SeaDart Test 1 (SDT1)
From the very first meeting with the client I expressed a concern as to how heavy we need to make the model so it sits at the correct waterline. To have it bobbing on the water like a bath toy just isn't acceptable. We are going to great lengths to assure scale fidelity and the sit of any model either on the ground or in the water is very important to the overall look and ultimately is part of the identity of that aircraft. We know from our research the exact waterline required which sits 0.46 meters (18 inches) below the root leading edge and flush with the back of the wing.
4562006374_e388004636_o by Alex Jones, on Flickr
We talked at length with Fighteraces about possible solutions and collectively came up with this basic plan.
With the basic principle that one liter of water is equal to one kilo in weight we can cut the hull along the scale water line to find the volume that we need to displace. This varies slightly with fuel state but not significantly. Having designed the hull in CAD while doing the ski retraction work we could easily cut across the water line and work out the volume. Assuming we make the Ski’s neutrally buoyant we need to displace 89 litres of water, hence the model needs to weigh 89kg (196lbs). We’d like the model to weigh 60kg (132lbs) as this gives an acceptable wing loading, any more and its starting to become a handful especially with our scale aerofoil sections. The reason for having the available thrust equal to weight is that scale models don't have the energy of their full size counterparts. With excessive power we can create the impression of energy using the thrust intelligently which allows us to fly the model in a scale manner.
We are now left with a model weighing 29kgs (63lbs) more than we would like. The solution with the obvious requirement for lots of tests is to build a 29 litre wet compartment into the hull. This would be shaped to distribute the water with regard to the CG. The tank would flood once the aircraft is sitting on the water and have several large openings concealed in the formers that the front and rear ski mechanisms will be mounted to. Its easy getting the water in but we now need to evacuate it during the takeoff.
Believe it or not the full size actually took on water, in one instance the ground crew forgot to remove the stoppers in the vents and the aircraft took off with over 1000kgs of water in the hull. The difference being is that the water wasn't wanted in the case of the full size but just accumulated from the open areas at the back near the jet nozzles.
4561387447_5b6c2e3150_o-2 by Alex Jones, on Flickr
Conveniently the SeaDart has a large water rudder at the back which doubles as a speed brake. The area available to evacuate the water from is large when the water rudders are cracked open so the hope is by the time the model has de-planed its skis the compartments water level should be in line with the current waterline of the model. We have lots of other ideas but this is by far the simplest so starting here seems the best choice, if this doesn't work we will move to the more complex ideas…
So this brings us on nicely to floating the pattern in our SeaDart Test 1 (SDT1)
The picture below shows the pattern exactly as seen sitting in the water at 42.35kgs. As you can see it is far too light and needs to displace considerably more water to get the correct waterline.
_DSC9231 by Alex Jones, on Flickr
_DSC9226 by Alex Jones, on Flickr
From the very first meeting with the client I expressed a concern as to how heavy we need to make the model so it sits at the correct waterline. To have it bobbing on the water like a bath toy just isn't acceptable. We are going to great lengths to assure scale fidelity and the sit of any model either on the ground or in the water is very important to the overall look and ultimately is part of the identity of that aircraft. We know from our research the exact waterline required which sits 0.46 meters (18 inches) below the root leading edge and flush with the back of the wing.
We talked at length with Fighteraces about possible solutions and collectively came up with this basic plan.
With the basic principle that one liter of water is equal to one kilo in weight we can cut the hull along the scale water line to find the volume that we need to displace. This varies slightly with fuel state but not significantly. Having designed the hull in CAD while doing the ski retraction work we could easily cut across the water line and work out the volume. Assuming we make the Ski’s neutrally buoyant we need to displace 89 litres of water, hence the model needs to weigh 89kg (196lbs). We’d like the model to weigh 60kg (132lbs) as this gives an acceptable wing loading, any more and its starting to become a handful especially with our scale aerofoil sections. The reason for having the available thrust equal to weight is that scale models don't have the energy of their full size counterparts. With excessive power we can create the impression of energy using the thrust intelligently which allows us to fly the model in a scale manner.
We are now left with a model weighing 29kgs (63lbs) more than we would like. The solution with the obvious requirement for lots of tests is to build a 29 litre wet compartment into the hull. This would be shaped to distribute the water with regard to the CG. The tank would flood once the aircraft is sitting on the water and have several large openings concealed in the formers that the front and rear ski mechanisms will be mounted to. Its easy getting the water in but we now need to evacuate it during the takeoff.
Believe it or not the full size actually took on water, in one instance the ground crew forgot to remove the stoppers in the vents and the aircraft took off with over 1000kgs of water in the hull. The difference being is that the water wasn't wanted in the case of the full size but just accumulated from the open areas at the back near the jet nozzles.
Conveniently the SeaDart has a large water rudder at the back which doubles as a speed brake. The area available to evacuate the water from is large when the water rudders are cracked open so the hope is by the time the model has de-planed its skis the compartments water level should be in line with the current waterline of the model. We have lots of other ideas but this is by far the simplest so starting here seems the best choice, if this doesn't work we will move to the more complex ideas…
So this brings us on nicely to floating the pattern in our SeaDart Test 1 (SDT1)
The picture below shows the pattern exactly as seen sitting in the water at 42.35kgs. As you can see it is far too light and needs to displace considerably more water to get the correct waterline.
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