From: raveling@Unify.com (Paul Raveling)
Newsgroups: rec.aviation.soaring
Subject: Re: Thermalling technique revisited -- a philosophers' stone?
Date: 21 Jan 93 20:42:43 GMT
Organization: Unify Corporation (Sacramento)

In article <11752@uqcspe.cs.uq.oz.au>, colin@cs.uq.oz.au (Colin Millerchip) writes:
> I'd like to ask the net's opinion of a thermalling technique that's
> been told to me recently. In a previous thread a "standard" thermalling
> technique of "a little out-of-turn aileron, a little bottom rudder" was
> discussed in detail.  

	That's just the combo of control inputs that's required
	to maintain a coordinated turn.

> The technique that I'm now referring to involves a nose-up attitude,
> with top rudder ...

	In other words, fly a slipping circle.

... (which I assume is to speed the inner wing, and thus
> avoid a stall). ...

	It only speeds up the inner wing when you're increasing the
	yaw input -- in a steady state turn it makes virtually no
	difference in speed (translating to AOA) on the inner wing.

	Having said that as a 1st order model, there's a second-order
	effect that would have the opposite result.  The translational
	motion that the slip produces toward the center of the turn will
	decrease the turn radius slightly.  This will increase the turn
	rate slightly, which increases the yaw rate, and that will
	DECREASE airspeed over the inner wing slightly.  On the other
	hand maintaining the slip requires having the inside aileron
	up instead of down, significantly reducing AOA at that wingtip.

	Aileron deflection probably dominates to give you a VERY slightly
	increased margin over stall, but I wouldn't swear to it.

 The associated wisdom is that this technique almost
> causes the glider to fall into the core of a thermal, which would seem
> to imply that this is the philosophers' stone of gliding.  
> 
> Any thoughts/explanation? 
>  -- is this method efficient? Any effect on natural sink rate (sounds 
>     like this should increase)? 

	Your words were appropriate about making the glider FALL into
	the core of the thermal.  This will improve efficiency if and
	only if the thermal is very small in diameter -- Your mild
	decrease in turn radius has to put you in enough additional lift
	to make up for your additional sink due to the slip.

	"Your milage will vary" depending on how your particular airframe
	slips.  Personally I'd be shocked if this produced a better
	climb rate than a coordinated turn -- my intuition says a slip
	that shrinks your turning circle much is likely to at least
	double or triple your sink rate.

>  -- does the glider really have a tendency to move towards areas of
>     better lift, and if so, why? 

	No -- in fact I'd speculate it'll do the opposite.  There's
	no aerodynamic reason for this to steer you toward lift; lift
	will still try to raise your inside wing, and sticking that wing
	into the faster-rising air of the core will try to kick you
	out of the thermal.  If this technique reduces turn radius it'll
	increase that roll moment that tries to take you out of the core.

>  -- any hints on correct/proficient execution of this technique? 

	Not of THAT technique.  My advice when thermals are small but
	have strong cores is to be sure you have enough knots of airspeed
	to stay above stall and fly your thermaling circle in a coordinated
	60-degree bank.  When necessary to adjust the turning circle
	shallow out briefly to no less than a 45-degree bank, then
	roll back in to 60 degrees.


------------------
Paul Raveling
Raveling@Unify.com
Newsgroups: rec.aviation.soaring