Basics of RC Flight

In this section, we cover the basics of RC airplane flight. We will learn about how the airplane moves, and all of the correct terminology. This is not meant to be a lesson in aerodynamics, but rather a simplified and practical explanation how the plane moves during flight.

The Parts of a Plane


We will start with a review of all the surfaces found on a REAL airplane, as the construction is nearly identical - with the exception of the lights, radio antenna and fuel tank. The above image does a good job of illustrating where all of the main parts are located. Working from the front of the plane backwards, we have the propeller mounted on the engine at the very front. The propeller and engine give forward thrust to the plane, allowing the wings to generate lift. The engine, and everything else, is attached to the main body of the plane. This body of the plane is called the fuselage. Lets move on to the wing structure and start with the "control surfaces".


Ailerons

     

If you notice, on the edge of each wing there is something called the Aileron. The aileron is a moving surface that can be used to "roll" the plane. When an airplane "rolls" it will rotate on an axis that causes the wings to spin around the center of the plane. This is illustrated by the arrows in the above diagram. The ailerons will move in opposite directions of one another - if the aileron on the left wing goes up, the aileron on the right wing goes down. When the aileron surface is moved below the wing, that forces the wing up. When aileron surface is moved above the wing, it forces that wing down. The picture on the left has an illustration showing a plane moving away from you. In that picture, there are two illustrations showing what happens depending on whether the pilot rolls left, or rolls right.


Flaps


Flaps are also found on the wing, and for RC flying purposes they are a non-necessary surface in most cases. They are not available on many RC airplanes, so I will cover them briefly. If you notice, the flaps are on the inside of the wing (closer to the plane), while ailerons were further on the outside of the wing (away from the plane). This is because the inside of the wing exerts a much smaller roll force on the plane. The purpose of flaps is to slow the plane down, while simultaneously giving lift. They are typically used at low speeds, and will aid in the landing by allowing the plane to go slower without stalling. Unlike ailerons, flaps move in the same direction, and they only move below the wing. When not in use, they are parallel to the wing. When they are fully engaged, they will point downward at a steep angle.


Vertical Stabilizer and Rudder

     

The vertical stabilizer is the large fin that points upward on the back of the aircraft. The purpose of the vertical stabilizer is to keep the tail of the plane from being overly responsive to yaw forces (we will get to what yaw is in a second). Its also worth noting that the size of both the horizontal and vertical stabilizers impact how well the aircraft rolls (bigger stabilizers mean the aircraft will be harder to roll).

Attached to the Vertical stabilizer is the rudder. This surface controls the yaw of the aircraft. The best analogy to the rudder is the steering wheel on a car. If you turn the wheel right, the car points to the right. In other words, using the rudder moves your plane the same way as the steering wheel moves your car - its sort of like driving on an invisible freeway in the sky. Movement on this axis is called the "yaw". When you adjust the yaw of your plane, you are essentially moving the nose of your plane to the right or left, depending on the direction you choose. If this is still confusing, the above right hand picture illustrates how it works from a top down view. When the rudder moves to the right, the nose of the plane moves to the right. Its also worth noting that in many planes, the rudder also produces a small amount of roll. This is merely a side effect of some of the less prevalent forces on the plane. If this effect is undesired, then rolling the plane (slightly) in the opposite direction will keep the plane level.


Horizontal Stabilizer and Elevator

     

The horizontal stabilizer looks like a second set of small wings on the tail of your aircraft. The purpose of the horizontal stabilizer is to prevent the plane from being too sensitive to pitch forces (we will cover pitch in a second). As mentioned before, the size of both stabilizers will also effect the speed at which the plane can roll due to increased air resistance.

Attached to the horizontal stabilizer are the elevator controls. These surfaces control the pitch of the aircraft. The pitch of the aircraft will move the nose up or down, depending on the direction you choose. When the elevator is raised upwards, it pushes the tail down and nose up. This is an "upwards pitch". When the elevator is lowered down, the tail is raised and the nose points down. The illustration above shows a plane pitching up.


Throttle and 3CH vs 4CH


Like in any other motorized vehicle, the term "throttle" refers to the control that manages the output of the motor. In your car, the throttle is the gas pedal. In your RC model airplane, it will be the left stick (vertical axis). Without getting into the electronic specifics, your throttle lets you manage how fast your engine and propeller spins.

Counting up all of these controls - we have throttle (engine), roll (ailerons), pitch (elevator) and yaw (rudder)... That is a total of four separate controls. When someone refers to an RC airplane as being "four channel (4CH)", they are referring to these four controls. So then what is a 3CH plane? I'm glad you asked - a 3CH plane only has three controls. Most of the time a 3CH plane is a beginner plane, and does not give a pilot the ability to roll. 3CH RC airplanes are usually high wing planes (Cessna, Super Cub, etc), as they will naturally level themselves and remove the need for the pilot to have to use the ailerons/roll control. Its one less thing the pilot has to worry about.


Wing lift, Wind and Stalling


Without getting too technical, lets quickly go over how a plane stays in the air. The short short version is that the faster you go, the more lift the wings will be able to provide, and the plane will be more responsive to your controls. So, how does a wing provide lift? Lets forget for a second that our plane moves. Lets pretend its a kite. If you were to turn the kite so that the body was parallel to the ground, would it fly? Obviously it wouldn't. In order to make it fly, you'd need to tilt the top of the kit upward, so it catches wind. Believe it or not, airplane wings operate in mostly the same fashion as the kite does. The leading edge of the wing is tilted upwards, so when the airplane moves into the wind most of the air is deflected off the bottom of the wing. The air is actively impacting the bottom of the wing at a slight angle. Just like a kite, an upward force will push up against the plane and negate the force of gravity. Some lift is also provided by an effect called Bernoulli's Principle, but the effect is secondary to the angle of the wing.

This leads into a slightly more complicated conecpt: the the speed of the plane relative to the ground doesn't matter as much as the speed of the plane relative to the air. You might think the air should be stationary like the ground, but its not. The speed of the air is the same thing as the wind speed. Think about this - if the air/wind is moving at the same speed as your plane, then you will not generate any lift. In fact, in that scenario, you will drop like a rock. When you lose lift due to lack of speed, that is called a "stall".

Let's take an example: if there is a 20 mph North wind, then if the plane is flying North at 20 mph, then its speed relative to the air/wind is 0 mph. This will surely cause a stall, as the plane is flying WITH the wind and is not travelling fast enough to maintain any lift. Now lets view the same example, but with the plane flying in the opposite direction (against the wind). The wind is moving 20 mph North, and the plane is moving 20 mph South. While the plne is only moving at 20 mph, the wind is moving against it at 20 mph. The difference between the two is a total of 40 mph. Obviously flying against the wind will produce more lift. This is why its recommended you land into the wind. Landing in the same direction as the wind, or "with the wind" will almost surely cause you to stall.


Ok, so what happens in a stall? Essentially, your wings are not providing much upward force due to lack of airflow, and the pressure of the air on the control surfaces isn't enough to let the pilot control the plane. Simply put: the force of gravity wins, and your plane starts to fall. Most new pilots think you'll only notice that you've stalled once your plane starts to fall. This isn't true. The first thing you'll notice is that your control surfaces aren't moving your plane. Reduced movement and control of the aircraft is the first sign of a stall. This is typically before your plane starts falling, and if you notice this early it will give you time to apply more throttle and regain control of the aircraft.

Its worth noting that when you use the controls on the airplane, you are distorting its aerodynamic shape. You are effectively creating "drag"(resistance), in very specific places in order to manipulate the course of your plane. This manipulation and extra drag will slow the plane down and decrease lift. As the picture indicates, if you stall you should apply power and let the plane nose down a bit before you apply any up elevator. Letting the plane nose down allows it to gain airspeed again. If you resist this action by trying to pull up too early, you will make it more difficult to recover from the stall.


Center of Gravity


The center of gravity is a very important concept to new pilots. Many beginner crashes are caused due to a lack of knowledge on the center of gravity (CoG). The CoG is where the weight-center of the plane is. In simple terms, its where you can balance the plane on two fingers, without it tipping forward or backwards. The CoG (the point where you can balance the plane) changes based on where you put things inside of your plane. After you put in all of your components and parts, you will typically have to adjust your battery or add counterweights to adjust where the center of gravity falls. Usually the manufacturer of the RC aircraft will tell you where the optimal CoG should be, and its up to you to try and balance it as close to that point as possible. The optimal CoG will often be somewhere between the front leading edge of the wing and the center of the wing. Its very important to have the plane balanced as close as possible to the suggested CoG, otherwise the flight characteristics of your aircraft can change dramatically.

So, what happens if your center of gravity (CoG) is not adjusted to the optimal position? Well, there are two possible scenarios: your plane cold be too heavy towards the tail, or it could be too heavy towards the nose. If your plane is too "nose heavy", it will have a tendency to pitch downwards. This will be most noticeable at slow speeds and during stalls. If its extremely nose heavy, the effectiveness of the elevator will become negligable and your plane will not be able to pitch up at all. While this probably sounds bad, its actually worse to be "tail heavy". If you stall with a tail heavy plane, its first tendency will be to tip backwards, thus slowing you down even more. Eventually, the plane will just tip over to one side and probably lead to a catastrophic crash. Being extremely tail heavy will most likely lead to a very poor launch and a very quick crash.

If your plane is properly centered, it will fly level even during a stall. This is generally regarded as the optimum behavior for the plane. For new pilots though, I'd recommend being *slightly* on the nose heavy side. It will actually help you a bit when you do stall, as it will tip down and regain some airspeed. You'd always like err on the side of being nose heavy, just in case you misjudged or misread the optimal center of gravity set forth by the manufacturer.



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Supplemental Videos From Youtube



Theory of Flight


This is from the same guy who does RCModelReviews. He goes over all the principles of how an aircraft flies and maintains lift. Its a very educational video.



Center of Gravity


This video gives a brief overview of the Center of Gravity and the effects on your model aircraft. The only thing I'd like to add is that instead of adding weights to your aircraft, it is usually far more useful to move the position of the battery if you can. You should only add weight if its unavoidable.



How Control Surfaces Work


This video was meant to explain how real airplanes fly, but it applies just as much to our foam/electric RC planes as well. It will give you a clear picture on how the flight surfaces effect the movement of the aircraft.




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