Secondary Flight Controls - Flaps
ever-present search for realism in our sims, we continue to
ask developers for more and more realistic flight models.
They have done a good job in responding to these requests.
One aspect of the flight model issue has been the introduction
and refinement of flap and trim use. Today, we routinely use
these features to bring out the most in realism in our sims.
Thats the good news. The bad
news is that our sim documentation is pretty lacking in any
significant information on how to use these features properly.
A second potential problem lies with the programming of the
sim itself. Flap or trim systems for our sims should replicate
real life since the sim pilot will most likely attempt to
use real life procedures and techniques to operate these systems.
To date, these two problems
of documentation and improperly coded flap or trim programs
detracted from "realism" in our game play. Too often,
players try to use flaps and trim in ways that are incorrect.
The result is that flap and trim systems are coded so that
the aircraft can be flown outside of its real world flight
My objective in this second of three
articles on flight control systems is to give you some background
info and suggested techniques that will permit you to operate
the flap system correctly and effectively. To do that, Ill
start off with some academic info that explains what flaps
are designed to do. Then, Ill suggest some tips on how
to use flaps in a typical sim.
Before we jump into the meat of the
matter, lets remember that these are articles on secondary
flaps are intended to supplement our
primary flight controls (stick and rudder)
them. To put it simply, we do not fly with flaps
flying easier and more effective by using them. Lets
not lose sight of that very basic fact as we get into the
But before we start, let me admit
that I am no aero expert. The information in this article
is simplified to help you enjoy these features in your favorite
not to make you the next engineer hired by
Section One - High Lift Devices: Flaps,
Slats, and Slots
What is a flap?
A flap is part of a family known as
"high lift devices". These consist of flaps, slats,
slots, and boundary layer control devices. All of these increase
lift and decrease stall speed.
The airfoil is the general curved
shape of the wing. Flaps operate up and down to change the
shape of the airfoil and, in some cases, the shape of the
planes have flaps that may be lowered in several stages. As
a rule the first one or twostages are used for takeoff and
possibly combat maneuvering, and the latter stages of the
flap range are normally used for landing.
What does a flap do?
go back to that picture of an airfoil and talk briefly about
what an airfoil does. The purpose of an airfoil is to produce
a difference in pressure between the air going over the top
surface of the wing and the air that flows under the wing.
This difference in pressures results in the formation of lift
of course, is what makes an airplane different from other
means of transportation! Our purpose in this article is not
to teach aerodynamics, so Im going to simplify matters
We associate the difference in air
pressure with the distance the airstream has to follow as
it goes over the and under the wing. Generally, the greater
the distance, the greater the difference in the two pressures.
The resultant difference in pressures creates lift.
few words about lift are in order. Why? Because lift is what
turns an airplane
and turning is what we do to perform
a combat maneuver. Better control of lift means better turning,
which in turn leads to better and more effective maneuvering
which then results in you being the winner and not the loser!
To better understand what a flap does,
lets look at the relationship of an airplanes
angle of attack to the amount of lift being produced. We begin
with the term "angle of attack".
Lets draw a line between the
leading and trailing edges of an airfoil. Call this line the
"chord line". When an airplane flys through the
air, the airstream meets the wings leading edge at some
angle. The angle between the airstream (also known as the
relative wind) and the chord line is the "angle of attack"
(AOA). Note this well! The AOA is a function of the "chord
line angle", not the "fuselage angle"!
Now, a graph to show the effect of
increasing AOA on the production of lift. Note the airplanespitchattitude
as its AOA increases.The vertical axis is calibrated in units
of "coefficient of lift" which is a fancy term that
refers to the amount of lift being produced. What we see here
is that the coefficient of lift (Cl
sub ell") increases as AOA increases
up to a point
where it stops increasing. As AOA increases, the airflow over
the top of the wing becomes more and more curved. Eventually,
the airflow cannot continue to follow the curvature of the
wing, and the airflow breaks loose and separates from the
This is the point at which the airplane
stalls. An airplane stalls at a given AOA
an airplane stalls as a function of AOA,
not airspeed! Stall speed will change with weight but stall
AOA remains the same for a given wing configuration!
That implies that changing a wings
shape may change its stall characteristics, to include its
True fact! Thats what flaps
are all about. Here is another graph that shows how flaps
change the lift picture.
In this diagram, you can see that
putting the flaps down creates an increase in Cl for a given
AOA. Also note that the stall AOA is less than for the "clean"
wing but has a greater Cl value. It is also important to see
that this new stall AOA occurs at a lower speed.
At this point, here is what we know:
flaps decrease the stall speed and stall AOA and increase
lift. The next subject covers the types of flaps.
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