Angle of attack and speedbrakes

[JammedStab]

Perhaps I should know better but I am having a bit of difficulty figuring out the why's for this statement in the 737 manual from Boeing. Anyone care to clarify for me?

"For any airspeed, the angle of attack is higher with the speedbrakes up. This increases initial buffet speed and stick shaker speed but has a lesser effect on actual stall speed"

[safetypee]

‘Speedbrakes up’ is essentially a new wing section, thus it will have different (degraded) lift characteristics. To achieve the same lift for a given speed the AoA will have to increase.

[JammedStab]

Thanks. It makes sense that more lift and therefore more AOA will be required to maintain the same airspeed. I guess I was reading it wrong and thinking that raising the speedbrakes immediately changed the AOA, whether at a fast speed or a slow speed.

[ampclamp]

Jammed stab.If the airflow across the wing changes (splrs flap etc) so does the lift characteristic.
Something must give and or change.

[PPRuNeUser0180]

Think of the chord between the wingtip and the tip of the spoiler when extended.
If you then draw the relative wind (a little under horizontal when in ground effect I'd suppose) you will see the new chordline will have a negative AOA when flying horizontal or at a slight negative angle.
The effect will be pressure buildup on the top surface of the wing, creating downforce.

[FCeng84]

The name "speedbrake" is really a mis-nomer as implemented on most transport airplanes. Extending the speedbrakes deploys surfaces on the wing that do far more to reduce lift than to increase drag. As such, the flight deck lever would be more appropriately named "lift dump".

As correctly noted in earlier posts above, deploying the speedbrakes reduces lift and requires an increase in AOA. The AOA increase is required to compensate for the associated lift loss. The AOA increase is not required to maintain airspeed, but rather to maintain flight path.

Pilots have long learned that a rapid change in speedbrake position in flight can be rather upsetting to the passengers. At cruise, full speedbrake deployment can result in dumping as much as half of the wing lift and thus rapidly changing normal load factor from the nominal 1g to 0.5g. Speedbrake deployment in air is best done slowly and in coordination with an increase in pitch attitude (i.e., AOA) to balance the associated lift loss. If speedbrakes are deployed at initiation of decent, the AOA increase can be accomplished by simply not pushing the nose over as far as would be needed for a clean wing decent.

Some airplanes include in the control systems automatic compensations to avoid any sharp flight path upset as a result of speedbrake deployment. This feature is provided on Boeing's fly-by-wire airplanes such that speedbrake extension is automatically balanced by the required pitch-up maneuver to maintain normal load factor. Rate limiting the speedbrake command also helps smooth the ride.

It is important to remember that when used on the ground, speedbrakes provide only a minimal increase in drag. The real benefit of speedbrakes on ground is to increase weight on the gear making wheel brakes more effective earlier in the landing or RTO roll.

[FCeng84]

The name "speedbrake" is really a mis-nomer as implemented on most transport airplanes. Extending the speedbrakes deploys surfaces on the wing that do far more to reduce lift than to increase drag. As such, the flight deck lever would be more appropriately named "lift dump".

As correctly noted in earlier posts above, deploying the speedbrakes reduces lift and requires an increase in AOA. The AOA increase is required to compensate for the associated lift loss. The AOA increase is not required to maintain airspeed, but rather to maintain flight path.

Pilots have long learned that a rapid change in speedbrake position in flight can be rather upsetting to the passengers. At cruise, full speedbrake deployment can result in dumping as much as half of the wing lift and thus rapidly changing normal load factor from the nominal 1g to 0.5g. Speedbrake deployment in air is best done slowly and in coordination with an increase in pitch attitude (i.e., AOA) to balance the associated lift loss. If speedbrakes are deployed at initiation of descent, the AOA increase can be accomplished by simply not pushing the nose over as far as would be needed for a clean wing descent.

Some airplanes include in their control systems automatic compensations to avoid any sharp flight path upset as a result of speedbrake deployment. This feature is provided on Boeing's fly-by-wire airplanes (777 and 787) such that speedbrake extension is automatically balanced by the required pitch-up maneuver to maintain normal load factor. Rate limiting the speedbrake command also helps smooth the ride.

It is important to remember that when used on the ground, speedbrakes provide only a minimal increase in drag. The real benefit of speedbrakes on ground is to increase weight on the gear making wheel brakes more effective earlier in the landing or RTO roll.

[spud3]

This increases ...stick shaker speed but has a lesser effect on actual stall speed"

The above from Jammed Stab's initial query. So...why would Boeing increase stick shaker speed to a greater degree than the change in actual stall speed?

[Pugilistic Animus]

For any airspeed, the angle of attack is higher with the speedbrakes up.

it will help to remember that aoa = the difference between where an airplane is going and where it appears to be going...so use of speed brakes to slow the plane means a higher aoa with decreased airspeed --or an steeper decent path.... both conditions perturb the flight path in such a way as to increase the effective angle of attack ---or if fuselage attachment angle is accounted for incidence...:)

[Brian Abraham]

Think of the chord between the wingtip and the tip of the spoiler when extended.
If you then draw the relative wind (a little under horizontal when in ground effect I'd suppose) you will see the new chordline will have a negative AOA when flying horizontal or at a slight negative angle.
The effect will be pressure buildup on the top surface of the wing, creating downforce.Helldogbe, not how it works at all I'm afraid. All I can suggest is a bit of study on aerodynamics, as it would consume too much space here to dispell the errors in your explanation.

[JABBARA]

http://i1222.photobucket.com/albums/dd499/Jabbara1/PagesfromA330-340FCTMREVDATE15062010MASTERR.jpg?t=1293072791

This is my first try to post an image, so it is too big:(
It is extracted from A 330 FCTM, and I guess explains everything

[StratMatt777]

Your image posted successfully.

To a previous poster (and everyone else here who knows more about flying airliners than I do) If the stickshaker is based on angle of attack instead of airspeed (I'm not sure if that is the case) that would explain why stick shaker occurs earlier with the spoilers up even though stall speed is not increased. It would be due entirely to the speed brake increasing angle of attack- not due to any decision made by Boeing to increase stick shaker speed even though stall speed is not increased.

Of course I have no idea what I'm talking about (I'm still a CFI due to the economy) because I don't know if the stick shaker is based on AOA or IAS. Wait a minute... why am I posting then if I am clueless? :oh:

Oh yeah- to tell you that your image did in fact post successfully! :ok:

[JABBARA]

In fact, approaching to stall is only measured by AOA; in Boeing case Stick shaker is triggered with AOA.

Airbus FBW normally can not be stalled, due to protection (Flight Control Computers do let you to fly close to AOA max but that is all, you cannot control airplane to fly at bigger AOA). However if Flight Control Computers (PRIMs, SECs as named for A 330) fails in some combination, then stalling is possible, but in this case stall warning is heard as "STALL, STALL".

[Castle Don]

Think there may be some terminology and theory disconnects here. Chord line from wingtip? And I don't think the AOA concept is correct.

No disrespect intended, just seeking well reasoned discussion.

[Brian Abraham]

Here is a patent filed by Boeing

The present invention is directed to a non-normalized aircraft angle-of-attack indicating system. An angle-of-attack vane produces a signal α' indicative of raw aircraft angle-of-attack. A pitot-static system (68) produces output signals representative of aircraft static pressure and total pressure, Ps and Pt, respectively. The signals α', Ps and Pt are fed to an air data computer (66) which correspondingly produces an aircraft actual angle-of-attack value α and a calculated Mach number, M. Configuration sensors (80) produce output signals representative of gear position, g, flap position, f, and speed brake position, sb. The signals α, M, g, f and sb are fed to a display computer (76). The display computer includes a submodule (90) which utilizes a lookup table to determine a value of angle-of-attack stick shaker, αss from the values of M, g, f and sb. A second submodule (92) utilizes a lookup table to produce a second angle-of-attack reference value αref from the values g, f and sb. The display computer (76) feeds the α, αss, αref signals to an angle-of-attack indicator which includes a pointer (102) which is moveable with respect to a fixed gauge (104). The α signal is used to drive the pointer (102) to indicate the aircraft's actual angle-of-attack. Bug drivers are utilized to position the first bug αss and the second bug, αref, at their appropriate positions on the opposite side of the gauge.

Aircraft non-normalized angle-of-attack indicating system - Patent 6131055 (http://www.freepatentsonline.com/6131055.html)

A fuller explanation may be found at Aircraft non-normalized angle-of-attack indicating system - US Patent 6131055 Description (http://www.patentstorm.us/patents/6131055/description.html)