Airfield Models - Engineering RC Aircraft for Light Weight, Strength & Rigidity

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January 21, 2009

Engineering Radio Control Aircraft Structures forLight Weight, Strength and RigidityThe name of the game for model aircraft airframes, at least theones meant to fly, is high strength to weight. That meansmaking engineering choices that result in an airframe that isonly as strong as necessary to withstand flight stresses andground handling.

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

Engineering ChoicesHeavy models are almost always poorly engineered. A well engineered model can stillend up heavy due to poor material and equipment selection or poor buildingtechniques. 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 drawingboard 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 comesup 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 themanufacturer. Additional cheap and heavy material is added to compensate forlack of strength which in turn adds more weight.

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

Builders have become so lazy over the years that any time they discuss a model havingbuilt-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 modelsfly better.

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

What I can't figure out is how the new generation of 3D models that have just a handfulof ribs having large cut-outs and a profile fuselage weighs as much or more than a

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Airfield Models - Engineering RC Aircraft for Light Weight, Strength & Rigidity

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"real" airplane that I build that has 20+ ribs in a larger wing. Actually I do know whyand here's a clue. Contest balsa ribs don't weigh anything. You could put 50 of them ina 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 ribsmake a more durable wing having a more accurate airfoil. You get all of this at noweight penalty.

To counter this supposedly lightweight wing, the designers take a thick (heavy) slab ofbalsa, slap on some plywood (heavier) around the nose and call it a fuselage. So muchfor 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 these3D planks. They are much larger and heavier than what is normally used on moretraditional wings. Heavy spars weigh more than light ribs.

Shown to the left is a rib set fora biplane having a 7" chordincluding ailerons. The setincludes 42 full ribs and 32 halfribs. I cut more ribs thannecessary because the design isnot finalized.

Approximately 3/4 of the ribsare cut from contest balsa. The rest are medium or hardbalsa to be used in the wingcenter sections.

I intend to space the full ribs 2"apart with one half rib betweenfull ribs. Therefore there will bea rib every 1" at the leadingedge.

The set of 74 ribs weighs a totalof 30 grams (approximately1.06 ounces). What that meansis that if the design couldsomehow use no ribs at all theweight savings would be only alittle over an ounce.

Nevertheless I will remove theinterior of the ribs for a fewreasons. First, I am payingattention to grams in thismodel. By doing so at everyjuncture, the overall weightsavings will be more significant.

Additionally, the cutouts willmake it easier to pass servoleads. Lastly, the ribs will lookmore attractive undertransparent covering.

The weight has dropped to 21grams (approximately 0.74ounces) after removing theinterior areas. The overall

Airfield Models - Engineering RC Aircraft for Light Weight, Strength & Rigidity

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weight savings is approximately1/4 ounce. This savings isinsignificant when taken on itsown.

More ribs provide a moreaccurate airfoil as well as astronger and more durablewing.

Skimping on ribs doesn't makeany sense at all — especiallywhen the design includes othercomponents that are heavierand weaker than necessarysuch as a profile fuselage builtfrom slabs of plywood epoxiedto a balsa plank.

Here's the wing almostcompleted. It still needsailerons and a few small piecesof plywood for the cabane andinterplane strut mounts.

The spars are Sitka Sprucewhich is probably the best sparmaterial there is short ofcarbon fiber. They are light,strong yet will flex significantlybefore breaking.

The spars coupled with full spanshear webs make an extremelystrong beam. This model isbeing built for up to a .30 glowengine, but could easily handlea .40 four-stroke which is aheavier engine than I woulduse.

If you want your models tohave stellar performance, thenhigh strength to weight is thename of the game.

That means a lot of strengthand very little weight. It can bedone. Attaining this goal ismore work, but it's worth it.

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

Airfield Models - Engineering RC Aircraft for Light Weight, Strength & Rigidity

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Designing a Lightweight Model AirplaneThe most important thing to keep in mind when designing a model is to learn to uselightweight materials arranged such that they spread loads rather than using plates andsheets to over-build a structure. My guess is that this poor technique is mostly used bydesigners who don't really understand the loads on a model so they just make surethere's lots of material in there to ensure nothing breaks. It works, but adds lots ofdead weight.

Wood has grain in only one direction. More often than not, loads come from multipledirections. That's why a lot of designers use a lot plywood. The ply's in the sheet arearranged such that the grain of each ply is 90 degrees to the adjacent ply. Plywood isgreat 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. Itshould not be relied on as a crutch simply to ensure something is strong enough thatcould 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 andother small, lightweight reinforcements when necessary rather than slapping onplywood plates.

Clamp joints or use weight whenever possible. You would be surprised how little glueis 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 glueback off as you can get. I'm not exaggerating. Put the parts under a lot of weightwhile 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 beingsmaller or having large openings.

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

When I design a wing, I always start by drawing the airfoil to determine the thickness ofthe wing. The next thing I do is determine how far apart the spars can be. The fartherapart the spars are, the stronger the wing. What I mean is the vertical distance

Airfield Models - Engineering RC Aircraft for Light Weight, Strength & Rigidity

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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 wingwhile keeping in mind any sheeting used will reinforce the wing somewhat. Forexample, leading and trailing edge sheeting do add to the strength of the wingassuming the center is glassed.

After the spar system is designed, I build the wing around it. This approach keeps mefrom continually adding more and more weight to the wing to strengthen it "just incase." I already know the spar system is going to be strong enough so everything elseadded to the wing is just to provide shape or anchor points and can be as light aspossible. 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 toengineering choices and especially due to wood selection.

Wing Construction Example

Fuselage DesignPeople who think fuselage sides should be made from lite-ply shouldn't be designingmodel 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 aboutsupporting these loads. The heavier way is to build the sides from thicker balsa. Thelighter way is to build the sides from thinner balsa and reinforce the inside with verticalsupports and, in some cases, diagonal bracing between the verticals. Essentially you'rebuilding a truss that's sheeted on the outside.

The lightest way to build afuselage (which also gives thebest strength to weight) is tobuild truss-work sides withgussets at all joints and nosheeting. It is more work, butlighter, stronger and more rigid(for their weight) than any othermethod in use.

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

Most slab-sided fuselages have cross-grain sheeting on the bottom. This sheeting doesnot need to be very thick because of the way the grain is arranged. For the samereason, it can normally be contest balsa or slightly heavier weight. But it does not needto 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 typicalkit fuselage that is generally identical is usually much heavier due to the wood providedwhich is not hand selected and graded for best strength to weight.

Fuselage Construction Example

Airfield Models - Engineering RC Aircraft for Light Weight, Strength & Rigidity

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Tail and Flying Surface DesignThe tail is often a slab which isheavier and has a lower strengthto weight ratio than a built uptail. For example, the horizontalstabilizer on Rustik is 3/4" thick,has about (20) 1/16" ribs, (2)1/8" square hard balsa spars and1/32" shear webs for the entirespan. 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 isstrong, 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. Thebuilt-up and sheeted assembly will not. If you want to build a slab stabilizer, then usecontest balsa and cap the ends with medium balsa to help prevent it from cupping.

Add a firm balsa trailing edgeabout 1/4" wide to put hinges in. These pieces will strengthen thestabilizer while keeping the bulk ofthe part light. Do the same forthe rudder and fin.

If you really want to build flatplates for the tail surfaces, butwant them even lighter than slabs, then build truss work. You can leave them open orsheet 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 sheetingwill add tremendous rigidity and strength which is why the whole thing can be contestbalsa.

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

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

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