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Old Nov 23, 2009, 12:20 PM
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Dallas, TX
Joined Dec 2007
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CG location and effects on speed?

Yesterday was a great time to go flying, so I spent a great deal of time with the Flash2. I took the opportunity to play around with the CG location as I usually prefer to put it as rearward as possible for increased pitch sensitivity (in this case 73mm on the Flash2).

Anyways, I was wondering if moving the CG actually affects the speed if all other factors remain the same (decalage, trim, dead wind, full charge LiPo)? I understand that shifting CG location affects the load on the tail, however for some reason, the farther back I moved the CG the slower "power off" speed I noticed - or was I just imagining things?
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Old Nov 23, 2009, 01:22 PM
The props of tomorrow
Varna , Bulgaria
Joined Feb 2007
962 Posts
On my Enigma I now fly at 78-80mm CG or 44%.
The plane now retain the speed on turns , flying faster and the thermaling capability are increased.Before I was hardly flying 10 mins on 3 climbs. Now I can make then with maximum 2 climbs , and when there is thermal with only one.

The CG location is very specific on different angles of attack on the wing and stab.
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Old Nov 23, 2009, 02:01 PM
Needs to do 52 legs !!
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Verenigd Koninkrijk, Fareham
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If you don't change the trim or stab incidence then just moving the CofG back will result in the plane being stable at a slower speed. You need to put a couple of clicks of down trim in or change the stab incidence to get the speed back up.

Conversely, move the CofG back to the original position but put in a few clicks of up trim and the plane will now be "trimmed" at a slower speed.

Just imagine if you where to move the CofG all the way to the tail, it would point the nose up vertical!

In terms of getting the most out of your plane (minimum rate of sink for any given speed).

I don't think the drag produced by the stab from a forward CofG is that big in a strait line. It is a slightly different story in the turns. But much more important is that the plane fly's like it is on rails. There should be no unwanted attitude changes as this will only lead to a less the perfect flight patch and extra drag from all the corrective inputs.
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Old Nov 23, 2009, 05:19 PM
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Dallas, TX
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Thanks for the feedback. I'll be trying different CG locations and see how things look.

It would be nice to see what others have to say about this too as it is something I've not "played" with too much with since flying hotliners.
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Old Nov 23, 2009, 08:17 PM
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Winnipeg, Canada
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As you move CG back decalage has to decrease otherwise the plane will want to ride nose high (wanting to climb as throttle increases). Some thing else I do is to trim the plane for level flight at 1/2 throttle. Now with this trim state as you apply more throttle the plane will want to climb. From here, 50% throttle and up, I mix in a slight amount of down trim (master throttle - slave elevator) so that at full throttle it flies straight as an arrow and goes exactly where you point it. You can have the mix is turned on by a switch if you want. Really depends on the type of plane you are flying.
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Old Nov 24, 2009, 07:13 AM
Needs to do 52 legs !!
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Quote:
Originally Posted by Batmanwpg View Post
Really depends on the type of plane you are flying.
I would say that again! You should never fly with 50% throttle on a hotliner or f5b.

The plane should be trimmed out without power.

Then if it does something funny with power on then the thrust angle might be off. Need to shim the motor.
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Old Nov 24, 2009, 07:25 AM
Boar? I'm a Thermal Pig!
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Yes, my comments are directed more to a pure pylon racer. It was more just to give an idea of what can be done.
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Old Jan 20, 2015, 10:50 AM
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Quote:
Originally Posted by Batmanwpg View Post
Yes, my comments are directed more to a pure pylon racer. It was more just to give an idea of what can be done.
Hello All,

So what is the CG on a Enigma?
Thank You,
Dave
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Old Jan 20, 2015, 03:53 PM
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Simple answer.... No.

The effect noticed is not root caused by the different cg but the not matching angles for wing and stab.

You have a highly dynamic system between planes neutral point, cg and angles to produce the lift you need for the specific flight situation and the stability. There is no chance to get the "one solution" - it is depending on own favourites or flying styles, it is always a compromise. The one likes flying on the edge, the other one needs a freeflight model.
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Old Jan 20, 2015, 04:45 PM
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New Zealand, Tasman, Richmond
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As I understand it the airplane can fly faster (at full throttle) with an aft C of G location because of reduced drag. There is less drag because a smaller angle of attack and less downward deflection of the H. stab are required to support the plane and overcome nose-down pitching tendency.
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Old Jan 20, 2015, 10:43 PM
Carbon fiber is our friend
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United States, CA, Lodi
Joined Feb 2001
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That should be true, but in practice it's usually a little faster to have the plane a little nose heavy because it will be stable and you won't need much control input to keep it straight.

The best I ever did in slope racing was with a plane that was super stable and I barely had to fly it. It just showed me that all I do with the sticks is mess things up!
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