8 hours ago, Bad_T said:

.So I was just wondering if this is just normal and everyone gets on with it just fine

I don't know. But I think the guy flying this one is laughing his butt off thinking is that all you got?

Whether or not an aircraft is allowed to use a runway is not only a function of whether it can scape off the runway at full thrust.  Also factored in are things like:

- if something really bad happens just before V1 can it get stopped before the end of the runway?

- if it loses an engine just after V1 will it have enough power to climb over obstacles?

- even if it doesn’t lose an engine, will it have enough thrust to climb over obstacles safely?

Plus lots of other factors that people smarter than me could better explain.

Another issue with a 737 taking off at EGLC is how the aircraft would get on the ground in the first place since they’re not certified for the steep approach into the airport.

All that to say I don’t think being able to take off from EGLC in a fully loaded 737 is necessarily an indictment of the physics in MSFS.

Edited May 11, 20224 yr by regis9

An interesting discussion. Some have stated that it’s impossible to answer the question whether this takeoff would be possible in reality, but I believe that at least Boeing should be able to answer this question based on their flight test data, and I think we can even get some of the way using only publicly available information.

There are two questions we need to answer:

1. What speed will the aircraft reach by the end of the runway?

2. At that speed, how much lift will the aircraft produce in the maximum attitude that can be achieved on the ground (i.e. tail strike attitude), in a clean configuration?

If the lift is greater than the weight of the aircraft, the takeoff should be possible (assuming no obstacles, perfect execution etc.), though of course not necessarily legal.

Boeing’s test data should be able to answer these questions.

Regarding question 1, they will have performed extensive takeoff acceleration tests. While they will likely not have performed these beyond MTOW, the data for these tests should extrapolate reliably beyond MTOW: Aerodynamic drag is not affected by the TOW, and there’s a simple inverse relationship between mass and acceleration. Tyre rolling friction will increase with TOW, but it’s a simple physical principle that should extrapolate reliably, and its influence is likely small anyhow.

Regarding question 2, the answer to that does not depend on TOW, and Boeing will certainly have precise lift polars for every configuration.

We’re not Boeing, but let’s see how far we can get with publicly available data.

First, let’s set some parameters. I’m going to assume a sea level ISA day with no wind. I don’t know the TOW that the OP used, so I’m going to assume MTOW.

From http://www.b737.org.uk/techspecsdetailed.htm, we find the following relevant data for the 737-700:

Static thrust 89.0 kN per engine, so 188.0 kN total
MTOW 77564 kg
Wing area 124.58 m^2

(Static thrust data is often quoted for sea level ISA conditions, so I’m assuming that’s the case here.)

The runway length at EGLC is 1508 m.

Let’s answer question 1 first. In doing so, we’ll have to make some simplifying approximations:

• We will neglect drag and tyre friction
• We will neglect the increase in thrust due to the ram effect, i.e. we will assume that thrust remains constant at the static thrust

These two approximations have opposite effects, but all in all, I expect that we will be overestimating the speed. Acceleration tends to reduce during the takeoff roll, which implies that drag and tyre friction have a larger effect than the ram effect. What this means is that the calculation will be too optimistic: It may predict that a takeoff is possible when in reality it is not. Conversely however, if the calculation says the takeoff is not possible, we can be pretty sure that it would not be possible in reality.

With that, let’s plug in some numbers and see what we get.

Acceleration is

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

This gives us the following speed at the end of the runway:

v = a sqrt(2s / a) = 2.3 m/s * sqrt(2 * 1508 m / 2.3 m/s) = 83.3 m/s

That’s 162 knots, which seems like it might be barely enough.

Let’s continue with question 2. I’m going to assume that the 737-700 can reach clean CLmax in the tailstrike attitude. Unfortunately, it’s hard to find a clean CLmax for the 737-700, so I’m going to use the typical CLmax ranges found in this source; again, to be optimistic, I’ll use the top end of the range, giving me a CLmax of 1.8.

With an air density in sea level ISA conditions of rho = 1.225 kg/m^3, the lift we can achieve becomes

L = CL rho v^2 / 2 A = 1.48 * 1.225 kg/m^3 * (83.3 m/s)^2 / 2 * 124.58 m^2 = 783.6 kN

The weight of the airplane at MTOW is

Fg = m g = 9.81 m/s^2 * 77654 kg = 761.8 kN

So we’re achieving just slightly more lift than the aircraft’s weight, but keep in mind that we’ve made various optimistic assumptions and approximations. Without these, it’s possible we wouldn’t quite be able to achieve the required lift.

We could, however, make the conditions more favorable than sea level ISA, nil wind. Lift increases with the square of the airspeed, so a ten-knot headwind, say, is going to help a lot. We could also lower the temperature, which would give us both more thrust and more lift. Changing both of these environmental conditions could be enough to compensate for the optimistic assumptions we’ve made. (@Bad_T, I'd be interested to know what the weather conditions were for your test.)

In the end, the results are close enough that we can't really make a definitive statement either way, but I'd say we can conclude that getting a 737-700 out of EGLC at MTOW is at least not wildly unrealistic.
 

16 hours ago, regis9 said:

Plus lots of other factors that people smarter than me could better explain.

And there it is 😀:

11 minutes ago, martinboehme said:

An interesting discussion. Some have stated that it’s impossible to answer the question whether this takeoff would be possible in reality, but I believe that at least Boeing should be able to answer this question based on their flight test data, and I think we can even get some of the way using only publicly available information.

 

On 5/11/2022 at 5:22 PM, jcomm said:

I did not but the 737 because I am actually waiting for the FBW A380 which I believe will be payware, but I am sure PMDG programmers have done their best to make it perform close to the numbers...

Not sure where those rumors come from, but the A380 will be open source just like the A32NX. It's basically impossible to commercialize fbw code since it's all gpl3 - which is also one of the reason why it can't be on the marketplace.

On 5/11/2022 at 6:33 AM, abrams_tank said:

With respect to the PMDG 737 for MSFS, multiple real life 737 pilots have commented on it now: https://www.avsim.com/forums/topic/617802-collection-of-real-life-737-pilots-opinions-on-pmdg-737-here/

In fact, flightdeck2sim, another real life 737 pilot, said that the PMDG 737 for MSFS handles and performs better than the PMDG 737 for P3D on his Youtube live stream yesterday.

I would think real life 737 pilots know what they are talking about if they are commenting on the PMDG 737 in MSFS.

I know a lot of real life pilots who say an add-on is great…but as far as accuracy in many areas, it can be dismal. They’re either paid to say it, or accept it’s nothing like the real airplane but if it has the same layout, switches in the correct place, then it’s ‘amazingly accurate’. 
 

But that doesn’t mean it is. 

On 5/11/2022 at 7:25 AM, sd_flyer said:

I'm pretty sure no real pilot tried this  to deliver you correct result, so nobody would know. However there is history of experimenting and putting things to the limit. For example

https://www.youtube.com/watch?v=uM5AI3YSV3M

That’s not correct at all. Their take off data, from the testing, their programs they use for flap settings, V speeds etc would tell them if it could. 
 

A quick flip through google (not a super valid source I know)shows for Flap 5, standard day a 737-700 would be limited to around 125,000lb (57,000kg) using a 5000ft runway. 
 

Another source says at 154,000 lbs it needs 5500ft. 
And I’m guessing that’s at flap 5 as well. 

So heavier, at zero flap? It wouldn’t easily take off at all. You’d be hard pressed to believe 5000ft would be enough 

 

Edited May 12, 20224 yr by Doug47

On 5/11/2022 at 11:36 PM, sd_flyer said:

Here is another example. Ever heard about Bob Hoover? Yes it's impossible unless you of course Bob Hoover! LOL

 

Great pilot and good enough to not get near an assymetric spin or would have been lights out most likely

How would we even know?  Most of us are using basic or semi-basic flight controls, sitting in front of a monitor.  I quit chasing trying to get everything right from a feeling standpoint a long time ago...  There are too many user variables.

It is worth noting that any sensible dev will spend most of their time tweaking behaviour within normal flight parameters.

As a slightly silly example, spending weeks getting a 737 to behave accurately while looping, doing barrel rolls and climbing vertically like it's in an airshow would not be a good allocation of resources. It is unlikely many devs in civilian sims have put much dev time into perfecting inverted flat spin dynamics for example, whereas in a mil sim that may actually be important,

I'm sure this would be physically possible to achieve (albeit with a premature rotation), even with a 737 at EGLC..... but, it's not certified for EGLC on the basis that your V1 would have taken place too far along the runway length*.   That's what restricts certain aircraft at certain airports, rather than an absolute ability to be able to physically take off within the runway length

*Based on a sensible V1 speed.   Or.... for a V1 that leaves enough runway for a full stop before the end of that runway, your V1 would be around 70kts!   In other words, too impractically slow to make a true go/no go decision.

Edited May 13, 20224 yr by JYW

There's nothing impossible or even unreasonable about this. London City is a 5,000ft runway. That's only 800ft shorter than Burbank, and we take full -800s out of there no problem.  -900ERs too, though it probably would not work full. 

The -700 is the rocket of the 737 NG series.  If you happened to have activated the 27k engine option, even more so... because most airlines have 24k engines on the -700.

Taking off without at least flaps one (slats being more important than trailing edge flaps) would not help you, but sure it would probably still work).

Remember that in the airline world, data for every takeoff is planned with an engine failure at V1.  So you can think of any takeoff where an engine does not fail as having twice as much power as actually necessary. 

TL;DR - a -700 taking off of a 5,000ft runway is not even a stretch. 

3 hours ago, Doug47 said:

A quick flip through google (not a super valid source I know)shows for Flap 5, standard day a 737-700 would be limited to around 125,000lb (57,000kg) using a 5000ft runway. 
 

Another source says at 154,000 lbs it needs 5500ft. 
And I’m guessing that’s at flap 5 as well. 

So heavier, at zero flap? It wouldn’t easily take off at all. You’d be hard pressed to believe 5000ft would be enough 

Not enough if an engine fails at V1, which is what this data assumes. But if an engine does not fail...

28 minutes ago, Stearmandriver said:

There's nothing impossible or even unreasonable about this. London City is a 5,000ft runway. That's only 800ft shorter than Burbank, and we take full -800s out of there no problem.  -900ERs too, though it probably would not work full. 

The -700 is the rocket of the 737 NG series.  If you happened to have activated the 27k engine option, even more so... because most airlines have 24k engines on the -700.

Taking off without at least flaps one (slats being more important than trailing edge flaps) would not help you, but sure it would probably still work).

Remember that in the airline world, data for every takeoff is planned with an engine failure at V1.  So you can think of any takeoff where an engine does not fail as having twice as much power as actually necessary. 

TL;DR - a -700 taking off of a 5,000ft runway is not even a stretch. 

Andrew, we talked about needing real world data before. Perhaps you are in a position to provide that for us?

I’m 95%+ sure you’ll be fine, even at flaps zero. Let us know how it goes!

47 minutes ago, OzWhitey said:

Andrew, we talked about needing real world data before. Perhaps you are in a position to provide that for us?

I’m 95%+ sure you’ll be fine, even at flaps zero. Let us know how it goes!

Unfortunately not, because all our performance handbook tables and calculator assume the ubiquitous engine failure at V1.  I'm pretty sure there's no data I have access to that shows unfactored data with all engines operating. I'll take a dig through later though... maybe there is a table in there. We would just never use it.