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Thread: Aft C of G

  1. #1
    cgoldy's Avatar
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    Aft C of G

    I thought this was worthy of a new thread. In another thread one of our Gurus said,

    Quote Originally Posted by skywagon8a View Post
    You will be very happy with the flying characteristics if you can keep the CG somewhat aft. Your short field operations would be greatly improved with this capability.
    I have heard this said many times and I am not doubting it. But could the aerodynamics / physics of this please be explained
    Back Country O-375 wide body extended wing cub

  2. #2
    RVBottomly's Avatar
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    I think the simple way to look at it is that when you are forward cg, you need more up elevator to get the nose up. That increases drag.

    The elevator is producing upside down lift, but lift of any kind increases drag.

    Sent from my SM-J320V using SuperCub.Org mobile app

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    cubdriver2's Avatar
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    Hold a gallon of milk at arms length in front of you, now hold it close to your chest. Which is easier

    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"
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    CG location effects your stall speed, due to the negative lift required by the tail. The further forward the cg the higher the stall speed.

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    skywagon8a's Avatar
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    Normally the horizontal tail is producing down lift. This increases the amount of weight that the main wing must lift. The two are fighting each other.
    The stabilizer must be moved to trim out the two forces. This increases drag.
    By moving the CG aft, the down load on the tail is reduced. Thereby the stabilizer can be mover to a more streamlined position when compared to the wing reducing the induced drag. = more speed.
    An airplane with a trim tab will have the tab displaced which displaces the elevator in relation to the stabilizer creating drag. When the CG is in the correct position these three items will be lined up reducing drag.

    When the CG is aft, less up elevator is required to raise the nose, thus less drag, less weight (tail down force) for the wing to lift, shorter take off run, better rate of climb, and a bit higher cruise speed.

    This will require a bit more down elevator to initially raise the tail on take off. But that is at the low drag end of the speed range and you don't need to get the tail very high for the optimum lift off angle and distance.
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    Eddie Foy's Avatar
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    That's why some big airliners have a wet tail. After takeoff, fuel is pumped aft to put the CG at or near the aft limit. Saves fuel.
    "Put out my hand and touched the face of God!"
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    In a Cessna 150, if you both slide your seats from full forward to full aft in cruise your airspeed will change about 10 mph.

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    Glad I never flew an airliner with a balance tank. We always hung a flag on the glareshield for crossfeeding, and even then sometimes we had to crossfeed back the other way.

    I am going to install a "cleared to land" switch in the Cubs. "Did we get a clearance yet?" daily!
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  9. #9
    texmex's Avatar
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    Cgoldy, it's to do with the distance between the CoG and the CP (Centre of pressure) and the controling arm of those two.

    The further apart the CoG and CP are, the more tail force required to balance the seesaw. The more tail force, the more drag with elevator displacement. (obviously less so with a trimable stab)

    Take all the above with a grain of salt, as I'm no aeronautical engineer, but my son is and we had a quick discussion the other day after reading the C180 developement link put up recently on this forum.

    Glad I never flew an airliner with a balance tank. We always hung a flag on the glareshield for crossfeeding, and even then sometimes we had to crossfeed back the other way.
    Mostly they do it all automatically. At least on the A330 I'm flying. But if the memory is correct the 78 did the same (I think). Yes the old 73, balancing of wing tanks was common. Must of had shite check valves.

  10. #10
    Speedo's Avatar
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    Quote Originally Posted by bob turner View Post

    I am going to install a "cleared to land" switch in the Cubs. "Did we get a clearance yet?" daily!
    Cleared for the approach: Taxi Light . . . . . . ON
    Cleared to land: Landing Light . . . ON
    Speedo
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    Quote Originally Posted by bob turner View Post
    Glad I never flew an airliner with a balance tank. We always hung a flag on the glareshield for crossfeeding, and even then sometimes we had to crossfeed back the other way.

    I am going to install a "cleared to land" switch in the Cubs. "Did we get a clearance yet?" daily!
    It is completely automatic and works solidly. Never had a problem. If you do you can gravity feed back to the main tanks. I sometimes see CGs of 39% of MAC in cruise flight. Before you start down it transfers forward automatically. Nor can you tell a handling difference in fly by wire as the plane always handles the same. Quite frankly the Airbus fuel system is the most trouble free and mindless fuel system I have every flown with. About the only trouble it runs into is the fuel in the outer wing tanks in the winter gets too cold and you have to manually dump it into the mains. Two button pushes and it is done. Have never had to cross feed except in the sim with an engine out, since the trim tank balances the mains.

    As for light planes, aft CG within the envelope makes it fly real nice. Neutral to slightly positive stability. Too far forward and you have excess stability. I remember I used to fly a Comanche 400 that was a real bear to land if you did not put a case of oil in the baggage compartment. Mooneys can be tricky too with forward CG. On both airplanes you have to carry excess speed for the elevator to be effective on flare. Not a recipe for good landings with laminar flow wings.
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  12. #12
    stewartb's Avatar
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    Too far forward and you have excess stability.
    Can you explain that? What is excess stability?

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    Gordon Misch's Avatar
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    Quote Originally Posted by stewartb View Post
    Can you explain that? What is excess stability?
    Greater tendency to return to the trimmed condition than is necessary, after a pitch displacement from trimmed pitch. i.e. greater stick forces and reduced stick response in pitch.
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  14. #14
    stewartb's Avatar
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    Not true in my airplanes. Resistant to pitch changes at slow speeds, maybe, but the same is true for aft CG. There's more to stability than pitch.

    GeeBee? I'm genuinely interested.

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    Gordon Misch's Avatar
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    Do some research Stewart. I'm referring to pitch stability, which longitudinal CG affects.
    Gordon

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    stewartb's Avatar
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    I'm trying. Your definitions don't make sense in my airplanes. That's why I asked.

  17. #17
    Gordon Misch's Avatar
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    Ok, so I'll adopt the conventional definition of positive stability, and that is the tendency to return to a stable condition. In other words, "strong" positive stability is a pronounced tendency to return to neutral, whereas "weak" positive stability is less pronounced.

    As CG moves aft, pitch stability becomes "weaker", until at a critical CG, stability becomes neutral, and as the CG moves further aft the stability becomes negative and a tendency to depart from the trimmed condition becomes more dominant.

    With well aft CG you'll notice your stick forces decreasing in a given flight regime.
    Gordon

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  18. #18
    pa11driver's Avatar
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    Quote Originally Posted by cgoldy View Post
    I thought this was worthy of a new thread. In another thread one of our Gurus said,



    I have heard this said many times and I am not doubting it. But could the aerodynamics / physics of this please be explained
    While I understand the theory here, I’ve never found this to be a true or a desirable situation while working off airport in any of the various cub type airplanes I’ve flown. I always load as far forward as possible. What are others thoughts?
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    Quote Originally Posted by Gordon Misch View Post
    Greater tendency to return to the trimmed condition than is necessary, after a pitch displacement from trimmed pitch. i.e. greater stick forces and reduced stick response in pitch.
    You got it!
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  20. #20
    Gordon Misch's Avatar
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    Post#5 by Skywagon nails the physics of it.


    Edit: Stewart, this link might be helpful?
    https://en.wikipedia.org/wiki/Longit...atic_stability
    Last edited by Gordon Misch; 09-21-2018 at 12:12 AM.
    Gordon

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  21. #21
    stewartb's Avatar
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    Nope. But I’m out.

  22. #22
    Gordon Misch's Avatar
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    Tried to help, but those who don't want to be helped can't be. Thought you were sincere in asking - sorry to have bothered you.
    Gordon

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  23. #23
    www.SkupTech.com mike mcs repair's Avatar
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    there's allot of generalizations here, trying to be applied to different, planes types.... with different personalities....

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    Before we quit - any comments about spins when the CG starts toward the rear limit? Do you think an ordinary Citabria can go flat in a spin?

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    And one more thought - I don't remember any transfer problems in the A320, but we were transferring all the time in the early 737s.

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    skywagon8a's Avatar
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    Quote Originally Posted by bob turner View Post
    Before we quit - any comments about spins when the CG starts toward the rear limit? Do you think an ordinary Citabria can go flat in a spin?
    Since it is a certified airplane, when operated within the certified limits it should not. However that does not guarantee that it will not. I have done research spin tests in a certified airplane of a different type, loaded within and not near the CG limits when it turned a normal nose down spin into a fully developed flat spin. Why? I was unable to duplicate the maneuver. It is still a mystery to me as to why or how it did transition from a full nose down spin into a fully developed flat spin.
    N1PA

  27. #27

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    Think of a gyroscope. The there is a reason why gyros try to place a much mass as far as possible from the rotational axis. When you have a far aft CG you have done just that.

  28. #28
    jnorris's Avatar
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    Years ago, we were on our way home from Sun N Fun in our Cessna 180. Got weathered in for two days in Mattoon, IL. When the clouds finally lifted up enough for us to leave, we were facing a stiff wind right on the nose. I was flying as low as I could, to stay out of as much wind as I could, but we were still going really slowly across the ground.

    So we were somewhere in northern IL when my wife had to use her "relief" system. When she slid her seat back, the airplane picked up 3 mph. I made her stay back there for the duration of the flight! Every little bit helps!!

  29. #29
    texmex's Avatar
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    Think of a gyroscope. The there is a reason why gyros try to place a much mass as far as possible from the rotational axis. When you have a far aft CG you have done just that.
    You lost me on that. (my failure not your example)

    Pictures always welcome.

  30. #30
    hotrod180's Avatar
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    I understand why an airplane is faster with an aft CG,
    but not why the stall speed is lower. ??
    Cessna Skywagon-- accept no substitute!
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  31. #31
    skywagon8a's Avatar
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    The down load on the tail is lower. Thus the total weight which the wing needs to support is lower. Since the wing is carrying less weight it will have a lower stall speed.
    N1PA
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    cubdriver2's Avatar
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    Quote Originally Posted by hotrod180 View Post
    I understand why an airplane is faster with an aft CG,
    but not why the stall speed is lower. ??
    Post #3 is a simplified example

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

  33. #33

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    Quote Originally Posted by texmex View Post
    You lost me on that. (my failure not your example)

    Pictures always welcome.
    Here is a good explanation

    https://www.flyingmag.com/safety/acc...overable-spins
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  34. #34
    skywagon8a's Avatar
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    Quote Originally Posted by bob turner View Post
    Before we quit - Do you think an ordinary Citabria can go flat in a spin?
    Bob,
    Why did you ask this question? Did you have an incident? Curious minds, as I'm into wondering why something happens if for no other reason than to prevent a reoccurence. Flat spins could be nonrecoverable especially if the fuel in the tanks moves away from the outlets and the noise maker goes quiet.
    N1PA

  35. #35
    Eddie Foy's Avatar
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    If it was out of CG with two pilots, imagine how far out it would be with one large one in the pit!
    Quote Originally Posted by GeeBee View Post
    "Put out my hand and touched the face of God!"

  36. #36

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    No - I rarely do more than 3/4 of a spin.

    But I have heard stories of folks who like to count the turns - I understand the impulse to make/break records (yesterday I did 42 Stearman landings) - but my impression is that after two or three turns a spin will tighten up.

    We had a Citabria fatality last month - extremely experienced instructor; object of the lesson was CFI spin certification. I have heard almost nothing - one pilot suggested a low level engine failure and an unsuccessful landing - but I initially wondered if it hadn't spun in?

    I own a Decathlon - not quite the same. I am not wild about multiple turn spins, so will be doing no exploratory testing. Just wondering if these fine airplanes can go flat? Pretty sure a Cub can get into a difficult-to-recover spin with a heavy guy in the back. Not gonna try that either.

  37. #37
    skywagon8a's Avatar
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    Apologies to Colin for the thread drift however I don't believe that you will mind.

    The airplane in which I had the flat spin episode normally spins with the nose nearly or sometimes, beyond vertical. This one time the nose went down to the normal position, wobbled a bit and rose to level. The plane was absolutely level in pitch and bank. One wing was going forward and the other back. All of the controls had no feedback whatsoever. The same as though it was parked in the hangar while checking the controls. It made seven turns loosing 700 feet. The only control which had any effect towards recovery was the throttle blasting the tail pushing the nose down to break the stall. Why did it do this? That is a big mystery which I was unable to duplicate. With this experience I feel that any airplane could get into a flat spin. If you are into a normal spin and the nose starts up on it's own, stop the spin immediately.

    My Cub will only spin if I load it to the aft CG limit. Steep spiral yes, but spin no. I found no unusual spin characteristics in the Cub. That doesn't mean that something unusual will not happen. I've also done extensive spins in the 185 on amphib floats with no unusual characteristics found.
    N1PA
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  38. #38

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    A lot of people get carried away with spins. First of all, if you are spinning a normal category airplane, all bets are off. If you are spinning in the utility category, read the AFM or TDCS and placards very carefully as the envelope is narrower than you may think. If you are in an aerobatic airplane and you exceed 6 turns, unless greater than 6 turns is the certification basis, you are a test pilot. FAR23-2150

  39. #39
    skywagon8a's Avatar
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    Don't misunderstand me, all three of those planes were or are licensed Experimental-Research & development or Amateur built. The spins were accomplished completely in accordance with the FARs and in some cases were accompanied by an FAA test pilot.
    N1PA

  40. #40

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    None intended, I was just reviewing what the pilot of a certified aircraft, even one certified aerobatic should know before getting carried away. I was talking to a pilot of a well known certified aerobatic aircraft a while back, a real spin freak and he was stunned to know he was operating the aircraft outside of the certification parameters.

    In your case you really were a test pilot. Suffice to say that one incident would require investigation and remediation if one was going for a certification. The stall stability rules have to be exhibited every time, all the time.

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