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Thread: Which would be more efficient?

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    Member RainGull's Avatar
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    Default Which would be more efficient?

    Which "boat" would be more efficient:

    1) A boat that planes on the surface of the water.
    2) or a "boat" that rides the ground effect a couple of feet above the water without touching the water, in a sense, flying.

    Just thinking.
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    The Russians have the most experience with question 2 , The Caspian Sea Monster, see here http://en.wikipedia.org/wiki/Ground_effect_vehicle.

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    Funny, I was going to add that link to the OP but didn't want the distraction. Really I just want to find data that shows a better efficiency for one or the other.

    It seems that air resistance v.s water resistance is a no brainer, but then the prop efficiency in compressible air vs. non-compressible water may or may not negate that.
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    Quote Originally Posted by RainGull View Post
    Funny, I was going to add that link to the OP but didn't want the distraction. Really I just want to find data that shows a better efficiency for one or the other.

    It seems that air resistance v.s water resistance is a no brainer, but then the prop efficiency in compressible air vs. non-compressible water may or may not negate that.


    Fricton (drag) is the bigger factor. Yes, there is "friction" in the air, but the reason that cat-boats were designed was to overcome much of the hull friction created by all that area in contact with the water. The same is true of the props, of course. Friction is simply a bigger factor in the water. Comprssibilioty isn't a factor in the air unless you're flying the big iron or other pure jets where airspeed is pretty high. While a mathematician can calculate cmpressibility for a light aircraft, its effect is less than minimal. Enter a couple different problems on your calculator or flight computer and you'll see what I mean.

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    With my little 95 horse power Cub on floats, it takes most of that horsepower (and 2,600 rpm) to get up (on step).
    Then you can reduce power to 2,000 rpm to plane along in a step taxi configuration. This keeps you planing along just below flying speed, (30 mph) with added lift via the wings.

    However,, I can also become airborne in ground effect over the water. (float planes have more drag than wheeled aircraft in many instances so they do not receive the full ground effect benefit in some cases. ) Anyway,,, I can remain in ground effect flight at 1 to 2 feet above the water at 1,600 rpm at about 38-45mph

    So with my aircraft I can use less power to remain in ground effect flight compared to step taxi.
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    Perfect response, Thank you!
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    Quote Originally Posted by Float Pilot View Post
    With my little 95 horse power Cub on floats, it takes most of that horsepower (and 2,600 rpm) to get up (on step).
    Then you can reduce power to 2,000 rpm to plane along in a step taxi configuration. This keeps you planing along just below flying speed, (30 mph) with added lift via the wings.

    However,, I can also become airborne in ground effect over the water. (float planes have more drag than wheeled aircraft in many instances so they do not receive the full ground effect benefit in some cases. ) Anyway,,, I can remain in ground effect flight at 1 to 2 feet above the water at 1,600 rpm at about 38-45mph

    So with my aircraft I can use less power to remain in ground effect flight compared to step taxi.
    Do you think with a low wing seaplane that effect would be much greater? In other words how much does the ground effect increase as you descend to a runway or water. It seems from watching planes land, that the maximum ground effect is just off the tarmac. From what I've read, the effect begins at wingspan height off of the ground and is maximized over hard ground, but it sure seems to stack up the lower a plane descends.

    I'm stuck in Juneau having another kid so I've had weeks to walk the trail and watch planes land...
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    I have wondered about that myself. The only semi-low wing sea-plane I have flow was a Lake Buccaneer amphibian. In that case the hull is the fuselage, and it seemed to be more effected by ground / water proximity. But just as I have wondered about regular floats, is it really ground effect, or is the air becoming compressed between the hulls and the water? My experience with regular floats is that you have to be within a couple feet of the water with the floats bottoms. Regular floats create a bunch of drag and some odd turbulence effects at certain speeds and attitudes. Sometimes you can feel the elevators being slightly buffeted.
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    The slowest I can fly an airplane is while in ground effect. Floats and skis seem to enhance the effect. Whether airplane or boat the required horsepower necessary to achieve acceleration is greater than that required to maintain operating speed. Maximum efficiency depends on the mission and the load. It's hard to discredit the efficiency of a 737 but it wouldn't make sense for a one mile trip to a fishing hole.

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    Quote Originally Posted by RainGull View Post
    Do you think with a low wing seaplane that effect would be much greater? In other words how much does the ground effect increase as you descend to a runway or water. It seems from watching planes land, that the maximum ground effect is just off the tarmac. From what I've read, the effect begins at wingspan height off of the ground and is maximized over hard ground, but it sure seems to stack up the lower a plane descends.

    I'm stuck in Juneau having another kid so I've had weeks to walk the trail and watch planes land...
    Ground effect begins at the ground [or water], and diminishes as the aircraft climbs. It's completely gone when the airplane reaches a height equal to its wing span. That's a "rule of thumb" since some wing designs will lose the benefits of ground effect before reaching that altitude, but it's a close thing.

    Wingtip vortex, when in normal flight [at cruise for instance] is almost cylindrical in form when viewed from the front. We're not talking about wingtip vortex only, but a vortex surrounding the entire airplane. When near the ground, this vortex is "flattened" because the surface distorts it and creates a more horizontal oval vortex shape, thus increasing the effectivespan of the wings. As the airplane rises, this vortex begins to become more cylindrical, decreasing the "effective" wingspan to reach its true span, thus decreasing lift a bit. This is more noticeable in low-wing aircraft, which is why a "ground cushion" tends to build up beneath the wings, making landing speeds a bit more critical in those aircraft designs.

    Both skis and floats add a little lift, as Mr. Pid points out. Floats are designed to add a bit of aerodynamic lift, though the added weight of the floats tends to diminish this added lift slightly.

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    Thanks so much.

    Does the ground effect aid in take off or is it moot at that point since you're pulling out so quickly? If it aided I would wonder why many floats sit lower from the fuselage than they seem to need to.

    I was fascinated to learn that helicopters also experience ground effect.

    While watching planes land I have been surprised lately to notice vortexes several seconds after a plane passes over eerily and very briefly blowing in the tops of the trees (or not in the case of Juneau) like a ghost wind. What's been fascinating to me is that the plane passes over (usually the morning AA jet) I can detect nothing for a few seconds and then as if some strange ghost wind is hurrying to catch up with the plane here comes a quick intense wind.

    Is it just the vortex closing and consequent disturbance upon settling, or does the Air Force have invisible planes that have to follow visible planes in to land
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    Quote Originally Posted by RainGull View Post
    Thanks so much.

    Does the ground effect aid in take off or is it moot at that point since you're pulling out so quickly? If it aided I would wonder why many floats sit lower from the fuselage than they seem to need to.

    I was fascinated to learn that helicopters also experience ground effect.

    While watching planes land I have been surprised lately to notice vortexes several seconds after a plane passes over eerily and very briefly blowing in the tops of the trees (or not in the case of Juneau) like a ghost wind. What's been fascinating to me is that the plane passes over (usually the morning AA jet) I can detect nothing for a few seconds and then as if some strange ghost wind is hurrying to catch up with the plane here comes a quick intense wind.

    Is it just the vortex closing and consequent disturbance upon settling, or does the Air Force have invisible planes that have to follow visible planes in to land
    It does aid in takeoff, wich is why the pilot must pay attention to the airspeed indicator. In a forced takeoff, I've pulled a C-180 off the ground when the airspeed needle hadn't moved off the peg. I suspect that Mr. Pid has too. Don't to that . . . . . Pulling up too quickly at a low airspeed will almost instantly place the airplane above ground effect, making airspeedf all the more important.

    Floats are positioned in accordance with the floats' manufacturer. The designer has calculated float installation for any given aircraft, and the rigging (mounting hardware) has been considered during design and fabrication.

    Helicopters aren't really acting within ground effect, I believe, but because of rotor downwasn. I don't fly helicopters, so I'm making an inexperienced statement here.

    Your last observation is correct: the vortex is settling, as well as disipating, which is why a plane landing behind a jet is cautioned to approach a bit high and to pass through any remaining vortex more quickly in order to lessen its effects.

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    The helicopter bit I took from the wikipedia article on the subject. I figured the floats would be mounted as much for structural strength and vertical stability as for any advantage with ground effect, but couldn't escape noticing that the floats raise the plane more than appears neccesary thus giving up some unmeasurable to me advantage.

    I remember watching Flying Wild Alaska, watching Ariel Tweto pull up to fast and Ponce scold her for not getting enough speed first, so I assumed that must be true. I still wonder though how much ground effect helps at takeoff as you still have to get up enough speed to fly above it and climb out. But I guess ground effect raises the effeiciency of the plane enabling you to reach that speed quicker, so it would be a transient effect on take-off but important none the less.

    Thanks again to everyone, it's been enlightening.
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    Quote Originally Posted by RainGull View Post
    The helicopter bit I took from the wikipedia article on the subject. I figured the floats would be mounted as much for structural strength and vertical stability as for any advantage with ground effect, but couldn't escape noticing that the floats raise the plane more than appears neccesary thus giving up some unmeasurable to me advantage.

    I remember watching Flying Wild Alaska, watching Ariel Tweto pull up to fast and Ponce scold her for not getting enough speed first, so I assumed that must be true. I still wonder though how much ground effect helps at takeoff as you still have to get up enough speed to fly above it and climb out. But I guess ground effect raises the effeiciency of the plane enabling you to reach that speed quicker, so it would be a transient effect on take-off but important none the less.

    Thanks again to everyone, it's been enlightening.
    Many - - - if not all - - - instructors teach new students to lower the nose a bit just after liftoff in order to build sufficient airspeed before emerging from the benefits of ground effect. Many bush takeoff acciddnts occur because some hotshost Super Cub driver began a showoff climbing turn just after liftoff, ending up in the cottonwoods.

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    The floats are rigged to help minimize the spray that gets in the prop and all over the plane as you take off or land. It is very easy to suck water into the prop and ruin it VERY fast even with the planes rigged as they are.

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    Quote Originally Posted by avidflyer View Post
    The floats are rigged to help minimize the spray that gets in the prop and all over the plane as you take off or land. It is very easy to suck water into the prop and ruin it VERY fast even with the planes rigged as they are.
    Careful now - - - - - the manufacturer adds "spray rails" for this purpose. The overall float/hardware design doesn't usually handle the spray very well, so those spray rails are added. The "rigging" doesn't consider the spray thrown up by the prop. And, by the way, that spray is much worse during a static runup or in downwind conditions at high RPMs.

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    Griz is correct:

    Speaking of water on the prop, here is a photo from my training manual of what happens when you use too much power in a heavy cross wind.

    And a second photo showing what happens when you use the wrong power setting while taxiing down-wind with a strong tail wind. The haze around the plane is water spray.
    Attached Images Attached Images
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    OK, so he asked why they sit up so high and not lower... think about it. If it was lower, what would that do to the prop. Spray rails help, but if the plane were mounted much lower, you would be out of a prop in under a hundred hours, if you were lucky. I have only manufactured float mounts for 5 or 6 planes, so I am by no means an expert at it, but I can tell you what does and doesnt work. Mounting them lower DOES NOT WORK because you will eat the prop, and you WILL blast your fuse and tail feathers with more spray than you can deal with. Extend those mounts and you can expect and get much better service life out of the prop. So, while it may not be the ONLY reason that it is done, it is indeed effective and it is deffinately taken into consideration when you are setting up a float mounting system.

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    Quote Originally Posted by avidflyer View Post
    OK, so he asked why they sit up so high and not lower... think about it. If it was lower, what would that do to the prop. Spray rails help, but if the plane were mounted much lower, you would be out of a prop in under a hundred hours, if you were lucky. I have only manufactured float mounts for 5 or 6 planes, so I am by no means an expert at it, but I can tell you what does and doesnt work. Mounting them lower DOES NOT WORK because you will eat the prop, and you WILL blast your fuse and tail feathers with more spray than you can deal with. Extend those mounts and you can expect and get much better service life out of the prop. So, while it may not be the ONLY reason that it is done, it is indeed effective and it is deffinately taken into consideration when you are setting up a float mounting system.
    Absolutely correct, avidflyer. And, as you know, raising the aircraft too high will only add to its overturning characteristics, and moreso during step turns, especially in windy conditions.

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