Flight Model

[PGleize]

Dear IXEG,

 

First of all, congratulations for the release of this wonderful simulation ! The job you have done is impressive and it made me going seriously on XP10.

It's a real joy to fly and to see that systems interacting realistically such as landing lights wattage impacting electrical sources loads.

As I am keen on flight modeling and aircraft performance and because you use your own model, I carried out several tests vs FCTM of the 737-300 to check the basic aero model.

It turned out that you the model is quite close to the real thing. However I noticed 3 small gaps you could fine tune :

- the wings generate too much lift: IXEG AOA are always below FCTM AOA by almost 1 deg

- this discrepancy may affect induced drag as a side effect. It seems that you fine tuned drag so that descent path match at heavy weight (121 000 lbs). If you carry out a 80 000 lbs descent test, a slight gap in flight path from FL 200 start to be visible.

- braking distance: I used unfactored figures at max manual braking + rev + spoiler. Landing distance are shorter than what FCTM says. I dunno if it's reverse thrust or wheel braking that is too strong. Further investigation is required there.

- 1 Engine out performance is quite good as you fall into an acceptable error margin for N1. A gap start to arise at high altitude and high weight (15000 ft/135000 lbs) but this may link to lift/issue cited above.

I didn't have much time to check engine model, but will do as soon as I can.

Please, take it as constructive critcism. I really enjoy your bird and wih you a great success.

 

Philippe

IXEG 737-300.xlsx

Edited May 9, 2016 by PGleize

[Morten]

Hi Phillippe, thanks for the report, not sure if less than 1 deg deserves a medal or a bugreport though  

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- the wings generate too much lift: IXEG AOA are always below FCTM AOA by almost 1 deg

You are jumping to conclusions here  

You have tested PITCH, not AoA (FCTM), which are two very different things.  So you do not know if it's wing Cl, fuselage lift, washout or incidence (deck angle) that might be just a *tad* off.  This is not FSX which I understand you have been working with, things are far more complex in XP.

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- braking distance: I used unfactored figures at max manual braking + rev + spoiler. Landing distance are shorter than what FCTM says. I dunno if it's reverse thrust or wheel braking that is too strong. Further investigation is required there.

We have not altered the default manual braking forces, airliners almost always use autobrakes for many reasons, but we might look into that later

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I didn't have much time to check engine model, but will do as soon as I can.

You probably do not have the data to do that (we do), so assume you just mean checking N1 vs pitch/weight values.  If so you need to take into account that the FCTM values assume some ideal engine and are only ballpark figures, so comparing those will not be completely accurate, we have a random aging model on our engines which means N1 numbers will vary slightly.  

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Please, take it as constructive critcism. I really enjoy your bird and wih you a great success.

No problem, we are offcourse open to finetune - if it makes sense    

PS: Make sure you port that Concorde to X-Plane one day!!  We really need a good one!

Edited May 9, 2016 by Morten

[PGleize]

Hi Morten !

I tested pitch and flight path angle and deduced AOA assuming angle between wing and fuselage is 0. The average offset between tables is approx 1 deg and seems fairly constant, so looks like a quick fix to be better.

Things may be a tad more hard for glide path at light weight.

To me the debate XP vs FS is more and more meaning less now that external FM in both world exist. Only the result and quality have a sense.

Concorde on Xplane is one of my dream

Cheers,

Philippe

[Litjan]

Dear Phillipe,

I am thankful for your very close scrutiny of our flight-model and the constructive criticism.

I am sure that you being a developer for the competing platform - FSX has nothing to do with you trying to pick holes into it: http://www.flightsimlabs.com/index.php/about-us/

You will find that real aircraft have a slight variance in performance, ESPECIALLY when comparing them to the official documentation. Almost all aircraft flying out there are a few percent "worse" than the manufacturers specification and sales catalogue - I wonder why?

I would bet that it is possible to find discrepancies between real performance and simulator performance in every simulation, X-Plane, FSX, PMDG and even the LEVEL-D simulators. I understand that developers from "that other" simulation may find our level of fidelity threatening, but we don´t come into your forums to pick apart your product, either.

Thanks, Jan

 

[PGleize]

Dear Jan,

Sorry if my comments sounds picky. I am an aviation enthusiast and really don't consider any competition between products or platform.

To me you did a really great job in capturing the spirit of the 737 and your flight model is really enjoyable. In fact I appreciate the hard core approach you had on this study sim and hope it will help to bring a new standard in simulation. 

i wish you a bug free day

Philippe

 

[Litjan]

Thank you, Phlippe, and sorry if I came across a bit snappy. We worked very hard on getting the flightmodel as close as this - within the confines of the simulation platform. I am sure you know what a battle this can be. Then it hurts a bit if someone says "you are good, but not perfect!". Of course we are not perfect, but I don´t think it is possible to be perfect with any of these simulators.

We had to twist, bend and tweak many aspects of X-Plane´s default flightmodel and engine model, and in the end it will always be an approximation.

From a pilots perspective I can only say that it is very rare to have an airplane that flies exactly according to the book. There are so many variations influencing this (atmospheric conditions, engine age, manufacturing tolerance, ...) and we are often surprised how much/little power an airplane needs in a give situation.

If we are within "almost 1 degree", "a slight gap" and "acceptable tolerance" coming from someone as knowledgable as you - then I am happy .

Of course we can tweak and improve - but for now we have bigger bugs to stomp and crashes to hunt down...

Merci, Jan

 

[PGleize]

No worries  happy hunting these damn bugs ! It is the perfect 737 sim so far.