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Stall speed question

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So I read the other day that the minimum manouvering speed of the aircraft in clean conditions was around 210kts (depending on payload etc of course, but in round figures). And doing a full flaps landing, VREF seems to usually be around 130 kts (again, very generalized). Now, I decided to play around a bit and see how the 737NGX behaved in a stall, so I took of from the runway and initiated a pretty steep climb and retracted the flaps. To my surprise I was able to maintain a steady climb without stalling at 130kts IAS, without any flaps, even making turns at about 30 degrees of bank without a problem. Is this really realistic? I mean, this is VO - 80 kts, or am I missing something here?


Peter Palotas

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If you're in the air you should be stalled and falling at 80 knots.


Kenny Lee
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I have also wondered about those stall speeds to. Once I was able to land the 738 (with passengers and over 70% fuel) at around 90 knots without stalling.


Jon Alexander.

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Landing is a bit of a different matter, you'd be in ground effect which would not only reduce the drag but also give you something of a cushion of air to ride upon, although at 90 knots I should think it would have been a miracle if that phenomenon had kept a 737 in the air for long, although if you approached at a much higher speed and bled off speed at low altitude, merely touching down at 90 knots, it's not wildly beyond what might just about transpire when in ground effect.

 

If you check out the details of the crash at Schiphol of Turkish Airlines Flight 1951 (which was a 737-800 NG), you will note that its speed dropped to 83 knots at one point, and it was doing about 95 knots when it hit the deck in a tail down attitude, having been at around 145 knots on the approach. That was of course by no means a gentle landing, since nine people were killed in the impact, including all those on the flight deck, and there were a total of 86 injuries among the 126 people who survived, 36 of those injuries being very serious indeed, which is understandable in an impact which broke the fuselage into three sections and tore off both engines.

 

But 40 people walked away from it almost completely unscathed, apart from shock obviously, so what it does tell us, is that a 737-800 NG is not necessarily going to plummet out of the sky and drill a crater in the Earth if it goes a lot slower than the recommended flaps approach speed. Sure it will start going down, but most probably not in a vertical nosedive.

 

Clearly one should not be able to climb an NG like a rocket at 130 knots with no flaps, but to imagine that would mean such a speed and configuration would make you drop like a grand piano with an anvil strapped to it is probably not the case either. So what is going on? Well, this is...

 

The explanation in relation to the performance of the PMDG NG in FSX, is that FSX is a great flight simulator, but not necessarily a great stall simulator. One can indeed spend a lot of time programming in stall and spin behaviours for FSX aircraft, and there are some add-ons which feature that; but PMDG did in fact state prior to the release of their FSX NG, that they did not want to, nor ever will want to make a crash simulator. As such, the PMDG NG is designed to behave like a real NG when one is flying it like a real NG should be flown, but if one goes well outside that criteria, then all bets are off. There are just some things which FSX does not simulate very well, and crashing is one of them.

 

So nothing is really amiss with the PMDG NG; it's just that it isn't something one could use to make crash investigations with, because it is for simulating normal flights, and it will exhibit the correct behaviour when you do that.

 

Al


Alan Bradbury

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So I read the other day that the minimum manouvering speed of the aircraft in clean conditions was around 210kts (depending on payload etc of course, but in round figures). And doing a full flaps landing, VREF seems to usually be around 130 kts (again, very generalized). Now, I decided to play around a bit and see how the 737NGX behaved in a stall, so I took of from the runway and initiated a pretty steep climb and retracted the flaps. To my surprise I was able to maintain a steady climb without stalling at 130kts IAS, without any flaps, even making turns at about 30 degrees of bank without a problem. Is this really realistic? I mean, this is VO - 80 kts, or am I missing something here?

A few times I have forgotten to set takeoff flaps in the NGX, and took off without any drama, following the correct V1, Vr, V2 schedule for the flap position I forgot to select. I doubt FSX models this particularly well. All an addon designer can do is make sure the aircraft flies well when in the correct config for the speed.

 

Vref speeds are 20% or 30% above stall speeds, so if you climb at 130 knots the stall speed (with takeoff flap) might actually be 100. So even with flaps up it might still be above stall speed. Don't forget that in a climb the engines are at high thrust, some of which acts in a vertical direction (due to climb flight path angle) and so adds to vertical lift. It would be rather different flying straight and level at flight idle.


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. I doubt FSX models this particularly well. All an addon designer can do is make sure the aircraft flies well when in the correct config for the speed.

 

 

Ever flown an accusim bird?

 

But I doubt there is much data for accellerated stalls for a 737NG out there haha


Andrew Greentree

 

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The explanation in relation to the performance of the PMDG NG in FSX, is that FSX is a great flight simulator, but not necessarily a great stall simulator. One can indeed spend a lot of time programming in stall and spin behaviours for FSX aircraft, and there are some add-ons which feature that; but PMDG did in fact state prior to the release of their FSX NG, that they did not want to, nor ever will want to make a crash simulator. As such, the PMDG NG is designed to behave like a real NG when one is flying it like a real NG should be flown, but if one goes well outside that criteria, then all bets are off. There are just some things which FSX does not simulate very well, and crashing is one of them.

 

Well, I fully understand that what happens around and when the aircraft actually stalls is not modelled correctly in FSX. And I don't expect this behavior to be realistic. Not that I would have any idea as to what would actually be realistic or not anyway. But I don't understand why the speed at which the stick-shaker sets in and the airplane starts to lose lift couldn't be modeled correctly. Clearly this must be a different thing from modelling the actual stall, no?

 

Of course, maybe this is realistic, maybe the 737-800WL with a fairly light load is able to climb steadily with engines at climb thrust, clean configuration (no flaps/slats) at 130 kts IAS? I just didn't expect this to be the case. Is there anywhere one might find out what the actual stall speed of the real aircraft is in a particular configuration, or is this information just not availble?


Peter Palotas

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Ever flown an accusim bird?

 

But I doubt there is much data for accellerated stalls for a 737NG out there haha

Accelerated stall doesn't need data as such, it should be inherent in any flight model. Under g loading the aircraft needs more lift, and so higher AOA, but will stall at the same AOA as in 1g flight.

 

As for Accusim, that is mainly about special effects such as detailed engine and systems models, sounds, etc. Surely PMDG systems simulation is on a par with anything A2A has produced, and no Accusim aircraft I kow of have LE devices to consider. It's these that make the most difference to stall behaviour comparing clean with a takeoff configuration.


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From pprune:

 

B737-800 stallspeed, flaps up, gear up and forward center of gravity at 10.000 feet:

@42.000kgs (empty) = about 126kts

@75.000kgs (MTOW) = about 174kts

 

Best Regards,

Bert Van Bulck

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From pprune:

 

B737-800 stallspeed, flaps up, gear up and forward center of gravity at 10.000 feet:

@42.000kgs (empty) = about 126kts

@75.000kgs (MTOW) = about 174kts

 

Best Regards,

Bert Van Bulck

 

I will have to verify the exact weight I was using in my test, but a stall speed of 126 kts pretty closely matches what I experienced, perhaps mine was a tad bit higher. I never thought that the stall speed would be that low, but if this is so then I'd say the PMDG NGX seems to be pretty much spot on. :) (Why would I ever suspect anything else anyway?) Thanks for finding those numbers!


Peter Palotas

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A few times I have forgotten to set takeoff flaps in the NGX, and took off without any drama, following the correct V1, Vr, V2 schedule for the flap position I forgot to select. I doubt FSX models this particularly well. All an addon designer can do is make sure the aircraft flies well when in the correct config for the speed.

 

Vref speeds are 20% or 30% above stall speeds, so if you climb at 130 knots the stall speed (with takeoff flap) might actually be 100. So even with flaps up it might still be above stall speed. Don't forget that in a climb the engines are at high thrust, some of which acts in a vertical direction (due to climb flight path angle) and so adds to vertical lift. It would be rather different flying straight and level at flight idle.

 

It's probably worth mentioning that aircraft can take-off without flaps. The Spanair flight (5022) which crashed on TO from Madrid crashed pretty much because they didn't extend flaps/slats and then didn't recognise when the aircraft was beginning to stall. If I recall correctly though, it was "common practice" for some MD-82 pilots to pull the circuit breaker for the take-off config warning (which caused the crash on another flight, NW255) so I don't doubt for a minute that it's the first time the respective pilots actually took off forgetting flaps. It's a pretty serious thing to attempt, but providing every other aspect of the take-off goes to plan, it isn't necessarily going bring the aircraft down.


Luke Harvest

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B737-800 stallspeed, flaps up, gear up and forward center of gravity at 10.000 feet:

@42.000kgs (empty) = about 126kts

 

Okay, so at a gross weight of 43.5 I was able to maintain a climb at 111 kts at 25.000 feet, and around 125 kts at 41.000 feet. So I guess this is a little lower than the figures you posted, but not as much off as I thought it was. And I don't know but maybe the PMDG numbers are more accurate. This plane really can fly pretty slow without stalling. That's good to know. :)


Peter Palotas

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fsx limits are too much.

If you want to see correct stall values you must use autothrottle.

To do it:

DO NOT SET a GW on the fmc.

This will prevent the MCP IAS field to be locked to the minimum manouvering speed.

Now, start to fly, level the aircraft, let it stabilize, then in alt hold, and with SPD active start to reduce speed. do it at little steps when approaching the maximum AOA (use the AOA indicator or activate it from the options) reach the limit speed and just lower a knots or 2, the plane will start to shaker, if you continue it will correctly start to descend.

If you try in a different way, you will be not able to see correct reaction due to the fsx limits.

Stall is not speed dependent, is a AOA dependent, use it to check if you're in a stall condition.

However the shaker will correctly work only as described above, in other conditions it will engage too late.


Regards

Andrea Daviero

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It's probably worth mentioning that aircraft can take-off without flaps. The Spanair flight (5022) which crashed on TO from Madrid crashed pretty much because they didn't extend flaps/slats and then didn't recognise when the aircraft was beginning to stall. If I recall correctly though, it was "common practice" for some MD-82 pilots to pull the circuit breaker for the take-off config warning (which caused the crash on another flight, NW255) so I don't doubt for a minute that it's the first time the respective pilots actually took off forgetting flaps. It's a pretty serious thing to attempt, but providing every other aspect of the take-off goes to plan, it isn't necessarily going bring the aircraft down.

I wasn't trying to say it should not be possible, some aircraft are certified for no flap takeoffs in fact, but usually these aircraft have no LE devices. However if you takeoff in an aircraft like an MD-82 or a 737 without LE slats deployed, and you rotate at the planned speed and to the planned attitude you will be in big trouble, in real life at least. You will certainly be on the verge of a stall. LE slats/flaps don't increase lift, they increase the AOA at which a stall will occur. This will be about 14 degrees with no LE devices, which is not much more than typical takeoff attitude, AOA will be slightly less than the pitch angle, but safety margins will be reduced to nothing and the stall warning will almost certainly be triggered. When I did my inadvertant flapless takeoffs I didn't get stall warning, but this is an FSX issue rather than a PMDG issue.

 

fsx limits are too much.

If you want to see correct stall values you must use autothrottle.

To do it:

DO NOT SET a GW on the fmc.

This will prevent the MCP IAS field to be locked to the minimum manouvering speed.

Now, start to fly, level the aircraft, let it stabilize, then in alt hold, and with SPD active start to reduce speed. do it at little steps when approaching the maximum AOA (use the AOA indicator or activate it from the options) reach the limit speed and just lower a knots or 2, the plane will start to shaker, if you continue it will correctly start to descend.

If you try in a different way, you will be not able to see correct reaction due to the fsx limits.

Stall is not speed dependent, is a AOA dependent, use it to check if you're in a stall condition.

However the shaker will correctly work only as described above, in other conditions it will engage too late.

This is true of the real aircraft too, it isn't an FSX limitation. If you pull too fast into the stall the stall speed at which the shaker activates will be higher than expected. In FSX you don't need to use autothrottle to demonstrate stall speed, in fact I'd recommend doing it with A/T off (and AP off) and with thrust at idle. Slowly increase elevator input, aim for the classic 1 knot per second deceleration for stall entry and don't retrim once airspeed is less than 1.3 Vstall. Don't try and maintain altitude, you are bound to lose height with idle power.


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