Aft CG limit(s)

Recent threads that discussed handling and performance have referred to the aft cg limit, perhaps implying that there is a fixed aft limit.

For the ASW-19b and the ASW-28, the gliders I have most recent experience in, the aft cg limit moves forward as the gross weight increases. I assume this is true for other glider types but I don't have data available.

For the ASW-28 the manufacturer recommends an optimum cg range of 300-310mm aft of datum but does not seem to mention that the aft limit of this 'optimum' range is behind the aft cg limit at max gross weight.

I have found that a cg position of 320mm aft of datum gives best climb performance at 9psf loading without compromising the high speed performance. That cg is at the aft limit for 9psf but well forward of the 345mm 'dry' aft limit. (9 pounds per square foot is a loading limit for USA standard class contests)

The low weight cg range for the 28 is 227-345mm aft of datum for those that want to consider percentages.

So how many of you know if the glider you fly has a weight dependent aft cg limit?

Andy (GY)

Andy, The CG limits are firm for any weight of your ship. Its up to you to add trim weights to bring the CG back to desired with a heavy pilot. I set up my ships for ideal CG without wing water. When I load water, I must add weight to the tail to compensate for the forward CG shift caused by the wing water. Many ships will have a tail water tank to accomplish this.

That is, unless I'm flying the Genesis 2, then I must add water to my nose water tank, because the wing tanks are aft of the empty CG and wing water causes the CG to shift aft, so I must compinsate with forward weight in the form of nose tank water. JJ Sinclair

Is it possible you're mixing apples & oranges? The July 1977 ASW 20 handbook Flight manual p.18 lists a firm *in flight* cg range of 240 mm (9.45' to 360 mm (14.17' aft of datum, with the datum being the leading edge of the wing at the wing root rib, excluding the fillet of the wing/fuselage fairing.

Assuming similar wording in the '19 & '28 manuals, I think you're confusing this with a table shown elsewhere in the manual, entitled 'Empty Weight c of g positions and limits.' Here, it does appear the minimum cockpit load increases (and aft cg shifts forward) at higher weights. Please note, however, that the x axis represent the EMPTY CG and the Y axis represents EMPTY weight.

Per wording in the '20 manual at p. 30a, this table is to be used when tail ballast requires a re-calculation of the minimum cockpit load. This re-calculation is needed because the *flight* cg must be within the *flight* limits.

Clear as mud, huh?

The text of section 2.5 of the ASW-28 flight manual gives a center of gravity range for flight of 222mm to 345mm. It does not relate this limit to flight mass.

The flight manual also shows the flight mass vs CG envelope in graphical form on page 5.10. It shows the aft cg limit constant at 345mm from 300kg to about 380kg. Note that 380kg is the maximum allowed flight mass without water ballast. The aft limit then slopes forward to about 315mm at 525kg. There is also a small forward movement of the forward cg limit as flight mass increases. I believe this diagram clearly shows a flight aft cg limit that is dependent on mass.

I re-checked the ASW-19 manual and I did jump to the wrong conclusion there. The data I looked at relate to empty mass cg as suggested by Judy.

I don’t know why the aft limit moves forward with increasing mass for the ASW-28. Could it be that Schleicher found the stall/spin recovery characteristics unacceptable at max GW at the dry aft limit.

Would a 27 owner please say if that glider weight/cg envelope also shows a variable aft limit.

Thanks

Andy (GY)

There is no weight dependant envelope of cg for the ASW27

Section 2.7 state the foremost limit is 0.210 m and the aftmost limit is 0.320 m.

In the non-approved part of the Performance section, point 5.3.2.3 there is a discussion of the optimal cg position.

Øyvind Moe

Somewhere tucked into the text of the manual, there must be some passing reference to using this graph!

Just guesswork on my part: (1) a forward-leaning aft CG limit line may reflect the flight CG of the glider shifting forward with the addition of water in the wing tanks, while (2) this shift is partially offset by carrying water in a tail tank, and (3) this balancing act is also reflected in the forward-leaning forward CG limit line. (It seems to me your graph *must* contemplate a tail tank... ballasting the wing tanks alone would cause a forward shift in the flight CG, NOT a forward shift in the forward CG *limit* - only the addition of weight well aft could do that!)

In all cases, of course, (1) the unballasted minimum seat weight must be met (otherwise, the pilot who dumps water ballast will zip out the aft end of the flight envelope. Not pretty.) and (2) no flying weight may exceed maximum gross weight (I assume, as in the good old/bad old days, that few pilots can actually fly with full ballast... they'll bump into max gross first.)

If my guesswork is correct, the next logical questions about the graph are (1) what assumptions are made about ballast distribution in wing and tail tanks and (2) as a practical matter, what glider systems & pilot procedures assure this distribution.

Not helping much, am I?

At 19:06 24 November 2003, I stuck my foot in my mouth by saying:

That last phrase should have read, 'only the addition of weight well aft could make the forward CG limit LINE shift forward.' (That's because the LINE represents the CG limit - a fixed value - being affected by the distribution of weight in wing & tail tanks.)

Apologetically,