And another one bites the dust ...

[Colin Bernard]

Well not a good weekend - first time out for two months and two prangs .

 

The first was my own fault - my little Mountain Models Cub caught the prop in a rut on the field and the motor mount failed. It was really too gusty for it, and a gust immediately after a hand launch caught me out before it had got proper flying speed. Minor repairs and it will be ready for next weekend.

 

The other has me foxed though. This was the maiden of a Seagull Innovator, which after some initial trimming was going quite well and coping with the gusts. After a few circuits I decided to have a practice approach and on the final descending turn to the left, all of a sudden it gave the impression of full right aileron and there was no response to my stick waggiling.

 

Despite the clouds of balsa bits, damage seems to be limited to the nose section which being modular meant that the rest of the fuselage has only suffered a few splits so will not long be in the air again.

 

What's throwing me is what could have happened? I don't think it was not my clumsiness with the sticks - I was in a steady descending turn just about to level out.

 

I thought there might have been a glitch and the failsafe cutting in, and it losing the settings I had programmed in - but when I checked out the Rx (Corona 35MHz Synthesised), it was working fine and the failsafe mode was not even switched on. In this mode if the Tx signal is lost then the servos just chatter - they do not go into any hard position.

 

The only other thing I can think of is that I let the speed drop and it stalled on the turn, but again I am not sure this could be the issue as I was in a gentle dive so if anything would be gaining speed, and the stall on the Innovator is supposed to be quite gentle.

 

So chums - any other suggestions? Or do I just chalk it up to experience and welcome the opportunity to fix my first ARTF ?

 

(Should explain have been in this game many years and up to now built from plans and kits!)

 

[Bob Cotsford]

Two points and a couple of questions -

If it had tip-stalled it would more likely have tipped into the turn, not out.

If failsafe had not been set there's nothing to stop interference sending one servo to an extreme position. If you have the facility, always set the failsafe.

 

Is the Corona an intelligent failsafe capable of recognising a bad signal, ie is it described as DSP or similar or does it only respond to lack of signal?

 

What was the weather like, and were you near any trees, hedges or anything else that could have caused turbulence? I once had a Hooligan completely invert itself as it hit the rollover from a hedge on a blustery day.

 

[Colin Bernard]

Hi Bob - Thanks for the feedback.

Yes I agree on the failsafe - and normally I have it set. I simply overlooked in the haste to get out on a Sunday afternoon where I could fly. (First since before Easter for me!).

 

The Corona is a DSP Rx so the failsafe condition detection should be good. It was blustery - I am guessing gusting to 10mph plus, but the Innovator was handling those pretty well, and I don't recall any at the time of the incident.

 

There are trees at the bottom of the field but I estimate I was about 100' away from them and the tree line was parallel to the wind direction so the wind was blowing along them not over them.

[Shaunie]

Hi Guys,

Not familiar with the aircraft but, with my understanding of aerodynamics although the higher outside wing on a turn is going faster than the inside wing it is also at a higher angle of attack due to the attitude of the aircraft. It also has a downward deflection on the aileron both of which can promote a stall in the higher wing causing a sudden (unexpected) wing drop. I know it caught me out a couple of times when first learning to fly, I complained to my instructor something was wrong with the plane to which his reply was along the lines of "nothing wrong with the plane, it's the pilot, you let the plane slow up too much". This was a PrecedentHi-Boy trainer.

Just wondering if a bit of local turbulence added to this effect may have been responsible.

Shaun.

[Biggles' Elder Brother - Moderator]

What you describe is quite possible Shaunie. In a very slow turn, all other things being equal, it is true to say that the inside wing is the one most likely to "let go". But if you add "into turn" aileron under these conditions, and the wings are close to the stall anyway, that down going outside aileron can - and often does - trigger a tip stall. The result is the plane appears to flick out of the turn, ailerons become ineffective and it is of the nature of the situation that you're usually too low to effect a recovery - result, splat!

 

Its another reason in favour of differential ailerons - although even that won't help you if you are really close to the stall. Some folk will advise the use of rudder in these circumstances - but that brings its own problems. If you are a tad heavy handed with the rudder you create excessive side-slip which increases the drag and slows the model still further. As your airspeed margin over the stall is minimal anyway under these conditions any further slowing is obviously undesirable. The only gaurenteed solution is to ease off the elevator thus reducing the AoA and building speed.

 

Reading the description Colin gives above TBH my money would be on "it tip stalled" just as you are suggesting Shaunie.

 

BEB

[Former Member]

[This posting has been removed]

[Lazygit]

Just another nit pick, altering the camber does alter the AoA.

[Colin Bernard]

Thanks guys - yes I am convinced on it being a tip stall.

 

Having thought about it more since the original post I am thinking this was also compounded by the conditions. It was not a constant wind with the plane reacting to gusts when I felt nothing, and then when I was feeling the breeze the plane seemed to be in smooth air.

 

This leads me to think that maybe I was caught out on the downwind leg with the plane being in a stronger breeze and my interpreting the increased ground speed as air speed and as a result I backed off the throttle too much.

 

Lesson learned - 'You can only land a plane that is flying, so keep the speed up!' 

 

Edited By Colin Bernard on 03/10/2011 09:04:03