Airfield Models - How to Build and Design Lightweight Model Aircraft

Engineering Radio Control Aircraft Structures for Light Weight, Strength and Rigidity

January 21, 2009



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Airfield Models (http://www.airfieldmodels.com/)Engineering Radio Control Aircraft Structures for Light Weight, Strength and Rigidity

The name of the game for model aircraft airframes, at least the ones meant to fly, is high strength to weight.  That means making engineering choices that result in an airframe that is only as strong as necessary to withstand flight stresses and ground handling.

Anyone can easily build a model that is strong.  We see it all the time in models that are not only strong, but also much heavier than they need to be.  This pitfall is avoidable if you understand how loads are distributed through the airframe and you target those specific loads and avoid the temptation to add more structure "just to be safe."

 
 

Engineering Choices

Heavy models are almost always poorly engineered.  A well engineered model can still end up heavy due to poor material and equipment selection or poor building techniques.  Even so, a properly engineered model at least gives you a fighting chance.  A poorly engineered model is usually a lost cause unless you take it back to the drawing board and redesign the entire model.

Poor engineering usually is due to one or more of the following:

  • The designer doesn't understand model aircraft structures and over-designs.

    The designer copies poor engineering concepts of other bad designers or he comes up with all new poor concepts of his own.

  • The design is compromised to ease building at the expense of additional weight.

  • Poor strength-to-weight materials are used to decrease costs to the manufacturer.  Additional cheap and heavy material is added to compensate for lack of strength which in turn adds more weight.

Any model having lite-plywood fuselage sides is weaker and heavier than the same model would be if it had properly engineered balsa wood sides.  Even if the plywood sides have large cut-outs, Warren truss fuselage construction is lighter while attaining a higher strength-to-weight ratio.

Builders have become so lazy over the years that any time they discuss a model having built-up fuselage sides, the kit is called a builder's kit meaning that the kit is only for "true builders."  Call it what you want, but one fact will never change lighter models fly better.

If you aren't willing to do the work to build a lightweight aircraft then don't be surprised when your models don't fly as well as those built by builders who are willing to make the effort.

What I can't figure out is how the new generation of 3D models that have just a handful of ribs having large cut-outs and a profile fuselage weighs as much or more than a "real" airplane that I build that has 20+ ribs in a larger wing.  Actually I do know why and here's a clue.  Contest balsa ribs don't weigh anything.  You could put 50 of them in a wing and it wouldn't make but (at most) an ounce of difference in the finished weight.

The lack of ribs in 3D aircraft is only to give the illusion of light weight.  More ribs make a more durable wing having a more accurate airfoil.  You get all of this at no weight penalty.

To counter this supposedly lightweight wing, the designers take a thick (heavy) slab of balsa, slap on some plywood (heavier) around the nose and call it a fuselage.  So much for the weight savings of having only 4 ribs in the wing!

And by the way, take a look at the size of the leading edge and spars on some of these 3D planks.  They are much larger and heavier than what is normally used on more traditional wings.  Heavy spars weigh more than light ribs.

A set of 74 ribs for a lightweight biplane. Shown to the left is a rib set for a biplane having a 7" chord including ailerons.  The set includes 42 full ribs and 32 half ribs.  I cut more ribs than necessary because the design is not finalized.

Approximately 3/4 of the ribs are cut from contest balsa.  The rest are medium or hard balsa to be used in the wing center sections.

I intend to space the full ribs 2" apart with one half rib between full ribs.  Therefore there will be a rib every 1" at the leading edge.

The set of 74 ribs weighs a total of 30 grams (approximately 1.06 ounces).  What that means is that if the design could somehow use no ribs at all the weight savings would be only a little over an ounce.

Nevertheless I will remove the interior of the ribs for a few reasons.  First, I am paying attention to grams in this model.  By doing so at every juncture, the overall weight savings will be more significant.

Additionally, the cutouts will make it easier to pass servo leads.  Lastly, the ribs will look more attractive under transparent covering.

Only one quarter ounce is saved by cutting out the interior. The weight has dropped to 21 grams (approximately 0.74 ounces) after removing the interior areas.  The overall weight savings is approximately 1/4 ounce.  This savings is insignificant when taken on its own.

More ribs provide a more accurate airfoil as well as a stronger and more durable wing.

Skimping on ribs doesn't make any sense at all especially when the design includes other components that are heavier and weaker than necessary such as a profile fuselage built from slabs of plywood epoxied to a balsa plank.

A very light wing that is designed to easily handle the flight loads imposed upon it. Here's the wing almost completed.  It still needs ailerons and a few small pieces of plywood for the cabane and interplane strut mounts.

The spars are Sitka Spruce which is probably the best spar material there is short of carbon fiber.  They are light, strong yet will flex significantly before breaking.

The spars coupled with full span shear webs make an extremely strong beam.  This model is being built for up to a .30 glow engine, but could easily handle a .40 four-stroke which is a heavier engine than I would use.

High strength to weight ratios are the name of the game if you want excellent flying model aircraft. If you want your models to have stellar performance, then high strength to weight is the name of the game.

That means a lot of strength and very little weight.  It can be done.  Attaining this goal is more work,  but it's worth it.

By the way, the wing ended up using 17 full ribs and 16 half-ribs.  I'll have a bunch of full ribs left over.

 
 

Designing a Lightweight Model Airplane

The most important thing to keep in mind when designing a model is to learn to use lightweight materials arranged such that they spread loads rather than using plates and sheets to over-build a structure.  My guess is that this poor technique is mostly used by designers who don't really understand the loads on a model so they just make sure there's lots of material in there to ensure nothing breaks.  It works, but adds lots of dead weight.

Wood has grain in only one direction.  More often than not, loads come from multiple directions.  That's why a lot of designers use a lot plywood.  The ply's in the sheet are arranged such that the grain of each ply is 90 degrees to the adjacent ply.  Plywood is great stuff for building houses and other structures that are not supposed to fly.

Plywood in models should be used only as a last resort when nothing else will work.  It should not be relied on as a crutch simply to ensure something is strong enough that could have been strengthened through significantly lighter means.

Always ensure that joints are a good fit.  They are stronger and lighter than an ill-fitting joint that uses excessive glue to fill the gap.  Make it a habit to use gussets and other small, lightweight reinforcements when necessary rather than slapping on plywood plates.

Clamp joints or use weight whenever possible.  You would be surprised how little glue is needed to hold a joint together if it is under pressure while it dries.

When laminating, for example, you can coat both parts and squeegee as much glue back off as you can get.  I'm not exaggerating.  Put the parts under a lot of weight while the glue dries and there will be no separating them.

I often make my own plywood so that the part will have structural integrity while being smaller or having large openings.

 
 

Wing Design

Wings vary wildly in weight for comparable areas.  Properly engineered, any wing can be very light and very strong.  Use contest balsa for everything but the spars and leading edge.  The leading edge can be contest balsa, but it's prone to dings and dents so harder wood will help with durability.

When I design a wing, I always start by drawing the airfoil to determine the thickness of the wing.  The next thing I do is determine how far apart the spars can be.  The farther apart the spars are, the stronger the wing.  What I mean is the vertical distance between a pair of spars (upper and lower).

Once I have the distance, I design a beam that can support the entire load of the wing while keeping in mind any sheeting used will reinforce the wing somewhat.  For example, leading and trailing edge sheeting do add to the strength of the wing assuming the center is glassed.

After the spar system is designed, I build the wing around it.  This approach keeps me from continually adding more and more weight to the wing to strengthen it "just in case."  I already know the spar system is going to be strong enough so everything else added to the wing is just to provide shape or anchor points and can be as light as possible.  These items do not need to do anything to keep the wing from breaking.  That's the job of the spars.

The following wing example is fully sheeted and has four servos, yet is very light due to engineering choices and especially due to wood selection.

 
 

Fuselage Design

People who think fuselage sides should be made from lite-ply shouldn't be designing model airplanes.

If the fuselage side needs to be sheeted, then a much better choice is lightweight balsa.  However, there are loads that go across the grain.  There are two ways to go about supporting these loads.  The heavier way is to build the sides from thicker balsa.  The lighter way is to build the sides from thinner balsa and reinforce the inside with vertical supports and, in some cases, diagonal bracing between the verticals.  Essentially you're building a truss that's sheeted on the outside.

Typical Warren Truss Fuselage Construction which is incredibly light, strong and rigid.The lightest way to build a fuselage (which also gives the best strength to weight) is to build truss-work sides with gussets at all joints and no sheeting.  It is more work, but lighter, stronger and more rigid (for their weight) than any other method in use.

Thick plywood doublers inside a fuselage don't do anything useful.  Often it is a good idea to have a plywood doubler, but it doesn't need to be 1/16" plywood.  That's the quick way to add several ounces of weight to the model.  Use 1/64" ply instead and keep it as small as possible.  I normally extend it just past the tank compartment into the radio compartment at most.  On smaller models, I don't use any doublers at all.

Most slab-sided fuselages have cross-grain sheeting on the bottom.  This sheeting does not need to be very thick because of the way the grain is arranged.  For the same reason, it can normally be contest balsa or slightly heavier weight.  But it does not need to be hard, heavy balsa.  That's overkill and unnecessary weight.

The following fuselage construction example looks robust and it is very strong.  However, it is also very light due to engineering choices and wood selection.  A typical kit fuselage that is generally identical is usually much heavier due to the wood provided which is not hand selected and graded for best strength to weight.

 
 

Tail and Flying Surface Design

Built up and airfoiled horizontal stabilizer assemblyThe tail is often a slab which is heavier and has a lower strength to weight ratio than a built up tail.  For example, the horizontal stabilizer on Rustik is 3/4" thick, has about (20) 1/16" ribs, (2) 1/8" square hard balsa spars and 1/32" shear webs for the entire span.  It is sheeted with 1/32" contest balsa and has solid block tips.

The entire assembly including the elevator weighed 2.1 ounces prior to finishing.  It is strong, very rigid, has an actual airfoil and probably weighs about the same as a 1/4" slab of contest balsa having the same area.

Additionally, a flat slab can easily bow or warp - even after the model is completed.  The built-up and sheeted assembly will not.  If you want to build a slab stabilizer, then use contest balsa and cap the ends with medium balsa to help prevent it from cupping.

Built up and airfoiled horizontal stabilizer assemblyAdd a firm balsa trailing edge about 1/4" wide to put hinges in.  These pieces will strengthen the stabilizer while keeping the bulk of the part light.  Do the same for the rudder and fin.

If you really want to build flat plates for the tail surfaces, but want them even lighter than slabs, then build truss work.  You can leave them open or sheet them.  For example, if the tail is 1/4" thick then you can build it from 3/16" square sticks (contest balsa) and then sheet it with 1/32" contest balsa.  The sheeting will add tremendous rigidity and strength which is why the whole thing can be contest balsa.

Use a glue that is not water based to apply the sheeting and sheet both sides at the same time.  I use slow-drying epoxy smeared on to the stick work in a thin film.  Put the assembly between 2 sheets of wax paper and put a lot of weight on it while the glue cures.  Let it sit for at least a day - longer is better.  It should be very flat and stay flat when the glue is cured.

Epoxy is heavy?  I guarantee I can build a tail the same size and thickness this way lighter than a slab tail can be built.

 
 

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Copyright 2004 Paul K. Johnson