Aircraft Pitch-Angle Question

[HighGround22]

.
I'm having a bit of trouble with a particular twin piston-pounder, and I can't seem to find the solution on my own.

Y'know how an aircraft normally pitches forward as you increase its speed? So the faster you go, the more nose-down the aircraft flies?

Well that's all well and good, and as it should be, but this particular aircraft seems to have way too much of it, and I can't seem to find the parameter -- aircraft.cfg or .air file -- that controls it.

My pea brain's logic tells me it should be something to do with the angle of incidence, but I can find no such item in the control files. So I'm flummoxed on this.

Does anybody have a decent idea a) what the heck I'm rambling-on about; and b) where to go to adjust it? (be kind)

Cheers, all.

 

[JohnC]

What you're looking for is in the .air file.

Your Cmo is first determined in the Cm vs AoA table in section 473
This coefficient can be offset in section 1101, top of the pitch section.
Mach based adjustments (in 0.2 increments) can be made in section 433

As a side note, the angle of incidence in the Aircraft.cfg is nothing more than a reference number, it has no effect in the simulation.

 

[Ken Stallings]

I cannot help you with finding how/where to tweak it in an airfile.

However, I can provide a decent answer about how it works on real aircraft.

At a given weight and no change in the wing (flaps deployed, retracted, ice accumulation, etc ...), as airspeed increases angle of attack for level flight goes down. Except for instantaneous changes, normally angle of attack equals aircraft pitch. But I'm sure you have seen video of aircraft pulling out of a dive, with the nose pointed up, but the plane still going down -- well that's a graphic demonstration that sometimes pitch and angle of attack is not the same.

In terms of whether the aircraft's angle of attack is changing too radically for changes of speed, to be honest I would have to experience that virtual aircraft and compare it to the real thing to know for certain. Some aircraft do have significant changes in AoA for changes in airspeed. So, it just depends.

The essential difference between pitch and angle of attack is that pitch is the angle of the aircraft relative the horizon. Angle of attack is the angle of the wing's chord line to the air mass the aircraft is moving through. The chord is a straight line drawn from the tip of the wing's leading edge to the wing's trailing edge.

Cheers,

Ken

 

[JohnC]

Ken is correct on most accounts. Just one slight correction. The definition for AoA is accurate for a local lift body, i.e. the main wing. However, FSX is concerned with the aircraft's angle of attack. The aircraft's Angle of Attack is the angle between the vector component of the ambient air velocity which lies along the plane of symmetry (X-Z) and an arbitrary axis along the X-Z plane. The fuselage reference line is commonly used for this arbitrary axis, however, in FSX it is the x-axis in the body fixed coordinate system. This is the same axis as is used for positioning longitudinal contact points, lights, etc.

Beyond the changes in AoA as velocity is increased, the efficiency of a lifting body changes with both Reynolds number and Mach number. Since there are typically three major lifting bodies on an aircraft (Wing, Horizontal Stab, and Fuselage), the resulting change in moment coefficient can be quite drastic over a given flight envelope. Calculating this changes can be exceedingly complex, even when the fuselage component is ignored. Best to use either an empirical data source (such as a flight manual, or ideally a pilot) or go with what feels "right" in the subjective sense.

 

[jmig]

Sounds to me like Ken is giving the "real world" aviation definitions and John the MSFS definitions. John illustrates, provided you can understand engineering speak , some of the differences between MSFS concept of flight and the real world physics.

 

[fliger747]

Unfortunatly FS computes the action of the stab and elevator not by applying the up/down forces acting on the appropriate arm wiht appropriate moments using the actual airflow, but by changing the pitch moments on the wing directly. The major effect thta this has is that speed stability is derived by appplying independent fudge factors. In a real plane stability is mostly controlled by having a downforce applied by the stabilizer ballancing an downward pitching moment on the wing caused by the COG being ahead of the center of lift. As the plane speeds up the tail downforce increases, causing a pitch up, slowing the plane, this makes it self correcting speed wise and with a postive controll feel. With an aft CG an upforce on the tail is necessary to keep the plane level. A very unstable configuration, for instance slowing down will cause a pitch up, causing a further up pitch tendency. Only a fly by wire can handle this very well.

FS does not react very directly to CG position stability wise. However generally a CG of about 25% MAC should work well. Check that for starters, also the wing and tail areas. I reccoment AFSD utility by Herve Sors as a way of checking out all of the control and trim positions as well as other aerodynamic properties that could be responsible. Start with the simple stuff and then start plowing about in the air file....

T

 

[Brett_Henderson]

Back to the original question.. a piston twin that has a a lot of negative pitch when power is increased..

Might that be the Goose ? I've never flown a real Goose, so I can't say for sure.. but considering how far above the CoG that the thrust is.. it "feels" realistic to me (as does the positive pitch when power is reduced).

In the aircraft.cfg file:
-------------------------------------------------------------------------------------
reference_datum_position = 2.991, 0, 0 // (feet) distance from FlightSim Reference position: (1/4 chord, centerline, waterline)
empty_weight_CG_position = -3.2831, 0, - 2.255 // (feet) longitudinal, lateral, vertical distance from specified datum



[GeneralEngineData]
engine_type = 0 //0=Piston, 1=Jet, 2=None, 3=Helo-Turbine, 4=Rocket, 5=Turboprop
Engine.0 = 3.589, -5.837, 4.41 //(feet) longitudinal, lateral, vertical distance from reference datum
Engine.1 = 3.589, 5.837, 4.41 //(feet) longitudinal, lateral, vertical distance from reference datum
fuel_flow_scalar= 1.07
min_throttle_limit = -0.25; //Minimum percent throttle. Generally negative for turbine reverser

--------------------------------------------------------------------------------------

The highlighted numbers are the vertical coordinates. I did not make the model, so I cannot confirm the model origin.. but you can see that the reference-datum, and model origin are the same (vertically).. The CoG is 2.255 below the reference.. the engines are 4.41 feet above the reference... meaning that the thrust-line is 6.66 feet ABOVE the CoG ! ... This will definately cause pitch changes when the power is changed.

 

[JohnC]

Y'know how an aircraft normally pitches forward as you increase its speed? So the faster you go, the more nose-down the aircraft flies?

The original question stated change in pitch with change in speed, not applied thrust. So HighGround can correct me if I am wrong, but I believe we are on topic.

 

[Brett_Henderson]

I didn't mean we were off topic.. and you're right.. his wording was about airspeed.. I read my interpretation into it, just in case that's what he meant

I'm probably way off base..

 

[JohnC]

I wouldn't say way off base, it's very plausible that this is a thrust induced moment issue. A good way to differentiate the two would be increasing speed with constant throttle + nose down, or through the rapid application of thrust in a high drag configuration to limit the increase in velocity.

 

[warchild]

what puzzles me is people saying a plane pitches downward the faster you go. thats kinda like, opposite of reality. The greater the speed, the more lift on the main wing, the more lift on the main wing, the higher the nose will climb. Thst doesnt change the angle of attack as thats related to the angle of the aircraft in relation to the flight path and with more lift, the nose rises and the flight path changes, not the angle of attack.

::EDIT:: ARRRGH some days my eyes just dont see everything ( like minus signs )... will have more later but to many people needing me at the moment .. i'm sorry..

 

[Killbilly]

what puzzles me is people saying a plane pitches downward the faster you go. thats kinda like, opposite of reality. The greater the speed, the more lift on the main wing, the more lift on the main wing, the higher the nose will climb.

This depends on the design of the aircraft. Increased lift on the main wing will not pitch the nose up or down unless the inreased lift on the horizontal stabilizer is disproportionate. If the lift on the wing is increased more than that on a rear horizonatal stabilizer the nose will pitch up. If the lift increase on the rear stab. is proportionately more than on the wing, the nose will pitch down. That is all assuming that the plane is flying straight and level in the first place. If the aircraft is in a dive, climb, or turn, the effect of increased airspeed on pitch will be altered. If the original post was about thrust and not just airspeed, then placement of the engines comes into play as well. There are lots of variables at play here. I don't think this is something that can be generalized to all aircraft.

 

[Brett_Henderson]

what puzzles me is people saying a plane pitches downward the faster you go. thats kinda like, opposite of reality. The greater the speed, the more lift on the main wing, the more lift on the main wing, the higher the nose will climb. Thst doesnt change the angle of attack as thats related to the angle of the aircraft in relation to the flight path and with more lift, the nose rises and the flight path changes, not the angle of attack.

::EDIT:: ARRRGH some days my eyes just dont see everything ( like minus signs )... will have more later but to many people needing me at the moment .. i'm sorry..

It depends where the CoG and wing-apexes are. If the center of lift is behind the CoG.. more lift from higher airspeed could cause a negative pitch. ( if the center of lift is in front of the CoG.. the plane will be very unstable).

If the plane is on autopilot and airspeed increases.. that too can cause a negative pitch as the autopliot tries to maintain altitude. Really.. it'a all more complicated than the MSFS model can account for..

 

[fliger747]

A suggestion, get the free utility, Aircraft Airfile Manager... Go to the primary aerodynamics section 1101 (or you could use aired) and down to the pitch section. Try the first entry there, CMO Pitch Moment-AOA. Also in this section are adjustments for pitch factors for pitch from flaps, gear , thrust etc.

Without looking at the file and test flying the plane it is difficult to evaluate the exact issue/issues. For a recent twin project I did, the value here was -0.01953. In cruise a negative value tends to move the nose up.

Good luck! T

 

[warchild]

yeahh, i'm quite accustomed to disproportinate lift :;shudders:: A lot of very old planes had a big twin radial stuck meters ahead of the wing along with a forward mounted pilot, and no weight on the tail plane.. It makes for some really interesting dynamics..

 

[bobmay]

.


Does anybody have a decent idea a) what the heck I'm rambling-on about; and b) where to go to adjust it? (be kind)

Cheers, all.

I find that adjusting the cruise_lift_scalar brings up or takes down the nose. Only a small + or - adjustment is needed.

[flight_tuning]
cruise_lift_scalar =1.00

Bob.

 

[Ken Stallings]

Sounds to me like Ken is giving the "real world" aviation definitions and John the MSFS definitions. John illustrates, provided you can understand engineering speak , some of the differences between MSFS concept of flight and the real world physics.

Precisely!

However, I was interested in hearing how FSX chose to model angle of attack. Of course, when I made my post I was thinking about a more traditional aircraft that derived its lift from a wing and countered the lift with a vertical stabilizer acting in opposition to maintain level attitude.

A lifting body aircraft would of course have to have its chord line computed in a different manner and I'm not going to put forth how that's done because I do not know.

Cheers,

Ken

 

[Ken Stallings]

Back to the original question.. a piston twin that has a a lot of negative pitch when power is increased..

Might that be the Goose ? I've never flown a real Goose, so I can't say for sure.. but considering how far above the CoG that the thrust is.. it "feels" realistic to me (as does the positive pitch when power is reduced).

In the aircraft.cfg file:
-------------------------------------------------------------------------------------
reference_datum_position = 2.991, 0, 0 // (feet) distance from FlightSim Reference position: (1/4 chord, centerline, waterline)
empty_weight_CG_position = -3.2831, 0, - 2.255 // (feet) longitudinal, lateral, vertical distance from specified datum



[GeneralEngineData]
engine_type = 0 //0=Piston, 1=Jet, 2=None, 3=Helo-Turbine, 4=Rocket, 5=Turboprop
Engine.0 = 3.589, -5.837, 4.41 //(feet) longitudinal, lateral, vertical distance from reference datum
Engine.1 = 3.589, 5.837, 4.41 //(feet) longitudinal, lateral, vertical distance from reference datum
fuel_flow_scalar= 1.07
min_throttle_limit = -0.25; //Minimum percent throttle. Generally negative for turbine reverser

--------------------------------------------------------------------------------------

The highlighted numbers are the vertical coordinates. I did not make the model, so I cannot confirm the model origin.. but you can see that the reference-datum, and model origin are the same (vertically).. The CoG is 2.255 below the reference.. the engines are 4.41 feet above the reference... meaning that the thrust-line is 6.66 feet ABOVE the CoG ! ... This will definately cause pitch changes when the power is changed.

Yep, I recall also that the pilots on the actual PBY Catalina's as well as the Twin Otter report the same issues. Pitch will change significantly with advancing or retarding of manifold pressure or RPM. As you said, it is because the centerline of thrust is above the wing chord line.

In a more traditional piston twin (like a Baron or 310R) the thrust centerline is pretty much exactly on the wing chord line. So, changes in thrust will not significantly change the aircraft's pitch.

Cheers,

Ken

 

[Ken Stallings]

what puzzles me is people saying a plane pitches downward the faster you go. thats kinda like, opposite of reality. The greater the speed, the more lift on the main wing, the more lift on the main wing, the higher the nose will climb. Thst doesnt change the angle of attack as thats related to the angle of the aircraft in relation to the flight path and with more lift, the nose rises and the flight path changes, not the angle of attack.

::EDIT:: ARRRGH some days my eyes just dont see everything ( like minus signs )... will have more later but to many people needing me at the moment .. i'm sorry..

Not true. As airspeed increases, the amount of angle of attack the wing needs to maintain level flight goes down. Therefore, if the pilot did nothing as speed increases, the aircraft would start to climb. Therefore, in order to maintain the same altitude, the pilot would need to apply nose trim down in order to reduce angle of attack on the wing.

This is also why, if you look at an aircraft's drag chart, it will show that as airpeed reduces, induced drag increases but parasitic drag will reduce. This is why when you chart the combined effects of induced and parasitic drag to determine at what speed the combination are at their lowest values, this speed is not at the stall speed, but neither at the max speed (Vne). It is normally betwen stall speed and the halfway point between stall speed and Vne.

One additional point is that as aircraft weight increases, for a given airspeed, you need more angle of attack to maintain level flight. This is why fuel economy increases as you burn fuel. Since stall always happens at a given critical angle of attack (regardless of weight) it is the cooresponding indicated airspeed where stall takes place that goes down as weight increases.

Cheers,

Ken

 

[warchild]

Not true. As airspeed increases, the amount of angle of attack the wing needs to maintain level flight goes down. Therefore, if the pilot did nothing as speed increases, the aircraft would start to climb. Therefore, in order to maintain the same altitude, the pilot would need to apply nose trim down in order to reduce angle of attack on the wing.


Cheers,

Ken

i believe we are describing the same phenomena, only you have a formal education in regards to flying and are capable of describing things in far greater detail.. i am but myself.. ..
Pam