Preventing groundloops in taildraggers

[CBLong]

Hi,

I've got about 75 hours on the standard C152/PA28 nosedraggers, but I'm hoping to do a tailwheel conversion over the summer, in anticipation of which I've been reading up on the peculiarities of the ground dynamics etc. I was interested in some of the technical fixes that have been used to make taildraggers more user-friendly, in particular castoring main gear (!), which seems to cause as many problems as it solves.

This got me thinking about an alternative solution, which I thought I'd post up here to see if it's been tried before and/or if it would work! My idea would be to allow each main wheel to castor out by a few degrees (maybe 5-10), probably against spring pressure, but to not allow it to castor in at all. By "out" and "in", I mean that the left main wheel would be allowed to "steer" anywhere from straight ahead to 5-10 degrees to the right, and vice versa for the right main wheel.

The reasoning is this: during a normal ground roll, any crab angle developed results in both main wheels crabbing at the same angle relative to the direction of movement, and hence developing the same frictional force. However, considering a crab to the left, the left wheel has a greater lever arm from the CoG, thus causing a further yaw to the left, and so on until you hit the nearest hedge. If the left wheel is allowed to castor to the right, but the right wheel is forced to remain pointing straight ahead wrt the aircraft, the left wheel would then develop a much smaller frictional force that the right, leading to a net restoring yaw torque to the right.

(Did I explain that well enough?? :rolleyes: )

Sounds reasonable to me? Does anyone know if this has ever been tried, and (assuming it has, which seems likely) why doesn't it work?

Cheers,

cbl.

[kabz]

I think I can guess at one effect, and that would be an almighty wheel shimmy ... that would probably take the plane apart after a few landings.

The shimmy from the tailwheel on a Citabria can give you a pretty severe shaking ... so I hate to thing what shimmy on a castoring main gear would be like ... :(

In practice, tailwheelers aren't so bad, just make sure you keep it all straight on landing, and you'll be ok ... there's a great point where steering down the runway with your feet suddenly becomes completely natural ... great feeling !

[Miserlou]

Hey, don't get your knickers in a twist about taildraggers.

You have probably heard horror stories about them, but mostly from people who don't fly them or from some-one who does fly them to some-one who doesn't.

They're not difficult to handle-just different!

They do, however, require a higher level of airmanship and this will make you a better pilot in any machine.

I trust you'll be doing it with the Tiger Club, bearing mind location.

You'll have a lot of fun. That's a real treasure trove of knowledge, there!

[CBLong]

Thanks for the replies. I'm not too daunted by the idea of learning to fly taildraggers - it should be a good challenge and will undoubtedly improve my general flying all round.

kabz,

Yes, I guess shimmy would be a potential problem, but you could build in quite a lot of damping without affecting the effectiveness of the system - and bear in mind, full castoring main gear has already been used, I wasn't making that up!

Thinking about it some more, an even simpler solution would be to have fixed main gear but with, say, 5 degrees of toe-in. That would achieve the same thing, although I guess it would only be suitable on grass - you wouldn't want your tyres being scrubbed away every time you landed on tarmac...

Miserlou,

Yes, I'm quite keen to get involved in the Tiger Club. There's an article about G-ACDC (the oldest Tiger Moth still flying, 70 years old!) in this month's Pilot which has definitely whetted my appetite. The Tiger Moth has been one of my favourite aircraft since my Airfix model, knee-high-to-a-grasshopper days - it's hard to believe I could be only a few months from not only flying one, but being checked out to do so solo!

Having said that, there's now a Stearman available at Manston (my usual haunt) which looks like great fun too, and it's got a big sexy radial...

Roll on summer, :cool:

cbl.

[QDMQDMQDM]

The simplest way to prevent groundloops in taildraggers would be to take the wheel at the back and put it in the front, just under the engine, then stick the main wheels a bit further back to compensate and get the whole thing to balance upright with the C of G forward of the main wheels. I'm told it's been tried, and with some success apparently, although much of the fun of flying is, of course, lost.

;)

QDM

[ChrisVJ]

CBLong

If the left main wheel is pointing slightly right that is TOE IN.

On old cars we used to have to check the toe in regularly to save on tire wear. The theory was with a degree or two of toe in the 'slack' in the rubber mounts etc would be taken up and the wheels would run true. Of course when front wheel drive came in the situation was often reversed and we set toe out. Problem with FWD and toe out was when you braked the wheels suddenly became even more 'Toe out' leading to altered steering chacteristics. Major advances in steering geometry have almost ruled out these things leaving todays cars duller but safer.

Depending on the type of U/c there may be a case for some toe out on a tail dragger, then if the plane pulled to the right there would be more weight on the left wheel which would then pull 'out' to the left but I don't think the effect would be nearly enough to change direction considering the cg and other forces at work, certainly not worth the extra wear on both tire and gear.

[flickoff]

CBL Yur 'anin' a larf; Conventional u/c is no harder than the nose wheel stuff, it's just differentish. All the scare stores are b(*&^%$$£cks put about by nose wheel drivers who couldn't hack it. Only one tip, remember to move your feet, the shorter the couple of the a/c the more foot movement needed and the quick it goes tits; do nopt stop thinking, as it's not over till you have stopped, and I mean stopped.

I think the earlier Spitfires had both toe in and camber and on tarmac, they chomped tyres with remarkable repacity.

[Prof Denzil Dexter]

Some tips to prevent a groundloop in a tailwheel aeroplane......

Rule 1 - Keep the bloody aircraft straight!!!!!!!!!!!!!!!

Look at the far end of the runway, and keep the aeroplane tracking straight - If it starts to swing 1mm either way, correct it immediately and then get ready for the next swing.

Ensure you reduce the swing correction before the aircraft starts to swing the other way..

Have high standards - dont accept anything but the best from your feet...

Be relaxed on the controls - tense pilots do not have a feel for the aeroplane.

Remember, nosewheel pilots - the rudder pedals are not just for resting your feet on!

Believe me, it ain't difficult, I have flown 50+ tailwheel types of all sizes and none of them have been a handful on T/O and landing, even with a bloody great big engine up front. Just keep the aircraft straight using whatever controls you need (Rudder, brakes and aileron if required), and dont give up 'til you are having a beer in the bar!

If the aeroplane has a lockable tailwheel, dont forget to lock it before take off and landing.

If you let ANY aircraft fly YOU, instead of YOU flying IT, IT will embarrass YOU in a big way..

P.S. You cant learn it from a book.. Get good instruction, and learn from your (inevitable) mistakes...

And have fun, 'cos thats what we do it for..:O :O :O

[FNG]

CBL hasn't suggested that it's particularly hard, just different. Tailwheel pilots (myself included) tend to say "it's no big deal", but this doesn't stop just a teeny bit of preening going on: "ha! I have tamed the wilful beast andf am a p-roper pilot or, indeed, an aviator, as in the days of yore").

I hope that Mr John Farley will not object if I cut and post here a very interesting short essay on ground loops which he posted on pprune a year or so ago. As in all his writings, he explains things very clearly. Here it is:-

***

Ground Loop

If you have a paper and pencil, the whole subject takes 30 secs to cover but I don’’t know how to put the appropriate diagrams up here, so a lot of words now follow
Ground loops happen because of the force due to friction that exists between a tyre and the ground.
When an aircraft touches down with a zero drift angle we can ignore the friction forces (sure they have to spin up the wheel but once that has happened the wheel just rolls and does not disturb the aircraft)
But when a wheel touches down with a drift angle, friction causes a drag force which acts 180 deg out to the direction that the aircraft is travelling (not the direction it is pointing). Visualising this is easy if you take an extreme case like 90 deg of drift (!)
This force is not in the plane of rotation of the wheel, so for our convenience we can split it into its two components, one in the plane of the wheel and the other at right angles to the plane of the wheel. It is the one at right angles to the wheel that creates the piloting problem.
Any force applied at right angles to the wheel is ACROSS the long axis of the aircraft and so will make the aircraft yaw.
Whether the yaw so produced REDUCES the original angle of drift or INCREASES will either make our day or spoil it.
So, and thinking only about the main wheels, if you land with the nose heading to the left of the runway and with the aircraft tracking down the runway, the nose will tend to snatch further left (bad) if the main wheels are in front of the CG (taildragger) but pull round towards the runway heading (good) if the wheels are behind the CG (tricycle).
This is the crux of the issue, weight is not strictly speaking a factor. What is a factor is the yawing inertia of the aircraft as this RESISTS the tyre induced yaw whichever way it acts. Thus, for a low inertia aircraft (probably light and short) things will get better or worse very much more quickly than for a high inertia aircraft (probably heavy and long)
I think it is important to recognise that during a ground loop the CG continues to move over the ground essentially in the direction it was travelling at touchdown but the aircraft ROTATES in yaw about the CG (as opposed to a notion of the CG being flung out sideways)
The last time I did a ground loop (it was a secret between me and about 6000 people at the airshow) the aircraft stayed on the runway centreline but remorselessly yawed to the right until I was going straight, but backwards, down the runway (at this point I knew what to do –– open the throttle, until I finished up going forwards and calmly taxy towards the turn off I had just missed……..)
If I have explained things well enough that you are happy with the above all the rest of the ground loop stuff follows easily:
They are less likely to happen on grass as it tends to produce less friction than tarmac.
If you have a tailwheel lock, then the side force from the tailwheel will be stabilising (behind the CG) If you forget to lock the tailwheel it just castors and produces no stabilising effect (bad)
Now you can see why tailskids provide nicer handling ON GRASS, as they tend to dig in more than tailwheels and so enhance their stabilising effect.
You can also see that ground looping on take off is perfectly possible if you let ANY yaw go uncorrected for more than a moment. If you do this the aircraft momentarily continues on its original tack, but points to the side, and bingo the tyres come into play. With a high powered piston, the initial yaw can easily happen due to a torque induced swing or from raising the tail a tad quick and generating a gyroscopic yaw from the prop.
My ground loop (in Bob Mitchell’’s Ryan PT 22) was caused by me being an idiot and lowerering the tail too fast after a main wheel only touchdown –– plus a strong cross wind with a tail component, trying to help the busy controller by expediting my arrival between displays and all that sort of stuff that you know you should not do.
JF

***

Alas, the only thing which my flying has in common with Mr Farley's is that I prefer to ground loop in front of an appreciative audience (all those nice spotters at an Old Buckenham fly-in waved and cheered so kindly, assuming that I was displaying the balletic grace of my aircraft on the ground just for them).

[CBLong]

Thanks for all the replies, everyone. It appears I have marked myself out as a scaredy-cat spam-can driver - bu**er! FNG has read me right - my musings were more out of a general curiosity than from any fear or concerns about losing my nose wheel...

Nevertheless, however much fun it is learning to fly tailwheel types, and however easy it is after a few(!) hours of practice, I still think it's interesting that such a popular landing gear configuration is fundamentally unstable on the ground, leading to "squirrelly" handling, lower cross-wind limits etc. Fair enough on a cub, but on a DC-3 or a Lancaster?? Blimey. I was just wondering aloud if any stability-enhancing tricks (such as my idea above) had ever been tried...

Would any of you fancy a go in a machine that was fundamentally unstable in pitch whilst airborne? "Don't worry, it'll teach you what your biceps are for..."! :D

[AerBabe]

Yikes... you mean *shudder* helicopters? :D

[Miserlou]

They weren't aces with thousands of hours those kids flying the heavy bombers or for that matter the fighters.
People are just not trained to the same standard these days; they're taught to operate in an environment!
That should give you a clue to how it really is.

PS. Helicopters are dangerous aren't they?

[Mike Cross]

No one seems to have mentioned this bit.

On a nosewheel aircraft the c of g is in front of the mainwheels, in a tailwheel it's behind (otherwise they'd both tip over).

So you land a tricycle and you have drag acting behind the c of g. The resulting couple (remember your physics) tends to keep the aircraft in a straight line.

In a tailwheel you have the drag taking place in front of the c of g, so get the slightest bit out of line and you have a couple tending to yaw the aircraft. Let it go too far and the back wants to overtake the front.

Braking increases the couple and hence the effect.

And of course as the direction the wheels are pointing in deviates from the direction the aircraft is actually going in, it introduces a sideways force on the mainwheels, which increases the turning moment. Higher friction surface, more sideways force, so it's likely to be worse on tarmac than on grass.

That said, it's not difficult, just different.

Mike

[Dan Winterland]

I used to instruct at a professional school where we used taildraggers. We didn't tell the students their aircraft was more difficult than a nosewheel type, we just showed them how to control the thing and let them get on with it. The fact that many of them used to solo within 10 hours shows that taildraggers can't be any more difficult.

Just different.

[Final 3 Greens]

QDMQDMQDM

much of the fun of flying is, of course, lost.

Yes, those nosewheels really do interfere with the handling at 2000ft :D :D :D :D :D

[FNG]

...though they do muck up yer drag profile somewhat...

Although of course you can't learn about taildragging or other aspects of flying just from books, David Robson, who has written a good book on aerobatics, has brought out an interesting book on tailwheel flying, which is worth a look: "Three Points". £20.

FFF has recommended Harvey Plourde's "Compleat Taildragger" (correctly spelled thus, as Isaak Walton fans will appreciate), although I haven't read that one.

[Final 3 Greens]

FNG

Not in an SF260!

That's as good as it gets for me. ;)

[AerBabe]

I'd also recommend FFF's copy of "Compleat Taildragger" (sic) ;)
But even more so I'd recommend the tiny booklet Stik sent me. It's incredibly simple, with straightforward explanations - all covered in just a few pages. Unfortunately, I can't remember any details and it's ca. 150 miles away... Stik?

[andrewc]

Maybe I'm being a little harsh...and doubtless will be told
so...but what actual advantages do taildragging bring to
an aeroplane?

More effected by wind on the ground...? yes

Worse view while taxiing...? yes

Less stable on landing...? yes

What are the flight advantages which make up for
these poor handling (insurance) issues?

-- Andrew

[Kingy]

Andrewc,

How about better short field performance and more speed?

What you should really be asking is what advantages taildragging brings to the pilot.... how about

Knowing what the rudder pedals are for...? Yes

The ability to land without drift....? Yes

Choice of landing techniques ....? Yes, wheeler or 3 point

Being able to fly the most interesting vintage, classic and aerobatic aircraft...? Yes!

As for this insurance nonsense - you may wish to check out some AAIB reports Here (http://www.aaib.dft.gov.uk/bulletin/bulletin.htm) ... mmm... seems like a lot of collapsed nosewheels eh? - I rest my case.

Kingy

[FlyingForFun]

Better short field performance, Kingy, I agree. And better soft-field performance, too, in most cases. More speed only applies for non-retractable types - once you fold the wheels away, it doesn't really matter where they are.

But I think you are right about the big benefits being for the pilot. I remember reading somewhere (may even have been here) a discussion about cross-wind landings, in which someone suggested that tail-wheel aircraft are better in cross-winds. After a bit of discussion, the conclusion was that this is not correct, and that tricycle aircraft can handle more cross-winds than tail-draggers - but that a tail-dragger pilot in a tricycle could handle more cross-wind than a tricycle-only pilot. All a generalisation, of course.

Incidentally, any fear I had of ground-looping was eliminated when a (very experienced) instructor demonstrated that a Super Cub can be landed quite safely pointing sideways. In still wind conditions, he flew the whole of the final approach in a crab, landed in the crab, and rolled out with just one wheel on the runway. In still wind. Then he got me to repeat it. You have to be very fast on the rudders, and I'm not suggesting that anyone try this without an extremely experienced tail-dragger instructor on board - but it proves that these aircraft will only bite you if you don't know how to handle them.

Have fun!

FFF
-------------

[stiknruda]

I'd recommend both Plourde's Compleat Taialdragger and Langesweiche's Stick And Rudder.

The wee booklet that I sent Aerbabe was acquired FOC from Cubair at Redhill.

Nosewheels are fine but I can't convince myself tha all the extra weight and drag are worth it for my type of flying, save of course when I'm in the back of an airliner!

Stik

[FNG]

Langewiseche is excellent on all aspects of flying, although it's fair to point out he was a promoter of the tricycle gear for everyday flying. He took the view that conventional (that is, tailwheel) undercarriage was good as takeoff gear but not so good as landing gear. It's no accident, though, that modern high performance aerobatic types are tailwheelers (retractable gear would add a lot of weight) and, as pointed out above, being able to fly tailwheel opens up access to a variety of interesting aircraft, old and new.

PS: The US edition of David Robson's book is called "Conventional Gear" and it's cheaper on Amazon.

[Kingy]

FFF,

I accept your point about retractable gear ... didn't think about that!

As for X winds, I think its all about a positive mental attitude and commitment. When flying with pilots not used to tailwheel strip flying I'm always surprised how they generally 'freeze' in the last stages of an approach - exactly at the time when liberal movement on the controls is often needed!

I've done unto 20tks X wind in the L4, the technique is to fly finals in a full slip right into the flare, then turn it into a tail down wheeler at the last moment making sure the upwind wheel is planted first. This is more than double the supposed limit, but the aircraft has powerful controls and it's achievable - you've just got to commit... I’m only an average pilot, but one who regularly flies old taildraggers into short strips in crosswinds.

The key to taildragging (for Me) is:

Be hard on yourself - drift on landing is not acceptable.

Practice, practice, practice (I do at least 1hr a week of circuits)

A 'can do' attitude - commit to an approach and use all your controls to make the aircraft do what YOU want it to do.

Elevator for airspeed, throttle for rate of descent - its got to be instinctive..

If the prop's turning - you’re flying. It’s not over till the planes tied down.

Kingy

[ianfallon]

If you want a good analysis of the forces involved, "The Compleat Taildragger Pilot" by H.S.Plourde is a good read. Goes well on the bookshelf next to "Stick and Rudder" :D

Ian (groundlooped a Cessna 120 :eek: but thankfully not the Auster :D )

[poetpilot]

...and in terms of owning a taildragger (and hopefully this appies to those organisations renting 'em out) it's probably obvious, but maybe needs emphasising, that maintenance and correct setup of the tailwheel assembly is critical in terms of keeping the aeroplane's ground handling characteristics predictable.

On the Jodel I part owned for many years, the difference when we replaced a tired tailspring was marked. The change in angle when the tail was sitting on the ground greatly improved its responsiveness.

[stiknruda]

Poetpilot's post prompted me to add this...

On both my taildraggers, the tailwheel and tailspring geometry is such that as the tailspring becomes "tired" and adopts a more relaxed shape, the rudder sits slightly nearer the ground and the chains that connect the rudder horns to the tailwheel steering horns become slacker.

My PFA inspector (also a licensed engineer) insists that I frequently re-tension these chains by removing a link.

On the odd occasion when I have flown similar types with slack tailwheel steering chains, the ground handling - especially when slow maneouvering close to other aircraft or obstacles is required leaves a lot to be desired! Obviously as speed builds up it is not such an issue as the rudder becomes energised but the amount of deflection required is greater than the correctly tensioned set up. You also experience a lot of free play which proves that you are not getting full tailwheel deflection.

On the Maule type tailwheel which has a "straight ahead" detente, slack springs allow "straight ahead" to disengage too easily.

I also regularly drop the tailspring and check that the spring and retaining bolts and saddles are in good nick as several years ago I made a very exciting excursion off a runway when the bottom leaf decided to fracture after a super smooth landing. The rudder and spring shook hands and this locked the rudder up. I exited stage left rapidly and nearly managed to stand somebody else's pride and joy on its nose as I stood on the brakes!!

Tail springs take a lot of punishment and do fail. It pays to make sure that they are well looked after.

Stik

[Kingy]

I discovered what a huge effect worn bungees can have on ground handling last year.

I was having real problems controlling the swing on take off on my Isaac's Fury. We operate out of a field with a marked camber so she often wants to run off left anyway, but I needed a combination of full right boot, half throttle till tail up and sometimes brake too - things were getting a little hairy!

To cut a long story short, a bungee snapped on landing (a non emergency as there are loads of turns..) One phone call to Light Aero later the new bungees arrived and once fitted totally transformed the ground handling - much, much reduced swing...

Turns out the squidgy old bungees were allowing the gear geometry to change on application of power making it veer off in an alarming manor...

A lesson learned

Kingy