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Aug 14, 2006, 09:41 PM
Watts Rule, Glows Drool
dogon1013's Avatar
Thread OP
here are some basics on props I have learned:

your max theoretical prop RPM is your motor KV (rpm/volt) times your pack working voltage (voltage while running) times your gear ratio. you might be able to use 80% to 90% of pack rated voltage as a decent estimate of working voltage.

Assuming rpm stays constant (which it often doesn't)
-larger diameter = more thrust
-larger pitch = more speed

-more RPM = more thrust AND speed

-you can usually trade dimeter for pitch, without changing amp draw much.
That is going form a 10*8 to an 11*7 should not change your amp draw noticeably.

-Larger props turning slower are more efficient than smaller props turning faster
-Prop efficiency tends to also go down when the pitch is near, or larger than the diameter (that's one reason it is difficult to find a 10*9 or 10 *10 prop)

There is a max speed that your model will go for a given prop pitch, spinning at a given rpm (this is the prop's pitch speed). if you have a very sleek, low drag model, you can usually get to this speed in a shallow dive. I high drag model usually can't, unless it has lots of thrust too, or the pitch speed is low.
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Aug 15, 2006, 06:18 AM
Registered User
Jimmy Hoffa's Avatar
Quote:
Originally Posted by guapoman2000
How about a screwdriver? Well, about 5 months ago at Moon Port Modelers someone with a Puffed up battery who perhaps got pretty peeved about the situation attempted to hammer in an old screwdriver in and everyone backed off quickly thinking they were going to witness some action.....well, NOTHING!!!!
Carlos,
Back when we used to fly at Blue Jacket, Ray had a bad Lipo and decided to "set it off" for entertainment. After repeated stabbings with a #11 Exacto knife it only smoked. NO fire whatsoever.
Phillip
Aug 15, 2006, 07:46 AM
Things are looking up...
mechrick's Avatar
Saw this on Good Morning America today - so timely. They showed all sorts of fires in laptops and cell phones (after shorting wires - duh). Looks like Dell needs to be educated on batteries grounding and isolation (just more fuel for the flame here):

http://abcnews.go.com/Business/wireStory?id=2314536

Rick
Aug 15, 2006, 07:50 AM
Things are looking up...
mechrick's Avatar
Quote:
Originally Posted by dogon1013
I would suggest you pick up a copy of "Basics of RC model aircraft design" by Andy Lennon.
Will be on my next Amazon order (I'm always getting something - DVD's or books):

http://www.amazon.com/gp/product/091...568745?ie=UTF8

Rick
Aug 15, 2006, 07:53 AM
Things are looking up...
mechrick's Avatar
Quote:
Originally Posted by dogon1013
Here is another usefull online calculator worth bookmarking:
It's often called P-calc
http://brantuas.com/ezcalc/dma1.asp
Is there a way to put in the motor data without having to pick a specific motor from the provided list? For instance, I have an Eflite 480 Outrunner, but the closest motor they let you pick is an Eflite 400 Inrunner - not really the same thing at all.

The fields are locked, unless I am missing something simple (I probably am).

Rick
Aug 15, 2006, 10:22 AM
Account closed
Quote:
Originally Posted by mechrick
Is there a way to put in the motor data without having to pick a specific motor from the provided list? For instance, I have an Eflite 480 Outrunner, but the closest motor they let you pick is an Eflite 400 Inrunner - not really the same thing at all.

The fields are locked, unless I am missing something simple (I probably am).

Rick
Rick,

Yes, correct this on-line P-Calc is locked from doing what you are asking but, if you e-mail the admin for this P-Calc with the Motor data such as Io, Ri etc.. he will add it to the list.

The "other" way is to use MotorCalc....I used it for 30 days...perhaps longer as my clock on my computer got slow... then, I opted to buy the dang S/W and loving it since as you can create your own motor just by obtaining the motor spec's.

Carlos
Aug 15, 2006, 11:13 AM
Watts Rule, Glows Drool
dogon1013's Avatar
Thread OP
There is another motor calculator out there just like P-calc that will allow you to put your own constants in. I have the link on my computer at home, I think. I will post it when I find it.


I found a pretty complex one for motor builders. http://www.badcock.net/drive_calculator/
Aug 15, 2006, 11:14 AM
Watts Rule, Glows Drool
dogon1013's Avatar
Thread OP
here's another good one to bookmark, It even has the custom motor field so you can put in your own constants: (this might be the one I have bookmarked at home)
http://www.csd.net/~cgadd/eflight/calcs_motortest.htm


sometimes these calculators give strange values when the prop pitch is near the prop diameter, Otherwise they are pretty accurate.
Aug 15, 2006, 11:19 AM
Things are looking up...
mechrick's Avatar
Quote:
Originally Posted by dogon1013
here's another good one to bookmark, It even has the custom motor field so you can put in your own constants: (this might be the one I have bookmarked at home)
http://www.csd.net/~cgadd/eflight/calcs_motortest.htm
That's the one! I have used it before with success but lost the link. Thanks for reposting int.

Rick
Aug 15, 2006, 11:43 AM
Watts Rule, Glows Drool
dogon1013's Avatar
Thread OP
Rick,

I just added those Motor calculator links to my Blog (click my name to get there). So if you forget it again, you know where to get it now.
Aug 15, 2006, 12:02 PM
Robot, RC Pilot & Builder :-)
kevinhines's Avatar
Toby, Carlos & Rick,

Thanks for all the helpful info! It will take me a little while to re-read your postings tonight and to absorb the info. I will also place an order for that book!

I appreciate all the helpful info.

Thanks,
Kevin
Aug 15, 2006, 07:39 PM
Watts Rule, Glows Drool
dogon1013's Avatar
Thread OP
just saw the nice news spot on the Dell Li-ion battery recall. They showed some nice pictures of laptops bursting into flames. I think it was a couple hundred fires they had...out of 4 million batteries being recalled.

I think they said that about 300 or so Cell phones exploded or caught fire out of 40 million or so, as well.

Of course nobody wants a fire....EVER, but you gotta admit, those are some pretty darn low odds (about 100,000:1 if you own one of the recaled laptops). I looked up some other odds ( http://funny2.com/odds.htm ) You are more likely to date a supermodel, or die in an explosion, or be considered possesed by satan, or become a pro athlete, or win an academy award, than to have your dell laptop burst into flames.
Aug 15, 2006, 07:47 PM
Aerowizz
aerowizz's Avatar
Quote:
Originally Posted by mechrick
Saw this on Good Morning America today - so timely. They showed all sorts of fires in laptops and cell phones (after shorting wires - duh). Looks like Dell needs to be educated on batteries grounding and isolation (just more fuel for the flame here):

http://abcnews.go.com/Business/wireStory?id=2314536

Rick
Rick,

I now have to return 4 lithium battery packs back to Dell due to this recall. Me and my fiance have Dell laptops we recently purchased. This recall was due in part to an investigation Dell performed after a Dell laptop exploded in a Japanese conference which was witnessed and photgraphed by many witnesses there.

My main point on Lithium safety is that the technology is much more volitile than any other battery technology currently available due to thermo chemistry of the cells. Most lithium cells use a cobalt oxide chemistry that can catch fire or explode if the cells are charged or discharged too rapidly or if it is physically damaged. This danger and risk is higher in lithiums because of the higher energy density when compared to all other battery technologies. This is due to the laws of physics and chemistry and not industry!

The bottom line is unfortunately there was nothing the user below could have done to prevent this explosion.
Aug 15, 2006, 08:25 PM
Account closed
Quote:
Originally Posted by aerowizz
Rick,

My main point on Lithium safety is that the technology is much more volitile than any other battery technology currently available due to thermo chemistry of the cells. Most lithium cells use a cobalt oxide chemistry that can catch fire or explode if the cells are charged or discharged too rapidly or if it is physically damaged. This danger and risk is higher in lithiums because of the higher energy density when compared to all other battery technologies. This is due to the laws of physics and chemistry and not industry!

The bottom line is unfortunately there was nothing the user below could have done to prevent this explosion.

Hey Aerowizz,

I am getting tired of all this scare and doom talk of a type of chemistry technology that has been around for quite sometime now....NASA has performed lots of research and testing with this technology for use on the MARs rovers, sensors devices and other Aerospace related items.

Industry has dumped many $$$ in doing the same for commerial uses and consumer electronics.

Here....read the article carefully!!!!

"In rare cases, a short-circuit could cause the battery to overheat, causing a risk of smoke and/or fire," said the spokesman, Ira Williams. "It happens in rare cases, but we opted to take this broad action immediately."

A short circuit to any battery device is not pretty no matter what it is...


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