Why does rudder effectiveness increase as AOA increases
David Fedors
My question for the newgroup is two fold: I am curious why rudder
effectivenes increases and aileron effectiveness decreases as angle of
attack increases. I am most interested in a physical
explanation. Thank you.
--
Dave Fedors
ShawnD2112
You've got a new one on me, Dave. I assume you're talking about the
aircraft alpha and not the local rudder alpha. If what you're saying
is true, I'd also be very interested in the answer because it
contradicts what I've been taught and experienced. If I've been that
wrong for that long, I want to know about it!
I've never heard of rudder effectiveness increasing with increasing
alpha. In fact, I don't know that alpha directly influences rudder
effectiveness itself but there may be some secondary effects from the
aerodynamics of a particular airplane that do so. You tend to lose
control effectiveness (with decreasing airspeed) in alphabetical
order: ailerons, elevator, then rudder. With enough propwash, though,
you can still have a pretty damned effective rudder - watch any
aerobatic airplane do a hammerhead - zero airspeed but full throttle,
and the plane just whips right over. Great fun if you ever get a
chance to do it.
We get pretty good rudder effectiveness on the Cub when taxying just
by giving it a shot of throttle to increase the propwash.
Additionally, in a full stall in both the Cub and C152, the rudder is
fully effective. In fact, even in a deep stall, you use the rudder,
not the ailerons, to level the wings. Using the ailerons in a stalled
or near stalled condition will give you unbalanced lift and drag
between the two wings, causing one to stall more than the other, and
may put you into a spin, so we only use the rudder. Taken to the
extreme, a snap roll is done by fully stalling the airplane then
whacking in full rudder. That control input alone produces an
extremely fast roll in the direction of the input.
As for the ailerons, I can't remember anything from my flying
experience or aero degree that talked about losing aileron
effectiveness purely with increased alpha. Now, you do lose them with
decreased airspeed (a common effect of increasing alpha and holding
it). Again, back to the aerobatic airplane, doing a barrel roll is a
case of increased alpha but the ailerons are fully effective (and
delightfully so!).
You do lose aileron effectiveness with decreasing airspeed because
they are effectively lift-increasing devices, subject to the same lift
equation as a pure airfoil. Using the ailerons is the same as
increasing the camber of that section of one wing similar to using
flaps, and decreasing (even spoiling) the lift of that section of the
other wing. So, as the airspeed drops, the lift generated by the wing
drops, the ailerons are less effective.
There's the additional problem that the ailerons are at the trailing
edge of the wing and, as you approach the stall, the flow begins to
seperate from the trailing edge and the seperation point moves forward
over the top of the wing. Eventually, it moves totally forward of the
aileron so it is getting only seperated airflow. To help prevent
this, most airplane wings are designed with a bit of wing twist so the
root of the wing stalls before the tips. Ideally then, enough of the
wing stalls before the bit with the aileron that the nose pitches down
and reduces the alpha to a sub-stall angle before the ailerons ever
actually experience a stall.
Anyway, the effects I can think of are more airspeed related, not
directly related to angle of attack. Hope that helps.
Shawn
Piper Super Cub G-AYPR
Stealth Pilot
wrote:
> My question for the newgroup is two fold: I am curious why rudder
> effectivenes increases and aileron effectiveness decreases as angle
> of attack increases. I am most interested in a physical
> explanation. Thank you.
cant agree with the question. as angle of attack increases in normal
flight the speed will decrease. if the speed didnt decrease you'd
climb and wouldnt achieve the increase in aoa.
as angle of attack increases and the speed falls away the rudder and
aileron effectiveness both decrease. if you enter stable very slow
flight with high angle of attack the controls are as mushy as hell.
my flying environment is homebuilts and little cessnas 40 to 120 knots
speed range.
do you have some particular flight environment in mind or are you off
on a misconception?
stealth
Dennis M. Straussfogel
> effectivenes increases and aileron effectiveness decreases as angle of
> attack increases. I am most interested in a physical
> explanation. Thank you.
>
Speaking as an aero engineer: aileron and rudder effectiveness don't
necessarily vary with AOA the way you describe. The effectiveness of
both decrease with airspeed, but not AOA, and for some configurations,
the rudder may lose effectiveness at high AOA if it gets blanketed by
the wing wake.
Speaking as a flight instructor: I'm thinking you might be asking why
one should use more rudder to keep the wings level when AOA is high
and approaching stall. That's because using aileron to raise a wing
may push the effective AOA of the low wing past stall and have
opposite the desired effect. At high AOA with airspeed decreasing and
approaching stall, it's prudent to use more rudder and less aileron to
keep the wings level, not only because adverse yaw is becoming more
pronounced, but also using the dihedral/side-slip stability to raise
the wing a little is more effective than using aileron. Of course,
making any sudden or large correction in this flight regime, with
either aileron OR rudder, may push you into a stall and incipient
spin.
I hope I didn't misunderstand your question.
---------------------------------------------------------------------
Dennis M. Straussfogel, Ph.D.
Aerospace Engineering Consultant
dm...@nh.ultranet.com
---------------------------------------------------------------------
WZMan1
This is not necessarily true on all aircraft, as my experience and
previous respondents have indicated.
What do you mean by rudder effectiveness? The ability to generate
yawing moment or something else? I am currently a T-38 instructor
pilot -- for that particular aircraft, the rudder is increasingly
effective as a roll producer (NOT a yaw producer) at higher AOAs,
while ailerons become less effictive roll producers at high AOA.
The reason for the latter effect, I believe, was correctly explained
by a previous respondent: At high AOA, the wing is operating closer to
CLmax; the icremental CL realized from the downward deflected aileron
is somewhat less than that which could be realized on the linear
portion of the CL vs Alpha curve. The upward deflected aileron may be
less effective because it is raised into airstream near the trailing
edge of the wing, an airstream which may have started to separate from
the wing surface at some point upstream of the aileron (highly
dependent on airfoil section, flight parameters and wing geometry).
The power of the T-38's rudder to generate yaw remains relatively
constant with AOA. The power of the rudder to generate roll, however,
does increase with AOA, and I believe the case would be true with any
SWEPT WING aircraft.
Consider a right rudder input: this causes the rudder to deflect to
the right and produces a left force on the vertical stabilizer. This
force, in turn, causes the aircraft to yaw tail left (nose right).
Because the wings of the aircraft are swept aft, the left wing is now
presented to the airstream at an angle somewhat less than it's sweep
angle, increasing the chordwise component of airflow over the wing.
The right wing is now presented to the airstream at a more oblique
angle, decreasing the chordwise component of flow.
In an extreme example, consider a notional aircraft with 45 degrees of
wing sweep. If I could yaw that aircraft 45 dregrees to the right,
the left wing would be perpindicular to the airstream and the right
wing would be parallel! Lift on the right wing would be GREATly
reduced (zero?). It is clear to see how Yaw can produce Roll on a
swept wing aircraft (there are addidtional factors, such as fuselage
interference, at work also).
Now that we've established this, why does this effect amplify with
AOA? Consider a wing operating at zero-lift AOA. If I yaw the
aircraft, one wing will be presented to that airflow more obliquely,
etc. Since the wing is producing no lift, however, the change in
chordwise flow will not change the AOA nor the CL. Lift will still be
zero and no rolling will be produced.
At an AOA other than zero-lift AOA, llift will increase on the wing
yawed forward and decrease on the wing yawed aft, producing a rolling
motion in the direction of yaw. By deduction, you can see this effect
increases at higher AOA.
One last note. The rudder itsefl, when deflected, produces a rolling
moment since it is located above the longitudinal axis of the a/c.
For a right deflected rudder, this means a LEFT rolling moment. The
T-38 rolls RIGHT in this instance, however, because the yaw-roll
coupling described above produces a greater rolling moment than the
rudder alone.
Warren Zelenski
Pilot, Engineer, etc
W Zelenski
brian whatcott
wrote:
>My question for the newgroup is two fold: I am curious why rudder
>effectivenes increases and aileron effectiveness decreases as angle of
>attack increases. I am most interested in a physical
>explanation. Thank you.
Illustrations of designers' responses to decreasing rudder effect with
increasing AOA are the inter-war Tiger Moth training biplane which
added a horizontal strake in front of the horizontal stabilizer to
reduce adverse airflow at AoA >40 degrees in the spin, and the post
war C150 which added a vertical strake in front of the vertical
stabilizer for similar reasons.
Brian Whatcott Altus OK
Ferrari Bugatti Aston-Martin Lotus
Julian Scarfe
fed...@earthlink.net wrote on 22/5/00 6:39 pm:
> My question for the newgroup is two fold: I am curious why rudder
> effectivenes increases and aileron effectiveness decreases as angle of
> attack increases. I am most interested in a physical
> explanation. Thank you.
As AOA increases in unaccelerated flight, airspeed decreases. Both
aileron and rudder effectiveness are likely to decrease at lower
airspeeds but the relative effectiveness of rudder as a means of roll
control increases.
Inducing a yaw with rudder has a secondary effect of rolling the
aircraft, as the outside wing travels faster than the inside wing,
producing more lift. The lower the airspeed, the greater
(proportionately) is the effect of the differential speed from a given
yaw rate. Thus it feels like the rudder is more effective.
Julian Scarfe
Vineet
David Fedors schrieb:
> My question for the newgroup is two fold: I am curious why rudder
> effectivenes increases and aileron effectiveness decreases as angle of
> attack increases. I am most interested in a physical
> explanation. Thank you.
Though I am not sure about the rudder effectiveness increasing with
AOA, I would try to explain the effect on aileron as follows :
The rolling moment generated by the ailerons is :
RM = 0.5*rho*v*v*(b)*S*Cl
where rho is the density of the air
v is the forward speed of the aircraft
b is the wing span and S if the wing reference area (usually
planform area)
Cl is the rolling moment coefficient generated by the ailerons.
Now we just need to look at Cl and v as all other parameters are not
controlled by us.
For a rigid airplane , Cl is primarily a function of aileron
deflection angle. It further depends on the aircraft AOA. As AOA
approaches stall, the airflow in the aileron region usually separates
and so ailerons are able to generate only a fraction of the Cl they
were generating at low AOA. So as the AOA increases, ailerons lose
effectiveness gradually. This phenomenon has nothing to do with the
speed, unless we start taking aeroelastic effects into
account. (Pilots always talk about speed mainly and aerodynamicists
mostly about AOA !).
The second factor, 'v', affects the total RM produced just as shown
in the equation. At low speed, low RM is produced and at higher speed,
very high RM is produced per deg of aileron deflection, whatever be
the AOA (below stall). For example, say for an aircraft, at 100m/s,
the change of aileron deflection from 5 deg to 10 deg increases the RM
by 400 units, then at 50m/s, the RM increase will be only 100 units!
Therein lies the explanation.
thanks,
vineet
david....@baesystems.com
news:39378284...@aero.tu-darmstadt.de...
>
> David Fedors schrieb:
>
> > My question for the newgroup is two fold: I am curious why rudder
> > effectivenes increases and aileron effectiveness decreases as angle of
> > attack increases. I am most interested in a physical
> > explanation. Thank you.
>
> Though I am not sure about the rudder effectiveness increasing with
> AOA, I would try to explain the effect on aileron as follows :
I would have thought the opposite is true, rudder effectivness
decreases as angle of attack increases, as more of the rudder is
'masked' from the airflow by the airframe.
cheers
David Rigg \ _ /
Mission Avionics Division, x__________\_(0)_/__________x
BAE SYSTEMS, Edinburgh, Scotland @ @ (](_o_)[) @ @
Tel : (+44) (0)131 314 8223
Don Stauffer
>
> Vineet <vin...@aero.tu-darmstadt.de> wrote in message
> news:39378284...@aero.tu-darmstadt.de...
> >
> > David Fedors schrieb:
> >
> > > My question for the newgroup is two fold: I am curious why rudder
> > > effectivenes increases and aileron effectiveness decreases as angle of
> > > attack increases. I am most interested in a physical
> > > explanation. Thank you.
> >
> > Though I am not sure about the rudder effectiveness increasing with
> > AOA, I would try to explain the effect on aileron as follows :
>
> I would have thought the opposite is true, rudder effectivness
> decreases as angle of attack increases, as more of the rudder is
> 'masked' from the airflow by the airframe.
I agree. Further, as AOA approaches high values, you get
more turbulence from seperated flow off wing, and this air
is less efficient in creating 'lift' from rudder. Remember,
most airfoils do not seperate instantly at a given AOA, they
increase seperation over a range of AOA.
--
Don Stauffer in Minneapolis
stau...@gte.net
http://home1.gte.net/stauffer/
GLPILOTSRV
> I am curious why rudder
>> > effectivenes increases and aileron effectiveness decreases as angle of
>> > attack increases.
While the aileron effectiveness decreases, I am not sure that the
rudder effectiveness acutally increases. It may be more effective than
the airlerons depending on aircraft type and/or configuration.
G. Lee
Burkhard Domke
>Vineet <vin...@aero.tu-darmstadt.de> wrote in message
>news:39378284...@aero.tu-darmstadt.de...
>>
>> David Fedors schrieb:
>>
>> > My question for the newgroup is two fold: I am curious why rudder
>> > effectivenes increases and aileron effectiveness decreases as angle of
>> > attack increases. I am most interested in a physical
>> > explanation. Thank you.
>>
>> Though I am not sure about the rudder effectiveness increasing with
>> AOA, I would try to explain the effect on aileron as follows :
>
>I would have thought the opposite is true, rudder effectivness
>decreases as angle of attack increases, as more of the rudder is
>'masked' from the airflow by the airframe.
OTOH, rudder affectiveness may be bolstered by vortices that are
generated by the airframe somewhere upstream...and these vortices
increase in direct proprtion to alpha. Also depends on yaw angle, even
speed may have to be taken into account.
Rudder masking is primarily a question of aircraft configuration.
Burkhard
Berlin, Germany
jim
Vineet wrote:
> David Fedors schrieb:
>
> > My question for the newgroup is two fold: I am curious why rudder
> > effectivenes increases and aileron effectiveness decreases as angle of
> > attack increases. I am most interested in a physical
> > explanation.
> Though I am not sure about the rudder effectiveness increasing with
> AOA, I would try to explain the effect on aileron as follows :
>
> The rolling moment generated by the ailerons is :
>
> RM = 0.5*rho*v*v*(b)*S*Cl
> where rho is the density of the air
> v is the forward speed of the aircraft
> b is the wing span and S if the wing reference area (usually
> planform area)
> Cl is the rolling moment coefficient generated by the ailerons.
>
> Now we just need to look at Cl and v as all other parameters are not
> controlled by us.
The letters to the editor in Aviation Week this week contain an interesting
comment from an airline jockey, ex IP for the USAF who says that on swept
wing aircraft as the beta(sideslip) is increased the leeward wing is more
nearly perpendicular to the flow vector than is the windward wing. Is this
possibly the effect that you are looking for?
Later
JEM
[Moderator's note: There is an extremely good article, "The Turn", by
William Langeweische, in the December, 1993, Atlantic Monthly, at
http://www.theatlantic.com/unbound/langew/turn.htm
You may also find the same author's article in the March, 1998,
Atlantic Monthly on the ValuJet accident interesting. The first part
http://www.theatlantic.com/issues/98mar/valujet1.htm and you can go to
the second and third parts by clicking the links at the end of each
part. MFS]
Burkhard Domke
> The letters to the editor in Aviation Week this week contain an
> interesting comment from an airline jockey, ex IP for the USAF who
> says that on swept wing aircraft as the beta(sideslip) is increased
> the leeward wing is more nearly perpendicular to the flow vector
> than is the windward wing. Is this possibly the effect that you are
> looking for?
No, that rather is about roll/yaw coupling of swept wing aircraft
(positive sweep):
Left rudder makes the aircraft yaw to the left, unsweeping the right
(windward!) wing. This induces additional lift on the right wing,
making the aircraft roll to the left. The resulting loss in total lift
in turn makes the aircraft sideslip to the left, which now unsweeps
the left wing, making the aircraft roll back into the opposite
direction. The latter effect is enhanced by dihedral.
Has nothing to do with rudder effectiveness as a function of AoA.
Burkhard
Berlin, Germany