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CG before the wings


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22 replies to this topic

#1 Luis Miguel Hernández

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Posted 10 February 2011 - 09:13 AM

After suggesting it to someone else, I've just reinstalled the ATR 42-500 by Claus Vendelboe Holmberg (well, the refinement he did of Francisco Sánchez-Castañer's work, atr425dk.zip in the "other site"). I'm liking a lot the final product, however I've seen some unstability in the pitch axis (always wanting to nose down). I checked the aircraft.cfg file and saw these lines (I'm showing only the relevant ones):
[WEIGHT_AND_BALANCE]reference_datum_position=000, 0, 6.800empty_weight_CG_position=000, 0.000, -8.000station_load.0=0.000000,0.000000,0.000000,0.000000empty_weight=32832.000[fuel]LeftMain=0.000, -14.500, -5.000, 758.000, 0.000RightMain=0.000, 14.500, -5.000, 758.000, 0.000[airplane_geometry]wing_root_chord=8.830
That's right, only one empty payload station, centered at the CG. The aircraft and its fuel tanks are also centered (in the fwd-aft axis) at CoG. The CG should be then at 25% MAC, right? Wrong: FS' payload & fuel editor shows a CG of -18% MAC (yes, negative, CoG in front of the wings), which explains the pitch-down tendency. I edited the Weight & Balance section so that there could be a distribution of the payload (i.e. not everything centered at CG). These are the lines I changed:
[WEIGHT_AND_BALANCE]reference_datum_position=000, 0, 6.800empty_weight_CG_position=000, 0.000, -8.000station_load.0=340.000000,26.000000,0.000000,0.000000station_load.1=4560.000000,-10.000000,0.000000,0.000000station_load.2=3420.000000,5.000000,0.000000,0.000000station_load.3=1400.000000,20.000000,0.000000,0.000000empty_weight=24802.000
I added 3 payload stations: one for flight crew, one for the forward and aft cabin each, and another one for the cargo. Doing my own math, the CG should be at 23% MAC. However, the CG seems unaffected by these changes, CG is still at -18%. How could I fix this balance problem?Best regards from Colombia,Luis Miguel

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#2 Cessnaflyer

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Posted 11 February 2011 - 01:52 PM

Unless FSX does physics differently the center of gravity is supposed to be in front of the aerodynamic center of lift.

#3 WarpD

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Posted 11 February 2011 - 03:59 PM

[WEIGHT_AND_BALANCE]reference_datum_position=000, 0, 6.800empty_weight_CG_position=000, 0.000, -8.000station_load.0=0.000000,0.000000,0.000000,0.000000empty_weight=32832.000[fuel]LeftMain=0.000, -14.500, -5.000, 758.000, 0.000RightMain=0.000, 14.500, -5.000, 758.000, 0.000[airplane_geometry]wing_root_chord=8.830

Ok... reference_datum_position is being set 6.8 feet below the aircraft center and empty_weight_CG_position is being set 8 feet above the reference_datum_position which would be 1.2 feet above the aircraft center. I don't think this is correct and would wonder how much this would mess up the aircraft's CG calculations.

#4 mgh

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Posted 11 February 2011 - 05:30 PM

The cg is where it is based on the disposition of the weights. It has no direct elationship to the MAC of the wing.For a trimmed aircraft the centre of lift is at the cg by definition.The positions of your stations loads looks strange. .0 is 26 ft ahead of the reference_datum: .1 is 10 ft aft of the reference datum. My calculations suggest that for the empty weight + fuel the cg is 0.618 ft aft of the reference datum: that with station loads .0 - .3 added it is is 0.312 ft aft of ther reference datum.

#5 Luis Miguel Hernández

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Posted 12 February 2011 - 09:38 PM

Problem solved, you all gave me a clue: there is nothing in the lines I posted that says that the empty weight CG position should be at 25% MAC. This is controlled by the wing_pos_apex_lon line:
[airplane_geometry]wing_root_chord=8.830wing_pos_apex_lon=-1.500
I understand this as where is CG respect to the leading edge of the wings. Therefore, this line would tell me that the CG is 1.5 ft forward of the wings. Now I'll change this value:
[airplane_geometry]wing_root_chord=8.830wing_pos_apex_lon=2.2075
Now my CG would be 2.2075 ft after the leading edge. I checked in FS' payload & fuel editor, now the CG is around 25% MAC. Now, some math just for verifying:OEW CG % = wing_pos_apex_lon / wing_root_chordFinal case: 2.2075/8.830 = 0.25 = 25%Initial case: -1.500/8.830 = -0.17 = -17%Gracias, muchísimas gracias por todo :( My ATR is now more balanced and easier to fly.Best regards from Colombia,Luis Miguel

#6 mgh

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Posted 13 February 2011 - 08:13 AM

There is a misunderstanding. The position of the MAC is determined by the aircraft geometry. the position of the cg is determined by the aircraft's weights (empty, load, and fuel) and their position. The position of the MAC is fixed: the [position of the cg varies with total weight.For example, the cg positions with respect to the reference datum for a couple of default FSX aircraft are: C172 SPempty cg -3.00 ft (12.4% MAC)loaded c -3.12 ft (15.0% of MAC) B747-400empty cg -90.50 ft (6.6% of MAC)loaded cg -96.00 ft (24.7% of MAC)

#7 Cessnaflyer

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Posted 16 February 2011 - 10:19 AM

For a trimmed aircraft the centre of lift is at the cg by definition.

Wrong. This is only if the CG was so far back that the tail would not be providing its usually downforce.

#8 mgh

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Posted 16 February 2011 - 01:22 PM

Wrong. This is only if the CG was so far back that the tail would not be providing its usually downforce.

The centre of lift is at the cg for a trimmed aircraft. If it were not there would be a pitching moment equal to lift multiplied by the distance between the centre of lift and the cg. The aircraft would not then be trimmed.

#9 Cessnaflyer

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Posted 17 February 2011 - 10:35 PM

The centre of lift is at the cg for a trimmed aircraft. If it were not there would be a pitching moment equal to lift multiplied by the distance between the centre of lift and the cg. The aircraft would not then be trimmed.

Again no. The tail produces downforce and offsets this difference in location between the center of lift and center of gravity. Also you have to remember as well as angle of attack increases then the center of lift moves forward. If the center of gravity is too far aft then they will switch places and the tail would not be able to produce enough lift to push the nose down and the aircraft would enter an unrecoverable stall. I attached a picture that also shows where the center of gravity and center of lift are in relation to each other.

#10 fxsttcb

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Posted 18 February 2011 - 07:43 AM

Gerry is quite correct! If the sum of the forces(Center of Lift) is applied anywhere except the center of Mass(CG) an imbalance will exist and the aircraft will RPY opposite the imbalance.For a stable aircraft, in level flight, the C of L and CG must coincide otherwise a seeking moment will occur.I have been trying to prove Sir Isaac wrong for over 40 years and have yet to succeed...Don

#11 mgh

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Posted 18 February 2011 - 09:44 AM

I don't know where you got your picture from, but it is wrong and completely misleading.(1) it shows the gross weight acting at the same position - an undefined "Centre of Lift" - regardless of the position of the centre of gravity(2) it show the oad imposed by the tail acting at the same point(3) it takes no account of pitching momentsThe following picture shows the real situation. For a trimmed aircraft the resulting pitching moment must be zero so the pitching moments from the wing and tailplane must be equal and opposite and cancel each other out. The net result is the the Centre of Lift must act through the Centre of Gravity for a trimmed aircraft. If it didn't there would be a resulting pitching moment and the aircraft would not be trimmed.[attachment=25857:Slide1.JPG]

#12 Cessnaflyer

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Posted 18 February 2011 - 08:13 PM

The picture is from the FAA's Pilot's Handbook of Aeronautical Knowledge. What you guys are attributing to CoL and CoG being in the same place is because of the artificial weight from the tail is included in that equation. If you notice in your own picture weight and lift are in two separate locations like I said but the aerodynamic forces make the CoL and CoG coincide to make the aircraft fly straight and level. I know that but what I am saying is that the actual CoG of only aircraft weight should always be in front of the CoL.

#13 WarpD

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Posted 18 February 2011 - 08:32 PM

Lift, Drag, Thrust and Weight are vectored values and thus do not require to be exactly opposite each other. Might want to think that over a bit.

#14 Cessnaflyer

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Posted 18 February 2011 - 09:48 PM

Lift, Drag, Thrust and Weight are vectored values and thus do not require to be exactly opposite each other. Might want to think that over a bit.

Thank you. You said it much better than I.

#15 dmaher

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Posted 18 February 2011 - 10:32 PM

Thank you. You said it much better than I.

I think you’re largely saying the same thing - but using different reference systems.