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Old Dec 21, 2014, 10:05 PM
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Joined Dec 2014
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Question
Hovercraft steering options.

Slowly beginning to make plans for my first foray into diy RC and i'm wondering which of the following configurations would result in the best steering for a hovercraft.

1. Fixed motor/propeller with a rudder behind it.

2. Motor/propellor on a servo.

3. 2 fixed motor/propeller on either side of the hovercraft.

4. a combination of the ones above ?

I'm looking to build a relativity fast and responsive hovercraft.
I havent decided yet on size and shape the hovercraft but i'm leaning towards a round hovercraft with a diameter of +- 60-70 cm simply because of the looks and the fact that u don't see them often.

Any ideas what would be best in my situation ?
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Old Dec 22, 2014, 12:43 PM
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United Kingdom, Scotland, Edinburgh
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Welcome.

It's a rather tricky question depending on what you mean by 'best' steering, given you don't want it to be fragile or difficult to build/tune.

You can't go wrong with a fixed prop and rudder, definitely use at least two or even three rudders linked together to catch most of the propwash for maximum turn authority.

Turning the entire prop and motor is possible but it introduces a lot of complexity. Also, rudders still work even when the motor is off when the craft is coasting, whereas with a moving motor/prop you'll always need to apply thrust to get any turning at all (unless you put a rudder fixed relative to the motor and prop, on the same pivot as them).

An issue with rudders is weathervaning - if you put them at the 'back' of the craft they will line up with the airflow and stabilise the craft to always point in that direction. This is good for high speed stability but can also be a major pain in winds or when you want to have the nose pointing in a different direction to where the craft is going. Depending on how much skirt drag you have this might not be a problem.

2 fixed motors is mechanically very simple (no pivots or linkages) and gives excellent turn authority if you get the differential throttle mixing right. It's possible with this set up to have too much control and end up with the craft spinning out if there's not enough skirt drag to damp the turns.

I mention skirt drag as my own hovercraft is skirtless so there isn't any at all, so it's very sensitive to steering. A small ducted fan and rudder is enough.

If you go for the round hull you could try having a fixed prop at each side for thrust and differential steering, plus a single rudder at the back that will help out at high speeds and give a little bit more stability.
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Old Dec 22, 2014, 06:36 PM
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Australia, VIC, Bass
Joined Jan 2013
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Another big factor to consider is the type of surface you are operating over, whether you have a skirt or not there will be some ground contact unless its just going on a perfectly smooth dry surface. -If you craft has a round planshape or is fairly wide, when it contacs the surface the drag will spin the craft making control very difficult. This becomes even more of a problem over water or wet surfaces where the drag is much higher. The longer the craft relative to width the more directional it should be.
If only operating on a smooth dry surface then a circular plan shape should be ok. - fitting a gyro unit also works well if set up properly. Have a look at my blog for some examples of different layouts I've used.
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Old Dec 25, 2014, 03:42 PM
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Thank you for the replies.

After mulling it over for a few days I think I'm going to abandon the circular design for maybe a future hovercraft and stick with a more conventional shape for this one.

High speed together with a wide design could indeed spell trouble with handling when friction with the ground increases.
And for steering I think I'll stick with the tried-and-true design with fixed motor + rudders.

I am having a bit of trouble deciding on motors for lift and thrust since that all depends on the weight of the craft and since I don't have any real world comparison that's kinda tricky to figure out.
But that's maybe a question for another thread
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Last edited by Scroo; Dec 25, 2014 at 03:48 PM.
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Old Dec 26, 2014, 02:21 PM
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Hi,
have a look at this German forum , http://www.rc-network.de/forum/showt...-indoor-Rennen .
there are some video ,doing indoor Hovercraftracing . Tought about some helicopter rear propeller that can counter flyforce in corners ,like ading grip with superglue tires . i have a iceglider that has only springsteel wire that give no direction . i steer it with a small propeller in the front that push the front inside the corner.
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Old Dec 28, 2014, 07:10 AM
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Hi - I'm +1 w/ FiftySlicks; " try having a fixed prop at each side for thrust and differential steering, plus a single rudder at the back that will help out at high speeds and give a little bit more stability."
Why take your valuable thrust energy & slam it into a rudder when you can trim power to one of the motors & save battery?
I think vectored thrust is simpler & lighter....

My 350mm vectored thrust mini...
RC Hovercraft from quadcopter (1 min 46 sec)


Early test photo...Ralph
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Old Dec 28, 2014, 07:36 AM
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Do you mean vectored thrust or just differential thrust ? They're very different. I've never seen a vectored thrust system that's either simple or particularly light . And of course the problem with differential thrust is that you have no steering unless you're under power, unlike a simple rudder.

Steve
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Old Dec 28, 2014, 06:36 PM
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Quote:
Originally Posted by slipstick View Post
Do you mean vectored thrust or just differential thrust ? They're very different. I've never seen a vectored thrust system that's either simple or particularly light . And of course the problem with differential thrust is that you have no steering unless you're under power, unlike a simple rudder.
Steve
Hi Steve - Guess I mean either one....in 25+ yrs of design/building/flying full size & RC hovers I don't think I ever heard the term "differential thrust"....but I like it

Pic's below are my "old school" 700mm vectored thrust craft that's about 15 yrs old......motor pylon is a pair of tubes...inner mounted to frame & outer can rotate. Since I got hooked on Quadcopters I've been neglecting these things...2 of my planned projects over the winter are to re-do the small one & convert the 700mm to 2.4 TX/brushless/lipo/etc.
One of my objectives is to keep the profiles to a minimum to lessen wind effects

Good to see some discussion on this forum............Ralph
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