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Author Topic: Kv-Pitch and Lap Time  (Read 615 times)

Offline Scott Jenkins

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Kv-Pitch and Lap Time
« on: April 14, 2008, 02:34:55 PM »
Dean,
Your formula has shed some light on what I am trying to achieve for an E-Speed Class A model
"nominal voltage of the battery times Kv then times 0.7" Your following statement was "This, together
with line length and desired lap time, dictates the pitch of the prop. Would you care to eloborate on this ? Assuming the line length is 42' and the lap time, in theory, was 14/10. How do you determine the pitch !

Scott
 
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Scott Jenkins
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FAI F2C VOLUME 2 SECTION 4, 4.3.7
m) During the refuelling and the restart of the motor, and until the time when he releases the model aircraft, the mechanic must keep the model aircraft in contact with the ground by at least one point and with the centre line outside the flight circle. During that time the pilot must be crouching or sitting inside the centre circle. He keeps one hand on the ground and his handle and his lines as close to the ground as defined by the F2C panel of judges until the model aircraft starts again.

Offline Dean Pappas

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Re: Kv-Pitch and Lap Time
« Reply #1 on: April 16, 2008, 11:37:44 AM »
Hi Scott,
The 70% figure is for folks looking to find out how to juggle the governor setpoint RPM versus Kv and battery voltage (the maximum possible RPM).
That's not the case for Speed.

You are running at W.O.T. with a normal full-throttle setting (no governor), correct?
So a shaft run would yield Vbattery times Kv RPM.
The problem is that no powewr is absorbed in a no-load shaft run.
The maximum power possible will be derived by running at exactly 1/2 of that same no-load RPM, but the motor current will probably be so high as to melt the windings!
In practical terms, a very, very, very heavy load will yield maybe 85% of the no-load RPM, at full throttle.

Now you can figure out what pitch prop you need for starters;
0.85 * Kv * Vbatt * 1/60 *Pitch[inches]/12 will approximately equal Line-length * 2Pai * (1/ Lap Lime)
The units of both sides of this equation are in feet per second.
The problem is that this does not predict speed, and it does nothing to help you figure out the diameter of the prop.
If you calculate the pitch based on your hoped for speed, then just try ever increasing prop diameters until you are running as much current as you dare. If you are making enough Watts for the drag of the ship, then life is okay, otherwise you may never get to the desired speed.

Hope it helped,
Dean
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Offline Scott Jenkins

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Re: Kv-Pitch and Lap Time
« Reply #2 on: April 16, 2008, 01:04:53 PM »
Dean,
First off, I liked your article in MA this month. But I am still confused, but thats my problem  n~. I have down loaded a copy of motor calc and have run several different scenarios I only believe about have of the output from the comparisons, the part of the comparisons that I do not believe are the flight characteristics. I have attached the comparison below. The part of motor calc I do not like is that I cannot specify the throttle % and the time of flight. See if you can make more sense out of this than me.

Scott
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Scott Jenkins
AMA 43122
FAI F2C VOLUME 2 SECTION 4, 4.3.7
m) During the refuelling and the restart of the motor, and until the time when he releases the model aircraft, the mechanic must keep the model aircraft in contact with the ground by at least one point and with the centre line outside the flight circle. During that time the pilot must be crouching or sitting inside the centre circle. He keeps one hand on the ground and his handle and his lines as close to the ground as defined by the F2C panel of judges until the model aircraft starts again.

Offline Dean Pappas

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Re: Kv-Pitch and Lap Time
« Reply #3 on: April 16, 2008, 02:29:18 PM »
Hi Scott,
This is Speed, right? Within the constraints of motor temp and battery peak current, maximum efficiency, and max speed, will always be had at full throttle, assuming you work with the prop to get it to the right diameter and pitch so that you can run at full throttle rather than part throttle.

Tha'ts because the ratio of peak current to average is lowest at full throttle, and worst at 50%.

OK, for laughs, try this: 75% throttle on 2 cells  equals 1.5 cells. Try putting 1.5 into the cell-count dialog box in MotoCalc. I know it worked on the old Aveox online simulator, and it works on the NeuMotors online job too!

take care,
Dean
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Offline walterbro

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Re: Kv-Pitch and Lap Time
« Reply #4 on: April 16, 2008, 02:43:17 PM »
Hi Scott

If you use MOTOCALC to "compare" and then close "compare", you will see a screen that allows you to change the percent power. It is in the upper right corner of the screen. It starts at 100% and goes down in 1% steps. When you reduce the power the screen automatically changes to show the new performance. You can see the
amps, thrust, RPM, pitch speed, and all the other info. calculated.
    Hope this helps.
Walt Brownell
 
 
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Offline Scott Jenkins

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Re: Kv-Pitch and Lap Time
« Reply #5 on: April 16, 2008, 03:22:43 PM »
Dean,
Moto calc will not accept a 1.5 connotation and yes this is speed we are talking about. Using the throttle bar on the inflight analysis window I can change the throttle setting down to any percentage value desired. Not sure about the ratios between peak current and average throttle. I have boiled it down to start with a 4.5 x 4 pitch prop and will go up to about 5.5 pitch without changing the diameter as soon as the plane is finished. It looks like a pregant F2A  #^ It looks as if I do not push the amperage past 42 amps, I will not fry the motor Scorpion custom HK2221-5 5400 kv or the batterys, I changed the batterys back to 7 Sanyo 2200 Nimh with a 70a esc. Lipos are kind of scary due to catching fire good power but still ?
Still confused !

Scott
« Last Edit: April 16, 2008, 03:40:38 PM by Scott Jenkins »
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Scott Jenkins
AMA 43122
FAI F2C VOLUME 2 SECTION 4, 4.3.7
m) During the refuelling and the restart of the motor, and until the time when he releases the model aircraft, the mechanic must keep the model aircraft in contact with the ground by at least one point and with the centre line outside the flight circle. During that time the pilot must be crouching or sitting inside the centre circle. He keeps one hand on the ground and his handle and his lines as close to the ground as defined by the F2C panel of judges until the model aircraft starts again.

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