Rate my Flaps...

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Foundationer, May 29, 2019.

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  1. May 29, 2019 #1

    Foundationer

    Foundationer

    Foundationer

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    I've read a bit on slotted flap design here, and gone through Theory of Wing Sections and the rule such as it is seems to be to make it look about right. What do yo reckon? Looks ok?

    I've gone for just over .01c gap on a 25% flap chord. Airfoil is Wortmann FX 79 K 144/17.

    Regarding actuating it... I can't quite get my head around whether I need two control linkages toward each end or if I can do it from one central one. What's the usual for a fairly long (2.4m / 8foot) slotted flap?
     

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  2. May 29, 2019 #2

    BJC

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    What is the wing loading and planned maximum actuation speed?


    BJC
     
  3. May 29, 2019 #3

    Foundationer

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    The wing Loading is 36.5Kg/m^2 (7.4lb/ft^2) and Vfe is 74 knots.
     
  4. May 29, 2019 #4

    wsimpso1

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    The flap looks like an airfoil and the lip appears to have enough overlap and depth.

    You could remove the lower lip and build a faired shape for the leading portion as shown in TOWS to get smoother flow onto the top surface of the flap. See the first picture.

    Actuation is a whole different issue. Most little airplanes (8 ft flaps indicate a spar somewhere around 30 ft typical of little planes) actuate a slotted flap with a pushrod near the inner end, and then accept that the outer end will be twisted off slightly by elastic deformation.

    Now if you are building with a full span flaperon, like the high performance sailplane guys (look up a discussion of such with BoKu) they have very compliant flaperons in bending so that it follows the wing pretty nicely. You can try to maximize torsional stiffness while minimizing bending stiffness, but the two are still related to each other. I suspect that these types are actuated at the middle of the semi-span, but then they have really long flaperons...

    Billski
     

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  5. May 29, 2019 #5

    BJC

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    Bill:

    I recall reading a technical report that showed that there is no advantage to fairing the lower wing surface into the flap cove. Is that what you are referring to? IIRC, the optimum slot dimension / flap position passes minimal flow compared to what is available at the opening into the cove. That was, at first, counter-intuitive to me.


    BJC
     
  6. May 29, 2019 #6

    Foundationer

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    Planform is an 8.2m hershey bar so good guess at 30 feet! I've got full span flaperons on my glider but this will be nice discrete ailerons outboard and flaps inboard
     
  7. May 29, 2019 #7

    BJC

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    Even at your relatively low flaps extension speed, the control force will be significant. If it is torsionally stiff, as mentioned by Bill, a single inboard actuator MAY be adequate, but, given the flap length, you likely will have three hinge points, so using a single actuator by the center hinge is what I would evaluate first.

    Tell us more about your design.


    BJC
     
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  8. May 29, 2019 #8

    BJC

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    Saw this after posting above comment. Will your wing deflection allow three hinges for an 8 foot flap?


    BJC
     
  9. May 29, 2019 #9

    Pops

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    All questions I had to think about and answer in the JMR. I made my flaps with a fully plywood sheeted flap for a higher torsional stiffness and since the flap just has a 5' span, I used a center pushrod. I would have liked to have built fowler flaps but wanted to stay as simple as possible in construction and take the performance hit of a less effected flap.
    3 hinges on the 5' span flap.
    So many things to consider.​
     
  10. May 29, 2019 #10

    Foundationer

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    I was planning for three hinges, flap construction is will be hotwired foam core, one layer of 265gsm (7oz/yd) carbon Bi-Ax at 45 degrees to chord to give it good torsional stiffness but limited bending stiffness.
     
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  11. May 29, 2019 #11

    wsimpso1

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    Could well be that the faired cove does not add to flap performance..

    If I were being brutally honest, I would have said that Foundationeer's lower skin extended beyond the the drag spar looks like it will both choke the air into the slot and looks fragile to me. Instead I was trying to get those fixed by suggesting the faired cove. I do think that the faired cove looks better too.

    One of the usual issues is the open slot leaks quite a bit of air when the flaps are stowed, costing significant drag. If you install the cove and make it run fairly close to the flap when flaps are stowed, you can put in a thin foam strip to seal the slot when the flaps are stowed. Sealed flaps! Sealing the flaps when stowed with just the open space is tougher to achieve but could be done...

    Billski
     
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  12. May 29, 2019 #12

    Foundationer

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    Brutally honest is usually the best honest as long as it comes with slightly ambiguous / meaningless encouragement both sides.

    I started with a nicely faired slot entry but then it seemed that it ups cruise drag slightly, Orion said it wasn't necessary in his thing about wings and most importantly the Gazaile 2 doesn't have one. Not that I've been nicking my ideas from there at all...
     
  13. May 29, 2019 #13

    BoKu

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    In general I like your design. With three hinge points I'd guess that you can safely drive it at the center hinge point. But I would recommend using two plies of 200g biax; that will improve its resistance to ground handling loads and almost double its torsional stiffness with only a modest increase in mass. You'll probably find that the hinge and drive arms and their anchors and hardware constitute about 30% the total mass of the flap, so an extra ply of carbon adds relatively little to the total, and it's well worth it if it lets you get away without needing an extra bellcrank, bearing, and anchor plus two more rod ends, etc.
     
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  14. May 29, 2019 #14

    Foundationer

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    I'm materials limited - I got a sweet deal on 265gsm bi-ax so EVERYTHING is 265gsm bi-Ax unless it's got double curvature in which case... I got a sweet deal on 265gsm 5 harness satin. Enough for two airplanes at a price of .85 airplanes. The biggest advantage of this is that when I screw something up I can put it in the bin and move on rather than agonise over all that expensive carbon.
     
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  15. May 29, 2019 #15

    wsimpso1

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    OK, here is my revised commentary.

    The flap shape looks nice, and I like the slot depth and overlap.

    I do not like the extended bottom skin, it looks like it will choke the flow to the top of the flap when deployed and be kind of fragile to boot. I am also kind of concerned about flap effectiveness when you just carry the wing shape on the flap - it sort of gets buried in the boundary layer. This is probably more important on the ailerons than on the flaps. I also do not like the idea of just letting the flaps leak air from the bottom and out of the slot when stowed.

    I suspect that a better scheme is to:
    Slightly thicken the flap, particularly on the top surface;
    End the bottom skin at the trailing edge of drag spar, fair the cove nicely, and make the faired shape come very close to flap when stowed. This gets rid of the fragile looking lower skin, reinforces the top skin, gives a faired flow from bottom skin to top of flap when down, and allows you to seal the flap when stowed by putting strip of foam rubber on the faired cove where the flap is closest.

    Billski
     
    Last edited: May 30, 2019
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  16. May 30, 2019 #16

    pictsidhe

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    I'm dubious about that bottom lip too. It could be worth having an internal fairing to take out the square corner under the top lip and make a better nozzle shape. A removable internal fairing would let you play with its shape. Bare XPS should be fine for experiments. Probably minimal advantage below the camber line.
    Have a google for 'slotted Blackburn flaps'
     
  17. May 30, 2019 #17

    BJC

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    I need to make it clear that I am referring to the lip / sharp edge at the lower aft edge of the bottom wing skin. A proper cove to guide the flow smoothly into the slot is important.


    BJC
     
  18. May 30, 2019 #18

    TFF

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    The Cirrus flap is similar without the top gap. The CRJ regional jet has a slat on the nose of the flap so any air in the cove is directed over the top of the flap.
     
  19. May 30, 2019 #19

    Foundationer

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    I'm not sure I follow you here. Are you suggesting for control surfaces you need to enlarge or 'fatten' the control surface in order to make it trip / push out into the boundary layer? I recall reading something along those lines somewhere but I forget where.

    Are you suggesting the line in red I've added here. Would be pretty easy to achieve in a variety of ways I think.
     

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  20. May 30, 2019 #20

    wsimpso1

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    There has a been a lot of discussion about this elsewhere.
    • Using cusped airfoils tends to increase control forces over un-cusped controls;
      • Running the control surfaces to a point is difficult to build in any type of foil - several approaches have good results:
      • Cut off last 1% of airfoil;
      • Use all 100% of airfoil, but thicken the trailing edge slightly, making the rest of the surface fair.
    • Control surfaces tend to have dead bands around center, and slightly thickening them tends to reduce or eliminate the dead bands;
    • Control surfaces tend to have separated flows at high angles, and slightly thickening them has been shown to keep flow attached to higher deflection angles.
    As to the cove, no we are suggesting that you cut the lower skin flush with the aft edge of the drag spar, then run a smooth curve from the lower skin to the bottom of the top skin that has clearance for the flap nose. Something similar to what I show in the attached photo. Many airplanes run without the cove faired at all. BJC mentioned above some work that indicates the fairing actually may have little to no purpose when the flaps are deployed. I like them for several reasons:
    • They look good;
    • They reinforce the upper skin;
    • They give me a hiding spot for my cables and pitot-static tubing;
    • They give me a place to put a seal for when the flaps are stowed and I do not want air going through the slot.
    Its your airplane. You get to weigh the arguments and do what you think is right in your bird.

    Billski
     

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