flight is x% greater than static - but I cannot find the reference again.

Any help/advice will be greatly appreciated!

Thanks and kind regards, Andy
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Old Mar 23, 2011, 04:47 AM
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Hello Andy, and welcome to RCG. The static draw is usually higher than the in-flight draw due to the forward motion of the plane helping to draw air to the prop. The exception is found when the prop is very high pitch compared to the diameter. 10*10 prop is called a "square" prop and a 10*12 is called an over square. Highly pitched props can be made to stall at low airspeeds, yes they will produce thrust, just not much. Then as the plane speeds up, the prop "bites" or unstalls, then the amp draw can really spike.

For most users, the static draw will be the worst case scenario for flight loads, hovering a 3-d plane is very similar to static test loads as the plane is not moving forward.

Does that give you an idea of what to look for?

Do give yourself some wiggle.room on the esc, don't try to get away with 34.5 amps drawn through a 35 amp esc. Go up in size to the next larger one (at least), 40 amps for that example.
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Old Mar 23, 2011, 05:37 AM
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Hi Jim,

Thanks for your welcome and very helpful reply!

I am using a 15 x 10 prop (so, that is not square), and I am flying a thermal soarer (so, no 3d hovering).

If I measure 43A static, then it sounds like I am ok with a 55A ESC.

Thanks again!
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Old Mar 23, 2011, 05:43 AM
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Sounds like you will be golden! A pitch to diameter ratio of of .5 to .75 is usually good for all around sport flying. To get an average draw, run the motor inflight at wide open throttle (WOT) for a known time and note how many amps are need to recharge the battery. We can help you with the math...just need the numbers.
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Old Mar 23, 2011, 05:45 AM
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Note... extended bench run more than a few second.de are frowned upon. Please keep you bench trial short!
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Old Mar 23, 2011, 05:55 AM
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Thanks again Jim!
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Old Mar 23, 2011, 11:31 AM
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Quote:
Originally Posted by andyfromoz View Post
How should I estimate the current drawn by a model "in the air", from measurement of current drawn "on the ground"?
From the Advance curves below, the Power Coefficient Cp illustrates the power absorbed by the prop at different airspeeds.

The X-Axis may be considered airspeed.

The curves are for constant RPM. For this reason, real life will be slightly different as the RPM of the motor changes as the airspeed increases
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Old Mar 24, 2011, 04:19 AM
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Hi Martyn,

Thanks for your post. I have studied the charts in your post. They're interesting although I'm not sure I fully understand them.

For each propellor there is a ration P/D shown, is this diamater/pitch?

Do you have a legend that describes each of the labels more fully?

Thanks again, Andy
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Old Mar 24, 2011, 09:13 AM
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The Advance curves were taken from NACA Technical Report TR-237.

http://aerade.cranfield.ac.uk/ara/19...report-237.pdf

P/D is the Pitch-Diameter ratio.

The Power and Thrust Coefficients (Cp and Ct) may be used to calculate the propeller Power and Thrust at various airspeeds in Imperial units.

The X-axis may be considered airspeed.

Pitch Speed (RPMXPitch) and maximum efficiency occurs at an Advance equal to the P/D ratio.

Zero Thrust Speed occurs at an Advance of P/D+0.2
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Old Mar 24, 2011, 09:24 AM
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P/D is pitch/diameter, which is 0.67 for your 15x10 prop. The closest of Martyn's graphs is "propeller C". The 3 curves show power ('Cp') thrust ('Ct') and efficiency ('n'). Current draw is proportional to power, which for this prop is almost flat up to half the pitch speed.

I have highlighted the power curve in red. Unfortunately this graph does not show the static power. Although it may look like power should be a bit lower at zero airspeed (due to the blades being partially stalled) depending on blade shape it may flatten out or even increase slightly. At high airspeed the prop 'unloads', reducing power and current draw (down to ~20% at pitch speed).

How much unloading you will get depends on how fast your model flies. A glider at max climb rate will probably be flying quite slow, and therefore will not unload significantly. However the battery's voltage will sag as it discharges, and the motor's resistance will increase as it warms up, so you can expect the average in-air current to be lower than the peak static measurement with a fresh battery.
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Old Mar 24, 2011, 12:57 PM
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I use motocalc, which tries to simulate in air performance http://motocalc.com/
Ground testing matches pretty well to the program's numbers, I don't have an in the air measurement system but subjectivly Motocalc seems to be close based on my experience.
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Old Mar 24, 2011, 03:14 PM
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Quote:
Originally Posted by Bruce Abbott View Post
At high airspeed the prop 'unloads', reducing power and current draw (down to ~20% at pitch speed).
Just a comment. For the prop in question, the Power and current draw are down to about 50% of static at PITCH SPEED.

They are down to about 20% at VIRTUAL PITCH SPEED or ZERO THRUST SPEED as a result of the profile drag of the prop.

There has been considerable confusion in the past about the definition of PITCH SPEED.

PITCH SPEED (RPM X PITCH) occurs at an Advance equal to the P/D ratio.

For a prop with a P/D ratio of 0.7 the PITCH SPEED occurs at an Advance of 0.7.

At this Advance, the vector sum of the forward and rotational velocities of the prop give an effective angle of attack along the entire prop blade of zero degrees and a maximum efficiency for flat-bottomed airfoils (propellers). This is why props are twisted. In full scale aircraft the propeller, plane and engine combination are designed to cruise at PITCH SPEED because this speed gives maximum prop efficiency.

ZERO THRUST SPEED or VIRTUAL PITCH SPEED occurs at an Advance of P/D + 0.2 which can be 20% to 40% faster than PITCH SPEED depending on the P/D ratio.

The 0.2 factor occurs because propellers still have thrust at negative angles of attack (about -5 degrees for flat-bottomed airfoils).

My point is that PITCH SPEED and VIRTUAL PITCH SPEED are two different entities.

It is possible to fly faster than PITCH SPEED because propellers still generate Thrust at negative angles of attack.

The X-axis of the Advance curves although normally considered to be airspeed may also be thought of as effective angle of attack with PITCH SPEED being zero degrees and ZERO THRUST SPEED (VIRTUAL PITCH SPEED) being -5 degrees.

For the prop in question, the STATIC effective angle of attack would be about 17.5 degrees.
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Old Mar 24, 2011, 04:28 PM
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Quote:
Originally Posted by Martyn McKinney View Post
Just a comment. For the prop in question, the Power and current draw are down to about 50% of static at PITCH SPEED.

They are down to about 20% at VIRTUAL PITCH SPEED or ZERO THRUST SPEED as a result of the profile drag of the prop.
Point taken. I was using the zero thrust point for pitch speed (ie. virtual pitch speed or true pitch speed) but this is usually higher than the geometric pitch speed.

The amount of difference between geometric and virtual pitch depends on the airfoil shape of the blades and how closely the blade twist matches a true helical pitch along the blade. Also, the designated pitch for a particular prop might not match its true pitch (eg. GWS EP7035 is supposed to be 3.5" pitch, but according to my measurements it's closer to 2.5").

Without wind tunnel testing it is hard to know exactly what the true pitch and loading of a prop is, so calc programs just have to make a guess. Unfortunately they don't always get it right. Here's a Motocalc prediction for 50mph pitch speed, showing thrust and power dropping to zero at only 48mph! (it does manage to get the efficiency about right though). This may explain why it often underestimates the top speed of fast models...
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Old Mar 24, 2011, 08:06 PM
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I'm curious about the Motocalc results.

From the NACA Advance curves it may be seen that at zero thrust the Power absorbed by a prop does not go to zero because of its inherent profile drag. Motocalc indicates zero power at zero thrust.

Also the Motocalc Thrust-Efficiency curves indicate the highest efficiency at zero power.

Is there something wrong with the inputs or should someone give Motocalc a call?
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Old Mar 25, 2011, 01:01 AM
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Quote:
Originally Posted by Martyn McKinney View Post
Is there something wrong with the inputs or should someone give Motocalc a call?
There's nothing wrong with the inputs. I always knew that it didn't take prop drag into account, but I hadn't tried plotting prop efficiency until now. This makes it even more obvious that something is wrong with the algorithm that Motocalc uses.

There are a few other improvements I would also like to see, so I might compile a list and send it to stefanv.
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