Landing Gear Design and Placement

When you build an RC plane you have two basic choices of landing gear, either tricycle gear with a nose wheel or the more old fashioned style with the main gear in front and a tail wheel in back.  In the World War II era the style with the tail wheel was called “conventional” landing gear, but most people today refer to this type of airplane as a “tail dragger”.

triketaildragger

If you are building a scale model you will be stuck with whatever type of landing gear is found on the original.  But a lot of RC sport aircraft offer you the choice of either tricycle or tail dragger setup.  Most RC pilots tend to choose tail dragger over tricycle because of the cool factor.  But there was a time in the 1960s and 70s when the sporty planes mostly had tricycle gear because it looked modern.  Maybe the style will come around again.

Regardless of your perception of style, tricycle gear offers the pilot the best and easiest ground handling.  Assuming that everything is adjusted correctly, the plane will handle very much like a car when taxiing.  During takeoff and landing runs there will be very few surprises.

Tail dragger planes require a bit more skill to operate correctly.  During a correctly executed takeoff the tail will rise off the ground as the plane accelerates to flying speed.  Gyroscopic precession, which causes a spinning mass to shift in a direction 90 degrees from the force that is applied, will cause the nose of the plane to turn to the side when the tail comes up, due to the spinning propeller.  A tail dragger pilot has to be ready to give the correct rudder input to keep the plane tracking straight down the runway.  Like all other aspects of the RC flying hobby, this is not a problem per se.  It’s fun.

There are a couple of problems that a pilot will encounter when flying a tail dragger, nose-overs and ground loops.   The term “nose-over” should be self explanatory.  Most of the time a nose-over isn’t a disaster, but it can cause some damage.  Ground loops tend to be a lot more troublesome.  They occur during the takeoff or landing run, if the plane starts to steer to one side uncontrollably, causing the tail to swing around in a tight circle.  This can have disastrous effects if the plane flips over as a result, which usually leads to the tail being broken off, or worse. There are a few ways to minimize the occurrence of ground loops and nose-overs, which I will discuss later.

DESIGN FACTORS

Assuming that you intend to be as good a builder of RC planes as you are a pilot, there are a few things you need to know about landing gear.  Regardless of which type of landing gear you have, you should install wheels that are large enough to roll over the bumps you will encounter on your runway.  For the best ground handling your main wheels should be wide-set.  Planes with narrow gear are more likely to tip over.

On a tricycle airplane the main wheels should be slightly behind the  center of gravity.  During takeoff the tail will rotate downwards at least a little bit, and to facilitate this action the main gear should be as close to the center of gravity as possible, ideally.  In the real world, you set the landing gear far enough behind the center of gravity to allow good ground handling without the airplane teetering on the main gear as it goes over bumps.  In other words, if it teeters and the nose wheel comes off the ground, move the gear rearward.  If you have a hard time lifting the nose when it’s time to take off, move it forward.  That’s pretty much all there is to main gear placement.  One other thing to consider when setting up a tricycle airplane is the length of the nose wheel strut.  If the nose sits high, the plane will be in flying position before it attains enough speed to fly above ground effect.  This could lead to a stall.  Also, a nose high stance will encourage bouncing when landing.  Set your nose gear low and the plane will be able to attain flying speed before taking off, and when it lands it will stick without bouncing.  A neutral stance will provide the best of both worlds, allowing the pilot to develop skills without unnecessary problems.

Tail draggers are more subject to design factors that affect performance.   This is because of an engineering flaw inherent in the tail dragger configuration, which is that the main gear is ahead of the center of gravity.  This is the cause of the ground loop and the nose-over mentioned above.   Whether the plane is being pulled by the propeller or being carried by its own momentum as when landing, the center of thrust is higher than the center of drag, which is where the wheels touch the ground.  As a result, the plane wants to flip over on its nose.   Both ground looping and nose-over can be reduced by lengthening the tail moment of the aircraft.  Assuming that you are dealing with an existing plane, this isn’t a factor that can be modified.  But if you are designing your own plane, a long tail will improve ground handling.

Setting up tail dragger landing gear can be a lot of fun for the RC enthusiast who enjoys engineering.  There are several factors that help to reduce nose-over.  The landing gear can be shortened to bring the center of thrust closer to the center of drag, the landing gear can be moved forward, and the wheels should be as free of friction as possible.  For instance, foam wheels have a lot of drag in the grass, so narrow, hard rubber tires should be selected, and they should spin freely on the axles.  Ground friction can also be reduced by flying from pavement rather than grass.  Pavement is the most effective solution to nose-over.

The other major tail dragger problem, ground looping, can be reduced by moving the gear rearward, widening the stance of the landing gear, flying from grass rather than pavement, and toeing the wheels inward at the front.  The reason why toe-in reduces ground looping is best demonstrated by pictures.  In the first diagram below, the wheels are toed inward 3 degrees each.  The plane is traveling forward and there is equal drag on both sides.

toein1

In the next diagram the plane has turned 3 degrees to the left, and the momentum is still straight ahead.  Now the left wheel is pointing straight ahead and the right wheel is turned 6 degrees.  The right wheel has more drag, which causes the plane to turn back to the right and center itself.

toein2

Setting your wheels straight ahead seems like an obvious thing to do, but it is clearly inferior.  Toe-out, which some people mistakenly recommend, encourages ground looping in the same way that toe-in reduces the tendency.

Flying from grass or pavement is the greatest factor in ground handling, and it will be based primarily on the flying site or sites that may be available to you.  For about ten years I had the good fortune to fly at a deluxe public flying field with a paved runway and a very smooth, well manicured grass runway side by side.  Planes that had a tendency to ground loop were always flown from the grass.  The ones that liked to nose over were flown from the pavement. Even if a plane is designed poorly and suffers excessively from either ground looping or nosing over, it can usually be corrected by runway choice.  But if you can’t choose, you will have to build your landing gear to suit the conditions you are stuck with.  The next major factor is fore-aft positioning of the main gear.  Generally, the main gear should be near the leading edge of the wing, but it can be adjusted for better handling.  Be sure that you understand the trade-off between ground looping and nosing over when placing your gear.  If you are restricted to only grass runway or only paved, this will determine where you put the landing gear for optimal performance.  Keep in mind that if your runway is rough, and you have a significant amount of ground handling trouble with a tail dragger, your best option probably is to use a tricycle landing gear.

One other factor I should mention, and this seems like the best place to mention it, is that a tail dragger with an excessively forward landing gear will tend to bounce when landing on pavement, unless the pilot has learned to execute a full-stall landing.  I believe that the other factors being discussed in this article are probably more important, but I should mention this for the sake of being thorough.  If you fly from a paved runway, you may notice this.

Besides runway conditions and fore-aft position, which are both compromises between ground looping and nose-over, the rest of the factors listed above will help without causing other problems.  No matter the type of landing gear you choose, it should be wide, tall enough for propeller ground clearance but not taller, slightly toed in, and free-spinning.