Airfield Models - Radio Control Aircraft Design

Evolution and Radio Control Aircraft Design Development

May 05, 2015



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Evolution and Radio Control Aircraft Design Development

Myself and others often design a model and take what we get with no intention of further developing the design.  It's not the way to create a design that is as good as it can be, but if the model is for our own personal use and we're happy with it then that's all that matters.  With enough experience and a relatively standard design it isn't too difficult to achieve fair success with an initial prototype.

Unfortunately, many who publish their plans as well as manufacturers appear to take this approach with their designs.  It's obvious the model hasn't been properly developed when the flight trimming phase reveals that the wing incidence, decalage, dihedral, thrust line and other items need to be significantly changed for the model to trim properly.

95% of all sport kits on the market can be built by the average builder on their bench with no plans.  They are simple, standard designs.  That's why you don't see many of those kits in my gallery.  Why pay for a kit that I can build from better materials without financially driven compromises imposed by manufacturers?

As soon as you begin building you should be looking at what you could do better the next time.  My designs are usually better developed when I draw even minimal plans rather than making things up as I go along.

Either way I often find that in practice I could have improved some things by doing them differently.  This aspect mainly involves engineering, contruction and control systems.

From an aerodynamics standpoint, design development begins with the first flight of each prototype.  It is important that you take the time to properly trim your new model.  Most guys get the plane flying straight and level, make an adjustment or two to the CG and then consider their aircraft trimmed.  It is all done in a weekend or two.  Talk to any aerobatic pilot and he'll tell you that it takes gallons of fuel to get an aircraft in trim.

Get a trim chart and use it.  If you fail to trim your model then anything else you do to correct it may be an improvement to your model, but not to the design because you are basically treating the symptoms and not the disease.

One thing that always bugs me is when I hear the comment, "<Insert aircraft here> is the best model I have ever flown" - especially when what I am looking at is basically a rehash of a Stik model.  My first thought is, 'It is probably the only one you ever built that was close to straight and close to being in trim.'  The fact is that most models will fly very well if built and trimmed properly.

After the model is trimmed fly the model as much as possible to learn about it.  Specifically, perform sets of maneuvers you want it to be able to do to determine which characteristics need improvement.  Dedicate entire flights or even flying days to learning about one particular characteristic of your model.

Always use the previous prototype as a starting point

If you want to build a 3D aircraft that compliments your flying style, then the wrong way to go about it is to design a model, decide it does not do what you want and then design a whole new model.  Work on improving the prototype and you will be more successful than you will be if you keep starting from scratch.

Make one change at a time to improve the prototype.  If that means cutting off the tail feathers and building new ones of different proportions then do it.  If the ailerons need to be a different shape, then make new ones.  Try different propellers, changes to the CG, thrust line, wing incidence, control throws/differential, etc.

Trim charts do not apply only to aerobatic models.  Use one for reference to help you determine why a model flies the way it does and what to do about it.

Build the next prototype only when changes to the previous prototype are no longer a significant improvement or changes would be impossible or impractical to make.

Normally it is a poor practice to make more than one change at one time.  However, if there are several flight characteristics you want to improve then you should make all the changes you think will improve the design unless you want to build one hundred prototypes (which would probably be better in theory, but a nightmare in practice).

Rule out  do not guess as many potential reasons for a performance problem as possible.  For example, the prototype may have a sluggish roll-rate.  This could indicate one or more of the following:

  • Control surfaces

    • Too large or too small
    • Excessive hinge line gap
    • Sloppy hinges
    • Flexible control surfaces
    • Surface deflection too small
    • Blow-back (weak servo or flexible linkage)
    • Slow servo
  • Wing Design or Construction

    • Aspect ratio is too high
    • Excessive mass in wings or heavy outboard sections/tips
    • Flexible wing (twists under flight loads)
  • Etc.

Don't give up on the prototype even if the problem is severe.  Several of the above possibilities can be ruled out without having to build a new model:

  • Seal the hinge line
  • Adjust the linkages for more or less control surface throw
  • Strengthen the linkages
  • Replace the servo with one that is stronger or faster
  • Replace the hinges with hinges having less play
  • Build new control surfaces

If the problem still exists, then the only way to resolve it may be building another prototype.  Before you begin building the next prototype you should continue ruling out process for each flight quality you want to improve.  Then when you build the next mark you can significantly improve the design.

Now that you have ruled out as many potential reasons for undesirable flight qualities as possible you need to consider remaining possibilities and choose which one(s) may be the cause.  This is where experience is helpful because you might have to guess, but if you eliminated items then the choices should be very limited which increases your chances of picking the correct fix.  No matter what you choose you don't want to make the problem worse but that is always a possibility.

In the above example of an aircraft having poor roll response, you may have eliminated all linkage/servo possibilities leaving only aerodynamic causes such as airfoil or control surface size and proportions.  If you are stumped at this point then look at other models similar to yours that have the response you want yours to have.  Adjust the control surface design to more closely match that of the other designs.

Model aircraft design is not as complicated as it may seem, but it does require some experience with different aircraft to learn how each parameter affects an airplane's performance.

 
 

The evolution of a design

How I turned an Ugly Stik into a Moderately Attractive Stik

A Midwest Model Mfg. Sweet N' Low Stik.  To avoid any confusion, I was not involved with the design or development of this model.  However I did use it as the starting point for...

an unnamed model that I did design.  Note the grossly large fin/rudder "improvement."  This was one of my early "serious" designs, but obviously I had a lot to learn.  The model had a lot of flight hours logged that provided plenty of information to build the next generation...

Ballerina This "missing link" was never photographed.  A further  development of the above, this model had an improved empennage, a tapered wing, and refinements to the fuselage shape.

After some flight testing it was lost due to devastating elevator flutter in a terminal velocity power dive that never should have been attempted.  I furthered the development of this series with the next prototype...

Hammerhead has no resemblance whatsoever to the Sweet N' Low Stik.  This is a good example of design evolution.  I now have a good design that I am proud of and can honestly call my own.

Also see

 
 

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A Radio Control Model Aircraft Design Case Study
How to Design and Build Lightweight Model Aircraft

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Copyright 2003 Paul K. Johnson