Weight & Balance CG limits

[skidoo]

While going over my weight and balance examples, I find that in some situations, it would be very close to or exceeding the forward CG limit. So, that got me to wondering how safe the handling characteristics are when CG is near the limits (but within) or how much margin is applied to the establishment of these limits.

I am wondering if best flight characteristics are achieved if the CG is precisely in the center of the envelope or if it is biased either forward or aft. Or, if there is any significant difference as long as it hits anywhere within the envelope.

Any thoughts?

The other question is that on my T182T POH, it has both CG moment envelope and also CG limits. With Pilot, Passenger, half fuel, no baggage, it is right on the limit of 104000 lb-in and 2730 lbs, but the CG limit of 38.1" is at least a couple of inches within the envelope. Why do both charts not agree with being right at the limit?

Are these two separate measurement of which both must be within? If so, then it is possible to have a CG within limits but the CG moment out of limits...

 

[BillTIZ]

While going over my weight and balance examples, I find that in some situations, it would be very close to or exceeding the forward CG limit. So, that got me to wondering how safe the handling characteristics are when CG is near the limits (but within) or how much margin is applied to the establishment of these limits.

I am wondering if best flight characteristics are achieved if the CG is precisely in the center of the envelope or if it is biased either forward or aft. Or, if there is any significant difference as long as it hits anywhere within the envelope.

Any thoughts?

The other question is that on my T182T POH, it has both CG moment envelope and also CG limits. With Pilot, Passenger, half fuel, no baggage, it is right on the limit of 104000 lb-in and 2730 lbs, but the CG limit of 38.1" is at least a couple of inches within the envelope. Why do both charts not agree with being right at the limit?

Are these two separate measurement of which both must be within? If so, then it is possible to have a CG within limits but the CG moment out of limits...

Need pictures of the graph and your calculations. Cannot be CG in limit and CG moment out of limit at the same time.. it's the same thing. ??

 

[skidoo]

I thought they should be the same as well. But, take a look at 2700 lbs at 36" aft. This is within the CG, but out on the Moment envelope (97.2).

 

[mantakos]

Those two charts should be equivalent, it just allows you to do the calculation in either of two ways.

There's no "optimal CG", it's a tradeoff. A forward CG is better for stability, an aft CG is better for performance.

It's not uncommon for the forward CG limit to begin to inch backwards as you get closer to max gross weight. As I understand it, this is because as the plane gets heavier, the downward tail force needed to balance a forward CG becomes too great.
-harry

 

[mantakos]

I thought they should be the same as well. But, take a look at 2700 lbs at 36" aft. This is within the CG, but out on the Moment envelope (97.2).

That's odd, I don't know how to explain that. The two charts don't seem to coincide perfectly. The moment chart should look like a "slanted" version of the CG chart, but the charts that you posted here, the CG chart has an extra corner (35.8" and 2700lbs) that the moment chart doesn't have.
-harry

 

[gismo]

While going over my weight and balance examples, I find that in some situations, it would be very close to or exceeding the forward CG limit. So, that got me to wondering how safe the handling characteristics are when CG is near the limits (but within) or how much margin is applied to the establishment of these limits.

I am wondering if best flight characteristics are achieved if the CG is precisely in the center of the envelope or if it is biased either forward or aft. Or, if there is any significant difference as long as it hits anywhere within the envelope.

Any thoughts?

Certification testing is generally done with the most adverse CG (or both max aft and max forward) for the criteria being measured.

As to the CG for "best flight characteristics" that depends on which specific characteristic you're concerned with. For instance, takeoff obstacle clearing, climb rate, and cruise speed are generally better with a CG near or at the aft limit due to the reduction in total lift required. OTOH, pitch stability, Stall and spin recovery/resistance is best at the forward CG limit. Landing distances are probably better with a forward CG but in many airplanes when the CG is at the forward limit, it's very difficult to hold the nose off the runway on landing.

The other question is that on my T182T POH, it has both CG moment envelope and also CG limits. With Pilot, Passenger, half fuel, no baggage, it is right on the limit of 104000 lb-in and 2730 lbs, but the CG limit of 38.1" is at least a couple of inches within the envelope. Why do both charts not agree with being right at the limit?

Are these two separate measurement of which both must be within? If so, then it is possible to have a CG within limits but the CG moment out of limits...

Methinks you are misinterpreting the charts and/or limits. There shouldn't be any conflict between them. In many airplanes, the CG limits vary with weight. Also you are saying "CG limits" but "104000 lb-in" is a moment not a CG and the moment limits are always dependent on weight.

 

[Let'sgoflying!]

Given general choices of "loss of performance" (longer t-o roll, reduction in climb) vs loss of control (inadequate aft elevator), most would take the first one because you might live longer.
If you are asking a specific physics question of will the 2" vs the 200# have a better outcome, I will leave it to the the slide rule guys.

 

[gismo]

That's odd, I don't know how to explain that. The two charts don't seem to coincide perfectly. The moment chart should look like a "slanted" version of the CG chart, but the charts that you posted here, the CG chart has an extra corner (35.8" and 2700lbs) that the moment chart doesn't have.
-harry

That looks like someone put the wrong moment graph in the POH. If you graph the corners of the CG graph on the moment graph it expands the left side considerably in the 2700 lb area. I'd go by the CG graph and check with Cessna. You could also look up the CG data on the TCDS, that almost has to be correct.

 

[skidoo]

I just sent a request to Cessna to look at the graphs and update if necessary...

 

[twdeckard]

I have always speculated that Cessna miscalculated the forward CG limits (defined by the margin against stalling the horizontal stabilizer in the landing configuration). Its why you see wrinkled firewalls on the Cessna's but not on the Piper Indian's.

The stabilizer interaction with the wing downwash is so much more complicated on the high wing Cessa. The dynamics of pitch and power and elevator inputs within ground effect can bait someone into a pilot induced oscillation - however I have always thought you are flirting with the forward CG limit with two fat kids in front. The last fatal plonk in the rodeo is really a tail stall for those unlucky enough to need an insurance agent after landing.

I'd be curious what you uncover.

 

[poadeleted20]

So, that got me to wondering how safe the handling characteristics are when CG is near the limits (but within) or how much margin is applied to the establishment of these limits.

Near but within the limits, you are guaranteed that the aircraft's pitch control meets the FAR Part 23/CAR 3 standards (as applicable), and that its handling characteristics will be safe enough for any pilot who can pass a practical test to fly. Beyond that point, all bets are off -- no guarantees whatsoever.

The problem for you if you exceed those limits is that you have no idea how much safety margin the manufacturer built in, or how fast things go downhill beyond those limits. There may be a considerable margin beyond the book limit, or there may be almost none. The only way to find out how much margin there is would be to review the certification flight test data or to conduct your own flight test series to figure that out. Are you a graduate of a recognized Test Pilot School? If not, you probably don't want to do that.

I am wondering if best flight characteristics are achieved if the CG is precisely in the center of the envelope or if it is biased either forward or aft. Or, if there is any significant difference as long as it hits anywhere within the envelope.

Depends on what you consider "best." You get the highest speed and lightest control forces at the aft cg limit, but you give up some stability. You get the best stability at the forward limit, but you pay for that with heavier stick forces and less speed.

The other question is that on my T182T POH, it has both CG moment envelope and also CG limits. With Pilot, Passenger, half fuel, no baggage, it is right on the limit of 104000 lb-in and 2730 lbs, but the CG limit of 38.1" is at least a couple of inches within the envelope. Why do both charts not agree with being right at the limit?

That shouldn't happen.

Are these two separate measurement of which both must be within?

No, just two different ways of measuring the same thing.

If so, then it is possible to have a CG within limits but the CG moment out of limits...

No, it isn't. Something must be wrong with either the depiction or your reading of one of the charts. If you can't figure it out, call Cessna Technical Support to discuss the matter -- number's on their web site.

 

[Dr. O]

Doing W&B is a good thing... Try not to push the corners - what is within CG at one weight can be out at another weight - it is just the way it is and Cessna cannot change the laws of physics... Know thy airplane...

denny-o

 

[Dan Thomas]

I have always speculated that Cessna miscalculated the forward CG limits (defined by the margin against stalling the horizontal stabilizer in the landing configuration). Its why you see wrinkled firewalls on the Cessna's but not on the Piper Indian's.

The 172/182 have their nosegears mounted to brackets on the firewall. The underlying structure isn't too robust and gets wrecked by clumsy pilots. The Pipers have the gear mounted on the engine mount (as does the Cessna 150) and so the mount rather than the firewall gets bent. Cheaper to fix.

That said, the early 177s (Cardinal) had stabilator-stall problems that Cessna fixed with a slot. Bent more than a few airplanes.

Dan

 

[Doggtyred]

I will take overweight, in balance, and a long runway ANYDAY before I consider flying outside of limits (published or extrapolated)

 

[Greg Bockelman]

I will take overweight, in balance, and a long runway ANYDAY before I consider flying outside of limits (published or extrapolated)

Well, by definition, over weight IS outside the limits.

Yeah, I put the smileys on but in reality, I am being serious.

 

[BillTIZ]

I thought they should be the same as well. But, take a look at 2700 lbs at 36" aft. This is within the CG, but out on the Moment envelope (97.2).

If we are going by your pencil marks... you plotted at 2700# and 39".
The other is pounds and moment arms.

Without the entire W&B data to plot the marks.. it's hard to tell where the math error is.

 

[skidoo]

If we are going by your pencil marks... you plotted at 2700# and 39".
The other is pounds and moment arms.

Without the entire W&B data to plot the marks.. it's hard to tell where the math error is.

The pencil marks show both in, but the CG limit is further away. I suggested a different point to look at in an attempt to keep it simple and demonstrate that a certain combination of data can be in spec on one graph and out of spec on the other.

Ignoring the pencil marks, if you look at 36" and 2700 lbs, that point is right "inside" the corner where the slope changes on the CG limit graph. Taking that same value, 2700 x 36" gives 97,200 Iound-Inches. So, plotting 97.2 on the other graph at 2700 lbs, it shows clearly out of the limits. Again, ignoring the pencil marks...

 

[gismo]

From the T182T TCDS:

C.G. Range Normal Category
(1) Aft Limits 46.0 inches aft of datum at 3,100 pounds or less.
(2) Forward Limits Linear variation from 40.9 inches aft of datum at 3,100
pounds, to 35.5 inches aft of datum at 2,700 pounds, to
33.0 inches aft of datum at 2,250 pounds; 33.0 inches aft
of datum at 2,250 pounds or less.

You can plot that on either graph (just convert the the CG&weight to Moments by multiplying them). These are the official limits regardless of what's in the POH you have.

 

[skidoo]

From the T182T TCDS:



You can plot that on either graph (just convert the the CG&weight to Moments by multiplying them). These are the official limits regardless of what's in the POH you have.

That is a great answer! So, it looks like the CG graph in the POH most closely fits that description and the CG moment envelope graph is incorrect. It doesn't show any slope change at 2700 lbs... I can draw my own graph and be more accurate relative to the description.

 

[Doggtyred]

Or, if there is any significant difference as long as it hits anywhere within the envelope.

In a conventional plane, with a conventional tail surface (as opposed to a Canard)...
EDIT TO CORRECT

The tail provides downforce.. the more forward the CG the more downforce required by the tail. Causing more trim drag, which decreases range/speed for a given power setting/fuel burn. Aft CG has the least amount of trim drag

In a canard, both the canard and mainwing are normally lifting surfaces. Forward CG increases the AOA on the canard, increasing trim drag, so in a canard, the fastest, twitchiest least draggy configuration is everyone/everything besides the pilot being stored aft, in the seats or baggage area.

As for margins on the CG, it is best to approach the subject as if it was a cliff than a gradually increasing slope. Dont go there.

 

[twdeckard]

In a conventional plane, with a conventional tail surface (as opposed to a Canard) ...

I'm not sure I read any of that right? The highest performance is achieved with the aft-most CG becuase the tail is generating the least-down force. Unloading the tail entirely would yield the best range for a given weight but would have neutral pitch stability.

Pitch stability improves as you move the CG forward and the limit is generally the most forward that is still unlikely to result in the tailplane reaching its critical angle before the wing does.

I still wonder if in the 182 with me and someone else my size in front we might be flirting with the forward limit and in the rodeo that I call a typical landing you might wind up stalling the horizontal stabilizer and dropping the nose.

 

[skidoo]

In a conventional plane, with a conventional tail surface (as opposed to a Canard)...

A CG that is over (or close to) the center of lift (which tends to be forward)will have the least trim drag and the greatest achievable speeds for a given power setting. It also will be squirelly in pitch.

In a conventional plane (tail in back, not a canard), you end up using lots of nose down trim with aft CG that adds drag and slows you, but adds stability by dividing the load between the main wing and the tail.

In a canard, both the canard and mainwing are normally lifting surfaces. Forward CG increases the AOA on the canard, increasing trim drag, so in a canard, the fastest, twitchiest least draggy configuration is everyone/everything besides the pilot being stored aft, in the seats or baggage area.

As for margins on the CG, it is best to approach the subject as if it was a cliff than a gradually increasing slope. Dont go there.

That sounds reasonable. The thing I must be missing then it where the center of lift is located. Is this typically published? Know where to find it?

 

[Capt. Geoffrey Thorpe]

That sounds reasonable. The thing I must be missing then it where the center of lift is located. Is this typically published? Know where to find it?

Somewhere near 25% of the wing chord will be the "center of lift" of the wing. I'm pretty sure you won't find anything in the POH or similar documents. You could look at Abbot and Von Doenhoff "Theory of Wing Sections" which contains data for the airfoils typically used on Cessna / Piper. And/or an on line search - somewhere there is a web site that lists what airfoil is used on what aircraft (a long, but obviously not complete list)...

 

[Doggtyred]

I'm not sure I read any of that right? The highest performance is achieved with the aft-most CG becuase the tail is generating the least-down force. Unloading the tail entirely would yield the best range for a given weight but would have neutral pitch stability.

Pitch stability improves as you move the CG forward and the limit is generally the most forward that is still unlikely to result in the tailplane reaching its critical angle before the wing does.

I still wonder if in the 182 with me and someone else my size in front we might be flirting with the forward limit and in the rodeo that I call a typical landing you might wind up stalling the horizontal stabilizer and dropping the nose.

Oops my bad.. got the conventional stuff backwards...