08-08-2012, 09:49 AM
Effect of ground on Aerodynamic Properties of a Wing
Effect of ground on Aerodynamic Properties of a Wing.pptx (Size: 1.2 MB / Downloads: 32)
Introduction
Deals with flow over the wing influenced by the proximity to the ground
Changes occurring to various aerodynamic properties of the wing
A study on 2D ground effect by ‘Chih-Min Hsiun and Cha'o-Kuang Ghen’
Defect in boundary conditions used was found, cannot be true to simulate the ground effect
Boundary conditions revised and simulations done using CFD
Results plotted
Advantages and Disadvantages
Advantages
Increased lift and decreased drag
Less fuel required for a specified range when compared to aircraft
Increase in payload capacity
Decrease in wing aspect ratio, hence may lead to less structural weight.
Disadvantages
High Structural loads during maneuvering
High pitching moment to be overcome
Larger control surface required
Engine and body sustainable to corrosive sea environment.
Effects
Chord dominated Ground Effect(CDGE)
Lift increased due to ‘ram pressure’ underside of airfoil. It’s a 2D effect
Span Dominated Ground Effect(SDGE)
It is a 3D effect
Strength of tip vortices is lost as they collapse
when come in contact with ground
2. Induced drag decreases
3. Lift increases due to increased effective angle
of attack
Pitching moment
Change in pressure distribution causes change in pitching moment.
Large pitching moments required due to increased lift
Overall efficiency may decrease.
Effect of height
Zone1:-between the boundary and a height of 20% of the chord of the wing.
Zone2:- between the height of one chord length of the wing to ten span lengths.
Zone3:- Combined effect of above two, between 20% of the chord and one chord height
Benefits
Directional control and maneuverability
Two choices
Raise to a height at which it can bank and then turn, involves complex designing
Use rudder to turn, results in large turning radius and inconvenient centrifugal forces to passengers.
Study:Effect of ground on Aerodynamic properties of a 2D Airfoil
Based on study by ‘Chih-Min Hsiun and Cha'o-Kuang Ghen’ titled ‘Aerodynamic Characteristics of a Two-Dimensional Airfoil with Ground Effect[1]’
NACA4412 airfoil chosen and flow
over it solved numerically.
Preliminary CFD simulation performed.
Problem found with boundary conditions
used in the above reference
Boundary conditions revised, flow was further
understood.
Effect on CL contd…..
Lift was decreasing for low ground clearance (H/C=0.05)
Lift decreases effectively at low Reynolds number
At low Reynolds no. boundary layer thickness is more so flow is influenced more by ground boundary layer for lower H/C.
Results:- Effect of Reynolds no.
Effect on Cd
1.Decreases due increase in
Reynolds no. which decreases
skin friction.
2. Reynolds no. increases
which increases suction
force and shifting to
leading edge
Effect on L/D
1.Overall increase in L/D due to decrease in
drag and increase in lift
2. Although for H/C-0.05 lift was less but
drag is also very less.
Results:- Effects by Ground Clearance
Effect on CL
Lift for high angle of attack is more because suction pressure is more (Cp).
The effect of increase in lift increases with decrease in α.
Decrease in lift coefficients for with H/C<=0.05
Effect can be explained by pressure distribution.
Results:- Effects by Ground Clearance
Effect on Cd
Loss in suction pressure with decreasing ground clearance and uniformity of pressure distribution below the airfoil leads to less drag
Effect on L/D
Overall increase in L/D
Catching points
Such a profile of velocity can only be observed for no slip with stationary wall boundary condition imposed on ground.
Zero velocity observed at
ground.
Hence the results depend lot
on boundary layer
Moving wall with no slip has
to be given.
Conclusions
Boundary conditions
To simulate the correct ground effect surface or ground below the wing should also be given an equal velocity which was not given by the author of reference 1
Effect by ground clearance
Lift increases with decrease in clearance from the ground due to rise in positive pressure under the airfoil, only till a certain value of H/C>0.25
Lift then reduces due to reduction in suction pressure considerably.
Drag decreases with the decrease in clearance from ground due to decrease in pressure drag. Hence the overall L/D is always increasing with the decrease in height from the ground.
Effect by Reynolds no.
Lift also increases with the increase in Reynolds no. due to rise in suction pressure as well as positive pressure under the airfoil when in ground effect.
Drag decreases with the increase in Reynolds no. due to decrease in skin friction drag and reduction in pressure drag with increase in Reynolds no.
Leading edge phenomenon
The effect of formation of circular region below leading edge at high angle of attack (α>100) and low Reynolds number (Re<5X105) can only be present when no slip or stationary wall boundary condition is imposed on ground.
Revised Results
Revised results shows that the lift will keep on increasing with the decrease in height due to increase in positive pressure under the airfoil which is uniform.
Coefficient of drag decreased with increase in height due to loss of suction pressure. Hence the L/D ratio increased with decrease in height.
References
Chih-Min Hsiun and Cha'o-Kuang Ghen, “Aerodynamic Characteristics of a Two-Dimensional Airfoil with Ground Effect”, AIAA-46949-490,Journal of aircraft Vol. 33, No. 2, March-April 1996.
Michael Halloran and Sean O'Meara, “Wing in Ground Effect Craft Review”, The Sir Lawrence Wackett Centre for Aerospace Design Technology, Royal Melbourne Institute of Technology, Contract Report CR-9802.