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#11
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Optimum CG Range
On Dec 14, 8:56*am, wrote:
On Dec 13, 9:19*am, JJ Sinclair wrote: Another useful approach is start at about 66% aft using manufacturer's CG range. When making the tightest turn you normally do, if you run out of elevator, you need to shift CG back a bit. You will probably end up around 75%.There isn't a huge benefit in having the CG way back, but there is a significant deterioration of handling which requires better pilot skills to offset.The last little bit of glider performance costs quite a bit in pilot workload until you are very proficient. I usually take a couple pounds out of the tail in the Spring and put it back in when my skills are back up to snuff. FWIW UH- Hide quoted text - Also FWIW and purely as a first approximation, I've noted that the following works reasonably well in most glass birds I've flown. - At altitude, trim the glider so that it flys at "about" best L/D airspeed. This is usually somewhere between 50-55kts. - If that trim position results in significant up-elevator, you need more weight in the tail. "Significant" in this context means that you have more than a small percentage of the available up-elevator travel dialed in to maintain best L/D airspeed. Obviously, this is only a secondary check after you've already run the numbers or done a proper W&B, but it seems to work pretty well. In my LS8, the above works out to about 80% of manufacturer's aft limit and results in very pleasant handling and obvious improvement in get- home performance on weak days. Anything wrong with this approach? P3 |
#12
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Optimum CG Range
On Dec 14, 7:56*am, wrote:
On Dec 13, 9:19*am, JJ Sinclair wrote: OR A percentage of MAC behind the wing LE Brian W Errrrr, not neccessarily so; The wing leading edge will be the zero MAC point only if the leading edge of the wing is a straight line, otherwise as in the LS-8, zero MAC will be located behind the leading edge. I know a guy that made this incorrect assumption on the first flight of an RS-15 and he flew the whole flight (rather short) with the stick full back because his CG was forward of the forward limit. He considered bailing out, but found he could keep the nose up if he flew 80 knots. He landed OK touching down at 80. I like to refer to the CG in a percentage of the allowable range. The Genesis likes to be about 85% of the allowable range which is; 0 to 5.25" aft of root rib and 85% is 4.5"aft. After adjusting the CG, give her a test drive. If you find you are trimming forward when entering a thermal, your CG is too far aft. Cheers, JJ Another useful approach is start at about 66% aft using manufacturer's CG range. When making the tightest turn you normally do, if you run out of elevator, you need to shift CG back a bit. You will probably end up around 75%.There isn't a huge benefit in having the CG way back, but there is a significant deterioration of handling which requires better pilot skills to offset.The last little bit of glider performance costs quite a bit in pilot workload until you are very proficient. I usually take a couple pounds out of the tail in the Spring and put it back in when my skills are back up to snuff. FWIW UH Here's another gem piece of advice: With my 3D model airplanes I roll inverted and check if I need down elevator to stay level. If so, the cg needs to be moved further back. A well set up model will happily fly inverted without elevator movement! Haven't tried that in my LS8, though. Seriously, Hanks and Eric's methods will both work well. As long as inside the book range, find the cg that gives you good handling and enough up elevator to stall the plane in a steep turn. It'll be at 75%-90% aft. Herb, J7 |
#13
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Optimum CG Range
On Dec 14, 3:34*pm, Herb wrote:
On Dec 14, 7:56*am, wrote: On Dec 13, 9:19*am, JJ Sinclair wrote: OR A percentage of MAC behind the wing LE Brian W Errrrr, not neccessarily so; The wing leading edge will be the zero MAC point only if the leading edge of the wing is a straight line, otherwise as in the LS-8, zero MAC will be located behind the leading edge. I know a guy that made this incorrect assumption on the first flight of an RS-15 and he flew the whole flight (rather short) with the stick full back because his CG was forward of the forward limit. He considered bailing out, but found he could keep the nose up if he flew 80 knots. He landed OK touching down at 80. I like to refer to the CG in a percentage of the allowable range. The Genesis likes to be about 85% of the allowable range which is; 0 to 5.25" aft of root rib and 85% is 4.5"aft. After adjusting the CG, give her a test drive. If you find you are trimming forward when entering a thermal, your CG is too far aft. Cheers, JJ Another useful approach is start at about 66% aft using manufacturer's CG range. When making the tightest turn you normally do, if you run out of elevator, you need to shift CG back a bit. You will probably end up around 75%.There isn't a huge benefit in having the CG way back, but there is a significant deterioration of handling which requires better pilot skills to offset.The last little bit of glider performance costs quite a bit in pilot workload until you are very proficient. I usually take a couple pounds out of the tail in the Spring and put it back in when my skills are back up to snuff. FWIW UH Here's another gem piece of advice: With my 3D model airplanes I roll inverted and check if I need down elevator to stay level. *If so, the cg needs to be moved further back. *A well set up model will happily fly inverted without elevator movement! *Haven't tried that in my LS8, though. Seriously, *Hanks and Eric's methods will both work well. *As long as inside the book range, find the cg that gives you good handling and enough up elevator to stall the plane in a steep turn. *It'll be at 75%-90% aft. Herb, J7- Hide quoted text - - Show quoted text - Herb means 75-90% aft of the CG RANGE not the MAC. |
#14
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Optimum CG Range
Herb wrote:
Another useful approach is start at about 66% aft using manufacturer's CG range. When making the tightest turn you normally do, if you run out of elevator, you need to shift CG back a bit. You will probably end up around 75%.There isn't a huge benefit in having the CG way back, but there is a significant deterioration of handling which requires better pilot skills to offset.The last little bit of glider performance costs quite a bit in pilot workload until you are very proficient. I usually take a couple pounds out of the tail in the Spring and put it back in when my skills are back up to snuff. FWIW UH Here's another gem piece of advice: With my 3D model airplanes I roll inverted and check if I need down elevator to stay level. If so, the cg needs to be moved further back. A well set up model will happily fly inverted without elevator movement! Haven't tried that in my LS8, though. Seriously, Hanks and Eric's methods will both work well. As long as inside the book range, find the cg that gives you good handling and enough up elevator to stall the plane in a steep turn. It'll be at 75%-90% aft. Why is the ability to stall in a steep turn a useful criteria? It sounds like a safety problem to me. -- Eric Greenwell - Washington State, USA * Change "netto" to "net" to email me directly |
#15
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Optimum CG Range
Herb wrote:
Another useful approach is start at about 66% aft using manufacturer's CG range. When making the tightest turn you normally do, if you run out of elevator, you need to shift CG back a bit. You will probably end up around 75%.There isn't a huge benefit in having the CG way back, but there is a significant deterioration of handling which requires better pilot skills to offset.The last little bit of glider performance costs quite a bit in pilot workload until you are very proficient. I usually take a couple pounds out of the tail in the Spring and put it back in when my skills are back up to snuff. FWIW UH Here's another gem piece of advice: With my 3D model airplanes I roll inverted and check if I need down elevator to stay level. If so, the cg needs to be moved further back. A well set up model will happily fly inverted without elevator movement! Haven't tried that in my LS8, though. Seriously, Hanks and Eric's methods will both work well. As long as inside the book range, find the cg that gives you good handling and enough up elevator to stall the plane in a steep turn. It'll be at 75%-90% aft. Why is the ability to stall in a steep turn a useful criteria? It sounds like a safety problem to me. -- Eric Greenwell - Washington State, USA * Change "netto" to "net" to email me directly |
#16
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Optimum CG Range
Herb wrote:
...find the cg that gives you good handling and enough up elevator to stall the plane in a steep turn. It'll be at 75%-90% aft. Herb, J7 Reminds me of the old rule of thumb, if you could call it that: CG too far forward, can't pull the nose up for takeoff. CG too far aft, can't recover from a stall/spin. This method can be expensive in airplanes though... :-) Brian W |
#17
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Optimum CG Range
Eric Greenwell wrote:
Herb wrote: Another useful approach is start at about 66% aft using manufacturer's CG range. When making the tightest turn you normally do, if you run out of elevator, you need to shift CG back a bit. You will probably end up around 75%.There isn't a huge benefit in having the CG way back, but there is a significant deterioration of handling which requires better pilot skills to offset.The last little bit of glider performance costs quite a bit in pilot workload until you are very proficient. I usually take a couple pounds out of the tail in the Spring and put it back in when my skills are back up to snuff. FWIW UH Here's another gem piece of advice: With my 3D model airplanes I roll inverted and check if I need down elevator to stay level. If so, the cg needs to be moved further back. A well set up model will happily fly inverted without elevator movement! Haven't tried that in my LS8, though. Seriously, Hanks and Eric's methods will both work well. As long as inside the book range, find the cg that gives you good handling and enough up elevator to stall the plane in a steep turn. It'll be at 75%-90% aft. Why is the ability to stall in a steep turn a useful criteria? It sounds like a safety problem to me. I expect it has to do with efficiency. If your CG is such that your control inputs are minimised - you reduce drag. In the case of steep thermalling, it reduces safety - because you can now stall and theoretically spin. There can be few things as frustrating as my experience with my (new to me) Kestrel 19. First flight I wanted to be cautious so set the CG at 35%. Then the day was booming - but with tight strong thermals, and I was continually running out of elevator. Stick against the back stop and the thermal is still tighter. My Cirrus with it's all flying tail never had that problem. Of course you could depart controlled flight if you got too enthusiastic... Bruce |
#18
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Optimum CG Range
On Dec 14, 7:55*pm, Eric Greenwell wrote:
Herb wrote: Another useful approach is start at about 66% aft using manufacturer's CG range. When making the tightest turn you normally do, if you run out of elevator, you need to shift CG back a bit. You will probably end up around 75%.There isn't a huge benefit in having the CG way back, but there is a significant deterioration of handling which requires better pilot skills to offset.The last little bit of glider performance costs quite a bit in pilot workload until you are very proficient. I usually take a couple pounds out of the tail in the Spring and put it back in when my skills are back up to snuff. FWIW UH Here's another gem piece of advice: With my 3D model airplanes I roll inverted and check if I need down elevator to stay level. *If so, the cg needs to be moved further back. *A well set up model will happily fly inverted without elevator movement! *Haven't tried that in my LS8, though. Seriously, *Hanks and Eric's methods will both work well. *As long as inside the book range, find the cg that gives you good handling and enough up elevator to stall the plane in a steep turn. *It'll be at 75%-90% aft. Why is the ability to stall in a steep turn a useful criteria? It sounds like a safety problem to me. -- Eric Greenwell - Washington State, USA * Change "netto" to "net" to email me directly- Hide quoted text - - Show quoted text - Gives adequate pitch authority to pull to max lift coefficient, thus tightest turn. From my experience, this is usually about 75-80% aft in manufacturer's approved range. UH |
#19
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Optimum CG Range
Bob, thanks for figuring out the MAC of the LS8. I wasn’t clear on
how to “fudge” (DJ Aerotech’s term) the winglet and the wing root to get the required area for use in their graphical calculation of MAC. Your values of 736 mm for MAC, with a MAC LE of 41 mm aft of the LE of the wing root, jibe quite well with my arithmetic chord of 700 mm and my arithmetic chord LE of 50 mm aft of the LE of the wing root. I’m not clear why you said it didn’t seem right. But thanks to you I can now calculate the LS8 CG limits in terms of MAC. The fwd CG limit is 280 mm aft of the LE of the wing root, and the aft CG limit is 400 mm aft of the LE of the wing root (both values from TCDS G14CE). So: Fwd CG limit = 100 * (280 - 41) / 736 = 32% MAC Aft CG limit = 100 * (400 - 41) / 736 = 49% MAC This result (a) makes the LS8 odd from the perspective of SE light aircraft (typical CG range from 15% to 35% of MAC) and the HP-18 (was 25% to 40%, now 25% to 35%), and (b) means that the 1981 Frank Irving optimum CG guideline of 30% to 35% of MAC isn’t useful. So that answers my original questions. -John On Dec 13, 10:30 pm, Bob Kuykendall wrote: Hmm... That doesn't seem right. Using the DJ Aerotech graphical MAC method, the same LS8 drawing, and information on the LS8 from Thomas' Fundamentals of Sailplane Design (thanks again, Judah!) that places the planform break at 0.6 semispan, I got: * MAC length of ~736mm * MAC LE location of ~41mm aft of the wing LE at side of body _or_ * MAC LE location of ~45mm aft of the intersection of the projected leading edge and the plane of symmetry (yeah, who uses that?) Given that the LS8's double-trapezoid planform gives it more MAC per unit area than the HP-18's eminently buildable single trapezoid planform, and that the LS8's wing is unswept along the 25% chord line as opposed to the HP-18 being unswept along the 41.25% chord, the MAC and MAC LE numbers I got sound about right to me. But, hey, I'm a college dropout with no engineering training, what do I know? As regards the suggested CG location for the HP-series, Dick Schreder typically suggested 25% to 40% MAC as the allowable range. Based on an analysis of the margin of static stability of the HP-18 done by Steve Smith (that's Dr. Smith to you Mythbusters fans), and based on my own experience flying an HP-18 with CG back around 40%, I currently recommend that HP operators limit their operation to 25% to 35% MAC. |
#20
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Optimum CG Range
Thank you UH, P3 and J7 for your thoughts on optimal CG location. My
current CG on the LS8 lies between 45% to 55% of the available range, depending on how well my diet is working. :-( I’ll start next Spring without any rear weight until my proficiency returns, then move the CG back to 65%, then 70%, etc. I’ll quit at 80%, which for me is 1 gal of water in the tail tank and thus serves as a very clear stopping point. -John |
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