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  #1  
Old 08-01-2011, 12:44 AM
Jeremy Clarkson Jeremy Clarkson is offline
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Air boat style airplane boat

I'm building a 12 foot long rectangular boat out of plywood with only a tall angled front. It has a flat bottom and is 3 1/2 feet wide.

The plan is to put a 4cly engine and trans from a rwd automobile with a prop in the rear mid of the boat.

then attach a mount with 2 sets of pillow bearings that allow a wing which sits on top of them to lean right and left and forward and back.

so it should be like light weight weight shift boat plane.

what do you think?
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  #2  
Old 08-01-2011, 02:08 AM
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Jeremy Harris Jeremy Harris is offline
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A 12ft long, 3 1/2ft wide hull won't carry much weight without being submersed to a fair depth, and the weight of your engine, prop, wing and yourself is almost certainly going to be too much for it.

At a guess I think you need to look at making the hull bigger, with a greater planing surface.

The wing will need to be carefully designed to maintain stability. I assume you're looking to operate in ground effect, as a wing-in-ground-effect craft (usually abbreviated to WIG). The controllability in the marginal area between WIG enhanced lift and non-ground effect flight is challenging and the wing ideally needs to be designed to be self-stabilising, both in pitch and roll.

The closest I can think of to what you want to do are the controlwing aircraft designs of George Spratt. Take a look here for some information you might find useful:

http://www.earlyaviators.com/espratt3.htm

http://speleotrove.com/wings/controlwing_plans.html

Jeremy
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  #3  
Old 08-01-2011, 03:15 AM
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JH still early in the morning for me but do I understand a boat hanging on a wig wing dont need a big standing tail?
with a coffee now i realise it would be more like a flying rib http://yachtpals.com/flying-boats-7022
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  #4  
Old 08-01-2011, 05:55 AM
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Jeremy Harris Jeremy Harris is offline
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Quote:
Originally Posted by yipster View Post
JH still early in the morning for me but do I understand a boat hanging on a wig wing dont need a big standing tail?
with a coffee now i realise it would be more like a flying rib http://yachtpals.com/flying-boats-7022
Yes, if the wing is self-stabilising in pitch, like a Rogallo wing used on flex wing microlights/ultralights, or the delta and reflexed aerofoil used on rigid flying wing designs, like those pioneered in the 1930's by Walter and Reimar Horten, then no tail is required.

It isn't that easy to design such a wing, though, particularly if you want it to work in ground effect, which is what I suspect the OP may be thinking of.

The main problem is that there can be significant change in lift coefficient and wing pitching moment as the wing approaches the ground effect height boundary from either direction (typically a height of about one wing span). WIG craft use an auto-stabilising wing design, where the pitch change due to the change in pitching moment acts to regain stability, rather than lose it, as a conventional negative pitching moment coefficient wing section as used on tailed or canard aircraft would tend to do if used without a tail (or canard). Many WIG craft have a wing area that is such that they cannot sustain flight above the ground effect boundary at normal cruise speed, which also helps with the stability issue.

The key to making a tailless wing stable is to use either a reflex aerofoil section that has a positive pitching moment coefficient (which is generally the opposite of the way a conventional aircraft wing behaves) or to use a fairly neutral pitching moment coefficient section and sweep the wing so that the wing tips act as a tail, in effect, by providing a correcting pitching force aft of the wing aerodynamic centre.

In addition, WIG craft need to provide a means of maintaining roll stability, but this is usually fairly easy to do by providing anhedral at the wing tips or even end plates. These will act to increase the effective lift of the side of the wing that moves down towards the surface, creating a rolling moment that tends to keep the craft stable.

The good news is that there is a lot of freely available design information available on both WIG and tailless craft, and the basic principles can be easily tested using models, before committing to a full size craft.

Jeremy
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  #5  
Old 08-01-2011, 09:44 AM
timabram timabram is offline
 
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Hmmm.... Weightshift Wing in ground effect. Sounds quite a challenge, As noted by others for a wing to be effective in ground effect you need a lot of pitch stability, Further with a weightshif arangement you've got a huge problem with control both in pitch and roll and maintaining that ground effect.

Flex Wing microlights and hanggliders spend very little time in ground effect. Take off and landing only ! ... and purely transition the effect on the way up or down so it's not so much of an issue.

Further be aware that in ground effect, the centre of pressure (dynamic lift) from the wing will be in a different place to where it would be in free air. so you will need to put the pivot in a different place than you would in a microlight wing. This is one of the reasons WIG's have a different wing section to aircraft and usually as suggested have reflex tips to assist with stability.

I think you are going to get serious control problems and serious stall issues for over/under pitching with a pivoting wing. The last thing you need in a WIG is high speed stall, caused by high angle of attack with a section that supports ground effect, close to the water as you'll get a nasty pitch down moment.

Given the thrust you need to get enough indicated airspeed over the wing, as you are suggesting a pusher (not a tractor) prop, a stall will pitch pole the craft really quickly with no time to recover and no way of picking up airspeed before the nose digs in. (Opening the throttle will increase your airspeed but you're unlikely to get a fast enough response to unstall the wing)

Roll is an issue too .... For two reasons one as previously discussed re stability, but also remember that rolling a wing dramatically increased the load on the wing structure. The lower wing as it is rolled naturally reduces the pitch and decelerates at the indicated airspeed given speed and consequently reduces lift and provides a relative increase in drag giving you the yaw effect your looking for in a turn. Fitting a rudder rather than rolling the wing will induce the yaw your looking for, and while the secondary effect is roll due to the deceleration on the inside wing of the turn effective anhedral will stabilise this out better. Just rolling the wing, given how close it will be to the water, risks dipping the tip and either throwing you into what in a plane would be called a ground loop, or snapping the structure because of the sudded increase in drag at the tip, leading to a catastrophic result.

For this you would be better using control surfaces wather than pitching and rolling the wing.

BTW wigs need a LOT of thrust to get them airborne and to keep them there, because you're relying in this to maintain airspeed over the wing not just to counter the mass. Remeber in Equilibrium flying LIFT=MASS and THRUST=DRAG ... there's an auful lot more drag from a flat bottomed hull than an aircraft undercarriage and you wount get much indreased lift fron the hull once airbourne if you're using weightshift as a control principle (i.e. no angel of attack fron the planing surface).

Most WIG's are deep V planers rather than flat bottomend like airboats.

But goold luck in comming to a solution...... be facinating to see this baby fly !
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  #6  
Old 08-01-2011, 11:12 AM
Submarine Tom Submarine Tom is offline
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OP Jeremy,

I admire your enthusiasm, but don't underestimate the complexity of your undertaking. These posts only scratch the surface of the challenges involved.

I would recommend going with a proven design, deciding which one will be challenge enough. Oh, and then there is building it of course...

The other option you may want to consider, and in my opinion, a far better one, would be an air effects tunnel hull or an airplane itself.

-Tom
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  #7  
Old 08-01-2011, 11:25 AM
Jeremy Clarkson Jeremy Clarkson is offline
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Quote:
Originally Posted by timabram View Post
Hmmm.... Weightshift Wing in ground effect. Sounds quite a challenge, As noted by others for a wing to be effective in ground effect you need a lot of pitch stability, Further with a weightshif arangement you've got a huge problem with control both in pitch and roll and maintaining that ground effect.

...

But goold luck in comming to a solution...... be facinating to see this baby fly !

Yes it should be just like the link yipster provided, but I don't see how the one I plan to build would handle much differently than one of those or any other flex wing trike/ultralight plane

the only real difference is that there is a long but pretty narrow flat boat hanging from the wing instead of a basket.

the whole thing should weigh about 750lbs loaded and the engine should be around 80-110 hp depending on what I can get from a junk yard
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Old 08-01-2011, 11:26 AM
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Jeremy Harris Jeremy Harris is offline
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FWIW, my background is aircraft-related and I've designed and built light aircraft, so understand the challenge that the OP has posed.

Having said that, getting something like a controlwing/free wing design to fly in ground effect only should be fairly safe and straightforward. George Spratt always reckoned that you needed little in the way of flying skill to safely fly a controlwing, as by its very nature it cannot ever stall. Provided roll stabilisation was adequate I think it should be reasonably safe.

For something pretty much off-the-shelf in terms of design, I think it would be tough to beat the UH Hoverwing. This works OK and has been flown safely by people with little aviation experience. It's remarkable stable in roll, which helps make it easy to fly.

http://www.hovercraft.com/content/in...index&cPath=53

Jeremy
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Old 08-01-2011, 11:31 AM
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Jeremy Harris Jeremy Harris is offline
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Quote:
Originally Posted by Jeremy Clarkson View Post
Yes it should be just like the link yipster provided, but I don't see how the one I plan to build would handle much differently than one of those or any other flex wing trike/ultralight plane

the only real difference is that there is a long but pretty narrow flat boat hanging from the wing instead of a basket.

the whole thing should weigh about 750lbs loaded and the engine should be around 80-110 hp depending on what I can get from a junk yard
Have you flown a flexwing? If so, then you already know that they can be a real handful near the ground. Pendular stability tends to reduce as wing lift coefficient rises, so in ground effect they tend to be less stable than when at altitude. An additional problem is that the whole wing anhedral (needed to reduce roll force in flight) makes them less stable in roll when very close to the ground.

There are already a few amphibious flexwing aircraft around, but all tend to use large wings with a slow stall speed, to ease some of the handling challenges and give them a reasonable payload. Typically they have a MTOW of around 450 to 500kg, so around 1000 to 1100lbs. With a car engine the weight would rise a lot, as this would be massively heavier than the typical sort of small engine used on flexwings (engine and gearbox weight for even a 100hp flexwing will be around 80kg or 180lbs).

Jeremy
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Old 08-01-2011, 02:33 PM
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Originally Posted by Jeremy Clarkson View Post
12 x 3 1/2 feet
what do you think?
I think that if you value your life, you will build and test a 1/3rd scale (4-foot long) RC model before building full scale.

Do you plan on having this licensed as an ultra light or watercraft?

If you plan on flying in ground effect only, I'd go with watercraft.

A few sketches would help.
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  #11  
Old 08-17-2011, 12:35 PM
Jeremy Clarkson Jeremy Clarkson is offline
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I figured that with this size boat bottom and at this weight it should only be about 3 inches in the water.

Becuase 10dft/12x 3.5ft = 35x .36 x62 lbs = 800lbs and .36 is about 3-4inches

Also how much surface area should the wing have I think about 150 sqft so 24x6ft
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Old 08-17-2011, 02:18 PM
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Jeremy Harris Jeremy Harris is offline
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What speed do you want to lift off at?

What cruise speed are you aiming for?

What total all up weight are you expecting?

How stable do you want this craft to be in flight? Are you planning on having a self-stabilising wing? What aerofoil section are you proposing to use?

Do you understand the impact that selecting an aspect ratio of only 4:1 will have on wing performance?

You need answers to these questions to determine the wing area and aspect ratio needed, you can't just guess and hope to get something that is either viable or safe.
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Old 08-17-2011, 02:35 PM
Jeremy Clarkson Jeremy Clarkson is offline
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Originally Posted by Jeremy Harris View Post
What speed do you want to lift off at?

What cruise speed are you aiming for?

What total all up weight are you expecting?

How stable do you want this craft to be in flight? Are you planning on having a self-stabilising wing? What aerofoil section are you proposing to use?

Do you understand the impact that selecting an aspect ratio of only 4:1 will have on wing performance?

You need answers to these questions to determine the wing area and aspect ratio needed, you can't just guess and hope to get something that is either viable or safe.
Thanks Jeremy Harris

A very low cruising speed

Weight of plane about 800 lbs

as stable as possible maybe a tall 10" airfoil

I dont want it to be doing stunts I understand that a high aspect ratio is good for gliding? What about 3:1 21ftx7?
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Old 08-19-2011, 05:31 PM
Jeremy Clarkson Jeremy Clarkson is offline
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jeremy man bump
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  #15  
Old 08-20-2011, 03:10 AM
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Jeremy Harris Jeremy Harris is offline
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I didn't answer before because you didn't give enough information to allow me to.

Aspect ratio controls wing induced drag. A high aspect ratio wing has less induced drag for a given value of lift than a low aspect ratio one, so needs less power to maintain flight at any speed up to the point where compressibility effects start to become noticeable (not a worry for slow craft like this). A reasonable minimum aspect ratio would be about 6:1. Going to 3:1 makes things worse than the 4:1 you originally suggested. Increasing it to 8:1 would be going in the right direction, but you need to be aware of the way increased span significantly increases spar loads.

By "very low cruising speed" what do you mean? An ultralight might cruise at anything between 30 to 50kts, a very light aircraft at 80 to 120kts, a typical flexwing can cruise at anything from 35kts to 80kts, depending on wing area and design. Lift is proportional to the square of airspeed, so choosing cruise speed (and more importantly stall speed) is critical to sizing the wing. You can't just guess it.

Your comment about the aerofoil section doesn't mean anything, as aerofoil sections are chosen on their L/D and pitching moment characteristics, not thickness. Once you know your target stall speed and cruise speed you can work through the numbers iteratively to balance available thrust (from power) with wing area and aerofoil section to get the best match for your requirement.

You need to aim for an appropriate wing loading (max all up weight divided by wing area) for the type of use you want to put this craft to. I would suggest that you choose a wing loading in the 25kgf/m to 35kgf/m region (~5lbsf/ft to ~7lbsf/ft). Going over about 40kgf/m (~8lbsf/ft)will increase the stall speed to an unacceptably high level (over about 30kts), so the lower the wing loading the better, up to a point.

All told you would be best reading up some basics on aircraft design and then making a model to test your ideas before risking a full size craft. The book "Design of the Aeroplane" by Darrol Stinton would be a good starting point, as it's pretty easy to understand and is written for a non-technical reader, in the main. It's not hard to do this stuff, but you do need to have a basic understanding of the way that lift is generated and how stability and control can best be achieved. You also need some understanding of the stresses in safety-critical components, like wing spars and attachments, to build something that will hold up without failure.
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