Question About Take Off Trim Value.

[scandinavian13 5,192]

Not sure if I should be labeling this discussion as semantic or pedantic...

 

The trim values themselves are not specifically defined, at least from a crew perspective. Sure, you can assign an equivalent value to the trim in degrees, but it's referenced in "units," specifically because it has no other direct and easily referenced unit.

 

Kinda like Airbus...I'm sure you could figure out the actual flap values if you really wanted, but Airbus wanted to simplify things and refer to it as 1, 2, 3, and Full. What dimension is 1? It's dimensionless. Yes, you can find an equivalent number of degrees, but the 1 itself has no specific unit.

[kevinh 409]

Not sure if I should be labeling this discussion as semantic or pedantic...

 

The trim values themselves are not specifically defined, at least from a crew perspective. Sure, you can assign an equivalent value to the trim in degrees, but it's referenced in "units," specifically because it has no other direct and easily referenced unit.

 

Kinda like Airbus...I'm sure you could figure out the actual flap values if you really wanted, but Airbus wanted to simplify things and refer to it as 1, 2, 3, and Full. What dimension is 1? It's dimensionless. Yes, you can find an equivalent number of degrees, but the 1 itself has no specific unit.

I agree but let's not confuse units with dimension. At the risk of repeating myself, all angular units are dimensionless. Degrees, radians, etc are ratios. They have no dimension in terms of mass, length or time. A unit of trim is just another angular measure.

 

Same goes for flap settings. In a Boeing you set flaps in detent degrees. A number approximating to the actual flap angle, rounded to a more memorable figure and consistent across the product range with minor exceptions. Airbus chose to move to an even more arbitrary flap configuration number. But these numbers do equate to actual flap and slat angles in practice. The flight crew don't need to know what these are, but the relationship exists. Just like stab trim units relate linearly and directly to stabiliser angles.

[scandinavian13 5,192]

I agree but let's not confuse units with dimension. At the risk of repeating myself, all angular units are dimensionless. Degrees, radians, etc are ratios. They have no dimension in terms of mass, length or time. A unit of trim is just another angular measure.

 

Same goes for flap settings. In a Boeing you set flaps in detent degrees. A number approximating to the actual flap angle, rounded to a more memorable figure and consistent across the product range with minor exceptions. Airbus chose to move to an even more arbitrary flap configuration number. But these numbers do equate to actual flap and slat angles in practice. The flight crew don't need to know what these are, but the relationship exists. Just like stab trim units relate linearly and directly to stabiliser angles.

 

I can agree with that. Makes sense.

[vololiberista 205]

Hi guys.

 

I understand the concept of setting the Take Off trim and why we do it. I've watched Kyles video on his You Tube page about Trim settings too.

 

What I don't really understand what trim value really is. For example the FMC may give me a Trim Value of say 5.25 but what is 5.25? I've heard pilots on videos call it Units of Trim. Is it an angle setting of a the stabalizer in degrees like flap settings? If it's measured in units of Trim how do we arrive at the figure 5.25?

 

Hope my question kind off makes sense, thanks for any help.

Take it to be degrees. Refer to my earlier post with the link to the graph for the VC10.  That's also in degrees. Less than that for the Boeing because the 10's tailplane is enormous plus the CoG is further back because of the rear engines. That said the calculation method is the same.

Again for the 10 this is the basic weight and balance to find %MAC

                                      Kgs                        Moment           Weight lbs        Arm                 

                                     1,648 Fin              214,925.57      3,625.60          59.28

max fuel       66,336    64,688 Fuel            448,287.84    142,313.60           3.15

                                  71,214 Empty       1,193,374.27    157,000.00           7.60111     32.14

                                    8,403 Station:0    - 572,423.49     18,525.70       - 30.89889

Zero Fuel     85,152     5,535 Station:1      499,079.06     12,202.75         40.89889

max fuel       94,800    13,938                 1,783,243.25    333,667.65       5.3443696   22.598     -1.052       3.89              

                                                                                                                                    %MAC =  +/-inches    = TPI                

                                                                                                                                                  from datum    

 

The basic weight is 23.65%  of %Mean Aerodynamic Chord and all calculations are based on that. 32.14% is the empty weight %MAC which includes equipments etc. The final calculation of 22.598% is the total arm divided by basic weight MAC.

-1.052 inches is 22.598-23.65. Therefore 3.89 degrees of tail plane incidence is

=ROUND((-1.052*-0.1176)+3.7647,2)

The numbers -0.1176 and +3.7647 represent the equation to find the slope line

Equation for the graph slope line is y=-0.1176x + 3.7647. Above x=15, y equals 2.  Below x=-27.5, y equals 7

The whole point of setting TPI is so that the PF will always have the same stick force at rotation Vr regardless of aircraft weight. In the above calculation there are only 2 station loads. This is because however many station loads the model may actually have, the sim takes an average so two will do just as well.

I'm sure that PMDG did the correct CoG for their model. Quite often as in the case of David Maltby's VC10 the developer will use the centre of the model as the datum. Because in practice the sim doesn't model CoG very well.

In the VC10's case the CoG was actually 15 feet more forward than it should be. Correcting it gave me a 3 knot bonus at Vat which is very close indeed to the real numbers.

Therefore the only material difference between the calculations for the VC10 and Boeing is the angle of the graph slope line to determine the TPI angle.

Hope you find this useful.

[kevinh 409]

Take it to be degrees.

No, you cannot take it to be degrees. The trim setting in units will be equivalent to the angle of the stabiliser, but you can't simply say trim units are degrees. Waffling on about another developer's CG model isn't helpful here either.

 

And why do you never sign your posts?

[vololiberista 205]

No, you cannot take it to be degrees. The trim setting in units will be equivalent to the angle of the stabiliser, but you can't simply say trim units are degrees. Waffling on about another developer's CG model isn't helpful here either.

 

And why do you never sign your posts?

Angles are measured in degrees period

[kevinh 409]

Angles are measured in degrees period

Angles are also measured in radians. Trim units are a measure of an angle too. But importantly trim units are not always equal to degrees of trim. In the case of a Boeing, for example, zero units is not zero degrees incidence. There is a relationship of course. But the OP asked what trim units are and the answer is not degrees.

 

Still no name I see.