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AFT vs. FWD CG

Discussion in 'CFI Corner' started by azaviator08, Mar 30, 2010.

  1. azaviator08

    azaviator08 New Member

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    Can someone help me come up with a simple way to explain the effects that Aft vs. Fwd CG have on takeoff, cruise, and landing.
     
  2. tgrayson

    tgrayson New Member

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    For takeoff and landing, the biggest issue is having enough control authority to rotate or flare; a forward CG can make this problematic. Since an aircraft already has a nosedown tendency in ground effect, takeoff and landing is the most critical flight regime for a forward CG.

    In cruise, a forward CG must be countered by increased download on the horizontal stabilizer; not only does this increase the drag due to the trimming necessary, but more importantly, the main wing must counter this download with greater lift, increasing the wing's induced drag.

    Moving the CG back eliminates both of these drag sources and if you can arrange it so that the tail has no download at all, you will maximize cruise performance.
     
  3. moxiepilot

    moxiepilot Well-Known Member

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    I believe this information is in the Airplane Flying Handbook, broken down into the categories you indicate.
     
  4. mshunter

    mshunter Well-Known Member

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    Arooo? This sound like you are saying you don't want a fwd CG, then later saying you do want it for t/o and landing.:confused:

    I think I follow your thought process, but can you eleborate.
     
  5. tgrayson

    tgrayson New Member

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    Hmmm, I see how it could be read that way, but I can't fix it now. I didn't mean "critical to have it", but "critical" as in risky, potentially dangerous, wheelbarrowing during takeoff or bouncing during landing.
     
  6. MikeD

    MikeD Administrator Staff Member

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    In the Huey, it's very important to check the weight/balance and CG forms when reviewing the helo's forms on preflight. The normal CG range is 134 to 143, however each helo is different as to where it specifically sits, and thats normally dependant on whether people/cargo are in the cabin or not with full fuel. When the cabin is empty, on some birds, the CG sits aft of 140; and fuel must be burned down to a predetermined level in order to be at 140 or less, otherwise there's a Warning that approaches to landing need to be made to a 4 foot hover, then touchdown; versus descending all the way to the ground, in order to not strike the tail/tail stinger on the ground when landing.
     
  7. dcaaviator

    dcaaviator New Member

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    AFT CG
    1.T/O... rudder is less efective do to the CG arm being shorter so directional control will decrease the aircraft will rotate before VR and cause you to have a pitch attitude greater than normal which could lead to stalling.
    2. Cruise... the rudder will always be less effective do to the arm being closer to the rudder. so having an aft CG will cause the aircraft to be less stable and harder to recover from stalls. but you will have better performance do to the reduced induced drag! (less down force on the horizontal stabelizer) there fore you will have better cruise speed. With an aft CG your stall speed decreases!
    3. Landing...Once again the aircraft will be less stable! T/O and Landing are the most critical phases of flight due to the slow speeds and low altitude so just use good preflight planing
    and its vice versa for Forward CG...Stall speed increases more stable lower performance. pilot handbook of aeronautical knowledge (PHAK) good book!!
     
  8. Xiph

    Xiph New Member

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    While you're correct about the rudder having less of a moment with an aft CG, that doesn't have a whole lot to do with stall recovery - the rudder works in the vertical axis (yaw), while CG effects mainly deal with the horizontal axis (pitch).

    It's the angle of attack of the wings and, to a lesser extent, the horizontal stabilizer that are important here. That's what affects your stall speed and performance with respect to your CG location.
     
  9. mshunter

    mshunter Well-Known Member

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    I knew what you meant, I just didn't want some people confused.
     
  10. js0305

    js0305 Well-Known Member

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    Would it still be within limits? I always thought the most aft CG was still ahead of the center of lift. (which would require some tail force)

    Thanks in advance.
     
  11. tgrayson

    tgrayson New Member

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    Great question. I'll say "maybe." Some airplanes under some loading conditions will have positive lift on the tail, so it follows that there is a loading condition that will have zero load on the tail. I can't say for sure that this is achievable in all airplanes.

    Although you are correct in saying that the CG must be ahead of the "center of lift", I'll tweak that and say that the CG must be ahead of the center of lift of the entire airplane. Most diagrams we see show the CG ahead of the center of lift of only the wing; however, when you attach a fuselage and empennage to a wing, the center of lift of the entire airplane is different from the center of lift of the wing alone. There is a special name for this center of lift: neutral point. In reality, the CG must be ahead of the neutral point for the airplane to exhibit positive static stability. In a conventional aircraft, the existence of the horizontal stabilizer allows the CG to move behind the wing center of lift, as long as it remains in front of the neutral point. This allows it to counteract some or all of the wing's normal pitch down moment, reducing the need for down lift on the horizontal stabilizer. I'd say that the longer the tail, the more likely there is a safe location of the CG that eliminates the download on the tail.
     
  12. Xiph

    Xiph New Member

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    You can have a CG aft of the CoL, as long as your horizontal stabilizer can produce enough upward lift to make the balance work out. As tgrayson pointed out, different airplanes will have different limits. I'd imagine that aerobatic aircraft are more tolerant in this regard - airplanes with cambered horizontal stabilizers, on the other hand, wouldn't be.
     
  13. tgrayson

    tgrayson New Member

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    I will draw a distinction here between "trim" and "stability". The criteria that you describe is that of being in trim, but that doesn't mean that the aircraft is stable. There are positions past the neutral point where you still are able to generate enough lift on the horizontal stabilizer to keep the airplane in trim, but the aircraft will still be unstable.
     
  14. shdw

    shdw Well-Known Member

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    I am not familiar with any, non computerized, aircraft where this is the case. I know some fly by wire military craft are inherently unstable and I'd imagine this is one of the ways it is made unstable.

    Is anyone here familiar with an aircraft where the CG is aft of the center of pressure? The CP occurs about 1/3rd back from the leading edge in sub sonic flight, if that helps people when viewing their POH for CG locations.
     
  15. MikeD

    MikeD Administrator Staff Member

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    Some just are aerodynamic anomalies. The F-117 is statically and dynamically unstable by design, and the quad FBW corrects these instabilities to certain necessary points. Take away the FBW, and the jet just tumbles, as has been demonstrated in the past.
     
  16. tgrayson

    tgrayson New Member

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    I addressed this in my post #11. And note that CP is not the correct concept here, it's "aerodynamic center". "Center of lift" doesn't have any official definition, but I arbitrarily assume it means "aerodynamic center".
     

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