FSX, P3D and XP density alt & leaning

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Turner
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Joined: Fri May 06, 2016 7:18 pm

FSX, P3D and XP density alt & leaning

Post by Turner »

Ok... never really paid attention to this as typically I fly GA at 6500 or below.

In a RW DA40, at, let's say, 4500, I have no issues coaxing 135-140 indicated out of the aircraft in smooth air, and will typically have to back off a little bit when it gets bumpy. Like, hot early summer afternoon bumpy.

All other things being about equal, and leaned out:

I was at 8500 in the Alabeo DA40 in P3D and found I couldn't get more than about maybe 115-120 indicated, but the problem is that checking ground speed, I was only getting a few knots more than that.

Tried the same aircraft, same altitude in FSX and got about the same results.

Tried X-Plane, and found I could claim about a 160 kts ground speed. That's more like it.

So: Any ideas here? I'm wondering if it's the aircraft or the sim.

I searched around for about 15 minutes, but haven't found any definitive answer, but it appears that, at least, "out-of-the-box", XP is correctly modeling or calculating density altitude whereas FSX and P3D do not.

Again... might be the aircraft, but that's the only one I have that is the same cross-sim.

Thanks!
Andrew
Kyle.Sanders
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Re: FSX, P3D and XP density alt & leaning

Post by Kyle.Sanders »

Rookie here.... So take this with a grain of salt but:

The way XP is mainly set apart from the others is that it has its own modeling engine that acts UPON the aircraft rather than the aircraft being modeled to perform a certain way in certain conditions like the others do.

This means that you could technically put the blueprints into X-Plane maker of an aircraft that has never been built before and the sim will have it perform very close to how it would real world. You simply can't do this with the other sims.... They rely on performance data input.

So maybe the developers of your DA never input the higher altitude variables into those aircrafts and therefore it gives you inaccurate performance.
Kyle Sanders
Turner
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Re: FSX, P3D and XP density alt & leaning

Post by Turner »

Hi Kyle-

I'm familiar with the aerodynamic aspects of XP, but not really engine modeling...

That said, I don't really think 8500 feet is considered "higher altitude" as there are plenty of airports with an altitude of a couple of thousand feet, and certainly lots of mountains and MEAs that require performance at higher altitudes much higher than this, even for normally aspirated single-engine aircraft.

If it's the case that it truly isn't modeled in FSX/P3D, I'd be really surprised that I haven't heard about it until now!
Kyle.Sanders wrote:Rookie here.... So take this with a grain of salt but:

The way XP is mainly set apart from the others is that it has its own modeling engine that acts UPON the aircraft rather than the aircraft being modeled to perform a certain way in certain conditions like the others do.

This means that you could technically put the blueprints into X-Plane maker of an aircraft that has never been built before and the sim will have it perform very close to how it would real world. You simply can't do this with the other sims.... They rely on performance data input.

So maybe the developers of your DA never input the higher altitude variables into those aircrafts and therefore it gives you inaccurate performance.
Peter Grey
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Re: FSX, P3D and XP density alt & leaning

Post by Peter Grey »

That said, I don't really think 8500 feet is considered "higher altitude"
It is for light GA aircraft. In general power will be down 20-25% at that altitude

Having operated from an airport at 5000' for about a decade I can tell you that a reduction in max IAS is correct with altitude due the lower pressure "tricks" the indicator into reading low. The reduction in density causes this, however that also means the aircraft can so "faster" (in a true airspeed sense) to generate the same air pressure (and therefore drag).

Using a C172 as an example, at 5000' 110 knots IAS is around red line. At sea level it'll do around 130 IAS.
So: Any ideas here? I'm wondering if it's the aircraft or the sim.
Comparing Ground Speed sim to sim is a bit tricky as wind varies that. If your sims had different wind patterns that would influence it. Try it again with either the wind calm or using a TAS value, if you still get different values that would warrant some further looks.
Peter Grey
PilotEdge Director of Quality Assurance and Operations
peter@pilotedge.net
Turner
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Re: FSX, P3D and XP density alt & leaning

Post by Turner »

Hi Peter -

Sorry for the late reply.

For sure, setting up with all things being (basically) equal, I am getting different performance in XP vs. FSX/P3D... It's not exactly scientific. Same plane/manufacturer though.


Hmmmm...
Keith Smith
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Re: FSX, P3D and XP density alt & leaning

Post by Keith Smith »

I was at 8500 in the Alabeo DA40 in P3D and found I couldn't get more than about maybe 115-120 indicated, but the problem is that checking ground speed, I was only getting a few knots more than that.
Groundspeed is not an indicator of aircraft performance. The number you do care about is TAS.

The only way GS is relevant is if you set the winds aloft to 0 kts, in which case, TAS and GS will be identical.

You should absolutely not be able to reach the same IAS at 8500 that you can when you're down low. That's ok, though, the number that matters is TAS.

That is even more true if you are still full rich at 8500, in which case, the engine is comically rich and is producing significantly less power that it would be capable of if it was leaned properly for that altitude.

For a more extreme example of IAS/TAS divergence, consider that the SR-71 used to saunter along at Mach 3 at FL800+ (1800kts+), it's IAS was in the mid 400s if memory serves. IAS is not great indicator of performance once you leave the ground. Most airliners at FL300-FL350 cruise along at 300-330KIAS even though their TAS is 430-480.
Turner
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Re: FSX, P3D and XP density alt & leaning

Post by Turner »

Hi Keith,

I'm still looking at IAS since it's right there, but otherwise, do have winds turned off and am using GPS groundspeed and DME on a VOR 20 or so miles away. I haven't done any TAS calcs though.

This is leaned for performance and getting about as much speed out of it as I could. P3D and FSX had a slower aircraft than FSX by (in some cases) 20 kts under otherwise similar conditions.

I know I might be missing something here. As noted, this stuff is pretty interesting so I'm going to experiment a bit. Probably will just build/save identical weather setups.
Keith Smith
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Re: FSX, P3D and XP density alt & leaning

Post by Keith Smith »

If the winds aloft are set to 0 and you're seeing ground speeds that are very close to your IAS up at 8500 then something is wrong with the software engine.

You can verify the wind situation by flying on 4 perpendicular headings (360, 90, 180, 270, for example) and seeing if the ground speed changes. If it increases or decreases across headings, then there is wind.

Otherwise, something is indeed a amiss.

I was originally responding to the fact that you were getting 135-140kias at 4500 in real life, but only 115-120 at 8500 in the sim. That would be normal.
Turner
Posts: 134
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Re: FSX, P3D and XP density alt & leaning

Post by Turner »

Oh - sorry if it wasn't clear; when I noted 115-120 at 8500, I was only getting a slightly higher ground speed (no wind, GPS used for ground speed.)

I'll look again and see that I clear up all weather just to make sure, and will try flying in cardinal directions as you suggest.

Thanks again.
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