Four main types of drag are encountered in aerodynamics-Namely

  1. skin-friction drag,
  2. form drag,
  3. wave drag,
  4. induced drag.

The methods in use for reduction for the reduction of each type of drag are discussed in turn


In broad terms, skin-friction drag can be reduced in one of the two ways. Either laminar flow is maintained by postponing transition, this is so called laminar- flow technology, or ways to found to reduce the surface shear stress generated by the turbulent boundary layer.The laminar-flow can be maintained passively by prolonging the favourable constant-pressure region over the wing surface. Active control of transition requires the use of suction, either distributed or through discrete spanwise slots. Often the suction is used in conjunction with the favourable pressure distributions. The basic principle of maintaining laminar flow by means of suction has been known for at least thirty-five to forty years. However, the problems with the practical implementation on aircraft, either real or perceptual, have prevented the widespread of the technology. It seems increasingly likely, however, that the considerable gains in efficiency which would result from the use of laminar -flow technology, will ensure that it will be much more widely exploited on commercial aircraft in the near future.Other methods for maintaining laminar-flow have been developed, but as yet, have not been seriously considered seriously for practical application in aviation.

A moderately effective method for reducing turbulent flow friction which has been developed in the recent years involves minute modification of the surfaces so that they are covered with riblets.Riblets can take many forms but essentially consist of streamwise ridges and valleys, as shown. These triangularly shaped riblets are available in the form of the film.

This corresponds to the actual spacing of 0.025 to 0.075 mm for flight conditions. This 3M riblet film has been tested on an in-service Airbus a 300-600 and in other aircrafts. The skin friction drag reduction observed as of the order of 5 to 8 percent and the skin-friction drag  30 to 40% of the total aircraft drag.Thus the overall drag reduction is modest but, nevertheless, represents a very considerable economic benefit with little in the way of a penalty.It is very likely, therefore, that riblets will be widely used on commercial aircraft In the future.The basic concept behind riblets had many origins, but it is probably the work at NASA Langley in the USA, which led to the present developments .he concept, was also discovered independently in Germany through a study of hydrodynamics of riblet –like formations on shark scales.  A plausible explanation for the effect of riblets is that they interfere with the rear wall structure of the turbulent boundary layer in the region where the turbulence is mainly generated.The flow field in the turbulent boundary layer is highly complex, but the form of near wall structures has been elucidated in the seminal study of Kline et al at Stanford university.It appeared from this work that ‘hairpin’ vortices form near wall as shown these vortices then continue to grow until a point is reached when the head is violently ejected away from the wall and, simultaneously, the contra-rotating streamwise oriented branches of the vortices come together, inducing a powerful downwash of high-momentum fluid between the vortices towards the wall. This sequence of events constitutes what is termed a ‘near =wall burst’. The riblets act as barriers which prevent the free spanwise movement of the hairpin vortices.It is thought that owing to this the vortices cannot approach each other closely thereby weakening the near wall bursting process. It should be noted, however, that this explanation is not universally accepted



Form drag is kept to the minimum by preventing boundary-layer separation. Streamlining is vitally important for reducing form drag. It is worth noting that at high Reynolds’s numbers a circular cylinder has roughly the same overall drag as a streamlined aerofoil with a chord length equal to 100 cylinder radii. Form drag is overwhelmingly the main contribution to the overall drag for bluff bodies like the cylinder, whereas the predominant contribution in the case of the streamlined body is skin-friction drag, form drag being less than ten percent of the overall drag.For bluff bodies even minimal streamlining can be very effective.



Induced drag falls as the aspect ratio of the wing is increased.It was also shown that for a given aspect ratio elliptic-shaped wings have the lowest induced drag. Over the past fifteen years, the winglet has been developed as a device for reducing induced drag without increasing aspect ratio.Typical example is illustrated in the figure .winglets of this type have now been fitted too much type of commercial aircraft.The physical principle behind the winglet is illustrated in the figure. On all sub-sonic wings, there is a tendency for a secondary flow to develop from the high-pressure region below the wing around the wing tip to the relatively low-pressure region on the upper surface. This is the part of the process of forming the trailing vortices if a winglet of the appropriate design and orientation is fitted to the wing-tip, the secondary flow causes the winglet to be at an effective angle of incidence, giving rise to lift and drag components Lw and Dw relative to the winglet, as shown in the figure. Both Lw and Dw have components in the direction of the drag of the aircraft as a whole.Lw provides a component counter to the aircraft drag, while Dw provides a component which augments the aircraft drag.for a well-designed winglet, the contribution of Lw predominates, resulting in a net eduction in overall drag, or thrust, equal to delta T.


To some extent in the discussion of a supercritical aerofoil, it was found that keeping the pressure uniform over the upper wing surface minimized the shock strength, thereby reducing the wave drag. A somewhat similar principle holds for the wing-body combination of transonic aircraft. This was encapsulated in the area rule formulated by Whitcomb. It was known that as a wing-body combination passed through the speed of sound of sound, the conventional straight fuselage, shown in the figure., experienced a sharp rise in wave drag. Whitcomb showed that this rise in drag could be considered reduced if the fuselage was Waisted as shown in the figure, in such a way as to keep the total cross-sectional area of the wing-body combination constant. Waisted fuselages of this type are now a common feature of aircraft designed for transonic operation.

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A complete guide for GATE AEROSPACE 2018 preparation.

How to study effectively for GATE AEROSPACE 2018?

As the big day is nearing, all those GATE aspirants are Working hard to step into a great future.All your dreams come true when you work smart rather than hard for your GATE AEROSPACE 2018. There are few effective strategies, I teach my students, When you follow with this method, you can increase your success rate.

Here is my complete guide for GATE AEROSPACE 2018 preparation. these are my best-known ways to prepare for an exam.

I have got 100% success rate with this methodology, even I use to teach using this methodology.

Planning your study time effectively for GATE AEROSPACE 2018:

So, when you come to the point of “time management “.You frequently  hear the boring proverb “time and tide waits for none “.However, I believe “time is just an illusion” all you need to do is concentrate. Even if you read for half an hour, just try to understand the context. A deeper understanding of the context helps you learn the application techniques.

Therefore, it is not that how many hours you read; it is how well you read that matters.

I call this technique as O-BAD”.

Yup, it’s really bad, because it gives you big bang results when you follow it seriously.

O-BAD Technique:

  1. Outline the” big Chunks”
  2. Break down and list the subtasks
  3. Analyze the tasks and subtasks
  4. Decide when each subtask has to be completed.

Outline the” big Chunks”:

Start with analysing in which subject you are strong. The subject that you choose will be your Big Chunk.Check for the major topics in that subject, these will be TASKS now.

For example, Big chunks in  GATE AEROSPACE are Aerodynamics, Aircraft Propulsion, Aircraft Structures, Mathematics.

If you are good at Aerodynamics, it will be your Big chunk now. you have to sort down the topics in that, to find out your tasks.

In Aerodynamics, tasks will be

  1. Task No:1 -Fundamentals
  2. Task No:2-Inviscid Incompressible Flow,
  3. Task No:3-Inviscid Compressible Flow,
  4. Task No:4-Viscous flow.

now you are done with the first step of this technique. Let’s move to the next step.

Breakdown and list the subtasks :

Identify and list the steps that need to be taken in order to completely Cover each task.

Example: if aerodynamics is the Big chunk, and choosing task 2 -Inviscid Incompressible Flow as your task, now you have to list down the subtasks in that particular chapter and analyse the importance of each subtask with the previous year questions asked in that topic.and list down all the subtopics.

so for the task 2 Inviscid Incompressible flow, Sub tasks are

  1. Introduction to Bernoulli’s equation
  2. Introduction to Pitot tube
  3. Introduction to Laplace equation
  4. Types of Flows
  5. Kutta-Joukowski theorem
  6. d’Alembert’s Paradox
  7. Kutta condition
  8. Kelvin’s circulation theorem
  9. Classical Thin Airfoil theory
  10. Induced drag and Downwash
  11. The vortex filament
  12. The biot savart law
  13. Helmholtz theorem
  14. Prandtl’s lifting line theory
  15. Three-dimensional flows.


Hope you have understood.

Analyze the tasks and subtasks:

Take out and dust the previous year questions for it is much needed to analyze and Segregate the important topics.

Ask: which section-or concept has got more problems?

And which section you struggle the most?

(highlight and remember these topics. Since you need more attention )

Example: when you analyze the previous year question papers, on the Subject of aerodynamics, boundary layer problems have got a subsequent place.

If you are weak in this you need to have deeper learning in this boundary layer topic.

Decide when each subtask has to be completed :

 Follow the seven-day rule for GATE AEROSPACE 2018 Preparation:

Day 1: organize your study material, that is you have to allow for a particular subject.Aerodynamics what are the materials you Have. Textbook or class lecture notes or some other, Downloaded from the web.

Day 2: Study with lecture notes

Day 3: Prepare with textbook

Day 4: Solve if you have got problems in class

Day 5: Try solving problems in book

Day 6: Go with previous year questions on the subtask chosen

Day 7: Review everything. If it is a highlighted topic all you have to do is to allot extra time on it.

Example: if you have chosen Classical thin airfoil theory as a subtask in Aerodynamics

You have to follow the seven-day rule allotting half an hour each day. If it is a Highlighted topic ( your weaker area) you have to allot one hour each day.

Make your own study timetable with this technique and follow it.This method will give you 100% success in GATE AEROSPACE

The books that I recommend for aerodynamics are

Last but not least tips
Study a maximum of 2 hours a night . Try to study 10-12 hours a day.

Review constantly before the test or exam.

you have signed for any paid mock tests, then try to accomplish the test

Try to complete all the topics you have planned for the day before you go to bed.

Sleep at least 6 hours in the night.

Exercise every day to begin fresh.

“Health is wealth”, eat healthy foods.Drink more water, stay hydrated.

Don’t multitask , try to avoid chatting while you are studying.

Vary your study location to boost your preparation.

The method I have followed to prepare the study materials for GATE AEROSPACE 2018 propulsion.


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