Stalls in ground effect....

[hawk37]

I briefly read through an article on the Gulfstream G650 Roswell crash in a magazine. I'm pretty sure it said that an airfoil in ground effect will stall at a lower angle of attack than the same airfoil out of ground effect.

Can anyone verify this?

Is there a somewhat technical explanation that makes this understandable?

I thought I understood Aerodynamics 101.....maybe I didn't

Hawk

[Lyman]

I was taught to increase the angle of attack in ground effect to slow, and stay airborne until the Stall. Full Stall landing? Lowering the angle of attack increased sink. How does decreasing lift (lowering the AoA) accelerate a Stall?

[Magnetic Iron]

Sorry to change the direction of this thread but...

1) I really notice ground effect landing on hot days, interesting how heat effects the lift in ground effect, if anyone has the formula of the relation of heat in the lift equation ?

2) I was watching a cormorant flying today and was particularly impressed with the skill at which this bird uses ground effect flying so low to the water and then landing in the water.

Does anyone have any on how birds use ground effect ?

Cheers

[italia458]

Lyman...

He's saying that the AoA in ground effect, according to the article, is lower. Example: out of ground effect the critical AoA is 17 degrees, in ground effect the critical AoA is 14 degrees.

hawk37...

There is research that actually says ground effect will increase the stall angle and research that says it will decrease stall angle.

It initially makes sense that the stall angle will decrease in ground effect as the airflow is restricted from following the curvature of the airfoil and, as a result will experience separation at a lower angle of attack. This explains it a bit: Ground Effect (http://avstop.com/ac/flighttrainghandbook/groundeffect.html)

There are a lot of effects that happen when a wing is in ground effect. Boundary layer effects in ground effect are very complicated. Sometimes lift actually increases as the wing leaves ground effect. If a wing has a bit of a curved lower surface, this effect can be attributed partly to the venturi effect created by air flowing between the ground and the lower airfoil surface. That would create suction that would decrease the lift of the airfoil - lift would then increase as you got further from the ground, with the venturi effect being lessened.

This research paper produces evidence that stall angle increases in ground effect: researchbank.rmit.edu.au/eserv/rmit:6319/Walter.pdf

A couple quotes from the paper:

"In all aerofoils, the stall tends to occur between 10o – 30o AoA. The effect on the stall characteristic is clearly seen in the curve at 0.167 chord compared to the curve at 3 chord lengths. All aerofoils show the stall occurring later when the aerofoil is in close ground proximity compared to when outside ground effect."

"In the case of the Clark Y and 6-Series aerofoils, the lift force did not vary as expected prior to stall, with force decreasing with decreased ground clearance. The aerofoil with the Gurney tab displayed the typical characteristic of increased lift force with decreased ground clearance. All aerofoils displayed a decrease in drag with decreased ground clearance. The stall of the three aerofoils occurred at a higher AoA at close ground clearance than outside ground effect. Post-stall, the three aerofoils showed a decrease in lift and drag force with decreased ground clearance. Further study is required to determine how the ground simulation has affected these results. It is possible that the trends in the lift and drag data are a result of interaction with the ground boundary layer, and further study conducted in the field or with more accurate ground simulation may produce alternate findings."

[John Farley]

hawk37

Of course I do not know what article you read. You quote the word 'wiil' . I would not agree that. I would accept 'may'.

The differences in airflow round a wing in the presence of the ground plane as opposed the same wing with no ground plane are considerable and further may vary with the type of aerofoil section, the way the aerofoil may vary along the span, any twist in the wing, its planform and it's aspect ratio.

While many pilots look for (and even expect or insist on) a simple 'one answer that fits all', to may complex questions regarding aerodynamics the fact is they are likely to be hoping in vain.

Plus in this case we have not even discussed the position of the AoA sensor on the aircraft and the resultant effect of the ground plane on its reading!

One of the reasons all modern airliners have to demonstrate a takeoff during which the fuselage rear end has been dragged along the ground from as low a speed as the controls will allow it to be placed there is to demonstrae that you cannot accidentally stall the wing on the ground and prevent unstick. This requirement being a legacy of a Comet 1 accident back in the 50s.

JF

[Gremlin1991]

It is correct, an aero foil in Ground effect has a decreased downwash and resulting from that the induced angle of attack decreases, with a reduction of induced drag.So, with the decreasing of downwash/induced AoA the angle of the stall is decreased effectively in Ground effect... My FI always tells me to think of a pillow put under the wing in Ground effect, that makes the "falling" (respective: landing) easier for the pilot (with sufficient runway available ;-) )

[zerozero]

One of the reasons all modern airliners have to demonstrate a takeoff during which the fuselage rear end has been dragged along the ground from as low a speed as the controls will allow it to be placed there is to demonstrae that you cannot accidentally stall the wing on the ground and prevent unstick. This requirement being a legacy of a Comet 1 accident back in the 50s.

JF

Overall an excellent response, but I was not aware of that last bit at all. Very enlightening. Thanks for sharing.

[westhawk]

An interesting subject, the discussion of which might benefit from more information.

The full NTSB report (http://www.ntsb.gov/doclib/reports/2012/AAR1202.pdf) on the G-650 flight test accident in Roswell is quite comprehensive in it's analysis and an interesting read for anyone seeking to better understand what happened.

The pertinent section of the report as it concerns the difference between stall AOA in ground effect compared to out of ground effect can be found beginning on page 26 (sec 2.3) of the report.

Enjoy!

westhawk

[Lyman]

Italia 458, howdy...

Oh yeah. I was not tracking with the op.

The easy way for me to explain it to myself is this:

At a constant AOA, the airfoil produces more lift in Ground effect than above it.

"Effect"ive AoA is higher. So it stands to reason the airfoil will produce its maximum lift sooner, maximum lift is produced at the Stall, therefore, the AOA (Stall) is lower in GE...

I also like the "pillow" visual.....:ok:

[hawk37]

So much to read...

thanks Westhawk for the G650 crash link, a very informative piece of work.

and Italia too, you certainly have some technical subjects at your finger tips.

I think I grasp the concepts....if not the finer details

Hawk

[compressor stall]

Could you imagine the ATSB completing a report like that?

What's the aerodynamic equivalent of Lead Oxybromide?:}

[westhawk]

I think I grasp the concepts....if not the finer details

Yeah me too hawk37!

Anyway you're welcome and I hope you enjoyed all exquisite detail the board included in this one. Once you slog through all the detail and analysis a couple of times though, the conclusions reached appear reasonable.

The sequence of events leading to this mishap point right back to some flawed assumptions on the part of the flight test engineering department regarding critical AOA in ground effect. They assumed a decrement in critical AOA in ground effect which was less than the actual decrement applicable for this specific wing. The impression I'm left with is that the mistakes made to enable this accident were in large part driven by organizational pressure to meet the "runway performance guarantee" and to do it quickly. I've always considered Gulfstream to be pretty solid organization and it saddens me to see them fail in this way.

This event will serve as a cautionary tale for all future part 25 certification flight test programs regarding best practices in general and mitigating the corporate version of "go fever" in particular. In this respect, it strikes me that the analysis of this accident shares much in common with those applicable to the Apollo 1, Challenger and Columbia disasters in that "go fever" was a notable contributor to faulty organizational decision-making in all cases. In fact the risk/reward equation comes into play in any worthwhile pursuit and nothing will ever get done without some risk. One just hopes that lessons learned from the study of past events might be transferable to better planning and control of future endeavors and their associated processes.

And for ordinary pilots? Just don't go yankin' the thing into the air!

westhawk