Weight/flaps

20 Answers

1. 
   

   
   

   Kris Kortokrax on Jun 14, 2015

   It is interesting that you would ask for the answer to your homework question rather than help finding material to help you understand and answer the question on your own.

   If you go to the download link on this website you can find “Aerodynamics for Naval Aviators” and “PHAK” (Pilot’s Handbook of Aeronautical Knowledge). In each of these texts you can find a discussion on flaps.

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2. 
   

   
   

   John D Collins on Jun 14, 2015

   Flaps are not used for maneuvering, so make sure your assignment really is in regard to flaps and not ailerons or other control surfaces such as the elevators or rudder.

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   KiwiInstructor on Jun 19, 2015

   Hi Chris.
   Hopefully this will help a bit more, avoid the “Aerodynamics for Naval Aviators” if your new to aerodynamics, might aswell tell you to go read Kermode (the instructors aerodynamic bible).

   Here’s the golden rule: The requirement for lift is dictated by weight.
   the more you weigh the more lift you have to produce to hold yourself up, easy right?
   Ok: so we produce lift in 2 main ways Speed of air over our wings and the angle of the air to our wings (Angle of attack or AOA)

   Flaps don’t manoeuvre as such, we lower them for certain phases of flight but what they do is increase lift (and drag); they do this by increasing camber (wing curve) and angle of attack at the the wing roots.
   How does that relate? well if you weigh more flap will allow you to produce extra lift for free!

   But….. they usually cant be used at higher speeds and produce alot of drag meaning we have to have more power available to pull ourselves through the air.

   Most flaps range from 10 to 40 degrees of travel but only the first 15degrees generally increases lift after that its all drag baby!

   Does this help?
   If not ask again 🙂

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4. 
   

   
   

   Kris Kortokrax on Jun 29, 2015

   To me, it is like fingernails on a blackboard when I hear or read that flaps increase lift.

   You alluded in your post to the fact that in unaccelerated flight, Lift = Weight. If our weight remains the same, our lift will remain the same whether we extend flaps or not.

   If you were not averse to reading Aerodynamics for Naval Aviators, you would see that on page 33 it states:

   “Trailing edge flaps and leading edge high lift devices are applied to increase the CLmax for low speed performance.”

   On page 37 it states:

   “The principal effect of the extension of flaps is to increase the CLmax and reduce the angle of attack for any given lift coefficient.”

   Nowhere does it state that flaps increase lift.

   You need enough lift to offset your current weight. If you are operating at or below Max Gross weight, you can create that lift without flaps. The flaps allow you to create the same lift at a lower airspeed, because they increase the coefficient of lift over the width of their span.

   You cannot produce extra lift for free. You don’t need extra lift and the penalty for extending flaps is an increase in drag, as you stated.

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5. 
   

   
   

   KiwiInstructor on Jun 29, 2015

   Incorrect sir I suggest reading kermode,

   Flaps Increase AOA & AOA is measured from leading to trailing edge inc the trailing edge of the flap.

   Break down your lift formula

   lift = Cl x 1/2 p V2 xs

   CL is made up of AOA and Camber.

   for a given weight if airspeed remains a constant and your AOA all of a sudden increases….

   I suggest going out in your aircraft holding the same NOSE attitude and airspeed
   and extending flap at a given speed and you will see your VSI shoot up as your lift is increased and exceeds your wieght. 172 is ideal with the 110kt VFE to demonstrate this…

   To to get snarky but we have a massive focus on Aerodynamic knowledge for our instructors ratings here.

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   KiwiInstructor on Jun 29, 2015

   it was an explanation for a highschool kid we don’t have to over complicate it or get into arguments

   relax

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7. 
   

   
   

   Kris Kortokrax on Jun 30, 2015

   You have not identified the statements that I made which were incorrect.

   Nowhere did I aver that there was no connection between AOA and C/L.
   I am also quite familiar with the lift equation.
   As I said, you made the statement “Here’s the golden rule: The requirement for lift is dictated by weight”. This seems to indicate that you are aware that in unaccelerated flight, Lift = Weight. If Mr. Kermode has written something contrary to this, could you please include a quote from his book.

   “for a given weight if airspeed remains a constant and your AOA all of a sudden increases…”

   This is what I would call a transient condition. The momentary increase in lift will not be sustained.

   “I suggest going out in your aircraft holding the same NOSE attitude and airspeed
   and extending flap at a given speed and you will see your VSI shoot up as your lift is increased and exceeds your wieght. 172 is ideal with the 110kt VFE to demonstrate this…”

   Next time you try this experiment, note what your VSI indicates 1 minute later, 2 minutes later, 5 minutes later, 15 minutes later. See how long you can sustain the indication without making a power change.

   Better yet modify your scenario. Instead of 110 K, try it a minimum controllable airspeed. See how much “free lift” you get and how long it can be sustained.

   “we have a massive focus on Aerodynamic knowledge for our instructors ratings here.”

   Are you implying that instructors certified by the FAA have some deficiency in knowledge concerning aerodynamics? If so, on what do you base your assumption?

   “it was an explanation for a highschool kid we don’t have to over complicate it or get into arguments”

   Apparently I give high school students more credit than you. I certainly didn’t think that a high school student would have any trouble downloading the suggested references from this web site and finding an explanation of the function of flaps. The language in those books is clear and not terribly complicated.

   Regarding your characterization of my disagreement with you as an argument, I would suggest that it is you who needs to relax. If you look at some of the other posts on this website, you will find that we who post here regularly often disagree. When we do, we don’t get emotional about it. We provide references to support our position or to refute another’s position.

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8. 
   

   
   

   Brian on Jul 01, 2015

   This massive aerodynamic focus needs to spend a few minutes identifying two ideas:

   1) Lift coefficient and lift are not synonymous.

   2) Angle of attack is defined as the angle between the relative wind and the chord line of an Unflapped airfoil. Conversely you might find analysis of flaps focused around the change in absolute AOA. However I’ve yet to find a worthwhile text that defines chord line for analysis based on the flapped airfoil.

   I suggest rewriting your lift formula to:
   W = 1/2 Cl rho S V2

   This will conform with your golden rule. Further, analyses in this manner might help avoid the pitfall mentioned in #1 above.

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9. 
   

   
   

   Kris Kortokrax on Jul 01, 2015

   Brian,

   We are all aware of # 1. The coefficient of lift is a term in the lift formula. Since the formula does not read “L = CL”, we have figured out that lift coefficient and lift are not synonymous.

   As far as #2, you are going to need to provide a reference for your definition that restricts AOA to an unflapped wing. The angle of attack can be different at localized areas of the wing. For instance, when executing a left turn, the left aileron is deflected upward and the right aileron is deflected downward. This changes the chord line (defined in Aerodynamics for Aviators as “a straight line connecting the leading and trailing edges of an airfoil) for the wingspan ahead of each aileron. The AOA for the area of the left aileron is decreased and the AOA for the area of the right aileron is increased.

   Also, if you take into account washout (as found on a CE-172, for example), then the angle of attack will not be uniform along an unflapped wing.

   As my Kiwi friend stated earlier, extending the flaps increases the AOA for the area of the wing defined by the span of the flaps.

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    Brian on Jul 01, 2015

    “We are all aware of # 1. …To me, it is like fingernails on a blackboard when I hear or read that flaps increase lift.”

    I listed number one because to me it isn’t clear when it’s used in this context. Saying flaps increases lift, to me, screams that one thinks they can use Cl and lift interchangeably.

    I don’t have most of my books here with me and won’t be able to get to them till Friday.

    However, here are a few things to get you started. I pulled up a NACA article that does take the time to briefly mention this, “For our purposes the value of the lift-curve slope a~ is taken as that for the unflapped aerofoil…”

    http://naca.central.cranfield.ac.uk/reports/arc/rm/3087.pdf — top of page 6.

    Most of the books I currently have with me are strictly dealing with design. As such their analysis deals with the whole airplane. For instance, Mechanics of Flight by Warren Phillips defines lowercase aplha as “geometric angle of attack relative to the freestream (for a complete aircraft, this is defined relative to the fuselage reference line)” This is on page 1105

    Further they use the greek letter, lowercase delta, as, “flap deflection.” P1107. If you skim through their analysis of flaps on page 39 onward you’ll notice that the two methods used for analysis are with these two terms. Conversely you can measure it using absolute angle of attack. (Alpha subscript LO)

    I vaguely recall reading it verbatim out of Aircraft Performance and Design by Anderson, but that text isn’t with me to verify so I will get back to you. However if you pick up any text, including AFNA you can see where they define AOA and notice that the chord line is always depicted on an unflapped airfoil.

    AFNA’s analysis isn’t defined, but the charts depicted are using the standard AOA definition. This is noted by the curve shifting reward and upward on the graph. If the graph were plotted using absolute AOA then the shift would be forward and upward. They do show on page 21 how AOA is measured. Notice they use an unflapped airfoil to do it. See if you can find anyplace where this isn’t the case.

    If you still aren’t a believer, read through page 45 where they discuss what happens as flaps are introduced and taken away. How could you define a change in AOA that is necessitated by flap use if the chord line change were not addressed. Unless of course you’re assuming the chord line is unchanged with the introduction of flaps, in which case this description makes perfect sense.

    Finally, you’re familiar with a flapped airfoil stalling at a lower AOA than an unflapped airfoil. Do you think this would still hold true if you were to measure that angle with respect to the flapped chordline?

    Well let me know what you think and I’ll see if I can’t dig up a source that explains this verbatim if you still need it. For what it’s worth, they all do if you care to rummage through the math. That first source lists over 500 variables, not one of them reads “angle of the aircraft relative to the flapped airfoil.”

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11. 
    

    
    

    Kris Kortokrax on Jul 02, 2015

    I should not have said “We are all aware”. I’ll try not to speak for the whole group in the future. Let me rephrase it. I am well aware that lift and coefficient of lift are not synonymous.

    You refer to a statement from an NACA report worded:

    “For our purposes the value of the lift-curve slope a~ is taken as that for the unflapped aerofoil…”

    The presence of the words “For our purposes” indicates that what follows is not a universal truth, but is an assumption that was made concerning the observations being made for the purposes of that specific report. If it were a universal truth, the words “For our purposes” would be unnecessary.

    On page 20 of AFNA (Aerodynamics for Naval Aviators), it states:

    “The chord line is a straight line connecting the leading and trailing edges of the airfoil.”

    On page 22, the following statements appear:

    “The mean-camber line is a line drawn halfway between the upper and lower surfaces.
    Actually, the chord line connects the ends of the mean-camber line.”

    “The angle of attack is the angle between the chord line and the relative wind.”

    None of these statements is qualified and appear to be stated as universal truths.

    This indicates to me that if a wing has flaps and they are deflected,

    1) the chord line extends from the leading edge to the trailing edge,
    2) the chord line connects the ends of the mean-camber line, which is altered by the extended flaps.
    3) the angle of attack for the portion of the wing affected by the flaps is defined by the angle between the chord line and the relative wind.

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12. 
    

    
    

    KiwiInstructor on Jul 02, 2015

    HAHAHAHA were all singing from the same song book just disagreeing on the key we’re singing in hahahaha

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    Brian on Jul 02, 2015

    Unfortunately you’re reading what I’m saying and instead of trying to see the point of it you’re trying to see how it fits your own perspective. That first article I linked is one of very few that take the time to explicitly define chord line. Most don’t bother because it’s clear in the math. If there chord line changed due to flap input there would be no need to define the flap deflection angle.

    On the other hand you could compute control deflection, with the same result, by knowing the spanwise chord length, the flap chord length and new chord line relative to a reference axis. However, this would mean all formulas used be rewritten. Furthermore, it would mean a discussion, like that on page 45 of AFNA, would be totally useless since the point of reference for AOA (Chord line) would be ever changing.

    Here is another source that won’t state what you want explicitly, but I think if you poke around them for a bit with an open mind you’ll realize the truth.

    –SOURCE Volume 1 DATCOM: http://www.dtic.mil/dtic/tr/fulltext/u2/a086557.pdf–

    This first is Volume I of the USAF Digital DATCOM. If you’re unfamiliar it is this program that is used to,

    -QUOTE-
    “Digital Datcom calculates static stability, high-lift and control device, and dynamic-derivative characteristics using the methods contained in Sections- 4 through 7 of Datcom. The computer program also offers a trim option that computes control deflections and aerodynamic data for vehicle trim.” (SOURCE – DATCOM Vol 2: http://www.dtic.mil/dtic/tr/fulltext/u2/a086558.pdf)
    -/QUOTE-

    You can google the reference “Hoak, D., Ellison, D., et al., “USAF DATCOM,” Air Force Flight Dynamics Lab., Wright-Patterson AFB, OH.” if you wish to see just how many articles in this field use this program and it’s literature as a reference.

    —

    Ok moving on, Page 149,

    -QUOTE-
    “This section has been included to acquaint the user with the section geometric definitions, and the NACA designation scheme (reprinted from Datcom Section 2.2.1). The airfoil section module has been written to conform as closely to these designations as possible. Exceptions to the NACA designation scheme are described in Section 3.5.”
    -/QUOTE-

    Read that and then view the next page where they give a picture of an unflapped airfoil used to define the, “geometric definitions” used within this program.

    —

    Next we have page 36,

    The picture drawn on this page notes a chord line from root to tip, again an unflapped airfoil and says, “Angle of Attack Reference Plane.” It is this picture that is used for the NAMELISTS in the following pages where the various accepted input/output data is denoted. If you check page 83 where they discuss wing/tail modeling they make mention of some of the NAMELIST WGSCHR and HTSCHR these models.

    —

    Finally page 57,

    You’ll notice this picture is here due to supports what I opened with. It demonstrates how control deflections are computed and inputed for the system to know how to calculate flap inputs. These same variables both as input and output variables.

    –SOURCE Airplane Performance by Anderson–

    On page 260 he says,

    Imagine that this flap deflection is locked in (i.e., fixed), and the flapped airfoil is pitched through a range of angle of attack alpha, where alpha is still defined as the angle between the original chord line and the free-stream direction, as shown in Fig. 5.31b.

    I had a friend look this up for me as I don’t have this reference. I’ll check my other reference tomorrow, though I suspect it won’t give you verbatim what you want. I think what will prove more useful in you’re realizing this is to poke through DATCOM Vol. 1 and AFNA and see if you can find a single picture that shows a flapped airfoil with a new chordline that is labeled for analytic used. I can tell you it doesn’t exist in DATCOM, and a cursory look of AFNA revealed nothing as well.

    While using this chord line change to make it clear to the laymen how control surfaces effect an aircraft may be useful. That approach is relatively useless when discussing in further detail, especially with graphical representations for instance. If chord line (the reference line) is not held constant it’s akin to trying to do a weight and balance with a moving datum plane. You could do it, but why?

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    Kris Kortokrax on Jul 02, 2015

    Except for that verse about flaps increasing lift, instead of coefficient of lift. That hit a sour note.

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    Brian on Jul 02, 2015

    Yes sir. That puts us 50 percent on the same page. I posted a response but it is awaiting moderation. Probably made it too long! I swear I wasn’t cussing you out. Haha

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16. 
    

    
    

    Kris Kortokrax on Jul 04, 2015

    You lead with this statement:

    “Unfortunately you’re reading what I’m saying and instead of trying to see the point of it you’re trying to see how it fits your own perspective.”

    What you are really saying is that I am trying to see how things fit my perspective and not acknowledging that your perspective is correct.

    Then you follow with:

    “Here is another source that won’t state what you want explicitly, but I think if you poke around them for a bit with an open mind you’ll realize the truth.”

    Implying that if I don’t discover your truth, I don’t have an open mind.

    You completely ignored the explicit definitions I quoted from AFNA. There was no qualifying language in their definition of chord line limiting it to an “unflapped” airfoil.

    Do you believe or not believe the following:

    Aileron deflection increases or decreases the angle of attack for the associated wing area.

    Rudder deflection changes the angle of attack of the vertical tail surface.

    Elevator deflection changes the angle of attack of the horizontal tail surface

    The Wright brothers used wing warping to control roll. As the wing was warped, the chord line would still be drawn from leading edge to trailing edge and the resulting change in the chord line would dictated a change in the angle of attack. Glen Curtiss came up with the idea of ailerons. Placing a hinge between the leading edge and the trailing edge changes nothing concerning the chord line.

    Calling a hinged section of the airfoil a flap, instead of an aileron, changes nothing.

    When the flap is extended, the chord line changes.

    This was brought home to me yesterday. I was giving instruction to a friend in his 1929 WACO. The airplane has undercambered airfoils to the extent that the chord line would not be enclosed by the wing surfaces. A portion of the chord line resides below and outside of the lower wing surface. This to me supports the idea that if I deflect the flaps 10 degrees, I have changed the camber of the airfoil and the resulting chord line is between the leading edge and trailing edge of the airfoil and not some imaginary point where the flap was when not extended.

    You have not provided an explicit statement from any text or reference material stating that the chord line definition is limited to “unflapped” airfoils.

    You base most of your assumptions on the absence of pictures depicting a chord line drawn with a flap extended.

    I base mine on the absence of any explicit statement limiting chord line to an “unflapped” airfoil.

    If and when you find such a reference, please share it.

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17. 
    

    
    

    Brian on Jul 04, 2015

    What I really said was the proof is right in front of you if you quit trying to quibble over words and look at the math.

    Say I draw a line, call it line 1, and then draw 5 other lines and use variables to reference their location in space relative to line 1. Next I give you some formula’s to calculate interactions. Do I really need to tell you that you that line 1 doesn’t move?

    Conversely, say I draw a vertical line, call it v line 1 again and then draw 5 move lines and label them relative to v line 1. Again same conditions as above with variables and equations. And again, do I really need to tell you that v line 1 is fixed and you can’t just make up a new definition or location for it as you see fit?

    The problem with asking for proof of an analytical problem is rudimentary texts tend to be broad in nature, lacking important specifics like L ≠ Cl, for instance. On the other hand technical texts assume you can understand that if a line is used to reference a dozen variables you won’t try to move it around just because it suits your view on the topic.

    (End vent)

    Aircraft Performance and Design – John D Anderson. Page 259-261.

    “The angle of attack α in Fig. 5.30a to c as usual represents the angle of attack of the basic airfoil, as shown in Fig. 5.31a. Now imagine that the basic airfoil in Fig. 5.31a has a plain flap, and that the flap is deflected through the angle δ as shown in Fig. 5.31b.”

    The picture on the next page shows a basic airfoil as an unflapped airfoil with V infinite (rw) and chord line. Image, 5.31b, shows a plane flap deflected at δ angle. It then draws a dotted line representing the (‘virtual’ angle of attack), but keeps the original unflapped chord. The chord (REFERENCE LINE) is used to show how alpha and delta (flap deflection) are found using this original unflapped chord.

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    Kris Kortokrax on Jul 18, 2015

    So, it’s OK for you to “quibble” over math, but not OK for me to “quibble” over words. I will always “quibble” over the meaning of words. Without defining terms such as “right angle” or “hypotenuse”, the Pythagorean Theorem would be meaningless. The mathematical relationships might still exist, but could never be explained without words.

    I find it interesting that you conveniently ignored my questions concerning how the various control surfaces effect a change in the airplane’s attitude.

    Instead, you chose to question my character by suggesting that I modified the definition of “chord line”, which I quoted verbatim from AFNM, which I would hardly consider to be a “rudimentary text”. It is you who is seeking to modify the definition, based on your assumptions, not by any stated definition that might support your idea.

    No venting, just stating my observations.

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    Brian on Jul 20, 2015

    Yes and what you quoted in AFNA shows a picture on the very next page of what the definitions refer to. Again an unflapped airfoil. I quoted you a text that was very clear, and used virtually the same pictures you see in AFNA all while using the text to describe it:

    “The angle of attack α in Fig. 5.30a to c AS USUAL represents the angle of attack of the basic airfoil”

    The later portion of this same text uses the exact verbiage of the quote I posted in one of my first replies that a friend of mine pulled up for you:

    “Imagine that this flap deflection is locked in (i.e., fixed), and the flapped airfoil is pitched through a range of angle of attack alpha, where ALPHA IS STILL defined as the angle between the ORIGINAL chord line and the free-stream direction”

    —

    Like it or not this was all very clear in the math. I don’t judge your character, however I do judge your reasoning in this case. You know a datum can’t be moved with regards to weight and balance calculations or it screws up the charts, graphs, and calculations. So why would the chord line, also used as a reference line, be any different?

    As for ignoring comments, I asked you a while ago: Find a source that depicts a chord line in reference to a flapped airfoil. I know there is none in AFNA, though I’ve looked in great detail for such an analysis and have yet to find one. I’d be interested to see one if you discover one.

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    Kris Kortokrax on Sep 26, 2015

    I had forgotten about this post. Just saw that you responded to a more recent post and it jogged my memory.

    While you may question my reasoning, you have posted no information about yourself indicating that you are qualified to judge my reasoning.

    In fact, you have posted no information about yourself at all, save your first name. What is your frame of reference? Are you a flight instructor? Are you an aerodynamicist? What are your qualifications to speak to the subject at hand? People like John Collins, Mark Kolber, Wes Beard and me, have posted our full names and in the course of various posts have commented on our experience. I invite you to do the same.

    If you are the least bit resourceful you can use Google to find a picture of a wing with flaps extended and a new chord line drawn. It’s not rocket science.

    You have consistently ignored the questions I posed concerning how an aileron, elevator or rudder effect changes in aircraft attitude. If you are or were some kind of teacher, I would think you would take the opportunity to enlighten us.

    Please tell me how deflecting an aileron causes and airplane to roll.

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