Increased stall AOA with speed brakes extended

[hawk37]

In the recent thread "How does altitude affect the aircraft performance", John Tullamarine posted the link below to a Boeing article on AOA. An excellent read, BTW, many thanks

http://http://www.boeing.com/commercial/aeromagazine/aero_12/attack_story.html#figure1 (http://www.boeing.com/commercial/aeromagazine/aero_12/attack_story.html#figure1)

One of the figures, figure 2, shows that if speed brakes are extended, the stalling aoa is actually GREATER than that without speed brakes deployed, shown here

http://http://www.boeing.com/commercial/aeromagazine/aero_12/attack_fig2.html (http://www.boeing.com/commercial/aeromagazine/aero_12/attack_fig2.html)

The text says "Wing-mounted speed brakes or spoilers have the opposite effect. They reduce the lift at a given AOA; they also reduce the maximum lift achievable but, surprisingly, increase the AOA at which stall occurs."

Anyone able to provide an explanation of why a wing stalls at a higher angle of attach with speed brakes deployed?

[vilas]

Flaps decrease the stalling AOA because AOA relative to the chord with flaps down is greater. May be spoiler since they are on top of the wing do the opposite.

[hawk37]

Hi Vilas

"AOA relative to the chord with flaps down is greater"

I suppose you mean at the same aircraft pitch angle?

I used to think so too, however Boeing, and I think all in depth aerodynamics discussions, take the aoa from a fixed reference line on the aircraft axis, which does not change when one changes the configuration.

The same article in the first link states:

"Angle of attack (AOA) is the angle between the oncoming air or relative wind and a reference line on the airplane or wing. Sometimes, the reference line is a line connecting the leading edge and trailing edge at some average point on the wing. Most commercial jet airplanes use the fuselage centerline or longitudinal axis as the reference line. It makes no difference what the reference line is, as long as it is used consistently"

thanks for the response.

[blind pew]

I would imagine that the speed brakes act like vortex generators which re-energise the boundary layer. (possibly a venturi effect along the ends?).
Without teaching anyone to suck eggs...this delays airflow separation.

I attached vortex generators in front of the ailerons on a 1960s glass glider which increased roll rate at low airspeeds.
I also flew another "hot" ship which had them mounted on both sides of the fin.

What was never explained to me was why it was not allowed to use wing mounted brakes with flap? (HS 121/ VC10 DC9/10).

[hawk37]

Blind Pew,

yes that seems to make sense, delaying flow separation. Good response

[Meikleour]

blind pew:

With the B707 we were always told it was because of the level of vibration set up on the tailplane. Flap extended speeds are relatively slow hence more reason for the vortices to hit the stabiliser.

[blind pew]

Thanks ....someone once murmured structural integrity to me - rather unconvincingly - which I thought was BS.... I could buy the 707 story although I recently asked an ex tech flight mgr from the fleet...he appeared deaf.

I have witnessed some combinations outside SOP on various aircraft which didn't exhibit any strange behaviour.

Personally it could have been that some of the early jet jockeys found the transition that difficult that the combination frightened them f@rtless and the myth was continued.

My first jet we had to be fully configured for landing by 3,000 feet. My last four 400ft.

Took some guests over to Howth head saturday and whilst I was pointing out the islands watched 1/2 dozen jets on finals....all had the gear stowed bar my old employer....obviously a red, white and blue bus has to be flown differently to a green one or a 73 with a harp on it's tail ;)

[Goldenrivett]

Hi blind pew,
someone once murmured structural integrity to me - rather unconvincingly - which I thought was BS.
After this B707 accident:
1977 Dan-Air Boeing 707 crash - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/1977_Dan-Air_Boeing_707_crash) it was considered prudent not to use speed brakes during normal flight, in an attempt to reduce the fatigue loading of the stabiliser spar.

[blind pew]

I live and learn but it makes sense.
Interestingly (to whom?)
I drove the LAK test pilot in South Africa just after the Worlds a few years ago and asked him about his bad limp.
He had done all of the pre production test flying in the Lak 17 and two years later, whilst doing a VNE beat up across the airfield, the wing fluttered and dumped him and the wreckage onto the ground at 125 knots.
At the time they were using Soviet produced carbon fibre and although the strength was similar to western material the accident revealed that the material fatigued much faster. Hence the wing flexed, fluttered and failed.

For those of you not familiar with the gliding scene - they are at the forefront of fixed wing development and experimentation. Most of this was led by the German Universities Atafliegs...composite gliders in the 60s...laminar flow wings...winglets; their electronic kits were similarly advanced as is the investigation into the atmosphere and it's effects. Probably the most extreme are mountain flying - soaring in the rotor; and dynamic soaring as practiced by the albatross (the bird and not flavoured for the Python fans):ok: