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It's Not Just Tailwheel Aircraft That Have Problems On The Ground

If you've ever flown a tailwheel aircraft, you know they can be more unstable on the ground than a tricycle gear. And while tricycle gear aircraft usually prevent the instability problem found in taildraggers, that's not always the case. Here's why...

Derin Allard

Why Tricycle Gear Airplanes Are (Usually) Stable

Tricycle gear airplanes are built in the 'Delta' configuration, with the airplane's center of gravity slightly forward of the main gear.

This makes tricycles more stable than their tailwheel counterparts for two reasons:

  1. 1) the CG is forward of the main gear
  2. 2) the CG is closer to the mains than in a tailwheel aircraft

Landing Usually Causes The Problem

Most directional control problems happen during landing in both tricycle and conventional gear airplanes, but tricycles are less susceptible. Why? Let's start with the tailwheel.

When you're landing in a tailwheel plane, if your plane starts turning right, two forces are created: 1) friction on your wheels turns the plane right, and 2) because of Newton's 3rd law, the CG is thrown in the opposite direction, which happens to be to the left. These two forces make your tailwheel plane want to spin in circles on the runway - which can cause a ground loop.

When you're landing in a tricycle gear plane, if your plane starts turning right, the same two forces are created: 1) friction on the wheels turns the plane to the right, and 2) the CG is thrown in the opposite direction, again to left. But because the CG is ahead of the main gear in this case, the two forces cancel each other out, and your plane tends to straighten itself out and keep rolling down the runway.

When Things Start To Go Wrong For Your Tricycle Gear

So this means that a tricycle gear plane should never have problems with directional control on the runway, right? Unfortunately, the answer is no, and it's because of a phenomenon called 'load transfer'.

In most tricycle gear planes, 80%-90% of the airplane's weight is on the main gear. But that all changes when you land and hit the brakes. Your brakes are trying to slow you down, but your airplane wants to keep moving forward. You feel it in the cockpit when it happens - you get thrown forward against your shoulder harness, and if it wasn't for your seatbelt, your face would practically be planted to the attitude indicator. The same thing happens with your tires. When you aggressively brake, a lot of the airplane's weight is 'transferred' to the nose gear, even though your CG hasn't moved an inch. In many cases, you could have more weight on the nosewheel than the main gear during aggressive braking.

This load transfer isn't usually a problem, as long as you're going straight down the runway. But if you start to turn left or right, even slightly when you're braking, things can get out of hand.

That's because as you turn, the load is transferred more to the wheels on the outside of the turn. And since you only have three wheels, that means the nose and the left main gear are taking on the majority of the load. If the braking and the turning forces are strong enough, your right main gear will have a load of 0lbs or less, your airplane will roll over, the prop will strike the runway, your left wingtip will strike the runway, and your day will be ruined...

How To Avoid It

So how do you avoid this mess? There are three ways. The first is to avoid landing fast. When you land fast, your wings are producing more lift, which means initially there is a lot less weight on all three tires, making it even easier to get out-of-balance and lift a wheel off the ground.

The second is to keep as much weight on your main gear during landing as possible. You can do this by using back pressure throughout the landing roll. When you touch your wheels down, keep pulling back on the yoke, and transfer the airplane's load away from the nose and back to the mains.

The third is to avoid over-controlling your rudder and nosewheel on the ground. By controlling your inputs on the ground, you can prevent a rollover with almost 100% certainty.

Combine all three, and you'll make a landing that you can walk away from...every time.

Colin Cutler

Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at colin@boldmethod.com.

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