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Stability & Control (HT Design (planform design parameters SA AR …
Stability & Control
HT Design
planform design parameters
SA
AR
taper ratio (lamda)
sweep angle
sweep and AR effects on lift curve slope
incr sweep decr lift curve slope (beyond a certain AR)
lift curve slope incr w AR (grad starts off at 2pi then decr)
a higher slope means that clmax aoa is lower ao that means flow is less attached
note that clmax doesnt vary much with AR
vortex lift
for swept wing, stable vortices stem from the inboard LE and translate diagonally along the span towards the TE of the wing tip
incr clmax because high vel inside the stable vortex leads to high supervel
this vortex generation depends on
LE rad
and
sweep angle
blunt LE; vortex form only at high aoa
eg, ventral fins near the tail use vortex lift to provide restoring force at high aoa/sideslip
can also be applied in the placement of vortex generators on fuse to prevent separation at fuse and hence tail ineffectiveness
IMPORTANT
AR effects
high AR leads to high lift curve slope and low stall aoa (vice versa for low AR)
AR has little effect on clmax
taper ratio effects
taper ratio has little effect on lift curve slope
effect on clmax depends on quart-chord sweep angle:
high quart-chord sweep: vortex lift and hence incr clmax
low quart-chord sweep: tip stall due to low Re at tip and hence decr clmax
quart-chord sweep effects
high sweep: low lift curve slope, high stall aoa (vice versa for low sweep)
effect on clmax depends on vortex formation
HT surface design
functions
provide long. stab in all flight conditions
provide pitch control in all flight conditions
(!!!)
provide state of equil in each flight condition
(!!!)
requirements
They shall provide a sufficiently large contribution to static and dynamic longitudinal, directional, and (lateral) stability.
requires high AR and min sweep (only if AR is high to begin with)
They shall provide sufficient control capability.
exact requirements as the first point
Control shall be possible with acceptable control forces
requires high AR of control surface
Shall be able to handle high angles of attack
requires low AR of control surface
*note: this contradicts the previous point!
sweep is beneficial
The tail surfaces shall be able to provide a maximum force sufficiently large to balance the total tail-off forces and moments so that static equilibrium is achieved in all flight conditions
requires sufficiently large tail surface
requires sufficiently high clmax for a range of rudder deflections and icing effects
They shall be able to handle high Mach numbers without flow separation
requires a design M which is below the point of extensive flow separation
aeroelastic and icing effects
aeroelastic effects
wing bending n torsion
fuse bending
decr tail surface lift curve slope
tail surface bending n torsion
decr tail surface lift curve slope
decr control surface effectiveness
deformation of movable stabiliser attachment
ice effects
decr stall aoa (so clmax decr)
so ice accretion can lead to sudden pitching moments cos easier to stall
Flight beyond buffet onset and MMO
flight characteristics bet MMO and MD
civil xport a/c usually fly < MMO
only in failure that causes pitch down then MMO is exceeded
fokker 100
lift decr w M
lift shifts more inboard w M
to balance P distr on top n bottom, incr in aoa req.
from MMO to MD
neutral pt shifts aft
wing lift decr n moves inboard
tail lift incr
leads to nose down pitching moment
VT Design
Functions
provide directional stab in all flight conditions
provide yaw control in all flight conditions
ensure safe handling during engine failure conditions
requirements
They shall provide a sufficiently large contribution to static and dynamic directional and lateral stability.
req high AR and min sweep for high AR
They shall provide sufficient control capability.
same req as previous point
Control shall be possible with acceptable control forces
req high AR to reduce the control force
to reduce control force, can use either horn balance or balance tabs
Shall be able to handle high angles of sideslip
rew lor AR and either sweep or dorsal fin
** dorsal fins incr clmax with vortex lift at high sideslip angles
shall provide sufficiently large forces to balance the OEI moments and provide equilibrium
req sufficiently large tail surface
req measures to ensure high clmax (eg sweep or dorsal fin)
They shall be able to handle high Mach numbers without flow separation
requires a design M which is below the point of extensive flow separation
so for high yaw characteristics..
large stalling angle
obtained thru
low AR
and
swept back/dorsal fins
Wing control surfaces
control surface design
requirements
high deflection w/o separation
linear behaviour over the entire range of deflections
functions
provide means to achieve equil
allow for manoeuvres in pitch roll n yaw
counteract gusts in flight path tracking
aileron design
aileron reversal
due to elastic wing structure, aileron deflection caused an
opposite
local aoa
Solution
use outboard aileron only for low speeds
inboard ailerons may be used at high or low speeds
Re effect
higher Re cause ailerons to lose effectiveness due to formation of shock induced separation
solution
vortex generators to prevent:
LE separation
separation at food of shockwave
separation at hinge line
Spoilers
functions
MEMORIZE
roll control
speed brakes
lift dumpers
"spoil" lift to allow for higher wheel loads upon touchdown
load alleviation
effects of spoiler deflection
decr lift
change in pitching moment (not much)
incr in D
closed vs open shoud spoilers
open:
strong non-linear behaviour
less complex (hardware)
closed:
decouples response from flap deflection