Crashed & Survived......

[Steve Hargreaves - Moderator]

At the patch on Saturday I was flying my Wot 5 (a 25% enlarged Wot 4...63" span, 8.5lbs 91 up front). This was only its third flight so I'm still learning the model.....

A few simple aerobatics & general stooging around & then recovering from a gentle dive the model passes overhead at about 80 feet heading into wind & away towards the boundary when I get that awful feeling that I'm not in control anymore (don't you just hate that feeling...). The model refuses to respond to the elevator & continues downwards....rolls quickly to the right & then to the left & continues onwards finally hitting the ground just out of sight with a thwack...

I go to retrieve the model expecting it to be smashed to bits & lo & behold its still in one piece. As I approach the model I see the elevator is flexed down by about 20 degrees "servo failure" I think. Then as I get nearer I see no evidence of a pushrod between the elevator & the servo (the model has a rear mounted elevator servo)...."The flippin' pushrod has fallen off!!" I think

I get to the model & pull the nose out of the ground & apart from a broken prop it all looks OK....(I might be a heavy builder but I build 'em tough !!!). Underneath the model I find the elevator pushrod....still intact & unbent.

I gather the model & take it back to the pits....apart from the broken prop & the plastic U/C bolts breaking & a small crease behind the nose ring its absolutely fine....I can't believe it ...I don't know whether to laugh or cry.

I take the wing off & there is a bit of a jumble inside.....the battery has come loose as has the throttle servo & Rx....the plate supporting the back of the fuel tank has broken but I put all these down to the impact of the "crash". I check the radio & everything works just fine...all servos respond perfectly.

So what caused the crash.....I don't know is the short answer & this is very worrying....

Back home & the lack of damage is confirmed.....I wash the mud off the engine & clear out the engine bay...no damage anywhere...firewall is sound, no tell tale cracks around the joints...I really can't believe it.

But what caused it to come down like that? I had no elevator control...it just went in despite full back stick....it appeared to glitch with a quick roll to the right & then the left but I'm willing to concede that might have just been me panicking!!! Could the pushrod have come detached at one end during flight & then flicked off completely during the impact? Seems unlikely....

The detached Rx intruiges me too....I initially put it down to the force of the impact causing the Rx to become detached (its held in place with sticky pads) but thinking about it I find this hard to believe....the Rx only weighs anout 10 grams so I can't see it building up enough momentum to pull the sticky pads apart. Might the battery have come loose (fixed just underneath the Rx) & knocked the Rx off its mounting? Might this have happened during flight? Certainly there is no evidence of any impact damage on the Rx case...

So I'm now very worried....I have to fly this model once again & I don't know what caused it to come down. Part of me would rather it had been smashed to bits but I knew the cause....flying a model that has crashed for no reason is the stuff of nightmares

 

I flew my Pitts Spesh for the rest of the session & this flew just fine & without a hitch....might be worth noting that this model flies on 35MHz (I have a Futaba TM10 2.4GHz module for the big Wotty)

So over to you, my fellow forumites....can anyone suggest what might have happened & how I should check everything before flying again?

Just for reference the model is fitted with a Futaba 2.4 Rx (617)...all the servos are new (apart from the throttle servo) & I have two 4 cell AA bateries in parallel to power the radio, both of which checked out fine under load after the crash. Whilst I have been working on the model on the bench I have had it in "servo test" mode which runs all the servos slowly through their full travel & then back again continuously. Obviously I haven't been watching each servo all the time but as far as I can tell not one has glitched or mis-behaved.

So now what do I do????

[Stephen Grigg]

It does sound to me as if the push rod became detached as Ive had models crash and been written off bu were still attached.I fly Futaba 2.4 and have never had any sort of glitch with a 617 and I have 19 of them

[NigelH]

Fuel tube keeper used ?

 

[Steve Hargreaves - Moderator]

Thanks guys....I have to say I'm leaning towards the push rod as a possible explanation.

 

It does seem unlikely though...the pushrod has M3 metal clevises at either end & these are not easy to open to click into the hole.

 

That said it did come off at least one fitting during a relatively low impact crash (as evidenced by the lack of damage...)

 

I hadn't used fuel tube keepers Nigel....I didn't think it necessary with these types of clevis.....I think I will in future though...

[Hogster]

I had a similar incident. A different model but my elevator was set up with m3 metal clevises at both ends. And it was quite clear to me that the locking nuts had come loose and the push rod rotated with the vibration and worked its way loose from the elevator horn clevice. I now thread lock all my nuts

[Steve Hargreaves - Moderator]

That "Gotcha" has happened to me too Hogster....luckily I spotted it before the pushrod became detached... A lesson learned.....

 

In this case I have a threaded clevis at one end c/w lock nut & at the other end the clevis is soldered onto the end of the pushrod. This stops it rotating....

[Bob Cotsford]

I've seen clevis where the pin has come loose because they haven't been rivetted properly. The latest were 3 out of 4 that were supplied with a pack of Sullivan snakes, the type with a small locking spring clip. Without a tube keeper these would have been an accident waiting to happen.

 

The other thing that comes to mind with it being on 2.4 is blanking from the UC, the battery or any wiring between you and the aerial with the model overhead.

 

As for rotating clevis, I usualy run some cyno into the threads on one end even if there are locknuts both ends.

[fly boy3]

Sorry to hear that Steve, always very upsetting to all. not only the pilot. I am of the old school, belt and braces.Theory being -if its possibe to happen it will happen, so we have to put best practice in place, as I'm sure you did. Keepers, and lock nuts, and when you are satisfied some thread lock. Cheers

[Steve Hargreaves - Moderator]

Thanks Bob...the clevises are both M3 metal & look fine...certainly both still have the pins attached but I will check them again just be sure.....

 

The aerials are set at the Futaba recommended 90 degrees apart with one running along the length of the fuselage & one across the width held in place in plastic drinking straws.....the model does have a carbon U/C but this certainly wasn't between me & the aerials at the time of the incident as the model was flying away from me.....

 

One thing I have just thought about however is the routing of the aerial...one comes out of the Rx case & runs through its straw no problem....the other comes out of the case & thenturns through 180 degrees to run under the Rx to the mid bulkhead before turning through 90 degrees to run away from the Rx.....is this OK do you think?

 

Certainly the model responded well at quite a height just before the incident so I assume it was OK....

[David Ashby - Moderator]

Steve - darn lucky there.

 

Models can do funny things when they hit the ground and the force and momentum of the impact, even gentle can dislodge all sorts of things.

 

The explanations here sound fine, I'm sure it's happened but check the elevator servo horn arm splines, just make sure they not partially stripped or slipping in the servo output teeth.

 

 

 

 

 

 

 

 

Edited By David Ashby - RCME Administrator on 03/10/2011 14:15:57

[Steve Hargreaves - Moderator]

The servo arm seems fine David....I wondered if it had cracked or something & allowed the clevis pin to escape that way.....

 

It really is a mystery......

 

& I HATE mysteries where crashes are concerned.....

 

Just as an extra thought how should I satisfy myself that the model is OK to take up again.....obviously I range checked it & everything before flying.

 

What I have thought to do is start it up & tether it in a remote part of the field & then switch it into "servo test" mode & let the servos cycle throughout their entire range for a few minutes (hours??) & see how it gets on.....

Edited By Steve Hargreaves on 03/10/2011 14:24:52

Edited By Steve Hargreaves on 03/10/2011 14:27:13

[Biggles' Elder Brother - Moderator]

Hi Steve,

 

I use the 617 rx a lot - the aerial arrangement you describe sounds fine to me. Remember its only the last "bare metal" bit that is really important from an alignment point of view - what it does along the rest of the length is immaterial.

 

Let's think through a couple of scenarios:

 

1. The elevator pushrod becomes disconnected at one end - for whatever reason - on impact the other end then disconnects. Mmmm, doesn't sound very likely with M3 clevises does it? But then it was disconnected at both ends when you found it - no disputing that. Is it a one piece, one servo, elevator?

 

2. It went into failsafe. Possible but with the 617 you only have failsafe reset on throttle. Have you tested the failsafe to see what happens? Did the model shut the throttle - assuming that is your failsafe?

 

3. The battery unseated in flight and disconnected itself from the rx. But you don't say that you found the battery disconnected so that's unlikely.

 

4. Then there are those rolls to consider. If the elevator just disconnected why did it roll in two different directions? You say that might have been you - but if it wasn't then it points the finger at the Tx or rx.

 

Unless there is something we are missing I'm forced back to the conclusion that unlikely as it seems the pushrod disconnected from both ends. It possible that one end was disconnected, it was hanging down and then the impact of the rod end with the ground disconnected the other end.Are the control horns plastic - or grp or whatever?

 

BEB

[andy watson]

Couple of points to chip in.

 

617s really aren't that fussy about aerial arrangements, although 90 degrees is best practise.

 

I always cable tie them in place, with the tie running between the servo wires across the top of the rx- this makes it impossible for anything to work loose. (I also cable tie batteries in place)

 

 

[ken anderson.]

steve - i've had elevator servo's die in the air and thankfully-both times the elevator was in the neutral position and i controlled the flight using the throttle to climb / dive...... and got the models down with a bit of a bump....so if your elevator connection went-did the model respond to the pitch using the throttle?.....

 

ken anderson ne....1.... pitch dept...

[Steve Hargreaves - Moderator]

Thanks guys....

 

@ Ken......I didn't get chance to find out.....all my time was taken up with staring in horror/disbelief as my new model disappeared over the hedge!!!

 

@ Andy.....thanks for the thoughts Andy....my batteries were cable tied in as well but I assume they came loose in the crash.....I assume...

 

@ BEB....

 

1.no disputing it...the pushrod had come away from the model but it must have arrived at the scene at the same time as the model so we can assume it was still attached when the model hit the deck...at one end at least!!!

 

2. Failsafe...thats a point...I'd forgotten about setting that......as you say the failsafe is on the throttle only & the throttle was (thank goodness) closed anyway.....be interesting to check....if it is currently set to say open the throttle then we can eliminate the Rx going into failsafe mode as the engine certainly didn't speed up.

 

3. The battery was still connected when I recovered it...it was loose in the model but still connected electrically

 

4. The rolls could have been me...as I mentioned I kind of got the feeling I wasn't fully in control (a kind of 6th sense we all have I think ) & was planning to turn right to get back on the strip asap!!! The Tx is about 18 months old & has been faultless on 35MHz....this is the first model I've flown on the 2.4GHz module though....

 

The control horns are plastic Robart type units like this. I bought these as I thought they would be better than the usual SLEC type plastic horns I usually use on smaller models. They are a very snug fit on the clevis pin....

 

I find it very unlikely that the elevator pushrod could just ping off.....it really is difficult to get off in normal use....I just can't believe it would come away so easily...& yet there it was just lying in the dirt.....

 

Interestingly it was near the front of the model when I found it.....the model was nose down in the mud with the wings level & the fuselage about 30 degrees to the horizontal. The pushrod was lying in the dirt at the front of the model so had obviously been thrown forward at the point of impact rather than getting dragged off....

 

To paraphrase Sherlock Holmes "...when you have eliminated the impossible, whatever remains, however improbable, must be the truth?...."

 

[Allan Bennett]

According to your narrative, your first reaction was that the model was not responding to elevator. So, it seems to me the elevator pushrod coming off in flight is the most likely cause and, as you say, the left/right rolls were your inputs, trying to figure out what was working.

 

That Robart horn looks to be thickened where the clevis goes through -- designed for a ball link to be screwed in. Is it thick enough to have prevented your metal clevis pin from going right through and snapping into its other side?

[Steve Hargreaves - Moderator]

Allan I have to agree with you......I put the radio gear back in place last night & screwed the plane back together (engine in etc) & everything looks fine....plugged everything together & left it running in servo test mode on the bench whilst I cut some letters out for my latest build.....after 30 minutes I switched off.....everything was working fine.

 

I have to conclude that the elevator pushrod has come detatched in flight. I can't really believe it because it is quite hard to clip into place both on the servo & the horn but the evidence is impossible to dispute.....the push rod was not attached to the plane at the crash scene....

 

The Robart horns have a kind of built in ball joint.....the horn contains a shere with a hole through it & the top & bottom shaved off....the clevis pin goes through the hole in the ball. Its supposed to allow for slight mis-alignment between pushrod & horn.

 

I have used M3 metal clevises through out the model so they don't so much snap together as rely on the "springyness" (is that a word...?) of the metal to keep the clevis closed. Fuel tube keepers are now in place throughout the model.

 

@BEB....I checked the failsafe & I must have set it correctly as it was set to close the throttle to idle....

[Martin Harris - Moderator]

Steve,

 

You say you were recovering from a dive - was the throttle still fairly wide open? It's possible that you encountered aerodynamic flutter and the forces involved if this sets in are pretty extreme. Was there a harsh buzzing or chattering sound?

 

I had just this happen when test flying a model for a clubmate that was prone to flutter. We'd made some improvements to the hinging/linkages and I was doing gradually faster passes to test it when the flutter came back - followed by instantaneous elevator failure. Luckily, the model (a Protech Cougar) was stable in pitch and I was able to control it to some extent with the throttle and after a very wide circuit (models don't like turning without elevator and I didn't fancy the secondary effects of rudder dropping the nose) I managed to get it on the ground fairly tidily.

 

On examination, the metal clevis (2mm in this case) was detached from the servo arm but the fuel tube keeper was still in place...

 

If there's the slightest chance it was flutter, then check the hinges and linkages very carefully for excess play, eliminate any excessive hinge line gaps and if the elevator is of fairly substantial construction (i.e.heavy) it would be worth lightening it and/or adding some mass balance weight ahead of the hinge line would be even better if it's practical.

[Steve Hargreaves - Moderator]

Hi Martin......it wasn't so much a dive as a decent if that makes sense...I'd put the nose down & reduced power to loose some hieght.....unfortunately I ended up losing it all.....

 

It was flying quite slowly & I certainly don't recall any buzzing or chattering & I think I would have heard it as the model passed directly overhead.. I have used the plastic flat plate & pin type hinges & buried the hinge part in the tailplane to get a good tight hinge line. Elevator is made of 1/4" balsa & is in keeping with the model...being Wot 4 derived it is a fairly conservative size....not a typical 3D barn door type of surface...

 

There is a tiny bit of play (maybe 0.5mm total, measured at the elevator TE) due to a bit of backlash in the servo gears...I don't think I can do much about this.

 

Rather than fuel tube I am wondering about using cable ties to keep the "jaws" of the clevis closed....cinch up the tie & then tack in place on the clevis with cyano.....the linkage wouldn't be going anywhere then. I'm thinking a more solid locking mechanism might be needed....if whatever happened was enough to open a metal clevis then it will probably stretch the fuel tube keepers too....

[Martin Harris - Moderator]

Posted by Steve Hargreaves on 04/10/2011 10:58:29:...if whatever happened was enough to open a metal clevis then it will probably stretch the fuel tube keepers too....

That was certainly my experience. I must say that I like the Sullivan clevises with their metal keepers in that respect.

 

I also had a near disaster with my 1/4 scale Cub recently when both 3mm clevises with fuel tube keepers let go on the bottom strut linkages for one wing (the original kit instructions specified 2mm ones!) and luckily there was sufficient strength in the piano wire wing mountings to complete a gentle (rather vintage looking with massive dihedral) circuit and landing.

 

I've since made solid steel clevises with through bolts instead... 

 

Edited By Martin Harris on 04/10/2011 11:13:03

[Biggles' Elder Brother - Moderator]

Steve, if you're looking for a more secure link than a clevis consider going over to ball-joints. These are very secure and with a bit of locktite on the nut will not come off. The problem with any clevis is that the pin is only so strong and they can "ride" out specially as Martin suggests if the surface is vibrating despite fuel tube or heat-srink over them.

 

BEB

[Allan Bennett]

Posted by Martin Harris on 04/10/2011 11:06:35:

That was certainly my experience. I must say that I like the Sullivan clevises with their metal keepers in that respect. ...

Sooooo ... that's what those little circlippy things are in the Sullivan snake packs I've been using their snakes for years, but never realised what those little bits were for.

[Martin Harris - Moderator]

They work well but can be a bit fiddly in the field.

[kc]

Sounds like a failsafe situation to me.

Everyone should note that it's mandatory under BMFA rules to have (as a minimum ) the failsafe set to low throttle in case of loss of signal. see page 21 of the BMFA handbook.

[Martin Harris - Moderator]

Actually, I have an issue with that as the wording has changed from "must" to "should" in the latest issue and it only says "should" in CAP658, which to my way of thinking makes it advisory and not compulsory - I have already taken the liberty of raising this observation with the BMFA....

 

I wholeheartedly agree that all failsafes need to be set to closed or a low idle at the very most...