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kv to rpm comparison

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kv to rpm comparison

Old 03-24-2010, 10:37 PM
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Default kv to rpm comparison

Hey gang, does any one have a comparison between Kv (electric motors) and RPMs (glow)? Is there no such thing?

Thanks
Old 03-25-2010, 01:18 PM
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Default RE: kv to rpm comparison


ORIGINAL: kargo

Hey gang, does any one have a comparison between Kv (electric motors) and RPMs (glow)? Is there no such thing?

Thanks
You are asking a question that compares two totally dissimilar things. One is voltage and the other revolutions. Voltage may result in revolutions just like glow power can result in revolutions per min

If the question is, is there a way to compare the two types of power, electrical versus gas/glow, of course there is. Approximately 750 watts of electrical power equals 1 hp of glow or gas power

hopefully that helps yopu

Matt Kebabjian
Old 03-25-2010, 06:12 PM
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Default RE: kv to rpm comparison

Thanks for the reply MTK, sorry for the confusion. I speaking of rpms per volt or a kV rating. I'll use a supertiger electric motor (random choice) as an example;
http://www.advantagehobby.com/produc...111941&cat=351
Super Tiger .10 sized
Specifications
Mounting Space: 1.5" (38.7mm)
Diameter: 1.4" (35mm)
Length: 1.18" (30mm)
RPM/V (kV Rating): 1250
Weight: 2.4oz (69g)
Input Voltage: 7.4-11.1V
Max. Constant Current: 29A
Max Surge Current: 33A
Max Constant Watts: 320W
Max Surge Watts: 370W
No Load Current: 1.2A
Suggested Prop Sizes: 10x7E - 11x7E
Connectors: 3.5mm Male Bullet


RPM/V (kV Rating): 1250 is what I'm referring to. Its an rpm rating, but not like an rpm range given with a glow or gas motor. I'm not working on a fast electric project now, but would like to in the future. Soooo knowing what electric numbers/terms to compare to a glow motor would be helpful. I know in general 2 strokes run at a higher rpm, turning a smaller pitch prop faster for a faster plane (please don't laugh at my ignorance, I need to educate myself before I get a screamn demon, although I think I'll be putting a glow motor on that one[>:]). Does that clarify it any? Or do I need to take in some approved reading material. Holding a sleeping newborn in my lap gives me some time...
Old 03-25-2010, 11:17 PM
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Default RE: kv to rpm comparison

the kv of an electric motor is the rpm it spins with no load at one volt input.
Old 03-26-2010, 06:36 AM
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Default RE: kv to rpm comparison

Kargo,
I'm not sure that you got the answer that you wanted.  I have been converting some of my older models, and have been unable to find a good guide.  I think the answer if you are looking for a number cruncher approach is more likely in the Wattage rating.  That .10 size is 320 watts continuous.  So, how does that stack up to a typical nitro like a Cox Golden bee?

Another serious consideration is that while the Cox Golden Bee swings a 5 inch prop at fairly high RPM's  your plane is not carrying a 1 pound LiPo in the cabin when you go electric.  So just matching RPM's won't cut the mustard.

Even the motor selection is nebulous at best.  I had been using E-flite 450's but recently purchased a CommonSense motor of approximately the same size and weight.  Turns out the E-flight is at least 70% more powerful.  What I plan on doing next is purchasing a Watt meter, and dragging out my old Tach.  I'm thinking that by building a spreadsheet that contains the name of the motor, the power drawn for a given prop at mid and full power, and the rpms the prop turns, I will have something that can be used to narrow in on the comparisons to nitro or gas.


KKKKFL

Old 03-27-2010, 10:54 AM
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Default RE: kv to rpm comparison

I don't understand the question, the listed motor is named .10 to indicate that it would be comparable to a .10 glow engine...

The electric would spin its recommended props at around 10000 rpm, while a .10 glow needs a smaller prop (around 7x4) and will spin that at about 15000 rpm.
Old 03-27-2010, 07:11 PM
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Default RE: kv to rpm comparison

Hey gang, thanks for the help. The better definition of kV is very helpful. Question; So does this mean that we can multiply that kV rating by the number of volts to get an rpm? Ex. 3S Lipo 11.1*1250=13875 sounds likely....

I hear you about the variations between electric motors. What were you using as a gauge of engine power?

I know you've been burnt by the "cheap" motors, but I'll throw this one out anyway. Hobbyking has some watt meters for about $25. I'm sure the more expensive ones are better, but so far I've found this to fit the bill.

http://www.hobbyking.com/hobbyking/s...power_Analyzer

Spec.
Operating voltage: 4.8~60V (0V with optional auxiliary battery)
Measures;
0~130A, resolution 0.01A
0~60V, resolution 0.01V
0~6554W, resolution 0.1W
0~65Ah, resolution 0.001Ah
0~6554Wh, resolution 0.1Wh
Screen: 16x2, backlit LCD display
Size: 85x42x25mm
Weight: 82g

Thanks again for the replys
Old 03-28-2010, 05:17 AM
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Default RE: kv to rpm comparison

I still don't quite get what you are after...

This forum is dedicated to putting glow engines into planes that were originally designed for electrics. Your questions are perhaps better suited in the electric forum?
Old 03-28-2010, 01:03 PM
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Default RE: kv to rpm comparison

opps, guess you're right. i'm looking at glow to electric rpm comparisons...
Old 03-28-2010, 08:24 PM
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Default RE: kv to rpm comparison

You are correct, it's kv times the voltage gives you a no load rpm for the motor.
Old 03-28-2010, 10:46 PM
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Default RE: kv to rpm comparison

Thanks, one more question; by no load I assume its meant no air load (bench run) but with a prop? Dumb question I know...

So if I found a motor that had 1621kV and used a 3S lipo, it would have about 18,000 rpms depending on where the volts were at (on a static test)?

The whole load unloaded thing is a topic for another day

Thanks again for the clarification.
Old 03-29-2010, 07:00 AM
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Default RE: kv to rpm comparison

I purchased an inexpensive Watt meter, now I am putting together a spreadsheet and plugging in the data.  I think we are headed down the same path.  For example here are two models.

Bolt at 20 oz spins a folding 10x6 @ 6100rpm
Robin at 26 ounces spins a fixed 10x7 @ 7300 rpm

The Airspeed is pretty similar, but the Robin leaps into the air.  The extra 1200 rpm more than makes up for the extra 6 ounces.

I would like to know what the Lbs of thrust are given the prop and RPM.  Suppose I could tie a scale to the tail ends.

KKKKFL
Old 03-29-2010, 07:07 AM
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Default RE: kv to rpm comparison

For thrust vs airspeed for different props and rpms, there is an excellent spread sheat here; [link=http://mvvs.nl/prop-power-calculator.xls]MVVS-prop-power-caculator[/link]
Old 03-29-2010, 12:04 PM
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Default RE: kv to rpm comparison

Yikes, that spreadsheet has everything except the phase of the moon...

All I am looking for is the following:

Have airplane weigs in at 20 0z  draws 8.2amps and 8.1 volts 70Watts  swings 10x6 folder at 6100rpm

How many lb's of thrust are being generated?

I'm still looking at the spreadsheet trying to figure out where to stuff those numbers and where to get the reading...


KKKKFL
Old 03-29-2010, 01:11 PM
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Default RE: kv to rpm comparison

Well there is factor that takes the particular prop model into account but I guess your folding prop is not in the list. If it is a wideblade one the factor will be larger than 1 and the resulting thrust larger. For generic (MAS) 10x6 spinning at 6100rpm the static thrust would be 0.51 kg (about 14.4 oz) and would require 53W of power to spin at that speed.
The remaining 17W are lost in the battery and engine, or possibly for wide bladed prop you would get a bit more thrust and are using a bit more of the supplied power.
Old 03-29-2010, 02:06 PM
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Default RE: kv to rpm comparison

no load for an electric motor is no prop on it at all and then spin the motor up. if motor spins at 15,000 with no load then put a prop on it it will drop to say 10,000 depending on prop size and pitch. this is where you need a watt meter or go with recomended prop and battery sizes that cam with the motor.
Old 03-29-2010, 03:38 PM
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Default RE: kv to rpm comparison

I simply assumed that those numbers where obtained with the prop on the engine, I don't have any electrics and would never check the rpm without a prop...

Also there is something not right with those numbers, 8.2 Amps at 8.1 Volts is 66.4 Watts...
Old 03-29-2010, 05:50 PM
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Default RE: kv to rpm comparison

lol he asked what the KV was for the motors so i explained what it was.
Old 03-29-2010, 10:03 PM
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Default RE: kv to rpm comparison

yup pretty much, thanks again for the reply
Old 03-30-2010, 05:39 AM
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Default RE: kv to rpm comparison

Thanks Mr. Cox,

The 10x6 reading that you provided gives me some data to work with.  Now given that I change only the prop, the battery, and motor stay the same, which way do I go to get more than 14 ounces of thrust?   I am thinking 10x4 might give better acceleration but would not have the top end and 10x7 might be slower to accelerate, but would push the model a little faster.  Or should I tinker with 11x6 versus 9x6?


Thanks


KKKKFL
Old 03-30-2010, 06:23 AM
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Default RE: kv to rpm comparison

Just use the file linked to above. It has everything you'll ever need and then some...

For higher static thrust you need a larger diameter and then you'll have to reduce the pitch to keep a similar rpm.

For higher thrust at speeds near the pitch speed you'll need a higher pitch and will have to reduce the diameter to keep the rpm at a similar level.
Old 03-30-2010, 06:33 AM
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Default RE: kv to rpm comparison

That linked file will take some serious study work on my part since it is so intensive.  I did not see where to just plug in the values for a 10x6 prop spinning at 6100 rpm  and then read out that it provides 14 ounces of static thrust.  I got lost with the instructions to fill in block 1, 2 and 3...

Will have to look at it again since I now have your data...

Thanks again..

KKKKFL
Old 03-30-2010, 09:00 AM
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Default RE: kv to rpm comparison

Yes, I guess it can be intimidating at first, but you only have to enter the prop constant, prop size and pitch, and rpm, the rest is calculated for you.

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Old 03-30-2010, 09:09 AM
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Default RE: kv to rpm comparison

Yup,
Was able to figure that out when I saw the data you provided...  kinda nice as you can put a 10x6 with specs in engine 1 data
then try 12x4 with appropriate specs in engine 2 and check the graph.

KKKKFL
Old 03-31-2010, 10:47 AM
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Default RE: kv to rpm comparison

if you are spinning a 10x6 then the APC 11x5.5 is a good prop too

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