"George Vranek" wrote in message
...
Does somebody know, how works its spoiler roll controll? Is this machine
suitable for a low time pilot?

George


I haven't flown the American Eaglet but I have flown another glider with
spoiler roll control. I also had the opportunity to discuss the AE's
handling with a pilot who had flown one. I wouldn't recommend it.

Spoilers, as roll control devices, have several problems when applied to
gliders.

First the basics. Ailerons produce roll and adverse yaw. (Note, "adverse"
used in this context doesn't mean "bad", just opposite). Roll spoilers
produce roll and "proverse" yaw meaning "in the same direction".

One would think that roll spoilers with their proverse yaw would be perfect
in that no rudder coordination would be required. However, the perfect
ratio between the roll produced and the yaw produced only happens at one,
fairly low airspeed called the auto-coordination speed.

Spoilers produce roll by spoiling lift on one wing but not the other.
However, the lift available to be spoiled is essentially constant over the
full speed range i.e. lift equals weight which is constant. The result is
that the roll authority is essentially constant over the full speed range.
On the other hand, the drag produced by a spoiler increases with the square
of the airspeed.

The result is that above the perfect auto-coordination airspeed, spoilers
produce a skidding turn. At high speeds, the skid is extreme and full
out-of-turn rudder may not be enough to keep the yaw string centered - not
good. In fact, it's really scary.

Further, in a thermalling turn roll spoilers make it difficult to hold off
overbanking. A bit of top aileron will just hold the bank where you want it
and the "adverse" provides yaw into the turn - meaning little rudder is
needed to fly perfectly coordinated circles.

To hold off overbanking with spoilers, the top spoiler has to be kept
deployed which creates an out of turn yaw, which in turn, requires
into-the-turn rudder, which produces still more overbanking, which requires
still more top spoiler... Trying to thermal with one spoiler open is not a
good thing.

So, substituting spoilers for ailerons is an all-around bad thing. But,
thinking about this a bit more, what differential spoilers really do is
replicate the rudder, not the ailerons. In fact, differential spoilers
connected to the pedals work like a MONSTER rudder which would be VERY
useful on a big flying wing. I've been trying to convince my friend Jim
Marske of this for 35 years.

Note that one thing I didn't say is that differential spoilers produce more
overall drag than aileron with rudder used to oppose adverse yaw. In fact,
it's about a wash with a slight edge to the spoilers. The drag of deflected
ailerons + deflected rudder used to oppose adverse yaw while rolling into a
thermalling turn is quite high. Roll spoilers actually produce slightly
less drag in the same situation.

Bill Daniels