How much pressure needed to deflect flaps, ailerons, rudder, elevator?

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by rtfm, Oct 5, 2019.

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  1. Oct 8, 2019 at 2:17 AM #21

    pictsidhe

    pictsidhe

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    Ailerons, about 40% chord, 45% span. All the way to the tip, that makes a significant difference to roll rate. Flaps, 55% span, chord depends on my wing folding mechanism, but probably 20-30%. I'm going to try and meet recommended forces and gradients across the whole speed range.
     
  2. Oct 8, 2019 at 4:35 AM #22

    mcrae0104

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    Let's expand your question to include some missing elements: How much force is required at a particular airspeed to create a particular rate of change in pitch, roll, or yaw? Next, how much does that force change as airspeed increases? How much additional force is needed to get more deflection/rate of change (gradient)? The scope of the question might be larger than one first suspects. (BTW I have not gotten this far yet on my project so I am learning like you, Duncan.)

    Billski's suggestion to start with TOWS is good. There is also the method of copying something that works--Brand Q has X% span ailerons with Y% chord, this many degrees up deflection and that many down, etc, and so many degrees of stick deflection, and so on...

    Also Gudmundsson has a pretty good (broad) overview of sizing control surfaces that might be helpful although there is a bit of basic calculus with the methods he suggests. Probably Raymer's big book too but I don't have it handy.
     
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  3. Oct 8, 2019 at 10:54 AM #23

    wsimpso1

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    Duncan's handle on here advises us to read the manual. I took him at his question. Silly me.

    As for copying somebody else is the potentially effective "Monkey-See, Monkey-Do" approach. It is usually no more effective than the original, but with a little insight can be better. When copying, start with a known good handling and good feeling design, then get to know it in detail before sheming out your own. Things like throw at the surface and throw at the stick and pedals need to work out. Many times, ratios of intermediate parts have to be played with to fit stuff inside the existing spaces... so insights on how it all works are still needed. Thus the value of running the numbers.An example seen on here is RockieDog2's discovery that the Zenith needed clearance beyond what the plans covered to get full control travels...

    Billski
     
  4. Oct 8, 2019 at 3:56 PM #24

    Aerowerx

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    IMHO the OP's question could have been worded better.

    The force to deflect the control surface comes from the pilot through the control linkages. I took a more simplified approach---the maximum force the pilot would apply would be at VNE (all bets are off if you exceed this!) and at maximum deflection of the surface. This is proportional to the dynamic pressure, surface area, and angle of deflection.

    What seems backwards to me from what others are saying here, is that the force being applied by the wind is trying to undeflect the control surface, which the pilot is fighting against.
     
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  5. Oct 8, 2019 at 4:58 PM #25

    BJC

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    Applying full control surface deflection at Vne is not wise, unless the design is an oddball with a Va = Vne.

    BTW, the question was about pressure, not force, which makes it ambiguous at best.


    BJC
     
  6. Oct 8, 2019 at 5:34 PM #26

    Aerowerx

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    I was not advocating that you should do full deflection at VNE. I used VNE as a good starting point for the absolute maximum stick force. Isn't VNE the point where things start breaking? No need to calculate higher than that.

    Something else I seem to recall from somewhere (MIL-F-8785C maybe). There is a different amount of acceptable stick force if you are pushing or pulling. Also side-to-side for ailerons.
     
  7. Oct 8, 2019 at 6:33 PM #27

    BJC

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    Good. I misinterpreted your intent.
    Required stick force for a particular control surface deflection is dependent on several things in addition to speed. As an example, an early Glasair with a GA(W)-0413 airfoil has aileron forces that start getting excessively heavy at 170 to 180 MPH, and by 200 MPH are too heavy for maneuvering. That may not be a problem for cross country flying. Glasair eliminated the cusp in the bottom of the ailerons in later kits, and roll control forces are light up to the Vne of 260 MPH.

    Nothing should “break” at Vne, but full control deflection should be done only at speeds below Va. The demonstrated maximum trouble-free speed is Vd. Vne is typically set at 0.9Vd.


    BJC
     
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  8. Oct 9, 2019 at 5:54 PM #28

    Steve C

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    How about Glasairs that are now going 400mph? They must have had overbuilt stuff to begin with. I know they are clipping the wings, so the ailerons might be shorter.
     
  9. Oct 9, 2019 at 6:33 PM #29

    BJC

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    The 260 MPH Vne cited above is for the Glasair II, which is the four cylinder version. The Glasair III Vne is listed at 335 MPH, but several have been flown much faster. Bob Herendeen never publicly said how fast he flew in his air show, but I’m willing to bet that he routinely exceeded 400 MPH. The factory developmental Super III reportedly exceeded 400 knots TAS at 30,000+ feet. Jeff Lovell holds the Reno Sport class record qualifying at about 410 MPH around the pylons. I believe that he has a standard wing, not clipped. The -III wing is the same dimensionally as the -II wing, but uses carbon rather than glass for some parts.


    BJC
     
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  10. Oct 10, 2019 at 5:01 AM #30

    PTAirco

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    That is a gem of a book. I'd love a hard copy for my collection.
     
  11. Oct 10, 2019 at 5:11 AM #31

    mcrae0104

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    So many classics, so little time.
     
  12. Oct 10, 2019 at 2:24 PM #32

    wktaylor

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    FYI...

    MIL-F-8785 Flying Qualities of Piloted Aircraft [98 pages, CX'd] was replaced by MIL-HDBK-1797 Flying Qualities of Piloted Aircraft [849 pages, fairly comprehensive].

    Also...

    SAE AS9490 Vehicle Management Systems - Flight Control Function, Design, Installation and Test of Piloted Military Aircraft, General Specification For
    [AS9490 replaces MIL-DTL-9490 Flight Control Systems - Design, Installation And Test Of Piloted Aircraft, General Specification For]

    https://quicksearch.dla.mil/qsSearch.aspx
     
    Last edited: Oct 10, 2019 at 2:50 PM
  13. Oct 10, 2019 at 2:57 PM #33

    pictsidhe

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    1797 doesn't have numbers to guide us non-experts.
     
  14. Oct 10, 2019 at 3:26 PM #34

    ScaleBirdsPaul

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    Hiscocks book Light Aircraft Design has a lot of good information on control surface loads. Sizing the control surfaces is a two part problem. You need to ensure there is enough deflection (authority) to meet the design load cases at both the forward and aft CG cases. You also need to ensure the stabilizer and elevator can handle the loads, and then work your way back to the control stick, accounting for the mechanical advantage of the control linkages.

    I find Hiscocks explanations to be the most mathematically simple, however the explanations and terminology can be confusing. He also likes to combine terms and not show units, mixing in data from his example design with conversion factors which means you have to rework the math by hand.

    The “basic lift” and “additional lift” terms used in calculating elevator loads especially kept throwing me for a loop because the tail loads chapter did a poor job of defining them. It wasn’t until I went back to the beginning of the book where he first defined them (in more general terms) and it started to click.
     
  15. Oct 10, 2019 at 4:08 PM #35

    Aerowerx

    Aerowerx

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    I ABSOLUTELY HATE when someone uses "Magic Numbers" without defining units or showing its derivation.

    I also hate when you have to go back and re-read the book to understand "what does that symbol mean". What ever happened to the good ole' fashioned glossary of terms?
     
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  16. Oct 10, 2019 at 4:35 PM #36

    ScaleBirdsPaul

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    Same here. To be fair, the terms are defined, but it’s not common phraseology. But the lack of units drives me up a wall.

    I will say though the reason his book is so valuable is that it shows you how to take your design and calculate real loads. Plenty of books show you how to determine lift with a given flap deflection, but his book actually walked through varying load cases/maneuvers to determine which ones are design limiting, so you can take that info and plug it directly into your detailed structural design.
     

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