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Old Dec 21, 2015, 07:27 PM
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C of G, and thermaling

Would anyone like to offer an opinion as to what difference the c of g position makes to thermaling, vis vis thermal detection, and gaining maximum lift rate.
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Old Dec 21, 2015, 07:44 PM
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The further back the Cog, the more sensitive the plane becomes to changes in air movement, but it also becomes less 'stable', IOW you have to fly it with more finesse.

Further forward, the plane penetrates (flies 'upwind') better, becomes more forgiving to controller input but less sensitive to the changes in the air around it.

Where is the best place? Depends on the plane, the flyer and the conditions they are flying in. Your best bet is to experiment, moving the CoG a little at a time until you find your birds sweet spot. Eventually you will learn how to adjust it to best suit the air you're flying in.
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Last edited by Fly Wheel; Dec 23, 2015 at 07:39 PM. Reason: made corrections
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Old Dec 21, 2015, 08:35 PM
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I like to push mine back little by little until I'm no longer comfortable with it, and then just bump it forward a touch. It is more sensitive to lift and other disturbances, and I seem to be able to turn it better and core lift better.
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Old Dec 22, 2015, 01:02 AM
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I start by doing a shallow dive test. Move the cg back until the glider will stay on a dive if put in one. I then see if I can handle the glider in calm air thermals. If it ok then I used that location for calm air. I generally move the cg forward as the wind goes up. I found one glider I have that I could not handle with the cg as a dive test showed. I moved the cg forward and the glider has much better to handle. The lost was I could not see the speed change like I did before when I moved the cg.

Art
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Old Dec 22, 2015, 10:53 AM
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OK, thanks for all your inputs.
Sounds as though the bird, (MPX Solius) needs some re-trimming. I've got 35 grams of ballast just behind the motor, so no problem there. (Balances on the dimples, as per instructions.)
Flywheel, when you say it becomes more sensitive to changes in air movement, does that mean if it flys straight in to a thermal, will the tail be lifted up further, and will it's speed increase more rapidly the further back the CofG?
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Old Dec 22, 2015, 03:17 PM
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CG has nothing to do with how fast an aircraft can fly (physically, safety is another issue.) An aft CG will, however, increase control sensitivity, which will make high speed flying more difficult.

Personally, I fly my birds right at the limit of the dive test; they may be phughoid unstable, really.
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Old Dec 22, 2015, 06:06 PM
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Mr_SMO

All I can say is on my glider when the cg was set to where it would say in a dive I could see a difference in speed of low wing loaded glider when it was in thermal. When I mover the cg to were I could fly the glider easier when in a thermal the speed difference was less when in a thermal. When flying over head the only way I can tell if my glider is going into thermal is by a speed change as I can not tell if the stab is flying high or low. As stated above is if one can fly with an after cg the plane is more sensitive to control inputs and which includes the thermal affect on glider speed. As a glider become heavier it is less sensitive to the affect of thermals on the glider speed. It is similar to the affect of inertia in the roll axis as to showing thermals. The heavier the outer wing panels the less response to a thermal.

As the turbulence in the air picks up to tend to move the cg forward to make the glider easier to fly and read the air. Better pilots just seem to be able fly smooth in rough air.

It amounts to flying your glider allot to understand how it responds to the air currents. It requires flying smooth to read the air current form control inputs.
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Old Dec 23, 2015, 02:25 AM
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Once again, thanks for all your answers. It seems to me that a summary would be:- that the further back the CofG, then the better you glider will be at thermaling, detecting thermals, but not as good to handle.
BTW, I had to look up " phugoid ". I'm wiser now.
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Last edited by Mr_SMO; Dec 23, 2015 at 02:27 AM. Reason: correct spelling phugoid.
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Old Dec 23, 2015, 02:31 PM
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Handling "suffers" because so many folks have no idea that elevator throw must be reduced when cg is moved back.
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Old Dec 23, 2015, 06:46 PM
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I wouldn't say handling with an aft CG is "not as good". it's more sensitive. Up to a point that's good. Beyond that point it's bad. The sweet spot will vary from pilot to pilot.
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Old Dec 23, 2015, 07:45 PM
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Quote:
Originally Posted by Mr_SMO View Post
<Snip!>
Flywheel, when you say it becomes more sensitive to changes in air movement, does that mean if it flys straight in to a thermal, will the tail be lifted up further, and will it's speed increase more rapidly the further back the CofG?
Not so much that it will lift higher as it doesn't take as strong of a rising current of air before it does react.
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Old Dec 24, 2015, 12:08 PM
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Not so much that it will lift higher as it doesn't take as strong of a rising current of air before it does react.

Flywheel, The above is exactly the info that I was looking for. Come next summer, I've got some trimming to do, and no doubt I should be able to detect thermals better as a result, or at the very least, weaker thermals.

Thanks for everyone's input, I've learnt a lot from this thread.

Oh, and a very Happy Christmas to you all.
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Old Dec 24, 2015, 12:20 PM
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I found when glider can fly hands off C/G must be pretty good. Each bird/ flyer has that sweet sopt they like best. With new Glider i like bird flying slightly nose down till happy with it.
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Old Dec 25, 2015, 04:47 PM
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Quote:
Originally Posted by cityevader View Post
Handling "suffers" because so many folks have no idea that elevator throw must be reduced when cg is moved back.
BINGO

And trim required is less. Overall, the glider is producing less induced drag so it's more efficient.

And it's not just more efficient, but it telegraphs lift warnings to us better.
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Old Dec 25, 2015, 06:20 PM
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Quote:
Originally Posted by cityevader View Post
Handling "suffers" because so many folks have no idea that elevator throw must be reduced when cg is moved back.
I agree completely. If you want to fly slow in thermals or whatever and be efficient you set it as far back as you can minimizing elevator deflection. That would be the cleanest most efficient slow flying. Make a moveable ballast. . if you want to then cruise most efficiently! ,
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