I have been reading some of the threads discussing electric motors, batteries, and prop combinations. One of the resources that I did not see mentioned was eCalc.

http://www.ecalc.ch/motorcalc.htm?ecalc&lang=en

We, Dave Denison, and I find the program a useful starting place for electrics. We have used it, and similar calculators for several years in RC. It does not really allow for throttle adjustments, making it difficult to adjust for desired speed. We first use the standard 4S or 5S LiPoly pack and run the data.  One of the ways to incrementally change the engine RPM is to approximate the voltage going to the motor by switching to NiMh cells and using 10 cells or 11 cells instead of the approximate 12 cells that would equate to a 4S with the lithium pack. Similar adjustments can be done for the five cell lithium packs. In general it will give a good approximation of full throttle pitch speed which approximates airplane speed. Some of the normally used electric motors are already in it. It is easy to put a custom motor in if you can find the parameters of the motor. Those parameters are easily seen as blanks to fill in.

I believe the pitch speed is calculated directly from the pitch mathematically. If the prop pitch is not actually that stated by the manufacturer than the calculations will be invalid. I have read on the forum that the 13 X 4.5 F 2 B prop is closer to a 5.5 effective pitch. The program seems to work fairly effectively with the 11 X 5.5 and standard 12 X 6 props. It seems to be correct for both rotations. When we have used the 12 X 6 F 2 B prop the pitch seems to be close to 6.7 effectively. Fortunately the calculator will allow for odd pitch sizes.

Battery amperage seems to run a bit high as done by eCalc versus what we see in flight, but it does not adjust for “throttle” or RPM settings as noted.

We have tried using a radar gun and calculating airplane speed using known parameters of radius and lap time. Once speed is known one can calculate the prop pitch necessary to generate that forward speed at a known RPM. The pitch speed and airplane speed seem to work out fairly closely disregarding factors such as drag.

An airplane on 60’ lines has a radius at the fuselage of about 64’ including arm length and with a 5.2 second lap is flying about 52.7 mph. The same plane on 65’ lines has a radius of about 69’ and is flying about 56.8 mph. Both examples are a direct correlation of mph at the fixed lap time.  The 64’ circle is 92% of the 67’ circle and 52.7 mph is 92.7% of 56.8 mph.  That knowledge helps when you look at prop speed to see where you are within eCalc.  You need to fly about 53 – 58 mph prop speed depending on line length and you need to have enough RPMs left as “headroom” to allow for gain.  If you can reach the needed/desired prop speed on a couple of NiMH cells less than the LiPoly equivalent and fit in the amperage/wattage parameters, then you have a good chance of the system working well for you.

A couple of other useful approximations are noted.
    A 6” pitch prop will turn 0.5’/rev and at 9680 rev/min will travel  9680rpm x 0.5’/rev divide by 60 sec/min = 80.6’/sec or 55mph.  If the pitch is effectively 6.7” (11% higher) about the F2B 12 x 6, the RPM can be about 11% lower for the same speed, or 8615.  This is about the RPM we see with the 12 x 6 F2B.
 

    Using about 5 – 5.2 seconds as lap time the change of about 150 rpm in the 8500 range and 200 rpm in the 9600 – 10,000 rpm range will change the lap time about 0.1 seconds.


ECalc and the approximations seems most useful when used in light of flyer's experience. The Thread on "List your Set Up" is a great value used along with the calculator.

I hope this is useful and helpful to others.  

Howard thanks for the read and reply

Of course trial and error and experience trump.  It is nice to be able to select a motor and get an approximation of whether or not it will work.  The actual prop pitch will help make better sense of the numbers on eCalc, especially prop speed but the ability to try motor, prop, and battery combinations seem important to many of us as we start in electrics. 

Actual effective prop pitch is easily obtainable if the numbers in "List your set" up are correct and complete.  Line length, rpm, and lap time will mathematically work backwards to the necessary effective pitch.  I note math, not true engineering as drag/slip and others factors are not applied, still the effective pitch for comparison.  The purpose is to test systems before buying and setting up airplanes, for those of us with less experience.

The term "pitch speed" is noted on eCalc, and I used that term because it is listed.  I like the term effective or calculated pitch, determined by knowing tachometer rpm, line length/circle circumference and lap time.  Not perfect, but the effective pitch that will make or allow the plane to act within the above parameters.  The 11 x 5.5 electric prop seems to act like a 5.5" pitch as I have flown it.  Also the electric 12 x 6 both directions seem to act like a 6" pitch and the 12 x 6 F2B acts like an effective 6.7" pitch.  If I use those parameters then the program seems to work.   I will eventually get data from the 13 x 4.5 F2B that I have heard is more like 5.5" effective pitch and then have most of the props that I am likely to use.  They are the electric props that I see commonly asked about.

Here is another similar program

http://adamone.rchomepage.com/calc_motor.htm

I use them to compare motors, Kv, props, and batteries when looking at set ups.  I also try to compare to known set up in "List ..." and what I have used previously.

One problem that I see is when the program is felt to suggest that a particular motor might work.  When you get to a specific motor brand then bearings vary a lot is size and quality.  Some of the inexpensive and light weight motors have small and poor quality bearings.  I have seen the difference in the bearings in an Eflite 32 and the generic 32 equivalent with the same motor parameters and therefore the same calculations on eCalc. Eflite bearings were 5mm wide vs 4 and much better tolerances to begin with.  Replacing the cheap bearings with Boca or similar helped a bit, but the housing did not allow a larger bearing.

Howard, are you not liking the term "pitch speed" or suggesting that the calculator is too misleading in gereral?  The prop data is supplied by the user as to prop length and pitch.  You can use the prop manufacturers data or better data as can be determined or calculated.  As you can see from threads in Stunt Hanger, there many questions about getting started and specific combinations.  I have not destroyed any equipment, yet, from using this program including several years of RC.

I am not sure whether you are stating a possibility or noting that the program gives that for your actual data.  If real data, what is your line length, rpm, lap time, and then eCalc parameters - prop, battery, motor and if needed motor parameters? 

I do not understand your statement "engine, battery, and prop will go 45 mph.  If I used that information and did something to make it go 55 mph, I'd hurt something."  Perhaps real numbers as above would help.

Thanks for the clarification.  What is your actual data, please?  What prop, what pitch, what rpm, actual line length 70" loop to loop, and actual lap speed when the program said 45 mph??  As noted, the usual culprit is calculated pitch vs manufacturer stated pitch.

The culprit is that it is a bogus calculation.  

"It" meaning eCalc or my calculated effective pitch.  I am not trying to explain airfoils or how propellers work, or design one as the link.  I am using rpm, lap time. and distance traveled to get the distance covered by one revolution, the effective pitch.  It does not compare airfoil or types of propellers, but I think that is the reason that you choose a "type of propeller."  I notice the "PConst" change by type.  How is that mathematical calculation bogus?

Sorry.  That wasn't clear.  The program says that my rig would go 45 mph.  I don't know the throttle setting they have in mind.  That's a 6.6-second lap on 70-foot lines, isn't it?  That's too slow.  If I believed the program, I wouldn't have tried flying that combination.  I'd change kV or something to make it go faster.  Then, when I tried running it, it would go really fast.  When I misset my throttle on a cool day last year and went too fast, I toasted a battery.

You used the program and it failed you, or you toasted a battery on your own but the program would have gotten you to try a different Kv motor??

Sorry, not trying to be argumentative, but I am actually trying to learn something and help others who have questions getting started.  I have used this program RC, Electric gliders with folding props and now control line.

The airplane is going a certain speed and distance with the motor and prop turning a given rpm in level flight.  That divides out to a distance per turn, an effective pitch, though distance per turn is probably a more accurate term.  How and why it generates that distance may not be so simple mathematically.  I use the manufacturers prop data in many cases and that stated pitch usually is about what I can calculate as distance per turn.  I have noted the 12 x 6 F@B difference and if I use 6.7 for that distance per turn I am close.  Perhaps the PConst accounts for some of the other factors.  When I buy a prop I am buying a stated pitch, not analyzing it.  I am just correcting that pitch to travel distance per rev and that seems to correlate with flight speed of the airplane and rpms.

I have seen it stated in one of the threads that the 13 x 4.5 F2B seem to fly about like 5.5 and I thought that was how others were describing it also.  Its effective distance per revolution.

You are convincing me that I may not understand all of the factors as to why it goes a certain distance per rev, but I still see that distance per rev.

I have broken enough props to look at the cross sections and recognize the complexity of design.  Some also seem to make better paint stirrers but in use, there is still a distance per rev.  Certainly the PConst shows that some use more watts to turn and act differently, but once turned and the plane flying level and fairly consistent for timing, there seems to be a d/r relationship.

The reason that I requested your data is to check what I am doing.  I have not seen anything in the program that would help me toast a battery.  When I see discrepancy it usually suggests more watt usage than I actually see.  It seems in that sense to have a safety factor.  

An illustration using the Eflite 25 Motor as an example of how eCalc may be helpful,  of course taking into account the above views on effective pitch and calculator usage.  Also the disclaimer at the beginning of eCalc covers this.

Using an Eflite 25 Motor on a 660 sq”, 62.5 oz airplane, and 64 ft lines and a 4s 3000 mah 30c battery gave lap times of 5.2 at  9600+ RPM.  That left little headroom or overhead for “gain.” Also, about 85% of the battery was used for a full pattern.  The prop used is an APC electric 12 x 6.  Looking in “List your setup” showed similar setups with more like 8600 RPMs.  It turns out that the real prop used, was a 12 x 6 F2B, and gave similar lap times on similar lines as the 12 x 6 at 9600+.  The proposed effective pitch of the 12 x 6 F2B is about 12 x 6.7, giving about the same speed for about 11% less RPMs.  An Eflite 32 Motor was then substituted with the same F2B prop at about 8600 RPMs and did produce 5.2 lap times. 

The E 32 was already available, in hand, and with 5s battery, 2700 mah 20 – 25C cells, has a similar weight to the 25 with enough battery to make it work.  As it turns out, the 12x6  F2B prop seems to be a great prop for the E 25 and on eCalc, uses less amps than the regular 12 x 6.  That seems similar to what is found in “List your setup.”

The F2B prop can help the RPM range for the E 25.  The other help is a battery with a lot higher “C” rating, less internal resistance and better output. This gives more RPM for the same prop by just a battery change.  You can find high ”C ” light weight batteries but they are generally wallet heavy.  We actually experienced real performance gains with 3 and 4 cell packs, by getting higher C ratings when flying other airplanes, not just eCalculated.

It is most helpful when List your Setup notes enough parameters: Motor, specific prop, tachometer rpm, lap time, line length, amp used and flight/run time and battery pack specifics mah and C rating.  It is also helpful to know which esc and timer or governor used.

That may be "effectively" correct, have not tried it.  The 3 references that I gave above use effective pitch, and I gave your disclaimer.  I have to admit that much of your information is over my head on propeller design, thus the effective pitch which seems to factor in a lot of the complex.  Perhaps you can use your expertise and energy to explain why effective or calculated pitch does not work, in a method that I can understand. What is the flaw in using math and measuring the forward progress related to pitch and rpm, not what is the complexity is prop design, accounting for the articles above.  Thanks.