arcdude

I can see your pulling out all the stops on this, bravo! Are you shaping the sponson's on the boat as your go or pre-cutting them to start? What are the overall dimensions, it looks short, or is it just the angle of the pic? That is a fantastic design I'll give you that

Jeremy_H

Foam parts are being cut on a bandsaw where possible before being fitted. I only have a hobby bandsaw with a three inch cut, so I can't pre cut as much as I'd like, but I'll make up a long board today as an aid to shaping fitted parts evenly, using ply guides. There's a few concave surfaces going in which are the trickiest to get right, I'm hoping this method eases that.

The dims are the same as last mentioned, in the pic with the SI3. Phone pics can give very different impressions of size when taking pics of things this close, it's a focal length/perspective thing. I'll take a picture of it alongside the SI3 through a long lens later to show the comparison better.

Jeremy_H

Here's a better pic of the Twin alongside the SI3:



And some of progress on the Twin itself. Here the pieces added last were shaped over, and after chamfering the ply aperture to suit the aft section of the duct has been added. The aft foam sections were also shaped in at the duct end before fitting:

arcdude

That's better, thought I was going big I do agree the cockpit area does look a little slim, however the overall contouring will determine the outcome, nicely done! What type of sheeting are you using, almost looks like mahogany veneer? will you be sheeting over all foam or cf?

Jeremy_H

Sheeting will be epoxy/glass with CF stiffening where appropriate, the CF will be structural only, no cosmetic stuff on this one.
The ply is simply Gaboon Marine, it's light, but far more flexible than Birch. The idea here is that it's part of a structure, the foam on the deck top will act as a former that once glassed will form a 'box' style construction, coupled with profiling on the tops, curves and elements such as the vent, resist bending far more than simply two parallel surfaces. There's evidence of flex issues on SI3's overcome by various forms of deck stiffening, mine gets around it through the CF/ply/glass being much more rigid than base Birch. So on this boat the hull sandwich as it were should give me a rigid boat. It will not be light for its size.

arcdude

So your glassing and or CF all the styrofaom then and not sheeting all foam area's? I just did a little read up on the Gaboon marine ply seems flexible from descriptions I've read. Hopefully it won't end up too heavy, as you've demonstrated with the SI3 weight can be of benefit to keep the hull on the water but as with any aspect too much may impede performance. I'm watching this build with interest and may consider something along this styling at a later date, looks good so far

Jeremy_H

Yeah there's no ply on the foam from here. Think of it as a GRP moulding out of a mould like any other RC boat, which you'd then partially fill with foam for capsize buoyancy, the moulding has the intrinsic strength. This one will just end up with foam formers and a ply stiffener left within it, the GRP alone could survive on its own as the foam is there simply to act as a shape to layup the structural glass/epoxy skin on to. The ply is there to act as a load spreader for the engine pylon, and as peripheral strength to side forces around the hull. It'll become clearer as the pylon fixings and laminate stiffening go in.

Jeremy_H

I've shaped a concave recess into the rear of the deck before adding the next layer of foam:



Whilst I was watching epoxy dry I had a chance to try a possible addition. This has always been in the frame but I needed the vent in place to try something to be sure. I'm still undecided on this but it's interesting to consider it. Adding that vent is fraught with risk of water ejecting through it straight into the props, this is another idea that's been in my head for many years, originally it was to vent a step on one of my 'flatty with sponsons' designs such as the tiger paint boat way back in this thread. Arguably that setup would not be such a problem, but with tunnels on this boat the pressure of water splashes in them which can clearly be seen in the little red boat will undoubtedly make its way up the vent.
The Budweiser T4 is a good example of the problem, on these 200mph boats the force through the vents was enough to blast bits off it, it was only raced twice as a consequence:

http://youtu.be/pIWOs9LlNYE So some card and a Kebab stick to check out my idea of a powered vent cover. In the pic below of it open it's not as wide as it would be, and I'd make them longer to close the front end a little more, but it's an easy thing to rig and could be controlled in a few ways. Open/closed, or degree movement, linked to throttle or rudder, or both. Whatever, it's dooable:

arcdude

A few thoughts if I may from a layman's perspective. Interesting concept, although I would think the faster you go the more air lift realized through the underside of the hull will equal less disturbance on the water surface so less chance of water getting kicked up into the vents. The idea of the vent covers could greatly assist at lower speeds but when even partially opened at higher speeds I would think the force behind the water being forced up into the vent will be minimally controlled no matter what you do. With the prop thrust causing a vacuum effect on the air from and over the vents it will inevitably bring the water along with it and if the foils are as I suspect the air flow at the trailing edge of the vent will be moving faster than the air flow above the vent in a disturbed upward direction and dragged into the prop wash. With an air engine there will be a ram air effect in and over the vents whereas the Miss Bud in the video the air flow is controlled passively I believe. I realize there is so many factors involved and the placement of the engine in relation to the vents and the overall aerodynamic properties are key to the success or failure is near impossible to assess from a keyboard and none the less intriguing to see how it all comes together. One thing I am certain of this is beyond my experience level and you are certainly challenging yourself with this one

Jeremy_H

Check airflow through propellers, you may be surprised, the airflow into the prop is largely passive until a slight pressure drop just before the prop.

arcdude

Fascinating stuff for sure, it would appear at a stationary position there is a greater vacuum effect and when actual air speed match's prop pitch speed the cleaner the airflow entering the prop causing a yaw effect at the forefront of the prop. Learn something new everyday, can't be afraid to step out beyond one's comfort zone not sure if you noticed, I sent you a pm.

Jeremy_H

The PM was there but I had no notification, so might have dropped that ball when answering other folks messages, done now.

This prop thing is sort of counter intuitive, the only way to really get ones head around it is to understand the relationship between air volume and pressure. With a blast of high speed air coming off the back of the prop it makes sense to see it has high pressure as well as high speed, and that the air must be replaced in front of the prop so it must be heading into it at similar speed. But in fact it's not, the air in front is moving very slowly, and accelerating it through the prop increases its velocity, but decreases its pressure in keeping with Bernoulli's principle, as such the volume of air behind the prop might well be moving quickly, but it's at a lower pressure, hence entering volume of air is effectively spread out over the high speed zone meaning it's not replaced in front of the prop as fast as one might think. The other common counter intuitive aspect of this is that the pressure of air coming off the prop is seen as the motive force, when it's actually more about the immediate region of the blades, where an airfoil section on the blade means that with the same principle as above the blades part the air forcing it to take a longer path over the front side, which means it has to speed up, resulting in pressure reduction, add that to a pressure increase on the underside of the blade where the air is compressed and it's the resulting pressure differential which makes the blade move forward, pushed by high pressure and sucked into low. One of the ways to witness this is to look at some of the apparently absurdly large cowlings behind some airplane radial engines.

Anyway, the net total of all of this is that airflow over the vent on this boat is created by the boats movement, not the incoming air to the prop. The issue with the boat being higher in the water at speed and therefore causing less problems with splashing is enhanced by the vent itself, it's purpose being to force air into the tunnels, adding to that entering under the front of the hull, in an effort to lift the rear end, which is what the Budweiser T4 was trying to do. When turning, and when at low speed, the potential for splashing is much greater, and that's why I want it active, it'll only be opened at full chat in a straight line, hence why it might be operated independently with degree control, switched open/closed, or rigged to the steering and/or throttles via the transmitter computer. Then again I might not fit it at all, this is just a ponder thing

arcdude

Sooo much pondering With all the aspects you have incorporated into this build what's one more challenge Considering you can run boats all year round time is of minor consequence whereas here in about 5-6 wks we will be blessed with that wonderful white stuff to contend with, yukkk!

Jeremy_H

Shaped the deck front and added the sponson tops this evening.
Before final shaping the topsides I'll be flipping it over to fit service tubes to run back to the pylon area. It's quite a list to account for. Four fuel lines, six servos, two glow plugs, two rev counter sensors, and two temperature sensors have their cables and tubes run through to the cockpit area. The tub will then be built over that.

arcdude

Your going to monitor rpm remotely as well?

Jeremy_H

Yes. One of the bits of unfinished business with the in line twin double boost idea is that it's tricky to tune the rear engine. In essence the rear engine can only be done with the front running, I'd like to have gone for on board mixture control, but that's a bridge too far. At least with this rig the rpm/temperature thing can be checked under way, as offloading changes tuning.

Jeremy_H

Made a start on the service tubes. A bit of thinking to do before finalising though, depending on where the rudder and possible rear wing servos go.

When thinking about the need to seal the mass of wires and tubing, it's pretty tricky to do in a way that would be dependable, and which allows easy replacement. Servo extension lead plugs will need to be pulled through, and as such plugs and inline sockets could be in a wet area (resulting in electrolysis issues when damp and powered up) I've chosen a different route. Only the receiver, gyro, battery and telemetry module will be in a waterpoof radio box residing in the front end of the cockpit. Outside of that the rest of the cockpit interior, the interior of the engine mount and pylon, and the fuel tank space will be a wet area. Round section three core cable will be hard wired to the servo's cables to extend them, the joins will be potted to seal. These cables will then enter the cockpit via the service tube just under the forward end of the tanks, then through glands in the radio box, hence why a swap to round cable. So, the service tubes can allow water in to them which will drain through the transom. This method negates the need for any form of seal on the tank cover/pseudo inlet (which has to be readily removable for starting), the pylon cowling, and all of the control connecting rods.

arcdude

Smartly done Jeremy_H, looking good! How's the progress coming along?

Jeremy_H

Ok mate, workshop time has all been paying work again, but I've been working on the fuel system.
The idea of a false air intake was nagging at me, so I've revisited the fuelling arrangements and I'm now looking at Perry pumps again. There doesn't seem to be a comparable need to mine out there so I've got a couple of pumps on the way to do some trials. I'm hoping that I'll be able to use a similar setup to what the heli guys seem to be sometimes using. It differs in using a header tank and a fuel pressure take off, and retention of exhaust pressure feed. There's drawings out there for this, but I'll do my own to show you what I mean.

Jeremy_H

Here you go:

arcdude

Complicated Are the blue squares fuel pumps? This is foreign ground for me my friend! Can one tank/pump in a pressurized system not be utilized for such a setup? For e.g. a pressurized system with a main and a buffer tank as depicted or single main and two buffer tanks rather than duplicating everything, just a thought Would the engine's not draw as much fuel as required if there were a check valve of some sort to prevent one engine from drawing throughout the system starving the opposite engine? May be worth some experimentation to find out

Jeremy_H

Yes the blue parts are the pumps. They've arrived, so here's the actual things:



They work on a pulse principle, which is taken from the engine crankcase (green lines on the drawing). I guess that inside there's a diaphram or maybe just a tube that changes volume with the pulse acting on it, and with a non return valve on the in and the out it'll pass fuel with each pulse. An adjusting screw changes the flow rate, but I'm not sure if that's pressure, volume, or both that's being controlled.

The first challenge is fitting the pressure take off to the engine. The back plate should be the easy option, but this one is only a 1.4mm wall with no boss or other element in the casting giving it some depth like many engines have. The next option is going through the mounting lug, but I don't think they are thick enough for comfort so the back plate it is, I'll probably through bolt a flange or similar.




The only thing nagging me about the current setup in the drawing is really knowing what head of fuel the pumps can lift. The heli's tanks are close to each other, and whilst the pump manual spouts on about having a tank in any position nowhere does it mention the head, which is lame for a pump as it's a basic spec. I'm figuring that's down to the unknown of the crank pressure it's got to work with, but some sort of example would have been helpful. The instructions generally are lacking to be honest considering they come from the Conley Engine house.

For me the only sensible way to suss this is to go through a sequence:
Run the engines with regular pressure fed tanks at carb height, as I have, to a point where they are running predictably both independently and together with the correct props on.
Rig the pumps according to the instructions and adjust so that the same running is achieved.
Move the tanks around to see what happens.
If that fails>
Rig a simple version of the tee'd off idea and up the pump output so that engine always sees fuel, in a free vented tank
If that fails>
Rig the header tank as in the rig above.
If that fails>
Think again.

What you suggest does make some sense. There are a few mild advantages to complete duplication, such as the obvious running different fuel types (possible because the smaller engine seems to like higher nitro), the pumping of fuel to a dead engine in the event of one cutting (fixable by programming a shut down function for each engine to close the throttle to minimise), and the loss of both engines if the one that powers the pump shuts down. But more generally the idea of the pictured rig seems to be to make fuel available at the carb at all times, with any excess pressure bleeding off, so in principle a single pump delivering enough fuel for both engines is viable.
With check valves on the exhaust pressure lines it may allow a single main tank, and I see no reason for a single header to be a problem at this time. The pumps themselves are light, so it's no biggy to have both there.

Trials really are the only way on this one. There are many variables to account for, tube material length and bore, and the head, height in particular are very important to whether this works. All being well I'll get them fired up on the weekend.

Jeremy_H

The pressure take off's proved to be easy, I just used a first tap to cut an undersized thread, some Loctite pipe thread sealant and an o ring on each went in and tightened firm.

I've mounted the pumps simply for now for trials. Waiting on some fuel filters to get going. They are in according to the instructions for now, so I'll start the test sequence above and only make up header tanks if signs are not good.

If I have time I'll fit the temperature and optical tacho sensors to check their use and operation.

arcdude

Are those servo's at the base of each engine? if so I've never seen them with a plug in at the base. If they are the servo's, is that the permanent location for them? Can the pumps not be put at a lower location to reduce the bulk up high, what is the effective distance they'll function?

Jeremy_H

Yes it's the throttle servos and that's their position. It's not plugs, they are sealed strain reliefs for these truly waterproof servos. Many claim waterproof servos which simply aren't, these are IP67 rated so can be immersed up to one metre. The pump position is temporary, I'd like to get them tucked in more but I have to keep air paths clear for cooling. The instructions recommend carb height and horizontal, again the trials will help.