by Frank Granelli
May 2005 33
IT HAS BEEN a long, but hopefully educational and not too
difficult, road to this point. In the preceding “From the Ground Up”
installments, Bob Aberle covered what an airborne radio-control
system is, how to choose one, how to install one, and how to keep
one working well. He also covered electric motors, batteries, and the
basics of electric-powered RC flight. The articles before those
detailed the mysteries of the modern glow engine in all its
incarnations.
If, at times, it seems that there is more information than you really
want, it is because the “From the Ground Up” concept is to present
everything that a new RC pilot will need to know in the first few
years. It is hoped that learning the technical aspects of model aviation
from these articles or from fellow pilots might be more fun—and less
expensive—than being educated by “Fractured Balsa University.”
With this in mind, let’s move on.
At this point we have an engine and a radio system. Nice, but we
need something to house these items. It would be convenient if this
housing for the engine and radio were also able to lift the assembly
off of the ground.
There is such a structure, and we call it the airframe, the model,
the airplane, or other less-polite names when all is not working well.
Whatever it is called, the airframe is the next topic. Specifically, I
will be discussing ARF and RTF aircraft.
Yes, there are many other types of model airplanes. Some are
built from wood kits—assembled one stick at a time—and then
covered. Some consist of fiberglass fuselages and Styrofoam (foam)
wing cores with balsa sheeting. Some are built straight from plans. In
that process, the builder cuts the wood into all the pieces of a kit and
then starts the assembly. Last, there are modelers who design their
own aircraft, draw the plans, create the kit, and then build it. Future
“From the Ground Up” installments will detail all of these airplanes.
The following articles will cover the types of airframes available
and which are best suited to learning the basics of RC flight. Along
the way I will include some thoughts about how to modify RTF and
ARF airframes to improve performance and durability. Proper
balancing is important, as is building the airframe straight and true.
RTF and ARF appearances are usually predetermined, but there
are some ways in which even beginning pilots can make minor
changes to the looks to set their aircraft apart from all the others.
Radio setup can be optimized on many models—even on some RTF
airframes.
The sheer number of ARF and RTF aircraft is astounding. There
are Scale models that replicate full-scale airplanes and competition
models meant for aerobatic-performance contests. There are basic
trainers, biplanes, second aircraft, sport models, 3-D performers, and
some that defy any logical description.
All of those accomplish their design tasks far better than their
compatriots of just 20 years ago. But new RC pilots will usually find
it best to begin with a “basic trainer”-type aircraft.
Why begin with a good basic trainer design? It is possible to learn
RC piloting on any type of model, but sport or Scale aircraft react
quickly to control inputs, leaving less time for the new pilot to plan
the next control inputs. Also, takeoff, in-flight, and landing speeds
can be higher with the more reactive models.
Sport and Scale airplanes demand more from the pilot than most
basic trainers. In addition, their higher airspeeds and quicker control
responses mean that this “more from the pilot” must happen more
quickly. This requirement that the pilot be “further ahead of the
airplane” makes learning to fly RC with a sport airplane more
difficult and time consuming.
There are so many excellent sport ARF designs that make great
advanced trainers or second models that many new pilots want to
use them as their basic trainers. This is possible, but sport aircraft
have certain drawbacks in that capacity.
Although high-wing advanced trainers such as the Midwest
Aerobat, which is shown, may resemble basic trainers and even have
similar wing loadings (the weight each square foot of wing area
must support), their design criteria are different, as are their wings’
airfoils.
The Aerobat and its cousins, which include Hobbico's Avistar,
Hobby Lobby’s Bonnie 20, and Hangar 9’s Arrow, feature
symmetrical (or semisymmetrical) wing airfoils that have less lift
than equivalent flat-bottom trainer airfoils. I will explain why
shortly. A symmetrical wing is curved equally on the top and
bottom, and a semisymmetrical wing is curved on the bottom but
more so on the top.
Although it resembles a basic RC trainer, the Piper Cub has a big wing with ailerons near the wingtips, causing adverse yaw.
Narrow landing gear results in poor ground handling. Mark Lanterman photo and aircraft.
34 MODEL AVIATION
Sport aircraft such as this Sig Four-Star 60, with its
symmetrical airfoil, are somewhat gentle but fly faster and
respond quickly. They are designed for aerobatic flight
regimes—not basic training chores.
Slower, yet honest and responsive, basic trainers such as
Hangar 9's RTF Alpha 60 can take a new pilot from starting out
to solo flight in just weeks. Notice the flat-bottom wing.
Hobbico's NexSTAR keeps flying even at 6 mph. This RTF
basic trainer features high-lift wing devices, flaps to control
airspeed, and three-axis flight-stabilizing system. It also comes
with PC-based flight simulator.
SuperStar EP, offered in RTF and ARF format, is three- or fourchannel,
electric-powered basic trainer that handles and
performs like a glow-powered aircraft. Flight times can reach
seven minutes.
The symmetrical wing allows for easy inverted flight, Outside
Loops (loops with the airplane upside-down, performed using downelevator),
faster aileron response, and good Snap Roll/Spin
performance. But landing speeds are higher and the symmetricalairfoil
wing is more responsive, making these second aircraft sensitive
to control inputs.
The symmetrical airfoil actually has slightly less air drag than an
equivalent flat-bottom wing. This means that the aircraft gains more
airspeed than a basic trainer if the student pilot lets the nose drop in a
turn. Aerobatic trainers will not “balloon” as much (raise the nose and
climb) in this situation as basic trainers would. That is a plus for them.
However, the increasingly rapid descent in the turn usually causes
the new pilot to input substantial up-elevator, causing the aircraft to
rocket upward or tighten the radius of the turn. Learning to make level
turns is the first step in becoming an RC pilot, and aerobatic trainers
make that more difficult.
These second airplanes are designed to take a newly soloed pilot
beyond basic flight and into the exciting world of aerobatic flying—
not to be the best basic trainer.
Some Scale aircraft would seem to be ideal basic trainers and do
use flat-bottom airfoils, but they are better suited as second models.
The Piper Cub, for instance, is extremely short coupled; the tail is
close to the wing, given its wing’s large span and width (wing chord).
Without perfectly set aileron differential, one aileron moves upward
much farther than the other moves downward, and the Cub needs
coordinated rudder/aileron input to make proper turns. Coordinated
turns and aileron differential can be daunting for most new RC pilots
and is seldom necessary in a basic trainer.
The Cub’s short fuselage and narrow landing gear make ground
handling troublesome for new pilots. Its light wing loading and large
wingspan compound landing difficulties because the aircraft tends to
land on the main gear and keep the tail up. This usually means that the
wing drops to one side, pulling the aircraft off in that direction, or the
model ends up on its back.
Yes, a new pilot can begin on a Cub or similar Scale model, but
learning is easier on a basic trainer that is designed expressly for
teaching RC piloting.
A good basic trainer has several design features to ease this learning
process. Most important is that the wing usually has a flat bottom and
a curved, or airfoil-shaped, top section.
As far as I know, only one basic trainer—Hobbico’s Hobbistar
60—uses a semisymmetrical airfoil, and its wing is made overly large
to compensate. Without delving into total eye-numbing detail about
why an aircraft flies, this type of wing produces more lift than a
similar symmetrical wing with equal airfoil shapes on both sides.
The air flowing over the curved top section of a flat-bottom wing
must move faster to cover the longer, curved distance than the air
Photos by the author except as noted
May 2005 35
HobbyZone's Firebird Commander RTF electric-powered basic
trainer teaches absolute basics of RC flight and seldom
requires an instructor. It is ideal for sampling RC flight before
making a full commitment.
High-performance advanced trainers can also be electricpowered,
such as Hobby Lobby's Bonnie 20, shown
temporarily on floats. With proper motor and batteries, it can
fly for as long as 16 minutes.
Midwest's Aero-Star 40, with flat-bottom and extra large wing,
is one of the prototypical 40-size trainers that many RC pilots
use to earn their wings.
Basic trainers are not necessarily boring. The Lanier Explorer 40
performs simple rolls after completing three consecutive loops.
flowing over the straight bottom. According to Daniel Bernoulli’s
theorem of gaseous density, the faster a given amount of gas moves,
the less dense it must become if all other conditions remain the
same.
If the air above the wing has reduced density, the pressure it
exerts on the wing’s top is lower. This means that there is a “lowpressure”
area above the wing. The wing tends to move upward into
the low-pressure area, taking the rest of the aircraft with it.
But then how does an aircraft with a fully symmetrical wing fly?
Daniel Bernoulli is on vacation where this wing is concerned, but,
fortunately for all sport and Aerobatics pilots, Sir Isaac Newton
remains in the house.
If the wing is pointed upward to the airflow even a few
degrees—called a positive angle of attack—much of the air striking
the wing’s LE is redirected downward. But Sir Newton demands
that for every action (here the redirection downward) there is an
equal and opposite reaction. Therefore, the wing is “pushed”
upward, creating lift.
This is all a gross oversimplification of why an aircraft flies.
There is still much debate about this subject even after 102 years.
However, this basic explanation serves to illustrate why a flatbottom
wing has more lift per square foot than a symmetrical airfoil.
Of course, Newton’s law also affects the flat-bottom airfoil--
maybe even to a greater degree than it does a fully symmetrical
wing. But Bernoulli and Newton work together on a flat-bottom
wing, generating that extra lift.
Extra lift means slower takeoff and landing speeds, lower stall
speeds (the speed at which the wing stops producing lift and begins
resembling an anvil), fewer bad habits such as snap stalls in tight
turns, and generally an all-round more rewarding learning
experience.
It helps if the wing is placed on top of the fuselage (a high-wing
configuration) and has some positive dihedral; the wing is bent in
the middle so that the wingtips are higher than the center-section.
The fuselage below the high wing imparts a slight pendulum
effect—but every bit counts in a good trainer. The dihedral
reinforces this pendulum effect.
Combining these two features helps to create an aircraft that will
tend to stay in wings-level flight during straight flight and loops.
They also help the pilot recover the aircraft from most turns. Fewer
pilot corrections are therefore required in all of these flight regimes.
Dihedral has other effects, but some are not so beneficial. It
allows the rudder to be used to bank and turn the aircraft better, but
too much results in a model that requires continuous aileron input to
remain in a banked attitude. This is bad because it teaches a new RC
pilot bad habits. On the other hand, a reasonable amount of dihedral,
such as the three inches on Hangar 9’s Alpha 60 RTF which is
shown, adds a great deal to a trainer's teaching abilities.
36 MODEL AVIATION
Midwest Aerobat is outstanding flier with semisymmetrical
wing. Twin aileron servos allow computer transmitter to use
ailerons as flaperons for extra lift, speed reduction, and
amazing attitude changes when coupled to elevator.
HobbiStar 60 Mk III uses slightly semisymmetrical airfoil, but
its overly large 875-square-inch wing area makes up for any
missing lift. Its airfoil does improve inverted flight
performance. Photo courtesy Ron Farkas.
Since air must travel a longer distance over the curved top of a
flat-bottom wing in the same amount of time, it must move
faster than the air below the wing. The faster the air moves, the
lower the density.
Top and bottom, air moves the same speed over a symmetrical
airfoil. Therefore, the wing must be pointed up to have any lift.
Total lift is less than with a flat-bottom wing, but the symmetrical
wing can fly equally well upright or inverted.
Power-plant selection is also important when studying a basic
trainer. As far as I know, all RTF basic trainers that are powered by
engines are two-stroke designs; these include Hobbico’s NexSTAR,
which is shown. If the pilot prefers to use a four-stroke engine, he or
she must select an ARF.
There are also a few electric-powered basic trainers. Some, such
as Hobbico’s SuperStar EP Select, which is shown, are complete,
four-channel RTFs. Others, such as Horizon Hobby’s HobbyZone
Firebird Commander, also shown, are basic, two-channel RTFs.
Engine-powered ARF basic trainers such as Midwest’s Aero-Star
40 and Lanier’s Explorer 40, which are both shown, offer the new
RC pilot choices of radio system and engines that are unavailable in
RTF models. However, this luxury comes at the cost of additional
assembly work that could require basic model-building skills. RTFs
require only that the new pilot be vaguely familiar with which end of
the screwdriver points toward the work.
The amount and type of assembly work involved in ARFs and
RTFs (it’s hard to call preparing those models for flight “building”)
is all that separates the two. As you can see in the photos, they look
almost identical. And the two types do fly the same since there are
few airframe-performance differences. There are performance
differences, however, if the pilot equips an ARF with more powerful
engines/motors and more capable radio systems than are usually
found in an RTF.
With the exceptions of the Hobbico NexSTAR powered by the
O.S. Max .46 Fxi engine, the HobbiStar 60 Mk III using the O.S.
Max .60 LA engine, and the Hangar 9 Alpha 60 equipped with the
Evolution .61 engine, all RTF glow-powered aircraft currently use
.40 cu. in. power plants. No matter how good a .40 might be, and all
of today’s engines are good, a hot .46 offers more performance.
Except for the Hangar 9 Extra Easy 2 with the five-channel JR
XF-421 computer radio system, all RTF basic trainers use analog
four-channel radio systems for control. The NexSTAR does have an
installed flight-stabilization system, which is similar to an autopilot
but without direction control, but its transmitter remains a good fourchannel
analog system.
Whether a pilot chooses an RTF or an ARF airframe as a basic
trainer is his or her choice. Both offer excellent aircraft and
performance. But as good as these aircraft are, there is always room
for improvement and for pilot individuality.
Next month I’ll build an RTF trainer and make a few easy
improvements. Following that, I’ll look at the ARF world and show
you how to make these fine aircraft look different and perform better.
Until next time, you can review many of these aircraft on Sport
Aviator, MA’s online magazine, at www.masportaviator.com. MA
Edition: Model Aviation - 2005/05
Page Numbers: 33,34,35,36
Edition: Model Aviation - 2005/05
Page Numbers: 33,34,35,36
by Frank Granelli
May 2005 33
IT HAS BEEN a long, but hopefully educational and not too
difficult, road to this point. In the preceding “From the Ground Up”
installments, Bob Aberle covered what an airborne radio-control
system is, how to choose one, how to install one, and how to keep
one working well. He also covered electric motors, batteries, and the
basics of electric-powered RC flight. The articles before those
detailed the mysteries of the modern glow engine in all its
incarnations.
If, at times, it seems that there is more information than you really
want, it is because the “From the Ground Up” concept is to present
everything that a new RC pilot will need to know in the first few
years. It is hoped that learning the technical aspects of model aviation
from these articles or from fellow pilots might be more fun—and less
expensive—than being educated by “Fractured Balsa University.”
With this in mind, let’s move on.
At this point we have an engine and a radio system. Nice, but we
need something to house these items. It would be convenient if this
housing for the engine and radio were also able to lift the assembly
off of the ground.
There is such a structure, and we call it the airframe, the model,
the airplane, or other less-polite names when all is not working well.
Whatever it is called, the airframe is the next topic. Specifically, I
will be discussing ARF and RTF aircraft.
Yes, there are many other types of model airplanes. Some are
built from wood kits—assembled one stick at a time—and then
covered. Some consist of fiberglass fuselages and Styrofoam (foam)
wing cores with balsa sheeting. Some are built straight from plans. In
that process, the builder cuts the wood into all the pieces of a kit and
then starts the assembly. Last, there are modelers who design their
own aircraft, draw the plans, create the kit, and then build it. Future
“From the Ground Up” installments will detail all of these airplanes.
The following articles will cover the types of airframes available
and which are best suited to learning the basics of RC flight. Along
the way I will include some thoughts about how to modify RTF and
ARF airframes to improve performance and durability. Proper
balancing is important, as is building the airframe straight and true.
RTF and ARF appearances are usually predetermined, but there
are some ways in which even beginning pilots can make minor
changes to the looks to set their aircraft apart from all the others.
Radio setup can be optimized on many models—even on some RTF
airframes.
The sheer number of ARF and RTF aircraft is astounding. There
are Scale models that replicate full-scale airplanes and competition
models meant for aerobatic-performance contests. There are basic
trainers, biplanes, second aircraft, sport models, 3-D performers, and
some that defy any logical description.
All of those accomplish their design tasks far better than their
compatriots of just 20 years ago. But new RC pilots will usually find
it best to begin with a “basic trainer”-type aircraft.
Why begin with a good basic trainer design? It is possible to learn
RC piloting on any type of model, but sport or Scale aircraft react
quickly to control inputs, leaving less time for the new pilot to plan
the next control inputs. Also, takeoff, in-flight, and landing speeds
can be higher with the more reactive models.
Sport and Scale airplanes demand more from the pilot than most
basic trainers. In addition, their higher airspeeds and quicker control
responses mean that this “more from the pilot” must happen more
quickly. This requirement that the pilot be “further ahead of the
airplane” makes learning to fly RC with a sport airplane more
difficult and time consuming.
There are so many excellent sport ARF designs that make great
advanced trainers or second models that many new pilots want to
use them as their basic trainers. This is possible, but sport aircraft
have certain drawbacks in that capacity.
Although high-wing advanced trainers such as the Midwest
Aerobat, which is shown, may resemble basic trainers and even have
similar wing loadings (the weight each square foot of wing area
must support), their design criteria are different, as are their wings’
airfoils.
The Aerobat and its cousins, which include Hobbico's Avistar,
Hobby Lobby’s Bonnie 20, and Hangar 9’s Arrow, feature
symmetrical (or semisymmetrical) wing airfoils that have less lift
than equivalent flat-bottom trainer airfoils. I will explain why
shortly. A symmetrical wing is curved equally on the top and
bottom, and a semisymmetrical wing is curved on the bottom but
more so on the top.
Although it resembles a basic RC trainer, the Piper Cub has a big wing with ailerons near the wingtips, causing adverse yaw.
Narrow landing gear results in poor ground handling. Mark Lanterman photo and aircraft.
34 MODEL AVIATION
Sport aircraft such as this Sig Four-Star 60, with its
symmetrical airfoil, are somewhat gentle but fly faster and
respond quickly. They are designed for aerobatic flight
regimes—not basic training chores.
Slower, yet honest and responsive, basic trainers such as
Hangar 9's RTF Alpha 60 can take a new pilot from starting out
to solo flight in just weeks. Notice the flat-bottom wing.
Hobbico's NexSTAR keeps flying even at 6 mph. This RTF
basic trainer features high-lift wing devices, flaps to control
airspeed, and three-axis flight-stabilizing system. It also comes
with PC-based flight simulator.
SuperStar EP, offered in RTF and ARF format, is three- or fourchannel,
electric-powered basic trainer that handles and
performs like a glow-powered aircraft. Flight times can reach
seven minutes.
The symmetrical wing allows for easy inverted flight, Outside
Loops (loops with the airplane upside-down, performed using downelevator),
faster aileron response, and good Snap Roll/Spin
performance. But landing speeds are higher and the symmetricalairfoil
wing is more responsive, making these second aircraft sensitive
to control inputs.
The symmetrical airfoil actually has slightly less air drag than an
equivalent flat-bottom wing. This means that the aircraft gains more
airspeed than a basic trainer if the student pilot lets the nose drop in a
turn. Aerobatic trainers will not “balloon” as much (raise the nose and
climb) in this situation as basic trainers would. That is a plus for them.
However, the increasingly rapid descent in the turn usually causes
the new pilot to input substantial up-elevator, causing the aircraft to
rocket upward or tighten the radius of the turn. Learning to make level
turns is the first step in becoming an RC pilot, and aerobatic trainers
make that more difficult.
These second airplanes are designed to take a newly soloed pilot
beyond basic flight and into the exciting world of aerobatic flying—
not to be the best basic trainer.
Some Scale aircraft would seem to be ideal basic trainers and do
use flat-bottom airfoils, but they are better suited as second models.
The Piper Cub, for instance, is extremely short coupled; the tail is
close to the wing, given its wing’s large span and width (wing chord).
Without perfectly set aileron differential, one aileron moves upward
much farther than the other moves downward, and the Cub needs
coordinated rudder/aileron input to make proper turns. Coordinated
turns and aileron differential can be daunting for most new RC pilots
and is seldom necessary in a basic trainer.
The Cub’s short fuselage and narrow landing gear make ground
handling troublesome for new pilots. Its light wing loading and large
wingspan compound landing difficulties because the aircraft tends to
land on the main gear and keep the tail up. This usually means that the
wing drops to one side, pulling the aircraft off in that direction, or the
model ends up on its back.
Yes, a new pilot can begin on a Cub or similar Scale model, but
learning is easier on a basic trainer that is designed expressly for
teaching RC piloting.
A good basic trainer has several design features to ease this learning
process. Most important is that the wing usually has a flat bottom and
a curved, or airfoil-shaped, top section.
As far as I know, only one basic trainer—Hobbico’s Hobbistar
60—uses a semisymmetrical airfoil, and its wing is made overly large
to compensate. Without delving into total eye-numbing detail about
why an aircraft flies, this type of wing produces more lift than a
similar symmetrical wing with equal airfoil shapes on both sides.
The air flowing over the curved top section of a flat-bottom wing
must move faster to cover the longer, curved distance than the air
Photos by the author except as noted
May 2005 35
HobbyZone's Firebird Commander RTF electric-powered basic
trainer teaches absolute basics of RC flight and seldom
requires an instructor. It is ideal for sampling RC flight before
making a full commitment.
High-performance advanced trainers can also be electricpowered,
such as Hobby Lobby's Bonnie 20, shown
temporarily on floats. With proper motor and batteries, it can
fly for as long as 16 minutes.
Midwest's Aero-Star 40, with flat-bottom and extra large wing,
is one of the prototypical 40-size trainers that many RC pilots
use to earn their wings.
Basic trainers are not necessarily boring. The Lanier Explorer 40
performs simple rolls after completing three consecutive loops.
flowing over the straight bottom. According to Daniel Bernoulli’s
theorem of gaseous density, the faster a given amount of gas moves,
the less dense it must become if all other conditions remain the
same.
If the air above the wing has reduced density, the pressure it
exerts on the wing’s top is lower. This means that there is a “lowpressure”
area above the wing. The wing tends to move upward into
the low-pressure area, taking the rest of the aircraft with it.
But then how does an aircraft with a fully symmetrical wing fly?
Daniel Bernoulli is on vacation where this wing is concerned, but,
fortunately for all sport and Aerobatics pilots, Sir Isaac Newton
remains in the house.
If the wing is pointed upward to the airflow even a few
degrees—called a positive angle of attack—much of the air striking
the wing’s LE is redirected downward. But Sir Newton demands
that for every action (here the redirection downward) there is an
equal and opposite reaction. Therefore, the wing is “pushed”
upward, creating lift.
This is all a gross oversimplification of why an aircraft flies.
There is still much debate about this subject even after 102 years.
However, this basic explanation serves to illustrate why a flatbottom
wing has more lift per square foot than a symmetrical airfoil.
Of course, Newton’s law also affects the flat-bottom airfoil--
maybe even to a greater degree than it does a fully symmetrical
wing. But Bernoulli and Newton work together on a flat-bottom
wing, generating that extra lift.
Extra lift means slower takeoff and landing speeds, lower stall
speeds (the speed at which the wing stops producing lift and begins
resembling an anvil), fewer bad habits such as snap stalls in tight
turns, and generally an all-round more rewarding learning
experience.
It helps if the wing is placed on top of the fuselage (a high-wing
configuration) and has some positive dihedral; the wing is bent in
the middle so that the wingtips are higher than the center-section.
The fuselage below the high wing imparts a slight pendulum
effect—but every bit counts in a good trainer. The dihedral
reinforces this pendulum effect.
Combining these two features helps to create an aircraft that will
tend to stay in wings-level flight during straight flight and loops.
They also help the pilot recover the aircraft from most turns. Fewer
pilot corrections are therefore required in all of these flight regimes.
Dihedral has other effects, but some are not so beneficial. It
allows the rudder to be used to bank and turn the aircraft better, but
too much results in a model that requires continuous aileron input to
remain in a banked attitude. This is bad because it teaches a new RC
pilot bad habits. On the other hand, a reasonable amount of dihedral,
such as the three inches on Hangar 9’s Alpha 60 RTF which is
shown, adds a great deal to a trainer's teaching abilities.
36 MODEL AVIATION
Midwest Aerobat is outstanding flier with semisymmetrical
wing. Twin aileron servos allow computer transmitter to use
ailerons as flaperons for extra lift, speed reduction, and
amazing attitude changes when coupled to elevator.
HobbiStar 60 Mk III uses slightly semisymmetrical airfoil, but
its overly large 875-square-inch wing area makes up for any
missing lift. Its airfoil does improve inverted flight
performance. Photo courtesy Ron Farkas.
Since air must travel a longer distance over the curved top of a
flat-bottom wing in the same amount of time, it must move
faster than the air below the wing. The faster the air moves, the
lower the density.
Top and bottom, air moves the same speed over a symmetrical
airfoil. Therefore, the wing must be pointed up to have any lift.
Total lift is less than with a flat-bottom wing, but the symmetrical
wing can fly equally well upright or inverted.
Power-plant selection is also important when studying a basic
trainer. As far as I know, all RTF basic trainers that are powered by
engines are two-stroke designs; these include Hobbico’s NexSTAR,
which is shown. If the pilot prefers to use a four-stroke engine, he or
she must select an ARF.
There are also a few electric-powered basic trainers. Some, such
as Hobbico’s SuperStar EP Select, which is shown, are complete,
four-channel RTFs. Others, such as Horizon Hobby’s HobbyZone
Firebird Commander, also shown, are basic, two-channel RTFs.
Engine-powered ARF basic trainers such as Midwest’s Aero-Star
40 and Lanier’s Explorer 40, which are both shown, offer the new
RC pilot choices of radio system and engines that are unavailable in
RTF models. However, this luxury comes at the cost of additional
assembly work that could require basic model-building skills. RTFs
require only that the new pilot be vaguely familiar with which end of
the screwdriver points toward the work.
The amount and type of assembly work involved in ARFs and
RTFs (it’s hard to call preparing those models for flight “building”)
is all that separates the two. As you can see in the photos, they look
almost identical. And the two types do fly the same since there are
few airframe-performance differences. There are performance
differences, however, if the pilot equips an ARF with more powerful
engines/motors and more capable radio systems than are usually
found in an RTF.
With the exceptions of the Hobbico NexSTAR powered by the
O.S. Max .46 Fxi engine, the HobbiStar 60 Mk III using the O.S.
Max .60 LA engine, and the Hangar 9 Alpha 60 equipped with the
Evolution .61 engine, all RTF glow-powered aircraft currently use
.40 cu. in. power plants. No matter how good a .40 might be, and all
of today’s engines are good, a hot .46 offers more performance.
Except for the Hangar 9 Extra Easy 2 with the five-channel JR
XF-421 computer radio system, all RTF basic trainers use analog
four-channel radio systems for control. The NexSTAR does have an
installed flight-stabilization system, which is similar to an autopilot
but without direction control, but its transmitter remains a good fourchannel
analog system.
Whether a pilot chooses an RTF or an ARF airframe as a basic
trainer is his or her choice. Both offer excellent aircraft and
performance. But as good as these aircraft are, there is always room
for improvement and for pilot individuality.
Next month I’ll build an RTF trainer and make a few easy
improvements. Following that, I’ll look at the ARF world and show
you how to make these fine aircraft look different and perform better.
Until next time, you can review many of these aircraft on Sport
Aviator, MA’s online magazine, at www.masportaviator.com. MA
Edition: Model Aviation - 2005/05
Page Numbers: 33,34,35,36
by Frank Granelli
May 2005 33
IT HAS BEEN a long, but hopefully educational and not too
difficult, road to this point. In the preceding “From the Ground Up”
installments, Bob Aberle covered what an airborne radio-control
system is, how to choose one, how to install one, and how to keep
one working well. He also covered electric motors, batteries, and the
basics of electric-powered RC flight. The articles before those
detailed the mysteries of the modern glow engine in all its
incarnations.
If, at times, it seems that there is more information than you really
want, it is because the “From the Ground Up” concept is to present
everything that a new RC pilot will need to know in the first few
years. It is hoped that learning the technical aspects of model aviation
from these articles or from fellow pilots might be more fun—and less
expensive—than being educated by “Fractured Balsa University.”
With this in mind, let’s move on.
At this point we have an engine and a radio system. Nice, but we
need something to house these items. It would be convenient if this
housing for the engine and radio were also able to lift the assembly
off of the ground.
There is such a structure, and we call it the airframe, the model,
the airplane, or other less-polite names when all is not working well.
Whatever it is called, the airframe is the next topic. Specifically, I
will be discussing ARF and RTF aircraft.
Yes, there are many other types of model airplanes. Some are
built from wood kits—assembled one stick at a time—and then
covered. Some consist of fiberglass fuselages and Styrofoam (foam)
wing cores with balsa sheeting. Some are built straight from plans. In
that process, the builder cuts the wood into all the pieces of a kit and
then starts the assembly. Last, there are modelers who design their
own aircraft, draw the plans, create the kit, and then build it. Future
“From the Ground Up” installments will detail all of these airplanes.
The following articles will cover the types of airframes available
and which are best suited to learning the basics of RC flight. Along
the way I will include some thoughts about how to modify RTF and
ARF airframes to improve performance and durability. Proper
balancing is important, as is building the airframe straight and true.
RTF and ARF appearances are usually predetermined, but there
are some ways in which even beginning pilots can make minor
changes to the looks to set their aircraft apart from all the others.
Radio setup can be optimized on many models—even on some RTF
airframes.
The sheer number of ARF and RTF aircraft is astounding. There
are Scale models that replicate full-scale airplanes and competition
models meant for aerobatic-performance contests. There are basic
trainers, biplanes, second aircraft, sport models, 3-D performers, and
some that defy any logical description.
All of those accomplish their design tasks far better than their
compatriots of just 20 years ago. But new RC pilots will usually find
it best to begin with a “basic trainer”-type aircraft.
Why begin with a good basic trainer design? It is possible to learn
RC piloting on any type of model, but sport or Scale aircraft react
quickly to control inputs, leaving less time for the new pilot to plan
the next control inputs. Also, takeoff, in-flight, and landing speeds
can be higher with the more reactive models.
Sport and Scale airplanes demand more from the pilot than most
basic trainers. In addition, their higher airspeeds and quicker control
responses mean that this “more from the pilot” must happen more
quickly. This requirement that the pilot be “further ahead of the
airplane” makes learning to fly RC with a sport airplane more
difficult and time consuming.
There are so many excellent sport ARF designs that make great
advanced trainers or second models that many new pilots want to
use them as their basic trainers. This is possible, but sport aircraft
have certain drawbacks in that capacity.
Although high-wing advanced trainers such as the Midwest
Aerobat, which is shown, may resemble basic trainers and even have
similar wing loadings (the weight each square foot of wing area
must support), their design criteria are different, as are their wings’
airfoils.
The Aerobat and its cousins, which include Hobbico's Avistar,
Hobby Lobby’s Bonnie 20, and Hangar 9’s Arrow, feature
symmetrical (or semisymmetrical) wing airfoils that have less lift
than equivalent flat-bottom trainer airfoils. I will explain why
shortly. A symmetrical wing is curved equally on the top and
bottom, and a semisymmetrical wing is curved on the bottom but
more so on the top.
Although it resembles a basic RC trainer, the Piper Cub has a big wing with ailerons near the wingtips, causing adverse yaw.
Narrow landing gear results in poor ground handling. Mark Lanterman photo and aircraft.
34 MODEL AVIATION
Sport aircraft such as this Sig Four-Star 60, with its
symmetrical airfoil, are somewhat gentle but fly faster and
respond quickly. They are designed for aerobatic flight
regimes—not basic training chores.
Slower, yet honest and responsive, basic trainers such as
Hangar 9's RTF Alpha 60 can take a new pilot from starting out
to solo flight in just weeks. Notice the flat-bottom wing.
Hobbico's NexSTAR keeps flying even at 6 mph. This RTF
basic trainer features high-lift wing devices, flaps to control
airspeed, and three-axis flight-stabilizing system. It also comes
with PC-based flight simulator.
SuperStar EP, offered in RTF and ARF format, is three- or fourchannel,
electric-powered basic trainer that handles and
performs like a glow-powered aircraft. Flight times can reach
seven minutes.
The symmetrical wing allows for easy inverted flight, Outside
Loops (loops with the airplane upside-down, performed using downelevator),
faster aileron response, and good Snap Roll/Spin
performance. But landing speeds are higher and the symmetricalairfoil
wing is more responsive, making these second aircraft sensitive
to control inputs.
The symmetrical airfoil actually has slightly less air drag than an
equivalent flat-bottom wing. This means that the aircraft gains more
airspeed than a basic trainer if the student pilot lets the nose drop in a
turn. Aerobatic trainers will not “balloon” as much (raise the nose and
climb) in this situation as basic trainers would. That is a plus for them.
However, the increasingly rapid descent in the turn usually causes
the new pilot to input substantial up-elevator, causing the aircraft to
rocket upward or tighten the radius of the turn. Learning to make level
turns is the first step in becoming an RC pilot, and aerobatic trainers
make that more difficult.
These second airplanes are designed to take a newly soloed pilot
beyond basic flight and into the exciting world of aerobatic flying—
not to be the best basic trainer.
Some Scale aircraft would seem to be ideal basic trainers and do
use flat-bottom airfoils, but they are better suited as second models.
The Piper Cub, for instance, is extremely short coupled; the tail is
close to the wing, given its wing’s large span and width (wing chord).
Without perfectly set aileron differential, one aileron moves upward
much farther than the other moves downward, and the Cub needs
coordinated rudder/aileron input to make proper turns. Coordinated
turns and aileron differential can be daunting for most new RC pilots
and is seldom necessary in a basic trainer.
The Cub’s short fuselage and narrow landing gear make ground
handling troublesome for new pilots. Its light wing loading and large
wingspan compound landing difficulties because the aircraft tends to
land on the main gear and keep the tail up. This usually means that the
wing drops to one side, pulling the aircraft off in that direction, or the
model ends up on its back.
Yes, a new pilot can begin on a Cub or similar Scale model, but
learning is easier on a basic trainer that is designed expressly for
teaching RC piloting.
A good basic trainer has several design features to ease this learning
process. Most important is that the wing usually has a flat bottom and
a curved, or airfoil-shaped, top section.
As far as I know, only one basic trainer—Hobbico’s Hobbistar
60—uses a semisymmetrical airfoil, and its wing is made overly large
to compensate. Without delving into total eye-numbing detail about
why an aircraft flies, this type of wing produces more lift than a
similar symmetrical wing with equal airfoil shapes on both sides.
The air flowing over the curved top section of a flat-bottom wing
must move faster to cover the longer, curved distance than the air
Photos by the author except as noted
May 2005 35
HobbyZone's Firebird Commander RTF electric-powered basic
trainer teaches absolute basics of RC flight and seldom
requires an instructor. It is ideal for sampling RC flight before
making a full commitment.
High-performance advanced trainers can also be electricpowered,
such as Hobby Lobby's Bonnie 20, shown
temporarily on floats. With proper motor and batteries, it can
fly for as long as 16 minutes.
Midwest's Aero-Star 40, with flat-bottom and extra large wing,
is one of the prototypical 40-size trainers that many RC pilots
use to earn their wings.
Basic trainers are not necessarily boring. The Lanier Explorer 40
performs simple rolls after completing three consecutive loops.
flowing over the straight bottom. According to Daniel Bernoulli’s
theorem of gaseous density, the faster a given amount of gas moves,
the less dense it must become if all other conditions remain the
same.
If the air above the wing has reduced density, the pressure it
exerts on the wing’s top is lower. This means that there is a “lowpressure”
area above the wing. The wing tends to move upward into
the low-pressure area, taking the rest of the aircraft with it.
But then how does an aircraft with a fully symmetrical wing fly?
Daniel Bernoulli is on vacation where this wing is concerned, but,
fortunately for all sport and Aerobatics pilots, Sir Isaac Newton
remains in the house.
If the wing is pointed upward to the airflow even a few
degrees—called a positive angle of attack—much of the air striking
the wing’s LE is redirected downward. But Sir Newton demands
that for every action (here the redirection downward) there is an
equal and opposite reaction. Therefore, the wing is “pushed”
upward, creating lift.
This is all a gross oversimplification of why an aircraft flies.
There is still much debate about this subject even after 102 years.
However, this basic explanation serves to illustrate why a flatbottom
wing has more lift per square foot than a symmetrical airfoil.
Of course, Newton’s law also affects the flat-bottom airfoil--
maybe even to a greater degree than it does a fully symmetrical
wing. But Bernoulli and Newton work together on a flat-bottom
wing, generating that extra lift.
Extra lift means slower takeoff and landing speeds, lower stall
speeds (the speed at which the wing stops producing lift and begins
resembling an anvil), fewer bad habits such as snap stalls in tight
turns, and generally an all-round more rewarding learning
experience.
It helps if the wing is placed on top of the fuselage (a high-wing
configuration) and has some positive dihedral; the wing is bent in
the middle so that the wingtips are higher than the center-section.
The fuselage below the high wing imparts a slight pendulum
effect—but every bit counts in a good trainer. The dihedral
reinforces this pendulum effect.
Combining these two features helps to create an aircraft that will
tend to stay in wings-level flight during straight flight and loops.
They also help the pilot recover the aircraft from most turns. Fewer
pilot corrections are therefore required in all of these flight regimes.
Dihedral has other effects, but some are not so beneficial. It
allows the rudder to be used to bank and turn the aircraft better, but
too much results in a model that requires continuous aileron input to
remain in a banked attitude. This is bad because it teaches a new RC
pilot bad habits. On the other hand, a reasonable amount of dihedral,
such as the three inches on Hangar 9’s Alpha 60 RTF which is
shown, adds a great deal to a trainer's teaching abilities.
36 MODEL AVIATION
Midwest Aerobat is outstanding flier with semisymmetrical
wing. Twin aileron servos allow computer transmitter to use
ailerons as flaperons for extra lift, speed reduction, and
amazing attitude changes when coupled to elevator.
HobbiStar 60 Mk III uses slightly semisymmetrical airfoil, but
its overly large 875-square-inch wing area makes up for any
missing lift. Its airfoil does improve inverted flight
performance. Photo courtesy Ron Farkas.
Since air must travel a longer distance over the curved top of a
flat-bottom wing in the same amount of time, it must move
faster than the air below the wing. The faster the air moves, the
lower the density.
Top and bottom, air moves the same speed over a symmetrical
airfoil. Therefore, the wing must be pointed up to have any lift.
Total lift is less than with a flat-bottom wing, but the symmetrical
wing can fly equally well upright or inverted.
Power-plant selection is also important when studying a basic
trainer. As far as I know, all RTF basic trainers that are powered by
engines are two-stroke designs; these include Hobbico’s NexSTAR,
which is shown. If the pilot prefers to use a four-stroke engine, he or
she must select an ARF.
There are also a few electric-powered basic trainers. Some, such
as Hobbico’s SuperStar EP Select, which is shown, are complete,
four-channel RTFs. Others, such as Horizon Hobby’s HobbyZone
Firebird Commander, also shown, are basic, two-channel RTFs.
Engine-powered ARF basic trainers such as Midwest’s Aero-Star
40 and Lanier’s Explorer 40, which are both shown, offer the new
RC pilot choices of radio system and engines that are unavailable in
RTF models. However, this luxury comes at the cost of additional
assembly work that could require basic model-building skills. RTFs
require only that the new pilot be vaguely familiar with which end of
the screwdriver points toward the work.
The amount and type of assembly work involved in ARFs and
RTFs (it’s hard to call preparing those models for flight “building”)
is all that separates the two. As you can see in the photos, they look
almost identical. And the two types do fly the same since there are
few airframe-performance differences. There are performance
differences, however, if the pilot equips an ARF with more powerful
engines/motors and more capable radio systems than are usually
found in an RTF.
With the exceptions of the Hobbico NexSTAR powered by the
O.S. Max .46 Fxi engine, the HobbiStar 60 Mk III using the O.S.
Max .60 LA engine, and the Hangar 9 Alpha 60 equipped with the
Evolution .61 engine, all RTF glow-powered aircraft currently use
.40 cu. in. power plants. No matter how good a .40 might be, and all
of today’s engines are good, a hot .46 offers more performance.
Except for the Hangar 9 Extra Easy 2 with the five-channel JR
XF-421 computer radio system, all RTF basic trainers use analog
four-channel radio systems for control. The NexSTAR does have an
installed flight-stabilization system, which is similar to an autopilot
but without direction control, but its transmitter remains a good fourchannel
analog system.
Whether a pilot chooses an RTF or an ARF airframe as a basic
trainer is his or her choice. Both offer excellent aircraft and
performance. But as good as these aircraft are, there is always room
for improvement and for pilot individuality.
Next month I’ll build an RTF trainer and make a few easy
improvements. Following that, I’ll look at the ARF world and show
you how to make these fine aircraft look different and perform better.
Until next time, you can review many of these aircraft on Sport
Aviator, MA’s online magazine, at www.masportaviator.com. MA
Edition: Model Aviation - 2005/05
Page Numbers: 33,34,35,36
by Frank Granelli
May 2005 33
IT HAS BEEN a long, but hopefully educational and not too
difficult, road to this point. In the preceding “From the Ground Up”
installments, Bob Aberle covered what an airborne radio-control
system is, how to choose one, how to install one, and how to keep
one working well. He also covered electric motors, batteries, and the
basics of electric-powered RC flight. The articles before those
detailed the mysteries of the modern glow engine in all its
incarnations.
If, at times, it seems that there is more information than you really
want, it is because the “From the Ground Up” concept is to present
everything that a new RC pilot will need to know in the first few
years. It is hoped that learning the technical aspects of model aviation
from these articles or from fellow pilots might be more fun—and less
expensive—than being educated by “Fractured Balsa University.”
With this in mind, let’s move on.
At this point we have an engine and a radio system. Nice, but we
need something to house these items. It would be convenient if this
housing for the engine and radio were also able to lift the assembly
off of the ground.
There is such a structure, and we call it the airframe, the model,
the airplane, or other less-polite names when all is not working well.
Whatever it is called, the airframe is the next topic. Specifically, I
will be discussing ARF and RTF aircraft.
Yes, there are many other types of model airplanes. Some are
built from wood kits—assembled one stick at a time—and then
covered. Some consist of fiberglass fuselages and Styrofoam (foam)
wing cores with balsa sheeting. Some are built straight from plans. In
that process, the builder cuts the wood into all the pieces of a kit and
then starts the assembly. Last, there are modelers who design their
own aircraft, draw the plans, create the kit, and then build it. Future
“From the Ground Up” installments will detail all of these airplanes.
The following articles will cover the types of airframes available
and which are best suited to learning the basics of RC flight. Along
the way I will include some thoughts about how to modify RTF and
ARF airframes to improve performance and durability. Proper
balancing is important, as is building the airframe straight and true.
RTF and ARF appearances are usually predetermined, but there
are some ways in which even beginning pilots can make minor
changes to the looks to set their aircraft apart from all the others.
Radio setup can be optimized on many models—even on some RTF
airframes.
The sheer number of ARF and RTF aircraft is astounding. There
are Scale models that replicate full-scale airplanes and competition
models meant for aerobatic-performance contests. There are basic
trainers, biplanes, second aircraft, sport models, 3-D performers, and
some that defy any logical description.
All of those accomplish their design tasks far better than their
compatriots of just 20 years ago. But new RC pilots will usually find
it best to begin with a “basic trainer”-type aircraft.
Why begin with a good basic trainer design? It is possible to learn
RC piloting on any type of model, but sport or Scale aircraft react
quickly to control inputs, leaving less time for the new pilot to plan
the next control inputs. Also, takeoff, in-flight, and landing speeds
can be higher with the more reactive models.
Sport and Scale airplanes demand more from the pilot than most
basic trainers. In addition, their higher airspeeds and quicker control
responses mean that this “more from the pilot” must happen more
quickly. This requirement that the pilot be “further ahead of the
airplane” makes learning to fly RC with a sport airplane more
difficult and time consuming.
There are so many excellent sport ARF designs that make great
advanced trainers or second models that many new pilots want to
use them as their basic trainers. This is possible, but sport aircraft
have certain drawbacks in that capacity.
Although high-wing advanced trainers such as the Midwest
Aerobat, which is shown, may resemble basic trainers and even have
similar wing loadings (the weight each square foot of wing area
must support), their design criteria are different, as are their wings’
airfoils.
The Aerobat and its cousins, which include Hobbico's Avistar,
Hobby Lobby’s Bonnie 20, and Hangar 9’s Arrow, feature
symmetrical (or semisymmetrical) wing airfoils that have less lift
than equivalent flat-bottom trainer airfoils. I will explain why
shortly. A symmetrical wing is curved equally on the top and
bottom, and a semisymmetrical wing is curved on the bottom but
more so on the top.
Although it resembles a basic RC trainer, the Piper Cub has a big wing with ailerons near the wingtips, causing adverse yaw.
Narrow landing gear results in poor ground handling. Mark Lanterman photo and aircraft.
34 MODEL AVIATION
Sport aircraft such as this Sig Four-Star 60, with its
symmetrical airfoil, are somewhat gentle but fly faster and
respond quickly. They are designed for aerobatic flight
regimes—not basic training chores.
Slower, yet honest and responsive, basic trainers such as
Hangar 9's RTF Alpha 60 can take a new pilot from starting out
to solo flight in just weeks. Notice the flat-bottom wing.
Hobbico's NexSTAR keeps flying even at 6 mph. This RTF
basic trainer features high-lift wing devices, flaps to control
airspeed, and three-axis flight-stabilizing system. It also comes
with PC-based flight simulator.
SuperStar EP, offered in RTF and ARF format, is three- or fourchannel,
electric-powered basic trainer that handles and
performs like a glow-powered aircraft. Flight times can reach
seven minutes.
The symmetrical wing allows for easy inverted flight, Outside
Loops (loops with the airplane upside-down, performed using downelevator),
faster aileron response, and good Snap Roll/Spin
performance. But landing speeds are higher and the symmetricalairfoil
wing is more responsive, making these second aircraft sensitive
to control inputs.
The symmetrical airfoil actually has slightly less air drag than an
equivalent flat-bottom wing. This means that the aircraft gains more
airspeed than a basic trainer if the student pilot lets the nose drop in a
turn. Aerobatic trainers will not “balloon” as much (raise the nose and
climb) in this situation as basic trainers would. That is a plus for them.
However, the increasingly rapid descent in the turn usually causes
the new pilot to input substantial up-elevator, causing the aircraft to
rocket upward or tighten the radius of the turn. Learning to make level
turns is the first step in becoming an RC pilot, and aerobatic trainers
make that more difficult.
These second airplanes are designed to take a newly soloed pilot
beyond basic flight and into the exciting world of aerobatic flying—
not to be the best basic trainer.
Some Scale aircraft would seem to be ideal basic trainers and do
use flat-bottom airfoils, but they are better suited as second models.
The Piper Cub, for instance, is extremely short coupled; the tail is
close to the wing, given its wing’s large span and width (wing chord).
Without perfectly set aileron differential, one aileron moves upward
much farther than the other moves downward, and the Cub needs
coordinated rudder/aileron input to make proper turns. Coordinated
turns and aileron differential can be daunting for most new RC pilots
and is seldom necessary in a basic trainer.
The Cub’s short fuselage and narrow landing gear make ground
handling troublesome for new pilots. Its light wing loading and large
wingspan compound landing difficulties because the aircraft tends to
land on the main gear and keep the tail up. This usually means that the
wing drops to one side, pulling the aircraft off in that direction, or the
model ends up on its back.
Yes, a new pilot can begin on a Cub or similar Scale model, but
learning is easier on a basic trainer that is designed expressly for
teaching RC piloting.
A good basic trainer has several design features to ease this learning
process. Most important is that the wing usually has a flat bottom and
a curved, or airfoil-shaped, top section.
As far as I know, only one basic trainer—Hobbico’s Hobbistar
60—uses a semisymmetrical airfoil, and its wing is made overly large
to compensate. Without delving into total eye-numbing detail about
why an aircraft flies, this type of wing produces more lift than a
similar symmetrical wing with equal airfoil shapes on both sides.
The air flowing over the curved top section of a flat-bottom wing
must move faster to cover the longer, curved distance than the air
Photos by the author except as noted
May 2005 35
HobbyZone's Firebird Commander RTF electric-powered basic
trainer teaches absolute basics of RC flight and seldom
requires an instructor. It is ideal for sampling RC flight before
making a full commitment.
High-performance advanced trainers can also be electricpowered,
such as Hobby Lobby's Bonnie 20, shown
temporarily on floats. With proper motor and batteries, it can
fly for as long as 16 minutes.
Midwest's Aero-Star 40, with flat-bottom and extra large wing,
is one of the prototypical 40-size trainers that many RC pilots
use to earn their wings.
Basic trainers are not necessarily boring. The Lanier Explorer 40
performs simple rolls after completing three consecutive loops.
flowing over the straight bottom. According to Daniel Bernoulli’s
theorem of gaseous density, the faster a given amount of gas moves,
the less dense it must become if all other conditions remain the
same.
If the air above the wing has reduced density, the pressure it
exerts on the wing’s top is lower. This means that there is a “lowpressure”
area above the wing. The wing tends to move upward into
the low-pressure area, taking the rest of the aircraft with it.
But then how does an aircraft with a fully symmetrical wing fly?
Daniel Bernoulli is on vacation where this wing is concerned, but,
fortunately for all sport and Aerobatics pilots, Sir Isaac Newton
remains in the house.
If the wing is pointed upward to the airflow even a few
degrees—called a positive angle of attack—much of the air striking
the wing’s LE is redirected downward. But Sir Newton demands
that for every action (here the redirection downward) there is an
equal and opposite reaction. Therefore, the wing is “pushed”
upward, creating lift.
This is all a gross oversimplification of why an aircraft flies.
There is still much debate about this subject even after 102 years.
However, this basic explanation serves to illustrate why a flatbottom
wing has more lift per square foot than a symmetrical airfoil.
Of course, Newton’s law also affects the flat-bottom airfoil--
maybe even to a greater degree than it does a fully symmetrical
wing. But Bernoulli and Newton work together on a flat-bottom
wing, generating that extra lift.
Extra lift means slower takeoff and landing speeds, lower stall
speeds (the speed at which the wing stops producing lift and begins
resembling an anvil), fewer bad habits such as snap stalls in tight
turns, and generally an all-round more rewarding learning
experience.
It helps if the wing is placed on top of the fuselage (a high-wing
configuration) and has some positive dihedral; the wing is bent in
the middle so that the wingtips are higher than the center-section.
The fuselage below the high wing imparts a slight pendulum
effect—but every bit counts in a good trainer. The dihedral
reinforces this pendulum effect.
Combining these two features helps to create an aircraft that will
tend to stay in wings-level flight during straight flight and loops.
They also help the pilot recover the aircraft from most turns. Fewer
pilot corrections are therefore required in all of these flight regimes.
Dihedral has other effects, but some are not so beneficial. It
allows the rudder to be used to bank and turn the aircraft better, but
too much results in a model that requires continuous aileron input to
remain in a banked attitude. This is bad because it teaches a new RC
pilot bad habits. On the other hand, a reasonable amount of dihedral,
such as the three inches on Hangar 9’s Alpha 60 RTF which is
shown, adds a great deal to a trainer's teaching abilities.
36 MODEL AVIATION
Midwest Aerobat is outstanding flier with semisymmetrical
wing. Twin aileron servos allow computer transmitter to use
ailerons as flaperons for extra lift, speed reduction, and
amazing attitude changes when coupled to elevator.
HobbiStar 60 Mk III uses slightly semisymmetrical airfoil, but
its overly large 875-square-inch wing area makes up for any
missing lift. Its airfoil does improve inverted flight
performance. Photo courtesy Ron Farkas.
Since air must travel a longer distance over the curved top of a
flat-bottom wing in the same amount of time, it must move
faster than the air below the wing. The faster the air moves, the
lower the density.
Top and bottom, air moves the same speed over a symmetrical
airfoil. Therefore, the wing must be pointed up to have any lift.
Total lift is less than with a flat-bottom wing, but the symmetrical
wing can fly equally well upright or inverted.
Power-plant selection is also important when studying a basic
trainer. As far as I know, all RTF basic trainers that are powered by
engines are two-stroke designs; these include Hobbico’s NexSTAR,
which is shown. If the pilot prefers to use a four-stroke engine, he or
she must select an ARF.
There are also a few electric-powered basic trainers. Some, such
as Hobbico’s SuperStar EP Select, which is shown, are complete,
four-channel RTFs. Others, such as Horizon Hobby’s HobbyZone
Firebird Commander, also shown, are basic, two-channel RTFs.
Engine-powered ARF basic trainers such as Midwest’s Aero-Star
40 and Lanier’s Explorer 40, which are both shown, offer the new
RC pilot choices of radio system and engines that are unavailable in
RTF models. However, this luxury comes at the cost of additional
assembly work that could require basic model-building skills. RTFs
require only that the new pilot be vaguely familiar with which end of
the screwdriver points toward the work.
The amount and type of assembly work involved in ARFs and
RTFs (it’s hard to call preparing those models for flight “building”)
is all that separates the two. As you can see in the photos, they look
almost identical. And the two types do fly the same since there are
few airframe-performance differences. There are performance
differences, however, if the pilot equips an ARF with more powerful
engines/motors and more capable radio systems than are usually
found in an RTF.
With the exceptions of the Hobbico NexSTAR powered by the
O.S. Max .46 Fxi engine, the HobbiStar 60 Mk III using the O.S.
Max .60 LA engine, and the Hangar 9 Alpha 60 equipped with the
Evolution .61 engine, all RTF glow-powered aircraft currently use
.40 cu. in. power plants. No matter how good a .40 might be, and all
of today’s engines are good, a hot .46 offers more performance.
Except for the Hangar 9 Extra Easy 2 with the five-channel JR
XF-421 computer radio system, all RTF basic trainers use analog
four-channel radio systems for control. The NexSTAR does have an
installed flight-stabilization system, which is similar to an autopilot
but without direction control, but its transmitter remains a good fourchannel
analog system.
Whether a pilot chooses an RTF or an ARF airframe as a basic
trainer is his or her choice. Both offer excellent aircraft and
performance. But as good as these aircraft are, there is always room
for improvement and for pilot individuality.
Next month I’ll build an RTF trainer and make a few easy
improvements. Following that, I’ll look at the ARF world and show
you how to make these fine aircraft look different and perform better.
Until next time, you can review many of these aircraft on Sport
Aviator, MA’s online magazine, at www.masportaviator.com. MA
