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FS2004 Help with the Maths

vololiberista

vololiberista

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italy
I am trying to interpret %MAC into the required take off setting for Tail Plane Incidence.
mac.jpg

So some questions!! This first image is from AirWrench. Does fuelled CoG represent empty weight+full fuel? Or fully loaded+full fuel?
Therefore is Loaded CoG max payload without fuel?
On the basis that they do I did some calculations which seemed to fit but I can't be certain.
mac2.jpg


The next problem is to convert the difference between the calculated %mac and empty %mac. In this case 5.9 inches. So now I have to calculate the required TPI.
WB_TPI_settings.jpg


You can see here that 0 representing 28.56%mac is 3.7 degrees incidence. Whereas 5.9 inches represents 4.5 degrees incidence. I'm assuming that is correct or should it be 3.1 degrees? When flying the model anything above 3.8 causes a snatched rotation. So I'm guessing that the settings in the sim may be different to real life.
Any ideas welcome.
Ps The calculations that I come to provide a similar control column load for rotation at all weights.
 
Volo
There is an explanation of the CG on the Weight page in AirWrench. Empty is obvious, Loaded is without fuel and Fuelled is loaded and fuelled.
I gather that the VC10 has an elevator and a tailplane that is moved in incidence for trim purposes, same as the Hunter.
The best explanation of trim forces in the simulator is that on page 17 and 18 of Yves Guillame's "Flight Dynamics in MS Flight Simulator".
He basically describes the simulator trim concept as being the same as your VC10 has, elevator and trim from the tailplane. No elevator trim tab.

However, horizontal stabilizer incidence and trim deflection are not the same in the sim. Horizontal stabilizer incidence is a fixed value in the aircraft.cfg file.
Trim setting is Elevator Trim Position, degrees and the limits are set in the aircraft.cfg as degrees total. The cfg file also has a trim effectiveness scalar
The air file has two entries for trim. CM0 which is the pitch moment at AOA=0 or nominal zero trim/cruise setting. Adjusting CM0 should give you zero trim at cruise speeds.
The other setting is CM_dt =pitch moment per radian of trim deflection. This basically adjusts the effectiveness of trim.
In reality a degree of horizontal stab deflection would have much more effect than a degree of elevator trim deflection,
so you need to make CM_dt a bit stronger than you might at first think so that your gauged elevator trim deflection matches actual tailplane deflections.

Now the difficult bit is getting zero trim at the speeds you want and that involves the overall airplane pitching moments dealt with in Record 473/1546 Cm versus AOA

There really are no trim datum or trim physical limits in the sim.

As an aside, I usually try to get a zero trim setting for take-off and sufficient nose-up trim that the autopilot will have enough authority to fly level down to the beginning of a dirty stall.

Roy
 
Many thanks Roy,
A lot to chew over! As for you last point if the TPI on the 10 is zero during the take-off run then there is no take-off! So that's correct in both worlds as it were!
Currently CM_dt is -4.5. The TPI is usually flat-ish during level flight. Perhaps 1/2 to 1 degree depending on airspeed etc. So in flight it behaves correctly it seems. It's just trying to match the take-off setting that's the problem.
 
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Incidence is best set to zero according to the SDK since it's already factored into the lift and drag.
 
Incidence is best set to zero according to the SDK since it's already factored into the lift and drag.
Click to expand...

DG, the incidence you refer to is wing incidence. Normally this would be a factor in calculating Angle of Attack (AOA), but in the sim (and I think it was the same in FS2004) AOA is referenced off the fuselage axis and so the wing incidence is actually ignored.
The incidence we have been talking about in this thread is horizontal stabilizer (tailplane to us Brits) incidence. If this is non-zero it has an effect on trim through Cm_dh pitch moment due to H stab. However if the incidence is zero then Cm_dh does nothing.
Both factors are a bit artificial but many people set the H stab to zero because it simplifies the trim moment calculations.

Roy
 
CG are always measure in EW, ZFW, and GW.
That's Empty CG (no fuel or cargo), Zero Fuel CG (EW+CARGO), Gross CG (EW+CARGO+FUEL)

Yes, the sim can match the real airplane if the input info is correct...

That being said, there are a few very important things here.

- Aerodynamic Center of Lift must be correct
- Airplane geometry must be correct
- CM vs AOA must be correct

special emphasis:
- horizontal stab incidence must be correct

I recommend setting CL_dh and CM_dh to zero.
I recommend setting CMo to zero.


If the above are set correctly you can find the real airplane's trim load but first you need a few variables...

Plot all of the following from a real airplane:

CG, trim setting, weight, and the standard items you should always log (mach, true/IAS speed, pressure altitude, sea level altimeter, total air temperature)

Then find these variables:
xCG = Distance from CG to AC (aerodynamic center)
xCGt = Distance from CG to Horizontal stab AC

Delta = sea level pressure / ISA pressure
Q = 1481.4 * Mach^2 * Delta
S = Wing area, feet^2
c = wing MAC, feet


Take gross weight * xCG = Moment force from wing

moment force from wing / xCGt = Moment force from tail (for level flight)

Moment force from tail / ( Q * S * c ) = CM_dt Coefficient at trim setting x

Map as many trim settings as you can. Try to map a narrow airspeed and mach range.


Now, the complicated part...separating the stab load at zero trim from the trim load.

MSFS considers trim an addition or subtraction to H.tail incidence (ih_stab). This is only a factor for airplanes with elevator trim tabs and a stationary horizontal stab. For airplanes with moving stabs you can skip this step.

You can take the zero trim CG range (map CG to zero trim) and use that CG to find the trim moment using the same steps above.

The Zero Trim condition can be added via CMo for elevator trim tab airplanes.


Notes:

CL_dh only affects the Net Lift of the airplane (thus AoA, thus CM vs AoA). I don't use it, because your Lift table SHOULD be Net Lift (Body + Wing - AoA related Losses).

Pitch Trim vs Dynamic Pressure or vs AoA are important here. Most modern systems have artificial limiters for structural reasons. Most older systems are aerodynamically limited. The curve is different for each airplane and will cloud your results.

Remember, the trim position the sim uses is in addition to the ih_stab moment. If the stab has incidence it automatically produces a moment (using CM_dt). Five degrees of incidence will produce five degrees of trim. The cockpit indication will usually be zero trim at the incidence angle.

Lastly, all these tests must be done clean! Flaps change the trim loads significantly and should not be worked on until the clean trim is correct.
 
Your right i was talking about the main wing. I usually put zero for the Htail if the horizontal stabilizer sits flat. But adding a few degrees should improve pitch!
 
In flight the tail plane behaves absolutely correctly with always the correct trim for the various phases of flight. Even for approach and landing keeping the aircraft nose fairly level. Both with the AP and manually. Using the correct n2 setting on finals the aircraft behaves exactly as published even down to a tendency to float if you over flare. So I don't feel I need to change any numbers in the airfile.
The only thing I need to establish is the TPI take-off setting which as per the graph above is based on inches fore and aft of datum.
If no incidence is set then the aircraft won't take-off (which is also true in real life). Generally setting a range of 2.8 to 3.5 will get the a/c off the ground depending on weight and fuel. But these numbers are not the actual numbers. The curve as it were is a bit steeper with 3.8 being typical for a full load with full fuel.
The graph illustrates the problem with 1.Fuelled 2. Loaded 3. Empty
real.jpg

So I need to find the inbetween numbers. Itàs a bit of a headache for a mortal!
 
if you cant use the real numbers, your input is wrong. MSFS is perfectly capable of matching the real thing if input data is correct. Up to you though if you want to put in the extra work (headache) to get it just right.

If clean performance is good, then look into the Flap moments.
 
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