Prop unloads

[kyleservicetech]

Originally Posted by JetPlaneFlyer

I'm not really sure. i.c engines have very different charecteristics to electric power. Generally on i.c. when holding open throttle the power of the engine increases as the RPM rises. This is opposite on electric motors where (input) watts reduces (eventually to zero) when the motor is allowed to rev out and is at a maximum when the motor is stopped dead.

Very good points. The gasser/glow engines are sort of a constant torque power plant, where the RPM will increase to the limit set by the engine torque available. And, diving or climbing out, these engines RPM will vary widely. Put to small of a prop on these engines, and they will over rev, potentially damaging them. Put to large a prop on these engines, and they will lug down, and possibly overheat.

Compare that to the quality electric motors, where they are more of a constant RPM power plant. And, diving or climbing out, the prop will stay fairly constant, but the current/watts will vary widely. Put to small of a prop on an electric motor, and the motor will simply turn over at its maximum unloaded RPM value. It may not put out enough power to taxi the model. Put to large of a prop on an electric motor, and it will turn the prop over very nicely, that is until the smoke pours out of the motors windings.

As such a properly set up electric model airplane will be pulling maximum watts while the model is not moving.

This is why having access to a wattmeter is critical when trying different props and battery packs. Even a small prop diameter change, or cell count on the battery can and will have a very significant effect on the Amps/Watts your motor is pulling.

[Larry3215]

Just for grins - and to see how different the results might be - I just ran the numbers on some data from one of my sailplanes.

This is an older design all balsa Mirage sailplane with a 112" span that weighs right at 33 ounces in this test. Power is an A20-20L Hacker, ICE 50 controller, 11x6 folding prop and a 1300 45C off brand pack that was fairly new and had a pretty good Ir at the time. Id call it an honest 30C pack.

This is a 30 second run at full throttle while climbing at roughly a 40-45 degree angle from a hand toss.

I had to look at a dozen or so different climbs to find this one that had a fairly straight rpm curve for at least a majority of the climb.

On most of the climbs I am constantly adjusting the climb angle and heading to compensate for gusty conditions, thermal activity and to dodge other models - this was during a contest.

If you look at the latter part of the graph you can see where I level out and fly horizontally (perhaps even enter a shallow dive) for a while just before the motor cut off. The rpm climbs rapidly as the power drops off at the same time even though the model was still at full throttle.

A very graphic example of unloading

However, if you compare the battery voltage and power early (about 1 second) in the launch and then again just before I level out, the comparison is interesting.

The battery voltage drops from 10.9 to 10.6 volts over roughly 20 seconds and the power drops from 257 watts to 214 watts.

That voltage drop is works out to a power drop of roughly 8% dropping the power by 21 watts to 232 watts just because of the voltage drop alone.

That leaves an additional 15 watts of power loss to account for over roughly 20 seconds of flight.

Would that entire 15 watts be due to aerodynamic unloading or would some be due to motor and/or esc heating? How much effect did the battery warming up have on the results? Did I perhaps make a slight change in climb angle? If so, was it up or down?

Even if we lay it all on "unloading" you can see that the power loss due to battery voltage drop still dominates.

Of course, if you pick different reference points on the graph to compare, then the numbers come out somewhat differently.

This "test" was quite different from most of my earlier testing. This model is much more streamlined than the large foamies I used before, but probably has a similar total wetted area. Wing loadings were in the same ballpark.

These newer batteries are much better at holding voltage now than they were 2 or 3 years ago. The motors in all cases are hi quality outrunners.

Most importantly (maybe) - this "test" was done during a (more or less) constant climb rather than a (more or less) level flight.

In any case, there you go

[Larry3215]

Lets look at one other section of the graph - the first 3 seconds. Thats when I would expect most of the initial speed build up to occur. In other words, the model should be well on its way to a top speed by the end of the three seconds.

Thats the time when the power graph is dropping the fastest. It tends to level out after that 3 seconds.

I expected to see the greatest aerodynamic unloading during that time while the model was accelerating up to top speed. However, the numbers work out quite differently.

Power drops from the same 253 watts to 240 watts. Voltage goes from 10.9 to 10.7.

That works out to a power loss from the voltage drop of 14 watts. So the power should have dropped to 239 watts - but it actually only went down to 240 watts.

Why is there no sign of any aerodynamic unloading during the initial acceleration?

This is not the result I was expecting to see - which I think is cool

[soarrich]

Wow! I thought I was getting hitech when I bought a watt meter, rather than using the smoke test for motor selection.

[JetPlaneFlyer]

Yep, that's an excellent example of unloading

If you look at the power just before the plane levels out and the RPM rises (1245 seconds) power is about 205W. Four seconds later the plane is in level flight the prop has unloaded and power is about 150W (hard to be exact because the power line drops of the bottom of the graph).

That's a 25% power unloading just between a fast climb and level flight. now imagine how much unloading you might see between static and level flight

The contribution of voltage drop in the graph is very minor. If we exclude the initial fresh battery surge that lasts about a second, and a couple of momentary dips, then voltage was rock solid at 10.8V throughout.

All in all this are some of the reasons why i feel quite comfortable having my motors right on the stated limit, or even quite significantly over, during static thrust test with fresh batteries.
I've yet to burn a motor despite in some cases running at twice what the grams/3 rule of thumb would indicate was the motor safe limit (your mileage may vary)

[soarrich]

Originally Posted by soarrich

My motor is right at it max rating on the ground 55a and a little over 815 watts, (834w), so when it unloads in the air am I going to toast it? or am I safe?
Thanks

Just a follow up report:

I test flew the plane, it's a .40 Kaos kit converted to e, right after I landed the motor felt like it was about 80° F and the same for the battery, so I feel I'm safely in the comfort zone for both. I was getting about 8 minutes and the plane had a lot more vertical than I remember a 40 Kaos having 30 years ago. I'm going to switch to a 10x7, hoping to get more of the feel of a 70's 40 Kaos and get 10 minutes flight times.

[kyleservicetech]

Originally Posted by soarrich

Just a follow up report:

I test flew the plane, it's a .40 Kaos kit converted to e, right after I landed the motor felt like it was about 80° F and the same for the battery, so I feel I'm safely in the comfort zone for both. I was getting about 8 minutes and the plane had a lot more vertical than I remember a 40 Kaos having 30 years ago. I'm going to switch to a 10x7, hoping to get more of the feel of a 70's 40 Kaos and get 10 minutes flight times.

Yeah
One very rough test on how hard you are pushing your motor is putting a finger on it for 15 seconds immediately after landing. If the motor nameplate isn't branded into your fingertip, the motor should be OK.

I did check with Hacker last year about safe operating temperatures on their motors. Hacker indicated their motors are safe up to about 140 degrees F. And, 140 degrees F will be very uncomfortable to put your finger on for more than a few seconds.

As you've found out, it isn't hard to put together an electric power system with quality components that can match or outperform the same model with a glow engine up front.

[kyleservicetech]

Originally Posted by Larry3215

Lets look at one other section of the graph - the first 3 seconds. Thats when I would expect most of the initial speed build up to occur. In other words, the model should be well on its way to a top speed by the end of the three seconds.

Thats the time when the power graph is dropping the fastest. It tends to level out after that 3 seconds.

I expected to see the greatest aerodynamic unloading during that time while the model was accelerating up to top speed. However, the numbers work out quite differently.

Power drops from the same 253 watts to 240 watts. Voltage goes from 10.9 to 10.7.

That works out to a power loss from the voltage drop of 14 watts. So the power should have dropped to 239 watts - but it actually only went down to 240 watts.

Why is there no sign of any aerodynamic unloading during the initial acceleration?

This is not the result I was expecting to see - which I think is cool

Just looked up flight data on my new Kantana model, equipped with a Hacker A50-12S motor, 16X8 APC-E prop, 6S2P A123 battery pack.

On take off, the CC ICE ESC shows a maximum full throttle current pulled of 65 Amps, 17 Volts, 1100 Watts, 7550 RPM.

After the model leveled off, the full throttle readings dropped down to 37 Amps, 17.6 Volts, 650 Watts, with the RPM increasing to 8230 RPM. That is a power reduction of 50% with only a 9% increase in motor RPM. Typical of a high efficiency motor.

Note that the voltage on these A123 cells actually increases with reduction in current pulled out of the battery. This is true even after about 70% of the battery capacity has been used. On this run, the A123 battery pack put out 16.7 VDC at 56 Amps, about 20 seconds after takeoff. After pulling 70% of the battery capacity, that same A123 battery pack put out 16.4 Volts DC at the same 56 Amps during a climb out. This is typical of these A123 cells, their voltage discharge curve is pretty flat.

Guess this would be kind of expected with a big diameter, low pitch propeller on a fairly streamlined model such as the Kantana.

[Dr Kiwi]

Just for grins - and to see how different the results might be - I just ran the numbers on some data from one of my sailplanes.

now imagine how much unloading you might see between static and level flight

Larry and Jetplaneflyer, just to help complete the picture... have you measured WOT static... so that we can see how much unloading there is between static and actual flight (climb, level etc).

[kyleservicetech]

Originally Posted by Dr Kiwi

Just for grins - and to see how different the results might be - I just ran the numbers on some data from one of my sailplanes.

now imagine how much unloading you might see between static and level flight

Larry and Jetplaneflyer, just to help complete the picture... have you measured WOT static... so that we can see how much unloading there is between static and actual flight (climb, level etc).

On my models, the wide open static motor current is pretty close to the motor current pulled during a vertical climb out.

[kyleservicetech]

There has been a lot of comments on this "Propeller Unloading" of a model's propeller during flight.

Here is a very interesting site from APC on propeller data for most of their props.

Their charts cover forward MPH, Advance ratio, Prop Horsepower, Torque, and thrust.

Take a look:
http://www.apcprop.com/v/downloads/P...B/datalist.asp

On my Giant Big Sticks 19X12 APC-E prop, at 7000 RPM, the static HP input is 3.9 Horsepower. At a forward flying speed of 60 MPH with the same RPM, the Horsepower input drops to 1.9 HP, about 1/2 of the static HP input. Of course, the motor RPM will increase slightly, by a measured 7% in the case of my Hacker A60-16M. This 7% value came from the CC ICE ESC's downloaded flight data.

This electric motor characteristic is very different than the same prop with a glow/gasser engine. These engines will "wind out" as the propeller becomes unloaded. These higher quality, higher efficiency electric motors will not wind out nearly as much.