Flaps & stalling
 

Just a thought here guys,

I was reading a book titled "Flight Automation", (which i've since added to my christmas wish list) and at the beginnning of the book, it discusses the theory behind drag, lift and angle of attack etc etc...

We all know that when you increase the AOA, that you produce a creater lift coefficient and so climb etc etc, very basically. However more regarding a stalling situation:

When the critial angle of attack (anlge between the relative airflow and the mean chord line of the wing) is met and exceeded, the aircraft will stall.

If you maintain an angle of attack close to the crittical angle of attack, same powerr setting etc, will the aircraft stall if you were to abruptly lower flap?

Technically this increases the mean chord line angle and so i think it should. What do you think?

I by no means accept responcibility for pilots who attempt this! lol

Regards,

Antony

I can't be 100% sure as I'm only part-way through my flight mechanics course, but as I understand it the critical wing stalling angle is constant for a given wing profile or cross-sectional shape.

If you extend the flaps then yes, the mean chord angle relative to the oncoming airflow will change. However, the cross-sectional properties will also change (as that's the whole point of flaps) so the stalling angle would also change. I'm not really sure, but that's my tuppence anyway. Oh and I think there's a difference between wing stall and aircraft stall. Talk to me in 6 months time :p

Well,

by lowering flaps you increase camber, resulting in an increase of Cl / Cd while AOA crit decreases. At the same time, AOA is increased due to the change in chamber, as Antony already mentioned. So logically the wing would stall.... (Two causes leading to a stall)

That's my two cents but I might still be wrong...

If you were to ABRUPTLY apply full flap whilst maintaining the same pitch attitude, YES, the aircraft will stall. (You'll need some good degree of remaining elevator authority to achieve this).

Stalling a 'Clean' wing and a 'Flapped' wing will be at approximately the same angle of attack, hence, with Flap applied, the Pitch Attitude at the stall will be LOWER.

Regards,

Old Smokey

Hey Smokey,

so the decrease in AOA crit is considered to be insignificant?

microburst8265,

The decrease in AOA crit is VERY significant. If the pitch attitude, which is just below stall in the clean configuration, is MAINTAINED, the decrease in AoA crit as compared to the existing pitch attitude is the cause of the stall.

I have tried the very manoeuvre described here, very difficult to accomplish unless you have sufficient elevator authority to overcome the pitch down produced by lowering the flaps. When I was younger and much more stupid than I am now, I investigated stall speeds when inverted with Flaps retracted and Flaps Down (or is it Flaps Negative Down when you're inverted), oddly enough, it didn't seem to make any difference. Not too inclined at my age to investigate it further.

Regards,

Old Smokey

Ok, thanks for clearing that up...
I misunderstood your comment about the clean and flapped wing stalling at approximately the same speed.

cheers

btw: what type of a/c were you flying back then?

Hi microburst8265,

My main reply related to stalling the 'right way up', i.e. normal flight.

The inverted flight footnote related to flying in a Chipmunk (DHC1), and it still remains a mystery to me why, with or without flaps, the stall speed was the same when inverted. By all reasoning the stall speed should have been higher (when inverted), with the flaps Down (or Negative Down). All of the Inverted stall speeds were noticeably higher than for normal flight, which would be expected. Another Chipmunk oddity was that recovery from an inverted spin was inevitably faster than for 'right way up', perhaps because the fin and rudder received direct 'unblanketed' air flow.

Regards,

Old Smokey