Hot weather flying

[Bob Llewellyn]

Interesting then that Jabiru haven't added anything similar to their 120 and 230D POH, yet they're the same resin.

I suspect the 160 has slightly smaller structural margins than the earlier generations of Jabs; and the 120 uses (used to use?) an earlier gen wing design, albeit updated to somewhat later production techniques - I think - Jabiru don't share these little details with me...

 

 

[M61A1]

Could be engine approaching detonation limits, elasticity of airfame approaching flutter limits, reduced stiffness of spar boom approaching buckling limits, reduced atmospheric density exacerbating prop compressibility... the GTT is not a magic number, it is a critical boundary; then little thangs in the plastic change their nature before the GTT is reached, and there are other considerations (as listed, plus probably a few i ain't thunk of).

What sort of composite resins are they using? The ones I'm used to get painted dark green and sit in 40 deg heat long enough to burn you if you lean on it, then they spin it up over 300 rpm, and lift 6+ tonnes with it while it flexes all over the place.

 

 

[Bob Llewellyn]

What sort of composite resins are they using? The ones I'm used to get painted dark green and sit in 40 deg heat long enough to burn you if you lean on it, then they spin it up over 300 rpm, and lift 6+ tonnes with it while it flexes all over the place.

(a) sitting in 40 degree heat gives an elevated temperature cure, only takes a few hours or less;

(b) They're probably using epoxy, because it is the superior fatigue resistant resin at high stress levels (vinyl ester can be very good re fatigue, but it is subject to creep at high stress levels, and unlike epoxy, it is not specifically nominated for aircraft use). In fact, as the J-160 was designed against JAR VLA, it can only be epoxy.

 

 

[M61A1]

(a) sitting in 40 degree heat gives an elevated temperature cure, only takes a few hours or less;(b) They're probably using epoxy, because it is the superior fatigue resistant resin at high stress levels (vinyl ester can be very good re fatigue, but it is subject to creep at high stress levels, and unlike epoxy, it is not specifically nominated for aircraft use). In fact, as the J-160 was designed against JAR VLA, it can only be epoxy.

I was referring to things like main rotor blades that are already cured, usually in an autoclave, and definitely epoxy. But was interested to know what sort of stuff they might be using that is incapable of dealing with temps under 60 deg C, as that is usually the lowest temp for an accelerated cure, most I've done are around 120 C +.

 

 

[jetjr]

The issue as it was explained to me was that when the airframe was new, having been produced at low ambient temps had possible softning at fairly low temps too, relatively speaking, so to meet derating for tested loads there had to be a limit set around that 40 deg where everything was safe.

 

As the material ages it becomes stronger (to a point)

 

I think they use an araldite epoxy, numbers are at home. The airframe and materials isnt the weak point with jabs. If you want to start using materials autoclaved and high cost inputs youve lost the intended market.

 

The FRP epoxy is cheap and very easy to repair and live with

 

 

[Bob Llewellyn]

I was referring to things like main rotor blades that are already cured, usually in an autoclave, and definitely epoxy. But was interested to know what sort of stuff they might be using that is incapable of dealing with temps under 60 deg C, as that is usually the lowest temp for an accelerated cure, most I've done are around 120 C +.

Thermosetting plastics of the epoxy family have two modes of polymerisation. The fundamental mode forms large elongated molecules, with (relatively) rigid zig-zag shape, and a few cross-links to adjacent molecules. The GTT is, crudely speaking, when the "free" energy level exceeds the cross-link energy sum for each molecule - wtf? well, hotter = more amplitude of vibration of the atoms, and when they are stretching the intermolecular bonds too far, the bonds can "snap" across to the nearest adjacent attachment site. So, them there rigid zigzag molecules start to slide past each other.

Now, the secondary mode of polymerisation of thermoplastics, involves a lot of cross links forming close together; if excited by enough heat - as in staged elevated temperature curing - the zig-zag molecules can actually get a bit shorter, as the intra-molecular bonds are reformed into inter-molecular bonds.

 

The more inter-molecular bonds there are, the higher the GTT becomes. So, a room-temperature cured epoxy has a low GTT as cured; running it through temperature cycles - such as day and night through half a year - gradually elevates the GTT.

 

So why not autoclace the crap out of it? Because the maximum rupture strength of the epoxy matrix is reached at less than complete cross-polymerisation; AND, critically, the maximum fatigue endurance is reached at rather less cross-polymerisation than the maximum rupture strength. The Jabiru "system" - which includes chemicals, application temperatures and times, and cure cycles - achieves outstanding fatigue resistance, at adequate rupture strengths, using low-overhead production techniques (otherwise nobody could build LSA-E kits!).

 

Rotor blades are rather specialised, in that the centrifugally induced stresses dominate; their fatigue resistance has to be optimised only for a spanwise mean-plus-cyclic situation, and no triaxial restraint exists in the loaded area, unless the designer has gone to truly extraordinary lengths to screw up!

 

I have had the - pleasure? - of designing a repair scheme for a set of doors on a certain model of FAR-23 composite piston single, which at 1100 hrs TT (then the oldest in the world fleet!) had developed fatigue cracks above both door-mounted windows, from slipstream tip vortex impingement. They were autoclaved carbon fibre doors, strong enough to kill oxen, but they couldn't handle fatigue.

 

Jabiru put loads of work into structural integrity / composite fatigue research, and when it comes to resilience, they are the (composite) light aeroplane gold standard. Note that I hold no shares in Jabiru, do not own a Jabiru, and have no present intention to get one; but I have (somewhat grudgingly) acquired a high respect for Rod's approach to composite structure,