Why "Vortilons" on DC-9, MD-80, B717?

[ITCZ]

Anyone shed light on the real reason that DC-9, MD-80 and B717 have Vortilons on the lower wing surface?

I know that it is an aerodynamic fence. I also know that if damaged, it is a "no go" item. I have also heard the urban mythology about it being a legacy of DC-8 engine pylons for #2 and #3.

But I am curious... what is the purpose of an aerodynamic fence on the lower wing surface so close to the fuselage, at about 1/4 span?

[Capt Claret]

ITCZ

From Douglas Jetliners by Guy Norris & Mark Wagner.

The wing also featured "vortilons", small chord-wise fences on the lower wing surface that improved control at high angles of attack up to 30 degrees. In most attitudes, the vortilons were aft of the area where the airflow "stagnated," so they had little effect. However, when the aircraft was in a potentially dangerous, nose-up attitude, the vortilons poked past the stagnation point and triggered vortices. The vortices extend over the upper wing surface and limited the span-wise flow, thereby preserving lift on the outboard wing sections, so the inner wing would stall first. In a swept wing design, this makes the nose pitch sharply down, enabling the crew to recover control quickly. The vortilons also reduced the downwash from the wing on the tail, which helped crews recover from potential deep stalls.

I like the explanation given by one of the sim instructors, with years of DC9 time from TAA. During development the wing didn't develop as much lift as expected and some sprog aerodynamicist suggested that the lack of pylons for #2 & #3 probably allowed too much span-wise airflow, so that tacked on pseudo pylons, and, Bob's your uncle.

Remember, Google's your friend. "dc9 vortilons" = 5 pages of hits.

[safetypee]

IIRC the vortilons developed on the HS 125-800 (some of the first to be used?) were relatively small (also see Gulfstream a/c). The objective was to replace wing fences used on previous models, which minimised span-wise flow and tip stall, and predominantly maintained aileron effectiveness. The -800 used a larger wing than with previous 125s and the ailerons have ‘super-critical’ sections. The standard wing fence ‘fix’ on the -800 required more vortex generators just in front of the aileron hinge line; the combination added drag.
The vortilon solution had less drag than the wing fence and required fewer vortex generators. Furthermore (as above) there were advantages at low speed (lower stall speed?), and possibly with high speed cruise performance.
There was only one set per wing, whereas other aircraft have several. Photos: Raytheon Hawker 800XP Aircraft Pictures | !!!!!!!!!!!!!!

[ITCZ]

Cheers, Clarrie and safetypee. Yes, had done the google but had found nothing specific to the DC-9 wing that was from an authoritative source. Plenty of discussion of isobars and stagnation points in aerospace.org and some nice contributions on !!!!!!!!!!!!!! forum re the DC-8 wing.

Taking a look at the vortilon on our machine, it makes sense w.r.t. above discussions of leading edge attachment line. "Preserving lift on the outboard wing sections, so the inner wing would stall first" also makes a lot of sense.

I was curious as to why it was placed at 1/4 span, rather than further from root/closer to tip.

Am now reading DC-9 aerodynamicist Dick Shevell's article on 'his' wing.

[Flight Detent]

I seem to recall seeing them on the outboard couple of leading edge slats on an NG!

FD...

[rigpiggy]

My understanding was that it was simply a scaled down DC8 wing, they found some unsatisfactory stall behaviour and went back to the 8 to understand the engine pylons were acting as stall fences, they added dogtooths to the LE at similar stations and problem solved.