"Ron Natalie" wrote in message
m...
wrote:
OK, but if spoilers "spoil" lift over the wing isn't that an
undesirable condition when low & slow such as a crosswind landing where
the upwind wing is dropped?

What do you think an aileron does in the same situation?

The difference is one of degree, as well as of mechanism.

To address the previous question...

For a given roll rate, a certain amount of force needs to be applied.
Ailerons provide this force in a balanced way, by increasing the angle of
attack on one wing and reducing it on the other. A spoiler, rather than
reducing the angle of attack, simply disrupts the airflow and reducing lift.

So, compared to ailerons, the spoiler is more like using only a single
aileron on the opposite wing to induce the roll (which would put that wing
closer to the stalling angle of attack).

Of course, that all ignores a bunch of other aspects of the situation.
Ailerons, even as they increase the angle of attack, also change the shape
of the wing, which can change the stalling angle of attack. Also, many
ailerons are configured for asymmetric deployment, to help reduce adverse
yaw; the aileron on the down-going wing is deflected more than the aileron
on the up-going wing.

Another element that is actually in favor of spoilers (the previously
mentioned ones are, IMHO, pro-aileron) is that if for some reason deployment
of the spoiler should cause a stall, all that will happen is that the wing
going down will go down faster. The pilot's instinctive response will be
just fine, as reducing or reversing aileron input will simply restow the
spoiler, allowing that wing to generate the required lift again. Contrast
that to slow flight with an aileron, in which not only does the wing stall
induced by aileron input cause a reversal of the rolling moment, but the
pilot's natural reaction would be exactly opposite that required.

Of course, being able to put full-span flaps on a wing also helps compensate
for any potential risk of a stall due to roll control input. It seems
likely to me that, given the situations in which spoilers are often found
for roll control, it's a net gain for the spoilers.

One should not forget that in reality, at typical approach speeds (even
those required for a short-field approach), one ought not to be close enough
to risking a wing stall due to roll control input, whether it's spoilers or
ailerons that are being used. Obviously, spoilers work "well enough" and in
fact in many cases may be a superior solution to ailerons.

Pete