This week, we're attempting to unravel the mysteries of the Flying Wing in a single article.
When you look at a flying wing, it's not immediately obvious what actually makes it fly. Why doesn't it need a tail? Why are they almost always swept? Well, it's a little complicated, but hopefully this particular edition of Aerodynamics Simplified will be able to let you in on the basics. Let's get started.
In short: a flying wing works because it balances airflow and the center of gravity in such as way that no tail is needed. Where a normal wing would adversely flip forwards or backwards when exposed to the oncoming wind, a flying wing is able to counteract this effect with some handy aerodynamics. If you'd like to know how - read on.
Before you read more about flying wings, I recommend you check out these other articles first if you haven't already.
Explained In Detail
As we know, airplanes fly due to the balance of the four forces. Lift is the most critical of these for the flying wing. It's really very sensitive. If you want to know how lift works, check out this other article.
With a wing, it's all about the relationship between the center of gravity (CG) and center of pressure (CP). The CP is, in some ways, also where the center of lift is. The CG doesn't move much, if at all. It's fixed. Depending on where you position your battery, your balance point will remain the same. The CP, on the other hand, does move depending on the angle of attack. This is what an airfoil looks like at a neutral angle of attack.
This is what it looks like at an aggressive angle of attack. As you can see, the lift bubble is further forwards. This shows that the center of pressure moves backward and forwards depending on the angle of the wing relative to the oncoming airflow. This can cause problems when you have your CG in a fixed place as you can effectively have your airplane become tail heavy at an aggressive angle.
What Stabilizers Do
The shifting nature of the CP means we usually need to put tails on airplanes. With the CG and CP lined up, the airplane will fly straight and level. Shifting the CG and CP apart, however, means that the wing will either start and then continue to nosedive or flip backwards. The moving CP means that the wing is inherently unstable. You may have seen this phenomenon when trying to throw a flat wing forwards into the air. It will either spin backward on itself or front flip into the ground.
A stabilizer builds natural stability into an aircraft. This is because it equally opposes the forces made by the unstable wing.
Why Flying Wings Don't Have Stabilizers
Flying wings need a considerable distance between the CP and CG, a lot more so than normal wings on a tradition aircraft with a tail. To achieve this, the CG is usually moved forwards using the positioning of a battery. To keep the aircraft from being nose heavy, the elevons on the wings provide something called reflex. This is where they provide up pressure to lift the nose.
Reflex isn't very efficient. It creates more drag than a regular wing - but it does work. There are many shapes you could make a wing, but the swept shape we commonly see in RC helps to spread the CP and CG apart. Longer distances mean more leverage afterall.
Sometimes, though, we do see plank wings like the FT Goblin.
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Article by James Whomsley
Editor of FliteTest.com
YouTube Channel: www.youtube.com/projectairaviation