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Old April 27th, 2012, 07:56 PM   #1
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Weight and Balance Design Question

So looking at a C-182C Weight and balance information I show the attached chart (Blue Line).

Why does the forward CG limit change from 33.5 inchs to 40 inches from 2100lbs to 2650?
My theory was that it was to limit the maximum down force on the tail, so I plotted this on the Green line, But it actually appears that as the weight increases it is designed to actually have less maximum down force on the tail?

Anyone else have any idea why the change in forward CG limit?
Or at least why so much change?

Brian
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Old April 27th, 2012, 08:11 PM   #2
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Re: Weight and Balance Design Question

I have thought about that for a long time. I think you are on the right track as to why. But, the chart you show is for the moment envelope, not the CG limits that you reference for 33.5 to 40 inches...
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Old April 27th, 2012, 09:43 PM   #3
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Re: Weight and Balance Design Question

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I have thought about that for a long time. I think you are on the right track as to why. But, the chart you show is for the moment envelope, not the CG limits that you reference for 33.5 to 40 inches...

True, I did some of the math from Moments to CG location. It is just taking the Moment and dividing by wieght. I also looked at the TCDS to get the CG locations.

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Old April 27th, 2012, 11:42 PM   #4
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Could have had bad stall recovery behavior at higher weights in flight test?
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Old April 28th, 2012, 12:04 AM
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Re: Weight and Balance Design Question

Here is a chart of mine. It is very similar, but a newer model. One thing about it is the two charts do not agree. The CG limits chart has a vertex at about 2700 lbs and I believe that is the correct one.

It does make sense that with forward CG more elevator down force is required to maintain a certain attitude. So, at 36" CG, the max weight is about 2700 lbs. To go to 2900 lbs, you would need to move that CG rearward by at least 2". This probably amounts to the same down-force on the tail to maintain the same attitude. Just some reasonable speculation here...
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Old April 28th, 2012, 12:10 AM   #6
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Re: Weight and Balance Design Question

Have you considered that the graph is about force available rather than force allowable?
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Old April 28th, 2012, 10:30 AM   #7
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Re: Weight and Balance Design Question

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Have you considered that the graph is about force available rather than force allowable?
I though I had. But at a higher weight the stall speed will be higher which would provide more available down force on the tail. Which doesn't tell me why they are limiting it to less down force on the tail at higher wieghts.

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Old April 28th, 2012, 11:47 AM   #8
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Re: Weight and Balance Design Question

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I though I had. But at a higher weight the stall speed will be higher which would provide more available down force on the tail. Which doesn't tell me why they are limiting it to less down force on the tail at higher wieghts.

Brian
Are force available and force required linearly proportional with weight? Look at the low speed side of the LD chart, weight is induced drag, the one that peaks before stall.

This is why you always use full flaps, throttle is more effective on the tail than speed.
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Old April 28th, 2012, 12:04 PM   #9
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Re: Weight and Balance Design Question

Could it have something to do with how much weight ends up on the nosewheel? Does the 185 have the same kink in the envelope?
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Old April 28th, 2012, 12:19 PM   #10
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Re: Weight and Balance Design Question

Waitaminute. Utility category is bigger than Normal? I don't buy that chart at all...
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Old April 28th, 2012, 02:10 PM   #11
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Re: Weight and Balance Design Question

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Waitaminute. Utility category is bigger than Normal? I don't buy that chart at all...
Yes, ignore that. I had forgot to relabel the chart. Actually I hadn't figured out how to do it in Numbers on my Ipad.
C-182C isn't rated for Utility Category.
and the green area is a fictional area I calculated using about a constant load on the tail.

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Old April 28th, 2012, 02:20 PM   #12
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Re: Weight and Balance Design Question

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Could it have something to do with how much weight ends up on the nosewheel? Does the 185 have the same kink in the envelope?
Interesting Idea,
C-180A shows
Landplane (+40.0) to (+45.8) at 2650 lbs.
(+34.5) to (+45.8) at 2100 lbs. or less

So it doesn't allow one to go as far forward at lower weights (By 1") Perhaps this supports the nose wheel idea. I will have to look at this further?

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Old April 28th, 2012, 03:10 PM   #13
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Re: Weight and Balance Design Question

The forward CG is often limited at higher weights to limit the required stick force on landing. This is particularly true for airplanes that are nose heavy to begin with. In other words, it limits the forward CG to prevent running out of elevator on landing.
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Old April 28th, 2012, 03:36 PM   #14
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Re: Weight and Balance Design Question

Quote:
Originally Posted by brcase View Post
So looking at a C-182C Weight and balance information I show the attached chart (Blue Line).

Why does the forward CG limit change from 33.5 inchs to 40 inches from 2100lbs to 2650?
My theory was that it was to limit the maximum down force on the tail, so I plotted this on the Green line, But it actually appears that as the weight increases it is designed to actually have less maximum down force on the tail?

Anyone else have any idea why the change in forward CG limit?
Or at least why so much change?

Brian
CFIIG/ASEL
Remember that the moment envelope graph is in reference to an arbitrary datum plane well ahead of the actual CG. I would expect the forward CG limit reference to the datum to move rearward toward the actual CG as weight increases in order to limit maximum moment around the actual CG.
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Old April 28th, 2012, 10:37 PM   #15
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Quote:
Originally Posted by alfadog View Post
Remember that the moment envelope graph is in reference to an arbitrary datum plane well ahead of the actual CG. I would expect the forward CG limit reference to the datum to move rearward toward the actual CG as weight increases in order to limit maximum moment around the actual CG.
That is what I was kind of expecting to see, and if you hold the moment constant around what I guessed to be the center of lift you get the green line. But you do have a good point and this is what I was looking for, a few more things to look at.

Perhaps if I adjust my location of the center of lift I will get a better match to the actual published limits.

As mentioned in an earlier post I probably also need to look at the loading on the nose wheel.

Thanks for the ideas.

Brian
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Old April 29th, 2012, 02:13 AM   #16
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Don't discount that control force idea either. We all know one needs big ape-sized arms to land a 182 without trimming. Look at me!

Haha.
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Old April 29th, 2012, 05:24 AM
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Re: Weight and Balance Design Question

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Originally Posted by brcase View Post
That is what I was kind of expecting to see, and if you hold the moment constant around what I guessed to be the center of lift you get the green line. But you do have a good point and this is what I was looking for, a few more things to look at.

Perhaps if I adjust my location of the center of lift I will get a better match to the actual published limits.

As mentioned in an earlier post I probably also need to look at the loading on the nose wheel.

Thanks for the ideas.

Brian
The center of lift moves with CG though as the force on the tailplane changes.
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Old April 29th, 2012, 06:16 AM
Posted in reply to brcase's post starting "That is what I was kind of expecting to..."
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Re: Weight and Balance Design Question

Quote:
Originally Posted by brcase View Post
That is what I was kind of expecting to see, and if you hold the moment constant around what I guessed to be the center of lift you get the green line. But you do have a good point and this is what I was looking for, a few more things to look at.

Perhaps if I adjust my location of the center of lift I will get a better match to the actual published limits.

As mentioned in an earlier post I probably also need to look at the loading on the nose wheel.

Thanks for the ideas.

Brian
Except that I phrased it wrong as the weight and balance moment around the actual CG will always be zero. That is why it is called the CG .

The maximum moment of interest is the moment between the center of lift and the CG. And yes, the COL moves rearward as you increase AOA with increased weight. So that would carry all allowable CG's rearward with it. CG must remain forward of COL to have downforce on the elevator, which provides pitch stability.

The forward CG limit would be expected to move rearward faster than the COL with increasing weight to maintain a max downforce on the tail. It would be interesting to plot the COL position line as part of the weight/CG graph.

I find the change in the rear CG with weight more puzzling. Why does it move as much as the forward limit??
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Old April 29th, 2012, 09:59 AM   #19
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Quote:
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The center of lift moves with CG though as the force on the tailplane changes.
I think a better way to say that is that as the Angle of Attack changes the Center of lift moves. Directly the CG obviously doesn't change the Center of lift. But a change in weight will change the Angle of attack. I had kind of assumed that the elevator would have had enough power to get you near the Maximum Angle of attack at any allowable weight and CG and thus at about the same Center of lift. I will have to think about this some more, If the limit is the maximum down force on the elevator perhaps you can't get to the maximum AOA at certain allowable Weight and CG location. This might mean I need to model an airfoil to figure out how much the CG moves.

Understand this is just a mental excise, The Engineer part of me just likes to know how stuff works.


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Old April 29th, 2012, 10:19 AM
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Re: Weight and Balance Design Question

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I think a better way to say that is that as the Angle of Attack changes the Center of lift moves. Directly the CG obviously doesn't change the Center of lift. But a change in weight will change the Angle of attack. I had kind of assumed that the elevator would have had enough power to get you near the Maximum Angle of attack at any allowable weight and CG and thus at about the same Center of lift. I will have to think about this some more, If the limit is the maximum down force on the elevator perhaps you can't get to the maximum AOA at certain allowable Weight and CG location. This might mean I need to model an airfoil to figure out how much the CG moves.

Understand this is just a mental excise, The Engineer part of me just likes to know how stuff works.


Brian
Think about what CG position does to to tail force required at any given weight. When that force is changed, wouldn't the center of lift be changed by the difference in the component of lift the tail is providing, be that positive or negative, since the center of lift is the culmination of all lift moments of the plane?

The having sufficient force designed in is exactly why we have that envelope and do the calculation.
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Old April 29th, 2012, 10:33 AM   #21
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Re: Weight and Balance Design Question

I think it is simpler than all that. The forward CG is limited so that you can flare the sucker when landing.
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Old April 29th, 2012, 10:36 AM   #22
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Re: Weight and Balance Design Question

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I think it is simpler than all that. The forward CG is limited so that you can flare the sucker when landing.

Most definitely, all part and parcel of the same thing, one the outcome of the other.
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Old April 29th, 2012, 11:18 AM   #23
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Re: Weight and Balance Design Question

In steady state straight flight (does not need to be level), the CG will always be directly below and equal to the vector sum of lift forces. In the envelope, the tail is providing downforce so wing lift > weight and CoL behind CG. CoL refers only to the wing.
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Old April 29th, 2012, 11:23 AM   #24
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Re: Weight and Balance Design Question

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In steady state straight flight (does not need to be level), the CG will always be directly below and equal to the vector sum of lift forces. In the envelope, the tail is providing downforce so wing lift > weight and CoL behind CG. CoL refers only to the wing.

When the wing stalls, where is the CoL?
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Old April 29th, 2012, 11:37 AM   #25
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Re: Weight and Balance Design Question

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Remember that the moment envelope graph is in reference to an arbitrary datum plane well ahead of the actual CG. I would expect the forward CG limit reference to the datum to move rearward toward the actual CG as weight increases in order to limit maximum moment around the actual CG.

Not necessarily so... As fuel weight is added/removed, the CG location tends to move more vertical on the chart. For example, with two persons plus baggage and 87 gal of fuel, its almost at gross weight. If all that is done is remove fuel to 0, the CG location moves less than 1".

The front seats on my 182 has a range of 32" to 50". So, if I have 25 gallons and zero baggage, the CG location is at 38". Then, if both seats are set at 38", then any weight added to the front passengers seats will move the CG location by zero. With a 150 lb pilot, no passengers, it CG is 38". Or, with a 350 lb pilot and a 350 lb passenger, CG is still 38". You would be under gross, but 1" forward or the limit.

Now if you moved those seats all the way forward to 32", the CG location would move to 36.6" and you would be 2.6" forward of the limit. However, good luck being able to fit two 350 lb persons at full forward seat position and still being able to move the yoke.

At the opposite end, move those seats to full rear position, and the CG moves to 40.8", and you are well within the limits.


I tend to operate my seat 1" from full forward, and I can certainly tell the difference in trim required with just two persons, and no baggage, vs with baggage in the rear. It is worst, when I have an instructor aboard because he has to reach everything like I would. But, when I just have one passenger and little or no baggage, I always ask them to keep their seat back some.

So, I believe, as others have suggested, that it is related to the tail force required on landing, or the amount of trim available. I have run out of trim to keep my 65kts with instructor only aboard. Next time out with the instructor, I added weight in the back, and the trim had sufficient range.
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