Pitch vs. power

[PRB]

Has anyone here flown the FSD FSX P-38? Not the latest release of the stand-alone Red Bull racer, but the older one, that came with the P-38J, K, L, and the Red Bull ship. This plane behaves “backwards” in terms of pitch response to power application. If you're stabilized and trimmed at level flight (more or less) and you jam on full power, this plane pitches nose-down, with a corresponding negative vertical velocity. If, instead, you retard the throttles to idle, the nose pitches up and the ship climbs. As awesome as the P-38 is, it's still, basically, and airplane, and I'm pretty sure this is not how it works... I've seen a similar “effect” in the FSX Alphasim Albatross as well.

In the case of the P-38, I'm tempted to declare that it isn't the air file, because I swapped out the air file with another well known P-38 model out there, and the effect remains. This would have to mean the culprit lies somewhere in the aircraft.cfg, no? But what could it be?

[Ivan]

Check the location of your thrust line in relation to you CoG if power application causes nose down pitch.

Check your Stabilizer Angle of Incidence if increasing speed causes nose down pitch.

It may not be so simple, but that is where I would look first.

- Ivan.

[Dev One]

I suspect, like Ivan that the engine vertical position is the culprit in the aircraft.cfg. Its position vertically might be correct geometrically with respect to the real aircraft dimensions, but as far as I know there is no compensation in FS for the vertical angle of the thrust, if one exists,
so try changing the vertical dimension to zero. It should take away the power/attitude/trim effect of thrust, which, by the way, is present, as far as I know, on all prop aircraft.
HTH
Keith

[Bradburger]

Both Ivan & Dev One point out the most likely cause for this action.

Whilst most piston aircraft have some kind of change in pitch with power (some more than others), it can be a bit excessive on some FS FMs, even if the thrust location is in the correct position as Keith points out.

I just checked the FSD P-38 Aircraft.cfg, and indeed, the engine thrust location is above the Datum/MDL centre.

Here's the engine positions from the cfg : -

Engine.0= -5.330, -8.000, 1.55
Engine.1= -5.330, 8.000, 1.55

I plan to do a brand new FDWB FM for this at some stage (well, it's on my list!) so it flies correctly and with a higher fidelity than the default FM.

Btw, wasn't the new entry in the cfg (ThrustAnglesPitchHeading.0 = ) added so you could set the trust line angle?

Cheers

Paul

Cheers

Paul

[PRB]

Interesting. I shall try this shortly. Thanks all!

[Dev One]

Paul, (Bradburger)
Thanks for that heading entry, never seen it before! Must try it out.
Keith

Just looked in the 2004 SDK & the FSX SDK. That heading entry is only listed in the FSX SDK, so not sure if it would have any effect in FS9 - I will still try it though as I only work in FS9, with port overs for FSX.
K

[Bradburger]

Paul, (Bradburger)
Thanks for that heading entry, never seen it before! Must try it out.
Keith

Just looked in the 2004 SDK & the FSX SDK. That heading entry is only listed in the FSX SDK, so not sure if it would have any effect in FS9 - I will still try it though as I only work in FS9, with port overs for FSX.
K

Sorry Keith, meant to say FSX only!

Cheers

Paul

[PRB]

It was the engine locations. Set them to 0 and the "feature" goes away. I wonder why the stuck the engines way up there.

I had found a couple of entries in the propeller section that looked unusual:

propeller.0 = 13.0,0.0, -0.95
propeller.1 = 13.0,0.0, -0.95

There was a comment about having placed the props below where the engines were located, for a smoke effect. I commented out the two entries but that had no effect of the pitch behavior.

Thanks all.

[Ivan]

It was the engine locations. Set them to 0 and the "feature" goes away. I wonder why the stuck the engines way up there.

Part of the problem may be that the actual CoG of the aircraft without Fuel or Ammunition is often not listed anywhere. When it isn't the aircraft designer has to make a few assumptions and often the designer may not be all that knowledgeable about how aircraft really work. I have built around two dozen aircraft for CFS over the years and I can say that the majority have had their CoG located by guesses.

Sometimes, the loaded CoG is documented but then it is quite bit of work to work back to where the CoG would be with no Fuel or Ammunition. I just did this a few weeks ago with a P-39 Airacobra and even with the great amount of documentation I have on the aircraft, I still had to make some guesses and assumptions.

For example: The pilot sits quite a bit ahead of the loaded CoG, but how much does he weigh? The weight allowance for the pilot varies from 150 to 200 pounds. The lighter the pilot, the further back the CoG would be. The weight of the 37 mm cannon is known as is the location, but although the weight of the 20 mm cannon on the early D models is known, the exact location is not. My assumption is that its location was the same as the 37 mm. The CoG with this aircraft is pretty far back even with conservative estimates which may be why it had a reputation for tumbling and longitudinal instability.

Since other expendables such as Engine Oil, Hydraulic Fluid, Coolant, etc are not modeled, you have to assume a single capacity instead one that varies with duration of the flight or battle damage.

Another issue is that the location of the CoG often changes a bit with Landing Gear extended or retracted, but one state has to be chosen for representation. On something like the P-38 Lightning, ALL of the Whels retract rearward so the CoG shifts several inches aft and several inches up. This vertical change is about the same as the vertical offset of the Thrust Line, so you either have it correct for flight or doubled if you chose the CoG with the gear down.

Yet another issue is that at least for CFS, the wheel brakes are either ON or OFF. There isn't such thing as partial application of brakes. That means that at least for Tail Draggers, to avoid dropping the nose onto the runway too easily, you might want to push the CoG back a bit.

Yet another issue for CFS that doesn't happen with the more modern simulators is that the flight model CoG and visual model CoG are identical. There is no offset capability which means that if you have a very nice visual model with the CoG located too far back or too high (the typical errors), you either set the propeller and gear contact points to agree with the model OR the actual aircraft but not both.

Perhaps I am making excuses, but these are the issues I know about.
BTW, Glad things worked.

- Ivan.

[PRB]

Thanks Ivan. Good info.

[fliger747]

In the air file in the primary aerodynamics section there is an adjustment for pitch with power in the pitch section. For real planes engine thrust line is a prime determinate, however the location of the tail plane is also important. A lot of thrust over the horizon stab tends to increase downforce and cause nose up pitch. Twins are usually less effected here.

I would have to do some research to opine on the proper response for the P38.

T

[fliger747]

In the air file there is also a pitch adjustment for gear up and down which can somewhat replicate known pitch changes for gear operation.

T

[PRB]

Thanks Tom. Hadn't thought about it until now, but this behavior might make sense for the HU-16 Albatross. While the engines are inline with the wing, the wing is above the plane's CG. Still, if the engines are brought to idle and left there, the nose wouldn't continue to rise up while the plane slows, would it? At some point the loss of lift ought to start to bring the nose down.

[Milton Shupe]

One must also consider Center of Lift. Generally I like it near the CoG, maybe a little behind the 1/4 wing chord. Moving the CoL forward of CoG will give you that nose up issue. I try to keep CoL at or just slightly behind CoG for more stable flight envelope in all phases, and keep trim neutral for level flight at cruise. Just my preference for a good sim experience, not necessarily possible for all aircraft.

High wing aircraft with engine mounted wings also have tons of fuel in those wings so the CoG may be higher than you think. That mass is going to dictate most of the movement and responses and the rest of the aircraft will follow with only lighter influences. Canting of wings and engines offset most of the undesirables.

Lower speeds for deployment of gear and flaps ensure manageable reactions to drag and changes in CoL due to flap deployment, and CoG due to gear deployment.

There's a lot to chew on for us amateurs but its all fun and challenging.

[PRB]

Rgr that Milton. It's like cat herding!

[fliger747]

Paul

the smaller monohull floatplanes with pylon mounted engines above can have some very unpleasant landing charchteristics, such as a tendency to nose dive if go around power is added, or a pitch up which can lead to a stall if power for landing is suddenly reduced.

In real aircraft a CG behind the center of lift leads to dynamic pitch instability as an up force rather than a stabilizing downforce of the tail plane is necessary. In FS such stability is externally applied to the aircraft. An example of strange CG, the new F14 has the CG right at the leading edge of the. MAC, a real plane would have some real issues in this configuration.

cheers. T

[Ivan]

In the air file there is also a pitch adjustment for gear up and down which can somewhat replicate known pitch changes for gear operation.

T

Yes, there is a pitch adjustment for extending landing gear (which tends to make most aircraft pitch down) but I don't believe there is an adjustment for CoG changes as I was describing for the P-38 Lightning where all the wheels retract rearward.

Do you happen to recall the offset for Pitch with engine power?

- Ivan.

[Ivan]

Thanks Tom. Hadn't thought about it until now, but this behavior might make sense for the HU-16 Albatross. While the engines are inline with the wing, the wing is above the plane's CG. Still, if the engines are brought to idle and left there, the nose wouldn't continue to rise up while the plane slows, would it? At some point the loss of lift ought to start to bring the nose down.

I believe the proper way to replicate the behaviour of the Albatross would be to have a high constant pitch moment up to simulate the air resistance of the wing and engines way above the CoG and have that offset by a very high thrust line. As for the nose coming down with the loss of lift, it kinda depends on where the CoG is located. If the stabilizer stalls first, it would pitch up even more.

- Ivan.

[Ivan]

One must also consider Center of Lift. Generally I like it near the CoG, maybe a little behind the 1/4 wing chord. Moving the CoL forward of CoG will give you that nose up issue. I try to keep CoL at or just slightly behind CoG for more stable flight envelope in all phases, and keep trim neutral for level flight at cruise. Just my preference for a good sim experience, not necessarily possible for all aircraft.

Wouldn't the ideal CoG be at the Neutral Point so that changing airspeed doesn't cause a pitch up or down? Problem though is that this would also tend to make the aircraft a lot less stable.

High wing aircraft with engine mounted wings also have tons of fuel in those wings so the CoG may be higher than you think. That mass is going to dictate most of the movement and responses and the rest of the aircraft will follow with only lighter influences. Canting of wings and engines offset most of the undesirables.

The fuel in wings would be a disposable load, so you would need to lay out the CoG of the aircraft without any fuel. With CFS1, I am pretty sure that it isn't possible to do cant the thrust line or even to reverse the direction of propeller rotation. (Yeah, it IS a bit primitive.)

Lower speeds for deployment of gear and flaps ensure manageable reactions to drag and changes in CoL due to flap deployment, and CoG due to gear deployment.

There's a lot to chew on for us amateurs but its all fun and challenging.

Since I don't fly, I easily qualify as an amateur! Been at this game for a few years and am still running into a bunch of things I can't figure out.

- Ivan.

[fliger747]

Most of the things that affect stability for real aircraft have no effect in FS as the sim does not model the tail plane directly in any way. All effects are applied as separate adjustments and moments to the main wing. For example Dino's Tomcat has the CG right at the leading edge of the wing. The real plane could not fly this way. Like Minton I prefer the CG near the COL which reduces the magnitude of the various adjustments needed to a small magnitude.

as to aft retracting gear, the CG shift is only important in how it affects pitch, for which there is an adjustment which completely is capable of providing the appropriate effect. For example the S2F series shares this characteristic.

Another example how how the list and pitch moments is to observe a taildragger, racing along three point, pull back and the mains lift off first. Liftoff in a real plane can only occur if the elevator can increase the AOA by dropping the tail, not possible if the tail is still on the ground.


Tom