COG Agressor 240
05-03-2004, 03:27 PM
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COG Agressor 240
Currently building the agrssor 240 from TopModel / Bichler.
http://www.topmodelcz.cz/letadla/agg...ggressor_a.htm
http://www.topmodel.fr/product_detai...e70d8a&id=4113
http://www.modellbau-bichler.de/date...67&uebertrag=1
The manufacturer gives a COG of 270mm from the leading edge. Root is 575mm and tip is 380 mm.
When the geometry if the wing is fed to a programm this results in a COG at 41%
To my feeling this distance of 41% can not be right. A value of 30%, 215mm from leading edge would me more close to my feeling.
The Middel of the wing-joiner-pipe is also at 30% / 215mm.
A Mail to the manufacturer also confirms this value of 270mm
Can this value be used as a starting point?
The wingsection is NACA 64A014. This section has the highest point relative far to the rear. Is this section known for the 'rear' placed COG's?
http://www.topmodelcz.cz/letadla/agg...ggressor_a.htm
http://www.topmodel.fr/product_detai...e70d8a&id=4113
http://www.modellbau-bichler.de/date...67&uebertrag=1
The manufacturer gives a COG of 270mm from the leading edge. Root is 575mm and tip is 380 mm.
When the geometry if the wing is fed to a programm this results in a COG at 41%
To my feeling this distance of 41% can not be right. A value of 30%, 215mm from leading edge would me more close to my feeling.
The Middel of the wing-joiner-pipe is also at 30% / 215mm.
A Mail to the manufacturer also confirms this value of 270mm
Can this value be used as a starting point?
The wingsection is NACA 64A014. This section has the highest point relative far to the rear. Is this section known for the 'rear' placed COG's?
05-03-2004, 05:46 PM
#2
RE: COG Agressor 240
Hans,
If this is a flying wing put the CG at 15% chord.
IIF IT IS NOT A FLYING WING YOU HAVE WASTED YOUR TIME GIVING INCOMPLETE AND USELESS INFORMATION.
To work out a CG position you need to know the planform of the whole plane.
I have worked out that the MAC (Mean Aerodynamic Chord) is 484 mm, and that is as far as I can get.
We need to know if the wing is swept back. Is the LE at the tip further back that the LE at the root? And, by the way, the root is on the centreline of the aeroplane, not where it meets the fuselage. The semi-span or panle span or panel length is from the centreline to the tip.
We need to know if the model has a tail (or a foreplane) and how big it is and how far it is behind (or in front of) the wing. measure from the quarter chord point of the wing mean chord to the quarter chord point of the tail mean chord. That's the tail arm.
Tail Volume Ratio (TVR or Vbar) is
(tail net area divided by wing gross area) times (tail arm divided by wing MAC).
Vbar is usually between 0.5 and 0.7 for a pattern model.
CG position = 15 + 40 times Vbar as a % of MAC on conventional models
If Vbar is 0.6 that puts the CG at 39%, so your 41% is possible. Work it out.
And don't mention the high point of the section (max thickness?) in this context as it is irrelevant.
Alasdair
If this is a flying wing put the CG at 15% chord.
IIF IT IS NOT A FLYING WING YOU HAVE WASTED YOUR TIME GIVING INCOMPLETE AND USELESS INFORMATION.
To work out a CG position you need to know the planform of the whole plane.
I have worked out that the MAC (Mean Aerodynamic Chord) is 484 mm, and that is as far as I can get.
We need to know if the wing is swept back. Is the LE at the tip further back that the LE at the root? And, by the way, the root is on the centreline of the aeroplane, not where it meets the fuselage. The semi-span or panle span or panel length is from the centreline to the tip.
We need to know if the model has a tail (or a foreplane) and how big it is and how far it is behind (or in front of) the wing. measure from the quarter chord point of the wing mean chord to the quarter chord point of the tail mean chord. That's the tail arm.
Tail Volume Ratio (TVR or Vbar) is
(tail net area divided by wing gross area) times (tail arm divided by wing MAC).
Vbar is usually between 0.5 and 0.7 for a pattern model.
CG position = 15 + 40 times Vbar as a % of MAC on conventional models
If Vbar is 0.6 that puts the CG at 39%, so your 41% is possible. Work it out.
And don't mention the high point of the section (max thickness?) in this context as it is irrelevant.
Alasdair
05-03-2004, 06:24 PM
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RE: COG Agressor 240
Alasdair,
I thought that the pictures of the model to be found in the links are helpfull.
The printscreen of the program make it very clear how the planform of the wing is.
Did not add half of the fuselage width of 12cm to the panel length however.
Anyhow thanks for your time and effort.
Regards,
Hans
I thought that the pictures of the model to be found in the links are helpfull.
The printscreen of the program make it very clear how the planform of the wing is.
Did not add half of the fuselage width of 12cm to the panel length however.
Anyhow thanks for your time and effort.
Regards,
Hans
05-03-2004, 06:30 PM
#4
Senior Member
RE: COG Agressor 240
Obviously NOT a flying wing 
The 41% c.g. does seem aftish...for that sort of plane, but not impossible.
I'd start at 30% myself.
http://www.topmodelcz.cz/letadla/agg...ggressor_a.htm
The 41% c.g. does seem aftish...for that sort of plane, but not impossible.
I'd start at 30% myself.
http://www.topmodelcz.cz/letadla/agg...ggressor_a.htm
05-04-2004, 03:39 AM
#5
RE: COG Agressor 240
Hans,
Sorry, I was in a rush, didn't check the links. I see it is a 2 m pattern plane.
As it is 2 m long also the tail arm is likely to be long so a big tail arm likely. Do the calculations because the answer could well put the CG at 41%. There is no point ballasting up, or even worse positioning the equipment, to achieve a CG at 30% if it it wrong!!!
Manufacturers are sometimes wrong, but give them a chance - check their figures with the formula.
Alasdair
Sorry, I was in a rush, didn't check the links. I see it is a 2 m pattern plane.
As it is 2 m long also the tail arm is likely to be long so a big tail arm likely. Do the calculations because the answer could well put the CG at 41%. There is no point ballasting up, or even worse positioning the equipment, to achieve a CG at 30% if it it wrong!!!
Manufacturers are sometimes wrong, but give them a chance - check their figures with the formula.
Alasdair
05-12-2004, 03:04 AM
#6
RE: COG Agressor 240
Hi
I just got a big box with an Aggressor in it =) thinking it´s a big and light plane 41% seem fairly ok as you need Cg way back to do flips and tumbles, specially when it´s light and lack the mass.
just remember to use lot´s of expo, I always setup a "chicken" triplerate 100%/75%/50% just incase... you never know.
I just got a big box with an Aggressor in it =) thinking it´s a big and light plane 41% seem fairly ok as you need Cg way back to do flips and tumbles, specially when it´s light and lack the mass.
just remember to use lot´s of expo, I always setup a "chicken" triplerate 100%/75%/50% just incase... you never know.
