Why does B737-800 speed brake/spoiler use cause nose to pitch up

[jparnold]

I have noticed that whenever I use the speed brake/spoiler on the standard B737-800 the nose always pitches up regardless of the fact that the aircraft speed is greater than the autopilot speed setting.

Whenever I fly in real aircraft (and can see the wing) and notice the speed brake/spoiler being deployed to reduce speed and/or reduce altitude the nose does not pitch up.

Sure raising the nose will cause speed to be reduced but this action (in FSX) seems incorrect.

Am I missing some point?

[il88pp]

Spoiler spoils airflow over wing. Last part of wing does on longer give lift.

Result:

plane will descend.

Also:

extra turbulence behind wing gives drag and slows plane.

Also, weight of back of plane is not lifted up so much any more-->pitch up.

 

----------

In your case:

autopilot is on, holding altitude.

Slower while holding altitude means pulling elevator trim up to maintain altitude. Result: nose up.

 

Same as when flying manually, closing throttle, and pulling stick back to maintain altitude.

Nose will point up. Etc.

[mabe5454]

The autopilot is compensating what you did when bring the spoilers up. Normal.

 

Cheers,

[mallcott]

Aerodynamics. Simple.

[ScatterbrainKid]

Does it also nose up if the spoilers are popped when the autopilot is switched off?

[dspaulding]

Some jets have a function when speedbrakes are deployed the elevator trim automatically goes nose up about 1 unit from current position maybe more. Regardless of autopilot being used or not.

[jparnold]

Thanks.

I didn't explain myself enough.

Firstly yes I fully understand what the spoiler does to the airflow and the results however the nose pitch up even occurs when the autopilot altitude is LESS than current altitude and the aircraft is descending to set set altitude (and the speed is reducing to set set speed - that is why I am applying the spoiler to both make the aircraft reduce speed faster as in when ATC tells the pilot to reduce speed). Surely this is incorrect (the autopilot attempting to descend and reduce speed and the spoiler causes nose pitch up).

 

Just did a test with autopilot ON.

Altitude and speed set and aircraft at 9300 feet and 320

Altered altitude and speed to 2000 and 200

Aircraft begins to descend and slow.

Set spoilers on - nose pitched up even though the altitude and speed had only reduced to 8400 and 290.

No the nose doesn't pitch up when autopilot is OFF however why does the autopilot attempt to stop the descent by pitching up when the set altitude is much lower.

 

Also I would imagine that when I see spoilers deployed on real aircraft (and the nose doesn't pitch up) that the autopilot was still engaged (don't pilots use autopilot until close to landing?).

It doesn't make sense.

[il88pp]

You need to stop thinking ot the spoiler as a speed brake.

It kills the lift. That has concequences.

One is the plane descends. Another is the nose wants to pitch up.

 

What happens is above the spoiler panel a roll of air is created. (Imagine a wide roll of wire fencing material, laying parallel to the spoiler panel, on top of the wing. Only the roll is made of circulating air instead.)

This creates huge drag above the wing.

Now imagine grabbing that roll with a huge hand (is airflow pushing the roll back).

The engine is under the wing. The force of the engine, combined with the blocking on top of the wing, creates a rotation around the lateral axis. That means a pitch up moment.

 

But the main thing is: "all lift is lost". Youi fall from the sky. No autopilot can point the nose any where in that situation really.

 

Read up on some real worls articles about Lift and what a spoiler is and what it does. The lift is killed instantly.

 

 

You may ask: "Then why does it slow me down when 'driving' on the runway immediately after landing?" well, the cushion of air above the spoiler does create some drag. But the main thing at work is still the lift that is instantly lost.

That slows you, because without lift, the brakes are much more effective. Rolling friction of unbraked wheels increases too.

 

Really, thinking of a spoiler as a brake is wrong.

It does something very different from braking.

 

Actually, in flight the sequence is more: spoiler kills lift -- this causes nose to pitch up -- that slows you down.

[jparnold]

Thanks.

I am not a pilot and ready to learn as much as possible and realise that spoilers 'spoil' the airflow over the wing and disrupt the lift generated causing the aircraft to loose altitude. Also that spoilers are extended at landing (armed) to ensure that lift is lost to help prevent the aircraft lifting off again. I am aware what the control surfaces on a wing are there for and what they do.

 

I read with interest something written by a retired pilot and instructor -

https://answers.yahoo.com/question/index?qid=20130203073150AAb1ApW

 

I guess what is really what I am getting at is why does it seem that a REAL aircraft does NOT appear pitch up when I see the spoilers extended (I might be wrong with this but it is what I sense when I see the spoilers extended) yet it happens in FSX?

I realise that raising the nose slows the aircraft but once again never sensed that a real aircraft pitches up when the spoilers are extended.

 

I am only wondering if the FSX 'engine' is mimicking a real aircraft correctly or is it a setting in the aircraft.cfg file which affects it.

 

Thanks again for your replies.

[MBKHOU]

It may not be totally realistic with the default 737. I don't believe you see a massive

pitch up when using the speed brake in the NGX. But also, you have a flight detent

limit, which I'm not sure if that's the case with the default FSX 737. I can't remember..

 

But say if one were to use the full throw landing detent for spoilers while in flight,

which would be unrealistic vs real world, I imagine the plane would want to buck up a

bit... But say with the NGX, if I'm descending with vertical speed at a constant rate,

that rate will not change much if I add some speed brake.. If the FSX 737 does, I

doubt it's realistic as long as the amount of brake was within the flight detent range.

[jparnold]

Thanks

 

I just tried the Feelthere Embraer E190 and it either doesn't pitch up or if it does it is minimal (I usually don't use this aircraft as I find it difficult - waste of money for me).

 

I tried the default FSX B747 and it pitches up also just like the B737.

 

I wonder if it's in the aircraft.cfg file (COG or something else).

[mallcott]

I wouldn't gauge anything by a Feelthere aircraft. :pilot:

 

You have the aerodynamic reasons.

 

The operational reason is that it is generally regarded to be bad pilotage to deploy the spoilers in flight, so you've probably never been exposed to it in the real world. And the reason for that operating reason is because of the pitch tendency and rapid deceleration which might frighten passengers. Again, you have your answer, right there. :(

 

As for the reality of our sim, don't fully open the spoilers in flight - that's a weight on wheels limitation for most aircraft, so it sounds like you aren't using them right - you shouldn't be using them in flight anyway, but when you do you would normally already have disconnected the autopilot or auto-throttle and IF you do you wouldn't open them fully - they are a sliding control for a reason. Default control is a simple on/off button. Assign to a lever. :pilot:

 

In the absence of finesse, simply disengage the autothrottle and manage pitch manually or via the a/p to maintain an appropriate rate of descent for the speed. You don't `go down` and `slow down` at the same time. Not in any normal flight regime. In your poor pilotage above, you wouldn't be doing 320 kts at 9,400 ft anyway - speed limits are usually at around 250 kts IAS below 10,000 ft, and you would maintain that speed to the selected altitude, THEN slow the aircraft.

[jparnold]

Sorry your reply begs more explanation.

 

I NEVER as a rule 'go down' and 'slow down' at the same time and of course I wouldn't be doing 320 kts at 9400 ft anyway (isn't the max speed 'limit' under 10000 ft 250 kts? - well when using the FMS/FMC it automatically changes the autopilot set speed to 250 when descending through 10000).

 

Those figures were only used in a hurry to get the result - I should have thought about it more before placing the aircraft in that position anyway.

 

And yes when I have noticed the spoilers extended during a real flight it always is at level flight or after landing (but once again no pitch up which was the reason for the initial post).

 

I will have to try and work out how to assign the spoiler to a lever so I can have better control.

 

Thanks again

[dspaulding]

In real flight, a real good pilot doesn't fully extend the spoilers in the first place. That's why you don't notice the pitching up when flying in a real 737 or what ever Boeing you choose. Real pilots know the consequences of blowing out full deflection in level flight. It's not comfy. Unfortunately some aircraft you cant ease out the panels. So you have to compensate manually because the autopilot can't adjust fast enough.

[speedy_one50]

Just try the PMDG 738ngx and see diferrence with the standard one.

 

You can extend full spoiler any time, but a lot of passengers are gonna be in angry with you.

[mallcott]

Sorry your reply begs more explanation.

 

I NEVER as a rule 'go down' and 'slow down' at the same time and of course I wouldn't be doing 320 kts at 9400 ft anyway (isn't the max speed 'limit' under 10000 ft 250 kts? - well when using the FMS/FMC it automatically changes the autopilot set speed to 250 when descending through 10000).

 

Those figures were only used in a hurry to get the result - I should have thought about it more before placing the aircraft in that position anyway.

 

And yes when I have noticed the spoilers extended during a real flight it always is at level flight or after landing (but once again no pitch up which was the reason for the initial post).

 

I will have to try and work out how to assign the spoiler to a lever so I can have better control.

 

Thanks again

 

Better, learn how to fly without utilising the spoilers in flight, so you have better control.

[jparnold]

Great info.

I am going to assign one of my Saitek levers to control the spoilers.

[beany_bot]

Pretty simple really. The spoilers are on the top of the wing and extend upwards. Exactly the same as happens with the elevators when you pull back on the stick. or indeed exactly what the rudder does, all be it in a horizontal axis.

[MBKHOU]

Speed brakes are used a good bit on the 737's. It's better to avoid them as far

as not wasting fuel, but due to ATC mayhem they get used a good bit. I've seen

them use them a good many times on real world SWA flights.

Also have a RW video going into Buffalo, and I can hear them being used for a short

time before the last turn to final. You could see "drag required" on the CDU, and

then about a minute later I could hear the rumble which lasted a minute or two.

 

Myself, I try to avoid using them, but sometimes have no choice.

The 800's in particular are pretty slippery, and sometimes you need a little extra

drag to slow down. I'm a little less prone to need them with the 700, and even less

with the 600. The 900 is fairly slippery about like the 800 I guess..

[Stepolano]

Just did a test with autopilot ON.

Altitude and speed set and aircraft at 9300 feet and 320

Altered altitude and speed to 2000 and 200

Aircraft begins to descend and slow.

Set spoilers on - nose pitched up even though the altitude and speed had only reduced to 8400 and 290.

No the nose doesn't pitch up when autopilot is OFF however why does the autopilot attempt to stop the descent by pitching up when the set altitude is much lower.

 

The speed brake causes the plane to sink faster and the autopilot pitches the plane up in an attempt to maintain the set vertical speed.

 

For a fun challenged, see how late you can leave the descent and still make the runway with a speed brake descent.