Doublers

[Greyhead]

Doublers, usually but not exclusively ply, serve 2 purposes; they strengthen the construction around high stress areas and also transfer the extra energy to other parts of the airframe.

Firstly a bit of theory, but if you only ever build from kits or plans and never think about “tweaking” the construction then feel free to skip the next 3 paragraphs! Personally though I think an understanding of why we do the things we do the way we do them never goes amiss.

The first thing to grasp is the fact that energy can neither be created nor destroyed; we can only change its’ form, there is the same amount of energy in the universe today as on the day of creation. For example, when we open the throttle the energy stored in the fuel is converted by the engine / prop into Kinetic Energy (mass and speed) and Potential Energy (mass and altitude). Not all the output from the engine is converted usefully, there are some “loses” caused by friction (drag), this produces heat energy, not a lot in our case but just think of the Space Shuttle re-entry and sound energy, that lovely whistle especially with all the rigging used on biplanes.

A simple “box” fuselage is very strong but we have to have access to the radio gear etc. through a hatch or the wing seat and this produces a weak spot. Doublers are used to strengthen the fuselage sides and transfer energy from the firewall and often the undercarriage mounting across this weak spot to the rear fuselage. When designing doublers avoid ending with a single vertical joint, this will concentrate the energy at this point, try to spread the energy over as large an area as is practical.

In the example shown below there was no plan supplied with the kit so a paper template is traced around the fuselage and the internal structures drawn on, the proposed doubler is outlined in red. In the case of a heavy landing the energy is concentrated at the undercarriage mount just in front of the wing seat, the doubler greatly increases the strength of the balsa fuselage sides and the energy is carried over the weak wing seat area, when it gets to the fish tail (nice description Dave!) it is divided so half goes to the top longeron and half to the bottom.

When gluing ply doublers it is important to sand the faces of the ply. The manufacturers use a release agent on the presses which by its’ very nature is designed not to stick; the amount of release agent left on the ply varies greatly but generally the thinner the ply the more there is. “Contact” adhesive is often recommended but personally I don’t like it, if the fuselage has rounded corners and you sand down to the doubler the glue pulls out and generally makes a nuisance of itself. I use PVA, the only problem being that the sides have to be held flat overnight whilst the glue dries, just make it last job of the evening and put them under your building board.

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[wonwinglo]

Grahame there is a lot of common sense in what you say,amazing how we sometimes do commonplace things without thinking about the reasons for same,you have placed the humble doubler into its proper perspective and shown the importance of this item and how to go about things properly.

I learned my lesson about acute sharp edges many years ago when I had an experimental card covered foam wing break in flight,a neglected crease had weakened the wing at a point three inches out from the centre section bandage,the ensuing 'snap & crack ' in flight haunts me to this day ! after this it was bandage>then nylon> then a gradual blend to finish things off.

 

[Glider Guider]

I agree with you Grahame and glue the doublers with PVA. Another technique I came across was with the Glens Models CAP 232 which has liteply fuselage sides and doublers and that was to wick low viscosity cyano along the edges of the doublers which is then pulled in by capillary action. Glen says in the instructions that a lot of modellers are sceptical that this will give a satisfactory join but he assures us that it will.

 

[wonwinglo]

I agree with you Grahame and glue the doublers with PVA. Another technique I came across was with the Glens Models CAP 232 which has liteply fuselage sides and doublers and that was to wick low viscosity cyano along the edges of the doublers which is then pulled in by capillary action. Glen says in the instructions that a lot of modellers are sceptical that this will give a satisfactory join but he assures us that it will.

*** Yet another variation on this one Dave is to apply PVA adhesive in squares and then rub a piece of glasspaper together loosely over the wet adhesive,then run lines of medium cyno between the PVA,the glass remnants stop the halves from sliding and the cyno gives instant grab,used this successfully in the past,it sounds complex but is quicker to do than to describe.

You can also easily thin down the PVA to make it easier to brush on.

Old flat irons obtainable fron car boot sales make excellent weights to hold things like this whilst the glue cures.

 

[Greyhead]

Another Use For Doublers

It’s true that the lighter a model the better it will fly, the exception being a slope soarer in a good blow, and to this end it is always recommended that the tail end be built as light as possible. However, using very light balsa for the rudder and elevators can have unwanted consequences; I have had problems in the past and seen them on several occasions on other pilots models. Using “standard” bolt through control horns, the wood can easily be crushed on first assembly and / or work loose over time and piano wire elevator joiners can break through the balsa surface.

The answer is to use very thin ply, 0.4mm (for us oldies 1/64"), to spread the load. It is best to drill holes for the elevator joiner before fitting the ply doublers to both sides of the balsa. Cut the ply to shape and use these as templates to mark the balsa, make recesses for the ply, I use small files of this as it is easy to get a neat edge, then glue the ply in place. Once set, fill if necessary and sand over the entire area.

I tried to use a thumbnail for the photo but for some reason it wouldn't work; strange!

 

[wonwinglo]

Nice job Grahame,I am always surprised to see how people just screw the horns into balsa without any re-inforcement,just asking for problems,if I can just offer a few more embellishments on what you have said and beautifully illustrated here,before cutting the 'V' from the rear of the elevator sharpen up a piece of piano wire,bend twice at right angles and with a small round file sink a slot into the centre portion leading edge of the elevator,drill two slots with your Dremel tool and drive in the spiked joiner nice and flush,drop some thin cyno around the joint and allow capillery reaction to take place,it will not move believe me ! next cut out the 'V' and do your trick with the ply braces,used this myself on slopers and never had any problem.

By cutting out the 'V' at this later stage there is no chance of having a twisted elevator.

ARTF builders can easily incorporate this mod into their models,they are particularly prone to elevator horn problems.

 

[wonwinglo]

Grahame,to digress,did I miss the answer to the Albatross D.V problem ? if so can you please explain what the problem was that was related to the original aircraft ? Thank you,Barry.

 

[Greyhead]

Barry

To make the elevators as one piece is a great idea, even built up ones could be done this way with a little bit of forethought!

As for the DVa you were correct the wings were twisting resulting in an ever-tightening turn that was virtually impossible to get out of. My first reaction was to increase speed as I thought it was a tip stall, this of course was the worst thing I could do in the circumstances. If I’d done a better job of researching the project (hence the sub title) I would have discovered that the full size had the same problem, also how they corrected it, then I could have included the modification on the original model and saved myself a lot of heartache. If you check the DVa thread I’ve put a link to a site describing the development from D1 to DVa. My only excuse is that at that time I didn’t have access to the Internet, now research a lot easier.

Grahame

 

[wonwinglo]

Thanks Grahame,I thought as much,you were indeed lucky to get out of that situation,this is exactly how I lost my Pou De Ciel when the wings flexed in a bank,despite having flown her for hundreds of flights.

So many problems for scale models can be traced at source from the original machine,and as you say a little bit of research can pay dividends.

Aileron flutter is another thing,very frightening,no wonder a lot of full size machines have been lost this way,imagine a huge wing beginning to flutter and just break up in flight ? it happened to the Armstrong Whitworth AW.52 Flying wing design,the pilot made the first use of an ejector seat for real.