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Thread: Move the wing for better CG?

  1. #1
    Little_Cub's Avatar
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    Move the wing for better CG?

    Always thinkin' about that 'next one'.

    Couldn't find anything on moving the wing.. sooo

    Lotsa folks have issues with the CG of a hi-HP Cub and patches for this have been shortening the mount, shortening the firewall,
    moving the battery and even lengthening the tail.

    Anyone taken the effort to move the wing? It seems like it would help on both ends and might require far less
    movement to afford the same result.

    Thoughts? Calculations? Guesses all welcome!
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    www.SkupTech.com mike mcs repair's Avatar
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    great idea,

    the original cub, had a certain wing location that matched the weight of engine and extra junk...

    so as we add more junk and bigger engine, that ratio get out of wack, and we have to compensate with elevator/stab.. engine mount angle...

    much better to get back to a proper wing location...

  3. #3
    Gordon Misch's Avatar
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    Feeling dumb here - - - how is moving the wing different than moving the other stuff - engine, tail, etc?
    Gordon

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    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by gordon misch View Post
    how is moving the wing different than moving the other stuff - engine, tail, etc?
    exactly!!!

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    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by Gordon Misch View Post
    Feeling dumb here - - - how is moving the wing different than moving the other stuff - engine, tail, etc?
    not sure if i can express this clearly, it's late.. beer... I've been singing...

    kinda a static or dynamic thing?

    if the plane flies well, power off, or prop stopped.... its balanced....

    but if not balanced, it needs (engine/or stab/elevator) power/input to fly ok, to make up for that imbalance in load/CG...

  6. #6
    Gordon Misch's Avatar
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    I thought I heard somebody singing

    if the plane flies well, power off, or prop stopped.... its balanced....
    Agreed. But given that, the positions of all the components of an airplane seem entirely relative to me. In other words, for example, moving the wing back would be the same as moving the tail forward and moving the engine forward.
    Gordon

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    www.SkupTech.com mike mcs repair's Avatar
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    but... can you name the song or movie it's from?


    Quote Originally Posted by Gordon Misch View Post
    I thought I heard somebody singing

  8. #8
    skywagon8a's Avatar
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    Quote Originally Posted by mike mcs repair View Post
    but... can you name the song or movie it's from?
    https://www.youtube.com/watch?v=qVjCag8XoHQ
    N1PA

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    Moving the wing would limit the CG envelope. The extended airframe works better for carrying stuff. I guess it depends on what you want your airplane to do.
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    In my design and build I am now pretty well lock down on what the build is, as in power and overall fuselage geometry. The wing has a few more calculations till I am rally happy with it. I have set the incidence which is less than what most Cub guys would build in since my cruse speed is higher and I am letting the monsters double slot flaps take care of the slow speed needs. So my last big design need is setting the fore-aft location of the wing over the fuselage. I expect to have my answer to this in a few months at which time I will then lock it in the metal.
    So for me, yes I am considering just how the plane will hang under the wing.
    Frank, with your mission I expect this would be very important. Your needs are ultimate importance at a very low speed with minimal weight, you would need to take the pitch moment with flaps and slats extended and get the plane to hang with minimal trim at extremely low speed such as no trim at 30 where my plane is being optimized for no trim at 120 with twice the weight.
    FWIW I calculate the wing on most Cub design was optimized for 70 mph @ about 1200#. Obviously Cub designs have varied substantially but I have not seen changes to wing location to optimize for said changes.

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    Quote Originally Posted by stewartb View Post
    Moving the wing would limit the CG envelope. The extended airframe works better for carrying stuff. I guess it depends on what you want your airplane to do.
    Moving the wing will allow for carrying more weight aft in the plane. Our limit of what can be carried is keeping within the CG range of the wing. If you move the wing aft so the light CG is at the forward limit, then you can carry more weight in the aft of the fuselage.
    Notice I do not state empty CG since we do not fly an empty airplane.
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    Moving the wing to help with forward/back CG can be done but now you have to deal with the strut attachment/strut length/fuselage cluster/door/window/wind screen. Putting the fuel tank in the tail will do the job, but what happens when you use it. Fly from raised back seat with bubble top for head. This would be pretty simple and kind of how piper did it.
    DENNY

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    I assumed Frank was thinking of moving the wing aft to center his CG with minimum loads to benefit STOL performance. For most planes I've flown the light fuel load/single pilot CG is at or near the forward limit. Sometimes I have to put weight in back to achieve that. Since virtually everything I consider useful load moves the CG aft? I want to keep the empty CG forward. If aft CG becomes a threat the practical solution is a belly pod for heavy stuff and lighter stuff goes into the aft baggage.
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    I think he is nose heavy. You could just extend the topdeck forward a few inches, move and gusset wing fittings, maybe move a cross tube or two. Struts would angle forward a bit, might have clearance issue with bottom of rear strut, change to taylorcraft attachment at the bottom? Still dark and wet out I will keep thinking.
    DENNY
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    Reduce the wing angle of incidence, and you reduce the pitching moment, resulting in a nose-up condition. At full forward CG, the stab should be slightly lower in it's slot, and the elevator deflected slightly up. When my wing was mounted at zero degrees incidence, (and washout correct) the pitch trim was halfway to the full nose-down position, and the elevator down at a noticeable angle. Other than the vicious spin (seven turns minimum recovery) that resulted from a stall, it still flew pretty good!

    *only tried to kill me once.....

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    hotrod180's Avatar
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    Seems like I recall reading up on airframe design,
    it mentioned establishing the CG at xx% of MAC (mean aerodynamic chord)?
    Whether you do that by manipulating location of engine battery seats etc in/on the fuselage,
    or by moving the wing fore-or-aft on the fuselage,
    should be a matter of which is easier or more convenient.
    Cessna Skywagon-- accept no substitute!
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  17. #17
    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by DENNY View Post
    ... Putting the fuel tank in the tail will do the job, ...
    DENNY
    I've always wondered about having 2 tanks one in tail and one as far forward and small transfer line/pump...

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    That’s why the MD-11 and A-300 have tail tanks.




    Quote Originally Posted by mike mcs repair View Post
    I've always wondered about having 2 tanks one in tail and one as far forward and small transfer line/pump...
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  19. #19
    www.SkupTech.com mike mcs repair's Avatar
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    you could actually do this as a bolt on mod, as far as moving wings forward or backwards..

    strut at bottom will swivel, would need an adaptor/extender at top of lift strut to fix angles....

    cable routing issues? add pulley and extend?

  20. #20
    www.SkupTech.com mike mcs repair's Avatar
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    www.SkupTech.com mike mcs repair's Avatar
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    hey, you could even make that bolt on thing slide for and aft so you could play with it....(in flight? or ground...??)

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    Frank have you ever thought about a way to change the wing incidence in-flight like the F-8 Crusader? Lift benefits or just a change the deck angle during take-off or landing. A higher aspect ratio wing giving more lift at slower speeds may the only way to go as you've probably maxed everything one can get out of a Cub wing.



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    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by wldwzl01 View Post
    Frank have you ever thought about a way to change the wing incidence in-flight like the F-8 Crusader? Lift benefits or just a change the deck angle during take-off or landing. A higher aspect ratio wing giving more lift at slower speeds may the only way to go as you've probably maxed everything one can get out of a Cub wing.



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    there is an old thread on that on here somewhere... i describe how to do it, how to deal with the strut length issue...

  24. #24

  25. #25
    Little_Cub's Avatar
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    Quote Originally Posted by mike mcs repair View Post
    hey, you could even make that bolt on thing slide for and aft so you could play with it....(in flight? or ground...??)
    There are many ways to tweak things and we've tried a few.. tail weights, moving the firewall, fanny tank and even a tail tank. Our plane weighs 800+ pounds empty, with the firewall back 5" we still had nearly 20# at the tail post last year in Valdez. Unless it was adjustable like Mike suggests it will just need to negate the engine moment and rely on a chunk of tail weight for those STOL days.. without adjust-ability (added weight) it just won't fit all occasions perfectly. The effort here is to keep the original Cub balance without feeling trapped in a nose heavy machine or being cramped behind a modified rudder pedal location.
    On a fresh build moving the wing attach and even the lower fittings isn't that big a deal, we all like the HP but flying a C-90 frame with a 0-360 doesn't allow all fun to get out!
    1- How important is it for the center of lift to be behind the CG window? (stability?)
    2- Is it true moving the wing forward does a couple 'relative' things..
    a. Moves the center of lift forward
    b. Moves the relative CG back.

    More yammer please!

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    Both a&b are the result.
    How much does your trim change with the flaps deployed?
    Can the plane settle down hands off with full flaps? I would expect that would be a goal allowing you the most sensitivity for slow approaches.

    I expect the wing will only need to move an inch or two but I have not run any numbers I may be well off on that.
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  27. #27
    www.SkupTech.com mike mcs repair's Avatar
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    adjustable:
    big weight on a endless cable like on a piper cherokee trim system should be able to find salvage

  28. #28
    www.SkupTech.com mike mcs repair's Avatar
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    and the weight would run on a heavier cable, the little cable is just to move it, like on a mining ore tram setup..

  29. #29
    www.SkupTech.com mike mcs repair's Avatar
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    there's a guy on fire lake that can weave you a custom length continuous cable, mark kerpenski ....

  30. #30
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    Has anyone ever flown their empty cub in competion from the back seat? Usually empty cubs with 1 person in front are pretty forward cg. Maybe longer to get tailwheel off, but you could hank harder on the brakes for landing..... just thinking... my Scub is single controls, so I can't try it.
    "There are three things in life that people like to stare at: a flowing stream, a crackling fire and a Zamboni clearing the ice." Charlie Brown

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  31. #31
    cubdriver2's Avatar
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    My Pa11 would land and takeoff shorter and slower from the back seat

    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"
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  32. #32
    Little_Cub's Avatar
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    Quote Originally Posted by cubdriver2 View Post
    My Pa11 would land and takeoff shorter and slower from the back seat

    Glenn
    The further you go the better.. until its too far however we normally fly in front of a Cub, with a big engine it's not balanced
    even before flaps add to the equation.

    Anyone know the math or what the result of moving the wing will bring? If we are moving the whole fuselage
    back relative the forward moving center of lift it doesn't seem like it would take much to get to the sweet spot again.
    Yes the wings movement will add to the forward CG but would think its a limited effect with a short arm.

    What if the center of lift approaches the CG point? Normally the center of lift remains behind the CG.. what is that result
    as it gets closer?

    I know there are some engineers on board

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  33. #33
    Gordon Misch's Avatar
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    What if the center of lift approaches the CG point? Normally the center of lift remains behind the CG.. what is that result
    as it gets closer?
    The tail doesn't have to push down as much in order to keep the nose up. Therefore wing lift is not wasted counteracting the down-push of the tail. Not as much drag from either wing or tail. And of course, pitch stability is reduced. Again, I'm not an aero engr, so I realize this is way-basic. But it's a start?
    Gordon

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  34. #34
    skywagon8a's Avatar
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    Instead of moving the wing, Why not just move the seats and controls aft? After all we are just talking about a CG adjustment.
    N1PA

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    Quote Originally Posted by Little_Cub View Post
    The further you go the better.. until its too far however we normally fly in front of a Cub, with a big engine it's not balanced
    even before flaps add to the equation.

    Anyone know the math or what the result of moving the wing will bring?

    Thank you!
    The way to go about the math.
    Since you have handled as well as built all the components of your craft.
    With the wings off, do a weight & balance of your fuselage. Do this with you seated in the plane. You may need tail weight without the wings, maybe not but get the numbers.

    You know what your wings weigh but you need to know where their balance point is fore-aft.
    Your wings also provide where your center of lift is.

    I expect for the math, use the LE of the wing as your datum.
    Now run your W&B but on paper, move the full mass of your fuselage fore-aft till you feel it is where in the lift range you want the mass.

    I expect the wing will want to be further forward, the numbers will tell you.

    Also run numbers with the engine moved forward from where it currently is should you want to regain room up front. Might consider just moving the engine on it's mount for part of this allowing you to fine tune after the build such that if the plane is not comfortable it can be tuned with different engine mounts rather than adding tail weight.

    Not sure if you have done this yet, I am sure you are not running any washout but you might consider having adjustment room in your struts to put a degree or so of positive twist in the wing.

  36. #36
    hotrod180's Avatar
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    Quote Originally Posted by Little_Cub View Post
    The further you go the better.. until its too far however we normally fly in front of a Cub, with a big engine it's not balanced
    even before flaps add to the equation...
    Now that you brought up the big engine...
    I thought your early performances at Valdez were awesome--
    a lightweight Cub with a stroked 85 spanking the hotrods.
    The Jerry Burr lightweight thing carried out to the nth degree.
    I was kinda surprised (and I will admit, kinda disappointed) to see you show up later with a 320 bolted on.

    Now you're talking about what sounds like a real major mod (moving the wing attach) to alter the CG,
    due to the heavier Lycoming engine.
    Have you considered just going back to the Continental?
    Lighter & simpler is often (if not usually) better-
    and that why originally I found your Little Cub so intriguing.

    This kinda makes me think of the Cessna 180--
    over the years it got heavier & heavier as they "improved" it,
    but some of us still prefer the lighter early models.
    Cessna Skywagon-- accept no substitute!
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  37. #37
    Little_Cub's Avatar
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    Quote Originally Posted by hotrod180 View Post
    Now that you brought up the big engine...
    I thought your early performances at Valdez were awesome--
    a lightweight Cub with a stroked 85 spanking the hotrods.
    The Jerry Burr lightweight thing carried out to the nth degree.
    I was kinda surprised (and I will admit, kinda disappointed) to see you show up later with a 320 bolted on.

    Now you're talking about what sounds like a real major mod (moving the wing attach) to alter the CG,
    due to the heavier Lycoming engine.
    Have you considered just going back to the Continental?
    Lighter & simpler is often (if not usually) better-
    and that why originally I found your Little Cub so intriguing.

    This kinda makes me think of the Cessna 180--
    over the years it got heavier & heavier as they "improved" it,
    but some of us still prefer the lighter early models.
    Agree with the smaller-lighter C85 but it just didn't pull hard enough for the eventual goal.. F_U_N factor was over the top!
    We don't want to change Lil Cub we are talking about a new build.. HP is good and the nimble feel is still available..
    just gotta get to the sweet spot without messing too much else up.. like leg room or visibility. With Lil Cub we cut the firewall back 5"
    with a little weight in the tail it's in the fun zone again.

    We would like to build that perfect machine.. plenty HP, leg room and comfortable, lotsa visibility, fun flyer.. too many times we sacrifice something.

    Speaking of Jerry.. he was at Oshkosh this year and (flying his certified J-3) beat all the Carbons, HP Super Cubs and all except Steve with the hot rod Just Aircraft in Experimental class! A lot to be said for consistency.. one good stick.

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    Isn't part of the reason for extending the SQ-12 and Rev 2 to re-gain elevator and trim authority to take advantage of the high lift/high AOA wing mods? Why is extending the tail aft a bad thing? It provides a good benefit-weight ratio. Judging by the SQ-12 videos and the pirep gross loads it carried it was well balanced. The Rev 3 wing may escalate things to another level, too. Interesting stuff.
    Last edited by stewartb; 10-31-2017 at 03:01 PM.
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  39. #39
    www.SkupTech.com mike mcs repair's Avatar
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    what about, if talking of moving wing forward, instead add another layer of slats out front of existing ones??? do they effect the wing CL???

    since they came here all nicely stacked together in the crate....

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    Movable (forward extending) leading edges? Hell, dream big.
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