Jeremy_H

Knocked up a cowling from a mouthwash bottle

Jeremy_H

You can see it better now it's in a base coat. I trimmed it some more having seen it painted.

Jeremy_H

Just taken the masking tape off, it's a bit dated looking but some decals will jazz it up

Jeremy_H

Prop up of the parts so far including a Balsa core Carbon Fibre wing, then sketched over to see how things look:



The wing:

St.Marek

Hello.
Model looks great.

Jeremy_H

Thanks Marek.

Made a test rudder today, something that allows changing blades. This assembly weighs 1-1/8oz.

Jeremy_H

Anodised the aluminium parts for this boat, and have final assembled the engine mount:



Also made a changeable turn fin, and some alternate fins/rudders:

Jeremy_H

Mounted the wing on a crude but OK bracket for now, I've rigged a fuel filler point with a cap and avoided having to bolt through the wing by making Delrin threaded buttons:

Jeremy_H

I've given it to the end of January for this diversion from the twins. I'm running hot and cold with it's chances of working but it's served me well to remind me of so much I'd put to the back of my mind after some years away from the making side of RC .

I'm leaving big gaps in it's update here, there's more interaction on rcairboats.net but I'll continue to post here till the project is deemed finished (if such thing is possible )

Anyway, I've chosen the risky path of an underwater radio hatch and installed carbon tubes to carry the Control rods. I prefer this latter way as it allows the whole rod to be swapped out, too many kinked exit points in the past for me, hate external rods on a scale'ish type model, but they work. The radio box is sealed with a polycarbonate cover flush to the hull, this will allow me to see the radio indicator lights. It will be sealed by a single piece of sign vinyl, once fitted there should be no reason to remove it, but if necessary no problem as I have a sign plotter and can make neat replacement seals to carry as spares. The switch and charging socket are to be fitted in under a sealed cap at the rear of the cockpit. For now I've used a plastic bottle top, it's ugly but it's an evenings work to make a flush one and I'll invest that time if the boat works:



Of more interest to me is the paint, which is coming together but it's a steep climb sussing the acrylics I'm using for the first time. This is where it is, I've got more lettering to add, drop shadows to go on, and a line to put on the deck edge, then a few repairs to some dings it's picked up and final lacquer. As usual I've given it a wipe with White Spirit to show up the surfaces better :



A couple more posts and it will be pond ready

Jeremy_H

fudged this one somewhat but a step in the right direction, first lacquer on

Jeremy_H

Final coats of lacquer on:

dadkins

WHOOOA. That looks WAY to cool to go thrash on at the pond - That looks like a shelf Queen! Beautiful work Jeremy. Although i must say everything you have built so far leads me expect nothing less than perfection. but WOW that hull looks fantastic. You should be proud.

Jeremy_H

Thanks for your kind words dadkins. I'm certainly pleased that spraying techniques from the past have come back quite well, and the use of some new materials to me have given a sound result. But, perfectionism is a fault imho and I can not resist getting it better to my own cost, as to me this is full of flaws. I'll happily point them out if you like.

For instance, this evening I flatted, compounded, and polished the tops of the sponsons, now I am happier with them, the reflection of the workshop light is a good indicator in these pics. But you see those ridges where the stripes are? No good, more flatting to come, I like ridge free graphics. The lettering is flat now but heavy coats needed on the base colours has given me a line to deal with, I suppose that's what can be expected when using aerosols. If I wanted perfection I would have repainted the white stripes to build up the paint layer to the same height, then repainted the lacquer to do the same to that. I should have done that, it's less work than this risk filled heavy machine compounding. I will say though that this is what it's about to me, trying to get a finish like a brand new Snooker ball, if I ever actually do that I'll probably go and do something else .

I've just noticed that you can see the wing and the cowling hanging from the ceiling in the reflection




Another evening to sort the rest of the finish and then final assembly, I'll put up some pics taken with a decent camera when that's done, as it's pond time this weekend if the weather allows, and it may be the last time it's in one piece

QED!

Jeremy_H

Miserable weather today, so finalised the throttle setup and test ran the engine. It all seems pretty rigid, some vibration from the wing bracket at about 1/3 throttle but it gets through it. The cowling is being contacted by the carb lever, so need to eek out that a tad. I took a quick vid:

http://www.youtube.com/watch?v=cJ9IBUozsxY&feature=share&list=UUw8b4mq7g44FwvGq97q6aKw Having done that I could put on the radio hatch seal:



Try again for a run and decent photos tomorrow.

Jeremy_H

Record pics prior to running the boat today, just in case it went pear shaped



BUT! It didn't go pear shaped, considering the history of this boat it was superb. Sure, there's some tweaking to do, but it runs well, steers flat, just a problem with torque to overcome with some added sponson lift I think, which let's face it this torque issue that plagues us is one of the main reasons why I'm making twins anyway!

So here's some pics of the boat under way, and I'll put up a vid shortly, then back to the subject of this thread!!!

Jeremy_H

http://www.youtube.com/watch?v=OSfoFvv_gkk&list=UUw8b4mq7g44FwvGq97q6aKw&feature=share

Jeremy_H

A short vid of the hydro with a couple of improvements, a lift tab on the left sponson, and re-prop'd down to a 8x7 from the 8x8 originally used. A little better, but that hull is never going to stop splashes from getting to the prop. Definitely a Mill Pond boat


http://youtu.be/Utkjg2Vogvk

carlosam

So, Jeremy
Enjoyed the construction and Video
You are a very creative person, and these are the types of buildings that stimulate always improve, and not only stay on construction standards.
Curiosity: what engine is using and what the final weight?

Jeremy_H

Thanks for that Carlos.

The boat ended up at 4lb 2 3/8 oz.

The engine is a simple Thunder Tiger GP 25. A plain bearing engine of mediocre performance. But, I am putting in an Alpha .28 which is about twice as powerful, at some time in the future, but not just yet as I'm working on the twin engined boats again now.

arcdude

All I can say is fantastic workmanship, top notch quality! Now make me a motor mount will ya, kidding. I haven't had a chance to read through the whole thread yet, it was your parts mounts, etc, that caught my eye, very nice indeed.

Jeremy_H

Cheers arcdude, much appreciated.

I'm getting the buzz for the twins again, no doubt taking the little red one to completion has been worthwhile, with it performing well I'm rethinking some aspects of the hull design for the twins.

I'll explain. When I made the red one the idea was to try a way of building that was not very well known to me, and to achieve that the issue was never about what shape it was in detail. But, whilst it could have been shaped like an Aardvark for me to try glassing over foam, dealing with some shapes I was to encounter on the twins was more useful to me, so it has many elements that are in the frame for the twins. But a simple lack of foam at sensible costs meant the boat is too narrow overall, indeed too slim in many areas. Too short of lift through the centre hull being too narrow, too curved on the stepped lift surfaces on the sponsons (because I wanted to use the size and shape the twins would have, but the red one is only a quarter of the size ((volume)) of the twins, so theses surfaces don't straighten out in the wetted area as the should as the sponsons are too small for the shape used), and too small overall making it vulnerable to conditions, or so I thought.......

Consider this, the red boat is a quarter of the size of the twins, but it's over a third of the weight, a tiny fraction of the power, and with a CoG around 1.5 times the height. So this opens lots of opportunities to see why the red boat is working so well, and why it's not in some areas.

Let's take the negatives first.

Under powered. So replacing the engine with a revvy Alpha .28 will probably sort that, there's a tuned pipe to squeeze in, but I think I have that covered. The new engine is quoted at 32,500 rpm at peak power, if I can get a happy engine to 29,500 I'll be at the limit of commercially available propellers. From there each prop will have to be made, and it may even be necessary to consider single bladed propellers.

Prone to prop washes. The deck/hull side cut outs are too deep for this beam of boat, it's leaving a gap for splashes to get to the prop. Airboat hydros need to be very wide in comparison to water prop boats, mine have all previously had parallel sides, so on the red boat it was a scalloping and see how you glass that situation because I want the scallop the twins sides, but with a full width deck, at least originally. Now the width of boat is also sold to us as a need for stability on airboats, well this boat says different (apart from failing in torque which I'll come too), as the ride surfaces on the front are actually inboard of the boat side by a big margin, look again at the picture of the underside in the post above, the ride surfaces are actually at prop diameter spacing, think about that. The deck of the boat is also too short, it should go further forward along the insides of the sponsons, to provide splash protection from the front and inside edges of the sponsons.

Torque twist: This is quite painful on this boat, I'm putting it down to the narrow hull and centralised mass caused by the engine being upside down. I'm not dwelling on that though as the contra rotating twins negates the problem.

There's a few other minor negs but that's the three big ones. Moving on to the positives:

Lift. This was a huge surprise to me. After a float test in the bath I was gaining confidence that the boat would come up on the plane at the front, but I was nervous about the back end thinking there just wasn't enough lift surface there, but it is perfectly clear, when the water surface is flat for a true view, that the boat planes out almost fully from stem to stern, It's actually how it sits on the bench as the sponsons are lower than the rear end. The stepped surfaces on the sponsons are showing remarkable seakeeping. Yes, they dive sideways a tad because of the curved rail but the hull rides out waves very well, it's a pity the prop doesn't. One other aspect of lift which I should not ignore is the hull shape. This was another one of the key elements to be built into the twins, where the underside of the deck is a pretty flat route to the transom, but the top surfaces form a convex shape that forces the air to accelerate along the surface, decreasing the pressure, hence creating lift. In the past I've always sloped the sponson decks forward to create down force at the bow, but this one is geared so that the centre of lift on this deck/hull is in line with the CoB. Question is, is it the wetted areas creating the lift, or is it the hull?

Speed: Keeping things in perspective it's not doing too bad for a two foot boat with a .25 'sports' grade budget engine. The hull is handling what power there is comfortably, where the limits of the hull speed are is yet to be seen of course.

Handling: This is it's real asset, it's a nice thing to drive, seasoned racers have had a go and they report the same. I thought if it worked at all it would be skittish, that it could slap in turns, and that it would just flip over in winds and flip over in waves. So my thoughts created by this skinny hulled boat have been proven to be completely wrong as the boat turns left like it's on rails at the moment, hard right turns have it up on one sponson, but gentle turns at speed, or just a little less throttle and it's settled. Jinking to avoid obstacles etc is OK so long as it's lefted again promptly. But it's the feel of the steering that's the thing. Without using exponential settings it rides the stick centre position very firmly, and there's a progressive bite when you turn it, almost as if exponential was dialled in. Right now I'm putting that down to the tunnels, as the combined area of vertical surfaces resisting side slip on the inside of the turn, as you pour on more turn the hull sides make the boat ride higher at the stern, reducing the depth of the vertical surfaces, hence the resistance to side slip. So it kind of rails in wide turns and slides a little in tight ones and proportional variation between the two.

So a happy accident overall really, back to the twins where some re-drawing is going on as a result of this . If I modify this boat soon I'll put the updates somewhere else and leave this thread to the original concept from hereon.

arcdude

Jeremy_H Is it just my internet connection or is the motor possibly sucking up some water in the above video? Oh ya I'm a lot like you I collect plastic bottles from all sorts of things including flavored coffee which look a lot like an after burner tube, funny what we can recycle. What bottle did you make your cowling out of?

Jeremy_H

On the first vid session I was suffering engine stalls because of water, on the second session I had no stalls. What you can hear is the prop being slowed by water. But, the video gives a false impression of the conditions, check out when I'm trolling in to adjust the carb. In the better condition on the right side of the course it's fine.
EDIT: I fitted a guard to the carb for the second session, I'll get a pic
The cowling was made from a mouthwash bottle.

carlosam

So Jeremy,
Have you thought about using silicone exhaust deflector on the carburetor.
I am awaiting the arrival of mine, to test.

Jeremy_H

Yeah I've tried a few things, Silcone exhaust headers, silicone outlets just like the one in your pic, and simple barriers like the one I've fitted to the red boat. They all cause a problem. Aero engine carbs have air forced into them by the prop in most cases, any guarding on them tends to interfere with that. I've found that the symptom I get is that when running the engines with the boat stationary I can tune it till I'm blue in the face, but when the prop starts offloading under way it runs out of air and skips a beat. I get around this my setting the needle so it's just the clean side of four stroking at full RPM, but it's not optimal. When you think about it rearward facing inlets will indeed suffer from Bernoulli's effect. The only way around this would be to build a sort of plenum box which stabilises the air pressure available to the inlet. But I really can't be arsed with that and would prefer to put my effort into a hull that does not cause the problem in the first place, or run the boat in sensible conditions :-).

If you want to see an airboat hydro with optimal prop protection check out the Selph Inflicted SI3 ;-)

EDIT: just noticed you can see the little plastic guard I've fitted in the still image for the video link above.