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#1
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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) |
#2
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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 |
#3
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At 15:12 23 November 2003, Andy Durbin wrote:
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. 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? Judy |
#4
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Judy Ruprecht wrote in message ...
Is it possible you're mixing apples & oranges? 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) |
#5
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Would a 27 owner please say if that glider weight/cg envelope also
shows a variable aft limit. 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 |
#6
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Andy Durbin wrote:
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. The Take-off Mass vs In-flight CG range diagram for my ASH 26 E shows the CG range (aft limit AND forward limit) changing with mass. Above 490 kg, the allowable aft limit moves forward; below 480 kg, the forward limit moves rearward. I believe the explanation for this lies in the text preceding the table: "The C.G. position shift due to water ballast load have been included. This is to make sure that the ASH 26 E remains within the approved limits after the water ballast has been jettisoned." Apparently (as an example), if you have the CG at the chart's _unballasted_ aft limit with full ballast, it will move behind the aft limit when you dump the ballast. I'm guessing that on strictly aerodynamic considerations, the CG range would not depend on the mass. This would seem sensible, based on the text of section 2.7: "2.7 Center of Gravity The limits of the C.G. are as follows: forward limit 290 mm aft of BP aft limit 410 mm aft of BP" BP means "reference datum". -- ----- Replace "SPAM" with "charter" to email me directly Eric Greenwell Washington State USA |
#7
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At 03:24 24 November 2003, Andy Durbin wrote:
The (ASW 28) 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. 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? Judy |
#8
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Aha! Now it all makes sense. It seems to me that the
cg range should be fixed for a given airfoil no matter the weight. Providing for staying within the aft range after dumping ballast has to be the reason for the shift described and this should be true for all gliders. At 18:00 24 November 2003, Eric Greenwell wrote: Andy Durbin wrote: 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. The Take-off Mass vs In-flight CG range diagram for my ASH 26 E shows the CG range (aft limit AND forward limit) changing with mass. Above 490 kg, the allowable aft limit moves forward; below 480 kg, the forward limit moves rearward. I believe the explanation for this lies in the text preceding the table: 'The C.G. position shift due to water ballast load have been included. This is to make sure that the ASH 26 E remains within the approved limits after the water ballast has been jettisoned.' Apparently (as an example), if you have the CG at the chart's _unballasted_ aft limit with full ballast, it will move behind the aft limit when you dump the ballast. I'm guessing that on strictly aerodynamic considerations, the CG range would not depend on the mass. This would seem sensible, based on the text of section 2.7: '2.7 Center of Gravity The limits of the C.G. are as follows: forward limit 290 mm aft of BP aft limit 410 mm aft of BP' BP means 'reference datum'. -- ----- Replace 'SPAM' with 'charter' to email me directly Eric Greenwell Washington State USA |
#9
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At 19:06 24 November 2003, I stuck my foot in my mouth
by saying: (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!) 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, Judy |
#10
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Our DuoDiscus also has changes in the allowable forward and rearward CG
limits based on mass, looking at the graph in the maintenance manual (page 6.7). It even shows a different rear limit at heavy weights with ballast tank installed (not to say filled). Then, in the flight manual, page 2.7, it says that the limits are simply 45 mm to 250 mm aft of datum. The two clearly disagree. I agree with you, Eric, that aerodynamically speaking the limits should not change with mass. The DuoDiscus manual goes further, stating in regards to fin ballast: ....water ballast may be carried in the fin tank to compensate for the nose-heavy moment of -water ballast in main wing panels and/or -loads in the aft seat (6.2.6) Pilots wishing to fly with the center of gravity close to the aft limit, may compensate the nose-heavy moment of loads on the _AFT SEAT_ with the aid of the diagram... (6.2.6) Compensation of masses exceeding the placarded minimum front seat load is not allowed. (6.2.6) --- A lower (than minimum) front seat load must be compensated by ballast... (2.7) (the rear seat occupant may not be factored in, although clearly well forward of the CG) There are more examples but this post is already too long. I can see that a fat boy in the front seat and a full load of water in the tail, although weighing the same as with a skinny guy in front, no tail ballast, and a rear pilot, would have more rotational inertia, possibly becoming a spin recovery problem. That makes sense right up until you see that 200 liters of ballast are allowed well out in the wings! Not allowing one to be in CG limits with a below minimum pilot in the front seat and a rear pilot helping to make the difference has nothing to do with inertia and could only be that Schempp-Hirth has discovered American style product liability. I understand trying to prevent mistakes but the pilots that will make the mistakes won't read the manual anyway -- too damned complicated! The DuoDiscus buyers end up being test pilots after all... -Bob Korves "Eric Greenwell" wrote in message ... Andy Durbin wrote: 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. The Take-off Mass vs In-flight CG range diagram for my ASH 26 E shows the CG range (aft limit AND forward limit) changing with mass. Above 490 kg, the allowable aft limit moves forward; below 480 kg, the forward limit moves rearward. I believe the explanation for this lies in the text preceding the table: "The C.G. position shift due to water ballast load have been included. This is to make sure that the ASH 26 E remains within the approved limits after the water ballast has been jettisoned." Apparently (as an example), if you have the CG at the chart's _unballasted_ aft limit with full ballast, it will move behind the aft limit when you dump the ballast. I'm guessing that on strictly aerodynamic considerations, the CG range would not depend on the mass. This would seem sensible, based on the text of section 2.7: "2.7 Center of Gravity The limits of the C.G. are as follows: forward limit 290 mm aft of BP aft limit 410 mm aft of BP" BP means "reference datum". -- ----- Replace "SPAM" with "charter" to email me directly Eric Greenwell Washington State USA |
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