tutor 40 mk2 wing failure

[dlonrar]

I recently started flying as a beginner about two months ago on a discovery.I was flying a new tutor mk2 (third flight) under instruction on a circuit at about 150 feet on half throttle. The plane suddenly pitched upwards (no input from me). My instructor took control and reduced throttle, this was to asses any problems, the plane was ok. As the instructor was about to hand back control to me, the same thing happened but this time part of a wing sheard off and the plane crashed. On examination of both parts of the wing, the shear occurred across the wing and aileron at the end of the wing spare. Photographs have been sent to the model shop and distributor about a week ago. Nothing back as yet. This should not happen with a trainer and luckily no one was injured. Does a reply from the distributor take this long.

[kc]

If you feel this was a structural failure you should use the BMFA report form also follow their advice about taking it up with the retailer.

[dlonrar]

Thanks kc, I had had a look at the BMFA report form. As the retailer and/or distributor have not been forthcoming with information, I will complete a form and send the photographs to the BMFA for information.

[Plummet]

Does the Tutor have rubber banded wings? I have seen sudden pitch changes like this when the elastic bands are not up to the job.

Possible?

Plummet

[Martyn K]

My Tutor 2 had a wing failure following a heavy 'landing' (OK a crash) when I was relearning. The damage was worse than it should have been because the outer tube that holds the G/F wing joining tube did not reach the 3rd rib. ie, it was only supported on the root rib and one rib out. It sounds like your wing may have had a similar problem. You can easily check as there will be no glue on the outer tube. I repaired mine by binding the tubes back to the main spar and recovered the wing. I have had no problems since.

Martyn

[kc]

dlonrar, If you think the model was faulty then you should claim against the retailer ( not distributor ) as the BMFA suggest. If bought with a Credit Card they may also be liable. The Consumers Association / Which? sometimes report that claims against Credit Card companies get settled easier than retailers. Look into this if you feel the failure is clear enough to justify action.

First of all you might also post the photos here on ModelFlying to see if anyone else has had a similar failure point ( perhaps Martyn would comment after seeing the photos? ) You might compare the failure with the good side to see if the workmanship is similar. i.e. is it a design fault or poor workmanship.

It would seem not many people know about the BMFA form to report ARTF faults. I came across it by accident last week but when I came to search for it again yesterday it was not at all easy to find - - I had to go through every page and I would have given up long before if I had not been convinced it existed. Crazy website design ==ARTF fault reporting come under " Resources" (which seems to mean publications) and not under Safety Bulletins etc.

[dlonrar]

Thanks all,

Plumment, yes the instructor said the sudden pitch up is a charateristic a rubber band problem. However, the plane flew straight and true during the tests (responding to any direction changes). The rubberbands are the 6" white ones recommended by the plane manufacturer with 4 in situ (2 diagonal and 2 along the wing chord)

Martyn k, I further checked the wing, and the 3rd rib (the one at the end of the wing spar is broken and part detached from the wing spare (glue still in place). The plane cannot be repaired as it nose dived from 150' onto a tarmac runway. The plane has had no previous hard landings and no previous damage to the wing.

kc, yes I agree about the BMFA form, retailer etc. I think the information was sent to the distributor by the retailer because of the nature of the failure. I think a week is enough time to wait for answers and I am looking to take things further.The start was posting on the forum and I shall be informing BMFA.

[dlonrar]

[Martin Harris - Moderator]

Without a close examination by an expert it's impossible to tell whether there was a structural weakness but I'd be rather suspicious of the bands. The 2 along the fuselage would do little good so effectively you only had the full holding down power of the single crossed pair. In fact, really, all the other pair would do is provide a fulcrum for the wings to bend over!

The number of bands is irrelevant - bands lose tension with use, fuel soaking, UV exposure etc. and the only test is to see if you can lift the leading edge with a fairly hefty pull. Any movement will allow an instant increase in angle of attack when either applying elevator or flying through turbulent air - and if your airspeed is above a critical point the airframe can be overstressed.

Full sized aircraft have pre-determined airspeed limits for rough air and maximum movement of the controls to protect against overstressing - we don't have such luxuries and our models are usually orders of magnitude stronger to avoid mid-air failures.

[kc]

I have to say that most people use 6 wing bands and 4 are considered too few by most aeromodellers I know. I use 6 all diagonal on a 4 pound vintage model and i certainly wouldn't use less on a heavier model. Unless the manufacturer stated only 4 bands were required I dont think you will get anywhere unless something in the construction is proven faulty.

If the same size bands were used diagonally as those chordwise then the chordwise ones could not be as fully stretched as the diagonals so really you used 2 stretched and 2 partially stretched bands......not enough I reckon. One would also have to question the experience of an instructor who allowed a model to fly with the bands like this. Most instructors that I have known would have insisted on more bands. It is so common for the instructors to have to ask around to borrow some more bands to help some newcomer. It's the first thing you would look for on a new model - has it got enough strong rubber bands.

Crashes happen, you have to accept the risk that a model can be destroyed. Instructors do their best..........but you must have an airworthy model..

[dlonrar]

Thanks all,

Martin, Yes, I will change the wing band arrangement and test the the wing for excessive free movement before flying.

kc, I take your point about the wing bands. I have taken on board your points and added them to those of other club members, my instructor during the 'post flight debrief' and 'wing band topics' I have read.

Before I posted this reply, I was contacted by the retailer and they said that the distributor had agreed to a replacement aircraft. This is great, as the Tutor 40 mk2 is a well rated trainer and I enjoyed the short time flying the first one. I will bear in mind all the points above during the build.

Once again thank you all for your input.

[kc]

Well that's good news!

If you are able to keep the old plane try to repair it if you can.......repairing planes is a necessary skill to learn.

Make sure you do a very thorough range check with any radio gear that was crashed.

[Martin Harris - Moderator]

That sounds like a very reasonable response from the retailer (and probably the importer who he may have agreed the replacement with). That's a shop that deserves your continuing support - and I'd venture to say, knows how to keep it?

Instructors aren't infallible but generally pass on their experience in good faith so don't be too hard on him if he is open to the suggestion that the bands were the likely cause. I recall one incident as a fairly inexperienced instructor when I accepted the assurances of a new pupil that he had the recommended number of bands although I was a little doubtful. All went well until he got himself in a slightly hazardous situation (he was doing very well for a novice and was much lower than most would be at that stage) and I took over to "save" the model - a rapid application of up elevator in a well banked turn led to a rapidly increasing turn and instantaneous flick - with terminal results...

We both learnt lessons that day - and thankfully he's gone on to be a very competent and enthusiastic flyer.

[Martyn K]

Difficult to check without a close look and it's even more difficult to determine what broke in the air and what broke on the ground, but to me it looks like the rib no 3 has failed between the tube and the spar. I still think this is a structural design vulnerability - the wing joiner should be bound to the spar to ensure a smooth transfer of the load path - especially as there is no upper/lower sheeting in this area to support the rib

It does appear to be different to the failure that I had although it happened in about the same place

Martyn

[dlonrar]

Once again thanks all,

kc, yes the more I look at the plane and all the gear, there is a rebuild challenge and a wealth of expertise to be gained. See if I can build as well as in my early free flight days.

Martin, yes I will stay with the retailer.

Martyn, thanks for commenting on the wing. It was your wing rebuild that has given me the incentive to rebuild the tutor mk2.

Edited By dlonrar on 23/05/2013 19:57:45

[Ian Jones]

On the subject of bands I'd like to add an experience I had with a Seagull Boomerang.

I had been instructing on this model for some weeks and the trainee was doing quite well. I hadn't spotted any problems with the 6 wing bands (I always use 6). At the bottom of a (properly controlled) loop the aeroplane suddenly pitched upwards and once levelled out behaved quite erratically. The trainee informed me that this had also happened when someone else had been instructing him but that they had not found a cause. I nursed it back in to find the the wing had unseated and settled slightly forward and lodged on top of the fus. A pull on the wing did indeed show that the bands were too stretchy and they were replaced. All seemed well.

A week or so later the same thing happened again, I don't know why but the trainee had put the old bands on again so the cause was soon determined.

Some days later the aircraft was flying nice and steady in level flight when it shed one wing just beyond the fully sheeted section. The wing fluttered down and the rest went in nose first in the next very muddy deeply manured field - which saved it from extensive damage.

With benefit of hindsight I am certain the impacts caused by the wing lifting then bouncing back against the fuselage flexed and overstressed the wing at it's weakest point and the damage in the wing went undetected until the disaster occurred.

The trainee decided to bin it but there was little wrong with it provided a decent repair of the broken spar was made, which I must have done because it has behaved impeccably ever since. It bunts with no problem at all and managed to get me through my A & B tests. Needless to say the wing bands have been applied very tightly indeed, stretching them across fuselage from one peg to the other before going over the wing if necessary.

The Seagull Boomerang is a well made tried & trusted model with an excellent reputation, so to blame the model would be difficult to justify without some clear indication of an uncharacteristic weakness. So my conclusion is that in this case the stretchy bands were an identifiable cause of the wing failure and sometimes we have look at how the model has been used as well as how it was built.

[Alan Pennington]

I can see that there could be a problem with the wing and that you are getting a new model to replace the one you lost. If you crashed on to concret what about the radio gear and engine? Thay say you should have the radio gear checked before using it again and I hope you are going to get it checked and I think the retailer should pay for this as well as getting the engine checked out and if its found the model is at fault let them go after the makers

[Martyn K]

My thoughts are that if you have to band it down so securely that the wing can't move (under any circumstances) then you would be better using nylon wing mounting bolts. I have always used 4 large bands on my Tutor and the wing never moved during flight. I used one over the wing Left and Right and one pair crossed over the top (left to right etc) to make sure the first bands cant slip off.

I also change the bands regularly and always throw the old ones out. I buy my bands from Staples - the large blue ones.

Martyn

[Alan Wood]

I always recommend 6 bands to hold the wing on, any less allows the wing to move too much and is noticible in flight.

[dlonrar]

Just to update everybody. The replacement Tutor 40 mk2 is built and flying with new gear and engine. The previous engine, servos and rx etc although ok and useable are being further checked out. The plane has flown for longer and is proving to be what has been said by club members etc.... that the Tutor 40 mk2 is a well rated trainer capable of flying a B test'. Regarding the wing bands, I use 4 if light or no wind and 6 if the wind is above 10 to 15 mph. This might seem odd but that's learning by experience (the forces on the wing will be greater when flying in windy conditions than when flying in no wind). My thanks goes to the retailer and distributor for their prompt resolution to my emails and photographs of the wing etc

[fly boy3]

Re rubber bands, Am I correct in saying, the bands should be 2/3 of the cord in length. Seem to remember reading this on the forum some time ago. Cheers

[dlonrar]

fly boy3, thanks for the informaton I will investigate further.

[kc]

Only 4 wing bands instead of 6 when it's not windy? I don't think the number of rubber bands should be reduced on a calm day! If it needs 6 then it needs them all the time.......the strain on the wings is due to the manouvre being performed This is likely to be just as severe during training as during really advanced aerobatics. See Peter Miller's column in RC Model Flyer this month, he quotes some enormous G forces recorde by a gadget he bought and tested by doing various aerobatics. Then he multiplied the models weight by this G force and calculated. From memory he said something like the 4 pound model effectively ' weighed' 56 pounds during some manouvres.

[Modonaut]

...I think the problem here is that if you put too many elastic bands on your wing, the wing won't come off if you hit the deck hard. Surely with a trainer, where you do want the wings to 'spring' off if you have a prang, it would be best not to try doing severe aerobatics where the G force is substantial enough to lose the wing seating. There must be a happy medium, also it's obvious that 6 old and worn out bands are possibly equal to 4 new bands... interesting.

[Martin Harris - Moderator]

The only difference on a windy day is the amount of turbulence you're likely to encounter - however, flown at reasonable speed a sharp edged gust is likely to stall the wing therefore limiting the load into the airframe.

However, learners often get into situations where airspeed is rapidly increasing and either make panic corrections - or as in the case I mentioned earlier - so can the instructor. If this results in a lifted leading edge because of insufficient holding power then you're getting into a precarious situation.

I agree totally with kc - always use enough bands - and I very much doubt that the 2 across and one each side that was mentioned originally will give enough holding down effect at the leading edge.

The rubber band mounting system should minimise damage but not be looked at as an automatic wing release for a crash situation. A (typical) minor repair to a sprung apart rear fuselage after a landing mishap while training is far preferable to taking a binbag and spade to the scene of a wingless arrival.