Why is it hard to steer at takeoff?


Photo by Larry Costales on Unsplash

We’ve had a lot of windy days this last week at CYTZ. A big low pressure system (one of the last of the winter, I hope) to the north east has been pushing strong northerly winds over Toronto, and when the wind comes over the downtown core it creates a lot of turbulence and gusts around the east-west runway at the Island.

Yesterday, one of my students – let’s call him Brave Ben – suggested he should get some practice at crosswind landings. So out we troop for some circuits in the 15G25 direct crosswinds and the associated turbulence. Brave Ben and I bumped and lurched around through three harbour head-smackers before deciding that while we could – we absolutely could, let there be no doubt – continue safely taking off and landing our little plane in the prevailing conditions, we simply didn’t want to, any more.

On one of the takeoffs Brave Ben remarked to me he was surprised to find how much left rudder input was needed to keep the aircraft within a hair’s width of the centreline as we accelerated along the tarmac.

Which got me thinking why is ground steering on takeoff tricky for new students?

Most people are used to steering a car, which (unless you’re into rally cross) mostly goes where you steer it: there’s good traction between the front tires and the road and the angle of the front tires is firmly fixed by the angle of the steering wheel.

In the Grob, as in many tricycle-gear airplanes, the pedals control the angle of the rudder, and are connected to the nose wheel by a sort of squidgy rubber bungee arrangement. The pilot can influence the nose wheel but it has a good deal of freedom to caster, that is, to line up in trail to the direction the aircraft is moving. The faster the plane is rolling, the easier it is for the nosewheel to caster.

In addition to the nose wheel, about fifteen feet behind you is the vertical stabilizer sticking up high into the air. In the air it does a pretty good job of yawing the nose into the relative wind, which is what you want. However on the ground this tendency to nose into the wind is unhelpful when instead you want the airplane to track along the centreline and NOT turn into the crosswind.

Finally you have the rudder itself: when you use the pedals you also change the angle of the rudder.

The rudder and the vertical stabilizer actually form one aerodynamic surface which just like a wing has its own angle of attack (it’s called the sideslip angle). The sideways force from the rudder/stabilizer depends on the speed and direction of the airflow over it: when you’re stationary with a direct crosswind the sideslip angle is 90° – and the vertical surface is about as stalled as you can get – so it exerts a relatively small sideways force, and that sideways force cannot be adjusted with the rudder. As you move forward faster, the relative wind increases in strength and arrives more from the front. When you open the throttle you add a lot of airflow over the stabilizer but because of the spiral propwash that has a left-turning effect. As the plane goes faster the sideslip angle diminishes, the rudder/stabilizer unstalls, and the pilot gains the ability to adjust the sideways force by moving the rudder with the pedals. At some speed the pilot can reduce the sideways force to zero with the correct rudder pedal position.

When we start a takeoff run we start very slow, and the plane follows the angle of the nosewheel we set when we braked to a stop, because the ability of the nose wheel to caster is limited by the slow speed.

Then we open the throttle: the spiral airflow from the propeller arrives at the vertical stabilizer, helping to unstall it, but also providing a yawing force to the left.

With the first few knots of groundspeed, The force from the airflow over the stablizer becomes easily able to caster the nosewheel and yaw the aircraft, to the left, in light or no crosswind, to the right, in a strong wind from the right. The pilot has to counteract that by adding a lot of pedal so that the bungee mechanism keeps the wheel (and the the plane) straight. If they don’t, in a strong right crosswind the aircraft will rapidly turn and head off the right side of the runway, or in no- or a left crosswind, to the left side of the runway. There’s probably also just the right amount of crosswind from the right that will keep the plane straight with no rudder in put, but I’ve never found it.

As the speed builds, the propwash effect straightens out, the sideslip angle decreases, the stabilizer airflow get faster and the rudder becomes more and more effective: the same heavy pedal input that kept the nosewheel straight initially becomes too much and the pilot has urgently to relax their pedal displacement. If not, there’s a danger of the airplane now running off the other side of the runway.

You won’t be able to leave the ground and keep directional control until the airplane is moving at least fast enough for the rudder to have enough authority to null the sideways force on the tail. Until it can do this, you are relying on the bungee to keep the nosewheel angled straight and the nosewheel to keep the airplane straight. Pick the nosewheel off the ground by rotating the airplane in those circumstances and you lose the abililty to steer – even full rudder will not keep you straight. In essence this is the same consideration at play that determines the maximum crosswind component the aircraft can handle at landing time. A bigger rudder helps, as does a faster takeoff speed.

It’s worth mentioning that positioning the ailerons fully into the wind considerably assists the nosewheel to keep the airplane straight: in strong crosswinds and at speeds below those at which the rudder alone is sufficiently effective you may not be able to control the airplane without doing this.

For a novice pilot who’s still using the mindset of a car driver to control the airplane on the ground the rapid changes in steering inputs with the feet to prevent the airplane veering to one side or the other can be rather unexpected. This is another example of a pilot doing a lot of work with brain, eyes, hands and feet just to make it look like nothing is happening – the aircraft just rolled straight down the runway, what could be simpler than that?

Final note: there are many training airplanes (the Diamond DA20 comes to mind) that has a full castering nose wheel. No connection to the pedals at all. In that case you have to use differential braking – brake one maingear wheel and not the other) to keep the airplane straight at speeds before the rudder starts to work.

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