THINGS TO CHECK ON AN ARF
by Ed Anderson
aeajr on the
The next big step after an RTF is an ARF, or almost ready to fly
aircraft. ARFs are aircraft that are 70% to 90% built. Here you are typically
selecting and installing all the servos, ESC, battery, receiver and using your
radio. You may be expected to do some assembly of parts, put in pushrods,
control horns or similar accessories. You might be gluing wing halves together
or installing the tail feathers. Certainly there will be things that you did not
have to deal with when buying RTFs, BnF, or receiver ready, RR, models. So here
are some tips as to what to check and what to consider when buying and preparing
When looking for an ARF consider your
knowledge and experience. The more detailed the instructions the easier the
assembly. Unfortunately not all ARFs come with extensive instructions,
especially the cheap ones. It costs a lot of money to develop and document good
build instructions. Check before you buy. If you have an experienced builder to
help you this will be of less concern.
Some companies depend on or leverage build
threads on the forums. They will sponsor a build and have the builder document
the build as a supplement to their instructions. Often they provide a link to
such a thread from their website. If such a link is provided, go there and read
before you buy. People share their build and you benefit from their experience
and insights. Product defects or design weaknesses are sometimes found and
solutions worked out in the thread. Some of the things you learn in a build
thread can be vital to a successful first flight and long term
Oddly, some of the most expensive ARFs may
have little or no instructions as they may be targeted to high end buyers who
are expected to know how to put them together. In this case, if you are not
experienced you definitely want to find one of those build threads or someone
with experience to help you.
KEY THINGS TO CHECK ON AN ARF BEFORE YOU START
- Check the contents the of the package to
insure nothing is missing.
- Straight – Be sure the fuselage and wings are
straight with no twists.
- Where possible dry fit pieces to insure parts go
- Make a list of what you need to complete the
- Check glue joints to be sure they are tight.
- Check push rods for
flex or bind.
- Consider where you will run the antennas
- Consider parts
placement to get the right balance
– Make sure you have everything BEFORE you start building. Sometimes things get
left out of the kit. Or they forget to list something that you have to provide.
Do you have everything you need? It is better to know before you
– Examine every part to make sure it is straight and true. Things can get out of
alignment siting in the box or sometimes they come off the assembly line crooked
and are not caught. Make sure you check BEFORE you start to build. Foam and wood
are especially subject to warping in the box based on how they are sitting, or
whether they were subject to heat or moisture. But composite parts can be warped
too, so check them!
For the fuselage, I like to turn them over and site from nose to tail. Is there a bend or a
twist? If there is you will either need to take it out or return the package as defective. The
same goes for the wings. A warp in the wing will result in a poorly flying plane
or, in extreme cases a plane you won’t want to fly at all. Fix it or return it
before you start the build.
Also check the alignment of
preinstalled control horns. Sometimes the factory was not as careful as they
should be and the location or angle of control horns may lead to binding of
pushrods or surfaces. Sometimes a slight bend in the push rod can avoid binding
with the structure without compromising the strength of the push rod. Take a
look before you install the servos as you might want to change things
Dry Fit –
Make sure things that should slide together, like wing rods, do slide together
and come apart as they should. See how they will be secured whether by tape or
screws. See that all parts fit and you know where they go before the glue comes
Is there a sequence that you
must follow? For example, will you have to install a motor before the servo tray
is installed? Sometimes things that look the same are slightly different in size
or shape. Look for makings or numbers on them. Once you have glue on it, making
adjustments can be a beast.
Know what you
have to provide – Every ARF is going to need you to supply things. Are there
recommended servos or will you have to figure this out for yourself. Do they
provide a reference motor and speed control? Sometimes they include control
horns and push rods and sometimes you have to provide them, or you have to
If you select electronics
other than what is recommended, you may need to make minor modifications to make
them fit. If you are using a brushless outrunner, how will you secure the wires
so the spinning can does not hit them? If a modification has to be done, better
to plan for that before you start the build. And don’t forget to consider servo
extensions if they are needed. Don’t assume that what is in the instructions is
correct, measure to be sure, especially if you are not using the recommended
Where will the connections
go? Will you have to plug and unplug when you put the wings on? Can you make
that easier with a little planning now? Designing in a quick connection scheme
may be easier before you complete the build. Access to wires may be more
difficult after you have servo tray, battery tray or push rods installed.
If this is a pure glider, is
there provision for a ballast system? If not, would you like to design and
install one? This will be easier to work out before the build. As a glider pilot
I like to have a ballast system worked out for my gliders whenever possible. I
may never use it but if the weight gain is small the flexibility it can provide
can be huge!
Are glue joints tight? Is the glue adhering properly? A dab of glue now can
prevent a failure later, but don’t go glomming everything. Just a touch where
– This step is critical, especially for the elevator. Make sure your pushrods
won’t flex or bow to the sides. If you get that aircraft going into a dive and
hit the elevator, will the elevator move or will the push rod flex or bow
letting your aircraft dive into the ground? Check it now!
In the build thread for a
very nice glider ARF, one of the new pilots put his plane into a dive and could
now pull out. It was discovered that the push rod for the elevator had too much
flex and at higher speed it flexed rather than moving the elevator. All the
people in the thread learned from that and worked out a way to support those
Be sure the rudder push rod
does not bind. A sticking push rod could leave the rudder locked to the side
putting your plane into a spin. Just check it!
If the push rods are close
to the fuselage with a rod inside a sleeve, a dab of glue every few inches works
great. Goop is my favorite for this as it can hold them AND fill a small air gap
too. A glob of Goop on a stick can be applied deep into a fuselage if the
pushrods are already installed AND if the pushrods are inside a sleeve. Unlike
epoxy, Goop tends to stay put and not run. Or you might need to fashion small
wood blocks that glue to the fuse then to the push rod.
If the push rod is not in a
sleeve you may need to add supports that include a hole and a sleeve so the rod
is supported and can slide smoothly through the support without binding. Plastic
antenna tube, common in the 72 MHz days, works well in many cases as a guide
sleeve when passing through a support.
While preparing a $1,500
glider ARF I found the pre-installed push rods were binding. I had to remove
them and replace them or the glider would never have flown properly. It was easy
to do before final assembly but would have been a challenge after it was all
done. The cost was minimal but the benefits were huge!
Most of the time we want the final aircraft to be as light as possible while
Here is what having
everything before you start can pay off. If you can get things together and tape
in the servos, motor, battery and other parts, you can see if you will likely
have to add weight to get it to balance your plane. By shifting components you
can sometimes save weight. Note that, if you have to add weight to the nose,
saving a gram in the tail will save 3-4 grams in the nose. The nose is most
often where we need to add weight to balance.
I had an RTF package that
weighed 38 ounces. After a crash I rebuilt the nose, and changed to smaller but
equally strong servos that I could move more forward in the plane. This also
called for a change in push rods. That saved me 3 ounces and made a big
difference in how the glider flew.
Putting in a lighter motor
may save you nothing unless you can shift weight forward or take weight out of
the tail. I once built an ARF that was designed for a brushed motor that weighed
6 ounces. I replaced that motor with a brushless that weighted about 3 ounces. I
would have to make up all that weight savings with lead.
However the pushrods
supplied with the model were steel and were quite heavy, to offset the weight of
that heavy motor. By switching to carbon push rods I took almost a full ounce
out of the plane. About half of this weight was behind the CG so I was able to
take a full ounce out of the nose saving almost 2 ounces.
On this same ARF the plan
for this aircraft called for NiMh batteries right on the CG. By using a Lipo
battery I saved 4 ounces. Then I shifted the battery forward. I was able to
further reduce the amount of lead in the nose. This required that I modify the
servo tray and the servo location. That called for a change in the length of the
push rods. Overall savings was about 6 ounces, on a plane targeted at 48 ounces.
That is a big weight reduction. This was all pretty easy to do before I started
assembly. Afterward it would have been much harder.
Careful with reinforcement,
especially in the tail. If you have to add nose weight already, remember that
adding a gram to the tail will add 3 grams to the nose. If you must reinforce
behind the CG, figure out the lightest way to do it to save weight. If you are
adding weight to the nose anyway, reinforcement up front may not cost you any
weight at all.
Where will you put the antenna? In the days of 72 MHz, the antenna were long and
could be easily routed to be clear of interference. But 2.4 GHz antenna are
small. You don’t want them buried in a pile of servo wires or blocked by carbon
in the fuselage or a big fat battery. In some of my models I add antenna tubes
to help me get the antennas right where I wanted them or to route them outside
the fuselage. If you use remote receivers, where will they go? Will you need
extensions? If the fuse has any carbon the antenna may have to go outside. Make
preparations for this before you start the build.
Almost ready to fly kits are
great. All the big work is done for you. But there are still details to be
reviewed and worked out. Sometimes a part gets changed between development time
and production resulting in you needing to do something that may not be
reflected in the instructions. If you take the time to check the details,
consult the build threads and do a little planning, you will have a wonderful
experience with your new model.
LET THE DISCUSSION BEGIN!