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Old 09-25-2008, 05:02 AM
Look out for that tree!!!
Larry3215's Avatar
Join Date: Mar 2006
Location: Gig Harbor, Wa USA
Posts: 7,061

Originally Posted by Dr Kiwi View Post

I think the connection is that guys were trying to determine how much of the observed drop in in-flight amp draw was simply due to battery sag (voltage) and how much was actually due to "prop unloading".

Cheers, Phil

Originally Posted by dk_aero View Post
I don't think the inflight testing will produce any useful quantitative data. For instance prop unloading (power decrease) vs. airspeed for a particular prop or percentage of power drop from prop unloading vs. voltage drop.

That being said, we might get closer by -
1. Testing the battery statically. With a variable load, you can (snip)

All variables that affect the independent (measured) variable must be controlled for accurate measurement (characterization) of the independent variable.
All good points, but Id still like to get as close as possible with the tools we have. I think we can at least get in the ball park with the on board logger data.

Your absolutely correct on needing to limit the "unknowns" as much as possible. We obviously cant control or know every detail perfectly - but I think we can get 'close enough' to at least get a general idea of the relationship between how prop unloading and battery sag effect the power drop in flight.

Originally Posted by mred View Post
I know the battery has an effect on motor speed and it always will, but it can be taken into account if you have a big enough battery with a good charge on it so that the load on the battery is not so high. (snip)

The point that I was trying to make is, prop loading or unloading has nothing to do with voltage sag, motor load does. If the chart has amps and voltage recorder on it and the rpm can be logged, then you have a way to measure the overall effect of prop unloading. You must chart rpm, voltage, and current to be able to tell if the prop is unloading. If the load on the motor decreases, then the amps will drop and the voltage will increase. This cannot be done very accurately with a heave load on the battery, you need a lite load, or the power supply I was talking about.


Hi Ed,

I see your point and I dont disagree with it for the most part. Your comments Ive highlited above are almost exactly how I am proceeding.

Im not saying props dont unload in flight. I want to know how much of the observed power drop in flight is due to battery sag and how much is the result of prop unloading.

Untill today I didnt have any data that would really even start to answer that question. All the previous graphs and data were missing too many details.

Tram just sent me his data file where he flew the patern we discussed previously and recorded voltage, amps, watts and speed. The one data element thats still missing is RPM. In this case I at least know what the plane was doing and what the throttle setting was at each moment along the graph.

I'll have to wait untill I can fly my own planes to get that RPM data as well.

As soon as I look at the file I'll post some results.
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