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Thread: Control surface slots and seals.

  1. #1

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    Control surface slots and seals.

    Lets move forward with discussion that started over on Oli's thread.
    There is allot of good information available that should be brought to light, lets do it here.

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    Sky left off with a comment of Champion adding Spades to the ailerons. The intention for these is to reduce control effort when maneuvering. These do not do much to alleviate the sloppiness other than reduce control loads such as to mask the vagueness. The sloppiness is in the geometry of the system which goes back to decisions made on the drawing board that have never bee addressed since.

    My experience with the spades has been more with the 8K and B series wing where the planes routinely get flown to there limits and beyond.
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    skywagon8a's Avatar
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    OK Charlie, let's start with the Aeronca 7 series ailerons since that is the airplane which Gary mentioned.

    Any control surface requires clean airflow over it in order for it to efficiently do it's job.

    An up aileron dumps wing lift by creating a deflection of the upper surface airflow destroying some lift (acting like a spoiler) in order to make a wing drop and a down aileron increases lift in order to raise the wing. Due to the Aeronca/American Champion design when at low speed/high angle of attack the boundary layer on top of the trailing edge of the wing is separating leaving some dead air space above the ailerons. When the up moving aileron is deflected into that dead air space there is nothing for it to react against. The other down aileron is the only one doing any work. Since the down aileron will then be at an even higher angle of attack it will stall causing the down aileron wing to drop. Remember practicing stalls over the top from turns?

    The up 7 series aileron has a section which acts like a drag producing fence which protrudes below the wing. This is for the purpose of counteracting the adverse yaw drag generated by the down aileron.

    When this airplane is flying at low speeds above stall the ailerons can be moved from stop to stop with only a very little amount of rolling action of the airplane. Very sloppy.
    American Champion's fix is to add an optional set of spades to the ailerons which fly well below the wing in clean air. Thus they are effective at high angles of attack of the wing. They are Just a band-aid!

    IF the ailerons were redesigned so that the clean air below the wing was redirected through a slot between the wing and the aileron leading edge there would be clean air for the up aileron to push against. This would make both ailerons share their responsibilities rather than having just one doing the roll control.

    Ha!​ You beat me to the punch while I was typing.
    N1PA

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    RVBottomly's Avatar
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    Thanks for starting this, Charlie. I've been trying to sort things like this out on and off for that past year.

    Sky, your observations seem right on for low speed.

    Now I wonder if anyone has articulated ailerons so there is no gap at small deflections, but opens at large down deflections. I like the gap idea but it seems draggy at higher speeds for the ailerons. Flaps retract, maybe ailerons can too?

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    skywagon8a's Avatar
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    Vic, At the high speeds of our airplanes the ailerons move very little in order to perform their function. At high speeds there is a slight amount of boundary layer separation and very little pressure differential top to bottom at the aft portion of the wing. Thus any gap will not generate a differential airflow and it's accompanying drag.

    Ailerons need to be designed to function at stall speeds. That is why they are the size that they are. Cruise ailerons could just be small tabs at the wing tips.

    The next time you ride on an airliner look at the ailerons. The big ones at the wing tips only operate when the flaps are not up. There is a small separate set of ailerons inboard for flaps up higher speed roll control. There are also spoilers for high speed roll control supplementing the inboard ailerons. The inboard control is to reduce wing bending loads at higher speeds.
    Last edited by skywagon8a; 06-19-2019 at 08:09 AM.
    N1PA
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    Quote Originally Posted by skywagon8a View Post
    OK Charlie, let's start with the Aeronca 7 series ailerons since that is the airplane which Gary mentioned.

    Any control surface requires clean airflow over it in order for it to efficiently do it's job.

    An up aileron dumps wing lift by creating a deflection of the upper surface airflow destroying some lift (acting like a spoiler) in order to make a wing drop and a down aileron increases lift in order to raise the wing. Due to the Aeronca/American Champion design when at low speed/high angle of attack the boundary layer on top of the trailing edge of the wing is separating leaving some dead air space above the ailerons. When the up moving aileron is deflected into that dead air space there is nothing for it to react against. The other down aileron is the only one doing any work. Since the down aileron will then be at an even higher angle of attack it will stall causing the down aileron wing to drop. Remember practicing stalls over the top from turns?

    The up 7 series aileron has a section which acts like a drag producing fence which protrudes below the wing. This is for the purpose of counteracting the adverse yaw drag generated by the down aileron.

    When this airplane is flying at low speeds above stall the ailerons can be moved from stop to stop with only a very little amount of rolling action of the airplane. Very sloppy.
    American Champion's fix is to add an optional set of spades to the ailerons which fly well below the wing in clean air. Thus they are effective at high angles of attack of the wing. They are Just a band-aid!

    IF the ailerons were redesigned so that the clean air below the wing was redirected through a slot between the wing and the aileron leading edge there would be clean air for the up aileron to push against. This would make both ailerons share their responsibilities rather than having just one doing the roll control.

    Ha!​ You beat me to the punch while I was typing.
    Good stuff, I like this.
    The upward moving aileron on most airfoils gets much of it's force at higher speeds not from disruption of airflow on the upper surface, but from the pressure change on the lower surface. take roll control spoilers for example, they have a dead zone with just a small deployment due to the boundary layer thickness on the aft upper surface.
    The aileron is effective in tiny movement due to the pressure drop on the lower surface.

    In the low speed case of the 7A Aeronca, I have not studied how much twist it in that wing? I expect the full trailing edge is stalling while there is plenty of lift remaining to continue flight. This would account for the "dead" ailerons.

    Spades,
    I had not thought of their value at low speed but can see fully what you are bringing up here. They are small control surfaces that remain in clean air while the main surface is stalled in dead air.
    My little bit of time in Aeroncas since I sold mine before flying it, I just knew my instructor said to use rudder only when slow. Logical but in this plane it was for a more important reason than preventing an unwanted wing drop, it was because the wing was not going to respond.

  7. #7
    cubdriver2's Avatar
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    Rudder is your first and last friend in any airplane.

    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"

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    RVBottomly's Avatar
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    Quote Originally Posted by skywagon8a View Post
    Vic, At the high speeds of our airplanes the ailerons move very little in order to perform their function. At high speeds there is a slight amount of boundary layer separation and very little pressure differential top to bottom at the aft portion of the wing. Thus any gap will not generate a differential airflow and it's accompanying drag.
    I'm going to have to ponder this boundary layer idea a bit to understand it. I get the pressure differential is low at near level angle of attack. But I see from flap design that it seems preferable to have the upper surface flush when retracted, even with gaps when deployed. So I'm trying to figure out why it would be different for ailerons.

    Of course, I my idea of "high speed" in this context is around 120-140 mph.

  9. #9
    BC12D-4-85's Avatar
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    Good idea on the breakout. Oli's experiment will be fun to follow.

    Here's a dupe of the post on slotted flaps and ailerons so it doesn't get lost.

    https://ntrs.nasa.gov/archive/nasa/c...9930091708.pdf

    https://generalaviationnews.com/2015...rbon-cub-ex-2/
    http://cubcrafters.com/c/2017/02/g-s...ailerons-faqs/

    Note the cove gap tapering to the outlet and lowered flap hinge line.

    Gary
    Last edited by BC12D-4-85; 06-19-2019 at 12:41 PM.

  10. #10
    RVBottomly's Avatar
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    Gary, that nasa pdf link comes back: "The page you tried to access does not exist on this server."

    I'd love to see that.

    Vic

  11. #11
    BC12D-4-85's Avatar
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    Bellanca later offered aileron and elevator gap seals to close those areas. Presumably that improved the maneuvers they promoted as a sales option in the Acrobatic Category. Now think wood spars and slipping nails for rib fastening leading to AD.

    Piper and Taylorcraft changed their aileron coves post WWII, perhaps to coincide with newer CAR3 requirements? Piper modified the original Cub's design on the PA-12 (+14?) from one like Oli's to a similar design like the flap cove. Not sure what the preceding J-5's were like. Same for Taylorcraft. The ailerons were also modified on the 12 with a forward lip I assume to correct adverse yaw with the up aileron. Stinson followed a similar pattern post War with aft wing design.

    Just some observations. There may be more.

    Gary

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    skywagon8a's Avatar
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    Quote Originally Posted by CharlieN View Post
    Logical but in this plane it was for a more important reason than preventing an unwanted wing drop, it was because the wing was not going to respond.
    Isn't this basically the same thing?
    N1PA

  13. #13
    BC12D-4-85's Avatar
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    Quote Originally Posted by RVBottomly View Post
    Gary, that nasa pdf link comes back: "The page you tried to access does not exist on this server."

    I'd love to see that.

    Vic
    Fixed above also: https://ntrs.nasa.gov/archive/nasa/c...9930091708.pdf

    The wing in question is a 23102 airfoil but the principles still apply to others. The Discussion section post p. 27 is worth a read as it explains the overall effects on flow circulation and pressures above and below the surfaces.

    Gary
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  14. #14
    skywagon8a's Avatar
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    Quote Originally Posted by RVBottomly View Post
    Gary, that nasa pdf link comes back: "The page you tried to access does not exist on this server."

    I'd love to see that.

    Vic
    This one? Page 27 is a list of references.
    https://ntrs.nasa.gov/archive/nasa/c...9930083935.pdf
    or this one?
    https://ntrs.nasa.gov/archive/nasa/c...9930091708.pdf
    N1PA
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  15. #15
    BC12D-4-85's Avatar
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    Quote Originally Posted by skywagon8a View Post
    The first link shows loss of flow behind aileron slots plus this and that about twisting and LE devices on a Taylorcraft vs stall and AOA. The second is the early plain vs slotted flap analysis.

    Fred Weick (https://www.goodreads.com/book/show/..._the_Ground_Up) the lead author of the Taylorcraft analysis had a long career in aeronautics. His goal was reducing stall/spin accidents and the Ercoupe was one of his products.

    Gary
    Last edited by BC12D-4-85; 06-19-2019 at 12:59 PM.

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    RVBottomly's Avatar
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    Thanks for those links. More stuff to study.

    As for the Cub Crafter news item Gary posted: https://generalaviationnews.com/2015...rbon-cub-ex-2/ , I see from the wing photo the flap slot and also what looks like an aileron slot. The aileron looks balanced at the end, too.

    Which brings up another question regarding ailerons (I'm exposing my naivete each time I post anyway): would a balanced aileron extending beyond the wing tip be an useful thing in slow flight?

    I'm talking about, instead of spades, having the aileron extend past the wingtip with a forward-of-the-hinge airfoil like we see on the empennage of super cubs.

  17. #17
    skywagon8a's Avatar
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    Quote Originally Posted by RVBottomly View Post
    Which brings up another question regarding ailerons (I'm exposing my naivete each time I post anyway): would a balanced aileron extending beyond the wing tip be an useful thing in slow flight?

    I'm talking about, instead of spades, having the aileron extend past the wingtip with a forward-of-the-hinge airfoil like we see on the empennage of super cubs.
    I never gave that much thought but yes it should since it would be operating in undisturbed air.

    Like this.
    N1PA
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  18. #18
    RVBottomly's Avatar
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    Thanks, Sky. That's exactly what I was envisioning.

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    cubdriver2's Avatar
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    Curtis's put the aileron between the wings in 1911 and Fokker extended them past the end of the wing in the late teens, nothing new

    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"

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    flyrite's Avatar
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    Added my spades to lighten the aileron forces, it was especially useful after I mounted the 22” spoilers on the Tips . Spades can be used for a variety of different reasons, like on the extra aerobatic planes ,they are mounted canted outboard with small plates, not only does it reduce forces and allow you to Taylor the forces to your desires ,it also adds stability for cross country that was lost due to no dihedral in the In the wings. When you extend the ailerons out past the wingtip, it accomplishes the same thing obviously, and are known as elephant ears. A lot of ultralights have done this with great success.
    Click image for larger version. 

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    RVBottomly's Avatar
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    Quote Originally Posted by cubdriver2 View Post
    Curtis's put the aileron between the wings in 1911 and Fokker extended them past the end of the wing in the late teens, nothing new

    Glenn
    Right. So, other than them getting caught in willows while taxiing in riparian zones, I'm wondering if there is any particular disadvantage to them.

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    BC12D-4-85's Avatar
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    I've mentioned before extending the Cub's upper flap farings over the ailerons. I liked that change and had no adverse effects. Same for rubber elevator and rudder seals.

    Gary

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    skywagon8a's Avatar
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    Quote Originally Posted by BC12D-4-85 View Post
    I've mentioned before extending the Cub's upper flap farings over the ailerons. I liked that change and had no adverse effects. Same for rubber elevator and rudder seals.

    Gary
    no adverse effects. That is one of criterion that the FAA looks at for issuing approvals for modifications.
    Accepting that, What positive effects did you notice?
    I get the elevator and rudder seals which would reduce leakage through the gap at high angles of deflection. What degree of improvement is something I'm not certain of.
    Can you address the improvement to the ailerons other than you liked it?
    N1PA

  24. #24
    BC12D-4-85's Avatar
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    Quote Originally Posted by skywagon8a View Post
    no adverse effects. That is one of criterion that the FAA looks at for issuing approvals for modifications.
    Accepting that, What positive effects did you notice?
    I get the elevator and rudder seals which would reduce leakage through the gap at high angles of deflection. What degree of improvement is something I'm not certain of.
    Can you address the improvement to the ailerons other than you liked it?
    We've discussed this previously. My goal was stable slow flight, maximizing lift, and minimizing aileron induced wing tip stall. To test that I incrementally set the wing wash to 1.5*, squared off the wing's round end with the Hoerner tips at the existing span (like Air Tractor), extended the flap fairings over the ailerons, extended the flaps into the fuselage, end spill plated the outboard flap ends, sealed any hinge slots in the flaps and ailerons, added VG's, and finally sealed the tail gaps first with tape. The time span was 1987-97 and the PA-18A-150 was flown on wheels, skis, and floats in Alaska year round. I'd run a series of subjective non-engineering quality slow flight and performance tests between mods and note what I could in the pre-Internet and GPS world.

    In the case of the ailerons I felt that when faired they were more effective in slow flight (although the rudder was the preferred control) and the wing was difficult to force into bad behavior during slips and skids. Subjectively it felt more solid - little mush during takeoff and landing while ground effect. I liked the control feedback and solid non-surprise feel if provoked.

    In the end after all the changes I borrowed a GPS III (?) and noted a 28 GS stall in level flight averaged over a quad course in little wind across the front and rear CG range I could load to.

    I will say one thing. Every little bit helps and obviously one incremental change can later surpass or incorporate the performance of earlier mods. In a 10-15 mph breeze the plane on floats spent little time on the step. Takeoffs were simply plow to liftoff in one smooth motion. On landing it would touch down in the low '30's GPS and just stop with little forward planing - less than 150' I'd guess.

    It was quite impressive compared to other planes I have flown. Others that flew it noted it was "tuned in".

    Gary
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    Gary,
    You mentioned the wing wash being set to 1.5°, Had you flown with other twist settings as well during your development?

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    flyrite's Avatar
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    Quote Originally Posted by BC12D-4-85 View Post
    We've discussed this previously. My goal was stable slow flight, maximizing lift, and minimizing aileron induced wing tip stall.

    I will say one thing. Every little bit helps and obviously one incremental change can later surpass or incorporate the performance of earlier mods.
    It was quite impressive compared to other planes I have flown. Others that flew it noted it was "tuned in".

    Gary

    Gary, my goal was as yours ,to better the low-speed capabilities of my mount that was not originally built with that in mind. The single biggest improvement to the slow speed characteristics were the addition of the slats. Not only did they make they wing hang on at higher angles of attack , but improved the low-speed handling even at lower angles of attack. As was discussed in a previous thread, they make the wing lift more efficient at all angles.
    Before the slats, I could aggravate the wing and get some severe breaks into Spin entries. Now you cannot get it to break, it must be flown into the spin.

  27. #27
    BC12D-4-85's Avatar
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    Quote Originally Posted by CharlieN View Post
    Gary,
    You mentioned the wing wash being set to 1.5°, Had you flown with other twist settings as well during your development?
    Yes Charlie others were tested before any of the other mods. I tried flat to factory (~2.5* from fuselage center; the 3/8 block technique; one rear turn of the forks at a time while on wheels). I settled on 1.5* as a compromise between abrupt loss of lift (pre-other mods including VG's) versus what I felt was a "soft or mush" at takeoff and landing with the factory approved setting. When I finally bought a digital level I was able to measure the 1.5* but before had no real idea just turns of forks from stock and a bubble level.

    I might mention I set the ailerons to be level in flight which was about 1/4" down deflection static. The flaps were set to parallel the aileron deflection in flight.

    Gary

  28. #28
    BC12D-4-85's Avatar
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    Quote Originally Posted by flyrite View Post
    Gary, my goal was as yours ,to better the low-speed capabilities of my mount that was not originally built with that in mind. The single biggest improvement to the slow speed characteristics were the addition of the slats. Not only did they make they wing hang on at higher angles of attack , but improved the low-speed handling even at lower angles of attack. As was discussed in a previous thread, they make the wing lift more efficient at all angles.
    Before the slats, I could aggravate the wing and get some severe breaks into Spin entries. Now you cannot get it to break, it must be flown into the spin.
    Did you make any changes to the tail...like seals or increased surface area as a companion to the slats at very slow speeds?

    Today we have LE devices like slots, slats, VG's, and cuffs. I had a Crosswinds STOL kit on my Citabria which I liked very much after the installation, but removed the same from my PA-12-180 and liked the stock LE better for whatever reason.

    Gary

  29. #29
    flyrite's Avatar
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    Quote Originally Posted by BC12D-4-85 View Post
    Did you make any changes to the tail...like seals or increased surface area as a companion to the slats at very slow

    Gary

    Sealed the gap which did not make any difference ,do have VG’s on the bottom. I run out of back stick at slow speed’s and due to the high thrust line ,can’t ride the power at high angles of attack because it will push the nose over well Before stall. Have a J3 tail now, I understand the super cub tail is a drop in replacement with no mods, that is the next thing on the list.
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  30. #30
    BC12D-4-85's Avatar
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    Quote Originally Posted by flyrite View Post
    Sealed the gap which did not make any difference ,do have VG’s on the bottom. I run out of back stick at slow speed’s and due to the high thrust line ,can’t ride the power at high angles of attack because it will push the nose over well Before stall. Have a J3 tail now, I understand the super cub tail is a drop in replacement with no mods, that is the next thing on the list.
    I went from a stock PA-11 tail (unbalanced elevator) to a PA-18 stab/elevator when I installed flaps. It's heavier but I felt mo better for control especially when slow - plus its part of the flap STC. I was going to install a PA-18 rudder next but changed airplanes.

    Edit: Double trim cable pulley setup also required for the balanced tail.

    Gary
    Last edited by BC12D-4-85; 06-20-2019 at 01:55 PM.
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  31. #31
    flyrite's Avatar
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    Quote Originally Posted by BC12D-4-85 View Post
    I went from a stock PA-11 tail (unbalanced elevator) to a PA-18 stab/elevator when I installed flaps. It's heavier but I felt mo better for control especially when slow -

    Gary

    My Thoughts exactly!

  32. #32
    BC12D-4-85's Avatar
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    Sometimes it's good to step back and fly the stock plane enough to note any changes mods later make. As built Cubs are a fine airplane...not sure of the exact % but surely 3/4 or more as good as one later tricked out. Empty weight is very important and the first PA-18-150 was a fatty at upper-1100+ (I'm being gracious-she was a lady). It flew well but the weight could be felt especially the extended wings. The PA-18A-150 above was mid-1000 when done and I liked that feel. So maybe do a few improvements to keep the air under the wing and on top of the tail plus good rudder and keep it light as possible.

    Gary
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