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Monday, July 18, 2011

On Washout

Chris wrote;
"I don't have a good understanding of washout, its relationship to tip stalling and how the seemingly common practice of altering the default position of ailerons and flaps helps matters."

The Wiki definition that you quoted is close to accurate.  The bottom line is that washout strives to make the wing tips stall after the wing root stalls.  When a wing tip stalls first, the wing drops hard and the airplane could begin to spin (a spin is a fully developed flight condition where one wing is completely stalled and the other wing is not, so the un-stalled wing is literally flying circles around the stalled wing).  Since airplanes often fly near the limits of a stall when close to the ground (e.g. the landing phase), a wing dropping hard or a spin at low altitude can be especially undesirable.  

One way to build-in aerodynamic washout is by repositioning the ailerons (assuming they are near the tips).  By deflecting  both ailerons up a little, you effectively change the airfoil cross section near the tips so the tips produces less lift than the roots with the ailerons in their default position (trailing edge slightly up).  

This really isn’t a pure solution, because it is not designed in from the beginning, meaning that you sacrificing some of the design lift of the wing, and consequently, you are carrying a bit more structure than is required to build the wing since it isn’t producing as much lift as intended.  The end result is that you’ll experience higher landing speeds and require a faster cruise than the aircraft as designed.  Basically, you are flying with wingtip spoilers deployed, all the time.  

With real washout, designed from the ground up, the total lift of the wing will still be appropriate for the aircraft even with the washout built-in.

“What are the symptoms?”

The symptoms of no washout are different from plane to plane, depending mostly upon airfoil type.  A low performance airfoil, which is flat on the bottom and curved on top, or heavily asymmetrical, experiences a very sharp loss of lift upon stalling.  So low performance aircraft like Cubs and Champs and most high wing trainers, are more vulnerable to tip stalls, since the loss of lift is more dramatic once stalled. 
So a high wing trainer, with a flat bottom airfoil and without washout, would be expected to drop a wing hard as a stall develops, and maybe spin, because it is very rare indeed that both wing tips are experiencing absolutely identical flight conditions and that the aircraft is flying perfectly straight and true.  

An aerobat with a symmetrical airfoil (exact same curvature on the bottom as the top) and still without washout, loses lift more gradually as the wing stalls—it does all go away at once.  So a high performance aircraft doesn’t care as much about washout.

“What's the source of the problem? How does one fix/mitigate it? The PZ 109 has a reputation for tip stalling on approach. Is this a washout problem or something else?”

Hopefully the text above covered most of those q’s.   I’d have to look at the PZ 109, but it sounds like a fundamental design flaw that might be mitigated with some washout.  

Plane most susceptible to tip stalls are planes with asymmetric (low performance) airfoils with the most even lift distribution from root to tip.  IOWs, if you look at the total amount of lift produced by the wing and then chart the distribution of that lift from root to tip, a tip-stall prone aircraft will produce a lot of lift near the tip.  An aircraft that is not prone to tip stalls will produce proportionately more lift near the root, and less at the tip.
This can be done lots of ways, not just with washout.  For example, with taper.  If you taper the wing, the tip’s contribution to total lift is relatively small compared to the root, so even the tip does stall first, it’s not as big a deal because in relative terms, not that much of your total lift is at risk.
Another way is with aspect ratio.  A very long thin wing, with the same wing area as a very short and stubby wing, has wing tips that are farther away from the fuselage, making tip stalls more of a problem.

So in general..

Aircraft that are more vulnerable to tip stalls:
  • ·         Use low performance airfoils (flat bottom or similar)
  • ·         Have little or no taper , like a simple rectangular wing plan form
  • ·         Have relatively high aspect ratios, or long thin wings
  • ·         Benefit the most form incorporating some washout
Aircraft that are less vulnerable to tip stalls:
  • ·         Have high performance, very symmetrical airfoils
  • ·         May have tapered wings, or a smaller chord at the tip
  • ·         May have stubby, low aspect ratio wings
  • ·         Benefit less from washout
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