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Author Topic: Balancing vs. Stab Adjustments vs. Wind Incidence  (Read 712 times)
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OneArm
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« on: February 27, 2018, 12:11:01 PM »

Hi All,

For rubber power, at what point is balancing complete and stab adjustments (or/and wind incidence) take over? If a plan has a balance point (Prairie Bird), I balance at that point and then make stab adjustments for glide. What if there is no suggested balance point? How do I know when I have an optimum mix of balance and stab and wing incidence? I'm a little confused. Thanks everyone in advance.
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piecost
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« Reply #1 on: February 27, 2018, 06:29:41 PM »

The easiest way to estimate the balance point is to refer to the point given on a number of similar plans. They must be of similar proportion; tail size relative to wing and fuselage length. Also, similar model class.

The balance point or Centre of Gravity (CG) must be forward of a location known as the neutral point (NP). You do not know the location of the NP and it can be complicated to calculate it. It does not really matter, but the NP can be thought of as the point through which the aerodynamic forces act. The fins on a dart, combined with the heavy front end ensure that its CG is forward of its NP
 The dart is therefore stable and flys straight. An aeroplane is the same.

There is not a single best location for the CG but if it is too far forward then excessive up elevator or nose down tail setting is needed. This is inefficient and duration suffers. If the CG is too far aft then the model is less stable and struggles to recover from a distrubance or collision. But it is more efficient. So, it is up to the modeller to find the correct location for his style of trimming.

I recommend experimenting with a model, moving the CG location with nose or tail weight, adjusting the tail trim and noting how stable the model is in flight.

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OZPAF
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« Reply #2 on: February 28, 2018, 12:06:38 AM »

I would perhaps recommend building and trimming a HLG or CLG to help your understanding of trimming - ie CG position and decalage. They cost very little to make and also need to fly well over a wide speed range.

Calculations help establish starting points but ironically you need a fair bit of experience to use them well - they are only tools and not absolute.

Also you can't read too much about what experienced modellers have written. There is a lot here on HPA.

Good luck.

John
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Hepcat
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« Reply #3 on: February 28, 2018, 12:46:35 PM »

One arm,
An interesting query but more complex than one would think at first glance.  One simple answer is that the trimming is all right if the flights suit you. If you have aims to fly competition then the stop watch probably gives the answer. It is good to see that you have realized that CG placement is the first requirement.  If it is not on the plan then build something else.  You already have replies from two experienced modellers and I am not here to criticise their advice, rather to endorse them. I notice that OZPAF missed your mention of rubber driven but that does not make his advice that a CLG is a simple, inexpensive way of getting a lot of trimming practice any less valuable.
One thing that has not been mentioned so far is that Free Flight models almost always fly in circles which can have an enormous effect on the placement of the CG,the incidence angles of the flying surfaces and offsets of the thrust line.
Now I am going to say a few things in a very 'broad brush' manner and leave you to ask more specific question of the HPA experts as you come across them.
Trimming is just a matter of trying a lot of small adjustments, with care, so as not to break something.
First we hand glide. We can't help it! At least it tells us if the model is violently stalling or diving and we set the incidences roughly.
Next we seek a decent glide which circles in the way you want it to. This will need some finger winds to give a bit of altitude and probably rudder offset for turn.
Now you start increasing the number of winds.  The extra torque will try to turn the model left and the extra thrust will make the model climb.  If the extra torque and thrust destabilize the model the effect can often be mitigated by offsetting the thrustline. Please note also that if the model flies faster because of more turns then the aerodynamic surfaces such as the rudder, or a wing panel will generate more force.
I think this is a good place for me to finish when you reach this stage it would be better for you to ask more detailed questions for the experts to  answer.
A final word. Successfull flyers keep records and the offsets, packings,CG and warps are all worth noting.
John
   
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OneArm
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« Reply #4 on: March 01, 2018, 07:11:42 AM »

Right on, thanks everyone. It really boils down to experience for me. I've been flying maybe 6 years but rarely get out to the 30 acre field close by. My planes fly well, but I always feel that I'm not getting everything out of them. It I flew more often, it would be easier. Anyways, I wanted to ask the question and see what came back
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flydean1
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« Reply #5 on: March 01, 2018, 12:51:53 PM »

Where are you located?  May be others located nearby.
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OneArm
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« Reply #6 on: March 01, 2018, 02:27:14 PM »

Pittsburgh, PA. Fly at Hartwood Acres. 40.5680, -79.9257
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dlasich
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« Reply #7 on: March 12, 2018, 10:06:08 AM »

If a plane doesn't give you a CG and/or you design your own, you can use an online c.g. calculator .  Mathematically it's just calculation using tail area versus the wing and how far back it is to find the neutral point, and then you set the cg fwd of that point... how far fwd depends on how strongly stable you want it.

https://rcplanes.online/cg_calc.htm

But as a rule of thumb on a conventional airplane I have always set the horiz stab about 2-3 deg decalage and the c.g. about 1/3 back from the leading edge and find I am always pretty close.  I then test glide and add a bit of clay aft or fwd as needed for best glide.
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