Airfield Models - How to Build and Design Lightweight Model Aircraft

Selecting Lightweight Equipment for Radio Control Model Aircraft

May 05, 2015



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Airfield Models (http://www.airfieldmodels.com/)Selecting Lightweight Equipment for Radio Control Model Aircraft

Now you know two ways to build a heavy airplane use too much of the wrong materials or select a poorly engineered design.  Combine those two areas for a double-whammy, weight-gain bonus.

But why stop adding weight there?  There is a another area that can add lots of useless weight equipment selection.

 
 

A Cat Chasing his Tail (Wermacht not withstanding)

A power plant has a snowball effect in both directions.  Using a heavier, more powerful engine requires a more robust structure to handle it.  Now we not only have to beef up the structure to handle the engine but we have to beef up the structure to handle the beefing.

All of the sudden the landing gear is heavier, the landing gear mounting system is heavier, the wing spars are heavier, the wing mounting system is heavier, the fuselage needs to be more robust, the tail surfaces need to be stronger, heavier servos are needed, a bigger onboard battery is needed, etc.  The list goes on.

If we go the opposite route and use a lighter, less powerful engine, then we can lighten all the above.  This is why some guys can build monster airplanes that fly on a .40 and other guys build the same size airplane and stuff a chainsaw engine in it.  I'd rather fly the plane with the .40 in it.

A lighter aircraft has less mass which means there is less inertia which means the structure is more likely to survive impacts.  The adage, "Build to fly, not to crash" is absolutely true.  Learn to live by it.

A lighter airplane can maintain flight at lower airspeeds, will climb better, will have better stall characteristics and will perform better aerobatics.

A light airplane will begin and end maneuvers much more crisply than a heavy airplane.  Swing a baseball bat and try to stop it.  Now strap a couple bricks on it and do the same thing.  Which way was it easier to start and stop the swing?  Physics is physics airplanes behave in exactly the same way.

The "Right" Material in Every Circumstance

Many designers get stuck using the same material in an application regardless of the model size, intended power plant or flight envelope.

For example, a designer may use 1/8" light ply fuselage sides for any model from .20 to .90.  If the sides can handle a .90, then they are certainly overkill for a .20 size model.

Why is it that 1/4 x 20 nylon bolts are used to retain a wing on a .40 size model if they are also used to hold the wing on a 1.20 size model?  If a standard servo can handle a .40 size model, then why would you use it in a .15 model?  Take the time to think things like this through.

Check the specs of equipment and use nothing larger and heavier than necessary.  Don't install heavy duty servos unless they're needed.  So called "standard servos" are the most useless servos made.  They were good when they first came out, but they have become obsolete.

There are now mini servos that have more power, more speed and sometimes include a ball-bearing.  They're also half the size and weight of a standard servo.  If a standard servo will work, then so will a Hitec HS-85 and there are plenty of others.

Don't use a larger onboard battery pack than necessary.  A small 3-channel model can get several flights from a 250 - 350 mAh pack.  Having a field charger will allow you to top off the battery between flights.

Landing gear can add a lot of weight in a hurry.  Landing gear weights vary greatly. You don't have to buy expensive carbon fiber gear, but if you do you'll save more weight.  Practice learning how to fly your airplane to a landing and you can get away with a lighter landing gear.

A way to save a lot of weight inexpensively is carefully selecting wheels for the model.  Take a postage scale to the hobby shop to weigh different brands and styles.

Aluminum spinners are pretty, but heavy.  If you need the nose weight to balance the plane then fine, but if it's nose heavy, don't put lead in the tail before you switch to a plastic spinner.  Aluminum engine mounts are also heavy.  For most engines, a fiberglass-filled nylon mount is strong enough and much lighter than aluminum.

Always choose the lightest radio gear, hardware, control systems and other installed equipment that will do the job and be reliable.

 
 

The point to all of this is that it is possible to have strength and durability where needed and still build a light model.  If you have a poor design and make cut-outs in the ribs, then you've wasted your time for no real improvement.  If you pay attention to every detail to ensure the lightest material is used that will provide adequate strength and rigidity, then you'll see a big difference.

In general, always look for ways to use the lightest wood possible.  Normally that means building up a structure rather than cutting out slabs.  Again, it's engineering and material selection - not glue and not lack of ribs - that determines the weight of the finished model.

 
 

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Engineering RC Aircraft for Light Weight, Strength & Rigidity
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Copyright 2004 Paul K. Johnson