Aviation question, if you please.

[John Hill]

As our Link trainer replica gets closer to completion I am often asked questions which my aeronautical but non-flying career has not prepared me for.

For example, when writing the test software that will be used to prove the operation of the machinery I must include 3 axis of movement and there is debate about the Z-axis (yaw). My opinion is that the test software should move the Z-axis according to the angle of bank whereas others say the rudder is what is used for turns.

So, the question is can I realistically control the yaw movement of the Blue Box (aka aeroplane) according to bank angle? There are technical reasons why I do not want to use rudder input, just yet.

[k3k3]

I don't know the answer, but can you ask one about using the elevator just so I can say it pitches about the athwartships axis... Oh, I just did. ;-)

[Fox3WheresMyBanana]

Bank and yaw are interconnected. The secondary effect of one is the other.
The extent to which they are linked varies between individual aircraft.
The standard technique for turning involves setting a bank angle to obtain a particular rate of turn (change in horizontal heading, i.e. your z number), and correcting for any unwanted yaw with rudder.
There are some conditions with some aircraft where the rudder is the primary control for banking the aircraft, e.g. high angle of attack in a Phantom, but these are not what the Link trainer was designed to simulate.
The rate of turn is affected by the aircraft airspeed as well as the bank angle, as you probably know.
I would do as you are proposing, and use bank angle coupled with airspeed to calculate the rate of change of z angle.

Since the link trainer was designed for simulated instrument flying, the Rate One turn, of 3 degrees per second, should be your standard test parameter.

[PHXPhlyer]

The unwanted yaw in this case is Adverse yaw.
If trying to turn with ailerons alone the aircraft will bank in one direction however the nose of the aircraft will try to move in the direction opposite the direction of turn.
The instrument indication would be the turn indicator showing a turn in the direction the yoke/stick is moved however the slip/skid indicator show the ball out in the same direction as the turn resulting in a slipping turn.
The response with the rudder is to "step on the ball" to center it and counter the adverse yaw.
Using too much rudder would cause the ball to go out in the opposite direction of the turn resulting in a skidding turn.

PP

[John Hill]

Thanks for those very useful comments.

[Boac]

It is time for KISS. As Fox says, work out an algorithm for rate of turn with bank and speed. Forget 'Adverse yaw' - there is no such thing in a Link trainer since it has no ailerons! I do not see any need for rudder input either.* Stick with 2-axis control.

Rate of Turn = (1093*Tan(bank angle))/ IAS [IAS in Knots, answer in degrees/second]

(1093 good enough for government work)

*The only need for rudder would be turning while taxying in a Link. Easy then to work out a direct heading change for a given rudder input?

[John Hill]

Well of course our Link has ailerons and full flapping tail feathers too, as does our WWII original. This new one also has toe brakes and I suppose we could add gear retraction too. However, as I constantly remind people, our Link Trainer is not a fairground ride and should only be operated with the hood down.

I have put a 23" screen in to be the instrument panel and if we want to include taxying, takeoffs and landings etc I would have to install another screen above that for the windscreen view.

These things are all practical as at the heart of it all is MS Flight Simulator, the punter sits in the Blue Box with the hood down and flies the Flight Simulator program which of course does all the number crunching. Data is bled off from the program to move the BB to follow the attitude of FS's virtual model.

Thanks for the handy formula for rate of turn, in the final configuration FS will handle all that for us but right now I am just trying to get the pitch, bank and azimuth axes to operate under control of the custom electronics, rate of turn from bank angle will be adequate and if any of the volunteer advisors mention rudder again I will now know what to tell them.