How to explain flaps to a Student Pilot?

8 Answers

1. 
   

   
   

   Bill Trussell on Mar 14, 2012

   I start by refering to the simplified version of the lift formula:
   L=1/2 C(L)*V(squared)*S
   where C(L) is the coefficient of lift- determined by wing configuration and shape
   and S is surface area.
   V is velocity
    
   The point in going over this is to note that most changes in the flap settings affect the coefficient of lift C(L) in  a positive way.
   The next discussion is to realize you do not get something for nothing.  Recall that a byproduct of lift generation is drag.  We are trying to take advantage of this fact in using flaps in that a higher C(L) means a lower speed is required to generate the same L.  This gets us a lower approach speed with flaps extended.
   The types of flaps used is aircraft and mission dependent.
   Plain flaps are easy for Piper to use and gets you some drag, while the first 10 degrees gets you more lift than drag. This is handy for soft field takeoffs as one example.
   Fowler flaps on Cessnas are useable for creating more lift in the first 10 degrees as the total surface area of the wing increases, thus a higher C(L).  They also allow extension to higher degrees (say 40 on some models) that create lots of drag for steeper approaches without increases in airspace.
   Slotted flaps are useable for increasing the surface area of the wing for lower speed performance.  They also change C(L) as the shape changes
   Split flaps are useable for slick aircraft and are more drag devices than anything.  The camber does not change per se but the C(L) does change as the airflow changes in general
   Teaching using the lift formula makes it seem too technical but once the relationship between lift, C(L), surface area and speed is understood it is useful for discussions of needs for more power during some maneuvers as well as the topic here.

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2. 
   

   
   

   Nathan Parker on Mar 14, 2012

   A little explanation goes a long way. Expecting the student to know all the differences between the flap types is asking them to engage in rote memorization that serves no purpose. I’d generally limit the discussion to ensuring that they understand 1) that the primary purpose of flaps is to lower the stall speed of the aircraft and, 2) that they accomplish this by changing the effective camber of the wing, and 3) they increase drag.
    
   An exessive quantity of detail will simply be ejected from the brain to make room for more important things.
    
    

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3. 
   

   
   

   Micah on Mar 14, 2012

   I agree with Nathan’s simplistic explanation, but with a different order:
   1. The purpose of flaps is to change the shape of the wing (camber, chord)
   2. The new shape produces new lift and drag properties
   3. The intended effects include higher stall speed and steeper approach path

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4. 
   

   
   

   Matthew Waugh on Mar 14, 2012

   Flight or Ground Training? In a Ground School I’d take Bill’s explanation all the way, for Flight Training the other explanations are more appropriate.
    
   When I took the written oh so many years ago there were questions on the different flap types. I’m not saying that rote learning fits in Flight or Ground Training but in that third training option “getting ready for the written Training”.

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   Kyler Dalton on Mar 15, 2012

   Agreed with Bill and Nathan, though Bill is explaining to a more advanced student. I like to ask myself: what will help my student remember, and how will this knowlege help my new private make better decisions? Flap type is more trivial whereas changed glidepath and airspeed are more useful.

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   Brian on Mar 15, 2012

   I’ll agree with Nathan, though I don’t disagree with, later, giving the details Bill noted. Though I think that’s a job for ground training, not the flight stuff.
    
   I must strongly disagree with offering the purpose in the way Micah does. This would be the same as claiming the purpose of the elevator is to change the camber of the horizontal tail. When speaking to purpose, I believe it imperative we stick to the reason the design was implimented; in this case, to reduce stall speed. How we achieve this, on the other hand, is by altering wing’s camber. (Sorry Micah :-|)

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7. 
   

   
   

   Micah on Mar 16, 2012

   Brian–I’m not offended in the least, and appreciate the comments. I think the conversation should/can be useful even/especially when there’s a fundamental disagreement. Here’s the thinking behind my order: I want to teach students how cause produces effect, or action produces results. Obviously, you must know the desired results/effects before you take action, but you must also know what effects an action produces. I don’t want to shortcut my students by teaching them only the end result/effect of an action.
    
   In this case, Action/Cause = change the shape of the wing
   and Result/Effect = slower stall speed/steeper descent path
    
    
   And if a student asked about the elevator (not sure one ever has) I think I would explain it exactly the way you suggest (that I shouldn’t). If I tell my student that the elevator changes pitch, I fear I’ve tremendously shortcut him/her. Rather, if the student wants to understand pitch:
   1. Use the yoke to adjust the elevator
   2. Elevator changes the shape of the tailsurface
   3. Shape of the tailsurface produces downforce (or upforce) at the tailsurface
   4. This tailforce produces a moment, rotating the airplane/wing and changing the angle of attack.
    
   I liken this to a student who is on short final and is flying low and slow. Every student knows they “need airspeed” and most (unfortunately) react naturally (and incorrectly) by increasing power. We can discuss how/why a student makes this response, but ultimately the student, in his/her ignorance, doesn’t know what the he/she is doing when adding power. Of course what we know is that a student should:
    
   1. Push forward on the yoke >>> to decrease angle of attack
   then 
   2. Add power >>> to decrease descent rate/climb to a safe altitude
    

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8. 
   

   
   

   Brian on Mar 16, 2012

   “Result/Effect = slower stall speed/steeper descent path”

   Well then we agree, since the words result/effect define purpose. 😉 With one caveat; flaps purpose was not to change descent path, though it’s realized that falls under the result category. The purpose for the design was to reduce stall.

   Keep in mind, I’m not saying it’s wrong to mention the changes in flight path, benefits to short field takeoffs, etc. I’m merely reiterating the main purpose. Similarly, the purpose for the rudders design was to coordinate flight, not allow pilots to slip; though that was a result. 

   To your elevator comments: I didn’t and don’t suggest teaching the elevator’s pupose to be changing pitch; it’s purpose is the change the AOA. I suggested that it’s purpose is not to change the tail camber. The later being synonomous with saying the purpose of flaps is to change the shape/camber of the wing. 

   To summerize what I’ve said, agree with, and teach regarding flaps:

   Purpose: Reduce stall speed.
   How: Change the wings camber/shape.
   Other Results: Drag, takeoff/landing performance, improved sight picture on final, etc

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