Game 'physics' realism VS 'aircraft systems' realism

[Doug47 574]

6 hours ago, ShawnG said:

right.  this is what we've been talking about.  we have a 737 pilot on this thread who has pointed out that the numbers on the documentation assume an engine failure at v1.  And that there likely is not publicly available info on the actual performance of the plane if both engines are still humming.  And that, given his experience he felt it was likely that a loaded -700 could probably take off from EGLC just fine, as long as there were no problems.  So we're still in the realm of opinion, but I'd tend to side more for the guy who flies the thing for real every day over a simmer who read a stat page without understanding what the process behind creating that stat was.

 

Now to dip my toe into the "uninformed simmer" column:  Flaps 5.  Topcat being my source, it seems to recommend flaps 1 way more often than flaps 5 in situations with high weight and short runway, I'm assuming because less drag allows the aircraft to reach takeoff speed quickly?  is this accurate?

I only based on official data from Boeing. Not my own thoughts. And since anything else is pure speculation, that’s all, we have. Which I’d trust more than Topcat as well. 
 

Has nobody tried the test with the -700 yet? 

[ShawnG 1,823]

4 hours ago, Doug47 said:

I only based on official data from Boeing. Not my own thoughts. And since anything else is pure speculation, that’s all, we have. Which I’d trust more than Topcat as well. 
 

Has nobody tried the test with the -700 yet? 

the difference being I used topcat as a source in order to ask a QUESTION of someone more knowledgable,  I'm not using numbers determined using a methodology, rejecting the possibility of that methodology, and then using it to claim a superior knowledge position. 

there famously being three types of lies:  Lies,  word not allowed Lies, and Statistics

[Doug47 574]

12 minutes ago, ShawnG said:

the difference being I used topcat as a source in order to ask a QUESTION of someone more knowledgable,  I'm not using numbers determined using a methodology, rejecting the possibility of that methodology, and then using it to claim a superior knowledge position. 

there famously being three types of lies:  Lies,  word not allowed Lies, and Statistics

I’m not arguing or trying to ‘claim a superior knowledge position’. 
it’s just a game. 🙂

I was only highlighting official data from the manufacturer of the real airplane showing runway required based on weight and thrust ratings for the -700. 
At MTOW, 26k thrust rating, (flap unknown. 1? 5? 25?) LCY would be tight, possibly doable. Possibly. 
MTOW, 20K , seems to indicate not doable by a considerable margin. 
 

The OP strangely never did say what the weight was (well over is all) . Or what the thrust rating was (recall PMDG seemed to go with 24k for the -700). 

I’m surprised we’ve gotten this far and no one else has tested some more for comparisons sake. 

 

 

 

 

 

[Stearmandriver 2,282]

5 hours ago, Doug47 said:

I only based on official data from Boeing.

Yes, but the data you're using from Boeing is enormously factored, because it assumes an engine failure. It's not relevant to this scenario, is what we're trying to explain. 

No one in this thread has access to the actual Boeing test flight data that would be needed for this scenario.

[WestAir 2,522]

The OP should re do the test, but fail an engine at V1.

[Bad_T 581]

1 hour ago, WestAir said:

The OP should re do the test, but fail an engine at V1.

haha no, that would not work, my original test setup was already stupid but with 1 engine less it's a bit too much to handle 😜

this thread turned out very interesting and i learned a lot of things, was just wondering if in real life that would be theoretically possible and, if not, if maybe we were just facing some 'sim physics' limitation. I'm really enjoying all those nice aircraft we have now (pmdg, maddog, fbw, WT cj4, and really tempted by the 146 and 310 butt well i probably better wait because too many choices isn't probably good for me atm) which all go fairly deep in 'systems simulation' so the 'physics' part was a bit more of a mystery for me.

Thanks again everyone for your inputs !

🍻

Edited May 14, 2022 by Bad_T

[Doug47 574]

9 hours ago, Stearmandriver said:

Yes, but the data you're using from Boeing is enormously factored, because it assumes an engine failure. It's not relevant to this scenario, is what we're trying to explain. 

No one in this thread has access to the actual Boeing test flight data that would be needed for this scenario.

Well it is relevant. Because no doubt PMDG would’ve based their data also on something available like this. 
 

26k engines, MTOW. Optimum flap setting. It needs a runway longer then LCY. Take off a few hundred feet for the factors you mentioned, it needs the full length or very little left over to take off. 
 

Now go ‘well over MTOW’ and use zero flap and it seems very unlikely. If he had’ el used flap 5/25 or something then it may be considered reasonable. But zero flap uses a lot of runway. 
 

I guess nobody has tried it yet? Surprised as these are the scenarios i often use to test add-ons to see how realistic or accurate they are. 

Edited May 14, 2022 by Doug47

[Stearmandriver 2,282]

2 hours ago, Doug47 said:

Take off a few hundred feet for the factors you mentioned,

The "factors I mention" involve greater than a 50% loss of performance.  We're talking about much more than a few hundred feet.  😉

On every takeoff in an airliner where an engine doesn't quit, you've got more than twice as much performance as the takeoff data was based on. 

[mrueedi 702]

 

On 5/13/2022 at 10:44 PM, martinboehme said:

However, if I'm reading the diagram correctly, CLmax looks more like 1.15 to me (at an AoA of 11 degrees)? That would reduce the lift quite a bit.

Oops from my side 🙂 Yes, you are right indeed!

On 5/13/2022 at 10:44 PM, martinboehme said:

The source doesn't say which variant of the 737 this is for though. I believe the NGs have a different wing than the Classics and Jurassics, so it's not clear whether this applies to the  -700.

But no problem, I found another source, this time an excellent one:

BOEING 737 MIDSPAN AIRFOIL (b737b-il) (airfoiltools.com)

It is Re number based, but for the 737 at take off speed, I get Re = 34'263'391, so the resulting 1.6 as CL this time seem to be established solidly now.✔️

Now for the drag at take off speed, taken from here, I get (Cd also coming from the link above):

Summary
Density of Fluid (1.225 kg/m^3 for air)1.225
Cross-Sectional Area (m^2)124.58
Velocity of Object (m/s)85.4
Drag Coefficient (cd)0.04

Resulting drag: 22.26kN

So, finally, I made a little spreadsheet of the take off run, which considers every relevant figure discussed so far:

https://1drv.ms/x/s!AhyOKVx8D0DxoM0jSJto1I2F08x1vw?e=afoc0q

The take off distance I get is 1397m. A robust 100m reserve for the runway in question. So the OP's take off is proved as realistic ✔️ If you doubt, you are invited to find an error in my calculation.

 

17 hours ago, Doug47 said:

Well it is relevant. Because no doubt PMDG would’ve based their data also on something available like this. 

No, PMDG would be lost if Boeing's minimal take off distance would be the base for their calculations. I am sure, that they at least calculate the physics as specific as I did in my calculation. The resuting minimal take off distance itself is an outcome of that, but never input in PMDGs calculations.

 

Edited May 15, 2022 by mrueedi

[Donstim 109]

An interesting exercise, 😀. But, since you invited anyone to comment on errors in your back of the envelope calculation...

Thrust does not increase during the takeoff roll. It decreases with increasing airspeed, so assuming a constant thrust at the rated static thrust level is optimistic.  A fall off of about 20% between 0 and 160 knots would be in the range I would expect to see. So, not accounting for rolling friction is not offset by your thrust assumption, making the calculation even less accurate.

It is difficult to know what an appropriate flaps up lift coefficient would be since there may be tail clearance issues in trying to achieve the CLmax determined for the airplane with flaps up. In other words, you may not be able to achieve the angle-of-attack to reach CLmax. Your assumed CL may be okay, or it may not be. Your drag coefficient looks a bit optimistic as well. 

The liftoff speed for a 737-700 with flaps 1 at 160,000 pounds is around 150 knots, so I would expect the liftoff speed with flaps up with some considerably higher (unknown) weight to be significantly higher than your estimated 156 knots. With lower acceleration due to the errors in accounting for thrust and drag, it looks like your back of the envelope calculation shows this to be a very doubtful possibility.

As for @Stearmandriver's comment that accounting for an engine failure means a greater than 50% loss of performance, well that's only for the portion after the engine failure occurs. You don't have "twice the performance" for the whole takeoff. V1 and VR are only separated by a couple of knots for a minimum balanced field length takeoff for the -700.

These comments aside, I do agree that this is a fruitless exercise, both in terms of evaluating MSFS flight physics and the PMDG flight model. There is no reason for PMDG to investigate flaps up takeoff performance at all since it is not an approved flap setting. (This same reason also applies to Boeing. No Boeing test pilot would ever have attempted this, and there would not be any flight test data to determine takeoff parameters for a flaps up takeoff within Boeing. The only reason this would ever be looked at would be for an accident investigation ( for example, if someone tried to takeoff off with flaps up out of London City at well over MTOW). http://libraryonline.erau.edu/online-full-text/ntsb/aircraft-accident-reports/AAR89-04.pdf

[spitzer45 99]

At msfs, p3d and fsx, sometimes i'm feeling like flying paper airplanes... no inertia. i'm prefering flight dynamics and i think x plane 11 is still way better for this area.

Edited May 16, 2022 by spitzer45

[MarioDonick 752]

On 5/11/2022 at 5:26 PM, sd_flyer said:

We have to open rubric "stuff you don't do in real life because it's unrealistic except you still do on occasion " LOL

 

https://www.youtube.com/watch?v=gXL1qQMky2w

 

 

 

Cool, I saw that on TV news when I was a kid in East Germany.

[Doug47 574]

On 5/14/2022 at 1:19 PM, Stearmandriver said:

The "factors I mention" involve greater than a 50% loss of performance.  We're talking about much more than a few hundred feet.  😉

On every takeoff in an airliner where an engine doesn't quit, you've got more than twice as much performance as the takeoff data was based on. 

Not correct. There’s no way a 737-700 at MTOW takes off at much less than what’s quoted by offical Boeing sources. 
 

154,500 lbs. 26k. Optimum flap settings. 5200ft. 
There’s no way at well over the MTOW, and zero flap will it use less (LCY airport). 
 

Sorry, but either MSFS or PMDG or both have it wrong. No surprise as it’s a game. 
 

 

[martinboehme 492]

On 5/15/2022 at 11:36 AM, mrueedi said:

But no problem, I found another source, this time an excellent one:

BOEING 737 MIDSPAN AIRFOIL (b737b-il) (airfoiltools.com)

Nice find! I see you've chosen the midspan airfoil, rather than root or tip -- I assume you're hoping this will give a good approximation of the behavior of the wing as a whole?

On 5/15/2022 at 11:36 AM, mrueedi said:

Now for the drag at take off speed, taken from here, I get (Cd also coming from the link above):

Summary
Density of Fluid (1.225 kg/m^3 for air)1.225
Cross-Sectional Area (m^2)124.58
Velocity of Object (m/s)85.4
Drag Coefficient (cd)0.04

Resulting drag: 22.26kN

This is the drag at the speed achieved at the end of the runway. I assume you're simply conservatively applying this drag from the start of the takeoff run?

It looks as if this is just the drag produced by the wing though -- not the rest of the airframe?

14 hours ago, Donstim said:

An interesting exercise, 😀. But, since you invited anyone to comment on errors in your back of the envelope calculation...

Absolutely -- my hope was that I would end up learning something, which I did (see below).

And to be clear, I'm certainly treating this as an academic exercise. For one thing, the idea of trying to lift off at exactly the stall speed and remaining in control is pretty impractical.

13 hours ago, Donstim said:

Thrust does not increase during the takeoff roll. It decreases with increasing airspeed, so assuming a constant thrust at the rated static thrust level is optimistic.  A fall off of about 20% between 0 and 160 knots would be in the range I would expect to see. So, not accounting for rolling friction is not offset by your thrust assumption, making the calculation even less accurate.

Thank you -- I did not know that!

For those following along, I found an explanation with some more details here:

https://aviation.stackexchange.com/questions/90123/what-is-a-ram-recovery-point

By the way, going back and looking again at the numbers I used, I noticed that the 89 kN correspond to 20,000 lbf thrust. It looks as if the -700 typically uses the 24k engines, so that could at least partially compensate for the various effects I've neglected.

13 hours ago, Donstim said:

It is difficult to know what an appropriate flaps up lift coefficient would be since there may be tail clearance issues in trying to achieve the CLmax determined for the airplane with flaps up. In other words, you may not be able to achieve the angle-of-attack to reach CLmax. Your assumed CL may be okay, or it may not be. 

Yes, I was just handwavingly assuming that it would be possible to achieve CLmax on the ground. Tailstrike pitch attitude on the 737-700 is 14.7 degrees. From the data that mrueedi quoted, the critical angle of attack appears to be about 15 degrees. I don't know what the angle of incidence is; I'm guessing it's small but positive, so it seems pretty clear we won't quite be able to achieve CLmax.

14 hours ago, Donstim said:

The liftoff speed for a 737-700 with flaps 1 at 160,000 pounds is around 150 knots, so I would expect the liftoff speed with flaps up with some considerably higher (unknown) weight to be significantly higher than your estimated 156 knots.

A good point -- though those 150 knots are likely a significant margin above stall speed and below tailstrike attitude.

14 hours ago, Donstim said:

These comments aside, I do agree that this is a fruitless exercise, both in terms of evaluating MSFS flight physics and the PMDG flight model. There is no reason for PMDG to investigate flaps up takeoff performance at all since it is not an approved flap setting.

They wouldn't investigate flaps up takeoff performance per se, but I do think they would make sure that, among other things, the aircraft produces a) the correct amount of thrust during the takeoff roll, and b) the correct amount of lift at various angles of attack with flaps up. If they've done those things, I think you should see reasonable behavior for a flaps-up takeoff?

1 hour ago, Doug47 said:

154,500 lbs. 26k. Optimum flap settings. 5200ft. 
There’s no way at well over the MTOW, and zero flap will it use less (LCY airport). 

If those 5200 ft are a takeoff distance (which published numbers would typically be), then they assume an engine failure at V1 (or, more precisely, slightly before V1) and achieving a height of 35 feet at the end of the takeoff distance.

The OP's scenario, on the other hand, is assuming both engines operating and lifting off potentially right at the end of the runway. The question is whether that leaves enough "wiggle room" to do the takeoff with flaps up and above MTOW. As noted, it is of course a purely academic question.

[snglecoil 1,828]

I haven’t read through the whole thread, however there is real word precedent for the original scenario….sort of 🙂

From Wikipedia:

On the night of February 3, 1986, a Boeing 737-201/Advanced airliner operated by Piedmont Airlines landed on a 3,877 feet (1,182 m) long Daniel Field runway instead of much longer runway at Augusta Regional Airport as intended. After skidding to a stop still on the runway, the crew and all 106 passengers were unharmed.

My understanding is that the plane was stripped down of excess weight and fuel and flown out.