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Author Topic: Air Boat Advice ?  (Read 1873 times)

john54

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Air Boat Advice ?
« on: February 19, 2009, 11:53:14 AM »

Hi All
Picked up a fast leccy cat hull. Make & model unknown (Any ideas?) | was thinking it might be a good base for an airboat?
Dias are L 30in W 14in & 1.5 K.g. Cant go i.c so has to be outrunner & lipo. So need advice on prop size pylon size & height/position.
Any help be much appreciated :-)
Thanks
John
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Martin (Admin)

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Re: Air Boat Advice ?
« Reply #1 on: February 19, 2009, 12:08:15 PM »

I would say .19-.45in (3.5-6cc) if it was IC so what ever the Brushless equivalent to that is ?!?!?
Mount the motor as low down as the recomended propeller size will allow over the deck and leave an extra
10mm in case you need to increase prop size.
Batteries in the bottom of the hull and make sure they are fixed down.
Large Air type rudders, twin. (Water ruders is better but you can't do go over land or weeds!)
Propeller cage if possible, prop shroud if not.
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andyn

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Re: Air Boat Advice ?
« Reply #2 on: February 19, 2009, 12:09:38 PM »

I'm building one about the same size, fitting a car fan heater motor to it, taken from a written off car for me by Bill. When run up in his shed, it moved most of the contents of the shed to the other side of the shed...

Motor draws about 4.5A, so on a 3700mah NiMh run time should be 40-45 mins

Please see attached pic. By the way that is a 12" ruler...

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andrewh

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Re: Air Boat Advice ?
« Reply #3 on: February 19, 2009, 12:57:15 PM »

What is BL equivalent to 0.19 to .45ci in IC engine?

Especially if driving an air propellor  - this I can do!
Several ways, also :}

If we know the power of the IC engine we can match that:
If we assumed a 0.45 is claimed to produce 1HP at peak revs - thats 0.746 Kw or say, 750watts

The BL that matches that is not, actually a 750 Watt device, but probably about 5 to 600W*
So now we only need to pic the KV, and Job Done. 
This depends a little on the prop diameter available (on the airboat)
Ideal is the largest prop driven slowly (quietest, most efficient)  but practical reasons will rule out the 18x24 inch prop
so the prop diameter will be set by the structure of the boat, the revs are set by keeping the tips of the prop well sub-sonic (for noise reasons), and then the pitch wants to be the coarsest  consistent with the power available and acceleration required.

I don't have the figures in my mind, but if the boat will take a 12 inch dia prop it probably wants to be turned below say 10,000 RPM for noise reasons so on 10V that would be a 1000KV motor, and power is tuned by playing with the prop pitch, which in 12 inch props seems to be avaialble from about 4 inch pitch to probably 18 inch - in 1/8th inch increments (12 inch props are used by stunt and aerobatic flyers who need very fine adjustments for their (weird) requirements.

OR - if we know the prop and RPM for the IC engine (which equates to the exact power) we can use BL motor data tables to find the same RPM for the same or similar prop - and there is no more to do than that! :}
Example of prop data for a particular BL motor - in this case  Scorpion 3026 -8
http://www.micronradiocontrol.co.uk/scorpion_data/3026-8.html

* Why would a BL motor need less power???
Several reasons:
  • Electric motors produce torque only, not vibration - less weight of supports etc to resist shaking
    IC props are thick enough the resist the vibration and torque variation.  less efficient than Electric props at producing thrust
    Power for IC engines is quoted at peak revs - often silly ones.  The 0.46 might peak at 18,500 on a 7x6 prop  - not usable in practice because of prop noise, and not desirable either cos of short engine life (Ducted Fan engines are an exception  - they always run WFO)
    IC manufacturers are human and lie
    The fine print of the IC engine says "ISO conditions" and "new and clean"  this translates as "YMMV"


Sorry about the long and rambling response its actually simpler than the water-prop case (Cos no-one has admitted to having water propellor power/revs curves)

It happens, by chance that the example I picked at random just to illustrate the principle seems to have some good combinations of power/prop

Prop APC 12x6E
Volts 10.5
Amps 66.82
Watts 701.61
RPM 9725
Thrust  2913 gms Thats three KG of thrust :-))  This will fly a 2.5 KG plane vertical.  Forever!

The same motor is happy at smaller diameters and larger pitches - have a look at the data!
Hope the wordstorm helps a bit
Andy - please let us have a finger-count before you commit airboating :}
andrew




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john54

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Re: Air Boat Advice ?
« Reply #4 on: February 19, 2009, 01:08:17 PM »

Thanks a LOT chaps :-)) more than enough info to be getting on with. So the hull will do then?
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dreadnought72

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Re: Air Boat Advice ?
« Reply #5 on: February 19, 2009, 01:36:07 PM »

...This will fly a 2.5 KG plane vertical.  Forever!

Careful, Andrew - you'll be getting NASA on the phone with comments like that.

But 3kg of thrust is 30N in proper units. If the airboat masses, say, 3kg (1.5kg hull and 1.5kg all-the-bits) then it's going to have 1g acceleration. With hardly any drag.

This could be seriously quick. Or lethal.

Andy
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john54

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Re: Air Boat Advice ?
« Reply #6 on: February 19, 2009, 06:22:26 PM »

Been looking at ducted fan units  {:-{ x2 1200gm it will be different if it works :} :}
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Martin (Admin)

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Re: Air Boat Advice ?
« Reply #7 on: February 19, 2009, 06:35:10 PM »

Prove me wrong but has anyone seen a ducted fan unit on a boat that actually really works well?
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andrewh

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Re: Air Boat Advice ?
« Reply #8 on: February 19, 2009, 06:46:12 PM »

John

I will be brief :-))

Ducted fans substitute fan speed for the efficiency of a big prop - they are a good idea for high speeds - but sometimes have trouble getting there - the efficiency improves as the boat speed increases.
All Ducted Fans thrive on revs - look for 30,000 or more
So the BL motors are often inrunners with Kvs in the region of 3500 to 5000

All the hardware is often sold as a working set (duct, impellor,  motor and tailcone) and this will (OF COURSE) come with full volts/amps/power/trust information - have a search for EDF +brushless

andrew
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andyn

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Re: Air Boat Advice ?
« Reply #9 on: February 19, 2009, 07:31:42 PM »

I wouldn't bother with a ducted fan in a boat if I were you, unless you have a system of a 100ft bungee chord in your lake, or a very large source of highly compressed gas in your boat it's not worth your while. To get up to speed with a DF, flyers use bungees, that ping them up to a very high speed right from the start (about 40mph ish). If you try to hand launch one you will need a very strong arm and a good runup, or as we have proved it just drops to the floor in about 5 feet making one hell of a lot of noise {:-{
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john54

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Re: Air Boat Advice ?
« Reply #10 on: February 19, 2009, 07:37:27 PM »

OOPS it was just an idea. Thanks AGAIN Guys .so it back to an airscrew then :-))
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andyn

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Re: Air Boat Advice ?
« Reply #11 on: February 19, 2009, 08:04:21 PM »

I still think it'd be better off with a heater motor ;)
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tobyker

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Re: Air Boat Advice ?
« Reply #12 on: February 19, 2009, 10:35:27 PM »

The reason you bungee launch a df aircraft is that most are based on jet prototypes that have a high stalling speed - a hand launch can't get them over stalling speed so they fall out of the air. A boat wouldn't have that problem.
In order to get an air boat to turn, you need a very hard chine forrard, which digs in when it leans out of the turn, and a soft multichine (or round bilge) aft so the stern can skid round. Otherwise it will merely carry on in a straight line but with a 45 degree yaw on - like a hovercraft!
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DLM

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Re: Air Boat Advice ?
« Reply #13 on: March 01, 2009, 03:18:22 AM »

I would like to have that white boat or cat or tunnel cause i could make a good pleasure or racing boat out of it. Since your have your heart set on airboat. The cg should be  30% but i question this as it is build like a cat. Prop should be a 9 x 4 on a 700-8.4 on a 11.1 lipo in the 20c or 25 c range. I would beef up the inside with strips of 1/32 and blue foam insulation. The base for motor stand can be had with block of basswood about 4" wide and width of boat and 3/4" deep and epoxied to bottom. The stand can be made out of .064 aluminuum with base being wider than top and 1" above the cover. the motor should angle down 2 degrees and to the left , looking from the transom end. The 700 can be held in place with nylon straps. Now hears a suggestion on motor wires coming thru the cover; i would have connectors outside and on top of cover and 12 gauge wire coming thru and have connectors just inside cover so when doing replacing batteries, the cover can lay off to the side. Same thing with antenna; have spade clips on the outside and like motors wires, the same with antenna. The double airrudders will have a good base from picture shown. Now are you going the pusher type or puller type propulision. My GUT feeling on CG should be further ahead due to the type of boat. I would lay everything in as x cg and find a good balancing point without adding weight.
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