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| Beginners / Newbies Interested in getting in the hobby. Not sure where to start. Post your questions here and one of the RCCanada experts will be glad to help you out! |
Topic Review (Newest First)
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| 07-11-2016 03:09 PM | ||
| kcaldwel |
Re: Nose heavy or tail heavy I should never get started on CG position and incidence angles! There are so many myths and misunderstandings amongst RC pilots, and some of them make RC airplanes harder to fly than they should be. Most plans, ARF airplane instructions and rules of thumb like "on the spar" or " 25% to 33%" result in very forward, very pitch stable CG positions. These may be "safer", in that there is little danger of having the CG behind the aircraft neutral point, and therefore being pitch unstable, but I find forward CG positions (large static margins, over 10% for me) make RC airplanes hard to fly. A large static margin means the airplanes really wants to return to it's trim angle of attack and speed. Whenever the airplane speeds up, either through more throttle, a gust, or a dive, the nose of the airplane will try to come up to slow the airplane down. This results in the classic ballooning in gusts. The nose rises with increasing power, and falls when the throttle is reduced. I find this results in fighting the airplane a lot in rough air, or when changing power settings. It also requires a lot of down elevator inverted, which makes rolls and inverted flight far more difficult than they should be. With the CG closer to the aircraft neutral point (I like 7% static margin usually, or even less), the airplane tends to stay in the same pitch attitude now matter the speed change from a gust or power setting. This requires the pilot to manage the airspeed and aircraft attitude, because the airplane will just stay in a dive, or stay nose up until stall if put in a low power climb. I've never been convinced that trainers with more than about a 10% static margin are easier for beginners to fly. Big static margins cause the airplane to balloon or dive radically with every gust, and to nose up with every speed increase. It seems they end up fighting with the airplane more, and it certainly makes it harder for anyone to fly it on a windy day with the resulting turbulence down low. Low static margins are a necessity for discus launch gliders and sailplanes in general. DLG's must fly straight from 150kph to 20kph. If the CG is too far forward, they'll loop on launch. Having the CG too far forward in any sailplane means they plough through thermals without indicating where the lift is. Many people think flat bottom and under-cambered airfoils balloon in gusts, and require enormous amounts of down elevator to fly inverted. It is not the airfoil that causes that, it is having the CG too far forward. Having camber in an airfoil causes the zero lift line to rotate up from the chord line by about 1 degree for every 1% of camber. People then put the flat bottom flat on the top of the fuselage and end up with about 6 degrees of incidence in the wing. They then move the CG way forward to try and get it to trim. See attached drawing comparing the zero lift angle of a Clark Y airfoil to a symmetrical airfoil. I also made a little CG and trim demonstrator model with a highly under-cambered free flight airfoil, but with a 2% static margin. It flew inverted with very little down-stick, didn't balloon in gusts, etc.: https://vimeo.com/50261376 https://vimeo.com/50258747 The trimming procedure for any airplane should be: calculate the aircraft neutral point using either a simple on-line calculator (not exact), or the more sophisticated CG calculator I linked to before. Set the CG at a static margin % you like from past experience. CG location is personal preference, but 10% SM, or 15% if you are used to forward CG positions, is not a bad place to start. Lower static margins do require reduced elevator throw, and more expo. The elevator will be more powerful than with a more forward CG. If you are going to try a reduced static margin, reduce the elevator throw. Adjust the elevator for the trim speed in flight you want. If the elevator deflection is less than 5 degrees one way or the other, and the fuselage angle in flight is to your liking, you are done. If you end up with a lot of elevator deflection to trim, you can adjust the stab incidence to eliminate the elevator offset. Do not adjust the CG position for trim, only to change the pitch stability and elevator response you desire. Kevin Edit: I meant to include a link for one of the on-line CG calculators. All the ones I have seen use a fixed stab effectiveness factor and several other simplification that mean they are not quite as accurate as the spreadsheet one: http://adamone.rchomepage.com/cg_calc.htm |
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| 07-09-2016 10:18 AM | ||
| williame3590 |
Re: Nose heavy or tail heavy Excellent post!! thanks Kevin. Bill |
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| 07-08-2016 11:46 PM | ||
| kcaldwel |
Re: Nose heavy or tail heavy Wing and tail airfols, incidence angles, etc. do *not* influence the CG location. The CG must be ahead of the aircraft neutral point in order for the airplane to be pitch stable. The aircraft neutral point is determined by the wing planform, the stab planform, the distance between the 1/4c of the wing and stab, and the effect of the wing wake on the airflow velocity at the stab (usually a small factor). Airfoils and incidence angles do not affect the CG location. Quote:
http://www.johnanthonymodeldesign.co.../timeline3.pdf Most RC sailplanes are flown with CGs well aft of the supposed "magic" 33%. They have long tail arms that move the aircraft neutral point back, and therefore for the desired static stability margin the CG will be well aft as well. And the stab on an RC sailplane typically lifts slightly up or down in different flight regimes. This is the best CG calculator I know of (short of running AVL), and will handle almost any aircraft configuration. It will also allow you to move all the equipment weights around to forecast where the actual CG will be even before you build the airplane. I find it very useful, but it does take some time to understand: www.rcgroups.com/forums/showthread.php?t=1106300 An airplane with a large static margin (15% or larger, CG well forward of the aircraft neutral point), will be less responsive to the elevator, may run out of up elevator control for landing. Moving the CG back requires decreasing the elevator throw, since the elevator will become more powerful. An airplane with the CG at the neutral point may diverge up or down when disturbed, and will have no tendency to return to a trim speed. Most discus launch gliders are flown neutrally stable. A decent RC pilot can usually fly an airplane with decent pitch damping, like a sailplane, with the CG behind the neutral point. The best test is to roll the airplane inverted, and see how much down stick is required to hold the flight path straight. A neutrally stable airplane will not require any down elevator input. A nose heavy airplane will require a lot of down stick. Aerobatic airplanes are usually tested on a 45 degree up-line. A dive test also works, but is harder to interpret. Basically trim for a nice glide, and pitch the nose down. Release the elevator stick and observe how long it takes for the airplane to pitch up. If it pitches up quickly, it is very stable (forward CG). If it just continues along the line, it is neutrally stable (CG near the aircraft neutral point). From Dr. Mark Drela, famous MIT aero prof: http://www.charlesriverrc.org/articl...GMarkDrela.htm Kevin |
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| 07-08-2016 04:35 PM | ||
| jsparky |
Re: Nose heavy or tail heavy Welcome to the hobby |
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| 04-10-2016 01:51 PM | ||
| Eye Can Fly |
Re: Nose heavy or tail heavy Once! 
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| 04-10-2016 01:25 PM | ||
| Zask714 |
Re: Nose heavy or tail heavy THere is a say, a nose heavy plane fly badly a tail heavy plane fly worst... |
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| 04-27-2015 09:21 AM | ||
| VA6WGO |
Re: Nose heavy or tail heavy Quote:
Will |
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| 04-22-2015 01:07 PM | ||
| Sukhoi 68 |
Re: Nose heavy or tail heavy Excellent link! Thanks! Scott |
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| 04-22-2015 12:13 PM | ||
| Michael Hyatt |
Re: Nose heavy or tail heavy Here is the simple aircraft center of gravity calculator. Or the full aircraft C.G. calculator. |
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| 04-22-2015 11:54 AM | ||
| Sukhoi 68 |
Re: Nose heavy or tail heavy On a constant cord wing (no taper) measure the width of the wing, leading edge to trailing edge. This distance will be your cord. Then figure out what 25% would be from the leading edge. This will be your starting point for the CG. Another good rule of thumb, though not always, is that the CG will be on the main spar. Like the guys say check your instructions. Example 24" chord 25% would be 6". so you CG will be 6" from the leading edge. Scott |
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