[ken]

George said:

Ralph, in your opinion which contributes more to helping control plow-in during a strong tailwind, (1); backsided high pressure bow fingers with your special anti-plow flap or, (2); the hull step.

Which contributes more in helping recovery and controling plow-in?

Do you consider your design solutions preventive or reactive?

...........................................

Carlos, the wing you speak of is to be mounted on the outside, over the airflow of the bow?

Or is it under the craft in the pressurized air cushion or ductwork?

You asked Ralph, but I'm going to chip in with my own answers:

Plow-in is in my opinion the most significant event in hovercraft operation. Most drivers avoid it religiously, and most designers seem to treat it as a minor annoyance. As a result, most hovercraft implementations have really bad plow-in behavior.

Plow-in can be triggered by several things:

1. Skirt issues
2. Terrain (waves, dirt, etc.)
3. Weight shift
4. Aerodynamic conditions.
5. Maybe more?

These things can be accidental or deliberately induced. The hull shape, or at least the bottom shape, has nothing to do with (Edit: triggering) plow-in as far as I know. The triggers, listed above, often cause the skirt to drag, particularly in front, which if left uncontrolled will cause hull contact. At the moment of hull contact, you have a plow-in.

There are a lot of ways you can control the event or avoid it. FWIW, the fact that the front of your craft is dipping should not be a cause for terror. Adequate trim wing(s) on a fast machine is enough to induce, control, abort or prevent a plow-in. Weight shift can help on smaller machine but is too slow and cumbersome in most cases to actually prevent something already started.

Some crafts use multiple compartments on their lift system. Sevtec has a self-regulating multi-compartment lift system which makes plow-in extremely rare. That's all it takes if you want to avoid plow-in entirely--it won't completely remove the chance, just make it rare, and from what Barry says the hull shape is adequate to reduce the effects in the event it does happen.


So far, I've only talked about the events leading up to plow-in. Equally important is the way the craft handles a plow-in once hull contact has happened.

The main thing that dictates plow-in behavior is the hull. You need two things: Adequate plow planes to keep the craft from submarining, and a way to detach the water flow from the hull. The plow plane angle determines part of a braking force, but if the water is not detached from the hull shortly after the plow plane, the entire hull acts as a sort of water wing forcing the hull down, which can add tremendous force to stopping the hovercraft. If there is adequate detachment then the plow planes determine the rate of deceleration almost entirely.

Ralph has a two-stage plow plane. There is a shallow plow plane, and if that isn't enough then there's a steeper one to keep the craft from submarining in really bad situations.

Another thing that is important in controlling and recovering from plow-in is the lift system. On my UH-12r, the lift system had a fault in that during a plow-in the bag air feed was cut off. My lift engine had to overcome the pressure closing off the bag before it could recover from the plow-in. That made it a little cumbersome, because it wasn't a smooth transition from plow-in to normal state. A finger skirt craft could easily fix this problem because of the internal ducting to each finger. Regardless, if you don't have adequate hull detachment then the lift system can't overpower the suction forces pulling the hull down.

Controlling and recovering from an in-progress plow plane is not magic. My UH-12r did well enough with the lift system and with the trim wing at high thrust. A compartmented system with good plow planes is really all you would need at low speed, you could induce and control everything except blow-over. To prevent blow-over you need some sort of aerodynamic control surface IMO.

This post has been edited by ken: 31 January 2008 - 10:37 AM

[kach22i]

Along the line of thought here, I'm currently building a rather large hovercraft model which is intended to explore some issues which I can't do at full scale with my Scat II. One of the issues or design features deals with plow-in.

I'm not sure a 4-1/2 foot long model can address plow-in properly. Is plow-in a phenomena which can be scaled up and down?

Some designs such as Barry Palmer's Sevtec seem to cantilever the bow. What I mean is, a Sev has a very stabile cushion platform, then at the bow is a second chamber which operates in an semi-independent way. A way which will not lead to a cascade event very easily I would think.

Try and picture a Sevtec with the front drape removed, would it still fly? I think it could, although not as the designer ever intended it to. In this respect the front chamber would then seem to be independent or free to deal with conditions which lead up to plow-in, right?

There are many ways to divide up a cushion area, and with different intentions and results.

Just an exercise in perspective...............gotta go now.

This post has been edited by kach22i: 31 January 2008 - 10:40 AM

[ken]

Along the line of thought here, I'm currently building a rather large hovercraft model which is intended to explore some issues which I can't do at full scale with my Scat II. One of the issues or design features deals with plow-in.

I'm not sure a 4-1/2 foot long model can address plow-in properly. Is plow-in a phenomena which can be scaled up and down?

Absolutely, positively. Do you have Light Hovercraft Design? That gives scaling functions. You might need to sit down to figure out the results, but it can be done.

Some designs such as Barry Palmer's Sevtec seem to cantilever the bow. What I mean is, a Sev has a very stabile cushion platform, then at the bow is a second chamber which operates in an semi-independent way. A way which will not lead to a cascade event very easily I would think.

It's really fully dependent, the way he does it. The entire chamber is fed from under the partition from the main partition. Yes, it does prevent plow-in very effectively. My argument with that system is that at high speed, inducing plow-in is the fastest way to stop. I am not interested in a craft on which I can't very rapidly induce a plow-in in a controlled fashion, and recover from it at will. I realize that a lot of the people driving hovers don't care about that, mostly because they don't drive fast enough to make it desirable, or because their crafts don't handle the event well.

Try and picture a Sevtec with the front drape removed, would it still fly? I think it could, although not as the designer ever intended it to. In this respect the front chamber would then seem to be independent or free to deal with conditions which lead up to plow-in, right?

You would need to move the center of gravity back. The Sevtec front cushion runs at partial pressure normally, I think around 60% if I remember my discussions with Barry correctly. When the nose starts to dip, the air escaping that front partition is cut off faster than the air feeding it, just by the geometry of the system. That raises pressure in that front partition, which aborts the plow-in before it happens.

AFAIK, that system was developed solely to prevent plow-in, but I don't know for sure. Maybe Barry will chip in.

There are many ways to divide up a cushion area, and with different intentions and results.

Just an exercise in perspective...............gotta go now.

I'm very interested in a multi-compartment lift system. I wouldn't feed it passively the way Barry does, but I think you could come up with some really interesting configurations that could lead to some really fast machines.

Barry's setup is IMO suitable for low speeds where all you want to do is drive your leisurely way down the river, but not high speeds. This is not a slam on Barry or Sevtec. There is a big market for that style of machine, and a whole lot of designs which satisfy that market, including Sevtec, UH, and just about any commercial design. Each has its benefits and drawbacks. Each has a different way of handling the events which come up with hovering. Each has a market segment which it best serves.

[hman]

Along the line of thought here, I'm currently building a rather large hovercraft model which is intended to explore some issues which I can't do at full scale with my Scat II. One of the issues or design features deals with plow-in.

I'm not sure a 4-1/2 foot long model can address plow-in properly. Is plow-in a phenomena which can be scaled up and down?

Some designs such as Barry Palmer's Sevtec seem to cantilever the bow. What I mean is, a Sev has a very stabile cushion platform, then at the bow is a second chamber which operates in an semi-independent way. A way which will not lead to a cascade event very easily I would think.

Try and picture a Sevtec with the front drape removed, would it still fly? I think it could, although not as the designer ever intended it to. In this respect the front chamber would then seem to be independent or free to deal with conditions which lead up to plow-in, right?

There are many ways to divide up a cushion area, and with different intentions and results.

Just an exercise in perspective...............gotta go now.

The basic approach seen these days to preventing plowin involves giving the bow a very gradual slope. This reduces water drag (usually but not always) but mostly it means that there is no hull-water contact as the bow dips downward. All hovercraft sort of pivot near the center, the stern stays high as the bow dips down, but a lot more down movement at the bow is needed with this shape to produce water contact than a squarish bow like a scat. Sometimes a partitioned cushion space is added to this - which can cause increased lift at the bow as it moves toward the water surface.
This all makes good sense and it works, but only up to a point. Enough down pitch trim and the right sort of wave impact and major hull/water interaction results. The faster you are going, the more interesting this becomes.
Since I started off with a SCAT 2, I have been obsessed with plowin. With any high performance sport craft, I would like absolute, rather than partial freedom from worrying about plowin. It has major advantages, imho.
The skirts I use now are standard type open scat segment. I have tried about a zillion things with skirts over the years and that no doubt has confused others. But there is nothing altered about the skirts on my present design.
Absolute safety comes from:
1. Plow planes near the bow, which means inside the skirt.
2.A lifting shape that is optimized for lift vs drag, which means it must be shaped differently than the hull can be.
3. A major step behind the planes such that hull contact is impossible for the front 1/3 or so of the bottom.
So when the bow goes down, what meets the water first is an optimal ski shape that will hold the main-bottom away from the water NO MATTER WHAT HAPPENS. Which means massive hydrodynamic lift when required.
This can be done without changing cushion height or obstacle clearance. But it cannot be done without basically redesigning the hull from the bottom up. The stresses are totally different from what normal hulls are designed to deal with. The whole front of the craft is basically hanging over nothing but air when stationary.

[jarrob]

Some designs such as Barry Palmer's Sevtec seem to cantilever the bow. What I mean is, a Sev has a very stabile cushion platform, then at the bow is a second chamber which operates in an semi-independent way. A way which will not lead to a cascade event very easily I would think.

From the Sevs I've driven and observed it would appear that the partitioned cushion helps prevent hull contact. The main cushion (behind the partition skirt) remains fully pressurised even if the front of the hull is in contact with the water. This keeps tha back half of the craft on full cushion and makes it almost impossible for the majority of the hull to contact the water.

However, Sevs DO plough-in! You can induce a controlled plough by venting the front compartment aggressively while dropping power OR the craft can self-plough under certain conditions (running downwind under power into large wave troughs at too high a speed).

Try and picture a Sevtec with the front drape removed, would it still fly? I think it could, although not as the designer ever intended it to. In this respect the front chamber would then seem to be independent or free to deal with conditions which lead up to plow-in, right?

It does still fly. In fact I've seen craft with the front skirt plastered up against the hull at times. This seems to happen at speeds above about 35mph into wind. The question then is, what is holding the front up? I suspect that there may be enough aerodynamic lift generated by the underhull shape to keep the bow up. I've also experienced this effect on my Prospector at full speed (about 45mph) - although I couldn't see the bow skirt, opening the skirt vent had absolutely no effect - obviously because there was no pressure in the front compartment to vent!

[kach22i]

The whole front of the craft is basically hanging over nothing but air when stationary.

The large and stern biased landing skids I installed do this, but is still "untested".

Thanks for the update on the bow skirts and the good reading in general.

[kach22i]

It does still fly. In fact I've seen craft with the front skirt plastered up against the hull at times.

Thanks, that confirms my little theory for the moment.

[kach22i]

Absolutely, positively. Do you have Light Hovercraft Design? ................Each has its benefits and drawbacks. Each has a different way of handling the events which come up with hovering. Each has a market segment which it best serves.

I'll look at that book again.

I think the market segment where you never want to have plow-in would have to be a large ferry. Maybe Kurt will chime in when he is not so busy.

This post has been edited by kach22i: 31 January 2008 - 12:29 PM

[ken]

I'm really glad you guys all stopped by!

What we have is a small group of people who either have experience in the systems I'm talking about or who have experimented with skirts and plow-in technologies.

I'd love to have Barry and Kurt have a say, and anyone else who might be watching who has something to add or has a question.

I might add at this point that my main qualification is that I've spent a whole lot of time driving back and forth on a UH-12r at high speeds and nosing it in, then turning around and doing the same thing in the other direction. And then going home to fix the damage. Everyone else here has more experience in some way, either by having more broad experience (Jarrob has a Sevtec and a UH-18sp) or has actually started experimenting with construction.

[ulnutt1]

I'm really glad you guys all stopped by!

What we have is a small group of people who either have experience in the systems I'm talking about or who have experimented with skirts and plow-in technologies.

I'd love to have Barry and Kurt have a say, and anyone else who might be watching who has something to add or has a question.

I might add at this point that my main qualification is that I've spent a whole lot of time driving back and forth on a UH-12r at high speeds and nosing it in, then turning around and doing the same thing in the other direction. And then going home to fix the damage. Everyone else here has more experience in some way, either by having more broad experience (Jarrob has a Sevtec and a UH-18sp) or has actually started experimenting with construction.

Sorry for my ignorance guys... I have read all of your coments on this thread regarding "plow-in" but I am still unable to figure out exactly what it is your talking about. It sounds to be a typical and common problem from what I am reading... but what is it and what causes it?

Don

[jarrob]

Sorry for my ignorance guys... I have read all of your coments on this thread regarding "plow-in" but I am still unable to figure out exactly what it is your talking about. It sounds to be a typical and common problem from what I am reading... but what is it and what causes it?

Don

Don,

Plow(plough)-in is when the hovercraft switches from low-drag to high-drag mode. The sudden change to high-drag mode can cause a very abrupt deceleration.

There seem to be two main methods used to reduce this effect, making the craft less likely to plough-in in at all and secondly, make the transition time long enough so that the deceleration isn't too severe.

All hovercraft will plough-in given the right circumstances - the big difference is in how they handle the event. Some early craft (and some current race craft!) have such a low frontal area (no effective plough plane) that they can actually partially submarine when they plough at speed. The event can cause ejection of the driver and even a craft flip. Other craft have such a gentle plough that the effect is similar to braking a car and it can be used to provide positive control.

The other issue with plough in is how and when it is triggerred. Certain craft give no warning to the driver that a plough is about to occurr giving no chance of taking any avoiding action (like holding on tightly ). Craft like this can plough for no obvious reason and at any time. To my mind, this is unacceptable and very dangerous.

[HoverTrekker]

Sorry for my ignorance guys... I have read all of your coments on this thread regarding "plow-in" but I am still unable to figure out exactly what it is your talking about. It sounds to be a typical and common problem from what I am reading... but what is it and what causes it?

Don

Don - I'm sure you've experienced it already in one form or another. In laymans terms, it's when the nose of your Hovertrek drops down and the hull contacts the water making a big splash. If you stay on the throttle it will just "skip" on the water and bounce back up on cushion. You are most likely to experience this when travelling with a tail wind against the current going a bit too fast and/or you let your rpm's drop off too much. You can use your buckets to slow you down while maintaining rpm and cushion pressure. Lots to learn I guess

[ken]

The "high drag" mode John is talking about is when the hull makes contact with the surface you're driving over. It's also different from landing or dumping lift in that the hull is not parallel to the surface but rather the leading side of the craft is dipping down to make contact and the trailing side is still at full cushion, or at least is higher than the leading side.

Advice regarding handling plow-in from other Neoteric owners is going to do more for you than the advice someone else might give you. That machine has features almost nothing else in the recreational sector has, and that changes best practices for operation.

You'll need to figure out how to tell what your craft is doing. From the sounds of things, you already have the idea down on your other machines, you just need to get familiar with your hovercraft. I don't know any better way to start on this than getting the craft up in the air and start looking at bottom features. Don't just look, but touch. Take some skirts off, check it all out. It's much better to get familiar with that in your garage than it is on some beach in the rain. Also get a really good idea how your controls work, the functional end of them. See what is likely to go wrong and figure out how to deal with it. That last part is probably not so important on your craft as it is on others. Neoteric has a reputation for bulletproof design and very high reliability, but there still will be failures eventually.

[kach22i]

The "high drag" mode John is talking about is when the hull makes contact with the surface you're driving over.

Adding to what Ken said; when and if you do further reading on this topic, some explaination will include the "pre" or "leading" conditions which end up making this "high drag".

The theory goes that for what ever the reason, such as a tail wind pushing the bow down, the bow shirt makes contact with the water, the bow skirt is then dragged into the water (resistance) which pulls the whole bow of the craft down with it, then splash (plow-in) an even more substantial resistance! This can all happen rather suddenly, so it can appear to be a single event rather than two seperate events.

The warning you get in an old Scat-2 is that the constant and often annoying water spray coming up over the bow disapears. If the spay suddenly disapears, move your body way back on the seat ASAP and throttle up.

This post has been edited by kach22i: 01 February 2008 - 11:53 AM

[ulnutt1]

Don - I'm sure you've experienced it already in one form or another. In laymans terms, it's when the nose of your Hovertrek drops down and the hull contacts the water making a big splash. If you stay on the throttle it will just "skip" on the water and bounce back up on cushion. You are most likely to experience this when travelling with a tail wind against the current going a bit too fast and/or you let your rpm's drop off too much. You can use your buckets to slow you down while maintaining rpm and cushion pressure. Lots to learn I guess

Oh yea - those sudden stops I kept having in the beginning... Thanks ... now I know another name for it besides some of the choice words I was using 'til I learned to control it. Thanks Has anyone put a video togather of some of these things? It would probably be very helpful to trainees.

Don