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  1. #1
    Moderator aeajr's Avatar
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    Things to check on an arf

    THINGS TO CHECK ON AN ARF

    by Ed Anderson
    aeajr on the
    forums

    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
    an ARF.


    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
    enjoyment.


    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
    YOUR BUILD



    • 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
      together properly.
    • Make a list of what you need to complete the
      project.
    • 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



    Contents
    – 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
    start.



    Straight
    – 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
    first.



    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
    out.



    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
    install them.



    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
    servos.


    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!


    Tight
    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
    needed.


    Push Rods
    – 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
    push rods.


    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!


    Balance
    Most of the time we want the final aircraft to be as light as possible while
    maintaining strength.


    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.



    Antenna
    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.


    Summary


    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!
    Last edited by aeajr; 01-10-2014 at 08:20 AM.
    Long Island Silent Flyers
    www.lisf.org
    Eastern Soaring League
    www.flyesl.com

  2. #2
    Moderator aeajr's Avatar
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    RE: THINGS TO CHECK ON AN ARF

    I guess there is no one here interested in ARFs.  Or, perhaps I did such a good job that no one has any other tips to add.
    Long Island Silent Flyers
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  3. #3
    hugger-4641's Avatar
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    RE: THINGS TO CHECK ON AN ARF

    You did a pretty good job!
    There is one thing that has bit me a couple times on arfs when I get in a hurry, but it falls under your "dry fit" instructions, so if that is done properly you should recognize any problems before you make the mistakes I have.

    If the horizontal stab is to be installed captive in the fusalage thru a pre cut slot, make sure you don't install the hinges first like many instructions will say. Also if the elevator is split and connected with a tiebar, make sure the tie bar goes in when you install the horizontal stab or else make sure you will have room to insert the tie bar after the stab is in place! It's a PITA to forget this and have to remove a stab that's already been epoxied in real good!

    Out of the list you gave, the most common problem I've run into with Arfs is the push rod tubes not installed properly. They may look fine when you poke a rod thru them to check, but often they will put the rod in a bind when you actually hook up to the servo arm and control horns. I've learned to install my servo's and check this before I do just about anything else.
    Jerry
    AMA -922698 Nomal people scare me, but not as much as I scare them...

  4. #4
    Moderator aeajr's Avatar
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    RE: THINGS TO CHECK ON AN ARF

    Thanks for the insight Jerry.
    Long Island Silent Flyers
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  5. #5
    Moderator aeajr's Avatar
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    Something happened to the first post and the text got all jumbled. I fixed it. Let me know of you find any typos/issues.

    Questions/comments welcome.
    Long Island Silent Flyers
    www.lisf.org
    Eastern Soaring League
    www.flyesl.com


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