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Enthalpy

Water Bomber

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Did you play with matchsticks once again? Here comes the fire brigade, with tools of adequate size.

Many aeroplanes, especially military cargo planes, have desirable performance as a water bomber: sturdy, good capacity, good climb rate, reasonable flight speed... The Bombardier 415 adds the precious ability to refill its tanks by scooping water from sea or a lake:
http://en.wikipedia.org/wiki/CL-415
and I should like to explain how more usual airframes can obtain this ability.

 

post-53915-0-85264500-1381698743.png

 

The plane flies a few metres above water and lowers a boom that scoops water to the tank(s).

  • The boom is articulated at the airframe. Downlift comes primarily from the steering winglets. Its design is inspired by a refueling boom.
  • A lightweight ski(s) follows the water's height. The winglet-ski combination shall alleviate shocks at the airframe by the waves.
  • Elasticity can be built in between the winglets and the ski. Design anyway this place as the weakest mechanically.
  • The opening of the scoop can be lower than the ski.
  • The scoop - or its opening - is smaller than depicted. Its drag is smaller than the touchdown of a waterboat.
  • Two-axis articulation of the boom may be better when the wind doesn't align with a small lake.

Alternately, the lower tip of the boom can be well profiled to plunge deeper into the water without a ski. This may be better at rough sea.

The spherical tank is to give a visual impression of the volume that the aeroplane used in the illustration can lift. Putting it (or them) outside the fuselage, like before the wheels, over the wheels or under the wing is probably better, as flushing it quickly is very important, but shouldn't destabilize the plane.

Water flowing in quickly is dangerous. It needs jet breakers at the tank, and protections for the personnel. I'd let it spill over when the tank is full and provide it a wide way out of the airframe.

Yes, you can take a bigger aeroplane, of course. Maybe a C-130 Hercules. But to use smaller lakes, preferably a not too fast aeroplane. Or design one purposely. A remote control would be nice as well.

Marc Schaefer, aka Enthalpy

 

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Only the Canadair-Bombardier CL-215 (and CL-415 with turboprop) were built purposely. Scooping procedure is summarized by pilots:
http://bernard.dumas.perso.infonie.fr/Ecopage.htm
http://pelican.46.free.fr/Caracteristiques/body_caracteristiques.html
Splashdown at 75kts, give power once arrived at 60kts, then open the scoops to take 500kg/s water.
End scooping, accelerate up to take-off at 78kts, accelerate further to 96kts then climb.

Scooping takes much propulsive force, but less so in a seaplane moving slower than its stalling speed. The CL-215 flies rather slowly, as a further design advantage, enjoying a big propulsive force. In contrast, the Casa CN-235 flies faster, and would scoop in flight, hence above its stalling speed, or around 50m/s (100kts). Many cargo planes are a bit less favourable: faster, and their flatter ascent angle meaning less available propulsion force. Commuter airliners fit even less because they're faster and not as sturdy.

As a seaplane, the CL-215 touches down and takes off parallel to the swell, not against the wind direction. A cargo plane must scoop parallel to the swell also. Then, at 50m/s and 10% from parallel direction, with 3m swell period (at the Mediterranean), the advantageous ski oscillates with 0.6s period and can be kept.

I estimate the Casa CN-235 has over 10kN excess propulsive force available, from its 9.0m/s climb rate and 10.1t empty mass. This allows only to scoop 200kg/s at 50m/s, needing 30s and 1.5km to load 6000kg, instead of 0.3km. Irrelevant at sea, but a drawback on a lake - the penalty for a mass-produced aeroplane.

Forces at the boom and the airframe are nearly simple... I take a slope of 1:2 for the boom. 200kg/s water turning from -50m/s to +50m/s create 20kN drag and 5kN down at the rear of the boom. The airframe pulls the articulated front end of the boom with zero N*m, (nearly) 20kN forward and 10kN up, but water impinging at the tank pulls the airframe 10kN forward and 5kN up. As a sum, the airframe feels 10kN drag and 5kN down. The rear end of the boom is pulled up (by the plane minus the water) with 5kN, to be compensated by >3m2 airfoil. At least one nice aspect: if the boom articulates at the right height (suppress the door), the airframe won't feel a pitch moment.

Airborne scooping procedure could then be: stabilize at 50m/s and 5m, lower the boom, press the ski down with the airfoil, give power, then open the scoop.
The airframe feels immediately 5kN (0.5t) down force, which would lower it by 0.5m after 1.4s, but is to be compensated.
Water flowing in makes the plane heavier by 0.2t/s, to be compensated.
Then close the scoop, raise the boom, accelerate and climb away, you guessed.

The operation pulls toward water an airframe not designed for sea and this is more risky than with the CL-215.
On the other hand, the cargo plane doesn't have to splash down, a risky operation avoided.
I want that the pilot has a fast (typically pyrotechnic) emergency button to sever the boom from the airframe, in addition to the fast release of water.

When scooping (and when dropping water as well) it would be better to control the plane by its landing flaps instead of its elevator. Build faster actuators there if needed.
A pair of rods between the wheel fastenings, the water tank(s) and a point about 1/3 out at the wing would improve strength.
Keeping the cabin pressure constant, even near sea level, would be nice to the crew.
And improve all protections against corrosion...

Marc Schaefer, aka Enthalpy

 

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For safe operation, the ski should drag very little when not scooping. This must prohibit a scoop built deep below the ski, changed my mind. The ski may have a portion of its width reaching slowly deeper, where the scoop gets steadily water, as the best possible design.

I like the latest design of the retractable scoop at Canadair's CL-415. It's a sector of a cylinder that plunges into water by rotating around its horizontal axis. Fits the ski nicely.

 

post-53915-0-30539100-1381699008.png

 

The plane will not scoop into the wind but parallel to the swell, flying hence sideways, so the boom must move on two axis.

I'd place the emergency button that severs the ski or the boom from the airframe just like the trigger button on a combat plane: at the yoke, with a latch.

 

 

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The current fleet of water bombers is ageing: the original CL-215 was designed five decades ago, and existing planes show fatigue, the Canadair as well as modified airframes. Several airplane manufacturers think at a follower (including ultralight aircrafts, which may explain why I saw so many fly over as I was thinking at the design), and I just have to add my own frenzy to the existing mess.

Among the older CL-215, Spain has lost 7 from 30 acquired, France 4 from 16. Among the CL-415, 7 have been lost from 60 built. Breaks in flight seem to be the main cause of accident, due to low flight over difficult terrain, and turbulence by wind and fire. Unspecialized airplanes use to be less solid; the C130 Hercules as a water bomber has been abandoned in Europe after two of them broke.

So I consider a specialized design should have such qualities:

  • It must be controlled remotely. The pilot stays on the ground, possibly aided by a scooper-dropper shared among several planes.
  • The airframe must be solid and resilient against stalling.
  • It should better be slow and drop its water well above the stalling speed. Climb rate is important, finesse far less.
  • Carrying capability doesn't need room, as water is dense.

And this is how I imagine the design:

  • A biplane. Out-fashioned, but still naturally solid with cross struts.
    With more area, it lifts more weight at lower speed.
  • The landing flaps control the pitch (though the elevator must be controllable).
  • The water tank(s) can act as an emergency water rocket to lift the plane.
  • A single turboprop. But I'd add a parachute, as some ultralight planes have, to land the whole airframe not too hard.
  • Of course, scoop in flight as described above, without needing a sea plane. Though experience shall tell if it's better.

The landing flaps, with fast actuators, act more rapidly on the flight than the elevator does. This is useful near the water, the terrain, or if stalling. Accordingly, the elevator is at the rear.

With its wings more closely stacked than usual, and especially if the upper one is a bit before the lower, a biplane keeps more lift when stalling.

When using an added tank of emergency gas to put the main tanks at 30b, flushing 10t of water lifts (and pushes forward if desired) a 10+10t aeroplane during 4s, enough to recover from stalling. Use many small nozzles.

The parachute is fastened to many parts of the aeroplane, in case the frame breaks.

It's probably better to let the plane decide alone if using stall recovery processes or the parachute, since the pilot has only visual information. The remote control must be tamper-proof, and the plane may take over if remote control is lost.

The water tank(s) must be vertical and narrow to reduce the longitudinal movements of weight.

Marc Schaefer, aka Enthalpy

 

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Pictures of a biplane water bomber! Something like 10t water in the size of a CN-235 but flying slower - or scale up and down.

 

post-53915-0-16856700-1381699141_thumb.png

 

post-53915-0-08995100-1381699164_thumb.png

 

post-53915-0-55023700-1381699177_thumb.png

 

Fowler landing flaps would probably operate too slowly. A compromise would be simple flaps put permanently lower than the wing's trailing edge, as the Fieseler Storch had, but this increases drag.

Neither the size of the elevator nor the scheme to retract the main landing gear (to the front here) have been checked.

Marc Schaefer, aka Enthalpy

 

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This water bomber shall fly continuous 16h thanks to hydrogen, so it can operate far from its base and saves ground time. A vacuum-insulated tank stores 360kg liquid hydrogen, as I describe there
http://www.scienceforums.net/topic/73798-quick-electric-machines/#entry738806
enough for 12 fuels cells of Honda's FCX Clarity type (each 100kWe and 100kg, 60%) at mean 900kWe.

Full 11750kg include 6250kg water, 1200kg fuel cells, 700kg hydrogen and tank. With L/D=12 at 35m/s and 70% propeller efficiency, the plane climbs 4.4m/s or scoops with its ski. With L/d=15 at 60m/s and 75%, cruise takes 620kW.

 

post-53915-0-13456900-1381699251.png

 

post-53915-0-03812300-1381699263.png

 

post-53915-0-47295000-1381699273.png

 

Eight tanks limit water movements. Pairs of opposite tanks share pressure-gas and drop valves. Tanks in the wings would ease the pitch balance, worsen the roll a bit, but would be difficult to empty in 2s. Tandem wings would accept more pitch torque but have regular strength. Hydrogen mass creates a small torque. Put the fuel cells to adjust the center of mass.

The biplane strengthens this water bomber and packs 150m2 in little room (though looking like an old monster) while making stall less brutal. Tip walls are not sketched but bring much to biplanes. Four 2.4m propellers look easier than two 4m pieces, and electric motors are simpler than turbines anyway; they blow only the upper wing, so the lower wing should be more offset to the aft than sketched. Again, mainly the flaps control height and pitch, for swift reactions near the terrain and the sea.

The nose gear could retract near the fuel cells, and the main gear in the Kármán or behind the water.

Several remote-control crews should share the 16h; one scooper-dropper can operate several aircrafts. Previous considerations about automatic anti-stall manoeuvres, lift and acceleration by water jet... still apply. Infrared and radar imagery would widen the operation conditions, if desired to nighttime; bigger tanks would easily bring the continuous operation to over 48h.

Marc Schaefer, aka Enthalpy

Edited by Enthalpy

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What of the previous ideas could be retrofit on the CL-415 Canadair?

The remote control needs "only" control and video transmissions. Since the plane supposedly has hydraulic actuators at all controls, servovalves can be added as alternatives to the manual inputs. This avoids fatalities, since crashes do happen in this job, and lets share the scooper-bomber operator among several aircraft. It also improves the payload.

Fuel cells still weigh much: 0.5kg/kW
https://en.wikipedia.org/wiki/Toyota_Mirai
so replacing 2*1775kW turbines takes 1775kg fuel cells. The electric motors can be lighter, but the hydrogen tanks are heavier. So while the flight duration improves a lot, the payload diminishes. More interesting at a new design, especially if not a flying boat, to my opinion.

The in-flight scooping ski makes most sense if not a flying boat, to save structure mass and ease the design. At the CL-415, it would let use shallower waters and save kerosene. It's also safer against debris collisions.

Pressure-fed water tanks would avoid to spread the last water inefficiently, and above all, they are a life-saving means against stalling.

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