Ground loop?
 

I hear about landing accidents where the aircraft has "ground looped" Can somebody please enlighten me as to what act or omission results in this occurrence? Thanks.

For those pilots who regularly fly tailwheel aircraft the following is certain....

"there are those that have.. and those that will groundloop"

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.....it goes so quiet when the goldfish dies.

[This message has been edited by Algernon Lacey (edited 07 October 2000).]

I have seen a PA28 ground loop on a calm day, I wish I could do that!!

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WHOOP WHOOP
PULL UP!!!!!!

STP - a ground loop most usually happens on a tail wheel aeroplane because the center of gravity is behind the two front wheels.

While things are going normally everything is fine, but if the line of travel of the aeroplane starts to swing to one side then momentum makes the C of G keep going in the direction i.e.: out to the side!

This is why, when flying a tail wheel, you have to concentrate on keeping it under control. I was taught to use the rudder pedals as if they were the pedals on a paddle boat and that seems to work!

You can ground loop a tail wheel, even at low speed taking off or landing, if you don't keep on it. At least embarrassing... maybe dangerous. Obviously the heavier the aeroplane the harder it is to keep the C of G under control.

I am sorry I cannot believe an instructor would ask such a question? Has Flying instructors been infiltrated by Wannabees?

hear hear fallen eagle

I was under the impression this forum was more like a "staff room" for instructors, whereas Wannabes and R+N were the classrooms.

If I've got a question about what to do to/for/with a student, it comes here, if it's how to fly, it goes to Wannabes.

"But that's just one mans opinion." (Keith Quinn)

Are our ego,s getting so big that we wont answer a question from some one who is clearly not an instructor ?.
Or do we just think that it should not be asked on this forum ?.

I'm very sorry if I have invaded private territory, I genuinely had no idea what a "ground loop" or how it occurred. I thought it best to ask in this forum because this is where knowledgeable people are most likely to frequent. With many thanks to VIRGIN and OLEO, I am now wealthy in knowledge and moreover, entirely satisfied that I have received more accurate information than I may have received in the wannabees forum.

I am now leaving, banished into exile into the land of the wannabees!

STP,

Don't take too much notice of Fallen Eagle or Chicken 6. Those jackasses just found their way on to the perch! If you want to ask questions of instructors I would think that this is as good a place to start as any.




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Mount'in man,
I luv mountin' women!

chicken 6 / fallen eagle
Would you like to check my pass? There are closed forums you know, if you need to keep the riff raff out.

Don't discourage anybody else for f***s
sake

This forum is hardly used as it is..

.. Why is this so ?

Thats all fine, but when someone pitches up on the instr. forum with a question like that its OK. but state who he /she is I am so VERY SORRY. BUT, I just thought an instr would know the answer to that one, and it concerned me that that chap did not. ANYWAY I have nearly had enough of the misunderstandings and jumping on people arround here its getting to much like a real flying club. bye

STP

I was going to stay out of this one, but I am always a tad unhappy to stand by and see slightly duff gen being taken in by an aviator trying to learn. That way off message stuff gets perpetuated. Hence my U turn (sorry)

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

MOUNT;IN MAN Sorry if in some way I OFFENDED you, it was not intentional.By the way I got on my Perch in 1970. bye bye. guess this will all settle down soon.

When I first saw the name "John Farley" being used as a nom-de-plume I thought "what a good name for a pilot to choose", now I wonder if John Farley really is the great man himself. John Farley, are you THE John Farley, the test pilot of the Harrier?

If so, please sign this bit of paper for me. I remember seeing the Paris Airshow er.. how shall we say...errr..yes crash, that is the word I'm looking for.

In the present day idiom 'nuf respect Mr. Farley, and Capt Pprune for letting us listen to these pearls of wisdom.

Thank you very much John Farley for your detailed explanation, I am deeply honoured that you have taken the time to respond to my posting and I am extremely grateful to you.

STP.

One question, 16 responses, 3 answers. I have to ask why Algernon Lacey, Capt Crash, Fallen eagle, Chicken6. A and C, Mount'in Man, Tallbloke and New Bloke felt impelled to post comments that had sod all to do with the question that was asked. Could it be an ego problem?

Congratulations to Oleo, Virgin and John Farley for making creditable attempts to answer, in a text based medium, what really requires a diagrammatic explanation.

I shall now don my flack jacket and await the inevitable stream of vituperation from those whose over-inflated egos I have dented.

Some factors to consider:

A tricycle type is directionally stable ie tends to keep moving in the same direction because the CG is ahead of the centre point of the drag from the wheels.

This means that if the two get out of alignment the inertia of the CG (pulling forward) is acting opposite to the drag of the wheels (pulling backward). Within certain limitations, the combination act as a couple, pulling the a/c back into alignment with its direction of travel.

A tailwheel a/c has the reverse. The drag is centred ahead of the CG. The CG has a tendency to keep moving in the direction it was traveling, while the wheels are trying to do the opposite.

If the wheels & CG get out of aligment with the direction of travel then the a/c will have a tendency to continue to yaw - unless a correcting force is applied. Think of a car skidding.

The correcting forces or factors that are available include keel area & rudder input, tailwheel effects eg locked or not, or steerable, moment arm over which these forces can act, width between the main wheels (Is this called groundtrack? I forget), rolling resistance caused by the surface etc

Spacing of the mainwheels is a factor because the further the wheels are apart the more yaw must happen before the CG moves far enough to the side for sufficient misalignment to cause a ground loop.

Anything that induces a yaw away from the longitudinal alignment of the CG & centre of drag from the gear can precipitate a ground loop.

This can include:

A tricycle a/c can behave like a tailwheel type if it has a relatively light load on its mainwheels & a relatively heavy load on the nosewheel eg if you wheel barrow by landing on the nosewheel, or if you try to force it to stay on the ground too long during the t/off roll.

I have a feeling I've forgotten something in the above...


[This message has been edited by Tinstaafl (edited 11 October 2000).]

Rolling C this is the first incoming !, i asked "are our ego,s getting to big that we cant answer a question.........."
What i did not like was that some of the posts where critical of some one who clearly wanted to further his/her undestanding of a subject and who better to ask than an instructor? .
It would seem some did not like this on this forum a point that i disagree with.
As instructors we should all be willing to answer questions from students what ever forum we find them.

Finaly thanks to john f for an a post far more eliquent that i could have made.

New Bloke

Hi. On a point of detail I have just checked my log book and can find no record of a crash.

It was actually my Lord High Master Bill Bedford in XP831 at Paris. After the lads in the hangar at Dunsfold did their body work stuff they gave it to me to take to Bedford (aerodrome) to compare with the Short SC1 which we already had on Aero Flight at that time. There I managed 128 landings in the beast without further modification to its shape – so I was well chuffed. Off topic I know but accuracy is everything (well below 50 ft anyhow).

STP

My pleasure, but there is no need to feel honoured – I’m just another aviator like you.

JF

To all of you, if you were to hear a conversation and it "went off topic" would you leap in and try to bring it back? That is what is happening here and snide comments about egos are really uncalled for. This is communication.
Why is there such a desire for some people to jump in and be nasty to total strangers?

John Farley, thanks for the correction, I thought I remembered an interview with you post incident and you were saying you knew something was wrong after the landing, as(when the dust settled) you were several feet lower than you were used to being.

I must have the incident mixed up with something else.

Rolling Circle,

I looked damn hard in your post and could find ‘sod all to do with the question' asked there either. So should that have been seventeen responses to three answers?

Some twenty-five years ago I flew ‘conventional undercarriages' extensively and thought I knew it all. John Farley, your discourse has enlightened me on some concepts that I hadn't considered. But I will add that a frictionless runway may also result in a slow rotating ground-loop. I had an experience (in a Cessna 180) similar to yours (Ryan PT 22) on a smooth, clay strip in light drizzle. Just prior to the ground-loop I did sense that the wheels had locked up due to excessive braking given the surface conditions. The application of power (a brainwave) during the reverse roll certainly resulted in a reduced stopping distance! Fortunately in my case there were few spectators.

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Mount'in man,
I luv mountin' women!

All the way home I was wondering why I had got John Farley and Bill Bedford mixed up, http://www.pprune.org/ubb/NonCGI/redface.gif especially as both (along with Graham Hill, Brian Trubshaw, Virgil Tracy and James Bigglesworth)were boyhood heroes of mine.

I suppose it's lucky I didn't comment on his run in with Air Commodor Raymond or ask how Tracy Island is now :)

For the anoraks out there, what was the harrier (as sold by Airfix) before it was the harrier? I remember something like p11D.

edited for rouge smiley (carol)

[This message has been edited by New Bloke (edited 11 October 2000).]

MM

Glad to hear I am not the only bloke still alive who has groundlooped....... IMHO even with your very slippery surfce there was just enough friction to kick off the manoeuvre. I say this 'cos with literally zero friction I guess the normal weathercock stability would have come into play - at least by the time you were drifting at 90 deg! There had to be some small destabilising force from somewhere.

JF

New bloke, aha ! P.1127, and the proposed supersonic development was the P.1154 ( I think). Did you have anything to do with that, JF ? I'd also like to add a note of appreciation for your taking the time to answer queries such as this topic's. Actually while I'm at it there's a few other blokes who usually give good gen, like Watford and Tinstaafl. Thanks, keep it up. I appreciate it, anyway.
Cheers DB6

Rolling Circle. I know its off thread but,have you ever worn a flack jacket for the purpose for which they are intended, I have and they are bloody uncomfortable so dont bother you wil get no flack from me.As I said sorry I thought it was an intsuctor asking the question thats me done, IN FUTURE I PROMISE TO STICK ONLY TO THE SUBJECT MATTER. bye.

John Farley and MM could start a club, I guess it would be nearly exclusive, of people who have used reverse thrust in a single piston fixed pitch (I'm guessing the fixed pitch bit, but it could be) Aircraft.
:) :) :)

Yes that's it DB6, I remember the chinook was the "vertol 10711" before it became the chinook, just couldn't remember the Harrier.

JF

Fully concur. After the event I did walk the strip and found that the brown silt clay surface was interspersed with large granite lenses forming a naturally smooth surface. A locked wheel passing over such a lens was probably the friction catalyst initiating such a memorable ride. But that split second event has challenged my thought processes for years! Thanks again for the ‘heads-up' on the scientific issues.

New Bloke,

I guess there are a few others eligible for membership but we all probably prefer to remain anonymous on such lapses.


MM

DB6

Oh! what a temptation – all the way from groundlooping to supersonic vertical landers in a couple of pages. Does Danny allow that sort of thing?

I refuse to be drawn on that beast, beyond undoing the top button of my anorak to say there is never a full stop between the P and the type number of Hawker aircraft. So its P1154 please.

JF

PS IMHO it’s a jolly good job “they” cancelled that one – and the other three.

J

Hmmm, Is a seemingly genuine question always met with derision and negativity in this forum? Thanks to John Farley and some others for valuable responses. I certainly picked up a few points about ground loops which I was previously unaware of - and I have been instructing for several years.

It would pay some people to remember that most of us are still "wannabes" - but just to a lesser amount as we gain experience.

Replies in chronological order, and sorry for the delay

A and C

nothing to do with ego or personality or ANYTHING to do with ANY PERSON, I just didn't (more on that "didn't" instead of "don't" later) think it should be asked here.

STP

there's no such thing as a stupid question, more on that later too.

Mount'in Man

I almost agree with your last sentence, except this quote I found on the entry to the Instructor's Forum,

"A place for instructors to communicate with one another "

That's pretty much what I quoted, no need to get personal at me if you don't read the notice on the door.

Tallbloke

there are no closed forums for instructors.

RVR800

I'm not discouraging anyone with my post, just questioning the group whether it is appropriate to have it HERE (sorry about the caps, i can't do italics)

rolling circle

my post was related by appropriateness to the forum. Would you as a student walk into a teachers conference and ask how to write an essay?

A and C again

I wasn't critical of the person, just the place they posted that particular question. More on that later (read "at the end")

New Bloke

thankyou (I think)

BurningKeroNow

no derision in my post.

now here's "that later". i think I've found where the misunderstandings are coming from. That bit I quoted IS at the front door to the forum, however, after seeing R+N and wannabes I fully concur with STP's choice of here to ask the question. Maybe (new idea coming up) we should either rename this forum to "school" or something like that, or we could all frequent Tech Log a bit more (although it does say on their front door, "for airline pilots..."). I go to Tech Log and look for the posts I know something about but there just aren't many there. Why should it be limited to airline pilots?

STP, there's no such thing as a dumb question and I'm not happy with you feeling sorry towards anyone for asking it. If you've got any more, you might as well bring them in if this is your best field of responses.

Safe flying

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Confident, cocky, lazy, dead.

JF, P1154 it is - thanks for the correction, I shall push the kapok back in and sew up the holes. The other three ? Now you've got me really curious. Do tell, do tell. Anything to do with the Bristol B188 (or something like that)?

DB6

Sorry, my mistake, there were only three aircraft cancelled by Harold Wilson’s incoming government of late 1964

The P1154 supersonic vertical landing strike aircraft and the HS681 VSTOL tactical transport both got the chop on 2 Feb 65 and the TSR2 followed on 6 April 65.

The other one that I was thinking of in my earlier post was the Fairy Rotordyne but that actually was binned by the previous conservative government on 26 Feb 62.

As for your Bristol T188 that stopped flying about the end of 63 because it could not reach the speeds that the RAE boffins needed to do kinetic heating tests.

IMHO all four of the cancellations were correct on technical grounds (although the reasons quoted at the time were not technical)

All the aircraft had some really great features, but each had (in my view) a fatal flaw

P1154 (Exhaust temperatures and velocities too high thus requiring special sites/procedures for VSTOL take-off and landing. This precluded Harrier type operating site flexibility)

HS681 (Conceived to support P1154 in the field. In fact while I am sure it could have been made to fly it was on very dodgy ground so far as safety following engine failure and payload range was concerned)

TSR2 (Not enough wing. Too much of a straight line beast and as we have relearned there is no substitute for aerodynamics when you want to manoeuvre)

Rotordyne (On this proposed city centre compound helicopter the rotor was driven by tip jets - doomed by noise)

Humble apologies to all Flying Instructor types for hijacking your forum

JF

Thanks to JF for enlightening on the subject of ground loops and the reasons why.

I recall landing [as a student] at Southend on a greasy runway [before they redid the friction course a few years ago] and noticing the right wheel brake disc momentarily "snatch" during touchdown. This was very odd as the a/c did not have toe brakes and I hadn't accidentally touched one.

Anyway the resulting weaving down the runway must have been hilarious for onlookers - it certainly got my attention!

I now realise that I was very lucky to be in a tricycle aeroplane that tends more towards forgiveness.

chicken6
I know there are no closed forums for instructors. I was suggesting that there may be peoplehere who wish there were so that the knowless could be excluded, and exclusivity could be promoted.

Danny has set up closed forums for quite a few of the companies. If you really want a closed forum for instructors why don't you ask him to set one up. All the time it is an open forum, I don't see any harm in any of us posting to it.

Where else can these questions be asked? if you don't want to answer pupils (or PPLs) questions, refrain from opening threads that end in a question mark. There are plenty of knowledgable folk only too happy to help someone out.

I personally don't give a toss who posts what where, in fact I preferred it when there were less forums so I didn't miss some interesting topics. As an instructor myself I don't care whether the stuff on this forum is strictly about instructing or not, and JF, I find the non-instructing stuff in your postings very interesting so no apology necessary to me anyway. Now I have to find out what the HS681 was, never heard of that one !
Cheers DB6
Oh, and a closed forum for instructors is a crap idea, although I can't work out if anyone has actually suggested that or not.

[This message has been edited by DB6 (edited 17 October 2000).]

Hi, New Bloke. Welcome.

Re: your question about prototype Harrier,
the aircraft was originally the P1127.

XP831 first flew tethered 21oct1960 and then untethered hover on 19Nov1960.Flew conventionally at Bedford 13mar1961 and then ,about a further six months afterwards, the first transition flights were achieved.

Sometime around this time the aircraft became known as the Kestrel.
I seem to remember it achieved its first Carrier landing during 1963 on board HMS Ark Royal,flown by Bill Bedford.
I also remember,on the other hand,an amazing display by John Farley at Leicester in the two-seater Harrier,in the most appalling of weather conditions,when the first most knew of his arrival was when the spray cleared; none of the approach could be seen as the cloud was so low !!

I was on Hunter GA11s then and the whole episode at that time was unbelievable. Absolute PFM,John.

Remember the Leicestershire Aero Club sticker that you took back with you ??

Sail Army.

Bit late with that contribution,wasn't I?
Sorry, guys. Better late than never (even though it had little to do with ground looping !!)