Understanding Washout - 2012/03

58 Model Aviation March 2012 www.ModelAviation.com
Understanding the twist in your wing
Understanding
by David Andersen
[email protected]
Unt optatur, omniminveles moluptae nisim quodis everum saperiorunt
quianda dolor alissi odi aut dolore illor restota spernam aceatibus.
Bob Patton’s Cessna Aerobat uses drooped wingtips for stall control, typical of STOL aircraft.
Photos by the author58 Model Aviation March 2012 www.ModelAviation.com
Understanding the twist in your wing
Understanding
by David Andersen
[email protected]
Unt optatur, omniminveles moluptae nisim quodis everum saperiorunt
quianda dolor alissi odi aut dolore illor restota spernam aceatibus.
Bob Patton’s Cessna Aerobat uses drooped wingtips for stall control, typical of STOL aircraft.
Photos by the author60 Model Aviation March 2012 www.ModelAviation.com
curl around the wingtip and cancel the
low pressure air above the wing. This
further reduces aileron effectiveness.
It also increases wingtip drag and must
be controlled by the vertical stabilizer.
Washout reduces wingtip vortex and
its associated drag.
Although wing efficiency is generally
unimportant in model aircraft, the
reduction of wingtip drag via washout
improves lateral (yaw) stability. This is
especially important at low speeds and
high angles of attack. Washout, therefore,
improves lateral stability and rudder
effectiveness.
Wingtip vortices cannot be eliminated,
so ailerons are not effective at the tip
of the wing. For this reason, in addition
to the aileron reversal problem, ailerons
rarely go all the way to the wingtip.
In highly swept wings, the washedout
wingtips act as horizontal
stabilizers, increasing pitch stability.
When carried far enough, it is possible
to eliminate the tail. Some flying wings,
such as the Northrup N-9M, are based
on this principle.
Why Not?
Too much of a good thing can cause
problems. All the good that washout
does in upright flight can be detrimental
in inverted flight—such as loss of
aileron effectiveness, nonuniform roll
rate, adverse yaw, surprise snap rolls,
and aileron reversal. For these reasons,
washout is rarely used in full-scale
aerobatic aircraft. For these aircraft, it is
important for the airplanes to behave in
inverted flight as closely to upright flight
as possible. In addition, aerobatic aircraft
need to be predictably snap-rolled.
Constant-chord wings, such as those
found on the J-3 Cub or STOL (short
takeoff and landing) aircraft, benefit
least from washout. They are built to
maximize wing area and need all the
lift they can get. Instead of washout,
they may use stall strips to soften the
stall, and shaped wingtips to reduce
wingtip vortices in lieu of washout.
Typically, biplanes have their wing
incidences adjusted so that the forward
wing (typically the top wing in a
Stearman or the bottom wing in a
Beech Staggerwing) will stall before
the rear wing. The ailerons are usually
in the rear wing so good aileron
control is maintained even if the other
wing is stalled.
This is one of the advantages of
biplanes over monoplanes; usually this
configuration does not merit washout.
LE slats can also prevent tip stalls, but
slats are usually combined with washout
for an extra margin of low-speed control.
Flaps increase the angle of attack of
the wing in the flap area by rotating the
chord line. Flaps also increase washout.
All the good that
washout does in upright
flight can be detrimental
in inverted flight.
Understanding Washout
At high angles of attack, such as in a
climbing turn, there is the danger that
the down aileron, (left aileron in a right
bank) can provoke a stall in that wingtip.
Such a stall creates plenty of drag in
the wingtip, pulling it back and yawing
the airplane in the opposite direction of
the turn. If the airplane has significant
dihedral, a roll in the opposite direction
also develops. This phenomenon is called
aileron reversal or aileron snatch.
Unfortunately, a pilot’s instinct to
apply additional aileron deflection
makes matters worse. The remedy
is to correct with rudder, not more
aileron. Beware of this when flying your
warbird in an inverted climbing turn or
victory roll.
Wingtip vortex is the tendency of
the high-pressure air under the wing to
The glider-like wing of the author’s 114-inch span Focke-Wulf
Ta 152H requires washout for stability. The aircraft would be
uncontrollable without it.
The author’s 1/3-scale Grumman Lynx has a
constant chord wing with no washout. It instead
uses Hoerner wingtips and stall strips, which are
effective.
In case of engine failure, washout provides good yaw control in Greg Hahn’s B-17.www.ModelAviation.com March 2012 Model Aviation 61
This improves pitch stability and aileron
control at low airspeeds.
Models without ailerons steer with
rudder and use the dihedral of the
wings for banking. As the rudder yaws
one wingtip forward, the angle of
attack is increased, while the other
wingtip decreases its angle of attack.
Washout would partially defeat this
effect, so it is seldom used in aircraft of
this type, except in Scale models with
pointy wings.
Washout should be avoided in
lightweight wings that are not stiff
enough to resist further twisting in flight.
Imagine such a wing in a dive. The root
is creating positive lift while the wingtip
is generating negative lift because of
washout. This twisting force tends to
further increase washout if the wing is
not stiff enough to resist it.
As speed increases, drag increases, but
net lift becomes zero and vertical dive
equilibrium develops. If there is enough
elevator to pull the nose up, the washout
will suddenly reverse and the entire wing
will be lifted, possibly breaking it. If there
is not enough airflow over the elevator
to pull out, the airplane will plummet
to the ground. Many RC gliders have
crashed because of this principle.
How Much?
The optimum amount of washout
varies from zero to several degrees,
depending on the following factors:
• High aspect ratio (span/chord) wings
need more washout because their thin
wingtips tend to stall.
• Tapered wings need more washout in
proportion to the amount of taper.
• High wing loading requires more
washout because it is prone to tip
stalls.
• Underpowered aircraft need more
because they must fly at higher angles
of attack.
• Thin wings need more washout
because they abruptly stall at low
angles of attack.
• Multiengine airplanes need plenty of
washout for rudder effectiveness in
case of engine failure.
• Biplanes need less (see the previous
“Why Not?” section).
• Aerobatic airplanes need none to be
symmetric in flight.
• Washout becomes less effective as
dihedral increases.
For Scale models, use the amount of
washout used in the full-scale aircraft.
In general, RC warbirds use roughly
1° or 2°of washout, adjusted up or
down by the
aforementioned
factors. An RC
airplane rarely
needs more than 4°
of washout.
Left: Despite its thin, pointed wingtips, Dave Szabo’s Spitfire has
excellent handling in part because of 2.5° of washout—roughly
the same as the full-scale Spitfire. A low pass before a chandelle is
shown here.
A 90° sharp-edge stall strip is added to the LE of the Grumman Lynx to lower the stall angle in the
root area of the wing. This alternative to washout also works when inverted.
There’s no washout and no incidence
in Dave Deschenes’ Wildcat—typical
of constant-chord dive bombers.62 Model Aviation March 2012 www.ModelAviation.com
Where?
In most cases, the angle of wingtip
attack should be close to zero in level
flight, generating little or no lift in
level cruise position, so the washout
angle equals the root angle. Washout
typically is distributed uniformly from
root to tip, but not always. Consider
the following exceptions:
• The three-piece wings of the
Mitsubishi Ki-15 Babs, North
American AT-6, and the Junkers Ju
87 Stuka have no twist in their center
sections, but begin outboard of the
landing gear.
• The Focke-Wulf Ta 152H highaltitude
fighter’s high aspect ratio
wing has 2° of washout, all of it in the
aileron area.
• For some models, such as the nearly
constant chord Howard Pete, little
washout, if any, is needed. But a small
amount is included in the wingtips by
Understanding Washout
shaping the LE of the outermost rib bay.
There are several methods of adding
washout during assembly, such as
temporary tabs on each rib to hold it
at the required angle, shims of varying
heights supporting the spars, tapered,
full-span sticks upon which the ribs
rest during assembly, and setting twist
after assembly.
Sometimes the ribs and spars can
be assembled on a flat surface without
washout. The TE of the end ribs are
then raised, twisting before the sheeting
is applied. Open-structure wings can
sometimes be completely built and
covered with heat-shrink plastic film.
The wing is then twisted while heat is
reapplied with a hot-air gun.
What if you forgot to build in enough
washout, or flight tests suggest it needs
more? You might want to play it safe
and temporarily include extra washout
during those first few flights.
Unless the airplane has full-span
ailerons, washout can be increased 1° by
slightly raising the TE of both ailerons.
For a typical Giant Scale model, this is
less than 3/16 inch. Later, if stalls and tight
turns are acceptable, lower the ailerons
in small increments until they are back
to neutral.
Questions or Comments
I would be happy to respond to
your comments and questions. You can
contact me via the website listed in
“Sources” or through email.
Thanks to the following for technical
assistance: Joe Grice, Scott Russell, Tony
Paladino, and Jon Bomers.
—David Andersen
[email protected]
SourceS:
Minnesota Scale and Giant Scale r/c
www.mnbigbirds.com
Washout can be added after construction by
slightly raising both ailerons. This is recommended
for the maiden flights of a new model.
Washout in the Howard Pete’s wingtip is formed by shaping the LE in the outer rib bay.
Leo Spychalla’s Ziroli Stuka has a gentle
stall despite its pointed wings. The wings
have 4° of washout, starting outboard of the
landing gear.

58 Model Aviation March 2012 www.ModelAviation.com
Understanding the twist in your wing
Understanding
by David Andersen
[email protected]
Unt optatur, omniminveles moluptae nisim quodis everum saperiorunt
quianda dolor alissi odi aut dolore illor restota spernam aceatibus.
Bob Patton’s Cessna Aerobat uses drooped wingtips for stall control, typical of STOL aircraft.
Photos by the author58 Model Aviation March 2012 www.ModelAviation.com
Understanding the twist in your wing
Understanding
by David Andersen
[email protected]
Unt optatur, omniminveles moluptae nisim quodis everum saperiorunt
quianda dolor alissi odi aut dolore illor restota spernam aceatibus.
Bob Patton’s Cessna Aerobat uses drooped wingtips for stall control, typical of STOL aircraft.
Photos by the author60 Model Aviation March 2012 www.ModelAviation.com
curl around the wingtip and cancel the
low pressure air above the wing. This
further reduces aileron effectiveness.
It also increases wingtip drag and must
be controlled by the vertical stabilizer.
Washout reduces wingtip vortex and
its associated drag.
Although wing efficiency is generally
unimportant in model aircraft, the
reduction of wingtip drag via washout
improves lateral (yaw) stability. This is
especially important at low speeds and
high angles of attack. Washout, therefore,
improves lateral stability and rudder
effectiveness.
Wingtip vortices cannot be eliminated,
so ailerons are not effective at the tip
of the wing. For this reason, in addition
to the aileron reversal problem, ailerons
rarely go all the way to the wingtip.
In highly swept wings, the washedout
wingtips act as horizontal
stabilizers, increasing pitch stability.
When carried far enough, it is possible
to eliminate the tail. Some flying wings,
such as the Northrup N-9M, are based
on this principle.
Why Not?
Too much of a good thing can cause
problems. All the good that washout
does in upright flight can be detrimental
in inverted flight—such as loss of
aileron effectiveness, nonuniform roll
rate, adverse yaw, surprise snap rolls,
and aileron reversal. For these reasons,
washout is rarely used in full-scale
aerobatic aircraft. For these aircraft, it is
important for the airplanes to behave in
inverted flight as closely to upright flight
as possible. In addition, aerobatic aircraft
need to be predictably snap-rolled.
Constant-chord wings, such as those
found on the J-3 Cub or STOL (short
takeoff and landing) aircraft, benefit
least from washout. They are built to
maximize wing area and need all the
lift they can get. Instead of washout,
they may use stall strips to soften the
stall, and shaped wingtips to reduce
wingtip vortices in lieu of washout.
Typically, biplanes have their wing
incidences adjusted so that the forward
wing (typically the top wing in a
Stearman or the bottom wing in a
Beech Staggerwing) will stall before
the rear wing. The ailerons are usually
in the rear wing so good aileron
control is maintained even if the other
wing is stalled.
This is one of the advantages of
biplanes over monoplanes; usually this
configuration does not merit washout.
LE slats can also prevent tip stalls, but
slats are usually combined with washout
for an extra margin of low-speed control.
Flaps increase the angle of attack of
the wing in the flap area by rotating the
chord line. Flaps also increase washout.
All the good that
washout does in upright
flight can be detrimental
in inverted flight.
Understanding Washout
At high angles of attack, such as in a
climbing turn, there is the danger that
the down aileron, (left aileron in a right
bank) can provoke a stall in that wingtip.
Such a stall creates plenty of drag in
the wingtip, pulling it back and yawing
the airplane in the opposite direction of
the turn. If the airplane has significant
dihedral, a roll in the opposite direction
also develops. This phenomenon is called
aileron reversal or aileron snatch.
Unfortunately, a pilot’s instinct to
apply additional aileron deflection
makes matters worse. The remedy
is to correct with rudder, not more
aileron. Beware of this when flying your
warbird in an inverted climbing turn or
victory roll.
Wingtip vortex is the tendency of
the high-pressure air under the wing to
The glider-like wing of the author’s 114-inch span Focke-Wulf
Ta 152H requires washout for stability. The aircraft would be
uncontrollable without it.
The author’s 1/3-scale Grumman Lynx has a
constant chord wing with no washout. It instead
uses Hoerner wingtips and stall strips, which are
effective.
In case of engine failure, washout provides good yaw control in Greg Hahn’s B-17.www.ModelAviation.com March 2012 Model Aviation 61
This improves pitch stability and aileron
control at low airspeeds.
Models without ailerons steer with
rudder and use the dihedral of the
wings for banking. As the rudder yaws
one wingtip forward, the angle of
attack is increased, while the other
wingtip decreases its angle of attack.
Washout would partially defeat this
effect, so it is seldom used in aircraft of
this type, except in Scale models with
pointy wings.
Washout should be avoided in
lightweight wings that are not stiff
enough to resist further twisting in flight.
Imagine such a wing in a dive. The root
is creating positive lift while the wingtip
is generating negative lift because of
washout. This twisting force tends to
further increase washout if the wing is
not stiff enough to resist it.
As speed increases, drag increases, but
net lift becomes zero and vertical dive
equilibrium develops. If there is enough
elevator to pull the nose up, the washout
will suddenly reverse and the entire wing
will be lifted, possibly breaking it. If there
is not enough airflow over the elevator
to pull out, the airplane will plummet
to the ground. Many RC gliders have
crashed because of this principle.
How Much?
The optimum amount of washout
varies from zero to several degrees,
depending on the following factors:
• High aspect ratio (span/chord) wings
need more washout because their thin
wingtips tend to stall.
• Tapered wings need more washout in
proportion to the amount of taper.
• High wing loading requires more
washout because it is prone to tip
stalls.
• Underpowered aircraft need more
because they must fly at higher angles
of attack.
• Thin wings need more washout
because they abruptly stall at low
angles of attack.
• Multiengine airplanes need plenty of
washout for rudder effectiveness in
case of engine failure.
• Biplanes need less (see the previous
“Why Not?” section).
• Aerobatic airplanes need none to be
symmetric in flight.
• Washout becomes less effective as
dihedral increases.
For Scale models, use the amount of
washout used in the full-scale aircraft.
In general, RC warbirds use roughly
1° or 2°of washout, adjusted up or
down by the
aforementioned
factors. An RC
airplane rarely
needs more than 4°
of washout.
Left: Despite its thin, pointed wingtips, Dave Szabo’s Spitfire has
excellent handling in part because of 2.5° of washout—roughly
the same as the full-scale Spitfire. A low pass before a chandelle is
shown here.
A 90° sharp-edge stall strip is added to the LE of the Grumman Lynx to lower the stall angle in the
root area of the wing. This alternative to washout also works when inverted.
There’s no washout and no incidence
in Dave Deschenes’ Wildcat—typical
of constant-chord dive bombers.62 Model Aviation March 2012 www.ModelAviation.com
Where?
In most cases, the angle of wingtip
attack should be close to zero in level
flight, generating little or no lift in
level cruise position, so the washout
angle equals the root angle. Washout
typically is distributed uniformly from
root to tip, but not always. Consider
the following exceptions:
• The three-piece wings of the
Mitsubishi Ki-15 Babs, North
American AT-6, and the Junkers Ju
87 Stuka have no twist in their center
sections, but begin outboard of the
landing gear.
• The Focke-Wulf Ta 152H highaltitude
fighter’s high aspect ratio
wing has 2° of washout, all of it in the
aileron area.
• For some models, such as the nearly
constant chord Howard Pete, little
washout, if any, is needed. But a small
amount is included in the wingtips by
Understanding Washout
shaping the LE of the outermost rib bay.
There are several methods of adding
washout during assembly, such as
temporary tabs on each rib to hold it
at the required angle, shims of varying
heights supporting the spars, tapered,
full-span sticks upon which the ribs
rest during assembly, and setting twist
after assembly.
Sometimes the ribs and spars can
be assembled on a flat surface without
washout. The TE of the end ribs are
then raised, twisting before the sheeting
is applied. Open-structure wings can
sometimes be completely built and
covered with heat-shrink plastic film.
The wing is then twisted while heat is
reapplied with a hot-air gun.
What if you forgot to build in enough
washout, or flight tests suggest it needs
more? You might want to play it safe
and temporarily include extra washout
during those first few flights.
Unless the airplane has full-span
ailerons, washout can be increased 1° by
slightly raising the TE of both ailerons.
For a typical Giant Scale model, this is
less than 3/16 inch. Later, if stalls and tight
turns are acceptable, lower the ailerons
in small increments until they are back
to neutral.
Questions or Comments
I would be happy to respond to
your comments and questions. You can
contact me via the website listed in
“Sources” or through email.
Thanks to the following for technical
assistance: Joe Grice, Scott Russell, Tony
Paladino, and Jon Bomers.
—David Andersen
[email protected]
SourceS:
Minnesota Scale and Giant Scale r/c
www.mnbigbirds.com
Washout can be added after construction by
slightly raising both ailerons. This is recommended
for the maiden flights of a new model.
Washout in the Howard Pete’s wingtip is formed by shaping the LE in the outer rib bay.
Leo Spychalla’s Ziroli Stuka has a gentle
stall despite its pointed wings. The wings
have 4° of washout, starting outboard of the
landing gear.