Washout (twist and shout)

In the last post I covered some topics on analysis.  In this post I want to focus on washout, in this case structural washout sometimes called twist. This can be a tricky subject and one that some builders and manufactures avoid.  I will do my best to describe what washout is and why I chose to include it in my design.

There are 2 types of washout, structural and aerodynamic. Aerodynamic washout refers to the use of several different devices that can accomplish the washout effect without incorporating a structural change to the wing. These include modified airfoil section, vortex generators, leading edge wing fences, notches, or stall strips.  The main purpose of these devices is to reduce airflow along the wing span to reduce the likelihood of a tip stall.  I am going to focus this discussion on structural washout.

Structural Washout

I pulled together this diagram to try to explain washout. This discussion will focus also on lifting airfoils, airfoils with a cambered top surface.  Structural washout is the twisting of the wing design so that the tip profile has a lower Angle of Attack (AOA) than the root.  In the diagram above you can see that the red root chord line forms a greater angle with the dashed black line than the yellow tip chord line.  In my design this difference is 2 degrees.  So why would you want to add twist to the wing?  It turns out there are a number of reasons.

Lets take a look at the diagram above.  The vertical forces that must be balanced on a wing are lift and gravity.  In order to counteract the force of gravity pulling your wing to the ground the wing must produce lift.  The force of gravity can be shown to act through a single point, the Center of Gravity (CG).  The lift that is generated by a wing is distributed across the wing profile but can be assumed to act through a single point, the Center of Pressure (Cp).  An important thing to note is that the CG is fixed, it will not move in flight.  The Cp however, will move fore and aft during a flight.  For a flying wing, the Cp and the Neutral Point (Np) coincide.  I only mention this because you may hear the turn Neutral Point in discussions regarding conventional tailed airplanes.  I won't get into a full discussion on locating the CG in this post but suffice it to say that the CG must be forward of the Cp in order for the wing to be longitudinally stable (pitch axis). This is not necessarily the case with tailed aircraft but it is important for tail-less wings.  This being said, because gravity is pulling down at the CG and the lift is pulling up at the Cp the wing will a tendency to pitch forward.  This is called the pitching moment.  All airfoils that generate lift will have a forward pitching moment. This is why airfoil selection is so important.  Wings have very little elevator authority because of the very short tail moment arm.  

Why Include Washout?

Designing and building a model aircraft that flies is easy.  I have seen planes that are nothing more than a sheet of foam board and a motor that fly.  Designing a wing that flies well is a much more complicated endeavor.   One of my initial requirements was a very wide operating envelope.  By using washout I hope to negate some of the negative effects of  some of my design choices and better balance others.  An airfoil will generate a certain about of lift for a given speed and AOA.  As a wings AOA is increased, it will generate more lift for that given speed until it reaches the Critical AOA.  The Critical AOA is the angle at which the airflow over the wing separates from the wing.  The wing stops generating lift at this point and stalls.

A swept wing will tend to stall at the tip of the wing first, a tip stall.  This is because there is a certain amount of airflow along a swept wing instead of across it.  This spanwise airflow increases as it gets closer to the tip and increases the effective AOA of the wing in the tip region.  As we said earlier, once the airfoil reaches the critical AOA it stalls.  If the wing stalls at the tip first, then we lose lateral control because the ailerons have the most roll control at the wing tips.  By lowering the airfoil angle at the wing tips we can compensate for this effect, delaying the tip stall and allowing the wing root to stall first.  In a longitudinally stable wing design a root stall will cause the nose of the wing to drop, causing the speed to increase, generating more lift at a lower angle and allowing the wing to recover.  This is very important feature allowing me to lower the minimum airspeed of the wing.

Another reason to include washout is to reduce the pitching moment. As I said earlier the lift that is generated behind the CG contributes to the pitching moment trying to nose the wing down.,  To compensate for this we must add reflex, or an upward flex to the elevons to generate down force at the back of the wing.  The more reflex we add the more drag we create and the less efficient the wing becomes.  This can also make the wing "pitchy", meaning that the pitch control can be harder to keep steady.  If you look at the figure above you will see that the wing tips are the furthest section of the wing behind the CG.  Therefor the lift generated at the tip will have the greatest effect on the forward pitching moment.  To compensate for this we must decrease the lift at the tips.  To do this we design the tip to be narrower, thinner and, you guessed it, to have less AOA. 

OK, that was longer than I expected but there is one more thing to discuss.  The pitching moment mentioned above will increase with speed.  Remember earlier I mentioned that the Center of Pressure (Cp) is not fixed?  As the wing starts to fly at a higher speed less AOA is necessary to generate a given amount of lift.  This means that we can fly at a lower AOA.  As the AOA is decreased the Cp moves back.  This will cause the pitching moment to increase because the lift is acting further behind the CG.  With the washout added, the wing tip AOA is less than the root and therefore generates less lift behind the CG.  This helps to offset the rearward movement of the Cp.  In fact, the airfoil I selected can generate positive lift down to an AOA of -1.3 degrees.  If the plane is flying at high speeds it can fly at a 0 degree AOA.  This means that with 2 degrees of washout the tips are not generating any lift at all, possibly even negative lift.  This will help the plane to be less "pitchy" at high speeds.  

So there you have it, theoretically, washout benefits a wing design at both low speeds and high speeds extending the flight envelop at both ends.  So why don't more foam wings incorporate washout?  I believe it is for a few reasons.  First of all, these are foam models and we can get away with things that a full scale aircraft can't.  Also, if you are hand cutting a wing the introduction of washout makes the cutting process more difficult.  Some would argue that the added benefits aren't worth the extra complexity. I have run the analysis and built prototypes and have found that including washout definitely improves the performance of my wing.  I have also spoken with designers who feel strongly that adding washout is appropriate in a design such as mine.  

I hope this discussion was helpful.  It was longer than I intended but it is a fairly complicated subject. Be on the lookout for my next post where i intend to discuss vertical stabilization and why flying wings tend to "seek".    Later I intend to get into my prototyping process.