Friday, 15 March 2013

Stick and rudder

Talking about adverse yaw with Mike, yesterday, I was made to realise that I only half understood what causes adverse yaw in a turn. I have been reading what is now a very old flying manual, as it was written for RAFVR pilots at the start of the war (but on the premise that aerodynamics do not change). Perhaps their description was too simplistic, or perhaps I misunderstood it. But blurting out that I understood this, then going on to mis-describe it in earshot of other pilots was humbling.

I said that adverse yaw was created when the outside wing, travelling faster than the inside one, suffered induced drag, so pulled back in the opposite direction to the turn. Mike said that actually you get more induced drag when the wing is going slower.

Mike explained that yaw is not a component of speed but of lift, and now it is slowly becoming clear in my mind, a day later! I am devising a teacher’s way of explaining it to myself and possibly others (about which, more later).

It needs to be reiterated that any misunderstanding is my own, not Mike’s, obviously. And also, if I have not got it right since Mike explained it, that isn’t a fault of his explanation, but my slowness in getting something. I have always been like this. I need to go away and think things through, which is what I am doing now. And remember, this is not a manual I am writing here, it is me playing with what I am learning, and trying to get some sense out of it.

By patient Q&A, Mike eventually drew the words “angle of attack” out of me and then went on to show me the effects on the chord line (leading edge to tip of the aileron) of depressing the aileron. The increased angle of attack increases that wing’s lift, and it is the increased lift, not the speed, which creates the drag. On the opposite wing, the raising of the aileron (raising the rear of the chord line so that it is pointing down) reduces the angle of attack and therefore reduces its lift, so there is less induced drag on that wing.

Visually (as well as technically), there is another aspect to this, which is the direction of the lift (it is all flooding back to me now, but I must revise all this, as it has been 5 years since I did my Principles of Flight exam). Lift is perpendicular to the relative airflow.* So, the outside wing’s lift is pointing backwards, while the inside wing’s lift is pointing forwards and if I am right, it is that, too, which causes the direction of yaw.  I think what we are saying is that because the lift is pointing backwards lift drags that wing backwards; drag is the secondary effect of lift.

So that is where my original misunderstanding came in. I said that airspeed created drag. Actually, greater airspeed creates lift and it is the lift which creates drag. Here’s a thought, maybe it would be more helpful to talk in terms of directions of forces. If the force perpendicular to the relative airflow is upwards and backwards, it will try to pull the wing (and the aircraft to which the wing is attached) upwards and backwards towards that wingtip. And because you want to turn in the opposite direction, you need something to counter this force, which is where the rudder comes in.

I am getting used to pushing the stick and rudder in the same direction (stick right, right rudder pedal) and that doesn’t pose a great problem, conceptually, except on the ground, taxying - where this is opposite to a flexwing's steering). I have been getting used to using the clinometer (turn and slip indicator), which is a curved glass tube spirit level with a ball sitting at the bottom in the middle. In a turn, you have to use the rudder to keep the ball in the middle to achieve a balanced turn, where the aeroplane is not tending to skid outwards of the turn or slip inwards of it. When the ball rolls to the left, you have to apply left rudder to correct this and bring it into balance again. I was pleased that I got the hang of this.

(My copy of Stick and Rudder, by Wolfgang Langewiesche - recommended by Katie - just arrived. Am off to do some reading now) 

* Mike just rang to say that people in the clubhouse have just been discussing what I wrote and he asked me to explain what I meant by saying that lift was perpendicular to angle of attack. Oh dear! I got that bit wrong. I have changed it to perpendicular to the relative airflow. "I will leave you to ponder that one", he said.

A friend of mine who was on the same Masters programme as me at Imperial College, and who is very scientific, and was in fact a curator at the Science Museum said, after reading this, "Blimey, and I thought riding a bike was difficult!"

No comments: