Lift Fan Settings

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gone7
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Lift Fan Settings

Post by gone7 » September 13th, 2012, 10:19 pm

Just curious about something; On the lift fan drive ratio for the Explorer - the plans are generally for an 18 to 20 foot craft. I notice that no matter which is chosen, the lift fan drive ratio and degrees of pitch for the fans seems to remain the same. Obviously I would imagine that the larger craft should require more air than the other because there's simply more space to fill in the plenum.

My craft is stretched to 21 feet. What's your opinion about the lift fan drive ratio/pitch for this? Do you think I should step up the fan drive ratio or pitch a bit?
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bphillip2
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Re: Lift Fan Settings

Post by bphillip2 » September 14th, 2012, 7:09 am

The plans assume you still have the same engine.

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jchovernut
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Re: Lift Fan Settings

Post by jchovernut » September 15th, 2012, 8:43 am

I don't think it's plenum pressure you're worried about; it's the 'lubricating air' that escapes the plenum around the periphery of the skirt at the ground contact line. The fan/prop ratios are set up to properly distribute engine HP for the designated powerplant, and provide adequate lift under varying thrust settings. I'll defer to Bryan and Barry for the calculations, but I don't think I'd sweat it, myself. If you're using a higher output engine than called for, I'd probably keep the lift fan settings as per plans and apply the extra horses to thrust for plane-out and heavy wind. If I can recall, I believe the two fan setup gives you a decent margin for lift!

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gone7
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Re: Lift Fan Settings

Post by gone7 » September 15th, 2012, 11:17 pm

Thanks for the quick replies!

Sounds like a good approach John. That's probably what I'll do. I was mainly just curious.

I'm hoping to learn a little bit here though. So, for the sake of discussion: It just struck me that while the size of these crafts were increasing that there was no apparent change to the air volume/pressure settings recommended. I'd thought that the gross weight and cushion area of a craft were both factors in determining the correct lift air requirements for a particular sized craft. I can understand that for a specific engine there's likely a BEST ratio to distribute the available hp between the lift and thrust, but with the same engine being used on all of these crafts, a person doesn't really have much of a choice to make changes. But if it's true that the craft's size should dictate the amount of lift air required, then as these crafts increase in size then I would assume that their performance is probably somewhat limited by the horsepower range of this motor (the Subaru EA-82).

I'd noticed on the plans (DWG 10) that there was a performance table shown for "stretched versions" of the Explorer. The table shows that as the size of the craft has increased from 18.5 to 24 feet (and carrying identical payloads) that the top speed gradually drops. So, let me try to sort out the possible reasons for this from my point of view: As "stretched versions" (not scaled up versions) I would assume that the width of these examples hasn't increased and that the drag itself wouldn't be greatly increased from one version to the next. I say this because in all of these variations the craft basically stays the same as seen from the front, each is just a bit longer in the mid-section. Each size increment of two feet is shown to add an additional 50 lbs to the craft, but the increase in size is also estimated to improve the payload by about 300 lbs. With this trade off I would not assume that the added weight of the extended hull to be a factor for increasing drag but rather could improve upon it. I'm trying to determine just what is dropping the performance on these size variations. What actually gives you a determination of "top speed" in these cases? Is it when the front skirt collapses or when the craft simply won't go any faster? From my experience it seems like my top speeds are usually limited by how much speed the front skirt can handle. It would seem like in either case that a little more hp might help. If the front skirt is collapsing perhaps a bit more lift air could be an improvement. If the craft is topped out on thrust, then obviously a little more power might increase the performance.

I know I'm just rambling on here, but I'm hoping you might share your insights to some of my thoughts. I guess my dilemma is in feeling that perhaps the 18.5 foot craft has been fitted with what it truly needs to function well, while maybe the larger ones are kinda stuck with the same motor and setup when they might actually perform better with something a little more adequate for their size.
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Re: Lift Fan Settings

Post by bphillip2 » September 16th, 2012, 4:33 pm

As craft size increases you would like to have proportionally more volume flow to maintain some particular air gap. If you have more hp you typically need to divide it up between lift and thrust or the craft falls of cushion to early as you slow.

The craft gets slower in theory because skirt drag increases with skirt size, however if cushion pressure drops you might gain some back as you are lighter on the water.

The front skirt does not limit speed, unless you nose down do to improper trim or rough water conditions. I'm curious... I've heard this before. Help me understand this rumor. "Sevtec's can only go so fast till the front skirt folds back". Nonsense. When the craft's bow skirt lays against the hull it simply indicates the dynamic pressure from forward speed has reached front cushion pressure. Not until the partition folds back do we have a problem... lack of flight controls for take off!

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Re: Lift Fan Settings

Post by gone7 » September 16th, 2012, 9:35 pm

I appreciate the explanation Bryan!

The front skirt issue just seemed to be what I'd experienced on my previous crafts. I guess there could be the possibility that I didn't have proper skirt trim but I didn't seem to have any other issues that might be associated with that. For the most part I felt that the crafts operated very well compared to others I'd traveled with. I did once have a video showing my Vanguard in operation at higher speeds where the front skirt was flapping back toward the bow with apparently not much purpose at that speed. I was surprised to see this since I didn't even realize it while driving. It was all up to the partition skirt at that point I guess. So, maybe it would have been more appropriate to have said that the partition skirt might be the limiting factor.

I understand that rough water conditions will force the front end to drop. But from my observations, the faster you go, the less of a wave it takes to drop the front end. On my Geo powered, stretched Vanguard, I could push it fast enough to where even a series of small ripples would force it to drop. I had more power to apply, but I was limited by less than perfectly flat water conditions and it's effect on the partition skirt. I can hope that I could somehow improve upon this but I just assumed this was generally an inherent character trait of the design. I can't say that I've really had a long enough completely smooth surface to test this effect on ideal conditions.
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Re: Lift Fan Settings

Post by highlandhovercraft » September 17th, 2012, 1:17 am

Another issue you can have if the ratio between lift & thrust is not set up right is that the front brake becomes less effective. I had lift issues with my surveyor so I increased the number of blades in the lift fan to 9. Big mistake!!!!!!! :oops:

I should have trimmed the skirt using blocks as per plan, but I was told the easiest way to trim the skirt is to use a predetermined length of wood under the bow so you have the right hover height. Then while holding the craft down to this piece of wood trim the skirt.

This is not a good way to trim the skirt. All it does is set the front curtain at the right height. The hull may also not sit level side to side. After spending months messing with different things trying to get better lift I reverted back to the method for trimming the skirt as per plans. Within a couple of minutes I found the answer to my lift problem. I had an issue with the rear skirt section, which for some reason was shaped like a banana, which meant the the rear of the craft was not achieving full lift, so the front would never be right.

Once I had sorted the rear skirt and retrimmed the partition and bow skirts my front brake would not work effectively because I now had too much air because of the extra 3 blades in my lift fan. At the same time I had less thrust because more of the power was taken by the lift fan.

It is a fine balancing act getting the right proportions of lift & thrust. I am using a larger hp subaru engine in my explorer build, but I shall let Bryan figure out the gearing ratios as I wouldn't know where to start. I shall be using the air boat paddle that Bryan sells for thrust, and I have used the two 24" 9 bladed fans set at 25* as per plan.

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Re: Lift Fan Settings

Post by gone7 » September 17th, 2012, 6:09 am

It is a fine balancing act getting the right proportions of lift & thrust.
I completely agree. It can make the difference between a bum craft and one that operates very well. That's one reason I've always felt that a single engine craft makes so much sense. It will certainly save you time and money to just let Bryan figure out the proper ratios for you.
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