Or, Turn the Other Cheek
Story, art, and photos by Murry I. Rozansky, firstname.lastname@example.org
Ground loop. An ugly word no matter how you define it and perhaps the dominant fear among those without conventional gear time wishing to keep it so. The blame of course is on the single (little) wheel at the rear of the plane not being on the nose. But maybe it’s not the location of the wheel that’s at fault as much as it is the function of the wheel – that being directional control. That begs the question, could a taildragger configuration be tamed if the mains were steerable?
There are different kinds of truth. One type is the partial truth. An example was the statement by a BMW vice president of engineering on setting a speed record for hydrogen-powered cars. “Hydrogen is the most abundant element in the universe,” he said. Now, that is true out in space and in stars, but it’s not true here on earth. It’s a true statement but somewhat misleading, the type of statement you would expect from a politician, not a technical guy.
Another type of truth is conventional wisdom – an incorrect statement that is repeated so often it becomes accepted as the truth. That the world was flat and the earth was the center of the universe were accepted truths at one time.
I feel a little bit like Galileo or Darwin as they challenged the conventional wisdom of their day. Our subject, debunking the “conventional wisdom” concerning directional instability of taildraggers, should be less controversial than the teachings of Darwin or Galileo, and hopefully my fellow aviation enthusiasts will be open minded. Being tarred and feathered isn’t my idea of a preflight. I once believed the conventional wisdom, so let me step through the clues that led me to the startling conclusion that:
Taildraggers aren’t directionally unstable, because they’re taildraggers.
Anybody who has taxied a taildragger knows they do not want to go straight. Yes, the main wheels are in front of the center of gravity (CG), and any deviation causes an increasingly tight turn if not stopped with the rudder, tail wheel, and/or brakes. We learned this in ground school, and our experience shows this to be true. That is the conventional wisdom. But I’ve found it isn’t the whole story.
What we have been taught are the most important facts.
- A taildragger’s main wheels are in front of its CG.
- The mass and inertia of the aircraft act through the CG.
- The tires will resist the sideways motion.
All this will turn the aircraft’s nose opposite from the direction of the side motion, increasing the slip angle of the tires and therefore the side force the tires are experiencing. All this side force is produced in front of the CG, which Newton has told us would rather continue in a straight line but the airplane responds by rotating around the vertical axis. It’s a runaway situation. If not stopped early (with the rudder, tail wheel, and/or brakes), each bit of rotation increases the side force on the tires in front of the CG causing more rotation, producing a classic ground loop.
One of my first doubts about the conventional wisdom came from considering that a ground loop is much like a spinout of a tail-heavy car like the old rear-engined VW Bug or the Corvair. But airplanes aren’t tail heavy.
Another clue came from my research into roadable aircraft. (See KITPLANES, September 2008.) They all seemed to have an automobile type or motorcycle type landing gear. That in turn led me to investigate three-wheel road vehicles, which led to a key discovery.
There are two major types of three-wheelers: the trike with one wheel in front and the tadpole with two wheels in front and one in the rear. The more stable of the two is the tadpole with two wheels in front. The trike arrangement with the single front wheel leads to terminal oversteer, a spinout, when pushed to the limit. If the CG is high in relation to the wheels, they can easily tip over in a turn. That led to the production of trikes being banned as an all-terrain vehicle configuration.
When you look at the wheels and CG drawings of the road vehicles and compare them to the wheel and CG drawings of aircraft, you’ll be puzzled as I was. The trike arrangement of the road vehicle looks just like a nosewheel airplane. The tadpole arrangement of the road vehicle looks just like a taildragger aircraft. Why does the more stable of the road configurations look identical to the notorious ground-looping taildragger? What’s going on? What’s the difference between the ground-bound configuration and the airplane?
In doing research for this article, I checked other design and flying taildragger books in my library including the classic Stick and Rudder (1944) by Wolfgang Langewiesche. When I was learning to fly at Sky Manor Airport (Pittstown, New Jersey) in the early 1970s, one afternoon I found myself standing next to the famous author watching some of the other students practicing takeoffs and landings in the Cubs. He asked me, “How come you guys make such good landings?” I replied as a-matter-of-fact as I could, “Wolfgang, we read your book.” There are hints supporting my theory in his book as in others, but there isn’t one I’ve found that has put it all together. The Compleat Taildragger Pilot by Harvey S. Plourde was quite useful as it has a complete detailed chapter just on the crosswind gear and its piloting techniques.
The crosswind landing gear was another clue. Around World War II, attempts were made to tame the taildragger, and forms of the so-called crosswind gear were developed. It was recognized by their designers that the side force produced by the main wheels ahead of the CG was causing the initiation of the ground loop. A mechanism was developed to allow the main wheels to caster if there was any significant side force on them. Tires produce side force when they operate at a slip angle. When they’re free to caster and are more or less vertical, they can produce no side force and therefore no ground loops. The crosswind gear effectively tamed Cessna 190/195s and other taildraggers. They could touch down in a crab and it was automatic, unlike the B-47 and B-52 when the pilot sets the landing gear crab angle. The spring-loaded detents would pop out and let the main wheels caster. No side force. No ground loop!
Have you figured it out yet? If you haven’t, here goes.
Taildraggers are directionally unstable on the ground, not because they’re taildraggers, but because we’ve been trying to steer the wrong end.
Castering turns out to be “the magic.” If the wheel or wheels in front of the CG are free to caster, they can’t produce any side force, and it’s side resistance in front of the CG that causes the ground loop. A fixed or locked tailwheel or the main gear on a tricycle-geared airplane are behind the CG and help stabilize the aircraft on the ground just like the vertical fin does in the air. Our conventional tailwheels can do little to stabilize a taildragger. Without a castering main gear, even a fixed tailwheel can only help a little to stabilize a runaway taildragger. A deflected nosewheel on touchdown can trigger a ground loop in a tri-gear airplane, too. Most nosewheel steering linkages have springs to minimize the effect of misplaced feet.
Do you see it now?
It doesn’t matter how many wheels are where. If the wheel/wheels in front of the CG can caster (or steer) and the wheel/wheels behind the CG are fixed, you have a stable vehicle on the ground.
Repeat our video experiment for yourself. Take a shopping cart with good wheels to a parking lot with a slight down slope and launch it gently downhill, first forward with the castering wheels in front. Then try launching it backwards with the castering wheels in the back. See what happens.
The Costco Experiment
This is a simple experiment you can do at your local shopping center parking lot. Find a lot with few cars. A slight downhill slope is helpful, as you won’t have to push too hard to keep the cart going. Pick a cart with good wheels and casters that turn freely. Try it first with the casters in front. Give it a good straight push downhill; it should track like an arrow downhill. Now turn the cart around with the fixed wheels forward and the casters in the rear. It may take a few tries to get a straight launch. The casters in the back can do nothing to stop the increasing turn rate. It’s a classic ground loop. Sometimes, if the slope is steep enough to keep the cart going, it will make a 180 and go the rest of the way down the hill as God intended, with casters forward and fixed wheels in back.
With a shopping cart facing in the normal direction and with the castering wheels up-front and the fixed wheels in the rear, we have a very stable platform, analogous to a tricycle landing gear with a castering nosewheel or the author’s proposed fixed tailwheel.
Now that we’ve solved the ground-loop problem, are we going to see a wholesale return to taildraggers? Even though the Concorde had a nice tailwheel assembly, I don’t think Airbus will convert the A-380 to a taildragger even though it would save weight. Gravity-assisted loading and unloading are interesting thoughts but would require a massive redesign of the aircraft and the airport gates.
Light-sport aircraft (LSA) and bush planes are the logical potential applications for my “New-Age Taildragger.” The weight and drag savings might be worth the effort. There’s no free lunch, of course. Castering and steerable main gear are going to add some weight and cost, and there are some other problems to consider. Our directionally stable taildragger can still be dropped in, and if the main gear rebounds, it increases the angle of attack leading to multiple landings or worse just like a conventional taildragger. This problem can be solved by proper landing gear design.
The simple spring gear wouldn’t be very suitable as it depends on the side scrubbing of the tires for what little dampening it has. The wheels going in opposite direction would make the design of a steering linkage problematic. Free-castering main wheels with differential braking might work. Differential main braking and castering nosewheels are used to steer some nose draggers, but I think the weight on the mains for my “New-Age Taildragger” may need too much braking to control the aircraft on a significant side slope. A telescopic or trailing arm gear, something with substantially vertical travel, would be most suitable.
My current guesses as to handling techniques are as follows:
- Land like in an Ercoupe, in a crab more-or-less wings level.
- If the fixed tailwheel is in the air and main wheels are down, maintain the crab and control track with the rudder.
- When the tailwheel comes down, it will align the fuselage with the ground track and you would transition to directional control with the main wheel brakes and/or steering.
- It might be best to land and take off from a less than stall angle, three-point attitude.
Until someone builds and starts developing the piloting techniques, we won’t have the answers to what might be the best way to handle steering and the transitions from wheel to rudder control and back again.
There are certainly challenges to overcome before we can bring back a taildragger an insurance company could love. If this isn’t enough of a challenge, maybe your editor will let me tell you why tricycle-gear airplanes should have nosewheel brakes.
Noteworthy experimental aircraft that have steerable mains:
On the cover of a vintage CONTACT! Magazine issue is Terry O'Neill Magnum bushplane powerd by a Ford 351 Windsor engine.
You never know what might show up at Flabob. Roadable aircraft might be all the buzz these days, but here’s an example that’s been around a while – a Nesmith Cougar with a retractable drive pod. Note that the mains are stearable.
Inside the Cougar we can see the dedicated steering wheel.