What is the Boundary Layer?

INTRO

In fluid dynamics, Boundary Layer is a THIN (less than an inch), SLOW MOVING (nearly zero velocity) layer of moving fluid which is in contact with the surface of any object (car, boat, aircraft, golf ball etc). It STICKS and tends to wrap around the object due to the VISCOSITY of the fluid.





Yes! You guessed it right ZERO VISCOSITY (ideal) means NO BOUNDARY LAYER!

Being Aircraft Mechanics we are naturally more concerned about Aircraft!
Let's see how Boundary Layer affects our airfoils and how we control it to change overall lift and controllability characteristics

AERODYNAMICS
When the free air stream comes into contact with the airfoil, the flow closer or in contact with the airfoil tends to slow down owing to it's viscosity and surface friction with the surface of the object. This thin layer of airflow is termed as the BOUNDARY LAYER. The boundary layer consists of several layers flowing over each other, each layer is a bit faster than the layer below it, hence there is a gradual increase in velocity from airfoil surface to the free stream (see Fig 1)

Figure 1 - Gradual Increase of Layer Velocity from Surface to the Free Stream


As the boundary layer is slow-moving in nature it has low Kinetic Energy. This lower energy results in it's separation from the airfoil (Fig 2).

Figure 2

NOTE: The static pressure increases from the thickest point of the airfoil to the trailing edge (See Fig 3), thus the boundary layer needs more energy to flow to the high pressure area or it will separate from the airfoil.

Figure 3

MORE KINETIC ENERGY means FARTHER SEPARATION POINT

Thus to delay the separation point, or to get the maximum positive effects from the airfoil (lift, controllability), we try to provide more energy the boundary layer.
To energize the boundary layer devices like SLOTS, VORTEX GENERATORS are used.


P.S. I have kept the post simple, there is much more to be added to this article, but this one is to give just some general concept of the Boundary Layer. You can always suggest improvements and help others!

What are Vortex Generators and What purpose do they serve?

There is a lot of information over the internet about the purpose of Vortex Generators. I will shed light on just some important notes you should keep in mind as an Aircraft Mechanic, I will try to explain more details later (if necessary) bu updating this post

Before proceeding I assume you are accustomed with the following terms:
Boundary Layer, Laminar/Turbulent Flow, Stall

VORTEX GENERATORS

Vortex Generators are small aerodynamic devices, generally fitted to the leading edge of wings (including ailerons, elevators etc).

The laminar flow tends to separate from the wing at some point (Transition Point) behind the leading edge, as the angle of attack increases the transition point moves forward, this can lead to a situation which will ultimately cause the wing to stall.

Transition Point - The point at which Laminar Flow leaves the airfoil causing a Turbulent Flow in result, so there is a transition from Laminar to Turbulent Flow at the Transition Point


HOW DO THEY WORK
The boundary layer of Laminar Flow is low in energy as compared to that of Turbulent Flow, thus to energize the boundary layer vortex generators are used.

Simple Operational Principle: As the name implies, when the airflow passes over the vortex generators they generate small vortices, these turbulent vortices energize the boundary layer and helps it stick with the airfoil surface thus delaying the formation of transition point/region.

Saw this interesting photo on a website www.aerospaceweb.org which can help you understand the phenomenon

Note: I tried to keep it short and simple. More information can be provided upon your request. Kindly suggest me updates for this article to make it better! 

Carbon Brakes or Metallic Brakes: What is the better choice?

Brief Intro:

Brakes are used to stop the Airplane (simple)
Operationally, the brakes convert the Kinetic Energy of the moving airplane to Heat Energy (that is why they are also known as Heat Packs) which is dissipated into the atmosphere by air flowing over the brake units.
Heat results in increased temperature, which directly impacts the life of braking unit.

METAL BRAKES

Steel pads are used between the Pressure and Backing Plates, when these plates are forced into each other the friction of the brakes cause the wheels to stop (this process in turn generates a lot of heat)

Steel Brakes are generally cheaper in price than their carbon counterparts and this was the primary reason they were being used.
But the problems with these brakes is their higher frequency of overhauls and also their heavy weights.

CARBON BRAKES

As it has been previously discussed that the brakes are kinetic to heat energy converters, thus their efficiency depends upon the amount of energy that can be absorbed by the brake.
In carbon brakes, the friction is produced by carbon fibre discs, which are capable of absorbing much more energy than the conventional metal brakes, thus they are more efficient in braking applications.
Other major advantage of the carbon brakes is they are much lighter as compared to metal brakes having same efficiency, this weight saving (of around 200-400 kg) is a of great advantage in terms of less fuel consumption. Less emission of CO2 by Engines is another by-product.
Carbon Brakes are more expensive than metal brakes yet they justify their cost by increased time between overhauls.

SUMMARY

Carbon Brakes are preferred on aircraft because following aspects:

1. Higher braking efficiency (greater energy absorption capability)
2. Weight saving (Fuel saving)
3. Less pollution (less emission of CO2 due to decreased fuel consumption)
4. More time between overhauls

Why is the Radome of Airplanes made using composite?

You should have seen aircrafts with fully aluminium skin fuselage but only the Radome fabricated from composite materials, before knowing the reason you must know what a Radome is and why is it called a Radome.

Radome is at the nose of aircraft and it houses a Weather Radar. It gives it the name RADOME, as RAdar and DOME (for it's dome shape)

RADOME

The purpose of using composite material is only the reason that composite does not cause hindrance in transmission and reception of Radio Waves from/to the Weather Radar, which aluminium or any other metal used could cause.
But the associated problem in using Composite is the accumulation of static charges, well this problem is driven away by using conducting strips (the strips you see on the Radome in the picture above) on the composite Radome, These strips carry away the charges to the fuselage thus conducting charges away as the bonding jumpers do.

WEATHER RADAR


P.S. correct me where I am wrong!

Why are Roller Bearings used in the hot section (Turbine Drum) of a Jet Engine?

Unlike Ball Bearings, the Roller Bearings allow some axial movement. When a turbine engine is operating the temperature of the hot section is very high, which result in linear expansion of the shaft in the hot section. To allow this linear expansion we use roller bearings in the hot section. If this was not the case, and we would have used the ball bearings which do not allow axial movement of the shaft, the linear expansion due to heat could deteriorate the engine.

To see the difference between Thrust and Radial loads, follow this useful link: How Stuff Works

P.S. Correct me where I am wrong

About This Blog

Hello Everyone,
I came across an idea to share information related to Aircraft Maintenance. Although there is huge amount of information all over the internet but I have planned to pack most of this into a single blog. The main theme of this blog is that you may ask any question, regarding Aircraft Maintenance and I (or the fellow community members)  will try to answer those, by which we can have a large number of FAQs on a blog dedicated only to Aircraft Maintenance. This is mutually beneficial for all!

In vain have you acquired knowledge
if you have not imparted it to others.
Deuteronomy Rabbah
(c.900, commentary on the Book of Deuteronomy)