5
$\begingroup$

In comparisons between conventional gear vs. tricycle gear aircraft, it is generally explained that the CG location relative to the mains is a necessary condition for ground-looping. But there's very little more explanation than that.

I'd like to know more about the design traits which affect the propensity for a ground-loop. The following is an uninformed list of some things which I could see affecting a taildragger's ground handling, but I wouldn't hazard a guess as to relative importance.

  • CG location
  • Gear width
  • Gear shape
  • Gear springiness
  • Tire pressure
  • Tread traction
$\endgroup$
3
  • $\begingroup$ I would love to know the answer to this for improving my RC plane $\endgroup$ – ThatCoolCoder Feb 8 at 8:58
  • $\begingroup$ TIL that R/C planes also had ground looping problems. $\endgroup$ – Kenn Sebesta Feb 8 at 16:54
  • $\begingroup$ They have a stupendous number of problems in common with their larger brethren, at least as far as physics is concerned. It's embarrassing how long it took me to join the EAA after the AMA, to learn from those with more at stake. $\endgroup$ – Camille Goudeseune Feb 9 at 2:36
7
$\begingroup$

CG Fore/Aft Location

The further forward the main wheels are in front of the CG, the greater the propensity for ground looping.

Main wheel separation

The closer together the main wheels, the greater the propensity for ground looping.

Main wheel Toe-in and Toe-out

The following text and images come from the book "Landing Gear Design For Light Aircraft" by Ladislao Pazmany, obtained on-line via this web page on www.vincesrocket.com:

The "toe-in" and "toe-out" of a wheel is measured either in degrees or in inches as shown in Figure 3-9.

3-9

How the "toe-in" or "toe-out" affects the directional stability and control of an aircraft is a controversial subject. The author consulted with a number of aeronautical engineers and landing gear experts. The answers were mixed. Some indicated that with "toed-out" wheels as shown in Fig 3-10, the wheel on the outside of the turn tends to run straight and counteract the ground loop.

3-10

Other opinions were that the "toed-in" wheels result in more drag (due to skidding) in the outside wheel than in the inside wheel, therefore counteract the ground loop as shown in Fig 3-11.

3-11

The recommendations in some Service Manuals are that a very small amount of "Toe-in" (0 to 1/8 inch) measured at empty weight, is desirable. Essentially the wheels should run straight (zero toe-in) at normal operating weight. The PL-4A (Figure 1-11) designed by the author has 0 degree toe angle and exhibits excellent ground handling. Any tendency to ground loop initiation is easily countered with the rudder and steerable tail wheel. The same comments apply to tricycle gear airplanes.

Here's a post from an on-line discussion forum for rc airplanes where the author describes a particular plane where the landing gear can be quickly removed and re-attached to convert toe-in to toe-out or vice versa. The author notes that any toe-out seems to greatly increase the difficulty of keeping the aircraft going straight when the aircraft is rolling at high speed on the main wheels with the tail in the air.

The entire thread from which the above post was taken may be of interest-- the opening question is very similar to the question here: "What design aspects affect ground handling?"

Here's another post from another related thread on the same forum where the author-- a very experienced builder and flier of rc planes-- advocates some toe-in.

Camber

Camber is a significant factor. If the bottoms of the main wheels are closer together than the tops, the wheels have positive camber. If the tops of the wheels are closer together than the bottoms (like on a Me-109 or Spitfire), the wheels have negative camber. Note that if a tailwheel aircraft has zero toe-in but positive camber when it is an a fuselage-level attitude, then the camber will automatically create toe-in when the tailwheel is on the ground. If a tailwheel aircraft has zero toe-in but negative camber when it is in a fuselage-level attitude, then the camber will automatically create toe-out when the tailwheel is on the ground. This may be part of the reason why negative camber has generally been found to contribute to an increased propensity towards ground looping, and positive camber has generally found to contribute to a decreased propensity toward ground looping. Note also that when a ground loop begins and the "leading" or outside wheel starts to get more heavily loaded than the "trailing" or inside wheel as the plane starts to tip over toward the outside wingtip, negative camber will make the "leading wheel" tend to "dig in" or experience an increase in traction, exacerbating the process, while positive camber will tend to reduce this tendency.

Tire traction

Tailwheel aircraft show much less propensity towards ground-looping when landing on grass or wet pavement, than when landing on dry pavement. This might seem to suggest that any tread on the tires of the main wheels--or at any circumferential grooves -- should increase an aircraft's propensity for ground looping, by increasing traction. However this related ASE answer indicates that tread or circumferential grooves don't actually increase a tire's grip on dry pavement.

It would seem that anything that increases a main wheel's "contact patch" -- which would include decreasing the tire pressure-- would increase the ground-looping tendency on dry pavement.

CG height

A high CG location increases an aircraft's tendency to tip over and drag the outside wingtip during a ground loop. Even before the wingtip touches, any tipping will load up the "leading" wheel and unload the "trailing" wheel, which will exacerbate the developing ground loop.

Fuselage length

An aircraft with a relatively short fuselage is called "short-coupled", and will experience a greater propensity for ground-looping than an aircraft with a relatively long fuselage.1 If the mass of the aircraft is concentrated near the CG so that the moment of rotational inertia is low, the aircraft will have less tendency to ground loop than if the mass of the aircraft is distributed far from the CG.

Annex

Here is a PDF from a course on landing gear design. Note however that it doesn't go into a great deal of detail. Toe-in/out and camber are not discussed.

Footnotes:

  1. This sentence naturally begs the question-- exactly how might we quantify how "short-coupled" an aircraft is? Probably not by comparing the fuselage length to the width of the landing gear tread, as this would seem to imply that increasing the distance between the main wheels is not helpful. There may be some room for further refinement of this part of this answer--
$\endgroup$
5
  • 1
    $\begingroup$ This is good info! One thing I wonder about is "The closer together the main wheels, the greater the propensity for ground looping." Gliders are an extreme case of this, where the wheels converge to the maximum extent, i.e. there's only one of them centered in the fuselage. Gliders have almost zero likelihood of a ground loop, so long as the pilot don't drag a wingtip. Thoughts? $\endgroup$ – Kenn Sebesta Feb 8 at 14:20
  • $\begingroup$ This is an excellent answer. @quiet flyer, all its missing is sources for some of the sections. Any chance you could get a source link into each major block? $\endgroup$ – Kenn Sebesta Feb 8 at 16:53
  • $\begingroup$ @KennSebasta -- some of it is just kind of common knowledge that's widely available-- I'll think about seeing if I can find a some sources-- $\endgroup$ – quiet flyer Feb 8 at 17:28
  • $\begingroup$ Note that a similar toe-out strategy has sometimes been used on cars as well, and for almost the same reason - this is what causes the car to revert itself back to going straight if you take your hands off the wheel while driving. (Obviously this is not the case with newer cars with lane-keeping or self-driving features.) $\endgroup$ – Darrel Hoffman Feb 8 at 18:49
  • $\begingroup$ @quietflyer, thanks. It would be especially good to see if a toe-in/toe-out consensus has formed since Pazmany published his book 40 years ago. The back and forth on that topic suggests that even the most fundamental questions about ground looping remain unresolved. $\endgroup$ – Kenn Sebesta Feb 9 at 20:07
2
$\begingroup$

Addressing only why gliders, even with only one main wheel, rarely ground loop. Point by point:

  • No toe-in/-out.
  • No camber.
  • Tailwheel only, and often landing on grass.
  • Low CG. The wheel's axle may be almost as high as the CG.
  • Mass concentrated very close to the CG, especially if still carrying ballast.

Large wingspan and lightweight wings make it easier to keep the wings level with ailerons even late in the rollout, so by the time a wingtip drags, groundspeed is too slow to cause damage.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.