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 Dec 31, 2007, 10:28 AM Registered User Joined Dec 2007 1 Posts Thanks for the Guide. I'm a 66 year old beginner and needed this kind of info to power up my J-3 Piper Cub with a 83" wing span. I hope I know what I'm getting into. Thanks Again Carl
 Dec 31, 2007, 05:17 PM Registered User Marseille, France Joined Apr 2002 169 Posts Good luck on your project. I have the World Model .25 size J3-cub. It is powered by a OS.25 LA. Another method to choose a power system is to compute the thrust to weight ratio. If you know the weight of your model and the amount of thrust that can be produced by the powerplant you have chosen, then you can tell whether or not that power system is good for the model by computing the thrust to weight ratio. However, this method can not be done without a thrustmeter. Some people just use a scale to measure the thrust of a motor with the right prop attached to its shaft. As a rule of thumb, you need at least 1lb of thrust to make a 3lb model fly. In other words, the thrust to weight ratio should be at least (1/3= 0.33 )to make a model fly. 0.5 is needed for acceptable performance and 0.57 is needed for agressive flying. A model aircraft can not be flown if the thrust to weight ratio is less than 0.33. I think you need 0.57 for your J3 cub because it is a scale. As we discussed earlier, 792 watts of power is needed to fly a 6 lb model. Now, by using the thrust to weight ratio formula, we need 2 lbs of thrust to fly that 6lb model. Therefore, the relationship between thrust to weight ratio and watts is that a 792 watt-motor that delivers approximately 2 lbs of thrust is needed to fly a 6 lb model Last edited by Hasina75; Jan 20, 2008 at 02:03 PM.
 Jan 15, 2008, 08:53 PM Crash test pilot trainee Medora,Il. Joined Dec 2007 183 Posts Ok. This may have been covered somewhere already but, I have really been looking. Once u have your motor and esc for said motor...What is an allowable battery amp wise that you could use? Is a system built around a 54 amp motor with a 65 amp esc for example be damaged by a battery with the capability of 100amps? If I understand this right, only the esc can determine how much of the amps are delivered to the motor? If not, what am I missing? Besides weight factors what is to much amp from a battery if there is one on any given build?
 Jan 15, 2008, 10:48 PM The Revegetator Melbourne Australia Joined Jun 2006 2,140 Posts Hi MUTDOG, the main part of the power system that dertermines the Amp draw is the prop size, because it is the prop that creates the load on the system. To determine the battery for your model you should have some idea of which motor you will be using. The specs for the motor will tell you the maximum Amp load and possibly the maximun efficiency. Your ESC should be determined by the Max Amp load of the motor (or the maximum Amp draw you intend using) plus a safety margin of 5 to 10 Amps. The battery requirements for your power system will be determined by the C rating and it's Capacity (mAh). Read through the C Rating and mAh sections again and it will show you how to work it out. A battery capable of 100 Amps will only deliver what the power system needs up to 100 Amps and no more so no damage will be done to the power system. You will damage a lipo pack if you exceed the C rating and/or over discharge the pack.
 Jan 16, 2008, 12:24 AM Crash test pilot trainee Medora,Il. Joined Dec 2007 183 Posts Thank you sir. That last sentence put it all together for me. Now its very clear.
 Jan 24, 2008, 10:03 PM Registered User Joined Jan 2008 9 Posts I am understanding how to figure out the sizes and types of motors and how batteries and motor create different amounts of power, but I am still confused on why you would need to use a gear box when there are many motors that seem like they would produce enough power for even large models. I just annot understand or "picture" why you would gear a motor. For instance. I am converting a GP Super Sportster from a 550 brushed geared motor to a brushless with 3 cell lipo. Why did they use a gearbox to decrease the rpms when useing the 550 can motor? Why would I NOT gear an inrunner? I have decided to use a Eflite Power 25 outruner to power it, but I am still uncertain why. I just followed the advice from many different sourses, and I would like to learn WHY? Just tryng to learn as much as possible Super sport weighs 3.5 lbs Low wing semi-symetrical 42" wing span Thanks for any help.I am trying to keep up. the more I read and in different ways the more I learn.
 Jan 25, 2008, 03:55 PM Registered User Joined Jan 2008 9 Posts Thanks for helping me visualize the situation. It realy helps to hear it as it pertains to you own personal questions. I understand gearing/proping much better. Now I just need to use it in real life situations. Thanks for the time you put into your response. The follow up question I have is Are thrust and RPM equal? It seems that you can spin a smaller prop faster and achieve the same amount of thrust compared to spinning a larger prop slower. I might be killing a dead horse!, but it just seems like the "same difference"
 Jan 25, 2008, 04:30 PM Proud member of LISF and ESL LI, New York, USA Joined Mar 2003 25,515 Posts I don't know if I can answer that perfectly. There certainly is a relationship. However the issue may be, can you spin the prop fast enough. Air is a fluid, like water, and can only move so fast. You can spin a prop so fast that the air can not keep up. I the world of water propellers, we call this cavitation. I am not sure if the same term applies. When a prop cavitates, it loses bite. A vacuum forms around the prop and thrust drops off rapidly. It is much more efficent to spin a bigger prop slower to generate thrust than to spin a smaller prop faster. Hope that helps.
 Jan 26, 2008, 10:53 AM Registered User Joined Jan 2008 9 Posts Once again I have not heard this discription before, and Now I feel like a moron because after hearing you explain it I realized the dumb question I just asked. The same principals apply to Nitro motors also, and as youi said Boats, and any other propeler. Thanks
 Jan 26, 2008, 04:16 PM If it flies, I will crash it USA, CO, Westminster Joined Jan 2008 3,095 Posts Well, I have been slowly poring over these forums for a while now and I have managed to gather allot of info. This thread may however be the best yet. Not by the amount of info in it but by the amount that I was able to understand after reading it. aeajr last comments really helped clear up a bunch of confusion and specifically as it regards props and gear boxes. Pitch and diameter and pitch speed all make sense now. I also finally understand the motor charts at the GWS web site and I am feeling a little less stupid for it. I now at least have some confidence that the random assortment of parts that I have can be assembled with the proper finishing parts to yield a working and flyable plane. Feeling better about this hobby all of the time. Thanks to all of the contributors in this thread. It was a great read. Rob...
 Jan 28, 2008, 02:41 PM Registered User Marseille, France Joined Apr 2002 169 Posts The follow up question I have is Are thrust and RPM equal? It seems that you can spin a smaller prop faster and achieve the same amount of thrust compared to spinning a larger prop slower. Poolpatrick> You asked a good question. I will give you an example, which is based on a real life situation in order to answer your question clearly. The amount of thrust produced by a 10 x7 " APC-EP prop that spins at 10200 rpm is the SAME AS the amount of thrust produced by a 5 x 3.5" APC-EP prop that spins at 20400 rpm. HOWEVER, the diameter of the airstream produced by the 10 x7" prop is larger than that of the 5 x3.5 " prop even if both props props produced the THE SAME AMOUNT OF THRUST. So, you may think that you can use both spinning props for a pylon racer and you'll achieve the same amount of speed , not really :-). A pylon racer will fly faster with the 5x3.5" than the 10X 7" prop EVEN IF both props produce the same amount of thrust . That because the diameter of the airstream produced by the 5x3" prop is SMALLER than that of the 10 x7" . In other words, for the same amount of thrust at different RPM, a small airstream produced by a small prop is needed for speed while large airstream produced by a large prop is needed for higher climbrates and to hover and to fly around. That's why, we use small props for pylon racers and speed wings; and, we use big props for slow flyers and 3D models. So, when we talk about thrust, we have to refer to the size of the airstream, which is in function of the prop diameter, but not to the amount of thrust itself. The question that we should ask is, " does this amount of thrust has a small or a large airstream? Then, the size of that airstream tells us for which type of model we have to use that thrust. Often, we are meaningless when we say a higher thrust means a higher speed without specifying the prop size and the type of model. Last edited by Hasina75; Jan 28, 2008 at 04:06 PM.
Jan 29, 2008, 01:22 AM
<- Balsa flies better ->
Gustafs, Sweden
Joined Feb 2006
1,978 Posts
A small note,
The discussion here is about static thrust, its not the same once airborne.

When choosing a prop I only give thrust a quick glance to see if it looks resonable, then I mock around until I find a nice prop with the right pitch speed compared to thrust suitable to the models weight and the specifications of the motor.

Playing with numbers can be fun but they're no that exact.
I would say that the 132W/lb power/weight recommendation is "too much" for a regular sports model, personally I like lots of power but to say that it's a "rule" and "needed" I can't comply with. Going for higher W/lb ratio makes your plane heavier too.

My experience tells me that the 100W/lb is a quite correct recommendation for a sports model, more is fun

I prefer to power after wingload instead, my old PC-9 had about 18oz/ft² which puts it between a trainer and a sports model.

To illustrate: My PC-9 @ 92W/lb (never mind the tree I got a proper airfield now).

 Seagull Pilatus PC-9 flight 2 (4 min 15 sec)

Peter
Last edited by wolw; Jan 29, 2008 at 09:33 AM.
Jan 29, 2008, 04:28 AM
Proud member of LISF and ESL
LI, New York, USA
Joined Mar 2003
25,515 Posts
Quote:
 Originally Posted by wolw My experience tells me that the 100W/lb is a quite correct recommendation for a sports model, more is fun I prefer to power after wingload instead, my old PC-9 had about 18oz/ft² which puts it between a trainer and a sports model. Peter
I have never heard this measure before. Peter, can you expand on this "power after wingload"?