Hands off thermalling

The aerodynamics of this are interesting.

The most likely reason that a glider exhibits the spiral stability to fly
thermals hands-off is that once it is set up to fly coordinated circles and
the controls released, the controls have just enough friction to keep the
aileron and rudder controls where they were - just a bit of outside aileron
and rudder. If you lube the controls, you may lose the hands-off
thermalling capability.

The other possibility is that the inner and outer wings are flying on
opposite sides of the polar curve with the outer wing on the high speed
side of the polar and the inner wing on the slower side. Discontinuities in
the polar due to separation bubbles may play a part too.

A perfectly symmetrical glider with a perfectly smooth polar and
frictionless controls SHOULD develop a spiral dive from a hands off turn.

Bill Daniels


"Buck Wild" wrote in message
om...
John Jones wrote in message
...
How many gliders can be trimmed for hands off thermalling?



LS4
-Dan

On Fri, 23 Apr 2004 20:14:02 GMT, "Bill Daniels"
wrote:


A perfectly symmetrical glider with a perfectly smooth polar and
frictionless controls SHOULD develop a spiral dive from a hands off turn.

How does this explain the behaviour of non-radio-controlled model
gloiders that return to level flight anytime? They are built perfectly
symmetrical, have a smooth polar and no controls at all...


Bye
Andreas

How does this explain the behaviour of non-radio-controlled model
gloiders that return to level flight anytime? They are built perfectly
symmetrical, have a smooth polar and no controls at all...

They have a lot more dihedral or polyhedral to help them return to level
flight.

Interestingly, a well trimmed hand launch free flight glider will climb almost
straight up, "flop" into a glide without stalling, and then start turning in a
stable, constant circle. Great fun!

Jim Vincent
CFIG
N483SZ
illspam

On Sat, 24 Apr 2004 14:04:11 +0200, Andreas Maurer
wrote:

On Fri, 23 Apr 2004 20:14:02 GMT, "Bill Daniels"
wrote:


A perfectly symmetrical glider with a perfectly smooth polar and
frictionless controls SHOULD develop a spiral dive from a hands off turn.

How does this explain the behaviour of non-radio-controlled model
gloiders that return to level flight anytime? They are built perfectly
symmetrical, have a smooth polar and no controls at all...

Free Flight gliders - at least in the F1A class, which I fly - are not
aerodynamically symmetrical: There is always more wash in on the inner
wing in the turn and the rudder is set into the turn, giving a
crossed-controls effect. Without the differential wash-in most designs
will have nasty spiralling tendencies.

The flight regime is somewhat different, too. The model is trimmed at
min. sink (about 8-10 kts) and is flying in a still air circle that
requires 40 - 60 seconds to complete. That's a 30 - 40 m diameter
circle being made by a 2.5 m span model, which is a relatively much
larger circle than we tend to use in our sailplanes and yet is small
enough to fit easily inside a thermal column.


--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

"Bill Daniels" wrote in message news:eOeic.21511$GR.2890579@attbi_s01...
The aerodynamics of this are interesting.


I was taught (and that was long enough ago for it to have passed from
fashion, apparently) that shallow, medium, and steeply banked turns
were defined by the control inputs required to maintain them, to which
the actual angles of bank are merely incidental. Shallow turns
require pro-turn aileron to maintain the angle of bank. Steep turns
require anti-turn aileron to preclude over-banking. As in level
flight, medium turns balance the converging lift vectors of wing
dihedral and require no control displacement whatever to maintain
angle of bank. This isn't to say that medium turns are stable, only
that at some particular angle of bank all lateral forces produced by a
wing with dihedral will balance about the c.g..

From an instructional standpoint, it's easy to demonstrate and serves
as a graphic intro to the subjects of stability and control. One
caution: Dont add this to the student's plate before he's in the
habit of (I mean "really learned") to manuever by applying whatever
force, rate and displacement of controls may be necessary to achieve
the desired result. This is one of those things that can get a
student to thinking too much and (except possibly for spin recoverier)
I've never seen anybody's flying improved by watching their hands and
feet while maneuvering.

Cheers,
Eric