Density altitude / performance in FSX and P3D

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Turner
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Density altitude / performance in FSX and P3D

Post by Turner »

Hi all -

Here's a post from another forum, where I've highlighted my non-scientific testing results. I'm posting here because of

1) Obviously careful treatment and discussion of performance physics, and
2) Hopefully, some insight to what I'm describing below...

Note: I find I am not getting the performance I'd expect with the 182 either; I will be going back and experimenting with the prop, leaning, etc. but wanted to ask here in the mean time.

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

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

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Nick - A2A
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Re: Density altitude / performance in FSX and P3D

Post by Nick - A2A »

Hi Andrew,

Yeah, there have been some quite interesting discussions here on density altitude and how it relates to FSX. My feeling was that something was a little off with the modelling because hot, high take-offs didn't seem to present the challenge they're reputed to (and I have no real world experience to draw on I'm afraid).

However, a bit of a closer inspection revealed that the relationship between indicated airspeed (IAS), density altitude and true airspeed (TAS) seems to be spot on. :)

[Slightly off-topic bit: Actually, I say 'IAS' but strictly speaking I mean calibrated airspeed (CAS). As far as my experience of FSX goes, the two only really diverge due to some custom A2A modelling of airspeed indicator error. This error or ASI under-reading is most pronounced at the lower end of the airspeed range as the angle of attack of the aircraft and its pitot tube increases. Anyway, for the purpose of this discussion, I think we can safely ignore ASI error and assume that IAS = CAS.]

You mention an altitude of 8,500 feet and a speed of 120 KIAS. Lets also assume an OAT of 0°C and standard atmospheric pressure. According to a simple online calculator such as this one, we should be seeing a TAS of around 137 KTAS. And this should be what you see in FSX. If we bump the temperature up to a scorching 35°C, TAS will increase to 145 KTAS. Again, this seems to be faithfully modelled in FSX. (So that 'hot and high' performance degradation is modelled, it's just not as 'bad' as I thought.)

Not sure about your figure of "160 kts" in X-Plane. Even with a very high OAT and a very low atmospheric pressure, I'm not sure this TAS would actually be achievable. Can't really speak for X-Plane I'm afraid, but I'd be surprised if it was wrong with its atmospheric calculations.

However, you mention "ground speed" and the missing part of the equation here is wind speed as you'll no doubt be aware if you fly a real world DA40. In other words, for your experiments in the sim(s), you need to ensure that ground speed isn't being affected by wind at the altitude you're flying. :wink:

Cheers,
Nick

P.S. You'd maybe get more responses to this if it was moved to the 'Flight Academy' forum or 'Pilot's Lounge'.
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Turner
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Re: Density altitude / performance in FSX and P3D

Post by Turner »

Hi Nick -

Thanks for the input!

Regarding forums - sorry, I haven't posted here much and thought maybe that the difference in expected performance vs. actual performance would be a factor closely related to Accu-Sim. That is, I'm not really looking for a discussion of density altitude, per se, but rather... ah... I guess I'd say maybe someone's already done the detective work! :)

Investigating the A2A 182 last night, I found I get performance closer to what I see in X-Plane with the DA40, i.e. performance more closely replicating what I expect to see in real-world flight.

(Side note: switched to the A2A 182 at 8500' and destroyed the crankshaft! Lol.. huh? Then, wouldn't start.. engine flooded. Couldn't un-flood in hangar, so autostarted... anyway...)

That said... I'm going through some other aircraft in P3D (Bonanza, another Iris DA40) and looking at some book performance data to see if I can figure anything out.

One thing I've just discovered (in P3D, anyway): Changing between the TAS/IAS settings in realism doesn't seem to have any effect...

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Nick - A2A
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Re: Density altitude / performance in FSX and P3D

Post by Nick - A2A »

No worries with the forum choice Andrew. It's just that the Accu-Feel forum is generally rather quiet so posts perhaps don't get the same exposure as they would elsewhere. Incidentally, "Accu-Feel" is a bit different to "Accu-Sim": it's an add-on designed to provide a more immersive environment (with sounds, vibrations and that type of thing) when using non A2A aircraft.

As regards the relationship between IAS/CAS, TAS and ground speed; well, that shouldn't be affected by your choice of sim plane at all - it's purely a function of the host sim itself. However, the performance of the individual aircraft obviously are. In this area, I reckon you'll find the A2A fleet matches the 'book' numbers pretty closely though it's not something I've personally checked.

You'll also get much better fidelity in terms of the way in which they operate. For instance, the guys at A2A have worked to create their own engine coding so the excessive requirement to lean with altitude in FSX is eliminated, as one example.

It's a friendly and knowledgeable forum too, and I've learned loads!

Nick
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Turner
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Re: Density altitude / performance in FSX and P3D

Post by Turner »

Nick M wrote:No worries with the forum choice Andrew. It's just that the Accu-Feel forum is generally rather quiet so posts perhaps don't get the same exposure as they would elsewhere.
Whoops!!! I was thinking this WAS the "Accu-Sim" forum... I think that's plane-specific, though.

Hmm...

Anyway - I'm afraid we may get spoiled with the Accu-Sim stuff. If only there were a way to convert other aircraft quickly :)

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Nick - A2A
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Re: Density altitude / performance in FSX and P3D

Post by Nick - A2A »

Yep, there's certainly a long list of types that folk would like to see "Accu-Simmed"! :)
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L.A.
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Re: Density altitude / performance in FSX and P3D

Post by L.A. »

Both FSX & X-Plane account for density altitude takeoffs. Both require a longer roll down the runway, and less engine performance if leaning procedures are not kept up with. At higher altitudes, the ground covered in the pattern will also be larger.

I know that default FSX starts to get critical around 6000'. I "sim" lean at my 4200 & 4600' airports. I do notice the effects, because my real life flying in 30+ different models of aircraft involved higher altitude mountain flying. I learned to lean on day one, and did so ever since. There is no doubt that different models in each sim, have variances of engine performance, versus how far the lean mixture is pulled. Some are closer to real life, and some seem off.

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