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Old 03-19-2012, 09:01 PM   #1
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Default Static run up: more amps to ESC compared to free flight?

When you are testing the amps for your aircraft during a static run up (using a watt meter), aren't you overestimating the amps? Reasoning: during a static run up the prop has to do more work to stay at the desired rpm compared to when it is actually flying. If this is true, is there anyway to compensate for this (use a strong fan blowing on the aircraft from the front)?
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Old 03-19-2012, 09:09 PM   #2
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I think it's a non-starter to be honest ......... on ground you will have xxxxx rpm / xx amps

In air you will xxxxx + a bit more rpm / xx amps as a result.

Anyway ... if on ground you get a higher amp reading - that to my mind is a bonus as it inbuilds a safety factor when deciding on ESC / Motor etc.

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Old 03-19-2012, 09:27 PM   #3
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Originally Posted by solentlife View Post
I think it's a non-starter to be honest ......... on ground you will have xxxxx rpm / xx amps

In air you will xxxxx + a bit more rpm / xx amps as a result.

Anyway ... if on ground you get a higher amp reading - that to my mind is a bonus as it inbuilds a safety factor when deciding on ESC / Motor etc.
What if you are also trying to determine a safe flight time? If you do this with a static run up, say at 50% throttle setting, will you get a realistic estimate of maximum flight time? It seem that the estimate would be less because there is more work being done by the propeller in a static run up compared to actual flight.
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Old 03-19-2012, 09:40 PM   #4
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Originally Posted by paul8 View Post
When you are testing the amps for your aircraft during a static run up (using a watt meter), aren't you overestimating the amps? Reasoning: during a static run up the prop has to do more work to stay at the desired rpm compared to when it is actually flying. If this is true, is there anyway to compensate for this (use a strong fan blowing on the aircraft from the front)?
Yes, you are over estimating the amps but it's always a good idea not to run your ESC/motor on the limit anyway as some of the published ratings are a bit suspect. I don't think you should worry about it too much, just don't go over the rated currents (even on a static test).
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Old 03-19-2012, 09:47 PM   #5
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There are too many variables. Low pitch props may give close read-outs while static. High pitch props may be no where near their flight values as they may be in complete stall. Props vary in their efficiency all over the map making them hard to compare.

While in flight the actual amp use will be quite variable depending on throttle position, prop efficiency, plane drag, wind, flying style, etc. That makes translating bench test to flying test an extremely rough comparison. It's a bit better than a WAG, but not a whole lot.

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Old 03-20-2012, 08:17 AM   #6
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Originally Posted by paul8 View Post
What if you are also trying to determine a safe flight time? If you do this with a static run up, say at 50% throttle setting, will you get a realistic estimate of maximum flight time? It seem that the estimate would be less because there is more work being done by the propeller in a static run up compared to actual flight.
50% .... now that is asking for trouble, as a BL ESC will be pulsing the power and the amps shown on a Wattmeter are generally regarded as inaccurate anyway. It's only WOT that a Wattmeter is generally regarded as remotely accurate.
Given that the Wattmeter is only a cheap meter and subject to errors regardless of what sales blurb says - is it then really a problem ?

A Brushed set-up is different as then you will have DC amps directly registered.

I personally ignore the innaccuracies and take the readings of the Wattmeter and use them for all calcs, assuming that they are overstated. It gives me a safety margin that so far appears to be true.

ie - My EDF on 4S and Wattmeter ... calcs give about 1m 20s at WOT on the 920mAh 4S 25C ... I flew for 1m 45s and just made it to land. That's good enough for me. Same EDF on 3S 1800 25C calcs are 6m 15s ... I get 6m 30s and still enough to do a couple of circuits to land.

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Old 03-20-2012, 12:45 PM   #7
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I used to do a short flight, measure battery voltage and repeat with a longer flight time until I hit my safe maximum time. I want 3.7-3.8 V/cell remaining.

Now I have one of these: http://www.hobbyking.com/hobbyking/s...mp_mAh_V2.html so I know whats really going on.

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Old 03-20-2012, 02:00 PM   #8
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As noted by others, there are too many variables to boil it all down to a simple 'one size fits all'' type answer. Fine to moderate pitch props do unload as the forward speed increases so amps drop. On the other hand coarse pitch props may show some initial increase in amps due to the blades 'un-stalling' as the plane moves forward.

Also it depends on the type of flying you are doing. If you are doing 3D type aerobatics then you will often have the plane stationary, or even travelling backward, so prop load may in theory at least, increase and amps increase even when you use a fine pitch prop. I tried a data logger on my 3D model and hit peaks that were equal to what I recorded in static testing.

In any case, whatever you fly and however you fly it, you would be asking for trouble to run your hardware so hard as to rely on it unloading (at all) in flight.
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Old 03-20-2012, 04:44 PM   #9
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Originally Posted by solentlife View Post
50% .... now that is asking for trouble, as a BL ESC will be pulsing the power and the amps shown on a Wattmeter are generally regarded as inaccurate anyway. It's only WOT that a Wattmeter is generally regarded as remotely accurate.
Given that the Wattmeter is only a cheap meter and subject to errors regardless of what sales blurb says - is it then really a problem ?

A Brushed set-up is different as then you will have DC amps directly registered.

I personally ignore the innaccuracies and take the readings of the Wattmeter and use them for all calcs, assuming that they are overstated. It gives me a safety margin that so far appears to be true.

ie - My EDF on 4S and Wattmeter ... calcs give about 1m 20s at WOT on the 920mAh 4S 25C ... I flew for 1m 45s and just made it to land. That's good enough for me. Same EDF on 3S 1800 25C calcs are 6m 15s ... I get 6m 30s and still enough to do a couple of circuits to land.
I should have been more specific in my follow up: I was wondering what the effect of estimated flight time (not watt meter reading) on doing a static run-up at 50 % throttle compared to flying at 50 % power.
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Old 03-20-2012, 04:48 PM   #10
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Originally Posted by pmullen503 View Post
I used to do a short flight, measure battery voltage and repeat with a longer flight time until I hit my safe maximum time. I want 3.7-3.8 V/cell remaining.

Now I have one of these: http://www.hobbyking.com/hobbyking/s...mp_mAh_V2.html so I know whats really going on.
I didn't know they made a product like that. No more guessing or trial and error. The price isn't too bad either.

Thanks,

Paul
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