|07-09-2016, 10:18 AM||#21|
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I am: William A
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Re: Nose heavy or tail heavy
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Excellent post!! thanks Kevin.
|07-11-2016, 03:09 PM||#22|
I am: Kevin C.
Join Date: Jan 2007
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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.:
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.
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:
Last edited by kcaldwel; 07-11-2016 at 05:54 PM.
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