Propeller load changes with diameter

[Brent Williams]

Looking for a bit of guidance regarding propeller load change calculations with diameter.  I have read a bit about the diameter changing the load something akin to the 5th power. 
In simple brain terms, if a person had a 10x6 prop that pulled 38amps at 9600rpm, is there enough info provided there to enter into the formulas to roughly figure what power a diameter increase to a 10.5x6 or 11x6, or a diameter decrease to 9x6 would require at the same RPM?  A change in prop diameter equals X increase in load.

Thanks in advance for any helpful answers, (please forgive my lack).

https://stunthanger.com/smf/gettin-all-amp'ed-up!/propeller-calculations-more-than-you-want-to-know!-pointy-head-alert!/

[Brett Buck]

Looking for a bit of guidance regarding propeller load change calculations with diameter.  I have read a bit about the diameter changing the load something akin to the 5th power. 
In simple brain terms, if a person had a 10x6 prop that pulled 38amps at 9600rpm, is there enough info provided there to enter into the formulas to roughly figure what power a diameter increase to a 10.5x6 or 11x6, or a diameter decrease to 9x6 would require at the same RPM?  A change in prop diameter equals X increase in load.

Thanks in advance for any helpful answers, (please forgive my lack).

https://stunthanger.com/smf/gettin-all-amp'ed-up!/propeller-calculations-more-than-you-want-to-know!-pointy-head-alert!/

   Note that it goes with the 4th power of diameter *only in a static test*. In-flight, there are many other factors, in particular, with more diameter and everything else equal, it is more efficient, so it might well take less power and reduce the shaft HP required to get the same in-flight speed. So in fact, it will put less load on the engine.

    The inverse also works. The 20FP setup usually works better with a 9-4 than a 10-4 precisely because it is less efficient, allowing/requiring the engine to run faster to get the same in-flight speed. Essentially, using less diameter slows the airplane down. A 10-4 is frequently enough more efficient that the airplane is too fast, and you have to try to needle it down, which puts the engine in a bad or unstable operating point.  You could manage it with a smaller venturi- but the last thing the world needs is another reason to start fiddling with engines.

    Brett

[Bill Schluckbier]

Some time ago I wrote about trying to do something similar for our club newsletter.  I fly both C/L and R/C so will see topics covering both aspects of the hobby.  Hopefully this helps.

[Tim Wescott]

If you're buying APC or similarly-priced props, get a handful at slightly different diameters and pitches, then try them out to see what works well.

Keep in mind that with electric it's really easy to trim the RPM up or down a bit, so the need to get a prop exactly right isn't as strong as with slime engines.

[Brent Williams]

Thanks for the good info.  I'll take some time and digest it.  I think my initial request looking for a simple factor for a diameter change is probably a bit incomplete considering the many factors that go into overall "load". 
 
Following a similar thought, Lucien at Innov8tive dyno tests the his motors with common APC props.  Attached is a cropped selection highlighting the full throttle load of 8x6, 9x6, 10x6, and 11x5.5 for the BA2820-1350KV on 3S.  We don't use them at full throttle RPM, but Lucien let me know how to relatively accurately calculate amp draw for a given RPM based on these full throttle numbers.

He indicated in an email that to calculate amp draw at a desired RPM, divide the desired RPM by the full throttle RPM and then square that number.  Then use that x² number to multiply against the tested full throttle amp load data.

If I wanted to get the approximate amp draw for this BA2820-1350KV motor on 3s at 9600RPM using Lucien's APC 10x6e test data: 9600rpm/12023rpm=0.798,  0.798²= 0.637, Full throttle 58.32amps x 0.637= 37.18amps at 9600rpm (loaded.), and even fewer amps as the prop unloads in-flight.

[Howard Rush]

I would be suspicious of most internet prop calculators. JavaProp is good. 

[Ken Culbertson]

This raises a question as to what parameter we should use and what we actually need.  I may be way off, but my gut reaction is that we should be looking to thrust and watts.  How much thrust do we need to get to the top of the hourglass on tired hot batteries and how many watts will it use.  The prop selection is even more critical with electric than IC.  I am in the minority here, but we seem to be trying to build lighter planes to accommodate small batteries rather than building larger batteries to accommodate heavier planes.   An IC has used 75% of it's fuel(weight) when it turns north in the Hourglass, an electric Zero and is pulling the same weight it did at takeoff.  IC still has 100% of it's power with less weight, battery has lost some power with the same weight.  Unless you have a stash of 4-5lb straight grained balsa in the basement of garage, I think the days of the 6s 2800 LIPO are behind us and we need to start looking in the 3200 range and building planes that will house them at realistic weights for the materials we have. 

Ken

[Brett Buck]

I would be suspicious of most internet prop calculators. JavaProp is good.

   This is my own ignorance speaking, but how to you get realistic prop parameters into a simulation? Or rather, sufficiently detailed parameters to be realistic? I have endlessly fiddled with props. I know that frequently, absurdly tiny changes make a huge difference for unclear or at best speculative reasons. Getting it right in the gross is not that difficult, getting it right in detail is slightly better than random luck.

    Brett

[Paul Walker]

This raises a question as to what parameter we should use and what we actually need.  I may be way off, but my gut reaction is that we should be looking to thrust and watts.  How much thrust do we need to get to the top of the hourglass on tired hot batteries and how many watts will it use.  The prop selection is even more critical with electric than IC.  I am in the minority here, but we seem to be trying to build lighter planes to accommodate small batteries rather than building larger batteries to accommodate heavier planes.   An IC has used 75% of it's fuel(weight) when it turns north in the Hourglass, an electric Zero and is pulling the same weight it did at takeoff.  IC still has 100% of it's power with less weight, battery has lost some power with the same weight.  Unless you have a stash of 4-5lb straight grained balsa in the basement of garage, I think the days of the 6s 2800 LIPO are behind us and we need to start looking in the 3200 range and building planes that will house them at realistic weights for the materials we have. 

Ken

Bigger and heavier is not always the best option. One needs to carefully balance the entire system. Knowing the discharge curve for your batteries is important. Li-ions are quite different than Li-pos as the Fall faster to a lower voltage without damaging the battery. With that information and battery cell count you can determine how much KV your motor will need to finish the pattern. Capacity of the battery also matters.

I used the 6S2800 Li-pos for YEARS. In my 68 ounce 700 in^2 plane, it would normally use 1900 to 2000 mah's. We'll within range of a long lifetime for that battery. Going to Li-ion's, there is no change in consumption but the voltage is lower when used in the same plane. It is required to reduce the min voltage in the ESC to 2.7 ish to get to the end of the pattern at "full" power.

What will make a difference is the prop. I mostly use the Igor 12*5 flat back prop. Nice drive and minimal drain on the battery. I have used the 12*5 Igor underchamnered prop and that will take more mah's for a pattern. I have burned up two motors and some bartteries using that prop. The motors because the average current level was above 30 amps and the motor only rated for 30. The batteries because it needed more capacity than they had.

Bottom line is I have used 2800's for years with no issues. Not sure why you have such a capacity issue.

[Ken Culbertson]

Bottom line is I have used 2800's for years with no issues. Not sure why you have such a capacity issue.

Thanks for responding.  Something is sucking the life out of the batteries and nothing is jumping up saying "Me, Me, Me"!   Since I cannot do anything about the weight (it is now 68oz after changing ESCs), I am stuck trying to stretch available mah.  Somehow I do not think that the battery draw of a twin on one battery is the same or less than a single motor of similar capacity.  I do know that for me, flying a twin is better than any single so I need to learn more about what I am doing.

A brief study of the prop chart was of no use for 6s.  Using the 5s chart tells me that the 10-6 that I have been using is a bad choice.  A 5 pitch is better and I now have a set to try as soon as work permits.  I have replaced the Spin 33 ESCs with FlyColor 50As which will support 6s batteries.  That and a careful rerouting of the wiring and airflow to get better cooling may be all I need.  If not, I am in uncharted waters.  I wish I could afford CF props but 2 @ even $50 is a nonstarter.  I am going to try various APC props to see if that helps.

Ken

[Howard Rush]

   This is my own ignorance speaking, but how to you get realistic prop parameters into a simulation? Or rather, sufficiently detailed parameters to be realistic? I have endlessly fiddled with props. I know that frequently, absurdly tiny changes make a huge difference for unclear or at best speculative reasons. Getting it right in the gross is not that difficult, getting it right in detail is slightly better than random luck.

It can be done, although I doubt if it’s worth doing.  One could cut up a Topflite 10-6, measure angle of attack along the blade, try to figure out the airfoil characteristics, then enter that into a program and see what it does. It is easier and probably more useful to have the program design a minimum-induced-loss prop from your inputs, then fiddle with diameter, blade number, and blade Cl to get insight as to how those things affect efficiency and off-design performance.  That should answer Brent’s question. Pick a speed somewhat lower than cruise speed, an RPM agreeable to your motor and battery, and a blade Cl of .5 for openers.

 I made a program awhile back from a paper by Adkins and Liebeck, put in Reynolds number and stuff, and even got discouraging advice from Larrabee himself.  I don’t remember what came of it.  I just use Igor’s props.

[Doug Moon]

>>> Getting it right in the gross is not that difficult, getting it right in detail is slightly better than random luck.

    Brett

You said a mouthful right there. I live on this road. Over the years I have learned that the worst part about a crash is losing the prop. That one you have messed with for years to get just right. I recently had one crack the trailing edge and it will NOT fly anywhere near the same. Such a shame too as it was an Eather finished green three blade... 

[Brett Buck]

It can be done, although I doubt if it’s worth doing.  One could cut up a Topflite 10-6, measure angle of attack along the blade, try to figure out the airfoil characteristics, then enter that into a program and see what it does.

     I actually did that, took a few props, sectioned them every 1/2" and then tried computing the AoA using the Stuart Sherlock's camber/zero lift line equation. What I found was very interesting, but not terribly surprising. The camber of the prop drastically affects the "experimental pitch" and it very very different at the tips between an Eather flat-back, Eather UC, and Bolly. It did tend to explain some of the results we got, but also showed the massive overreliance on/misinterpretation of the measured pitch using pitch gauges.

     Beyond that, even the very best examples varied a lot even from blade-to-blade on the same prop, much less one prop to the other.

     Brett