Firefighting Aircraft - Maximum Drop Weight Limitation(?)
Firefighting Aircraft - Maximum Drop Weight Limitation(?)
(OP)
Would anyone here know a good reason why a maximum drop weight limitation would be imposed on an aircraft modified for firefighting role (by means of a tank installation under the fuselage belly)?
--> The aircraft max drop weight limitation, imposed for some reason by the tank installer, is significantly less than the aircraft's max take-off weight.
What this results in is that if the aircraft takes off at max take-off weight to survey an area and the crew suddenly come across a situation where a drop is requested immediately, they cannot legally comply until they have burnt off a fair amount of fuel, or dump the fuel first.
I can't figure out why this limitation might be imposed. Any good ideas? Thanks in advance.
--> The aircraft max drop weight limitation, imposed for some reason by the tank installer, is significantly less than the aircraft's max take-off weight.
What this results in is that if the aircraft takes off at max take-off weight to survey an area and the crew suddenly come across a situation where a drop is requested immediately, they cannot legally comply until they have burnt off a fair amount of fuel, or dump the fuel first.
I can't figure out why this limitation might be imposed. Any good ideas? Thanks in advance.





RE: Firefighting Aircraft - Maximum Drop Weight Limitation(?)
Which begs the question: "If they could get down to the max drop load by dumping fuel, could they also get down to the max drop load by dumping water at the same mass flow rate as the fuel dumps?"
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
RE: Firefighting Aircraft - Maximum Drop Weight Limitation(?)
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RE: Firefighting Aircraft - Maximum Drop Weight Limitation(?)
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
RE: Firefighting Aircraft - Maximum Drop Weight Limitation(?)
But I'm still left wondering how dropping the water/retardant with less fuel in the wings will result in less load on the aircraft structure than dropping the same amount of water/retardant with more fuel in the wings (which would result in this limitation). There's no difference in how much, or at what rate, water/retardant is dropped in either case.
The aircraft, by the way, is a converted L-188 Electra.
As an aside, this maximum drop weight limitation has been in effect for several years, well before the C-130A incident.
RE: Firefighting Aircraft - Maximum Drop Weight Limitation(?)
RE: Firefighting Aircraft - Maximum Drop Weight Limitation(?)
The CG of the aircraft usually shifts as the retardant is being dropped. The control inputs that kept it in balance before the drop can make it climb. Actually, the aircraft is often already in a climb, and maybe faster than VA. It is also getting lighter, very suddenly. The wings are still at a high angle of attack (supporting the previous heavy load) so it wants to float up even more. The g-forces can be very high. Limit load has always been intended as an "occasional" occurrence, not a daily procedure. Herc's already have lots of fatigue problems with their spar roots. This mission just makes things worse.
Steven Fahey
RE: Firefighting Aircraft - Maximum Drop Weight Limitation(?)
Say you are standing on a bridge, and jump up and down on it.
No problem.
Now load the bridge up with a static weight to within 1 engineer's weight of breaking, and repeat.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Firefighting Aircraft - Maximum Drop Weight Limitation(?)
RE: Firefighting Aircraft - Maximum Drop Weight Limitation(?)
I suspect, as does SparWeb it seems, that this may have something to do with the CG shift that occurs during the drop. What I don't understand is how this situation is improved by first getting rid of fuel before dropping; or put another way, how does having more fuel in the wings during the under-belly drop make it less safe than with less fuel?
RE: Firefighting Aircraft - Maximum Drop Weight Limitation(?)
RE: Firefighting Aircraft - Maximum Drop Weight Limitation(?)
Compute the weight shifts starting at the extremes of the flight envelope and you may find excursions when the retardant is dropped.
If memory serves, many Electra conversions have been done by Aero Union, including firefighting. See what they say.
This question still sounds worth asking. There's sometimes the possibility that the answer will lead to a solution allowing greater utility.
Steven Fahey, CET
RE: Firefighting Aircraft - Maximum Drop Weight Limitation(?)
I think the reason might be acceleration loads on other parts than wings and spars. Imagine the wing lifting an a/c whose weight is suddenly reduced by a significant amount. The wings still produce the same lift so the result will be an upward acceleration imposing great loads on engine supports and other structural parts. Fuel in the wing tanks will most likely exert a large pressure on the bottom skins. I could go on, but I think you see what I mean.
Dan
RE: Firefighting Aircraft - Maximum Drop Weight Limitation(?)
RE: Firefighting Aircraft - Maximum Drop Weight Limitation(?)
RE: Firefighting Aircraft - Maximum Drop Weight Limitation(?)
The weight in the wings is called a "relieving" weight. That is, the weight is located where the lift is generated. That means fuel weight does not increase the bending moment on the wings.
The water bomber conversion locates all the water/retardant along the fuselage bottom. That weight DOES increase the bending moment on the wings, as the lift is generated out along the wings.
The gross weight limitation on the fire bombers is lower than the P-3 due to this loading configuration. Equivalent weights do not result in equivalent loads on the structure.
Dan
P-3 Structures Engineer