Monday, 24 August 2015

Some basic terms related to Aircraft Part 3 - Aerofoils More Explanations

More Explanations about Aerofoils.

  • Each family of Aerofoils consists of a large selection of airfoils but we use very few of them because of not having a clue about how to properly select a real airfoil.
  • Data on real airfoils won't apply in our model air crafts anyway.
  • The airfoils are tested at larger sizes than the average model aircraft and data changes as the size changes. However, the airfoils should scale down comparatively.
  • If you are purpose-building something competitive that requires the best possible airfoil for a particular application then I can't help you. But, if you approach a school of Aerospace then you may get help there.
  • From experience you can learn that the difference between one airfoil and another close to the same shape will make a very small difference,  that would only be noticed by an expert pilot. 
  • This differences are not enough to cause any problems in our construction unless we do some drastic changes like changing a round leading edge to one that is razor sharp.
  • There will be a huge difference between any under-cambered and any symmetrical airfoil.
  • If we want to construct unique planes that fly for fun, then make our airfoil close to the shape of airfoils used by other planes of roughly the same type.
  • If we can't decide, then copy an airfoil in use on a model and scale it to the right size for our model.
  • We can make small changes to original aerofoil design such as , thicken it to slow the model or make the leading edge radius smaller to allow sharper stall maneuvers.
Note: Any airfoil that is not symmetrical is a cambered airfoil.

The terms "flat-bottom" and "semi-symmetrical" are not used by the aerospace industry and they probably laugh at us when they hear these terms. Whatever it is, this article is for scratch builders like me, not for them.
Symmetrical Airfoils

  • Use for aerobatic airplanes - particularly monoplanes.
An aircraft constructed to do aerobatics should always have symmetrical flight surfaces (wing, horizontal stabilizer and vertical stabilizer).  Flat plates (which are symmetrical sections) work well for tail surfaces.
Semi-Symmetrical Airfoils
  • Use for secondary trainers, sailplanes and sport aerobatic biplanes.
If a beginner moves up too quickly and hasn't mastered his primary trainer, then a secondary trainer with this type of airfoil is a very bad choice. A lightweight, well behaved model with a symmetrical airfoil will be a good secondary trainer.
Sailplanes use highly refined and tested airfoils that provide the best lift to drag so that they can scoot across the sky quickly in their search for thermals and then climb easily in the lift.
Flat-Bottom Airfoils
In actual practice try not to use true flat-bottom airfoils for anything. Modified flat bottom airfoils are excellent for slow, gentle flight.
True flat-bottom airfoils are next to impossible to trim properly because they are extremely speed sensitive.  It may be possible to trim this out, but it means spending hours.
Modified flat bottom airfoils.
Modified flat bottom airfoils are used in model powered aircraft for slow flight or high lift capabilities.  They do not penetrate the air well but can stay aloft at very low speeds.
Under-Cambered Airfoils
Use for scale models, sailplanes and some high-lift situations.
They are mostly used for scale aircraft from the start of aviation.  They tend to have high lift and are sometimes used in free flight models.
Reflexed Airfoils
Use for flying wings.
A reflexed airfoil has a trailing edge that is turned up slightly.
Note that a true reflexed airfoil isn't necessary and often isn't used with flying wing model aircraft.  A lot of designers fake it by adjusting the elevons so they are slightly up.
  • The wing is so designed, and so inclined, that (in passing through the air) it will first attract the air upwards and then push it downwards and by doing so it experiences an upward reaction from air.
  • It is a simple example of Newton’s laws- “To every action, there is an equal and opposite reaction”.
  • The downward flow which leaves the aeroplane wing is called downwash.
Scratch Built Izicraft 400 Trainer - Full Balsa Wood Construction.

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