I was and still am impressed by the 1960’s vintage Cessna 185s I used to fly which were able to lift a useful load equivalent to their own empty weight. Rare even to this day. 19000 lbs takeoff weight for a 6000 lbs empty weight airplane is quite a feat owing to contemporary airframe structural technology. if true I’m more than impressed.
Not sure I will be happy flying a plastic plane low over fires… plastic melts.
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I work for a State forestry agency and I flew forest fires for 7 years in a Cessna 185. I’ve also flown some composite hull aircraft. I wonder about the effect of heat from a big fire on the composite. Tankers fly just above the tree tops to do their drops so they’re pretty close to the fire where it’s a lot hotter. 1,200 gallons of retardant or enhanced water makes for a really heavy airplane. Once they release the drop, the airplane suddenly becomes much lighter and the wings flex a lot. I’m wondering about long term airframe cracks that might develop with the heat and flexing.
My agency does use drones in firefighting and I’m a FAA Remote Pilot as well as a CAP Unmanned Aerial System Mission Pilot. Drones are mostly useful on a fire for providing an aerial view to the ground firefighters or detecting hotspots at night with IR cameras. However, their range and endurance are way too limited to replace light helicopters or scout planes for covering larger fires and staying airborne longer. They can’t carry much of a load so suppression is out of the question.
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As opposed to aluminum?
For aircraft designers, composites offer the ability to make large structures (like a whole wing or fuselage) in a single piece. That makes it structurally very efficient and light.
For an aircraft maintainer, those large single-piece structures can be difficult to repair such that the strength is sustained, and in the process the weight savings gets reduced.
Since these airplanes fly at low altitudes, I suspect damage from bird strikes or even tree branches, can be quite common? I think there could be a practical operational advantage to at least having removable/replaceable leading edges in the flying surfaces. Even at the cost of a modest weight penalty. Such a thing is inherent in most metal riveted wing structures, but there are exceptions like Grumman, where I believe it is all one wing skin from the rear spar all the way back around to the rear spar again.
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Well, it is cured resin of some kind but high performance reinforced with fibers.
Composite airplanes routinely operate in desert conditions of +130F ambient, plus sun heating of structure.
Well, if you drag them through treetops, as a DC10 tanker did, there may be damage to leading edges but I guess nothing different from metal structure.
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The damage is no different, but what happens next is very different. All of the leading edges on the DC-10 are replaceable parts. The composite wing has none. The metal airplane can be back in service as soon as the damaged parts are unbolted and replaced (typically by the following day). The airplane with a damaged composite wing will be out of service (typically for many days at least) while engineering figures out just what customized repair is necessary to restore it to a serviceable condition. Then the repair itself may take many days to complete, possibly at facilities with specialized tools (and backlog schedules of their own).
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STandard Composite materials are Epoxy based. These are thermosets and do not melt. They degrade w heat as do ALL materials. Aluminum (2024 T 810 Cannot endure any significant time above 250 F without over aging and losing fatigue life, Most modern epoxies can see 180 F HOT WET for indeterminate time with no aging and 250F Hot dry for similar. there is a lot of "hot air " concerning this but the FA 18 is made from early generation toughened epoxy and has an operating temp on th ground of 180 And in flight above 240 F. There have been NO structural failures due to these temps.
Yes I was a materials and process engineer on the program for a while.
Thankyou for providing facts.
So your uncommon scenario is a disadvantage partially offsetting the large productivity advantage of higher payload day after day?
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I suppose “uncommon” is in the eye of the beholder. I’m a mechanic, so I see broken/damaged airplanes every day of the week. What I’m saying is that an airplane with the greatest design purity isn’t always the one that is the most commercially successful. Look at the MU-2 and the early King Air. The MU-2 had greater design purity and more performance for less fuel, but the King Air beat it in the marketplace. Most airplanes that work commercially have design compromises that support operability. All I said was that I could see where a repairable leading edge might be such an advantage. Look at the Air Tractor products. They give up a bit of design purity in exchange for toughness.
Don’t get me wrong. I very much appreciate a design that can carry more than it weighs. That’s a real achievement. The Cessna 180/185 is remarkable in that way.
‘Design purity’ is a shallow term, the debate is actually about optimization and tradeoffs.
MU-2 was fast and less prone to propeller damage, which is why a utility in BC used it to visit its sites (gravel runways far from base).
The other big utility in BC used a Citation - could take an accountant to a site, spend the day shuttling parts and other people around, then bring her back home by nightfall.
I saw them come and go from YVR.
BTW, 700 MU-2s produced.
‘Uncommon’ was specific to dragging through trees. You see the result of a small proportion of flights. (Visit Field Aviation Calgary in the 70s-80s and you’d see Twin Otters with broken wings. Not the fault of the aircraft.)
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That C-130 had undetected cracks in wing structure.
Military and civilian-registered Hercs had advisories on wing structure after one fell out of the air after encountering light turbulence over the SE US. IIRC the aerial fire fighter had a patch on the bottom of the wing, that made inspections more difficult.
Neither case was what is suggested in this thread - intact un-cracked wings do not fail from sudden release of load.
‘Uncommon’ was specific to dragging through trees.
My mention of tree branches was never meant to be specific. I was representing the risks inherent in any low-level operation, be it firefighting or crop dusting or pipeline patrol or any of the like. Bird strikes are also a common hazard for aircraft that have high utilization at low altitude. So are power lines.
Planes that fly very low “hit stuff” more often than planes that fly high. And those hits tend to be on the leading edges. If I was designing a plane optimized for a low altitude use, I would either give it replaceable leading edges, or I would make them very, very strong. Even so, I would consider making them repairable.
‘uncommon’ is in reality - check accident statistics.
The Air Tractor was designed before serious structural composites became feasible for small airplanes. (‘fiberglass reinforced plastic’ was probably available but not suitable for wings, chemical tanks may be a good use of FRP.)
Life has choices - tradeoffs, the MU-2 and GB2 exemplify that with better performance and payload: that’s a large economic benefit.
I expect that tankers do not fly low above hot fires.
They may normally fly at 200 feet above trees, but Coulson upped to 300 if I understand correctly, after losing two airplanes.
It is important not to fly into rising terrain.
That’s true but the drop dissipates rapidly at too high an altitude and isn’t nearly as effective. Our Air Tractor AT802F single engine tankers fly 90 kts at 90 - 120 feet AGL depending on terrain and obstacles. We purposely bought the larger engines to allow them to pull up sharply once the drop is released. Larger tankers have a tougher time pulling up for rising terrain but they drop a lot more than the 800 gallons our Air Tractors drop.