The Immelmann turn

The Immelmann turn

f22 raptor

There it was...The british BE 2 was swooping into world war 1 skies after having successfully evaded the German Fokker EI. Oh but the poor brit in the cockpit had little knowledge of the aerobatic tricks the german pilot had in store for him. You see the German, was Max Immelmann. And Max Immelmann was the Mann....wait for it....
Who gave birth to THE Immelmann turn.

Image result for Fokker EI
Never seen before photo of Immelmann thinking about how he's going to do some twisted turns

You're kidding right? You don't know?
What is the Immelmann turn? Ugh noob. It's alright I'll tell you.

Take a look at  Immelmann turn first,




The half loop, half roll beast of a manoeuvre was executed usually after an attack to reposition the aircraft for another attack . Fighter pilots just couldn't get enough of the chase. 
How do you execute an immelmann turn? Either you get yourself a fighter capable of withstanding such engine power OR you have a really vivid imagination.
Let's go with the latter. But in order for this imagination ride to begin you need some intro into a couple of concepts.
Kinetic energy: The energy associated with a body in motion. Anything which moves has kinetic energy.
Potential energy: The energy associated with a body displaced to a height from a reference position.

Rollercoster
With the analogy of a rollercoaster, the ride has maximum potential energy and zero kinetic energy at the top as it relatively at a standstill and at a considerable height from the ground. This potential energy is traded for kinetic energy when the ride goes down the slope gaining momentum, and losing height.

Angle of attack: The angle between the chord line and free stream velocity.
Stall angle of attack: the angle of attack beyond which the aircraft begins to stall i.e. Lift is lost. WHAT? How does this happen? As the angle of attack increases we know that the lift increases (because angle of attack and lift coefficient CL are directly proportional) but too much of something is good for nothing. When you keep on increasing this angle there’s going to be a point (refer graph) where there will be separation of flow, and once flow separates there’s going to be no possibility of lift. (There are stall reversing manoeuvres’ though)

Related image
the first image shows attached flow over airfoil and lift (Coefficient of lift) linearly increases up until 16 deg (second image). This is when the flow starts separating and after this angle, flow becomes detached and lift becomes negative. NOT GOOD.
Rudder: The control surface on the vertical stabilizer which yaws the aircraft (learn about yaw here
 : LEAVE )
Related image
This swooshes the aircraft left or right
Ailerons: The control surfaces on the wing to roll the aircraft. (go learn : we know you want to.. )
Related image

 “Energy is neither created nor destroyed, it can only be changed from one form to another” – someone important.
This energy conservation principle drives many things, our little manoeuvre included. 
Let me paint your imagination with physics.

Image result for immelmann turn
The Immelmann turn
You first accelerate to gain kinetic energy (the energy associated with a body in motion), then he pulls the aircraft to a climb trading this kinetic energy for potential energy (the energy associated with a body which is at a height from a reference point). This climb happens when the angle of attack of the wings are constantly increased up until just before stall . MAD RIGHT? So then you apply full rudder to yaw the aircraft and the corresponding aileron to roll the aircraft to bring it back to level flight. This brings you back in position to get another shot at your enemy. 
And now you know.

Thanks for reading!

What is the dutch roll?

SKIES HAVE EYES

What is Dutch Roll?

It’s the 7th of June 1967, the six-day war is unfolding on the ground. But our eyes stay fixed to the skies where the swept wing greats, the Mirage 3 battles it out with the MiG 21 in a dogfight which is sure to make it to the books. Diving into the battlefield from the cloud ceiling, Giora Romm in the cockpit of the Dassault Mirage 3 engages in a fight for air superiority against the MiGs. A few minutes in, he is seen with an enemy MiG on his tail shooting for the kill. He then sets the Mirage to rock about its axis, to dodge the gunfire and escapes the danger successfully. PAUSE.


Dutch roll- Mirage 3
Hey, wanna check out my cockpit?

What he did there,  is a brilliant maneuver in military aviation, called The Dutch Roll.
This might sound like a mouth-watering puff pastry which tests the prowess of some cute dutch bakers, but all it tests is the roll and yaw stability of an aircraft....which is way cooler.
Before we go into how a dutch roll happens, let’s look into a couple of crazy circus acts our aircraft can do:
Dutch Roll - Aircraft Moments
The rotational motions of an aircraft
Rolling (banking) – Aircraft rotation about the longitudinal axis. This can be brought about by one wing having a higher lift than the other wing. (ailerons jutting in opposite directions).  For example, if the left wing has more lift, right-wing tilt down and the aircraft rolls right 

Yawing- Aircraft rotation about the perpendicular axis. Rolling and yawing come as a package deal. So your aircraft rolls left. The lift vector which was initially perpendicular to the wing jutting upwards is tilted to the left. Thanks to vector algebra, this tilted lift vector has a horizontal component associated with it called Sideslip (somebody makes this a dance move already). This skids/ yaws the aircraft in the direction of the roll in our case yaws left. 

Pitching- Aircraft rotation about the lateral axis. Can happen when the elevators on both the wings are operated in tandem. We don't need pitching motion in the dutch roll so it isn't explained here. But feel free to check out how pitching works. Knowledge is boundless. 

SO WHAT REALLY HAPPENS IN A DUTCH ROLL?

Put on your imagination caps so I can paint you this three-dimensional moving picture.



Dutch Roll - Swept angle
Rolling right creating the horizontal sideslip
Assume you roll your Mirage right, and the lift vector tilts right. The horizontal component of the tilted lift, sideslip yaws the aircraft to the right. This yawing causes the free stream velocity or the velocity of the wind to hit the wing at an angle. Now, pay close attention to the sweep (Wing swept angle) of that beauty. (learn about swept back wings here: yay information) This angle at which the wind hits the wing causes more chordwise component of wind on the right wing. (lift which matters is only generated when the flow is parallel to the chord. Go learn about lift here: click diz ). So more air is flowing just how we want it (parallel to the chord) on the right wing and not as much on the left. This translates to more lift on the right wing than the left. Still with me? This unbalanced lift, higher lift on the right side and lower on the left creates another roll, except now it's to the left. The right wing with more lift brings with it drag. This is the lift-induced drag explained in our article on the types of drag. : Drag your butt over here 

Dutch Roll - Swept Wing
Yawing right, the relative wind hits the leading edge like this
This right wing drag pulls the plane right (yaws right). This is when the vertical stabilizer (fixed vertical wing on the tail end of aircraft) does its thing by reversing this yaw, bringing it lined up with the nose. Remember our Mirage is still rolling left? This induces a left sideslip. This gives a yawing to the left. And this goes on as a repetitive rocking and rolling motion until of course the rudders are employed to bring it back to stable flight. You can think of the Dutch roll as you when you're 5 drinks in, and the rudder is your sober best friend. 

Although military aircrafts are thrown into dutch roll intentionally, their commercial cousins go into a dutch roll due to turbulence or external disturbance. They are not built for such instability: because that's what this is,  an instability condition. Which is why we say fighter jets are built for maneuverability (aka instability) while commercial aircrafts are built for stable level flight. (if they weren't our flights would be one hell of a ride). In this constant tradeoff between stability and maneuverability, as a design engineer you have to ask yourself this question: 

Is your aircraft equipped to rock and roll? 

Here is a visualization of the Dutch Roll 

Thanks for reading!

Video Credits: Padpilot