LEMAC

[iGoApp]

Hi. I have found that Saab's Mean Aerodynamic Chord is 2.08 m or 81.89 inches. Does anyone know what LEMAC is?

Also, if you happen to know %MAC at BOW this would be very helpful too!

Many thanks!

Edited April 25, 2014 by igorland

[Goran_M]

LEMAC - Station 412.3 in.

 

%MAC is in the AOM somewhere, but that's about 4000 pages.  You'll understand if I don't particularly want to dig through that.  

 

Just out of curiosity, what are you going to use this information for??  

[iGoApp]

Goran. Thank you very much. I tried to get %MAC by going to the plane in PlaneMaker and looking at the CG value there. However, the value there is -4.42 ft (i.e., -53.04 in). It is not clear to me what this value is and why it is negative. Guys, can you please explain it to me? I expected to see the CG as a distance from datum (say, the nose of the airplane) and calculating %MAC at BOW as:

 

(CG in X-Plane - LEMAC) / MAC * 100.

 

Thanks! Your help is very much appreciated!

Edited April 26, 2014 by igorland

[Goran_M]

Check your inbox.

[tkyler]

 It is not clear to me what this value is and why it is negative. Guys, can you please explain it to me? I expected to see the CG as a distance from datum (say, the nose of the airplane)

 

For other's interested, when creating an aircraft in plane-maker, an aircraft author can make any point they choose a datum (0, 0, 0, point).  Many developers make the Cg of the airplane the datum which means that points aft will be positive values and points forward will be negative values as that is x-plane's aircraft coordinate systems (+z towards the rear)   One reason it is convenient to make the Cg the datum is that when custom programming, x-plane uses the Cg of the aircraft to represent the aircraft' position in space.   It is mostly a personal preference, I myself use the nose (or thereabouts) as the datum as I typically try and use the same data found in maintenance documentation but any point can be a datum, even at the tip of the tail.

 

TomK

Edited April 27, 2014 by tkyler

[iGoApp]

Goran and Tom, thanks! With Goran's help, I have figured out what I was looking for.

 

Best,

[Vantskruv]

If I understand it correctly:

 

As I was in the planemaker, I saw that the endpoints of the fuselage was the same (though opposite sides), so therefore I guess the aircraft reference point is in the middle.

So total length of fuselage is 776.88 inches.

Default CG point -53.04 from center point.

We know also LEMAC Goran_M told us here: 412.3

We also know the MAC from iGoApp: 81.89

I have also found the displacement of fueltanks in Planemaker: -61.2

 

We get also help from this document telling us how to calculate the weight distribution in the aircraft:

http://www.seaborneairlines.com/portal/wp-content/uploads/2014/08/WBH-Weight-and-Balance-Handbook-Saab-340-@-R2.pdf

 

We get also help from this SEEGEE plate to know which trim settings we should have from table on the middle left hand side:

http://209.59.151.19/~seegeesv/wp-content/uploads/2014/03/OS2A-SAAB-340A-v8-v12.jpg

 

Though I don't know how to handle this plate, I've been trying to understand how to utilize it practically, and I've not found any videos or pictures on how it should be done.

 

Anyway with all the data we can calculate the %MAC of the aircraft, and then we can do the recommended trim for takeoff.

 

Note this is from understanding, and I may been getting everything wrong.

 

So, hoping I've been lucky, these are my calculations:

 

CG (using this as datum): 433.04 (776.88/2 - 50.34 + 98)

LEMAC: 412.3

MAC: 81.89

Fuel tanks STA: 425.4 (776.88/2 - 61.2 + 98)

 

Calculating the negative weight distribution N from datum (CG) where W is weight at current row:

Row 1: 126.1*W

Row 2: 96.1*W

Row 3: 66.1*W

Row 4: 36.1*W

Row 5: 6.1*W

 

Calculating the positive weight distribution P from datum (CG) where W is weight at current row:

Row 6: 25.9*W

Row 7: 55.9*W

Row 8: 85.9*W

Row 9: 115.9*W

Row 10: 145.9*W

Row 11: 175.9*W

C1: 221.6*W

C2: 276.6*W

 

To get the weight offset O in inches:

O = (P - N) / ZFW

 

We can then use the value of O to set the CG in X-Plane weight and balance manager.

Note that fuel is not counted in, as of what I know X-Plane automagically calculates the CG point depending on the fuel in the aircraft.

 

For calculation of the %MAC with fuel, we need to have fuel weight incorporated in the calculations.

 

So just add this to the sum of the above negative weight distribution N calculations:

Fuel weight: 8*W

And to get the weight offset in inches:

FO = (P - N) / TOW

 

To calculate the %MAC:

 

%MAC = ( (FO + 433.04) - LEMAC) / MAC

 

 

Correct me if I'm wrong!

Edited January 8, 2016 by Vantskruv

[birdy.dma]

In fact, the fuel and payload thing of X Plane is not very sexy.

 

This bird deserve its own feature.

 

Claude.

[iGoApp]

Vantskruv. Let me explain how iGoDispatch deals with it:

We need to calculate %MAC with certain weight. Instead of determining arms and weights and adding all the moments of mass together, we will use Mass Index.

1) We know (from the actual Mass and Balance docs) that the DOI (Index at DOW, i.e. 17,132) is 26. Now, let us say we have 3 males in Zone A and 3 males in Zone B (i.e. 6 people with weight 200 lb each in the Forward Zones), and 7 males in the Aft Zones, C and D (totalling 1400 lb). Forward index for 6 people is -8 and Aft Index for 7 people is +13. Our current index is 26-8+13 = 31. Let us assume, we have 364 lb in the first cargo compartment (index +8) and 135 lb in the second compartment (index +4). Our total index now is 43. The Weight and Balance Chart shows that at index of 43 and weight of 20,231 lb, %MAC is 27.3. Let us add 3,016 lb of fuel. This is index +1. Our current %MAC is: Index 44 at 23,237 lb (at the moment the calculated fuel consumption is 10 lb before the take-off needed for engine start-up; therefore we extracted 10 lb) = 28.44113%.

2) Our %MAC at DOW is 14.49. We can calculate the distance from LEMAC to the DOW point: (MAC * MACDOW / 100) = 82.07 * 0.1449 = 11.89 in.

The distance from LEMAC to the new loaded CG point is 82.07 * 0.2844113 = 23.34 in.

And the CG offset is 23.34 - 11.89 = 11.44 inches aft.

Unfortunately, I cannot share the actual Weight and Balance Chart that I use in the application.

Hope, it made sense.

Edited January 9, 2016 by iGoApp