Aerodynamics Index

Definitions

Aircraft Axis
CG definition
Define Up and Down
Define Pitch, Bank, Hdg
Define: Lift, drag, etc.
Define Power

Physics Review

Newton's First Law
Newton's Second Law
Newton's Third Law
Reaction = Lift
Reaction = Drag
Conservation of Energy
What is a Vacuum
Action at a Distance

The 4 Forces

Spotting Forces & Moments

Performance

Drag Overview
Induced Drag
Induced Drag Equation
Total Drag

Jet Climb Performance
Prop Climb Performance
Range Jet
Range Prop

Forces in a Turn

Misc

Pitch Controls
Roll Controls

Configurations

Elevator

The pitch control most pilots are familiar with is called an elevator. An elevator is a moveable control surface that is attached to a fixed surface called a horizontal stabilizer, or simply a stabilizer.

The elevator also normally has a trim tab that functions as described below.

When the pilot moves the control column or joystick forward or backward the elevator is deflected from alignment with the stabilizer, as shown in the picture to the left.

When the air flows around the bend a low pressure area forms. In the picture to the left the pilot is pulling back on the control column and a low pressure area is forming below the tail. This pressure pulls the tail of the airplane down.

Note that the low pressure also tends to pull the elevator back into alignment with the stabilizer. The muscle force the pilot puts into the joystick holds the elevator in the deflected position.

In the simulation you can grab the "joystick" with the mouse and use it to move the elevators.

Initially the airplane is on the ground - the airspeed reads zero. In this situation if you let go of the joystick the weight of the elevator causes it to "fall down." As a result the joystick goes forward. This is the position you will always find the joystick in when you enter the airplane.

Click the button labeled "Try it in Flight" the airplane is now in flight cruising at 150 KIAS. The elevator and trim are both at zero. Notice that the elevator is staying level all by itself. You do NOT need to hold it up as you did on the ground. Why? I.E. where is the force coming from to keep the elevator from falling down?

The answer is that if the elevator drops a curve will form at the bend and a low pressure area will form above the elevator. This low pressure will not only lift the tail of the airplane it will lift the elevator. Since the elevator only weight a few pounds it takes only the most minor bend at the elevator hinge line to support the weight of the elevator. This bend is so small that you cannot see it.

Try moving the joystick. Notice that pulling back slows the airplane down and pushing forward speeds it up.

More important notice that as soon as you let go the airplane returns to the trimmed speed - 150 knots.

Use the up and down arrow keys to change the trim. As soon as you use the trim the tab bends relative to the elevator. This creates another area of low pressure. A blue circle appears to show where the low pressure occurs.

Without a trim tab the elevator would always return to alignment with the stabilizer under the influence of the large low pressure are shown in the picture to the left and above. The force created by the trim tab is needed to keep the elevator deflected. The tab makes it possible for the pilot to release the joystick and have the elevator remain deflected.

The force created by the trim tab is much smaller than that created by the elevator, but because the trim tab is at the trailing edge of the elevator it has a greater arm to the the elevator hinge (see the picture.) To keep the elevator from moving the moment from the trim tab (force x arm) must balance the moment from the elevator (force x arm.) For example, if the trim tab has an arm 25 time greater than the elevator, then an 8 pound force on the trim tab will offset a 200 pound force from the elevator. The result is a zero moment and the elevator is held in the position shown. It is important to note that the net lift created by this tail is 192 lb downward (200 - 8.)

It is important to know that every tim setting corresponds to the elevator being trimmed to one particular deflection angle, which in turn corresponds to one particular angle of attack on the main wing. This approximates to a particular airspeed. So pilots routinely say they are trimming the airplane to maintain a certain airspeed. You can certainly see that this is true by changing the trim and watching airspeed change in the simulation.

Stabilator

Elevators are not the only type of pitch control. Some airplanes use a control called a stabilator.

A stabilator combines an elevator and stabilizer into one moveable control surface. As a result it is also sometimes called a flying tailplane.

The simulation to the left shows a stabilator. Notice that just like the elevator it tends to fall to joystick forward when on the ground.

Stabilators are always mounted with the hinge point at the aerodynamic center. In the simulation it is marked with a dot labeled ac. We will discuss the aerodynamic center in more detail in the chapter on stability.

Just like the elevator a stabilator needs a trim tab to be useable. In the simulation you can move the stabilator by dragging the joystick and trim with the up and down arrow keys. You may notice that the stabilator is more effective than the elevator, i.e. less angular deflection of the stabilator is needed to change airspeed than with the elevator. This makes sense because the stabilator is larger than the elevator.

Theoretically a stabilator is more streamlined than a stabilizer elevator combination, and therefore should produce less drag. In practice the elevator is streamlined if the designer is wise enough to arrange things so that the airplane cruises at zero trim (as I did in the simulation above.) Therefore, an elevator only exacts a drag penalty when flying significantly off cruise speed. Generally when you do that, such as on approach to landing you don't care so much about drag, so the theoretical advantage of the stabilator is lost.

The stabilator design is very useful however if you need a lot of elevator power. You will need a lot of elevator power in an airplane that has a very wide speed range, such as a jet. Therefore, a stabilator would be a good choice for a jet, or any other very fast airplane.

Trimable Stabilizer
(aka Trimable Tailplane)

Despite the advantage of a stabilator it is more common for jets to have an elevator and a trimable stabilizer.

In the simulation to the left you can change the trim with the up and down arrow keys. Notice that there is no trim tab. Instead, as you trim the horizontal stabilizer angle adjusts. An image of the zero trim position remains so you can see that you are moving the stabilizer.

Try going into flight. Initially the airplane cruises at 150 Knots. Use the trim to speed up and slow down. You should notice that the trimming power of the moveable stabilizer is very great. That is its chief advantage.

Realistically a small (i.e. slow) airplane doesn't need a moveable stabilizer. Click the button labeled "Make it a Jet" to see the real advantage of the trimable stabilizer.

Initially the airplane is cruising at 300 knots. Use the joystick to speed up and slow down. Notice that you can only change speed by less than 100 knots even with full scale joystick.

Trim the airplane to 450 knots. Notice that you can do this with no problem. Experiment with the joystick. You won't be able to slow down nearly enough to make a landing.

Now use the trim to slow to 150 knots. Again, no problem. Now you can fly the approach for landing.