Hello everybody and welcome to another instalment of Technical Thursdays. This time I am writing an article that has been suggested by one of you! A special thank you goes to Sponz for the suggestion. If you’d like to see an article written about something you’re interested in, but don’t have enough knowledge to write one yourself, head over to the FliteTest forum’s Articles Section (<- Link) to put your idea forward to someone or even myself!
In this article we’ll discuss; what tip stalls are, how they occur, and what is done to prevent them.
Tip stalls are exactly what their name says they are... Stalls at the tip of a wing. They are notorious in old Warbirds and even some of the most modern, high-tech Fighter Jets around today. They can be very dangerous and can cause the airplane to be unstable and unpredictable, a pilot’s worst nightmare.
What Are Tip Stalls?
Before we talk about tip stalls you should go check out my other articles such as “How Do Aircraft Fly?” if you haven’t already. It’ll give you an in depth understanding of the forces in play on an airplane in flight.
To truly begin to understand tip stalls we have to know a bit about what a stall is to begin with. A stall, also known as a partial or full separation of the airflow over a wing, happens at a critical angle of attack. It is where the horizontal line of the wing is at too great of an angle to the apparent wind direction causing a reduction in lift, not necessarily a complete loss of lift.
Tip stalls are exactly that, however they have a few other characteristics that can come into play.
How do tip stalls happen then?
Tip stalls happen when the aircraft is either manoeuvring at slow speeds or rolls very quickly at high speeds. They also form differently with different kinds of wing shapes; we’ll visit that later.
First let’s find out what happens at slow speeds.
Set a scene in your mind. An airplane is coming into land at a slow speed. It is pitched up due to this low speed to make some extra lift and so the wings are at a moderate angle of attack; similar to the image below.
It is fairly breezy and the pilot is trying his best to keep the airplane on the centerline by rolling the airplane back and to (pulling the stick left/right). This is where tip stalls can suddenly and critically begin to form.
Let’s look a bit deeper. As the airplane is rocking back an forth certain forces are happening. The first and the most crucial is the tip on one wing is stalling and then isn’t regularly with each movement of the yoke by the pilot. This is because at the aircraft rolls it creates an apparent wind (shown in red in the image below). The apparent wind comes from directly underneath the wing. This has the effect of bringing the apparent wind direction underneath the wing more than before; increasing the angle of attack and causing the tip of the wing to stall. The tip stalls first in this instance because as the plane rolls, the tip is moving faster than the root of the wing.
Not only that but on the other side of the airplane the opposite is happening. The apparent wind being created from the top side is reducing the angle of attack on the ascending wing and so is causing more lift to be made. These two forces together can cause the airplane to quite quickly rotate even with a small amount of yoke movement. If you can’t quite visualise how the apparent wind is affected, here is an image below to show you that.
The black arrow is the wind coming over the wing of the airplane that we visualised before. The red arrow is the apparent wind being created by the rolling of the airplane and the blue arrow is the combination of those two wind directions.
How Does The Wing Shape Affect How Tip Stalls Form?
Tip stalls are bad because they cause the airplane to roll and rock rapidly and unpredictably almost always leading to the pilot losing control and crashing. Tip stalls have been fatal several times in history. Instead, designers and manufacturers design airplane wings to stall at the root. This is because the ailerons at the tips will still be in use giving the pilot maneuverability and the wing will stall gradually giving the pilot an ‘early’ warning sign.
Some wing designs such as the Swept Back Wing shown above stall from the tips inwards. In this format tip stalls are even more dangerous because they have the affect of moving the centre of lift forward; causing a sharp pitch-up movement, making the stall worse that it was. This is why swept back winged airplanes have to takeoff, land and fly faster than regular rectangular winged aircraft. Elliptical wings are also bad because they stall along the whole wing surface; because of this, the pilot will experience a very sharp loss of lift near the stalling angle reducing his/her chances of recovering. Remember, the gradual stalling gives the pilot time to react.
How Can We Prevent Tip Stalls? Or Any Kind Of Stall?
Various methods have been designed over the years to; completely prevent stalls, delay them, initiate them in a desired location on the wing and/or ensure they don’t form in a chosen place.
Vortex Generators: I’m sure most of you reading this know about vortex generators. For those of you that don’t; vortex generators are small strips placed along the leading edge of a wing. They create twisting airflows that like to stick to the wing better than normal airflow does… in simple terms.. What this means is that designers can include them on the wing tips to direct the airflow down over the ailerons; maintaining the pilot’s control over the airplane when it is stalling.
Stall Fence: A stall fence is a sheet that is placed vertically on the leading edge and top of the wing inside of the ailerons. When a stall happens and progresses outwards can reach the ailerons in some cases. What this fence does is it literally creates a barrier and stops that stalled airflow from moving over the ailerons and making them useless. This again gives the pilot control even when the main area of the wing is stalled.
Washout: Washout, or aerodynamic twist, is where the tips of a wing are twisted downwards. This is so that when the root of the wing is at a higher angle of attack and stalls; the tips are a lower angle of attack and so not only are the ailerons in use but the wing tips are producing lift. (F/A-18 Below). As you can see, the black dotted line is the wing’s centre line at the root and the red line is the wing’s centre line at the tip.
Stall Strip: A stall strip does the opposite of a vortex generator and a stall fence. It does not stop a stall; it encourages one. Stall strips are placed along the leading edge and at the root of a wing. They encourage the stall to happen here because as mentioned before; a stall at the root of a wing is gradual and allows the pilot to have time to react.
Modern Techniques: More modern ways of stopping a stall include; a stick shaker, and angle of attack limiter and a stall warning sound. A stick shaker, to begin with, simply shakes the stick of the airplane to warn the pilot that the critical stall angle is being approched. An angle of attack limiter does not allow the pilot to pull the stick hard enough to stall the wing and or course the stall warning sound is often a beeping noise to indicate to the pilot that he/she is near to the stalling speed.
I’d first of all like to thank you for getting this far! It’s been an honour writing this article and I hope you enjoyed it. As always the images I used in this article are not my own, most are modified. They are purely there for your educational benefit, not for advertisement or for just blatent plagiarism.
Discussed In This Article:
- Quick outline of what tip stalls are.
- Explanation of how tip stalls form.
- Development of how wing shapes affect stalling characteristics.
- And how we can design our airplanes to prevent, or even encourage, a stall.
As always please go check out my other articles and leave a score up at the top. Don’t forget to bookmark this article so that you can find it easier at a later date!