My Beaver's not very well.....

[Pete B]

 

Terry Walters isn't going to like this........

 

A few weeks ago, I had a moment with the E-Flite DHC Beaver when I stretched the flight time a bit too much and LVC caught me out. I had to head her off down the valley to land and, thinking I was just above the grass, pulled a touch of up to settle her and found I was actually about 6 feet off the ground....

 

That was a very heavy landing. The port u/c gear was badly twisted and the battery rather mashed the floor pan behind the cowling:

 

 

A couple of evening's fettling and she was looking as good as new ( I didn't take any pics of the repair process). Flying resumed shortly after

 

Yesterday afternoon, out she came again and I set off into the wild blue yonder - and then this happened:

 

As she came in on the final pass, about 50 yards away, I saw something white flutter from the aircraft and the port elevator began oscillating violently. I cut the throttle and pulled full up elevator but to no avail, she nosed-over and went in very quickly..........

 

First action on the scene was to disconnect the 4s pack which was visibly swelling and getting hotter by the second and that was dealt with later. The DX8 telemetry showed no holds and just a few fades, so no issues there.

 

I got her out for some piccies today.

Not a pretty sight. The port wing spars have snapped and the flap is rather shredded:

 

The front end is to all intents and purposes destroyed, although the cowl strangely enough will probably 'polish out', ho ho

Here is the cause of the crash:

The leading and trailing edge spars of the stab, supplied in one piece, have broken cleanly at the root. I suspect that the leverage must have caused the elevator joiner to break through the port elevator, thus freeing it to oscillate - the counterbalanced tips creating the violent oscillation.

 

Why should this fail? Well, when I initially constructed the model, I was careful to not cut into the wood when removing the film to glue the stab, so I'd rule out weakening the spars by cutting.

 

Could there have been a weakness in a spar, either initially or some unseen fracturing occurring during its working life?

 

I don't know but I'm not sure that normal flight stresses should cause such a failure - unless those counterbalanced tips induce stresses beyond that of a normal elevator.

 

It's in the garage at present. There doesn't appear to be any physical damage to the motor or radio gear but I've yet to test it. It's not looking good for a rebuild right now as there are a lot of complicated formers around the front of the fuselage......but it may look a bit more repairable in a few weeks

 

Pete

 

 

[Biggles' Elder Brother - Moderator]

That's a pity Pete - nice model too.

 

Hard to say from the pictures about the cause. In one picture it looks like there is an elevator control horn and a torque rod - is that right? If so the torque rod should have been taking much of the inter-elevator stress - unless it had been "caught" with some glue fastening it to the elevator - then it could have transfered some of the stress to the elevator itself - not good. But that all based on an imperfect understanding of the pictures - I could be completely wrong!

 

BEB

[Pete B]

No external horn, BEB - it's all internal. The ele torque rod has an internal horn controlled by a pushrod. The torque rods are set in balsa within the plastic elevator moulding and they've burst through the surface, releasing the elevator. I think the force applied by the counterbalance has done the real damage.

 

I've just had a quick look and it seems the elevator servo gears may have stripped. Whether that's pre- or post-incident damage, I'll leave it until tomorrow to investigate further.

 

Pete

[Lindsay Todd]

As BEB says its difficult to say from the pictures. Its not the first time I have seen a tail plane fracture and the fuselage joint, the elevator joiner would offer little structural benefit and its not clear if the wood used is simply balsa or something more substantial such as spruce as a leading or trialing spar brace. If this is simply balsa then I would certainly examine the quality of the basla which I would think should be medium to hard close grain and certainly not anything light or soft. I note in the flight the model happily loops and rolls, I agree that this should be considered normal flight but its impossible to tell how tight the loops were being pulled and therefore what stresses were put onto the tailplane. Did the flutter occure before the tailplane failure? if so that could also be the cause, it can induce truly horrendous forces. Slight futter might well have caused the elevator pushrod to break through the elevator setting off a series of events resulting in the crash. The dowind pass, if at higher than normal speed, may have induced the flutter. Anyway just my thoughts, always difficult doing a post mortem, very sad as its a great looking model

Linds

 

[Martin Harris - Moderator]

There's the possibility that the tailplane could have been weakened internally during the previous "arrival" and gone un-noticed...it did take quite a thump by the look of it.

[Josip Vrandecic -Mes]

What a pity......all what I saw is pretty weak tail section ...I .would have agreed with Martin

 

 

[Frank Skilbeck]

I've had mine for 3 years, thrown it round a bit (4s battery fitted) and bumped it a couple of times trying to take off too slow/steep while experimenting with the flaps on take off, but the elevator has held up fine (hope they aren't famous last words).

 

However if the motor, esc and radio gear are fine have you seen the really nice Platinum series Super Cub they are doing now..................

[Pete B]

I'm tending towards the possibility that a weakness has been introduced at some time. The front spar is softer than the rear but neither could be described as soft - both of adequate quality, I'd say.

 

It begs the question as to what I might have found had I given all the surfaces a good tug'n'test after the previous incident - others might benefit from this lesson!

 

I tend to fly my models as I drive my car - ie, they both need to last a long time so I don't stress them too much with high G manoeuvres, so I don't believe that I'd put any excessive strain on the surfaces.

Having said that, I was taking advantage of a fairly strong breeze to see what she looked like at speed and I opened her up after the turn and pushed her rather harder than I have done in the past.

 

It all happened very quickly but my impression is that the stab detached itself and the flutter then started - if it has started before I didn't notice anything amiss. Only 5 seconds passed between the point where I saw the stab come adrift, and the impact.

I haven't had a chance to really get stuck into the examination today so I'll add anything I find of interest.

 

Pete

ps Frank, don't tempt me like that..........especially at that price!

[Terry Walters]

Arrrrrrrrgh!! Just seen this Pete!! Your lovely Beaver! What can I say? I going out to mow 400 sq metres of grass to take my mind of it.

 

Heartfelt condolences - I feel like it was my own a/c that has been wrecked!

 

When I pick mine up in the UK in Oct you can have a go with any time you like.

 

Ring you later!

 

Terry

[Terry Walters]

Sorry make that 4000 sq metres of 'grass'! All done now!

 

TW