Adverse Yaw

Dec 15, 2011   //   by admin   //   Tech Info  //  No Comments

 What it is, and how to cure it

 Often, when I mention adverse yaw to other aeromodellers, I get a blank look as if I’m off another planet. It is a common aerodynamic condition that is rarely mentioned in the modelling press, almost as if it’s a dark secret. Pilots  full size aircraft rarely get a second chance to cure the problem, as the usual outcome is for the plane to just “ fall out of the sky” with teminal results for the plane and the pilot.. This is a very serious condition which can happen in an instant, usually just after after takeoff, or on final landing approach when flying low and slow. When flying slowly, just above normal stalling speed, it is always necessary to use “up” elevator to increase the angle of attack of the wing in order to generate suffivcient lift to keep flying. So far so good, nothing amiss so far.

On many models and full size aircraft, the ailerons move up and down by equal amounts. This is fine when flying at normal flying speeds……BUT, when the wing is at a greater angle of attack to the airflow when flying slowly, the downgoing aileron will then have more drag than the upgoing aileron. The result being, when right aileron is given to roll the plane to the right, the plane will YAW to the left, and vice versa. This is why many full size aircraft always require simultaneous rudder AND aileron input to roll to the left or right.  (Most First World War aircraft required this) This is why coupled aileron and rudder is recommended on many high wing models that have a flat bottom wings and little dihedral. So why do planes just “fall out of the sky” ?  As the plane yaws, the fuselage is then angled sideways to the airflow, creating a lot more drag and also, the air is angled across the wing section which raises the speed at which the airflow over the wing will stall. AND,  because of the increase in drag, the air speed then drops very quickly to below the now raised stall speed and gravity wins again !

This is also the principle of a “ sideslip approach “ whereby an aircraft can approach a runway with a very steep approach and keeping the runway in full view, without building up excessive speed. The increase in drag caused by the plane flying slightly sideways prevents the plane from speeding up. Full rudder is used in conjunction with the ailerons to keep the wings level. In this case, the steeper approach, assisted by gravity, keeps the aircraft well above the minimum stall speed.

How many full sized aircraft prevent adverse yaw.  The “Cure“ is to ensure that the upgoing aileron always has the same, or more DRAG than the downgoing aileron. Sounds simple doesn’t it ? In practice, it is……..usually by mechanical adjustment of the aileron linkages as in the full size, or by transmitter adjustment, if you have that facility. For example:

1: The Pitts S2A Special has 60% up aileron and 40% down aileron movement.

2: The Tiger Moth has significantly more up aileron movement than down aileron movement. (I’ve been told, as much as 75% up and only 25% down) 

I used the above settings on a 1/3 scale Pitts S2A and it was absolutely spot–on first go and never needed changing. Why did I take the trouble to find out the full size settings from a friend at the CAA ??………….because years earlier, I had built a quarter scale Pitts S1 Special that had ailerons only on the lower wing, and powered by a Webra 60. I thought it was a good idea to droop both ailerons like flaps to give more lift at take off and enable it to fly more slowly.

WRONG !…….It taxied and took off ok but the first aileron input saw it cartwheeling through the grass on each of three attempts.  I didn’t know what was happening, I could fly, but my plane didn’t ? So I asked a more experienced model pilot to try the next takeoff………yes, he got it up, and higher than me, and fought two or three vicious adverse yaw flicks on the climb before the airspeed dropped, gravity won, and re-kitted the plane. Then another modeller knowingly took me aside and slowly explained the cause……………He was so right. 

My next semi-scale biplane also had ailerons on the lower wing so I made sure they were both slightly “UP” when in the neutral position and adjusted to give 66% up movement and 33% down movement. The first flight was absolutely fine with the ability to steer the plane using ailerons only. The upgoing aileron having the most drag so the plane started turning in that direction without any rudder input. Rolls were a bit corkscrewey and lost height unless the plane was climbing at about 15 degrees before the roll. So tuning out SOME of the aileron differential was easy to do over the next few flights until rolls were more axial and, at height, full aileron just before the stall did NOT yaw the plane wrong way. 

I am currently building a YT Spitfire Mk XIV to a reasonable flying scale standard and will be incorporating 60% up and 40% down aileron movements for my first flights and NOT the recommended 25mm up and 25mm down. Especially from reading the YT Spitfire blogs, some have spun in on landing approach especially when built with many scale additions. So if my rolls don’t please me and it aileron steers too readily, I can always fine tune the aileron movement for later  flights………..I should get more than one chance to get it exactly right …………….     

Follow this link to the very similar KMP Spitfire build with it’s first and final flight…………….What do you think happened ??

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