Originally Posted by RCPilot1234

Hey everyone, I've completed 100 flights on my Extra and I'll soon post my review of the MKS DS95s! In summary they're awesome and I urge anyone to try them out But when the review is up please read it if you're actually interested in using them as there's certain important things to note about them.

One other thing, I'm tempted to try a 13x6.5 over the 12x6. Of course it'll pull too many amps, but I'm wondering if I just create a throttle curve that produces the same 45 amps as my current setup surely I'll be okay? For example, create a throttle curve that ends at 90%. I'm using an 18MZ which allows loads of points on a throttle curve so I should be okay

I'm going to lay the same speil on you as I've heard here many many times.
Quote " The ESC sees full amps at all times" Limiting the throttle limits it's output not the input" Unquote. I can't say I truly believe it but, it should be noted that some of the people that say it I believe are smarter than I when it comes to electrics. All that said My Edge has the stock 2814T/820kv and 45A Airboss ESC and I have been running the 13x6.5 on it since last fall with no issues at all. I do not limit the throttle with a curve but I do limit WOt to 3-4 second bursts and not too often. I have about 125 flights on it with the Xoar 13x6.5 and without a doubt it is a much better prop for me. The motor has not come down over very hot to date but I have not flown it this way in summer heat.

Thats not true! Have you ever hooked a watt meter up to your planes?

What's not true? And yes I have many times. Watts meters read and average the spikes. ESCs are simply switches that switch on and off very very fast. The motor is like a dead short. It tries to pull all it can, no matter how brief the switch is on it is on full tilt.

So, looks like my HS-65mg on my rudder is dead and it almost cost me the plane. I was half way through my second flight of day when the plane started to go crazy. I actually thought the battery came loose by the way it was flying. When back on the ground I went full right on the rudder and gently pressed against it. It did not resist at all and I was able to push it back with very little force. The elevator and ailerons held easily with the same amount of pressure. Does it sound like this servo is dead? Thank goodness I was able to get it down in one piece!! I'm probably about 45 or so flights on this servo...

EDIT: It looks like my ailerons also exhibit this behavior, but they have more resistance to than the rudder. The elevator holds very strong, but that is a savox and that's what I compared it to initially. I first noticed this while I was doing a knife edge and the plane started dropping. The more throttle, the faster it would drop. This is what leads me to believe it is the rudder servo. I have extra savox, so I suppose I could swap it out. Everything else looks normal, any other ideas of what to check?

Originally Posted by RCPilot1234

One other thing, I'm tempted to try a 13x6.5 over the 12x6. Of course it'll pull too many amps, but I'm wondering if I just create a throttle curve that produces the same 45 amps as my current setup surely I'll be okay? For example, create a throttle curve that ends at 90%. I'm using an 18MZ which allows loads of points on a throttle curve so I should be okay

I have not tried such a thing. But from a theoretical perspective, it could be bad. The reason is your meter is just reading average current. But at half throttle the FETs in the ESC should be getting hotter than at full throttle for a given current.

When the FETs are on, their internal resistance is low. When they are off, their internal resistance is very high. However, when they go from on to off, and the inductive current goes from on to off, this is the time that causes the ESCs to get most hot.

So even though the average might be below 45 amps, the FETs could go into thermal shut down, or worse, due to the R*I^2 power drops when the FETs are turning on and off more frequently, as they do at less than full throttle. (They turn on and off more frequently at half throttle than at full throttle, and the power disappated is greatest at this time.)

So many great responses! Thanks for all the detailed help, I'll weigh it all up and make a decision

Love your EXP wallpapers Doc i have one for each month

More about ESCs.
Lucuen Miller is the owner of Innv8tive Designs, the US distributor of Scorpion motors, and an Electrical Engineer.

Originally Posted by Innov8tive

I was checking out this post, so I thought I would inject an Electrical Engineering point of view that might help clear up a few things.

The FET's (Field Effect Transistors) that are used in brushless speed controllers are the little electronic switches that turn on and off, cycling the power to the 3 phases of the motor to make it spin. FET transistors have a very unique property that makes them well suited to this type of use. When they are turned off, their resistance is very high, measured in the millions of Ohms, so they conduct virtually zero current. When they are turned on, they have an extremely low resistance, measured in milli-ohms, so they are very close to being a dead short. Because of these two characteristics, when they turned of they draw almost no power, and when they are turned on, there is very little power loss through them.

The only time that a FET has any real measurable resistance is during the brief period of time while it is switching from either off to on, or on to off. During this time period, the FET has a measurable resistance, and has the full phase current passing through it. Because of this, the only time that the FET really has to dissipate any real heat is while it is in the transition period while it is either switching on or switching off.

When a motor is running at full speed, Each FET in the ESC turns on and off one time per switching cycle. So when the ESC is running at full speed this is the time when there are the fewest number of on and off cycles per FET.

When a 3-phase brushless motor runs at less that 100% power, the way the ESC slows the motor down is by taking each phase pulse from the ESC and chopping it up into smaller pieces. So when you run at less that 100% throttle, instead of the FET's switching on and off once per phase cycle, they might switch on and off 2, 3 or even 4 times depending on how high you have the PWM frequency set. Since most of the heat generated by the FET's occurs during the time where they are switching on or off, if you double or triple the number of switching cycles per phase cycle, you double or triple the heat output of the FET's.

Most people have a misconceived notion that as you throttle back, the current through the FET's decreases, however this is not the case. When the ESC turns on, the entire battery voltage is placed across the motor whether it is running at full throttle or 1/4 throttle. At full throttle it just does it for a longer period of time per cycle, and at lower throttle settings it does it for a shorter period of time. Because of this, the average current is lower at lower throttle settings, but during each pulse of power, the current through the FET's is the same.

The worst case scenario for an ESC is to run at around 2/3 throttle setting. In this case, the transistors are carrying a heavy load and switching on and off several times per cycle, with no time to rest between cycles. It is during operation in this range that the ESC will get the hottest.

Another thing that many people make a mistake at is in sizing their ESC to a motor. Many times people will take a motor that is rated for 90 amps max current, and run it on a small prop so it only pulls 50 amps. Then they figure since they are only pulling 50 amps, that a 55 amp ESC will be fine. BIG MISTAKE!

A larger motor has larger coils of wire in it around the stator, and because of this, it presents a greater load for the FET's to switch when they turn on and off. The amount of time that it takes for a FET to turn on depends on the size of the load. The coil of a brushless motor acts like an inductor, and inductors try to prevent rapid changes of voltage by their very nature. This is why they are used as noise filters in DC circuits.

If you have a small motor with small coils, the resistance to voltage change is less, and the FET's can turn on and off rather quickly. If you have a larger motor the inductance of the motor fights the FET's harder and makes it harder for them to switch on. This increases the amount of time that the FET's take to turn on, and since they FET's only get hot during the time they switch, it increases the heating of the parts. For this reason, if you use an ESC that is rated for less than the maximum motor current, even if you run the motor at less than the max current for the ESC, the ESC can still overheat because it is fighting such a large load.

So if you have a motor that is rated for 90 amps, use a 90 amp ESC, even if you plan on only running the motor at 50 amps. This way the number of FET's on the ESC is matched to the load that the motor will put on the ESC, and everything will run in an optimized fashion.

Hopefully I did not lose anyone there, but now you should understand the inner workings of a brushless ESC a little better.

Lucien

So, maybe my last post wasn't very clear. Last night during my second flight, my EXP became very hard to control and it was very hard to land. During a knife edge it just started to drop, and the more throttle, the faster it dropped. The best way to describe it would be if the battery fell off and into the back of the plane (but it didn't).

When it was back on the ground, I noticed I could move the rudder without a lot of force so I assumed it was the servo. The servo looks fine and still moves normally. Also, the gears are not stripped, so I'm not sure what might have caused it. For now I replaced that servo and and the wire extension. Does it sound like this would be the servo, or could there be an issue with the rx, etc? I've had planes go haywire midflight before, but it's always been obvious when I land what was wrong, but not in this case. I also use a BEC and run at 6v if that matters. What should I do to verify that the servo I took out is fine, other than connect it and see it move?

Broken hinges or binding tail wheel?

Originally Posted by Kris...

Love your EXP wallpapers Doc i have one for each month

Sorry, dead link. Got to my next post please.

Originally Posted by Doc Austin

Extreme Flight Extra EXP Wallpapers

Pretty cool Doc. How about doing one for the yellow MXS? It would be on my desktop in a heartbeat

Originally Posted by Liberty10

Pretty cool Doc. How about doing one for the yellow MXS? It would be on my desktop in a heartbeat

Start here, then go to the next article. I could not put them all on one blog page or it would have made it really, really slow.

Extreme Flight EXP Wallpapers

Originally Posted by _OZ_

More about ESCs.
Lucuen Miller is the owner of Innv8tive Designs, the US distributor of Scorpion motors, and an Electrical Engineer.

Thanks OZ! Very helpful