For CFI's: ground effect lesson

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echelon

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echelon
Hey guys...
I have an interview for a CFI job at Embry-Riddle next week, during which I will be expected to teach a 10 minute lesson on ground effect to a beginning private pilot student.
This will be (hopefully) my first CFI job so I have virtually no experience outside of training, but anyway... I'm trying to decide what to discuss, and what is too much for a private student and what isn't enough. At first I thought I would discuss wingtip vortices=>downwash=>effective relative wind=>induced drag, etc, but that all seems like a bit much for a private student. Then I thought maybe a private student only really needs to understand the practical implications of ground effect, with only a basic knowledge of the aerodynamics, unless he or she asks for more. But how can I describe ground effect very simply without compromising technical accuracy?

Thoughts... Am I headed in the right direction on this one, or not

Thanks in advance.
 
Are you teaching only the theory or is it in prep for "what to expect" when actually flying?
 
I'd suggest explaining what "ground effect" is in practical terms like "1/2 the wingspan from the ground you get a cushioning effect", what the operational considerations are i.e. "this can let you get airborne at a slower speed than normal when on a soft field" or "if you have too much energy approaching the runway you may find yourself floating in the ground effect for a long way", and then touch lightly upon the aerodynamics of it.

That's what I'd do for a private student. A commercial student would get the practical/operational stuff as a review, with more emphasis on the aerodynamics, as they're supposed to really understand it.
 
The main effect of the ground is that if you hit it too hard, you break the airplane.
 
It's only a 10 minute lesson so I would keep it fairly simple and practical, though I would still mention the reduction in induced drag (briefly). Try to relate it to a soft field takeoff and a normal landing. A few of my students tend to demonstrate this effect by flying 5-10 knots too fast on final while we keep floating at 20 ft AGL down the runway.
 
I AM NOT A CFI.

I explained this to someone recently by saying the wing works by making lots of air go down (enough to offset the weight of the plane). When you are close to the ground, the ground stops the air from going down which has the effect of making the wing work better (more efficiently).

Either it worked or they humored me, but they seemed to get it.

Did I mention: I AM NOT A CFI?

John
 
jsstevens said:
I AM NOT A CFI.

I explained this to someone recently by saying the wing works by making lots of air go down (enough to offset the weight of the plane). When you are close to the ground, the ground stops the air from going down which has the effect of making the wing work better (more efficiently).

Either it worked or they humored me, but they seemed to get it.

Did I mention: I AM NOT A CFI?

John
Click to expand...

Speaking from the perspective of a very new pilot, this explanation is very close to the one in my head. I think of the air being pushed down, and when I get in ground effect, that air gets more compressed instead of just pushed away. That compressed air holds the airplane up better than air thats being pushed away without the ground close beneath it. Keep it simple. Students will come to this with a very diverse spread of experience / understanding of the laws of physics. Keep it simple. Keep it simple. Be prepared to go deeper if you get questions from someone who remembers high school or college physics/dynamics.
 
Before you can determine exactly what and how to talk about it, you must first know what the student knows.

Even if it is a 'canned' lesson plan, you cannot just begin any approach until you know the student's knowledge and experience level.

For instance, relating ground effect to a soft-field take-off would be a distraction to a student who has not done or has knowledge of that task.

Number One is Keep It Simple, which also means to go from a Known to an Unknown.

That would be my advice to start with a real student, but you are talking about an Interview, so you have to satisfy an interviewer, not a real student, so I would suggest you ask about the student who is receiving this lesson, or start the lesson with questions to the interviewer as if he/she were an unknown student.

A very critical part of any lesson plan is to know your student.
 
darthnomster said:
Speaking from the perspective of a very new pilot, this explanation is very close to the one in my head. I think of the air being pushed down, and when I get in ground effect, that air gets more compressed instead of just pushed away. That compressed air holds the airplane up better than air thats being pushed away without the ground close beneath it. Keep it simple. Students will come to this with a very diverse spread of experience / understanding of the laws of physics. Keep it simple. Keep it simple. Be prepared to go deeper if you get questions from someone who remembers high school or college physics/dynamics.
Click to expand...

Ground effect has little to do with "compressing" air under the wings. It has to do with interfering with the upflow into the leading edge (GE reduces upflow and thereby reduces angle of attack) and the wingtip vortices are slowed, reducing induced drag.

Better get a good textbook and get it right. Kerschner comes to mind.

Dan
 
Thanks for all the replies...

I'm liking the "practical application and touch on the aerodynamics" approach
 
I should'a said I'm not a CFI either. There. Done.

In ten minutes, the practical + a bit of aerodynamics is all you have time for. You need 20 minutes with a Q&A opening for correcting misunderstandings to adequately cover it. It is usually mixed with other topics in ground school classes to make the day's overall Q&A more efficient, from what I've seen.

As you can see by the replies here in the thread, many PP folks are (too) quickly taught with that "cushioning" analogy and it sticks, but isn't accurate. Try to avoid continuing that old wife's tale.

It's accurate as to what it *feels* like in most aircraft, but not true to the aerodynamics involved. Unfortunately in short time-frame training on the topic, some instructors use it and never get a chance to go more in-depth than that or forget to... Or worse, don't even know themselves or aren't confident of the details. I don't know which. They just blow it.
 
"...quickly taught with that "cushioning" analogy and it sticks, but isn't accurate. Try to avoid continuing that old wife's tale.

It's accurate as to what it *feels* like in most aircraft, but not true to the aerodynamics involved. Unfortunately in short time-frame training on the topic, some instructors use it and never get a chance to go more in-depth than that or forget to..."

Nailed it. From having gone to the school for three years I know that the three words that will blow an interview at Riddle the quickest are "cushion of air," and yet while not actually accurate it makes ground effect quickly easy to understand for people with no prior exposure to aerodynamics.

I think I'll just tell the interview board that induced drag has been covered in a previous lesson and quickly review it, and then focus on how landings/takeoffs are affected.
 
OK. So it's "next week" now......... how did your lesson go?
 
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