Clevis pin failure

[Mark a]

Out flying my Wot 4 today, checked it over before i went and checked again at the field and all seemed well. About half way through my first flight when i noticed the tail giving a wag when flying across the wind so bought it in for a landing from the west and there was a 7-8 mph 90 degree cross wind about 5 ft from touch down when the tail swung tried to correct it but down she came. Not much damage it is repairable but i did find that the clevis pin for the rudder was broken so the rudder had no control. Would this have caused me to crash ?. The clevis is a metal one has anybody had this happen to them ?.

Mark.

[Flanker .]

Poor you! Never had a clevis go like that. Could be bad luck, but after rebuild I would check prop balance and change all the other clevises. Vibes are bad for metal parts.

May it never happen again.

F

[Biggles' Elder Brother - Moderator]

Yes Mike - almost certainly a contributing factor. In a crosswind landing the rudder is a essential control - without it you can't control your sideslip angle.

You say that the tail swung close the ground and that you tried to correct it - presumably with the rudder? If the rudder was, however, U/S then I assume you plonked down travelling in one direction - but pointing in another! Not good for your undercarriage!

I've had plastic clevis pins break - which is why I don't like them - but never a metal one (touch wood).

Look on the bright side - it could have been much worse, it could have been the elevator!

BEB

[Tim Mackey]

I've seen some real rubbish metal clevis' in ARTFS - made of chinese monkey metal or some sort of silver cheese. I usually bin em straight away, and replace with a decent sullivan or kavan job - as to plastic ones, if they are of decent material, I find them fine on almost anything.

[Biggles' Elder Brother - Moderator]

Which just goes to demonstrate - you can't draw any generalised conclusions in this hobby! I've never had a metal one fail!

I don't actually know what make the metal clevises are that I use - I buy a job-lot from SLEC at the Nats each year! The plastic one that went on me was a fairly solid looking job on a Kyosho model. Admittedly it had seen long service. again that was a rudder clevis.

BEB

[scott finnie]

The only time ive had a metal clevis fail was on my great planes siren, it was the elevator and i was in a full power dive towards terra firma to produce a lovely whistle , though the whistle ended in a thud that day, that was extreme forces against the crevis, from that day ive put a few other aircraft through similar manouvers but been smoother on the roll out and havent had any failures, its strange for it to fail on approach like that, yes theres force permitting through it but i dont expect that much, you can guess what im doing now! checking all of mine, hopefully you have her fixed and flying again soon

Scott

[Turbycat]

I've had quite few clevises loose their pins. I had 3 fail over time on my WOT4 XL. All lost the pin in flight. I've had one recently on my Wot 4 (that was the rudder!) . I've had a few go in other planes too. Mind you my planes get a lot of hammer and plenty of hours flying on them. I've not yet lost a model through one failing but had a few interesting landings as a result of the failure.

Andy

[Bob Cotsford]

I've seen quite a few Sulivan ones where the pin hasn't been rivetted at all - they drop out when spreading the clevis to attach it to the horn. They all came in Gold'n'Rod packs.

[PatMc]

Not sure why the model should come down because the rudder failed. Perhaps the rudder clevis broke as a result of the crash which was caused by some other failure.

[Biggles' Elder Brother - Moderator]

Pat, I suspect the landing was just a bit "heavy" and crossed-up because he was cross wind and without a rudder - that's how I read it anyway.

BEB

[PatMc]

It doesn't read like that in the OP.

Mark said "there was a 7-8 mph 90 degree cross wind about 5 ft from touch down when the tail swung tried to correct it but down she came."

Why should lack of rudder response cause it to come down the last 5ft ?

If it acted like my Druine Akrobat or Priory Silhouette response to rudder applied in the last 5ft could have caused it to nose in.

[Flanker .]

He saz he saw the tail wag and thus made an emergency landing. no rudder on a crossed approach would be bad. The wind speed will often change near the deck and it sounds as if the model reacted with that.

F

[PatMc]

You don't need rudder on the approach. It should only be used as the wheels are about to touch.

The tail wag is unlikely to have been caused by a broken clevis, turbulence due to a gust is more likely.

[Flanker .]

You don't use the rudder on approach !!? Hmmmm.

[PatMc]

I only use rudder on the approach if I'm need to steepen it to avoid an overshoot. There should be no reason to use it otherwise & plenty of reason not to use it.

[Tom Wright 2]

Is Pat saying that not using rudder avoids the pilot applying crossed control which could induce wing drop at low speed .

Tom.

Edited By Tom Wright 2 on 17/12/2012 17:49:24

[Martin Harris - Moderator]

Surely it depends on whether you're adopting a crabbed approach where rudder should only be used to balance power changes (not that that's practical with a model) and adverse yaw during aileron inputs or a wing down slipping approach where you need a constant rudder input.

[Tom Wright 2]

Side slipping to loose height on finals usually involves use of top rudder . but it would be usual to add some speed on top of minimum safe approach speed and even more so if there is the possibility of wind shear or turbulence. At the round out phase the speed is coming off rapidly and if the stall is imminent then crossed controls could cause a spin entry condition which is observed from the ground as yaw and wing drop .

Tom.

[PatMc]

Tom, I fly gliders/motor gliders a lot. They aren't always fitted with spoilers so in a potential overshoot situation I drop the nose to increase airspeed then "S" it in using rudder to yaw & opposite aileron to hold the wings straight.

The same technique is transferable to power models but it's worth checking in advance if rudder causes the nose to drop before applying this & making allowances if it does.

In my experience wind shear isn't that much of an issue with models because were making the landing approach within a much more limited altitude range than full size.

[Mark a]

Posted by Tom Wright 2 on 17/12/2012 18:30:40:

Side slipping to loose height on finals usually involves use of top rudder . but it would be usual to add some speed on top of minimum safe approach speed and even more so if there is the possibility of wind shear or turbulence. At the round out phase the speed is coming off rapidly and if the stall is imminent then crossed controls could cause a spin entry condition which is observed from the ground as yaw and wing drop .

Tom.

Your discription just about says it all Tom, with the plane on approach the nose was pointing approx 45 degree's from were i wanted it to be and when i couldn't kick it straight with the rudder i did apply some aileron and power as i did that she did stall in. All glued back together now will flight test the weekend if the weather play's ball.

Mark.

[Tom Wright 2]

Posted by PatMc on 17/12/2012 19:29:45:

Tom, I fly gliders/motor gliders a lot. They aren't always fitted with spoilers so in a potential overshoot situation I drop the nose to increase airspeed then "S" it in using rudder to yaw & opposite aileron to hold the wings straight.

The same technique is transferable to power models but it's worth checking in advance if rudder causes the nose to drop before applying this & making allowances if it does.

In my experience wind shear isn't that much of an issue with models because were making the landing approach within a much more limited altitude range than full size.

Pat

Fish tailing on finals is of course a recognised way of increasing drag , and results in a higher rate of sink in order to arrive over the boundary at the correct height . I think the situation Mark encountered is very different as it is likely that he raised the nose at 5-6 ft agl to check the sink ,perhaps a little bit sooner than needed ,which resulted in several possibilities contributing to wing drop including a form of wind shear or roll over turbulence which is a phenomenon frequently encountered at our field due to a line of trees adjacent to the normal landing direction.

Tom.

[PatMc]

So it seems like the impact was the cause & not the result of the broken clevis.

[Martin Harris - Moderator]

High AoA in the round out and largish application of aileron to try to correct the yaw sounds like the recipe for a wing drop.

I remember reading an article by Brian Lecomber back in the 70s that raised some interesting points about slipping. His recommendation for a slipping turn in an engine failure and turn back sounded totally insane until he explained that he'd developed the technique when instructing in the West Indies from runways built out into shark infested seas where there were no straight ahead/30 degree options. The logic was that the turn could be rapid without much G increase and was the safest way to get back onto a reciprocal heading with minimum height loss. He went on to theorise that with the blanking effect of the airflow past the fin/rudder the elevator power would be reduced so that it was virtually impossible to stall anyway.

I put some of these theories into practise in my 1935 Rhonbussard (at a good height!) and it was indeed possible to get into a full crossed control sideslip with the elevator on the backstop without any tendency to stall or flick - in fact, Brian's statement that any aircraft that would flick against the direction of the rudder input was a very odd one springs to mind and he thought a flick over the top would be equally unlikely - and he was extremely well versed in flick manoeuvres in his Pitts Special airshow routine as I recall! It certainly gave me the confidence to exploit sideslip approaches to maximum effect and earned me the nickname of Bomber Harris when an example apparently impressed someone who I respected greatly as an instructor and soaring pilot.

[Tom Wright 2]

Posted by PatMc on 17/12/2012 20:04:42:

So it seems like the impact was the cause & not the result of the broken clevis.

Pat.

Lets say the clevis failure is a possibility ,although unlikely , and at the end of the day it is impossible for anyone to reach a final conclusion .......unless a video of the incident is available, or further conclusive information became available.

Tom.

 

Edited By Tom Wright 2 on 17/12/2012 20:27:00

[Martin Harris - Moderator]

One other possibility - the tail wag might have been due to flutter if some high speed was being indulged in and that might well have been enough to overstress a clevis.