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Tip of the week - Setting CG from a DiveSubmit your Tip of the Week to contact@allthingsthatfly.com and you could win a free month of Flight Pass from Higher Plane Productions! CG or Center of Gravity is the location on a model where it should balance. When dealing with a kit, ARF or model from a manufacture, they will typically provide the location for CG in the instructions. You will find that the CG suggested by the manufacture can make the model a bit nose heavy and for good reason. A model that is nose heavy will still fly fairly stable, reducing the chances of a poor handling. A tail heavy model will be very unstable and unresponsive to control inputs. Typically the measurement provided will be about 1/4 to 1/3 of the wing cord (depth from leading edge to trailing edge). Depending on the model you may want it to be a bit nose heavy or more neutral. 3D and sailplane pilots will prefer a more neutral CG, and pilots flying fast sport models will likely want a touch of nose weight. The best way I have found to determine if CG is set properly is in the air. Yes you want to use the manufactures suggested CG point prior to flying the model, but once you have done that you can fine tune it a bit by doing a simple dive test. This is how I do it. 1. Get the plane ready for flight (typical preflight checks to include checking the CG) 2. Launch or take off and get the model to a comfortable altitude so you can get the model trimmed out. 3. Once trimmed to where the model will fly straight with no inputs (for gliders no power, for sport and 3D models mid throttle or a bit higher) Take it up high enough you can make a long shallow dive without hitting the ground. 4. Put the model in a dive around 30-45 degrees and center the controls and cut power. 5. Let the plane dive and determine if the nose climbs (plane levels off and decreases the pitch of the dive) or if it tucks (plane increases the pitch of the dive) This info will tell you if the plane is nose heavy or tail heavy. This always seems reversed but it isn't. If the nose lifts, then the model is nose heavy. If the nose falls, then the model is tail heavy. One of the best ways to adjust this is moving the location of the battery or increasing or decreasing the size of the battery used depending on its location. Many models put the battery location at the center of the CG so you can change battery pack sizes and not change the weight distribution and not affecting the CG. But in many models, the bay area for the battery allows you to move it towards the front or rear of the model to make adjustments. Weight can be added also, but depending on the model, you may want to keep the weight low. On a foamy 3D model and smaller park flyers you would be better off increasing the weight in the proper area by moving the battery or increasing or decreasing the battery size and weight. Gliders and 3D planes really should be setup so that it is as neutral as possible. In a dive test the model would hold the dive angle and not nose up or down. This will give you the best lift performance and allow a 3D model to hover and harrier well. 3D pilots will notice they don't need as much input while flying inverted, and during knife edges, the plane will also more likely fly straight. CG is a critical ingredient for your model to perform its best. If your CG is off, it will require more input from the pilot to fly well. On gliders the trim used to keep the model flying straight to offset the improper balance will introduce drag and reduce your flight time. Gliders glide due to their high lift, low drag design, so CG is very crucial. ATTF |
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This is a pretty badly written article, especially with regard to sailplanes. A glider is very difficult to fly smoothly if it is neutrally stable as you describe. Some amount of positive stability is needed to make it comfortable to fly, particularly in turbulent conditions where if neutral every gust will upset the model with no tendency to return to a stable condition.
Contrary to what you write, CG position has basically no effect at all on the glide performance of a sailplane. What it does effect is handling, and responsiveness (indication) of lift. |
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It's basics of CG, not an article. All makes since, would have helped me when I first started. Thanks to CK and many others for basics of getting there!
However I'm glad that CG helps handling, and responsiveness of gliders. But since gliding is the only important aspect of a glider.. then you might look elsewhere. There can be non-stop books written on the subject. |
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Last edited by GFBurke; Mar 31, 2010 at 06:26 PM.
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One thing I didn't make clear and should have is my experience with sailplanes is limited to powered models. I live in the flat lands and sloping (which I very much want to try) is something I haven't done. I should have made it clear that I was referring to powered gliders and not slopers. I am not typically dealing with windy conditions. I understand that ballast is crucial under those conditions, but would not even attempt to tackle that topic. I welcome more comment on this. I realize CG is a topic that encompasses many aspects of a models design. I have a lot to learn on this as well and would love to see input from others. Thanks, Diggs |
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Basics, article, whatever. Does it make a difference what it's called?
Gliding is most certainly not the only important aspect of a glider. How can you glide it effectively if you can't control it accurately and tell how it's responding to the air? It's all related. Quote:
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Disagree all you like. I said basically none, not none at all. The magnitude of changes to CG we make on our gliders have next to no impact on sink rate and L/D. We adjust CG for handling and stability.
Some of the info in the RCA videos is just wrong unfortunately, but for the most part they are a superb reference. Very large control surface deflections will incur some trim drag but normal amounts of trim will not perceptibly affect flight performance. And you don't adjust CG to make the trim neutral, set the CG first then if the trim amount is visually unappealing, shim the stabilizer to neutralize it. Quote:
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But thanks for the input! |
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Maybe this will carry more weight: http://www.charlesriverrc.org/articl...GMarkDrela.htm
If Dr. Mark Drela isn't enough of an authority, then there's nothing will change your mind. |
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Shimming the elevator (if there is one) will add drag, you are trying to compensate for flight controlls that are in the airstream. IMHO, get the flight controlls out of the airstream and do ANYTHING you can to get the plane to run straight and true WITHOUT adding drag. Ballast will play a HUGE part in trimming any plane for minimum flight controll drag |
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