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Difference between CG Limits & CG Moment Envelope

  • Dane1993
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Dane1993 created the topic: Difference between CG Limits & CG Moment Envelope

I was doing a weight and balance for a C172N and I was wondering; what is the difference between the Centre of Gravity Limits and Centre of Gravity Moment Envelope? My instructor said that one had to do with the front seats in the Cessna 172 being adjustable.

Could someone clarify which chart takes this into account and just explain it a little better please?

Thanks in advance.
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bobtait replied the topic: Re: Difference between CG Limits & CG Moment Envelope

The centre of gravity moment envelope defines the combinations of weight and moment that prove the aircraft is safe to fly. Any combination of weight and moment that falls within the envelope is OK. The centre of gravity limits represent the most forward or most aft position permitted for the centre of gravity. They are in fact the lines that define the front and rear edge of the envelope. Any combination of weight and moment that falls beyond the limits is considered unsafe. The seat position is not normally considered, the adjustment of the seat position is allowed for when the moment envelope is constructed. The only system that considers the seat position that I know of is the Alpha [which is actually the Cherokee Six family]. It allows for the row two seats to be either aft facing or forward facing.
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  • Dane1993
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Dane1993 replied the topic: Re: Difference between CG Limits & CG Moment Envelope

That makes sense, awesome, thanks Bob!
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  • John.Heddles
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John.Heddles replied the topic: Re: Difference between CG Limits & CG Moment Envelope

Perhaps I can offer some thoughts on this question and add a bit to Bob's reply.

I am presuming that the charts referred to are the

(a) gross weight by CG limit chart

(b) gross weight by moment limit chart. (The usual ICAO/GAMA style of POH presentation is pretty dreadful, but that is another story to discuss at some time ..)


What is the difference between the Centre of Gravity Limits and Centre of Gravity Moment Envelope?

In essence, there is no difference - other than the one talks about CG, the other the equivalent moment.

This can be checked quite easily by comparing the same points in each chart - the easiest are the various vertices (ie where the line sections change direction) and a low weight at both forward and aft limits as a check on the bottom part of the limit lines.

For each point selected in the GW by CG chart, read both GW and CG,
Then calculate moment = GW x CG/(whatever constant is used in the GW by moment chart)
Plot each co-ordinate on the GW by moment chart.

Each plotted point should overlay the equivalent printed point on the GW by moment chart.

(One sideline note - the upper forward limit in the moment chart is a curve rather than a straight line but, with the datum presentation in the POH, you probably would have trouble detecting this)


My instructor said that one had to do with the front seats in the Cessna 172 being adjustable.

Sack your instructor.


Could someone clarify which chart takes this into account and just explain it a little better please?

Generally, neither will.

The OEM POH charts normally will reflect the TCDS certification envelope data with the GW by moment envelope just being a calculated map from the GW by CG envelope.

It is up to the owner/operator to make any restrictions which might be required for operational use of the POH data. In US parlance, this is referred to as "curtailment". It is a matter of some regret that a great many Australian WCOs have no idea about doing this and, indeed, how to go about doing it.

For light singles of the 2-4 place variety, the actual pilot seat arm (considering usable travel range for normal operation) usually will have little effect on the loaded CG as the arm range generally sits very close to the calculated loaded CG anyway.

Engineering specialist in aircraft performance and weight control.
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  • ray9ed
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ray9ed replied the topic: Difference between CG Limits & CG Moment Envelope

John,

I'm confused. If there is no difference between the two graphs why have both ? Also, can you explain your abbreviations, please ?
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bobtait replied the topic: Difference between CG Limits & CG Moment Envelope

I don't think you will find any aircraft that uses both types of centre of gravity envelopes. It doesn't matter which one the designer chooses, they both tell you whether the aircraft is safe to fly. Americans often call this type of presentation a 'go / no go' graph. It doesn't give you a precise numerical answer, it just tells you whether it's 'go' or 'no go.

The choice of presentation is simply up to the designer. Bit like Apple or PC when it comes to computers. Some like one, others like the other.
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  • John.Heddles
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John.Heddles replied the topic: Difference between CG Limits & CG Moment Envelope

Ray, perhaps I might answer your questions in reverse order ?

Also, can you explain your abbreviations, please ?

My apologies .. I really should think more about trying to match acronyms to the audience at hand. A bit of a glossary follows, with a few more reasonably important terms thrown in –

AFM - Approved Flight Manual

CG - centre of gravity

Design Standard - Code specifying the minimum requirements for an aircraft design eg FAR 23

GAMA - General Aviation Manufacturers Association

GW - gross weight

ICAO - International Civil Aviation Organisation

NAA - National Aviation Authority eg FAA, CASA

OEM - Original Equipment Manufacturer – important as this entity holds all the proprietary information for a design (and, generally, protects its rights in such data with vigour)

POH - Pilots Operating Handbook – the usual AFM format for light aircraft.

TC - Type Certificate – attests that a design complies with the relevant Design Standards

TCDS - Type Certificate Data Sheets – subordinate document to the TC with lots of useful (ie required) information. A typical example is rgl.faa.gov/Regulatory_and_Guidance_Libr...FILE/3A12_Rev_84.pdf

WCA - Weight Control Authority – maintenance authority for weight control matters

WCO - Weight Control Officer (see CAO 100.28 www.legislation.gov.au/Details/F2013C00881/Download ) – WCA holder

A few comments to help you see where stuff fits in the scheme of things…

When an OEM decides to design and build a new plane, their design group management gets together with the relevant NAA airworthiness certification folks and, after a lot of talk, agree on a Design Standard for the activity. As the Standards keep changing, it is conventional to “freeze” the requirements at a particular revision status so that the goalposts don’t move through the exercise.

After a lot of dollars and effort by the OEM, both parties agree that the design meets the Design Standard and the NAA issues a TC for the design. This document has a variety of subordinate documents, including the TCDS and AFM/POH. After a few more hoops have been jumped through, the OEM can start building and selling the aircraft.

Each aircraft, when registered, has a Certificate of Airworthiness which basically certifies that the specific aircraft complies with the Type Design as approved via the TC and any other local NAA requirements.

So far as the POH is concerned, ICAO issued a recommendation at Doc 9516 (I’m sure there will be a non-ICAO source link on the net somewhere but I can’t find any just now) for light aircraft POH documentation. This is essentially similar to the equivalent GAMA document.

GAMA, which numbers as members the major GA OEMs, issued a POH specification ( www.gama.aero/files/gama_specification_1...6_pdf_498ca05388.pdf ), originally in the mid-70s, which is the style you see in your local flying school/club aircraft (unless, like most of us old blokes, you prefer ancient aeroplanes). You may hear references to a “GAMA format POH” .. this is where that expression originates. A typical example can be found at www.gaceflyingclub.com/Member%20Download...ual%20Searchable.pdf

If there is no difference between the two graphs why have both ?

From the above discussion, the TC imposes a prescriptive CG envelope. This has to be specified in the POH and, for the typical GAMA Spec 1 format POH, one sees a written definition in the Limitations Section with that information repeated as the more usable graph in the Weight and Balance Section.

While the loading calculations can be done using this graph, it is a bit of a pain as one has to calculate the actual CG each time before a co-ordinate can be plotted.

The GAMA spec recommends that the calculation bits are done using the normal moment calculation and provides some moment based charts to facilitate this. More importantly, if you consider the typical longhand weight and balance calculation, you add weights, add moments .. but NEVER add CGs. It follows that you can use moment charts conveniently as the addition of weights and moments then can be done graphically. A read through the Weight and Balance Section will clarify this.

For Australian aircraft, be aware that WCOs may bypass the POH loading system altogether and prescribe something different to suit the owner’s/operator’s requirements. Typically, this will be to call up a trimsheet loading system. If this is the case, be very wary of trying to do things your own way rather than as the WCO specifies as the trimsheet design datum typically is different to the OEM datum and it can all turn topsy very quickly if you don't know just what you are doing ... Use of a non-OEM datum is done to increase execution accuracy when using the trimsheet.

This is the long-winded answer for which Bob gave the short brief in his previous post …

Engineering specialist in aircraft performance and weight control.
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  • ray9ed
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ray9ed replied the topic: Difference between CG Limits & CG Moment Envelope

All the way through my flying (theory and practical) training, it has been pushed that I have to use the POH datum and there is no option to that. How can you use a different datum ?

I am guessing that you hold a weight control authority ?
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bobtait replied the topic: Difference between CG Limits & CG Moment Envelope

It doesn't matter what point is chosen for the datum. Measurements from any datum will still give the the same centre of gravity position as a distance from that chosen datum.

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  • John.Heddles
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John.Heddles replied the topic: Difference between CG Limits & CG Moment Envelope

… it has been pushed that I have to use the POH datum and there is no option to that. How can you use a different datum ?

Some observations, if I may -

(a) you can use any position for a datum as Bob showed, very nicely, in his previous post. Indeed, the datum is just a convenient nail on which to hang the end of your tape measure.

(b) it does NOT follow that any datum is as good (ie as useful) as another – it’s very much a case of horses for courses.

(c) with PCs now running 64-bit calculations, and scientific slide rules gone the way of the dodo, there is NO sensible reason for using other than the OEM datum if you are going to do your sums longhand (ie with a microprocessor helping out for the number crunching).

(d) if you are running graphical calculations of the sort seen in the Bravo or Echo systems, there is no need to change datum position subject to the following caveat.

If one uses the typical OEM datum - without some drafting tricks - you get the long, thin, dreadful moment graphs seen in both the Bravo and Echo systems – but, unfortunately, you have to put up with that for the exams. A better result is obtained by skewing and stretching the envelope (as you will see in many GAMA POH loading charts) which allows you to get around the problems associated with the simplistic charts in Bravo and Echo and provides a more easily used presentation.

(e) if you are running graphical calculations of the trimsheet variety, a far better result is obtained by using a datum for the trimsheet which is different to the typical somewhere-up-near-the-nose GA OEM datum. Generally, one will get a satisfactory result with a datum somewhere in the aft half of the CG envelope.

Based on a quick looksee at the Alpha system, the chart appears to be for the Piper Turbo Lance II. I would need to see the original DCA flight manual to check this, though, as the CG envelope has been truncated in the trimsheet compared to the US certification – this probably would have been done due to concerns within the DCA certification engineering group relating to stall speed data provided by the US for the Australian certification.

Putting that concern to one side, the Alpha system probably was lifted from an original VH- sheet designed and drawn by Norm Overmeyer (a now long-departed Sydney WCO) around 1980 or so. The datum for the sheet is FS (fuselage station) 93.6 (inches) – ie around 2.4 metres aft of the OEM datum (78.4 inches forward of the wing LE). The datum CG is a vertical line in the envelope at IU = 0, ie just a bit forward of the aft limit.


I am guessing that you hold a weight control authority ?

WCA No. AV1. As a sidenote of very little importance, mine was the first WCA issued when the present system was introduced in mid-1976.

Engineering specialist in aircraft performance and weight control.
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