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Old 08-27-2013, 03:39 PM
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Jeremy_H
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Hi all!

Searching around the net for some info on airboats I came across you guys, seems there's a good mix here so thought I'd join up. I'm from the UK, live in the South West near Plymouth, I'm 53 going on 12 .

I've been modelling for about 45 years off and on, always had one model or another. Ones which I would count as the most fulfilling though would be airboats, from diesel engined free runners to RC Glow a few have come and gone. Some time ago I made one with my eldest son, he was around eleven when this ocurred, maybe 16 years ago, with another youngling at the same age now it's time for another joint project. The first is in this terrible image (all I can find I'm afraid):


The plan for the new one is to go for a twin engine, centreline thrust so one tractor one pusher (just for the hell of it). Still on the drawing board whilst I get my head around fuel issues, but once the groundwork is done it will come together reasonably quickly along with a duplicate using brushless motors, something new to me, I'll keep you posted, and hope to pick your brains if I may be so bold .

Early line drawing of the new boat profile:
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Last edited by Jeremy_H; 08-27-2013 at 04:05 PM. Reason: Update
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Old 08-28-2013, 01:08 AM
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G'day from Australia Jeremy!

Plan looks very cool.

Let us know how you go with the pull/push configuration.... why not !!?

You can also check out RC airboat world
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Old 08-28-2013, 02:45 AM
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Thanks, I'll check out the other forum.

There is some science behind the CL thrust idea. It was driven by one of the properties the yellow boat above had, torque effects on steering. At full chat I had a banked turn in one direction, regular airboat style, and flat turns like a hydro in the other. As I'm using spare engines, an Irvine 20 Black Head and a Thunder Tiger GP 25, they will not cancel each other out fully, but I'm leaving provision for interchangeable mounts should I want to tidy that up with matched engines later.
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Old 08-28-2013, 07:42 AM
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Quote:
Originally Posted by Jeremy_H View Post
Thanks, I'll check out the other forum.

There is some science behind the CL thrust idea. It was driven by one of the properties the yellow boat above had, torque effects on steering. At full chat I had a banked turn in one direction, regular airboat style, and flat turns like a hydro in the other. As I'm using spare engines, an Irvine 20 Black Head and a Thunder Tiger GP 25, they will not cancel each other out fully, but I'm leaving provision for interchangeable mounts should I want to tidy that up with matched engines later.
I'd tend to think you could leave the smaller engine on there as the rear engine because less power would be needed at the back because the air is already moving. I don't really know, but the theory seems sound to me.
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Old 08-28-2013, 09:17 AM
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Air turbulence may negate the aft engine, especially if using a pusher prop. Two tractor engines offset would work better but you still have air turbulence to deal with. Ideally, you want each prop to have its own clean air stream. Meaning you want both props to be pulling "clean" air. Now, I'm not saying the fore puller and aft pusher wouldn't work but it won't be nearly as efficient and likely wouldn't run as fast as it could especially if using two different displacement engines.

This very subject had been brought up in the aircraft forum some time ago and the common concensus was to use equal displacement engines (identical engines is best) side by side so each prop had its own air stream.

edit: in reality, the Irvine may be as strong or stronger than the TT. I'd be test running them on a test stand with the same fuel, glow plugs, and props to see how their power compare.

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Old 08-28-2013, 01:20 PM
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I get the thing about efficiency on an in-line arrangement, but for me there are other issues side by side motors would have that in-line doesn't. Assymetry is an obvious one, but as two side by side engines should rotate in opposite directions, and most model engines don't, then I'm not sure how much the torque thing would be effected by having two rotating masses trying to roll the boat (I've not looked recently, but about twenty years ago I bought the engines for a twin boat. I chose Irvine marine 40's, as at the time it was the only engine available to me with a reverse crankshaft option. Pop in the new crank, turn the piston and cylinder through 180, one backwards operating engine. Still have them, never built the boat ). The motive for me really is just about fun with a different installation, with the advantage of neutral torque if I can get it, plus there's the grin of the sound of a twin, and the unique chopper note in-line's give as the pulse from the front prop hits the rear one . If I was after full efficiency I'd be doing everything here very differently.

I've been testing the motors for the last week, but both on pullers at the moment, pusher props are seriously limited here with only one viable one around, APC 9x6. One is on the way, so I'll know more soon. It'll be interesting to see if the Irvine is more meaty than the TT, it has an early plastic carb and as it's only just been run in I'll have to see if it's any good, I had to replace these on the two marine engines as the throttle barrel thingy became sloppy in the carb bodies allowing air to pass, rubbish tbh.

There's a bigger potential problem I need to suss first regarding the pusher. I'm not sure if either engine can take a pushing force on the crank, it can mean that the prop driver runs on the front of the crankcase, wearing it down, eventually the con rod can be hit by the crank web which will equal a total loss. If this is the case I'll have to machine a thrust bearing arrangement (big prop driver running through thrust bearing to collar on crank case) QED.

Today I think I sussed the fuel system. There's not enough space between the engines, or an attractive way for me to put two tanks in the pylon, so the tanks are going in the rear of the 'fuselage' section, so the head is only about 4". This means a long fuel run, and whilst the engines coped with this I found the drain back after priming an annoyance so I've made a couple of non return valves. They work fine, but I need next to try it under way, so a rough build is on the cards, maybe a boogie board. How the weight shift through consuming fuel effects the boat may come to light too perhaps.
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