CFD ANALYSIS OF THE PROPWASH

AND ITS EFFECTS ON THE

AERODYNAMIC EFFICENCY AND

STABILITY OF THE MAVs

RAJASEKAR P

REJISH J

SATHEES KUMAR B

SILVA FRANKLIN S

Live project in – Experimental Aerodynamics Division “NATIONAL AEROSPACE LABORATORIES”, Bangalore. Under the guidance of Dr. Ramesh and Mr. Hemanth Sharma

OUTLAY OF PRESENTATION

INTRODUCTION TO MAV

PROPELLER EFFECTS

• PROP WASH

• P- FACTOR

• TORQUE

• GYROSCOPIC EFFECTS

PROPWASH EFFECTS ON AERODYNAMIC

EFFICIENCY AND STABILITY

LITERATURE SURVEY

BASICS OF CFD

INTRODUCTION TO MAV

• Micro Air Vehicle or Micro Aerial Vehicle is a class of Unmanned

Aerial vehicle that has size restrictions and may be autonomous

• Modern MAV can be as small as 15cm.

• The range of Reynolds number at which MAVs fly is similar to that

of an insect or bird (103 - 105)

CLASSIFICATIONS OF MAV

MAV

BASED ON WING

ROTARY WING

FIXED WING

FLAPPED WING

BASED ON PROPELLERS

PUSHER TYPE

TRACTOR TYPE

PROPELLER EFFECTS

Prop wash: p-factor:

Gyroscopic Precession: Torque effects:

PROPELLER EFFECTS

Spiral prop wash:

The turning propellers sends a spiraling column of air rearward that strikes the left side of tail and tries to push the tail to the right and yaw the nose towards the left is know as Prop Wash

The Prop does not throw the prop wash straight back there is some drag on the prop, and that tends to make the wash behind it come off in a spiral fashion and the problem comes when that spiral flow meets the rudder. If the rudder is mounted high, the plane will turn (yaw) left because only the top part of the spiral hits it.

NEUTRALIZING PROP WASH

Flight path due to prop wash

Right thrust

Neutralized flight path

ASYMMETRIC PROPELLER THRUST ( P- factor)

P- FACTOR

When a plane is pitched up into a climb or loop ,the angle of attack is made greater than the actual flight path At positive angle of attack the propeller blades on the right side of the plane bites more air than the blade on left side, resulting in more thrust on the right side, trying to push the nose left side

TORQUE

Since all propellers turn to the right, that means there is a force trying to twist (roll) the airplane to the left. Note that this force is about the ROLL axis- the torque forces do not by themselves turn or yaw the plane as do the other effects

Prop wash effects on MAV

This figure shows the contour of velocity in X-direction for propeller design condition (8000rpm and 12m/s inlet velocity). As air passes through the propeller wakes are formed behind the propeller due to the rotation

The axial velocity created by the propeller continues to be higher than the free stream velocity even up to a distance of 5 diameters from the propeller plane in the axial direction. This difference is indicates that, axial velocity is 5% higher than the free stream velocity at the end of downstream.

Variation of axial velocity

This diagrams shows the velocity vector behind the propeller for design condition

Velocity vector diagram:

Prop wash effects on MAV Aerodynamics

This figure shows the separation bubble which formed on the upper wing surface without the propeller effects

Without propeller:

This shows the bubble has been reduced due to the effects of propeller flow and the bubble becomes asymmetric this triggered the side force.

With propeller:

Flow over an MAV at 17 ° angle of attack

This shows the pressure distribution and streamlines for the stationary, elliptical airfoil • The dark region under the leading edge shows a region of

high pressure• The lighter area in the wake region shows a region of lower

pressure.• The streamlines plotted in the figure shows the beginnings of

separation in the wake region.

Flow over an MAV at 30° angle of attack

This shows a screen shot of a stationary, elliptical airfoil with a thirty degree angle of attack. Re =1000.• The model shows dark pockets of low pressure in the wake

region that get progressively lighter as the distance from the trailing edge of the airfoil increases.

• These regions of low pressure creates vortices that detached from the airfoil and flowed through the wake region.

Future work:

1. By using CFD we are going to analysis the prop wash effect for the MAV with tractor type propeller at the design condition by comparing with experimental results.

2. For the various angle of attack, we are going to calculate

• Aerodynamic efficiency (L/D ratio)• Coefficient of drag CD

• Pitching moment coefficient CM

• Side force coefficient CF

BASIC ASPECTS IN NUMERICS OF CFD

Discretization:

It is the process by which a closed form mathematical

expressions, such as a function or a differential or integral equation

involving functions, all of which are viewed as having an infinite

continuum of values throughout some domain, is approximated by

analogous expressions which prescribe values at only a finite number

of discrete points or volumes in the domain.

Discrete grid points:

Analytical solutions of PDE gives the variation of the dependent variable continuously throughout the domain

Numerical solutions can give answers at only discrete points in the domain, called grid points.

Structured grids :

The grid points are placed in a regular intervals (i.e.) if ∆x and ∆y are constant along x and y direction

Unstructured grids:

The grid points are placed in irregular fashion (i.e.) if ∆x and ∆y are not constant along x and y direction

Note : ∆x does not have to equal ∆y

Discretization techniques

Finite difference

Finite volume

Finite element

Finite Difference Methods:• The PDE is replaced with algebraic equations which

prescribe values at only a finite number of discrete points

• Difficult for complex geometries

Finite Volume Methods:• Convert the integral equations to a system of

algebraic equations.• Any flow domains can be solved.

Finite Element Methods:• Convert partial differential equation (PDE) as

well as integral equations to a system of algebraic equations.

FINITE DIFFERENCE METHOD

If u denotes the x component of velocity at points (i ,j ), then the velocity u at point (i+1,j) can be expressed in terms of Taylor series expanded about point (i, j) is given by

Taylors series:

Example problem:

The function is given by f(x) = sin 2πx

At x = 0.2; f(x) = 0.9511Corresponds to point 1 At x = 0.22; f(x) = 0.9823 Corresponds to point 2

• Now by using just first term on the right hand side Taylor series expansion

f(0.22) ≈ f(0.2) = 0.9511This corresponds to point 3 in the fig

The percentage error in this estimate is [(0.9823 – 0.9511)/0.9823]*100= 3.176% • By using two terms in the series

f(0.22) ≈ f(0.2) + 2π cos [2π(0.2)](0.02) ≈0.9511+0.388 = 0.9899This point corresponds to 4 in the fig

The error is 0.775%

The finite difference representations of derivatives is given by

Truncation error: This error tells us what is being neglected in this approximations

First order forward difference

First order backward difference

Second order central difference

Second order central second differences

Second order central difference for the mixed derivative

Explicit and Implicit Methods Explicit Method :

Flow properties at previous step are used to calculate new values at current time step

Implicit Method: Flow properties at previous and current time step are

used to calculate current time step