Center of Gravity "sweet spot" -- How to find? Published reference?

AggieMike88

AggieMike88

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The original "I don't know it all" of aviation.
A discussion about calculating weight and balance and the hazards of a CG's too far forward and to far aft had me wondering something...

During the discussion, I asked about how reducing the weight of a particular aircraft might offer an increase in ground speed. Another pilot answered, "Probably faster, but it depends on the weight removed, where you put everything, and the exact characteristics of the plane. While you have reduced gross weight, which will increase speed, you’ve also moved the cg forward, which decreases speed. Sometimes you actually go faster by adding weight way in the back as this moves the cg aft – my airplane racing friends often use ballast of 50-75 lb in the back of the baggage compartment to increase speed"

Currently, I'm piloting a 1975 C182P. We've talked on the forums a few times about 182's that adding ballast to the baggage area can improve handling when the scenario is single pilot or pilot plus passenger and unoccupied rear seat and empty baggage area. Essentially we're moving the CG aft a few inches from where it would be without the ballast. And what this accomplishes is finding a handling "sweet spot"

To me, it appears reasonable that the following items are true
  1. When the aircraft was designed, the designers determined there was a certain angle of attack on the wing that produced the greatest amount of efficiency while in cruise. And this particular AoA would provide all the lift needed, and also achieve a "least drag" state.
  2. As you move the CG forward and aft, you're also changing the AoA on the wing by a particular increment that might be outside of this "least drag" state.
  3. Additional drag reduces speed and fuel economy.
  4. If you knew how far from the datum this "least drag state" CG was, you could work your loading problem to achieve a best speed and best fuel economy condition.

For different aircraft (not just limited to Skylanes) is there any published reference that says, "If you load your aircraft so the CG is ____ from the datum, you are in the sweet spot least drag state"?? And therefore, for a particular power setting (and winds not withstanding), you're getting the most speed and fuel economy out of the setting...
 
It couldn't make that much difference, as long as you are within the published limits. Maybe if you are racing... Just trim up the plane and it should handle fine. Granted, my experience is mostly in trainers, which are pretty docile, anyway, so maybe I am just blowing smoke.
 
The defined "sweet spot" (highest lift for least drag) is defined as your best glide speed -- best L/D ratio -- max range speed ((((FAR from fastest speed))))) --- puts a 182 below 150 cruise speed

CG position, while being influential, is minor, however
 
Your argument is false. Moving the CG does not noticeably change the AoA. Moving the CG forward makes the total weight of the plane a little more so the AoA would change a little. But in general, a given AoA is required to maintain altitude for a given speed and weight regardless of CG.
 
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Would it not always be right at the aft limit?

In calm air you can see the effect. Slide your seat back and you'll have to trim nose down to compensate. Then watch the speed increase.

I remember being told that Mooney salesman would take advantage of this fact!
 
Captain said:
Your argument is false. Moving the CG does not noticeably change the AoA. Moving the CG forward makes the total weight of the plane a little more so the AoA would change a little. But in general, a given AoA is required to maintain altitude for a given speed and weight regardless of CG.
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I think the influence he is referencing is from the displacement and resulting force vectors from the stabilator/stabilizer-elevator due to displaced "balanced" cg location. And, I think it is correct that the least added force does come from the cg being about 15-20% aft of the aft-cg limit --- a place you don't want to be --- "squirrely" would be a gross and conservative understatement
 
You'll find the most efficient CG sweet spots if you simply fly an airplane with a canard rather than a tail wing.
 
Jim Logajan said:
You'll find the most efficient CG sweet spots if you simply fly an airplane with a canard rather than a tail wing.
Click to expand...

After talking to a few owners of the Peterson 182's, they are on my "I won the lotto" purchase list.
 
Aft is good, especially in a 182 that folks seem to have issues managing to flare when loaded forward. Just stay within limits.
 
The AOA of the main wing doesn't change as you move the CG forward but the tail AOA (elevator deflection or stabilator angle) does increase to compensate, creating additional drag. The most ideal place for the CG is at the aft limit.
 
oldShar said:
I think the influence he is referencing is from the displacement and resulting force vectors from the stabilator/stabilizer-elevator
Click to expand...

I think that this is the main influence. Lift generates drag and when the lift is negative, like from the tail in all or nearly all certified aircraft for inherent stability, it is a double whammy. If the tail is exerting 20 lbs of downward force, it is like carrying an extra 20 lbs of weight. The wing has to generate an extra 20 lbs. to make up for it. It creates drag as if it were 40 lbs. Every lb. of downward force that you can relieve the tail of needing to generate by shifting CG aft reduces drag as if you had unloaded twice that weight from the plane. As the CG moves aft, longitudinal stability decreases.
 
Sac Arrow said:
The most ideal place for the CG is at the aft limit.
Click to expand...

Sort of. The most ideal place is in line with the center of lift of the wing, so the elevator doesn't have to do any work (i.e. generate lift, and the induced drag that goes with it). The aft c.g. limit is set so you don't go past that, so the plane doesn't become unstable in pitch.
 
It makes a difference the sailplane guys play around with CG optimization. Iirc they usually end up 85% aft. There have been some ideas of moveable ballast aft for cruise forward for climbing, the technical hassle has so far outweighed the performance gain.
 
Van Johnston said:
Sort of. The most ideal place is in line with the center of lift of the wing, so the elevator doesn't have to do any work (i.e. generate lift, and the induced drag that goes with it). The aft c.g. limit is set so you don't go past that, so the plane doesn't become unstable in pitch.
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No, the aft CG limit is set up so you don't put the plane in to a position where you can have an unrecoverable stall or spin. That normally requires some amount of forward CG. But in theory you are correct.

Well and plus you cannot achieve stable flight without some level of forward CG.
 
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AFT CG is where it's at. We had a customer who wanted a 75# removable ballast as far back in the tailcone as we could get it in his Astra SPX biz jet. He figure he would be able to make his desired flight with better margins and skip a fuel stop.

I too enjoy loading the baggage up. Every plane I've flown flys better with weight back there.
 
Thanks for the comments so far.

The club keeps several cases of oil in the hangar. Looks like tossing one or two into the baggage area would be worth a try until I figure out what provides the desired flying performance.
 
Sac Arrow said:
No, the aft CG limit is set up so you don't put the plane in to a position where you can have an unrecoverable stall or spin. That normally requires some amount of forward CG. But in theory you are correct.

Well and plus you cannot achieve stable flight without some level of forward CG.
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Yes, I was just answering in context of the OP's original post about cruise. The consensus here is further back is better, and at the aft limit is best that can be achieved legally and safely, and I agree with that. But I think aft cg limit is forward of the S&L cruise 'sweet spot,' so if you stay within limits you will never achieve the sweet spot.
 
Van Johnston said:
Yes, I was just answering in context of the OP's original post about cruise. The consensus here is further back is better, and at the aft limit is best that can be achieved legally and safely, and I agree with that. But I think aft cg limit is forward of the S&L cruise 'sweet spot,' so if you stay within limits you will never achieve the sweet spot.
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In racing sailplanes, the aft limit is too far aft for optimum performance.
Depending on the sailplane, 80%-85% aft provides the best performance.

Remember , aft CG increases difficulty of recovery from stall/spin.
 
Van Johnston said:
Yes, I was just answering in context of the OP's original post about cruise. The consensus here is further back is better, and at the aft limit is best that can be achieved legally and safely, and I agree with that. But I think aft cg limit is forward of the S&L cruise 'sweet spot,' so if you stay within limits you will never achieve the sweet spot.
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Yes...
 
I think it was the 120 cessna's that had a sliding ballast to get the CG aft for cruise, but could be cranked back forward for TO and landing.
 
Sac Arrow said:
The AOA of the main wing doesn't change as you move the CG forward...
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Are we sure on that statement?

Since the main wing would have to carry more load as the cg moved forward (because of the additional tail down force required), would that not necessitate a slightly larger AOA, all things considered?

Which is why induced drag would increase, which is why the plane would be slower.

Right?
 
FastEddieB said:
Are we sure on that statement?

Since the main wing would have to carry more load as the cg moved forward (because of the additional tail down force required), would that not necessitate a slightly larger AOA, all things considered?

Which is why induced drag would increase, which is why the plane would be slower.

Right?
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.....or travel faster. Yes to maintain a given airspeed you would be correct, but consider that one of the effects of moving the CG forward is to increase stall speed. Increasing the AOA in cruise is an option, but increasing the critical AOA for stalling isn't.
 
Captain said:
Your argument is false. Moving the CG does not noticeably change the AoA. Moving the CG forward makes the total weight of the plane a little more so the AoA would change a little. But in general, a given AoA is required to maintain altitude for a given speed and weight regardless of CG.
Click to expand...

The CG shift is not relevant to the AOA of the wing, rather of the tail. The further forward, the greater the downforce on the tail not only adding drag but also adding to the force the wing must lift. When you shift the CG aft of the wing CoE the tail now provides lift reducing drag from the wing. This is why Canards are more efficient. Even with a partial canard like the Katmai 182 you get a great benefit in speed and economy.

Adding weight aft to change the equation though does little to no good unless the weight added is aft of the horizontal stab CoE. In another thread is discussed the MD-11, in that case they added depleted Uranium to the vertical fin to bring the CG aft.
 
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Fun fact: some large transports (I know the A330 for a fact but I'm sure there are more) have an extra fuel tank in the tail. Fuel is transferred back there after takeoff to move the CG close to its aft limit....to increase gas mileage.
 
Henning said:
The CG shift is not relevant to the AOA of the wing, rather of the tail.
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Still not getting this.

Would not a wing providing a total lift of 3,600 lbs have to fly at a larger AOA than one providing a total lift of 3,500 lb (because of the add'l 100 lbs tail down force due to a more forward loading)?

Let me look for an image on Google to clarify…

Here you go…

chapter_3-3_img_41.jpg


So, more weight supported by wing needs more lift needs more AOA - right?
 
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FastEddieB said:
Still not getting this.

Would not a wing providing a total lift of 3,600 lbs have to fly at a larger AOA than one providing a total lift of 3,500 lb (because of the add'l 100 lbs tail down force due to a more forward loading)?

Let me look for an image on Google to clarify…

Here you go…

chapter_3-3_img_41.jpg


So, more weight supported by wing needs more lift needs more AOA - right?
Click to expand...

The argument was in reference to Captains statement that the CG shift alone causes a nearly imperceptible change in AoA which at cruise speed is accurate enough. I was pointing out why the AoA shift is not the greatest factor in the equation.
 
Agree the drag is the larger factor in moving the CG forward. I was only responding to point number 2 of the OP which seemed to imply the AoA changes with a change in CG position (actually, that's what he said).

I was in no way offering opinion to drag or speed, just pointing out AoA change is imperceptible. If asked ill be happy to point out drag goes up and speed down with a forward movement of CG. AoA would increase for a given speed and altitude but you'd need a micrometer to measure it.
 
FastEddieB said:
Not to belabor the point but isn't the increase in drag directly due to the increase in angle of attack i.e. induced drag?
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Increased AoA of the elevator. Until this post all my refrence to AoA has referred to the main wing.
 
FastEddieB said:
Not to belabor the point but isn't the increase in drag directly due to the increase in angle of attack i.e. induced drag?
Click to expand...

Of the elevator, the increase in drag on the wing from the extra weight will come mostly from the wingtip vortices.
 
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