The flaps on the C172 are single-slot type flaps powered by the aircraft’s electrical system. The amount of flap lowered is measured in degrees, and in the C172, the different amounts include 10º, 20º, 30º, and in some earlier models 40º.

From first hand experience, putting down 40º of flap is almost always unnecessary due to the huge amount drag that it produces.

I mentioned above that the C172 has “slot” type flaps, specifically single-slot. I’ve included a diagram below of the different types of common flap systems utilized on a wide range of aircraft.

The difference between a slot flap and plain flap is that with a slotted flap, there is room for air to flow between the leading edge of the flap and trailing edge of the wing. In other words, air can flow between the wing and flap. WIth a plain flap, the flap simply drops down and is, in essence, an extension of the wing itself.

Slats are common on larger aircraft and you’ve probably seen them on commercial airliners. Slats act to maintain laminar airflow over the wing at a high angle of attack. Check out this diagram:

In the C172, the flaps are powered by an electric flap motor located in the wing. The system is protected by a 15 amp circuit breaker, labelled “FLAP”, in the cockpit. All diagrams are taken from From The Ground Up.

To follow up with my last post on the carburetor, I’ll be talking a bit about the carb heat lever and what is happening inside the engine when you engage it.

Icing is a problem that can affect an aircraft in many ways: added weight, change in airfoil shape, increased drag, and reduction of airflow to the engine. While all of these effects of icing are dangerous, this post will focus primarily on the last one.

As I explained in my last post, the carburetor mixes incoming air with fuel and feeds this mixture to the engine. Icing can occur when moist air enters the carburetor and freezes to the walls and butterfly valve. The eventual build up of ice can ultimately cut off the air supply or render the butterfly valve inoperable, and it is for this reason that we use carb heat.

Carb heat works by taking unfiltered, outside air and passing it through tubes wrapped around an exhaust muffler. This effectively preheats the air before it reaches the carburetor. If ice is present in the carburetor, this air will melt the ice.

There are a few side effects of using carb heat however. The C172 POH says that carb heat will reduce engine by 100-225 RPM at full power. This is due to the fact that hot air is less dense than cold air, and with less air molecules to aid in the combustion process, power is reduced. Also, if ice is already present and carb heat is applied, it may be observed that the engine will run rougher than it was previously. As the ice melts, the resulting water enters the cylinders and causes the roughness. It is extremely important that you leave carb heat on and let the water get out of the system. Some people apply carb heat and notice that the engine is running rougher than it was – they then turn carb heat off. Ice builds up even more and engine failure becomes imminent.

The moral of this story is to leave carb heat on and let it do its job! Give it some time and engine power should return to normal, even if it runs a little rough at first.

Below are a few diagrams, courtesy of From The Ground Up that show the effects of icing, the carb heat system, and the carb icing range. Enjoy.

In this post, I’ll be detailing the carburetor in the Cessna 172 and talking about what it does and how it works.

To start, one should know that an engine works by burning a mixture of fuel and air in a specific proportion. This miniature explosion drives a cylinder down, and then back up again, providing power. This power is used to turn the wheels on your car or the propeller on your airplane.

So what proportion is best when mixing fuel and air together? Approximately 1 unit of fuel to 12 units of air is an ideal ratio. So why aren’t planes manufactured so that there is always this 1:12 ratio of fuel to air? The answer has to do with altitude. As you fly higher, air becomes less dense, and due to this decrease in density, the 1:12 ratio on the ground may actually be closer to 1:10 or 1:8 at altitude. In order to balance this out, we need to modify the fuel-air mixture so that the ratio returns to an optimal level. This process is called “leaning” and simply decreases the amount of fuel mixed with the incoming air – this returns the ratio to normal.

All of this happens in the carburetor. In the C172, the carburetor is described as an up-draft, float-type carburetor mounted on the bottom of the engine. “Up-draft” simply means that air enters the bottom of the carburetor, then flows upwards through the system, and out the top to the intake manifold tubes. “Float-type” refers to the fact that a float system regulates the amount of fuel that sits in the float chamber of the carburetor. The float chamber is a small chamber of readily available fuel that is waiting to be mixed with incoming air.

The diagram below may provide a better visual of the process inside of a carburetor. Essentially, air flows up through a tube, called a Venturi, and is mixed with fuel coming from the float chamber. A pin inside the float chamber controls how much fuel is sent to the Venturi, and thus controls the mixture, either leaning or enriching it as necessary. When the mixture knob is moved to the idle-cutoff position, this pin fully blocks any fuel flow to the engine, which in turn starves the engine and shuts it down.

Last but not least, a butterfly valve inside the carburetor controls the amount of this fuel-air mixture that is allowed to go to the engine. This is your throttle control.

Diagram: From The Ground Up

If you have any questions, feel free to post in the comments. I hope I’ve done an adequate job explaining this system, but if there’s something I’ve left out or you feel is important, let me know and I’ll add it in.

I’m starting a new series called, appropriately, “Learn A System”. I’m currently coming close to my private license flight test, and in an attempt to learn all of the systems for the oral portion of my test, I plan to teach them to you.

In my mind, if I explain it to someone else, even if no one reads it, I’ll somehow magically absorb it and know it a little better than I did before. I fly the Cessna 172N, so here we go: the fuel system. (see the diagram at the bottom of post)

To begin, the C172 has the option of two different types of fuel tanks: standard and long range. Standard tanks are 21.5 gallons each totaling 43 gallons. However, 3 of these gallons are unusable (meaning 3 gallons will sit in the bottom of the tanks and can’t be sucked into the fuel lines). Therefore, standard tanks have 40 gallons of usable fuel.

Long range fuel tanks hold slightly more fuel in each tank at 27 gallons per tank. In total, both tanks can hold 54 gallons between them with 4 gallons unusable, meaning long range tanks can hold 50 gallons of usable fuel.

If you haven’t already deduced it yet, the C172 has two fuel tanks located in each wing near the root. From there, the fuel is gravity fed to the fuel selector valve, located at your feet in between the two seats in the cockpit. You can manually select to feed the fuel from the left, right, or both tanks. The off position stops the fuel feed to the engine.

From the fuel selector valve, the fuel flows to a fuel strainer which filters the fuel for any debris or foreign particles. This is also where the primer draws the fuel from where it is injected directly into the cylinder intake ports.

From the strainer, the fuel flows to the carburetor where it is mixed with air and then to the cylinders through intake manifold tubes.

So now you know how the fuel gets from fuel truck to cylinder, but the fuel system includes a few more things. There is space for air to move between fuel tanks, and in the C172N, the right fuel cap is vented to allow air to flow freely. Ventilation is essential to the fuel system as it allows fuel to flow freely.

So how is fuel measured you ask? Well the same way your toilet knows to stop filling once you’ve flushed it. A float sits in each tank and is connected to electrically operated fuel quantity indicators in the cockpit. These indicators are known to be very inaccurate, so always dip your tanks before you fly!

This sums up the basics of the fuel system. Sure, you could get into the locations of the drain valves or the proper position of the fuel selector during certain maneuvers, but I won’t bore you with things that you’ll learn in the cockpit anyways. This first lesson is supposed to be directed at the theory of it all, so I hope this helps.