Advanced Trimming

Trikes vs Taildraggers...

One of the milestones you pass as you progress in the sport of flying is moving to your first taildragger, and many a flier who is getting ready for his or her first attempt is often curious as to what to expect.

I am writing this in the hope of giving you a good understanding of what is going to be different - but I am going to keep it simple so as not to overwhelm you with a lot of confusing physics.

 

 

Example of a

1. Tricycle Landing Gear RC Model Aircraft...

 

 

 

 

 

Example of a

2. Tail-dragger Landing Gear RC Model Aircraft...

 

 

 

 

Take Off

Most people (including myself) find the difference between trike and taildragger is most noticeable during takeoff.

So let's compare the two:

Takeoff with trike gear:

Your plane is idling in front of you. As you add power, the plane exerts more pressure on the nose wheel thereby giving it a good grip on the ground. Steering is DIRECT. That is, steering is controlled by solid, wheel-to-ground contact. Turn the wheel, and the plane turns with it. The plane rolls down the runway on its solid 3-point stance and as speed increases and lift is generated, a little "UP" elevator is applied and (providing other factors like incidence and main gear placement are correct) the plane will rotate on its main wheels and lift off.

It doesn't get much simpler than that.

Now let's look at the same scenario with a taildragger. 

Takeoff with taildragger:

Just as in a trike, as power is applied, the drag of the main wheels causes the nose to want to drop - but there is no longer a nose wheel to prevent this. So if there is any drag on the wheels (Like grass or wheels that don't turn freely), the wheels stay put, the nose drops, the prop hits the ground, the engine quits, and you're heading back to the pits to either restart the engine, or replace a broken prop.

Take two: Now you know that you must hold the tail down as you power up, so you apply full "UP" elevator, and you slowly add power. But as the power increases, the pull on the nose is greater than the airflow over the tail, so the plane tips forward, and you're heading back to the pits to replace prop #2 (while quietly wishing you had built a trike).

So how do you prevent this from happening again? Being aware that it can happen is the first step. By being aware of the problem, you can be prepared for it. Start out by applying full "UP" elevator and raise the throttle a little, if the plane doesn't move, go back to idle, and "goose" the throttle. That is, give it a quick burst of speed, but quickly lower the power again.

How much? That's for you to figure out. It would be like me asking you, "If I were balancing a broom on my hand and it starts moving away from me, how far should I move my hand to re-balance it?" 

Once the plane is rolling, the air over the tail is usually enough to keep the tail down (Keep in mind that you are still holding full "UP"). If the plane simply refuses to taxi without nosing-over,you may need to move the wheels forward, switch to larger wheels, or cut the grass.

Now you're lined up at the end of the runway. Same rules apply here - get it rolling first, and slowly add power while holding full "UP". As speed increases, you will need to relax the elevator to prevent premature liftoff.

Now comes one of the greatest balancing acts known to mankind! (That guy on the Ed Sullivan Show who balanced 20 spinning plates on top of sticks would be amazed at what YOU are about to do).

First, let's talk about the elevator. As I said, you need to relax the full "UP" that you have been holding up until this point, but relax it too much, and you'll be replacing prop #3.

So now you need to use elevator to balance the tail until airspeed is achieved. If you lift off too soon, you could easily find yourself in a torque roll to the left that you often cannot get out of and you'll have a lot more than a broken prop to repair.

But the elevator is only one third of your concern (You haven't forgotten that you are also still on the throttle, did you?).

Next comes the rudder.

As soon as the tail lifts off the ground, you no longer have that steady 3-point stance. You have also lost your positive (Wheel-to-Ground) steering and you are steering only by the turbulent airflow over your rudder. So the plane will immediately start to "weather-vane". That is, if there is a wind blowing from the right, it will push the tail to the left (and vise versa). Also, even with no cross wind, as soon as the tail lifts, the plane will pull to the left. (NOTE: There is debate as to how much of this pull is derived from torque and how much is from prop wash, but for this discussion it is irrelevant - Just be aware that the plane will pull to the left).

To counteract the left turn, you'll need to apply right rudder. How much? Go back to the broomstick analogy. The amount will vary with every plane in every wind condition.

So now you are doing 3 things at once:

1. Increasing throttle

2. Balancing pitch with Elevator

3. Balancing Yaw with Rudder

This balancing act is often referred to as "Flying the Tail", and it's a very good description.

Once proper airspeed is achieved, you just add a little more "UP" and she will lift off the ground - but don't relax just yet. You will probably want to maintain a little rudder for a few seconds after liftoff, and relax it slowly to prevent an unsightly tail waggle.

Easy...

Flying

What is the difference once the planes are airborne? There isn't one. The wheel configuration plays virtually no part in flying, with only one minor exception - a taildragger has a little less drag due to having one less large wheel.

 

Landing

Some people claim that landing a taildragger is more challenging that taking off, but I sure don't know how.

It's only slightly different that landing with a trike gear. 

  • Your approach is the same and your flair is the same and for all intents and purposes you need not do anything different until your wheels touch the ground.
  • But once the wheels touch, your speed comes into play.
  • You no longer have a lot of throttle, so torque is pretty much a non-issue, but remember that your CG is BEHIND the wheels.
  • So all that can happen is the dreaded "Ground Loop" where the tail swings out in front of the nose.
  • Although truthfully, ground loops are a fairly rare occurrence and they're usually confined to a specific plane rather than pilot.
  • In other words, you may have no problem with ground loops on your Cub, while your Fokker Tri-plane does them all the time. 

And it may be worth mentioning that a slight "toe-in" of the wheels can help prevent ground loops on a taildragger.




 

In either case, the best way to avoid a ground loop is to do a proper 3-point (there's that phrase again) landing

  • To do this, once you have flared the plane for landing, you want to hold the plane just inches above the runway while you slowly feed in more elevator.
  • As the plane slows down, the tail will drop but due to the slow speed, the plane will not gain any altitude (Note: If you cannot get your taildragger to lower its tail, this can be a sign that is nose-heavy).
  • At this point, you are VERY close to stalling, but since you are only a few inches off the ground, as speed decreases the plane will just settle in on all 3 wheels at the same time.
  • If executed properly, the plane no longer has enough speed to produce lift so it will stay put on the ground, and since so much speed has been bled off, it will shortly roll to a stop (But keep holding that "FULL UP" elevator to help prevent a nose-over!) 

There are a lot more in-depth details and physics involved than I laid out here, but when it comes to the type of gear setup you have chosen, this should give you a basic understanding of what is happening and why.

Good Flying!