flaps and stalling

[adverse-bump]

to save me getting the p of f notes out. can any one surgest why flaps make an ac more likely to drop a wing whilst stalling?

[ddmsabre]

Imagine trying to balance a ruler with 2 pieces of string. (close to the center of the ruler)

Now let the string go further towards each side. The further the piece of string move outwards, the easier it becomes to balance.

Swap Lift for the string, and wings for the ruler.

More lift is produced by the flaps near the center. Less on the outsides. When no flaps, it's a lot more stable.

Well, that's what i was told 2 weeks ago. I'm now having second thoughts...

[sir.pratt]

just guessing here but.....

with flap extended, the outer wing is much closer (if not already stalled) to the the stall than the wing forward of the flap.

ANY yaw or (even minor) aileron input at the stall with flap will more deeply stall the retreating or 'aileron up' wing (depending on the input - yaw or roll), create lift on the opposite advancing or 'aileron down' wing, and dropping the retreating wing accordingly.

[foxmoth]

In designing a wing you design it so the wing stalls from the root first out to the tip (washout, wing shape etc.) this helps stopping the wing dropping since if it stalls at the tip first you have a larger moment arm if one wing stalls before the other. If you put flap down then that part of the wing will not stall as soon as it otherwise would, since the flaps are normally positioned inboard for structural reasons this moves the point of stall outboard, increasing the moment arm and the chance of wing drop.

[rotorfossil]

Looked at simplistically, when the flaps are down, the chord line is from the leading edge to to the rear edge of the flaps. Therefore effectively the flapped area of the wing is operating at a greater angle of attack than the tips and therefore theoretically would stall first. In reality though, because the wing has now become in effect a more cambered section and you throw in the effects of flap slots, vortices off the edges of the flaps etc there isn't a simple answer.
Having stalled a lot of types in the process of doing flight tests for magazines, some stall the same flapped or clean, a slightly higher higher proportion do drop a wing more enthusiastically but it is not predictable by just looking at the wing. The whole airflow pattern is I suspect more complex than simple diagrams would suggest.

[jamestkirk]

The rule analogy is probably the best. Hold the ruler near the tips and ask someone to try a defelct the ruler in roll then it's difficult. Hold the ruler in the middle and ask someone to do the same, it is more likely to deflect.

So; As flaps are lowered, the C of P moves inwards (and back). Thus its a alot easier to defelect the aircraft from level flight. For example, the slipstream effect of the propeller hitting the fin and making the aircraft easier to wing drop, generally to the left in a C152

Or on finals with a crosswind where it would be easier for a gust to drop the wing with 2 stages of flap over three.

Generally, flaps reduces lateral stability along the longitudinal plane due to the inward movement of the C of P.

[Kit d'Rection KG]

...not to mention the variability of rigging, and the paucity of genuinely high quality air testing...

[adverse-bump]

cheers guys, i like the ruler idea from a teaching point of view!:)

[foxmoth]

The ruler explains the reduction in lateral stability but this also combines with the reduction in tendancy to stall at the root as I described which is what actually causes the wing to drop so as in many things it is a combination of factors, possibly including the rigging.

[pilgrim flyer]

Why not just go and rent a 152 and try it....?

FWIW I reckon that you get a greater tendecy to drop a wing with flap, but then usually with a stude one would have power applied when stalling in this cofiguration.

Which certainly gets the eyes of some bulging.

PF

[adverse-bump]

i fly the tomahawk, so i know wat a wing drop is! everytime u stall a wing drops, flaps jst make it drop quicker!

[old,not bold]

Flaps down, so stall speed lower, so less rudder authority, so more yaw likely, so outside wing drop more likely ...???

Just guessing, I really don't know the answer.....

[adverse-bump]

only 4kts diff between on the pa38. Vso 52kts vs1 48kts

[Islander2]

If you put flap down then that part of the wing will not stall as soon as it otherwise would,Foxmoth, I think you will find that's incorrect for most light aeroplane designs. Flaps tend to increase the lift coefficient for a given alpha, but reduce the max alpha available (although this isn't the case with some of the more complex Fowler designs). In other words, with flaps down the aeroplane stalls at a slighter lower angle of attack (typically 1 to 2 degrees lower). Therefore, with flaps down, the inboard sections of the wings should stall with a greater margin over the outboard sections.

My explanation would be that, with flaps down, a larger proportion of the wings' total lift is on the inboard sections and there is, therefore, a greater loss of lift in the early stages of the stall. This then exacerbates the rigging/asymmetrical issues that actually cause a wing drop.