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Old Feb 25, 2012, 12:40 AM
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Curving relative wind, how much slip is ideal in turns, yaw strings videos

I started this discussion in "sailplanes"-- "sailplane talk" but wonder now if it would have been better posted here. Please read and then join in there or here...

http://www.rcgroups.com/forums/showthread.php?t=1597991

Thanks

Steve
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Old Jun 16, 2012, 12:58 AM
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recent observations full-scale Schweizer 1-26

To center the yaw string, I needed significant inside rudder and significant outside aileron. No surprise.

If I took my feet off the rudder I needed only a touch of outside aileron. Yaw string blew to outside significantly, probably more than optimal.

With just a touch of inside rudder, I needed a little more outside aileron, but less than with yaw string fully centered, and yaw string blew to outside maybe about 10 degrees, maybe about optimal.

For constant-banked circles in many rc gliders I bet an average rudder input of zero would be not so far from ideal. Ideally you want some rudder to mix in with each change in ailerons but figure out some way to bias the mix according to turn direction so average rudder position is near centered even though you are still mixing with ailerons and likely holding a touch of outside aileron, especially on a scale ship. Or simplify by just mixing with ailerons to minimize adverse yaw while rolling, and accept that since you are holding some outside aileron you will also be holding a touch of outside rudder, strictly speaking not ideal but maybe close enough, and the outside rudder will minimize the need for outside aileron which is not such a bad thing...

For my 2 meter spider I mixed rudder as described immediately above, tailored mix simply to minimize adverse yaw while rolling, and the yaw string deflection in steady constant-banked turns seemed pretty near ideal, streaming maybe 10 degrees to outside of turn...

We know that fully centering the yaw string at the nose is NOT ideal...

Steve
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Old Jun 17, 2012, 08:22 PM
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There was a picture floating around that simply showed the longer the fuse and tighter the turn radius, the more rudder was needed to to keep the wing perpendicular to the airflow.

It oversimplified lots of things but it's reasonable to expect that when flying in an arc (especially a tight one) with a long fuse, the back of the plane isn't seeing the air comming from exactly the same direction as the front.
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Old Jun 18, 2012, 12:21 PM
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It may make more sense to keep vertical tail streamlined to flow especially if fuselage is slender and streamlined. Or leave rudder neutral so vertical tail adopts a position that is nearly streamlined to flow. Means there will be sideways flow over nose and also over wing. That's ok, that interacts with dihedral to give a helpful rolling-out torque, so less outside aileron input is required. Less outside aileron may mean less drag, even if wing is experiencing a sideways flow component.

Actually some inside rudder is needed to fully streamine the vertical tail to the flow. The inside rudder offsets the drag of the outboard, faster-moving wingtip. But maybe better to give less inside rudder than needed to fully streamline the vertical tail, so the vertical tail meets the air at a non-zero angle of attack and contributes some of the yaw torque needed to balance the drag of the outside fast-moving wingtip. Having the vertical fin share the load with the rudder may be more efficient than having the rudder create all the needed inside yaw torque all by itself, with the fixed fin fully streamlined to the flow. In the former case (fin at non zero angle of attack) the point where the line of the fuselage is tangent to the curving flight path and relative wind lies somewhere back behind the rudder. In the latter case (fin fully streamlined to flow) the point where the fuselage is tangent to the curving flight path and relative wind is at the fin. Not at the cg, not at the nose. A yaw string at the nose will blow to the outside.

On the other hand, with a big slab-sided fuselage and relatively small vertical tail, we have more motivation to move the point where the fuse is tangent to the flight path and airflow further forward, toward the center of lateral area which may be near the c.g.. So the yaw string at nose is nearly centered (but not fully centered). This will require more inside rudder and also more outside aileron.

To fully center the slip-skid ball, the point where the fuselage is tangent to the curving flight path and relative wind should lie near the center of lateral area of the aircraft. The yaw string at the nose would still blow slightly to the outside.

Steve

Quote:
Originally Posted by bjr_93tz View Post
There was a picture floating around that simply showed the longer the fuse and tighter the turn radius, the more rudder was needed to to keep the wing perpendicular to the airflow.

It oversimplified lots of things but it's reasonable to expect that when flying in an arc (especially a tight one) with a long fuse, the back of the plane isn't seeing the air comming from exactly the same direction as the front.
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Old Jun 19, 2012, 05:11 AM
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The problem in a sideslip is how much drag the fuselage is causing. I know that sideslip is used in full sized gliders to reduce speed in an approach (in addition to spoilers and airbrakes), though I don't know how much of the component of drag is caused by the tailfin in these maneuvers. probably there is a sweet spot where neither the nose nor the tailfin are exactly aligned to the airflow.
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Old Jun 19, 2012, 06:43 PM
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Yep, I suppose the order of priority would be :

(1) Keep the wing perpendicuar to the airflow for greatest efficiency
(2) Keep the deepest part of the fuse aligned with the airflow for minimum fuse drag.

A well designed glider would probably get close to acheiving both (1) and (2) at the same time.
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Old Jun 19, 2012, 10:03 PM
greg
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Quote:
Originally Posted by Brandano View Post
The problem in a sideslip is how much drag the fuselage is causing. I know that sideslip is used in full sized gliders to reduce speed in an approach (in addition to spoilers ...
not reduce speed ... just pull back on the stick to do that. The increased drag caused by the slip increases the sink rate allowing you to more quickly loose altitude. Many small powered aircraft have neither flaps nor spoilers and use a slip to control the glideslope without sacrificing airspeed and risking a stall.

so slipping is to be avoided while thermalling by centering the yaw string (or allowing just a small angle).

greg
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Old Jun 19, 2012, 11:58 PM
agnotology
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Dr. Drela's article on rudder and yaw while turning:

http://www.polecataero.com/handlaunc...thermal-flying

Kevin
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