| On ModelFlight #42, 'Grapher'
raised some interesting questions on electric flight, to which there is
some response on post box.
On the last two issues, we've also had some
pictures of really nice and fairly large electric scale models from John
Anderson from Georgia, USA. Clearly, John has gone some way down the
electric road, and so I asked him if he might have some useful comments
to make arising from his own experience. John has kindly responded with
some real practical information and advice, but he is most anxious to
emphasise that he in no way considers himself an expert and would not
wish me to portray him as such. |
Things I have learned about electric power for model
airplanes
By John A. Anderson
First I want to say I am not an expert
in the subject and don’t pretend to be. I am as confused as most
people when they first start with the Electric Powered Model airplanes.
What little I do know I learned the
hard way with several failures to find out what not to do. There are
many experts out there that know all about it and will tell you so. I
talked to some of them and they had me more confused than ever. There is
just no way to tell what power to expect from what motor and battery
combination except for a rule of thumb that does seem to work. It is
said that in order to fly good, a sport model has to use 50 watts of
power for every pound it weighs and 75 watts for good acrobatic
performance. I haven’t found a good way to determine the wing area
required though and have always guessed at the power needed.
I like building scale models and found
that most of the kits and the plans for glow plug powered models are
very heavy. The reason is that these engines vibrate quite a bit and the
models have to be strong. Also a lot of builders will tend to build an
airplane stronger than needed anyway.
The motors are rated very different
depending on who makes them. Some are called 280 and some 400 also 600
and so on. I found that Astro flight has a more simple rating system
such as .05 .025 .40 and so on. This is to compare with the Glow plug
engines. I have found that it is not at all accurate though, as the
batteries used have more to do with the power than anything else does.
The more cells you are able to use the more power the motor develops.
That is up to the maximum amp rating of the motor and over that you will
burn it up in very short order. Also the propeller has a lot to do with
the power developed. The biggest difference though is with geared
motors. The gear ratio changes the RPM of the prop. There are several
web sites that I find help me decide on what to use for power. One is www.astroflight.com/
another one is www.newcreations-rc.com/
- both give information on the products that are available.
Some people have had good results with
cheap so-called Can motors. I have not. I find that the way to save
money is to buy the best available that you can afford. Usually the
cheaper the product the less use you will get out of it before it goes
bad. Also some of the motors are not all that good.
 The
gear box that goes on the motors gives the motor the ability to turn a
bigger propeller and that in turn gives more pulling power. I have an
Astro .020 brushless motor with a 3.3 to 1 gear ratio that turns an 11x7
prop at 5000 rpm on 7 cells. This I have in my Eaglet, which weighs 3
pounds ready to fly. It is a great flying airplane although it calls for
a .05 for power. I think that the brushless motor is the way of the
future for electric models.
 I
have two Monocoupes that were designed by Frank Mizer for Glow plug
engines and should weigh about 5 pounds (Email truescale@aol.com
). I have the turbo 10/20 brushless motor in each of them. This is the
Max Cim motor sold by www.modelelectronicscorp.com/
It is a brushless motor with a 3 to 1 gear ratio. It is very expensive
and also very heavy but has a lot of power. The first Monocoupe that I
built has 14 1700 SRC mAh cells and weighs 7 pounds. The batteries alone
weigh 28 ounces so the motor, gear box controller and batteries make it
very heavy. With only 65" wing span it has a high wing loading.
However, to my surprise it flies great with a 14x8 propeller. I don’t
have any idea how many watts it is using or how much RPM it is turning
but it flies great at about half throttle in cruise.
The second Monocoupe has the same
construction from the same plans. It has the same kind of motor but has
16 2400 SRC cells and weighs almost 7½ pounds. The batteries alone
weigh over 2 pounds. It has not been flown yet but I believe it will do
well.
The main thing I learned is build what
you want to and keep it as light as possible. Choose what power you
think it should use and try it. The more capacity the batteries have the
longer the motor will run but the all-up weight will be increased. I
like the Astro motors because there are no surprises and the quality is
great. I know there are other motors that may be as good but with Astro
you always know what you are getting.
The cost of getting started can be
great but there is no messy airplane to clean up. It is quiet, always
starts easy if wired correctly and no fuel to buy and carry around.
There are new batteries becoming available that give more power and
weigh less so the possibilities are much more for the future. Every day
there are new products becoming available and more interest in the
electric power. The small so-called park flyers are the latest thing and
I must admit they are interesting. Most of the scale models are big and
heavy but when powered with the right motor will fly more realistic and
look like a real airplane when in flight. I guess it all depends on what
you decide you want to build or fly. There are so many possibilities
that nothing would surprise me any more with the electric powered
models.
Some good sites for learning much about
the electric power for flight are as follows. The
E Zone also New Creations
R/C and Astro Flight and The
Future is Electric.
I have always had good results with
the Astro motors and on their web site they have charts that state the
number of cells to use and what prop giving the power developed,
plus other information.
Motors
The motor with direct drive will give a very
high RPM with a small propeller. This would be good for a faster
airplane that does not weigh so much.
The geared motor will fly a bigger and heavier
airplane at slower speeds using much bigger propellers.
The brushless motors are more efficient and
will give more power for the weight and they can use smaller batteries.
In a speed controller you want to get one that will handle the
power that you want to use. A lot of them have a feature called BEC.
That is a battery eliminator circuit. Usually this will work with
the smaller battery pack, some up to 10 cells. When using this feature
you don't need another battery for your radio receiver. Almost always
when using over 10 cells the BEC won't work.
Props
Master Air Screw makes several propellers for
electric only. They have thinner blades and usually are more efficient.
The balance of the propeller is most important on the electric motors.
It is possible to fly big heavy models using gear ratios that let you
swing a bigger propeller also. The Hangar 9 Cub will fly with power to
spare with an Astro geared 40 and an 11x8 prop on 18 to 20 cells, for
example.
I really don't know how to tell what combination of
batteries and what motor for what airplane. This is very hard to tell
because the weight and wing area have a lot to do with it. Also there
are a lot of cheap junk motors sold now.
I will be glad to help anyone that has a question that
I can answer. I however am still very much in the learning stage and
have to do things by trial and error.
John
Thanks for a very
informative article, John.
If you want to
contact John, his e-mail address is elmodl49@aol.com but
be sure to put the word 'electric' in the subject box to ensure that John
will open it. More
input on this subject will be very welcome - maybe somebody knows of a
good, straightforward book that will guide us through the technical jargon
jungle! The Astro Flight site referred to above has a brilliant chart
showing motor and battery-pack recommendations that will take a lot of the
guess work out of it. |
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