2 aero questions about flaps please

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by cheapracer, Nov 18, 2019.

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  1. Nov 18, 2019 #1

    cheapracer

    cheapracer

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    I have 2 questions on flaps if some knowledgeable aero person has a moment please:

    1/ Flaperons, clear what they do etc, but why do I not see them in conjunction with ailerons as in being the outer half of the wing. They always seem to be full width on STOLs.

    It seems to me that having the flaps to the outer half would help to prevent outer wing stall.

    So what's the issue, if any, with having flaperons in the 'normal' aileron position only?


    2/ With standard type of hinged flaps seen on most of our homebuilt craft, what would the issue be with having air feed holes) in the lower skin, just in front of the flaps, fed through outlets in the rear spar, sort of a poor man's version of a Fowler Flap, or call it the Fouler Flap!

    The flap closed would seal off the flow of course.

    Blue holes, Green air flow ....


    Fowl Fowler.jpg
     
  2. Nov 18, 2019 #2

    Victor Bravo

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    Have a look at the control system on the AS-W20 sailplane. Full-span flap control, full span aileron control, but separate surfaces moving at separate deflections for different speeds and uses.

    THEN a high performance landing position where the ailerons goup and the flaps come down for maximum safety and glidepath control.

    Complicated, but VERY WELL worth it !!!
     
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  3. Nov 18, 2019 #3

    Dana

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    Slotted flaps are quite common.
     
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  4. Nov 18, 2019 #4

    wsimpso1

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    Think about it a bit. You need good roll control down close to stall AND you need the stall speed brought down as much as you can. You want a lot of down flap and you want a lot of aileron travel. Both! how to do this?

    You can make the entire trailing edge a flaperon and progressive droop the ailerons neutral position by pulling on the flap handle. Trouble is you can only get to about what would otherwise be the half flaps position before you can not add anymore flaps and still have enough aileron movement for low speed roll contol. So full-span flaps have their limits for CL increases. Work great on sailplanes where they use the flap adjustments to shift the laminar bucket for their airspeed and CL, not to lower landing speeds.

    Another way is to accept that flap effect is big near the root and decreases as you go towards the tips. Because lift is approximately elliptical in its distribution, we can put in flaps over the inner 60% or so of the wing and we can deflect them all of the way to get big CL increase for all of the flapped portion and for some of the unflapped portion, then droop the ailerons part way while still using the rest of their travel for roll control lift. Now you can get more CL improvement this way than with full span flaperons and you did not have to resort to spoilers and their problems for roll control.

    About this ducting air from botom skin ahead of the control surface and dumping it on top of the control surface - this IS a slotted flap. To make it work well, you need to profile the control surface and the upper surface lip for this and at maybe help the flow a little with the cove design. Flaps and ailerons are schemed out this way, and work well.

    Billski
     
  5. Nov 18, 2019 #5

    Jay Kempf

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    1. increasing camber of the outer half of the wing and not the inner half would mean the outer wing will stall first. Gentle stall is center first and slow progression to the outer and keep the ailerons working throughout the entire stall warning and break period. That is the heart of the whole spin resistant direction that the FAA is driving right now. That is why you see cuffs on a lot of designs where they didn't pass as is and the outer wing had to be recambered for proper behavior. Like VB said above you can get quite elaborate if you make a whole bunch of individual panels at the risk of having a lot of panel end leaks. Model gliders had a mode called "crow" flaps down, ailerons up long before full scale because it built big drag while maintaining all directional control but that was for "spot landing." We don't do spot landing in man carrying stuff. Always though spot landing was a joke in competition but it was a lot of points.

    2. That air flowing through those holes is going to be turbulent. Not what you want there. If your goal is to provide slotted flap like behavior you need to accelerate the air through the slot over the top of the flap max thickness top skin and get it flying like the wing, not separated. I think what you propose will just make a large separation on the top front of the flap. It will function like a big pressure leak.
     
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  6. Nov 18, 2019 #6

    Chilton

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    I assume you are looking to have an inboard flap and then droop the aileron outboard as a flaperon in addition, rather than having only the outboard flaperon? If so it would be similar to the Robertson STOL kit on the Cessna 206, I think also the DHC Beaver, Otter and Twin Otter.

    The setup works well for reducing stall speed but also reduces aileron effectiveness at low speed as it puts the aileron in a less effective angle. I never had an issue with it but I know many who disliked flying with the setup. Possibly why so many designs go with full span flaperons so that the size of the surface outweighs the reduced effectiveness at low speed.
     
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  7. Nov 18, 2019 #7

    mcrae0104

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    The issue is tip stall. Start with a graph of spanwise cl distribution (not to be confused w/ spanwise lift distribution). Now push cl up, but only out near the tips; see how they hit clmax first? Deploying outboard-only flaps ensures that the outboard portion of the wing will stall first. No bueno, señor.
     
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  8. Nov 18, 2019 #8

    Victor Bravo

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  9. Nov 18, 2019 #9

    Turd Ferguson

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    From a practical angle, I used to fly a C-185 with a full Robertson STOL kit and part of that kit was that when the last 10 degrees of flaps were selected, the ailerons would booth droop like flaps. As soon as selecting full flaps, roll control went from good to poo poo. No idea how that was ever approved.

    Mitsubishi made the flaps the entire length of the trailing edge and used spoilers for roll control. They seem to work at all speeds.
     
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  10. Nov 18, 2019 #10

    Topaz

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    We don't? STOL pilots and every single solo student at my old soaring club would like to differ with you. ;)

    I think this is what VB was talking about in post #2 (I haven't flown an ASW-20), but it's not unheard of in the soaring world to have the ailerons and flaps move together for a certain small portion of "flap" travel (10-15°), and then only the inboard sections ("the flaps") go down beyond that. The ailerons are the only "flaperon" portion of the system - the inboard sections don't move differentially.

    The inboard section going down as "flaps" more than the ailerons gives you more aerodynamic washout to the wing, additionally protecting against tip stall.

    It's a complicated mechanism, but nearly "the best of possible worlds" for a wing. Most power-planes don't need the fine control of span-wise wing camber, which is why they have entirely separate ailerons and flaps. For 95%+ of small-plane applications, the traditional arrangement is certainly "good enough."
     
    Last edited: Nov 18, 2019
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  11. Nov 18, 2019 #11

    pictsidhe

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    I have little to add to the previous posts.
    Plan A for my project is inboard flaps, outboard ailerons. My spreadsheet thinks that will meet the stall speed requirement. If not, I will add a flaperon feature to the ailerons, probably only to be used to show stall speed compliance as it will detract from stall behaviour.
     
  12. Nov 18, 2019 #12

    BJC

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    The first RV-3 had ailerons that drooped with the flaps, but that feature was later abandoned.


    BJC
     
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  13. Nov 18, 2019 #13

    BBerson

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    Right.

    For full span flaperons, the outboard half could be somewhat less incidence. Or if ailerons are desired instead of flaperons, by drooping the ailerons some but not nearly as much as the flap when the flaps are deployed. (DHC-2 Beaver)
     
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  14. Nov 18, 2019 #14

    Dan Thomas

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    The Robertson 185 I flew would droop its ailerons in the first 20 degrees of flap deployment, then the ailerons would come back up a little as you approached 40° flap. That was to control stall behavior, though stalling one of those things was NOT advised. Could be nasty.

    You airplane might have been misrigged. Easy to do with that system.
     
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  15. Nov 19, 2019 #15

    cheapracer

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    Well that key sentence keeps things simple, not to take anything away from the other terrific responses today.

    Guys, thanks again for 'world-wise' responses, the real world experience pool at HBA just beats other forums hands down. It's just wonderful to leave a question late at night, go to bed, and then open up in the morning to see the quality responses.
     
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  16. Nov 20, 2019 #16

    Doggzilla

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    When they work. The Mu-2 has one of the worst records of any commercial aircraft because the spoilers dont work over a wide range of angles.

    Anything even remotely aerobatic should stay the hell away from spoiler control.
     
  17. Nov 20, 2019 #17

    Tiger Tim

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    Bold statement. Got much rice rocket time?
     
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  18. Nov 20, 2019 #18

    Doggzilla

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    No, I’m just smart enough to know 188 accidents with over 200 deaths out of 700 aircraft is an EXTREMELY poor record.

    That’s like WWII level loss rates.

    I’m betting a lot of those dead pilots said the same thing you just did. Didn’t work out too well for them.
     
  19. Nov 20, 2019 #19

    Victor Bravo

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    The AS-W20 glider did have full span camber and full span roll.

    When you move the stick to the right, the right flap came up a little and the right aileron came up a lot.

    When you moved the flap lever forward, both flaps and ailerons came up.

    When you moved the flap lever all the way back, past a detent, the flaps came way down and the ailerons went back past neutral and up a little.

    Then, you had a completely different lever for the spoilers, on top of everything else.

    It was a very complex system, but it worked magnificently.

    I measured a real-world high performance off-airport landing in a regional soaring contest once. Hard surface, real-world conditions, unplanned and unwanted forced landing, never been to that landing area before (asphalt mixing pad next to a major 2 lane highway in the Mojave desert).

    Landing distance: 256 Size 11 tennis shoes :)
     
    Last edited: Nov 20, 2019
  20. Nov 20, 2019 #20

    Tiger Tim

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    It’s a terrible record, but has nothing to do with spoilers (or ‘angles where they don’t work) or really any handling quality of the airplane. If anything the flaps are more dangerous than the spoilers but that’s just a procedural thing. In the mid-2000s the MU-2 went from the worst accident rate to the best nearly overnight without changing a single physical thing about the airplane. Interestingly the Merlin has a much worse fatality per fleet hour record but you never hear about it.

    I do agree with the main point though, even if your justification is irrational, that roll spoilers are neat but probably not suited to a light single.
     
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