# How Big is Skin Friction?

The total drag of a body moving through air (airplane) or water (ship) consists of

• form/pressure drag + skin friction drag.

It is generally believed from experiments dragging a plate through water, that for an airplane and ship skin friction may be 50-70% of total drag. Experiments are performed with (i) untripped/free transition and (ii) tripped/forced transition to a turbulent boundary layer, with (ii) showing a bit bigger drag than (i).

Tripping is done e g by mounting a rib along the upper part of the leading edge of a wing. Its effect of creating a thick turbulent boundary layer is illustrated in the above image.

Computations with DFS Direct Finite Element Simulation with zero skin friction (slip boundary condition on wall) shows drag in close accordance with drag experiments with free transition.

The DFS results thus show total drag as pure form/pressure drag with zero skin friction, in accordance with free transition experiments. This gives evidence that drag with free transition has very little contribution from skin friction, and further that the measured (small) difference between tripped and untripped drag can be used to assess the skin friction, which is forced by tripping and is thus absent without tripping.

Now, a real airplane is not equipped with tripping devices on wings or fuselage since that would increase drag for no use, and DFS with slip shows close correspondence to experiments with free transition.

Altogether, there is strong evidence that skin friction drag for an airplane or ship is an order of magnitude smaller than that commonly used based on experiments from tripping. The results indicate that what is believed to be a thick turbulent boundary layer forced by tripping with substantial skin friction, in fact is absent i reality without tripping and thus that the interaction between fluid and solid acts as slip/small friction (without boundary layer to resolve computationally).

Obviously, if skin friction in reality is less than 10% of total drag, instead of an unreal tripped imagination of 50-70%, the design of an airplane or ship will work from different premises.

DFS with slip makes CFD computable, whereas std CFD with no-slip tripped boundary layers is uncomputable.

Why is then tripping used in experiments if in reality not? This is to make experiments fit with the boundary layer theory of Prandtl as the Father of Modern Fluid Mechanics tracing drag to the presence of a thick turbulent boundary layer. But to fit unreal experiments to theory is opposite to the idea of of real science where theory is fitted to real experiments.

The effect of fitting experiment to theory is that standard CFD is calibrated to a skin friction of 50-70% of total drag, which with the evidence from DFS means that standard CFD underestimates form/pressure drag, and thus gives wrong input to design.

The plot below shows drag coefficients for NACA0012 by Ladson with free and tripped transition. Note the small dependence on Reynolds number for free transition and that difference between tripped and untripped drag is about 0.001 as about 10% of total tripped drag as an estimation of skin friction drag.

# Boeing 737 Max Crashes vs New Theory

Is the reason for the two fatal 737 Max accidents a flaw in the design of the airplane, making it prone to stall (compare with this post), which was compensated by a possibly over-reacting control system, which the pilots could not turn off? Did FAA authorise the plane without proper safety evaluation?   Questions are piling upFBI is joining criminal investigationWikipedia,  Boeing,  New York TimesPilot training, Stability, Aviation expert, Kludge, Aviationcv, Pilots viewThe Case Against Boeing.

Computational software used by developers of airplanes do not seem to allow simulation of the dynamics of stall and so the impact of stall on aircraft design and safety assessment must be done solely by expensive and time-consuming experiments in flight, and then also the design of the apparently needed control system. This may show to have been insufficient to make the plane safe, something which FAA did not have the capacity to check.

With our New Theory of Flight and new technology of Automated Computational Mathematical Modeling presented on this site (some references also collected here), the full flight of an airplane, like the 737 Max, including the full dynamics of stall, can accurately be computationally simulated as a unique capability shown in the HighLiftPW-3 Workshop, see also front page of Icarus Simulation. Such advanced technology could allow airplane makers better and faster simulations to design aircraft and assess their safety.

# Book Update into Publishing

The draft of the book The Secret of Flight is now under revision into completion for final publishing. Suggestions for publisher are welcome! The revised version will in particular include recent results presented at the High Lift Prediction Workshop III (see previous post) and new analysis of the critical phenomenon of stall: see movie.

# High Lift Prediction Workshop III

As one of 40 teams competing at High-Lift Prediction Workshop III, Hoffman-Jansson-Johnson showed for the first time computational simulation of full time-dependent turbulent flow around a jumbo-jet in full landing configuration, including the following unique features:

• automatic turbulence simulation
• computational efficiency
• superb agreement with experiment including stall

as the overall best result. The flow simulator by HJJ opens the possibility of constructing a first real flight simulator based on the real physics of turbulent air flow. Such a simulator will open to new forms of realistic pilot training of e.g. take-off and landing of a jumbo-jet under severe wind conditions or machine failure.

This ground breaking work is presented here.

# New Theory of Flight Published in JMFM!

Our New Theory of Flight (together with Johan Hoffman and Johan Jansson) has now been published in Journal of Mathematical Fluid Mechanics (online behind paywall and free originally submitted (2012) preprint) .

New Theory of Flight springs from our resolution of d’Alembert’s paradox formulated in 1755, published in JMFM 2008, and shows for the first time that both simulation and understanding of the miracle of flight is possible with present computer and brain power. As such it has every chance of becoming a landmark article.

New Theory of Flight is published by JMFM with a special Editors Foreword inviting scientists, mathematicians and engineers to scrutiny and discussion.

# Introduction

Dear Reader, Student, Pilot, Air Passenger, Engineer, Scientist, Interested Layman…:

Welcome to follow the resolution of a scientific riddle which has baffled human imagination since the dawn of science into present time, formulated by Kenneth Chang in New York Times on Dec 9 2003 in Staying Aloft: What Does Keep Them Up There? as follows:

• To those who fear flying, it is probably disconcerting that physicists and aeronautical engineers still passionately debate the fundamental issue underlying this endeavor:
•  What keeps planes in the air?

## 1. Old Theory with Old Answers

You will find that the answers to the question what keeps planes in the air, which you can find in classical books and articles in science and popular science, fall into one of the following categories:

• non-trivial but incorrect (for educated)
• trivial and incorrect (for uneducated)
• correct but trivial (for uneducated).

To see how this Old Theory is presented to uneducated as explaination of observations, it is instructive to watch:

## Take your time, watch with amusement knowing that you soon will understand much more than The Department of Defense did in 1957 and does even today.

In short you will be able to explain the secret of flight by understanding the flow around the airfoil as

1. potential non-turbulent flow before the trailing edge,
2. non-potential turbulent flow with 3d rotational slip separation at the trailing edge without pressure rise/drop,
with 2. the crucial new element of the New Theory.

## 2. New Theory with New Answers

You will be led to a real answer to the question with the following characteristics:

• non-trivial and correct (for both educated and uneducated)
• reveals the real physical mechanism of flight.

The New Theory has been submitted to AIAA, The World’s Forum for Aeronautics Leadership, as the article New Theory of Flight which should generate lively debate to be reported on this site: It contains new mathematics and physics, and is highly controversial by challenging the very foundations of the ruling paradigm defined by Ludwig Prandtl as the Father of Modern Fluid Mechanics 100 years ago.

You will find that the New Theory is based on the following new discoveries, which bring fundamental changes to the science of high Reynolds number flow:

1. It is possible to computationally solve the Navier-Stokes equations in the case of high Reynolds number of aerodynamics, using a slip boundary condition as a model of a turbulent boundary layer with small skin friction.
2. It is possible to theoretically understand high Reynolds number bluff body flow as potential flow modified by rotational slip separation.

## 3. The Secret Revealed in Four Basic Steps

1. The flow being incompressible with a slip boundary condition can only (as potential flow) separate at stagnation, which cannot occur before the trailing edge (before stall).
2. The flow above the wing is thus redirected downwards, which requires low pressure or suction peaking on top of the leading edge, which generates 2/3 of the lift with 1/3 from high pressure under the wing.
3. Main drag is created by high pressure (positive) on the leading edge by low speed flow in accordance with Bernoulli’s Principle.
4. Lift and drag from the leading edge are preserved by a specific flow separation pattern at the trailing edge with alternating high and low pressure with zero mean.

Here 1. and 4. are the new elements of the New Theory, with 4. the most surprising and intriguing, which are combined with the classical elements  2. and 3. relating to Bernoulli’s Principle from 1738 bridging over 280 years.

## 4. Understanding

You will discover that the resolution of the riddle can be expressed in basic mathematical terms which opens to understanding why flight is possible. This is because in physics real understanding can only be reached by understanding a mathematical model of the phenomenon under study. You will thus be able to understand what birds have understood since the Archaeopteryx as the first bird took off into the air some 150 million years ago, but man has not until very recently been able to grasp.

## 5. Ingeniuous Invention

You will find that the resolution is surprising and ingenious but also so simple that you will be able to understand why flight is possible so well, that you will be able to explain to anybody asking the question, from professors to school mates, friends and family members.

You will thus understand the cleverness of birds and why also human beings have been able to lift from the ground up into the air with proper equipment.

## 6. Incorrect Mathematics

You will discover a thrilling story of mathematical mistakes made by great mathematicians and scientists,

• Newton followed by d’Alembert 1652 (incorrectly) proved that flight is impossible,
• Kutta and Zhukovsky (incorrectly) proved in 1904 that flight is possible after the Wright brothers successful powered heavier-than-air flight in 1903 by connecting lift to circulation.
• Prandtl (incorrectly) connected drag to thin boundary layers in 1904.
• Aviation developed during the 20th century based on the (incorrect) theory of Kutta-Zhukovsky-Prandtl.

The fathers of the (incorrect) modern theory of flight: KuttaZhukovsky and Prandtl.

In short

• Kutta-Zhukovsky developed a (incorrect) theory for lift without drag,
• Prandtl developed a (incorrect) theory for drag without lift,

while what was needed was a correct theory for lift and drag.

## 7. Correct Mathematics

You will find that the miracle of flight can be explained from the following properties of the slightly viscous incompressible fluid flow characteristic of aerodynamics:

1. slightly viscous flow can be simulated with a slip (zero skin friction) boundary condition
2. potential flow satisfies slip and can only separate at stagnation
3. slightly viscous flow undergoes 3d rotational slip separation without the high pressure of potential flow.

1-3 are carefully explained in on this site, and the explanations are not complicated. Yet they combine into the marvelous invention of flight.

## 8. Presentation: Overview

To get an overview you may browse the following Powerpoint presentation:

## 9. The Dream of Human Powered Flight

The Gossamer Condor pedaled and piloted by Bryan Allen won the Kremer Prize in 1977 as the first successful human powered flight. This was the dream of Icarus and Leonardo da Vinci which thus ultimately came true only recently, and then after seemingly endless trial and error.

# 10. Short History of Flight

A short history of the theory and practice of flight is as follows:

• The flight of birds was intensely debated already by the Neanderthalers.
• Newton proved that powered human flight is theoretically impossible by using incorrect mathematics.
• The Wright brothers showed in 1903 that powered human flight is possible in practice.
• The mathematicians Kutta and Zhukovsky proved in 1904 that powered human flight is possible by using incorrect mathematics.
• In 2008 we proved that powered human flight is possible by using correct mathematics. Theory and practice eventually met in a happy marriage.

## 13. Take Off: Browse the Menu

You are now ready to in more detail browse the Menu including the Survey of posts.