wings
wing placement
high wing
most military aircraft have this so the fuselage can be close to the ground, meaning its easier to load / unload cargo without special equipment
engines and propellers above ground meaning there is protection against debris and damage from rough land
wing tip clearance - reduces risk of striking the ground in a high angle of attack roll
shorter landing gear reduced weight
allows addition of struts which lowers wing length but increases drag
wing box on top of fueslage compared to passing through it which reduces weight as the fuselage would be stiffened around the cutout which increases weight
large flaps can be used which increases lift coefficient for STOL
prevents floating due to ground effect - bad cuz difficult for landing or take off
improved downward visibility on smaller planes
easy entry into cockpit
gravity fed fuel system so no need for pumps
fuel tanks generally in the wing so a ladder is require to fuel which may not be available in all air fields. larger aircraft have fuel points in fuselage where fuel is pumped under pressure, but is not practical for smaller planes
fuselage strengthening needed for landing gear support which adds weight cuz landing gear on fuselage
external blister housing landing gear in fuselage increases drag
flattened fuselage bottom for storing cargo adds weight
visibility issues for small aircraft in banked turns or upwards
mid wing
least amount of drag for a circular fuselage whereas others need fairings for acceptable interference drag
some ground clearance benefit which is why most fighter aircraft have it, allows for armament attachment
best suited for aerobatics as it has a neutral roll stance and therefore more manoeuvrability. low wing has a dihedral shape for handling but when upside down the dihedral act in the wrong direction
can’t be used for cargo / passenger aircraft because of the wing carry through box since they’re usually lighter/ a ring frame can be used but they’re heavier
low wing
landing gear can be stowed inside the wing and usually attached to the wing box which is already strong and doesn’t need reinforcements
no external blister required which reduces drag
lesser aft fuselage upsweep which reduced drag
wing carry through box splits cargo compartment only meaning the passenger compartment is uninterrupted on top of cargo compartment
greater upward visibility
poor downward visibility
fuel pumps required
small aircraft require walkways on the wing meaning there is an externals step exposed to airstream
struts on low wings are subjected to compression and buckling failure is possible
increased risk of ground strike
longer landing gear required for sufficient ground clearance which adds weight
special ground equipment needed for loading and unloading cargo / passengers, which is why most low wing planes are used in established airfields
things to keep in mind
loading / unloading requirements
ground clearance requirements
landing gear weight, length, stowage
manoeuvrability
field of view
structural issues
aerodynamic drag
uninterrupted passenger cabin
wing tips
high pressure air below moves around wing tip to the top low pressure air
undesirable effects
pressure difference reduced therefore reducing lift
wing tip vortices and increases drag due to the lift
round
looks streamlined but actually makes it easier for air to move around the tip
wingtip vortex (at the vortex core) moves onto the upper surface which reduces the distance between vortex cores therefore reduces the effective aspect ratio. effective aspect ratio is the distance between the vortex cores and not the geometric span of the wingtips - therefore the further the vortex cores are from each other the higher the effective span and the less induced drag
aspect ratio can decrease by up to 20% in some cases
therefore not ideal for subsonic lift and drag
square
simplest form of wingtip
offers less induced drag than a round wingtip
the sharp edge forces the wingtip vortex to form on the outboard side, which pushes the vortices farther apart and increases the effecting AR slightly. span gain - the distance between the vortex cores that exceeds the geometric span
round - more induced drag, less parasitic drag. square - more parasitic drag, less induced drag. therefore the amount of drag reduced by wingtip depends on the configuration of aircraft such as design lift coefficient and aircraft speed - therefore wingtips need to be changed during research to determine the most effective
sharp
makes it harder for air to flow around the tip
which increases lift and reduces induced drag
most modern low drag wingtips use some form of sharp edge
hoerner
named after german aerodynamisit sigmund hoerner
more effective than a round wingtip and widely used for low drag
sharp edge but reshaping only on the lower surface. upper surface follows the same airfoil design to the tip which is important as upper surface produces about 2/3 of wing’s lift
lower surface undercut and canted 20-30 degrees
may also be concave to match the upper surface where they meet at the tip
drooped / upswept
uses an upward or downward curve tot rap air
increase lift and reduce drag by increasing effective span without they don’t increase actual span
slightly increase wetted area and parasitic drag
also add weight and increase torsional loads
poor design can lead to flutter
raked
effectively increases effective AR
increases wingspan and wing bending moments
larger span can cause parking and hanger issues
higher bending moments increase airframe weight
doesn’t increase interference drag
may increase skin friction if added to an existing wing
cut off forward swept
sometimes used on supersonic aircraft
tip is cut at an angle equal to the Mach cone angle
wing area within the shock cone contributes less to lift
reduced torsional loads and helps with flutter issues
endplate
make wing behave more like a two dimensional airfoil
increase drag due to added wetted area
effective span increase is only about 80% of actual span increase due to added height
increasing span is usually more effective than adding endplates
but are useful when span must be limited
winglets
advanced versions of endplates
use energy from tip vortex to improve lift-to-drag ratio by up to 20%
reduce induced drag but increases skin friction and interference drag. only works aerodynamically when the reduction in induced drag > skin friction and interference drag
vertical increase acts as a little wing generating lift inwards. vortex at wingtip interacts with the air and causes local flow striking winglet to be angled inwards. cuz of this, winglets lifting force which is perpendicular to local flow direction gets a forward component of tilted lift vector acting as ‘negative drag’ which reduces total wing drag
can be viewed as an effective increase in span. winglet produces a downwash as all lifting surface do. because downwash is angled vertically, downwash is actually an ‘outwash’ which pushes vortices further apart so winglet increases effective span therefore effective AR
most beneficial when wingtip vortex is strong. for an aircraft with high aspect ratio the benefit may be little to none
better for wings with an aspect ratio lower than normal or a wing that carries more aircraft weight then originally intended (operating at a higher lift coefficient)
can worsen flutter due to added weight behind elastic axis
can cause structural issues when added to wings not intended to have winglets
winglet shape must be optimised for one speed as it may increase drag at others
winglets tend to be used more as add on devices for existing wings requiring a little more efficiency without major redesign, whereas for a new aircraft its better to be designed with an already increased effective AR but yk you find out when testing
different winglet styles - blended, whitcomb, scimitar
blended on many passenger airliners
how to design
crucial for wing design
aspect ratio
taper ratio
sweep
twist
incidence
dihedral
wing area - total aircraft weight / chosen wing loading value
wing loading value - based on stall speed requirement, based on wing loading values of similar designs
create a reference wing
not an actual wing cuz it extends through fuselage and does not consider fillets and wingtip shaping (always square), root airfoil of reference wing considered in centre line of the fuselage as the root chord and not at the fuselage wing attachment
we need it to start with basic wing design and layout for the real wing
need to select effective aspect ratio from this reference wing to start planning
aspect ratio can be seen in birds - longer aspect ratio for gliders whereas short and thick wings have greater manoeuvrability but need more effort to stay up
when selecting aspect ratio we’re actually selecting wing span
wingspan main factor determining drag due to lift
the higher the wingspan / AR the less drag, but the longer the wingspan the more weight and bending moments, so a compromise must be made
also longer wingspan constrained by hanger and storage requirements
lower aspect ratio wings stall at higher AOAs as compared to higher ratio wings
can be selected based on historical data - such as exisiting gliders, commercial, aerobatic, fighter, trainers
taper ratio - ratio of tip chord / root chord
root chord considered fuselage centre line
tapering can improve spanwise lift distribution
spanwise lift distribution - how much lift occurs at which span wise location
too much lift at wingtip means more drag
ideal spanwise lift distribution is elliptical as that means there is not too much lift near the wingtip and therefore gives the minimum drag due to lift value. gives this when the wing is designed as elliptiical such as on the spitfire. not popular nowdays cuz difficult and expensive to manufacture and. ratio of 0.45 is almost ideal and is easier to make
taper also reduces wing weight therefore able to handle bending moments better
if taper ratio is too high, so tip chord is too small, problem of tip stall
untapered wing tends to start stalling at root and therefore better control in a stall cuz ailerons will still be in use
ideally taper ratio selected due to drag, stalling pattern, weight, and complexity of manufacturing
sweep
more aesthetically pleasing
but decreases lift by cosine of sweep angle, increases structural weight, reduces authority of flaps and ailerons, more prone to flutter
sweep advantages start at high speeds, speed increases speed at which shocks first form
sweep has stabilising effect like the dihedral
2 types of sweep angles
leading edge sweep - mainly for supersonic flight. swept so leading edge behind the mock cone to reduce drag. not also true as too large a sweep is impractical for structure
quarter chord sweep - mainly for subsonic flight, measured in reference to the quarter chord
forward sweep can also be used for the high speed advantage as it increases the speed at which shocks first form, but problem of divergence as wing can twist uncontrollably and may break off
mean aerodynamic chord MAC
decide where to put the wings on the fuselage so its stable
centre of gravity and wing must be in the right location with respect to each other
to do this, find out MAC - length of chord from a particular distance from the centre line
mac acts as all the wing’s area is concentrated in that chord. wing will have aerodynamic centre at the quarter chord point of the mac
if total pitching moment measured from here, it doesn’t change when angle of attack changes
2 ways mac can be found - using a formula and via a graph
mac critical for stability
cg placed at the quarter chord of the mac
twist
use to prevent tip stall and improve lift distribution
changes the spanwise lift distribution as it changes the local angle of attack seen by each air foil, which influences the drag due to lift
typical twist values - 0 to -5 degrees (minus meaning leading edge is twisted downwards)
in most cases, tip airfoil at a negative AOA compared to root airfoil, called a washout. wing with washout tends to stall at root before the tip which improves control
different airfoils at root and tip
two types - geometric (actual change in airfoil angle of incidence measured in respect to the root air foil) and aerodynamic (angle between zero lift angle of air foil with respect to zero lift angle of root air foil)
if only one air foil is used, then aerodynamic twist = geometric twist
wing with no geometric twist can have aerodynamic if, for example, root is cambered but tip is symmetric
incidence
pitch angle of wing with respect to the fuselage
if untwisted, the pitch angle is between the fuselage axis and airfoil cord lines
if twisted, then measured with respected to mac or root of exposed wing
diagrams often name root and tip incidence which defines the twist
wing incidence chosen to minimise frag during cruise. done so when wing is at correct AOA for lift, fuselage is at an AOA for minimum drag
for typica fuselages, incidence is such that the fuselage is a few degrees nose up
different formulas to determine incidence angle depending on whether the wing is swept or un-swept
dihedral
angle of wing with respect to the horizontal when viewed from the front
stabilising effect during a roll as a positive dihedral angle will roll it back to level
wing position matters while setting a dihedral value (high wing position has an increased dihedral effect), which is why many high wing aircraft have a negative dihedral, anhedral, to counter excessive effect of dihedral
excessive dihedral can lead to stability issues like a Dutch roll
generally, low wing aircraft need more than high wing cuz high wing already have a dihedral effect and therefore need a smaller geometric effect
when high wing plane rolls, will sideslip down to that side. fuselage pushes air over and under itself. air moves above the fuselage in a way that generates more lift, higher AOA, on the windward wing near the root, causing a rolling moment that stabilises the aircraft. opposite is true for low wing aircraft therefore low wing aircraft need more dihedral