It was something that was beaten into me from the beginning of my training; understanding chord-line, and how it changes with the additional of drag devices, such as flaps. The problem I see when teaching new CFIs is that there is a large misunderstanding of chord-line, how it changes, and how the no-flap landing is different aerodynamically than a traditional “full-flap” landing.
So here is the back story. I had a PHENOMENAL instructor for my CFI initial training, and he required a lot from me. During one flight, he asked me to demonstrate a no-flap landing. After a successful landing, I was asked (during the post-briefing, of course,) why the airplane performs differently during no-flap landing. At the time, I needed a little guidance to muster up a decent explanation. Fast forward three years later, and I have a student who was struggling with the same explanation. After watching one of my colleagues give an excellent description of aerodynamic misunderstandings with student pilots, this particular subject included, I figured it was time to get typing!
So here’s the thing; performing a no-flap landing usually means that you will be holding a higher pitch attitude throughout the maneuver, and you will have float longer during the landing roll out. But why?
Higher Pitch Attitude, Not Necessarily Higher AOA
Let’s review what chord-line is. The chord-line of a wing is the measurement from the leading edge of a wing, to its trailing edge. With a wing in the clean configuration, this is drawn as a straight line from the leading edge to the trailing edge at their longest portions.
(Of course, this is a generalized wing and condition. Remember, your relative wind is opposite of the direction of your flight path.)
Now what happens when we add flaps? Now that we changed the camber of the wing, and the position of the trailing edge, we also changed the chord-line!
(Note how with the same pitch attitude, with flaps added the chord-line changes, and as a result the angle of attack increases.)
An important concept of where the chord line falls into is Angle of Attack (AOA.) The measurement from the chord-line to the relative wind (direction opposite of your flight path) is your AOA. However, student pilots tend to overlook how your angle of attack changes, as a result of chord line changes, from the addition or subtraction of flaps.
So let’s look at it this way. You are in a Cessna 172, doing a traditional landing with 30 degrees of flaps. You are established on final at the right airspeed, maintaining a familiar sight picture, and you squeak the airplane right on the runway.
Now what would happen if upon moving the flap position lever to the 10-degree position, the flaps didn’t move? Your only options would be to troubleshoot, or more suitably, fly the airplane first and make a landing. Throughout the traffic pattern and on final approach, you realize you have a much higher pitch attitude than normal to maintain a desirable airspeed and decent to the runway. Why?
Simply put, you using less power and a higher pitch attitude to obtain the same angle of attack you would have with a full-flap landing. Now that the chord-line has decreased, due to the lack of flaps, a higher pitch attitude is required to position the chord-line at the same angle to achieve the same angle of attack you would have with flaps!
This part will not need as much explanation. Due to the lack of flaps, you have an overall decrease in parasitic drag. As a result, it is much harder to maintain a slower airspeed, and in the interest of safety, and not stalling, it will be better to be 5-10 knots faster on final approach to landing. Since you are faster, and have less drag, the airplane is going to float considerably throughout the landing. The issue? Landing too long, and not being used to it.
People get in their own comfort zone when it comes to landing a light general aviation aircraft with everything working correctly. However, when things don’t work correctly, and the airplane doesn’t perform as “normal” I notice people tend to focus more on continuing with procedure, rather than doing what’s safe.
So as a closing statement I will mention this; now you know that the airplane will have considerably longer float with a no-flap landing as opposed to a traditional full-flap landing. If you are starting to float farther down the runway than you are used to, do not be complacent, consider if a go-around is the best decision!
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