Mini-HowTo How to choose a power system

[Chris F]

Hi FF,

I use a Hyperion E-Metre. It is the best little gadget you will ever buy for e-flight. It has a tachometer built in and it can calculate all sorts of stuff you might never even need to know. Check it out!

About pitch.

The higher the pitch the higher the load on the motor. For the same size motor the higher pitch prop will spin slower and create less thrust. As the model builds up speed the prop will become more efficient. It's like riding a bike in 1st gear (= low pitch). It gives the equivalent to high thrust but low speed like if you want to hover 3D model. Move to top gear on your bike and the load on your legs makes it almost impossible to take off, but once you get up to speed your legs can take and maintain the load. A prop with high pitch is the same unless you use a larger motor.

I hope this helps. If not let me know. I think I will revise the guide to clear this up. Thanks for pointing it out.

[Ferndale Flyer]

Thanks Chris I think I get it now. So for a given RPM, that is the part I didn't catch on to, the higher pitch prop will give higher thrust, assuming the motor is already up to speed.

But considering taking off, the higher pitch prop will not generate the speed of the motor as easily and so won't develop thrust as well as a lower pitch prop.

Thanks, I just love this forum. There is so much information available it makes me like a kid again. I really love flying or anything that flies and at the moment I am building a new solar home. It is a huge job and getting a plane in the air will be a wonderful temporary diversion from all of the work.

Here is one more question.
Is there any way to calculate the stall speed of a plane or do you just have to find out by flying?
Thanks much.
Solar Dave

[Chris F]

Hi Dave,

What you say would seem correct to me. I say seem because I'm not an expert and someone is sure to correct me. Having flown some of my models with various pitch props of the same diameter I notice the following with a higher pitch:

It takes longer for the model to accelerate to take off speed.

Top speed of the model is higher.

The prop creates a higher load and the power system runs at a higher amperage.

The model takes a lot longer to decelerate for landing. I expect because the higher pitch prop creates a lot less drag when spinning freely when the throttle is closed.


I am envious! I wish I could afford to go solar too. Is this your first RC plane?

As far as stall speed goes, even if you can calculate the stall speed you don't have an airspeed indicator like in a real aeroplane so you have to judge it. The stall will of course be at a lower ground speed when flying into the wind too, so it is common to practice stalling any new model nice and high up to get a feel of the stall characteristics and speed so you have plenty of altitude to recover if something nasty happens. Some aeroplanes have nasty tip stalling tendencies at low speed, which means a wing will drop suddenly and the model will go into a spin. Most trainers won't have this problem if the centre of gravity is in the right place.

Are you in Perth? If so you should go and fly with Smokin Beaver!

[Ferndale Flyer]

Thanks Chris for all of your help. I am in Ferndale Washington in the USA and I feel really lucky to be able to build a new solar home. I built a passive solar home in Oregon and it worked very well. This one will be better because I have learned more about solar design since the last home building project.

I am really stoked about getting a plane into the air again. It will be such a release from all of the work and I just love flying and watching anything that flies. I am doing most of the work on the house so I am going to need some fun in my life besides building which I also love to do.

Thanks again,
Solar Dave

[Chris F]

Dave,

I should have read that you are 9941km away . I spent a month in Seattle many years ago when my father was training on the B 767. It is a very nice place. About the same size as Melbourne. It’s alot wetter there though .

[rjm24]

I have a question about the BEC. On my receiver i have a 4 plug ins. Ailerons, Rudder, Elevator and ESC. I have only one battey. Do my servos get power through the ESC? I read this on top but was not completley understanding. I guess my real question is that i have look at many servos on the net and most of them say that they are 4.8 volts. I know that my battery is 11.1. Does my ESC lower the voltage for the reciever? or do i have 11 volt servos and reciever?

Do some planes have 2 batteries one for motor and one for radio?

[wolw]

Originally Posted by rjm24

I have a question about the BEC. On my receiver i have a 4 plug ins. Ailerons, Rudder, Elevator and ESC. I have only one battery. Do my servos get power through the ESC? I read this on top but was not completely understanding. I guess my real question is that i have look at many servos on the net and most of them say that they are 4.8 volts. I know that my battery is 11.1. Does my ESC lower the voltage for the receiver? or do i have 11 volt servos and receiver?

Do some planes have 2 batteries one for motor and one for radio?

rjm24,

Your ESC powers the receiver and if you are using high voltage and many servos or just want to be on the safe side you can use another battery to power the receiver. Most servos run on 4.8V and many can run on 6V for extra power and speed.

Most likely you have a ESC with a linear BEC, it take the voltage down to 4.8V and bleeds the excessive voltage off as heat, as you can understand it's inefficient and heat up the ESC. Some little more expensive ESC's use a switching BEC circuit, it switches the current on and off very fast to give a average 4.8V. These ESC's can handle more current and still be able to power servos without getting too hot and shut down. My personal limit is:
Linear max 3S(11.1V) and 3 servos, Switching 3S(11.1V) 5 servos or 4S (14.8V) and 3 servos. For anything above this I use a separate BEC circuit (UBEC) or a Rx (receiver) battery pack.


Hope this helped you.

Peter

[aeajr]

Originally Posted by rjm24

I have a question about the BEC. On my receiver i have a 4 plug ins. Ailerons, Rudder, Elevator and ESC. I have only one battey. Do my servos get power through the ESC? I read this on top but was not completley understanding. I guess my real question is that i have look at many servos on the net and most of them say that they are 4.8 volts. I know that my battery is 11.1. Does my ESC lower the voltage for the reciever? or do i have 11 volt servos and reciever?

Do some planes have 2 batteries one for motor and one for radio?

This may be helpful:

THE ROLE OF THE BEC IN YOUR ELECTRIC PLANE
by Ed Anderson
aeajr on the forums

In the world of electric motors the electronic speed control, ESC, takes the
place of the throttle used on fuel powered planes. It regulates the speed of the
motor by pulsing the power to the motor to achieve the desired motor speed.
However most ESCs also have two other functions, the LVC and the BEC.

The LVC, low-voltage-cutoff circuit, will cut power to the motor and preserve
power to the radio system so you can land your plane safely when the motor
battery is getting too low. In the case of lithium batteries, the LVC, can also
save your battery packs by preventing them from getting too low. If you started
with NiXX packs and have switched to lithium packs, be sure your LVC is set
properly or you could damage your lithium packs.

The BEC, the battery elimination circuit supplies power to the receiver and the
servos. It is the BEC that will be the main focus of this discussion.

The name, battery elimination circuit, comes from the fact that, in the "old
days" of electric planes, you had a battery pack to power the motor and another
one to power the receiver. In order to save weight, the BEC was introduced to
eliminate the need for that receiver battery pack.

In most of our radio systems, the receiver is designed to operate between 4 and
6 volts. To match this, the typical BEC supplies power to the receiver at about
5 volts by stepping down the motor battery voltage. However the higher the
voltage of the motor battery, the harder the BEC has to work to get the voltage
down to 5 volts. In doing this work the BEC generates heat. The greater the
voltage reduction, the more heat the BEC generates. As a result most BECs have
to be disabled if the motor battery pack is over a certain voltage. ESCs that
are designed specifically for high voltage use often do not have an integrated
BEC.

BECs are also rated by how many amps they can deliver to the receiver. The
greater the number of servos installed the greater the amperage the BEC must
deliver and the more heat it generates in the process. However, with most
integrated BECs, the higher the voltage of the motor battery pack the lower the
amperage the BEC can deliver. This is often where problems occur. It is this
heat load that leads to the need for a compromise as to how many servos an
integrated BEC can support.

For example, if the motor pack is 8.4V, then a given BEC might be able to
support 4 servos. If the motor battery voltage is higher, say 11 volts, then
the same BEC may need to be derated to handle only 3 servos. Since more heat
will be generated by the larger step down from 11 volts to 5 volts, the amp load
has to be reduced or the BEC will overheat.

Note that the voltage rating for the ESC may be different than the voltage
rating for the BEC. Your ESC may be rated for 14.8 volts but the BEC may have
to be disabled over 12 volts and you will have to power the receiver separately.
If you don't take note of this and pop in a four-cell lipo, your ESC may be fine
but your BEC may be heading for a failure, resulting in a crash.

According to Dimension Engineering, a maker of BECs, "Many people don't realize
that their ESC's BEC rating is misleading. With the linear BEC built into most
speed controls, the current rating decreases as pack voltage increases. For
example, several popular 25A ESCs with "3A" BECs are only capable of supplying
0.5A when running from a 3s pack".

If you are flying an RTF or "receiver ready" model, there may not be ESC/BEC
documentation included. As an example, the manufacturer of the plane may
designate that the plane takes an 8.4V pack. At that voltage the included BEC
may be fine. However, if you decide to pop in a three cell lipo, a problem may
only be a launch away. The BEC may do fine for a couple of flights, or maybe 5
minutes or may fail 100 feet out, and down you go.

We also have the variable of which servos are being used. Different servos draw
different amounts of current. If the current draw gets too high, the BEC will
get too hot causing a thermal shutdown of the BEC. This protects the BEC and
prevents a fire, but cuts the voltage to the receiver. The net effect is that
you lose all power to the radio system and you lose control of the plane.

In the case of an overheated BEC, if there is enough cooling air going through
the plane, the BEC may come back quickly as it cools. This could look like a
radio glitch, but it could be the BEC operating on the edge of total failure.
If your ESC is very hot when you land, the cause could be the BEC operating at
the edge of its capacity. When we see these glitches, we often think the
problem is the radio system, but in fact the cause could be the BEC.

A CASE STUDY

This pilot was flying a new Spektrum 2.4 GHz system. All was fine till the plane
suddenly went dead and crashed. All sorts of speculation were offered about what
the cause could be and much of it was focused on the Spektrum 2.4 GHz system.
After the plane was recovered, everything seemed to work OK so it must have been
a radio hit, right? However, due to the diligent work of the pilot, it was
determined that the BEC had failed due to overload. You can read the actual
account at this link in posts 2986 to 3006.
http://www.rcgroups.com/forums/showt...04621&page=200

This is not the only account of this type that has been reported, but this was
one that was worked out over a short time with a very clear outcome. Note also
that the pilot had to run his test for several minutes before the failure
appeared. Thus, everything seemed fine at first; it seemed that the BEC was
handling the load. But over several minutes' heat built up in the BEC. Combine
this with the heat from the motor and the battery and, perhaps not enough
cooling airflow and the BEC shut down.

BE COOL FOOL!

With good airflow a BEC overload may be avoided. Regardless of what radio
system you are using, make sure you have enough cooling air going through your
electric plane. This is especially true of foam planes as the foam acts as an
insulator. You may have a cooling air vent in the front somewhere, but the heat
can't get out unless there is an exit air hole large enough to allow good
airflow. If you are pushing the limit on any part of your power or radio system,
not enough cooling air can cause damage or failure to your motor, ESC, BEC or
battery packs. The receiver could overheat or you could cook your servos.

How you fly your plane can also cause heat build-up. For example, an Easy
Glider that is flown for 1 minute to get to altitude might have enough airflow
to eliminate the built up heat. But if you fly it constantly for 10 minutes,
the heat build up could be enough to cook your BEC, your battery pack, or some
other part of the plane.

Be cool fool, and make sure you have enough airflow in your plane. If your
battery is very hot, or if your ESC is very hot, you may need more cooling.

OTHER CAUSES OF BEC PROBLEMS

You could be configured properly. Your BEC may be rated to handle your servo
count and you could have plenty of cooling air but still have problems. If you
have a servo push rod that is dragging or is otherwise placing a high load on
the servo, this can increase the amp draw of that servo. If that servo gets
stuck, the amp draw will go way up!

Servo loads are expected to be variable. A servo will move, put a load on the
BEC then come back to neutral and the current draw will drop. In between loads,
the BEC has a chance to cool. However a jammed servo will draw a lot of power
and that draw will be constant. You can see why it is very important that your
servos move freely, without binding. Check those control rods for kinks,
obstructions or things that could get in the way.

ENTER THE COMPUTER RADIO

In the past it was common to have 2 ailerons run off of one servo, so three
servos were typical of a 4-channel electric plane. With more and more people
using computer radios, there is a tendency to put 2 servos on the ailerons
meaning more load on the BEC.

Also, with a computer radio it is easy to add a little aileron to rudder mixing,
moving 3 servos at once. Now add a little up elevator in the turns and all four
servos are pulling power. Go to a full house electric sailplane, with flaps
following ailerons, rudder mixed in and a little up elevator in the turn and you
now have 6 servos, all moving at once. We begin to see how the BEC can become
challenged to keep up.

WHAT IF YOU NEED MORE?

If you need more power than the integrated BEC in your ESC can supply, or if
your motor battery voltage is higher than the BEC can handle, you will need to
disable the integrated BEC and put in a separate receiver pack or a separate
BEC. Many companies make after market BECs that can handle these higher
voltages or higher servo loads.

Note that there are different kinds of circuits that are used to create the BEC
function. There are linear BECs, which seem to be primarily what is found
integrated in with the ESC. These seem to be low cost but are more affected by
the voltage of the motor pack. Then there are switch mode BECs that seem to
tolerate these higher pack voltages better than the linear BECs. It appears
many of the after market BECs are of this type.

Regardless of what type you have, follow the instructions carefully or risk
losing your plane. And be sure to provide plenty of cooling air.

Listed below are some examples of after market BECs.

The Ultimate BEC
http://www.hobby-lobby.com/ubec.htm

Novak 3 amp BEC
http://www2.towerhobbies.com/cgi-bin...?&I=LXSGC1&P=7

For very large servo counts - 6 amps
http://www.horizonhobby.com/Products...ID=SQBDBEC008V

Dimension Engineering has several BECs
http://www.dimensionengineering.com

The SMART BEC - Combines BEC and LVC that is Lithium aware
http://www.dimensionengineering.com/SmartBEC.htm

SUMMARY

The ESC is the heart of your electric power system. The BEC is the part of the
ESC that powers your radio system. Keep it cool and make sure you read the
instructions so you don't overload it. Forget these tips and you may be
picking up pieces of your plane, wondering what caused that crash.

[wolw]

That's what I said

Peter

[nodirbek13]

Thanks for the detailed post Ed,

I have a Protech Extra 330L RTF which took off quite okay but the electronics shut down after 20m flight..hit the grass.
I assumed that i did something wrong and tried again, but the same issue happened in the second trial.

The ailerons were leveled and the elevators were nearly leveled, 70-75% throttle so I do not think that i was pushing it anyway near its limits.

Do i need to rip the ESC apart and add a heatsink?

Probably should not buy RTFs anymore..

[realtimerecon]

CHRIS F ...Thanks MUCH this is a great help!...R.T.R.

[Chris F]

Hi nodirbek,

It sounds like your esc is overheating and cutting out. A heat sink may help, but it would be advisable to test your power system and see if you are running your components close or above spec. Does your esc get any cooling? If not open up some cooling holes to get some ventilation to the esc. If the esc is being run at or above spec, get an esc with a higher rating. Some people mount the esc with a heat sink sticking outside the fuselage in the airflow for cooling, but you may not need to do this.

I'm sure you will probably get an answer from ED too .

[aeajr]

Nodirbek,

If that is a new plane, I would pack it up and retun it for a replacement. You have a defective part somewhere.

Are you passing range check OK?

Wiggle the antenna wire and the radio antenna - see if you have an issue

[nodirbek13]

wow...thanks a lot for your instant replies.

i have checked the range, and the while on the ground the plane operates at 200-250meter (didnt go far)

I checked to see if theres a loose contact so while the motor is running at low rpms i tried to shake the plane, rolled upside down, and the connectors seem tight.

After reading your post today i beleive it might be the BEC problem.
The plane is not in returnable state, as the second crash was a bit harsh and ripped open the tail (still repairable). What I can do is, try to ground test the electronics, and see if the temperature raises on the ESC..with and without the prop

The ESC is an Apollo 25A Brushless and the brushless motor itself has never been mentioned in the documentation or the product page.

Has anyone had any experience with this plane and what motor+prop+esc I should get for it (primarily, for moderate speeds, rather than acrobatic flight)

your input is appreciated.
n.

[aeajr]

Nodirbek,

1) if this is an RTF, then the only way the BEC can be a problem is if you changed the battery or the BEC is defective.

2) How did you range check the radio? Did you do it with the antenna down? What procedure do they provide in the manual?

3) You may have a flat spot on the throttle. I have that on some of my very heavily used Aerobird radios. You hit a spot where the throttle cuts out. it is a very very tiny window but it is there.

4) you could have a bad contact on the throttle. If you lean on it or push on it, the motor could cut out.

Again I say, if this is a new plane, take it back or call customer service. You know, if hte throttle cuts out and CAUSES the crash, it should be covered under warranty.

Anyway, that is about the best I can offer.
.