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Things to check on an arf

Old 03-05-2013, 02:34 AM
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Cool Things to check on an arf

by Ed Anderson
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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.

  • 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.


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.

Clear skies and safe flying!

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Old 03-05-2013, 03:28 AM
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Great post, Ed. Thank you!
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Old 03-05-2013, 03:33 AM
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Thanks Ed......that info will not fall on deaf ears on this end........hope the newbies take the time to dig in and absorb.....

Great stuff!
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Old 03-05-2013, 03:34 AM
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Thanks guys. Use this as a resource when helping new pilots and those moving to their first ARF.
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Old 03-05-2013, 03:41 AM
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Originally Posted by Rockin Robbins View Post
Great post, Ed. Thank you!
2nd that. With so many ARF's out there, and the comparative lack of kit and scratch builders these days, this is a very timely and practical thread. Only had one ARF myself, (an SDM "Skyhawk") and I had to totally strip it and do a complete rebuild before it would fly properly. Without the prior experience of building, many novice pilots need the extra help to get their plane sorted. Good job Ed.
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Old 03-10-2013, 06:00 AM
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If it uses a wing tube... check wing tube length.
Too short is easy to miss and it can cause the aircraft to come apart in the air.

I have run into 2 ARFs with short tubes. One I found out when the tube started coming out the bottom of the wing because it didn't make it to the plywood rib doubler that was supposed to support it.
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Old 03-30-2013, 05:34 AM
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I forgot to include checking for damage. Cracks in the fuselage. Or evidence of cracks in any part of the wing. A little glue can save big headaches on minor stuff. Big stuff should go back.
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Old 08-13-2013, 10:32 PM
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Read some good tips about using expanding insulation foam to help reduce flex in push rods.
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Old 08-13-2013, 11:29 PM
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Expanding foam can be good but must be used with care. You can expand your fuselage to destruction.

If trying to foam fill a tube type pushrod to stiffen it then look at what happens using the expanding foam and not cleaning the clear tube nozzle. you end up with a glob of foam at the tip and a glob at the valve and an otherwise empty tube.
There is a boating pour-in expanding foam that will fill the tube... but you have to buy about $100 worth to get it and its hard to mix in small batches.

For anchoring Nyrod sheath...
Tie the tube at the anchor spots with a piece of twist tie or copper wire.
mix 4 drops of Gorrilla glue (Polyurethane) with 1 drop of white school glue. (can make any size batch needed)
The white glue has water in it to kick off the poly. Water separates from gorilla glue... white glue does not separate.
Apply the glue mix at the places you tied the tube down.
This will form a ball of glue-foam around the tube and hold it very firmly.
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