Ask any tailwheel pilot what the most important control axis (aileron, elevator, or rudder) is during a landing and you will most certainly get an emphatic “Rudder!”. But in that quick response lies a problem, and the source of so many bent, ground looped, or otherwise damaged, taildraggers.
Read the NTSB reports for tailwheel landing accidents and common themes appear in the pilot accounts: “the airplane veered”; “corrected with opposite rudder and brake”; “overcorrected”; “the aircraft departed the runway surface”; “the airplane ground looped”. What strikes fear in us as tailwheel pilots reading these accounts is the suddenness of this “veer” that these pilots all speak of. And the apparent powerlessness that they encountered as they relied on their tried-and-true instincts to correct for what was happening. But alas, with the rudder pedal at the stops, and maybe even some timely brake inputs, they were unable to save the airplane that day. They lost it.
Funny though, in most all of the accounts you read, never is an aileron input mentioned. Sure, what is happening, as the pilot can plainly see, is a sudden change in the yaw rate of the aircraft. Yaw equals rudder, and the instincts we’ve been taught tell us rudder is the input that controls yaw. A full boot of opposite rudder is immediately commanded, and through the magic of reversible flight controls, results in an immediate output at the tail. Just as we’ve been trained to expect.
But, let’s back up a few potatoes. To the moments before the pilot detected the yaw rate, in fact, let’s back up to precisely the moment before the the yaw rate initiated. At this moment, the condition of the aircraft is such: it is traveling forwards in space, and, by our bounding conditions, is not rotating about its vertical axis. So what is going to cause the aircraft to develop yaw rate? Well in the case of our demon the ground loop, it’s usually one of two scenarios (or the mix of the two if you really want to bend a plane that day):
The first is the most familiar to the tailwheel pilot: not touching down straight. It’s the fundamental credo of the tailwheel pilot that under his or her command, the aircraft WILL touchdown straight. In technical terms, straight means keeping the longitudinal axis of the aircraft parallel with its track across the ground – in other words, keeping the wheels aligned with the direction of motion. If not straight, the wheels being forward of the CG, when in contact with the ground, will create a yaw moment. If at anytime during the landing roll things aren’t “straight”, corrections must be made by the pilot. Pretty straight-forward (ha).
The second scenario though isn’t as often discussed. And it’s the one that really gets people, because like all good gotchas in aviation, it’s insidious. For the astute tailwheel driver, it’s the piece of information that tells so much about what is soon to happen to the aircraft: The bank angle.
Now, the bank angle scenario of course goes hand-in-hand with a crosswind landing, so the discussion will start there. When a plane lands straight (as previously defined) in a crosswind, there will inherently be sideslip (Beta as we call it in Aerodyamics) on the aircraft. Explicitly, the relative wind is coming from a direction not parallel to the aircraft’s longitudinal axis. What this does is expose the upwind wing to a very clean expanse of air while the fuselage partially blanks the flow over the downwind wing. Additionally, a side force develops on the fuselage. The net result of these two phenomna is a rolling moment away from the wind. Of course, all the while the weathervane effect of that big rudder surface is also doing its part to yaw the aircraft into the wind, but you’re a good tailwheel pilot and the lizard brain is already taking care of that with the rudder (or so it thinks).
Now, say the astute pilot notices this bank (caused by crosswind or otherwise), what do they do? Apply aileron to counter it of course! And a happy easy landing is made. But the less astute pilot doesn’t notice it. The aileron remains neutral. And perhaps the landing works out, perhaps it’s a little exciting, or perhaps it’s too exciting and around the plane goes. Sounds like the aileron did something, but the bank angle was so small, how did that really make all the difference trying to correct for it?
It’s only partially about the bank. The bank is a sort of red herring. A few things happen when that bank develops, and it affects pilots differently. For one, there’s an illusion. As the aircraft develops that slight bank, for many tailwheel flyers, especially novice ones, it messes with perspective. The ability to discern what straight is becomes out-of-whack, and without knowing it, they are no longer tracking straight. Not straight, yaw moment develops, and the classic scenario begins to unfold. But physics also comes into play, and those familiar with weight transfer in car racing will catch on quick here. You see as that bank develops, more of the weight and aerodynamic forces that are being reacted by the landing gear are shifted to the downwind wheel. And tires being what they are, always slipping, will slip less as the weight on them increases. As the tire becomes loaded up, it more efficiently reacts those side forces back into the gear leg; thus for a given amount of tire slip angle, the amount of lateral force (yaw moment) generated is greater…perhaps too great for the rudder to overcome! (Damp grass runways sound so nice right about now).
Oh, but there’s more to these silly ailerons when landing. And when in doubt, always try more. Remember back to the early days of your flight training as you learned about coordinating the aircraft’s 3 axis in the air. As with all thing in life and flying, it was a give and take. The 3 control axis, while independent in the surfaces they move (usually), each affected the response of the aircraft in all 3 axis in one way or another. Deflecting those ailerons almost always caused a little bit of yaw opposite the bank. Adversely as we call it. The simple answer to this holds true, the downward deflecting aileron, way out on the end of the wing, creates more drag than the upward deflecting one, resulting in a yaw moment opposite of the bank. Hey, wait a minute, yaw moment opposite of the bank? That sure sounds like exactly what is needed in a crosswind landing. Bingo!
Of course, the amount of yaw from the ailerons varies from plane to plane. In the modern era, adverse yaw has been beaten out of the aerodynamics of those craft to the best of aero engineer’s capabilities. But thankfully our old trusty taildraggers are usual not in that category and many of our ilk have gobs of adverse yaw for us to use. So use it! Every time you land, note the wind and pour on that aileron. And don’t relax it till you roll to a stop…why would you give up free directional control?
My method: put in all the aileron, if the downwind wheel begins to lift, back off. Increase your input as the airspeed decays and hold to the stop until the plane is slowed.
I have ranted enough. Time for some demonstration. YouTube, through the efforts of some planespotter and some very honest pilots, affords us some fabulous footage of such ground loop events. Let’s take a look:
Almost Looping a Luscombe
From short final we can see the plane is not straight. But also note the bank. By the combination of the two, we can surmise the wind is from the left (later confirmed in the video). As the plane touches down, it is already pointed slightly left of its track bearing, resulting in a further tendency to track left and yaw to the left. The bank away from the wind is perceivable by comparing the spar to the horizon. And, or course, the pilot, as seen does not put and aileron into the wind. The result? nearly runs off the upwind side of the runway after realizing all the rudder he has is still not sufficient. Many thanks for this honest debrief video; may it serve many of well!
That 170B at Santa Monica Video
This one has made the rounds over the years for sure. We can see in the first fe seconds that the winds are light, slight right to left crosswind. Probably the most common ground loop cause is a light crosswind (if I had to guess). Touchdown while bouncy appears straight. The slight pitch oscillations however may have caused some oscillatory yaw moment from propeller gyroscopic. The frame of the video cuts in too close during those next key moments, but soon we can see the aircraft start to bank away from the wind, and the yaw rate develop. Now as the full airframe comes into view, we see the ailerons squarely centered, and possible minimal rudder inputs even. By this point our pilot is having a bad day as he proved out the resilience of Cessna spring gear. Some structural inspections hopefully happened before the next flight.
The Stearman that Didn’t Even Use What Little Aileron it Has
Stearman are not known for being crosswind airplanes. And their lack of aileron control at low speeds is keen to why I am very nervous in gusty crosswinds! The wind is coming at him left-to-right. Strangely the downwind wheel contacts first, even though the rudder is clearly showing the crosswind that is being fought. A slight bank away from the wind is visible throughout the roll out (and no aileron into the wind). As speed decays all is lost, far too late aileron is put in, but around they go. The grass thankfully saves a full lower wing panel replacement.
Be a Champ and Share a Mistake so we Can All Learn
Another honest debrief video. While hard to see due to the camera’s wing location, it appears there is a wind from the right. The aircraft lands for a wheel landing (as evidenced by the pilots immediate forward stick, prop gyro causes a left yaw in this case) and begins to yaw to the right. Pilot stick input remains squarely centered in roll as he soon runs out of rudder and goes around. Into the grass, plane isn’t too hurt…but still not great on that very old Aeronca gear.
You Can’t Win ‘Em Maule
And finally, a Maule fights a gusty crosswind at Stellenbosch, South Africa. The gusts seem to be exciting a dutch roll mode in the aircraft, possibly amplified by a PIO from out of phase corrections by the pilot (Haven’t flown a Maule, so hard to say specifically about the handling qualities). But that being said, at the moment of touchdown the pilot appears to have hit straight. And then soon relaxes to neutral aileron. A bank develops away from the wind and soon the aircraft veers into the wind. A large reversing input is made and soon the aircraft groundloops away from the wind. Thankfully it’s a tough plane and seems to fair alright. Thanks for sharing, lots to learn!
[Disclaimer: The above is all opinion. Free advice. Perhaps you got what you paid for. Perhaps you got more. Maybe less. Discussion is the goal. These are my thoughts after 1800 groundloop-free tailwheel hours in one of the windiest places in the US; and after coaching a lot of folks in the tailwheel art.]