737 Vortex generators/APU inlet
 

Heres a question I was asked recently but didnt know. "Why does the Boeing 737 have vortex generators at the APU inlet?"

Help will be much appreciated!!

Intresting question, are there any?

The massive 'bicycle seat' is supposed to slow down the local air flow and force feed air into the inlet to enable the APU to be started and used at any speed and altitude as far as I know - other aircraft types though seem to do without...

Edit: Oh wait, that was too obvious an answer - is that even what youre referring to? You specifically wrote generators... Are you thinking of the vortex generators scattered around the tail?

The "bicycle seat" is a vortex generator. It's a big one, and not shaped like we normally see them, but that's what it is. Edit: I may have read that in "Ace the Technical Interview." I'm looking for my source. I was surprised that it was called a VG, too.

The answer lies in SL 49-10 Dec 1977 “APU High Altitude Starting Improvement”. To summarise, Boeing added the vortex generator from April 1977 to improve inlet ram recovery and thereby improve the pressure difference across the APU during the flight. It goes on… “The vortex generator successfully reversed the former negative pressure difference across the APU so that aerodynamic cranking could assist the starter motor during in-flight starts.

The original VG did not work as well as expected so they added the trailing edge articulated flap which folds inboard 25 degrees when the APU is started and running. It is unfolded (and fared) when the APU is not running to reduce drag.

Boeing said that with these mods and a few others, the APU will start at 35,000ft after 3 hours cold soak.

S&L

Tail closure
 

I think the question was why all the vortex generators on the tail near the APU inlet. The 737 fuselage cross-section is the same as the original 707, a much LONGER A/C. As explained by my now long retired Boeing engineer father-in-law, the VG's were an afterthought, added to help smooth out the slipstream around the tail. Nothing to do with the APU. The 737 was meant to be an .80M aircraft but when it first hit the market, .72M was about all it could do before it started to buffet. Wind tunnel tests confirmed the buffeting above .72M was caused by the abrupt change of airflow at the tail. One solution was to add a stern fairing, but it would have to extend far beyond the tail, looked hideous and Boeing thought it would negatively affect sales. Hence the vortex generators. Note: a certain number of them must be present per the MEL.

Those are the Aft Body Vortex Generators.

Info and photos here

As said, it's to improve high altitude starting by directing airflow into the intake. Don't forget, most other modern types have retractable scoop intakes which force air into the APU at high altitude, which the 73 lacks. Why the NG didn't get one of those I don't know - probably a lack of space in the bay for the intake to retract into...

Thanks very much for the info guys, very helpful!!!