21 minutes ago, OzWhitey said:

But seeing you think “your scientists” are all superior:

"My scientists?"  When did I acquire scientists lol? 

The data being presented here has a number of holes.  Many assumptions are being made, and some factors are being ignored.  You yourself say it leads to a "probable" conclusion... which isn't a conclusion.

Now, if the actual performance engineers at Boeing give me good data for a particular takeoff in a particular configuration... why wouldn't I make that takeoff? I mean it wouldn't be an experiment, right? It would be Tuesday at work.   😉

19 minutes ago, Glenn Fitzpatrick said:

OK - you have 4900 feet at EGLC.

These idiots from Southwest landed at the wrong runway (3700 feet) and managed to get out again.

 

That... is fantastic. 

They didn't just manage to get out. They were airborne basically halfway down the runway!!

Edited May 13, 20224 yr by Christopher Low

3 minutes ago, Christopher Low said:

They didn't just manage to get out. They were airborne basically halfway down the runway!!

Granted, they were no doubt empty maybe even with some of the interior removed. But yeah, the -700 goes.  😉

Good discussion, I've learned a few things reading this thread. From a simming perspective, the more realism, the better. Since MSFS came out, there has been an understandable lack of "study-level" aircraft (whatever that means 😁), so its great to see that we have some options like PMDG moving the realism bar forward in this platform.

Since realism is mostly dependent on the aircraft developer, it would be interesting to see what their opinion is on the OP. 

6 hours ago, gb09f said:

He did say that, but he rates the freeware Zibo 737 as having the most realistic flight model. 

Absolutely. One only needs to look at the taxi and crosswind simulations to get a feel that MSFS still has to work on those aspects.

But ASOBO has aeronautical science majors, pilots, and a programmers on the team who can make magic happen. The only question is if PMDG will tweak the flight model as ASOBO adds more variables. They'll have their hands full on the 777, 747, and 757 by then. We might be stuck with inadequate trim, excessive roll authority in crosswind, etc, until PMDG starts work on the PMDG 737 Max.

2 hours ago, Glenn Fitzpatrick said:

OK - you have 4900 feet at EGLC.

These idiots from Southwest landed at the wrong runway (3700 feet) and managed to get out again.

 

And there seem to be plenty space left. I know we are talking about a fully loaded aircraft in the sim, but considering this makes the idea seem more plausible.

4 hours ago, Glenn Fitzpatrick said:

OK - you have 4900 feet at EGLC.

These idiots from Southwest landed at the wrong runway (3700 feet) and managed to get out again.

 

Not fully loaded. Bad comparison. 
 

Boeing says a -700 needs around 5200 feet at max take of weight, 154,500lbs with optimum flap settings for fake off. 26,000lb engines. Standard day. 

20,000lbs thrust, same weight 9200 feet for take off.

 

 

 

On 5/11/2022 at 10:02 AM, WestAir said:

Dozens a year! I edited my comment to remove that part because I wanted to stick to feasibility, but since you caught me before the edit, I want to link another video:

There is no way any of us would see the below landing done in MSFS and think for a second it was realistic. Turns out...

 

Would not get in an aircraft with that dude.

1 hour ago, Doug47 said:

Boeing says

Boeing probably says things by introducing safety margins. I think the aerodynamic boundaries are further stretched than what a manufacturer specifies. Only a test pilot could probably comment on this.

Edited May 13, 20224 yr by tweekz

43 minutes ago, tweekz said:

Boeing probably says things by introducing safety margins. I think the aerodynamic boundaries are further stretched than what a manufacturer specifies. Only a test pilot could probably comment on this.

I only quoted from documents at Boeing.com. 
 

The OP did a test on a runway shorter then what’s required according to Boeing for a -700 at MTOW. He also said the airplane was ‘way above take off weight’. And he used zero flap. And he said it did it easily.
 

That doesn’t exactly sound realistic but ground friction, flight dynamics  etc etc etc never are in any sim. As it’s fake and not real world. Expected. 
 

But what would be interesting is if someone actually did the test using the -700 at MTOW (not above) and flap 5, max thrust (TO), no derates etc. 
And see how it performs. 
 

Come on, this should be able to be done before the day is out. And we want pics! 😛
 

 

21 minutes ago, Doug47 said:

I only quoted from documents at Boeing.com. 
 

The OP did a test on a runway shorter then what’s required according to Boeing for a -700 at MTOW. He also said the airplane was ‘way above take off weight’. And he used zero flap. And he said it did it easily.
 

That doesn’t exactly sound realistic but ground friction, flight dynamics  etc etc etc never are in any sim. As it’s fake and not real world. Expected. 

 

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?

13 hours ago, mrueedi said:

You say CL = 1.8 but use 1.48?

Oops... thanks for catching that!

I originally found the value of 1.48 here:

https://forum.flightgear.org/viewtopic.php?f=4&t=20647&start=30#p198182

But that value applies to the Classic, and besides, the value is reconstructed from stall speeds taken from an FCTM. I then decided to be more conservative and use the optimistic best-case value of 1.8 for jet transports but forgot to update the calculation... 

13 hours ago, mrueedi said:

Why not use this source:

Solved For the airfoil polar diagram of a Boeing 737 shown | Chegg.com

It suggests that 1.6 should be fine.

Good find!

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.

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.

13 hours ago, mrueedi said:

The only thing I am not sure, is whether drag in the second half of the take of run would start having a considerable impact.

Agree - I'm pretty sure drag is not negligible, which makes the calculation "optimistic". I wasn't able to find good numbers for the parasitic drag on a 737 though.

12 hours ago, flying_carpet said:

Apart from that and additionally ...

On 5/12/2022 at 2:30 PM, martinboehme said:

Static thrust 89.0 kN per engine, so 188.0 kN total

Acceleration is

a = F / m = 178 kN / 77564 kg = 2.3 m/s

 

Thanks to you too for checking my numbers. This is getting embarrassing... I should have checked my calculations better.

With the correct numbers, the results become:

a = F / m = 188 kN / 77564 kg = 2.42 m/s

v = a sqrt(2s / a) = 2.42 m/s * sqrt(2 * 1508 m / 2.42 m/s) = 85. 4m/s

L = CL rho v^2 / 2 A = 1.8 * 1.225 kg/m^3 * (85.4 m/s)^2 / 2 * 124.58 m^2 = 1001.7 kN

That's a fair bit more than the weight of 761.8 kN, so the picture is starting to look quite favorable.

The source that @mrueedi quoted suggests though that the assumed CLmax of 1.8 may be very optimistic. With the CLmax of 1.15 from that source, we get a lift of only 640 kN. That would not be enough, and that's without taking drag into account.

So in the end the picture seems inconclusive. The main point that the answer hinges on is getting an accurate value for the CLmax of the 737 NG wing.

5 hours ago, Doug47 said:

Boeing says a -700 needs around 5200 feet at max take of weight, 154,500lbs with optimum flap settings for fake off. 26,000lb engines. Standard day. 

That data assumes an engine failure at V1, and subsequent ability to continue the takeoff and climb out over second-stage obstacles while on single engine.  If an engine doesn't fail, actual distance used is DRAMATICALLY less, because you have more than twice as much performance available (an engine failure creates not only a 50% thrust loss, but additional sideslip and control deflection drag. )

3 hours ago, ShawnG said:

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?

It depends on the limiting factor (in reality; I don't know how Topcat may simplify this.)

In general, you have two possible limiting factors: the length of the runway itself, and the ability to out-climb any obstacles that penetrate the second-segmet climb region. 

Usually if runway length is limiting (say a runway that ends at a large body of water), a higher flap setting will get you off the ground sooner, but at the expense of second-segment climb perf due to more drag on the aircraft for longer.