On 19 Aug 2005 14:33:29 -0700, "5Z" wrote:

Once it starts moving, even slowly, the wing is producing lift. The
more lift is produced, the more difficult it is to cause a wing to
drop, as this lift distributed over the whole wingspan damps a tendency
to roll.

Eliminate this damping by opening the spoilers and now the feeble
aileron is able to cause a roll.

... text deleted ...

-Tom


This topic of why opened spoilers assists low speed roll control
really is one of those questions that comes up again and again. From a
post 5 years ago on r.a.s.

Bob

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From: Bob Gibbons )
Subject: Dropping a wing on takeoff
Newsgroups: rec.aviation.soaring
View: Complete Thread (63 articles) | Original Format
Date: 2000-11-02 21:18:53 PST


This topic, that is, why deploying spoilers seems to help roll control

at low speeds, has come up often in RAS during the
years. Unfortunately, we seldom seem to get responses from
knowledgeable aeronautical engineers as to the real cause for what is
generally acknowledged as a real effect.


Earlier today I asked Dick Johnson the cause of this effect. I will
try to summarize his reply, but I may miss some of the finer details,
so any aerodynamicists, feel free to correct me. BTW, for those who
are not familiar with Dick credentials, in addition to his long
history of contest successes, Dick is a professional aerodyamicist
with 50+ years of experience.


Dick's response was that the effect of increased roll effectivness
with spoiler deployment is real, and is caused by a 3-dimensional flow

effect initiated by the spoiler deployment disturbing the normal low
pressure area over the top of the wing in the area of the
spoilers. The breakup of the normal low pressure in the spoiler area
results in a lateral spanwise flow in the airfoil forward of the
ailerons. This lateral flow modifies the normal pressure distribution
on the airfoil forward of the ailerons and allows the airfoil in the
aileron section to operate at a higher angle of attack (without flow
separation, i.e., stalling) than would otherwise be possible, thus
allowing greater aileron effectivness during the takeoff roll.


The effect is more pronounced in standard class gliders since without
the benefit of flaps to modify the airfoil camber, standard class
airfoils are often set on the fuselage at a higher angle of attack
than the corresponding airfoil on a 15m flapped ship. Furthermore, 15m

ships, with interconnected ailerons, can decamber their airfoil
(select negative flap) and generate increased resistance to flow
seperation at the ailerons during the early ground roll.


Hope this helps, and, as I said, this is an restatement by a
non-professional, corrections by practicing aerodynamicists welcome.


Bob

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