Hovercraft- Let the cutting begin!

[kach22i]

Originally posted by fastpat
See if you can determine his power to weight ratio. I'd think looking for an engine that overpowers your experiment would be just right.

Part of my experiment is the restriction that I get it to work with the same engine and fan (power) of the original 20-30 year old design.

Adding more power is like cheating, except that I've added weight to the craft (hull repairs, sound deading, skirt weight, added floation) and it may not be apples to apples.

Of the three elevators I installed, the largest top one slip down the aluminum post at some point. I don't know if this helped me or hurt me.

I was really working to keep the bow up with my last set of modifications which included adding weights.

All this bow up effort translates into actual "stern down" results..........................is that it? This would answer the wake starting at the tail, flight is not level.

I feel a need to start over on the stern skirt, but maybe taking off one of the dumbell weights will help. I'm going to think about it, I should have a few months of hard winter to do this in.

[lendaddy]

What is the experiment exactly? The bananna-split style skirt?

[fastpat]

Originally posted by kach22i
Part of my experiment is the restriction that I get it to work with the same engine and fan (power) of the original 20-30 year old design.

Adding more power is like cheating, except that I've added weight to the craft (hull repairs, sound deading, skirt weight, added floation) and it may not be apples to apples.

Of the three elevators I installed, the largest top one slip down the aluminum post at some point. I don't know if this helped me or hurt me.

I was really working to keep the bow up with my last set of modifications which included adding weights.

All this bow up effort translates into actual "stern down" results..........................is that it? This would answer the wake starting at the tail, flight is not level.

I feel a need to start over on the stern skirt, but maybe taking off one of the dumbell weights will help. I'm going to think about it, I should have a few months of hard winter to do this in.

Hmmm, well just guessing, I'd say you need a more adjustable weight changing apparatus. You might think about adding a bar, bathroom towel rack in appearance, on which you can add or subtract weight easily to achieve balance by trial and error. Another thing I think I saw in the video is plenty of forward thrust, but marginally adequate cushion air volume. Is there a method to change the ratio of forward/down thrust split?

[kach22i]

Originally posted by fastpat
Hmmm, well just guessing, I'd say you need a more adjustable weight changing apparatus. You might think about adding a bar, bathroom towel rack in appearance, on which you can add or subtract weight easily to achieve balance by trial and error. Another thing I think I saw in the video is plenty of forward thrust, but marginally adequate cushion air volume. Is there a method to change the ratio of forward/down thrust split?

1. Yes, I need better balance, both with the weights and the thrust angle via elevators.

2. Lots of thrust launching right off the boat ramp, but water drag on the skirt contacting the water builds up rapidly in turns.

3. Head winds have always been a problem with this low powered craft.

4. I've stolen as much lift air from the thrust air as I dare. Original was 33%, now at 40% give or take.

5. The experiment is the skirt (yellow part) and a few things in the ducting you don't see. The original finger skirt was more of a racing skirt allowing you to lean the craft for banking in a turn, but this meant it was unstable front to back as well as side to side. For cruising a stable skirt is nice to have. The experiment started when making major repairs to the hull from an incident I had.

Video 7c:
http://www.youtube.com/watch?v=6Js7ivM12x8

[kach22i]

As with any craft the center of gravity shifts with speed.

When you accelerate it's thrown back, when you brake it shifts forward.

Even though I used calculations and a pipe to balance the craft, this balance is at static hover, not at speed.

I added too much weight in the back, but how do I determine the center of weight/gravity at speed?




Let's say you have a body in motion, a car, an airplane or hovercraft - does not matter, looking for basic laws of physics here.

1. You know the weight.

2. You know the center of gravity at rest.

3. You know the center of lift (if applicable).

Let's say our object weighs in at 600 lbs.

Lets say our object is going 30 mph.

Q1:
How much does the center of gravity or weight shift back?

Q2:
Lets say I need to start or stop very quickly, how do I calculate the location shift of CG backwards and forwards?

I think the key is taking the the actual weight and turning it into mass, then take into account speed the horizontal force, which in turn may reduce the gravity force or weight over a given time peroid.

An actual example would help, then I can apply it to my situation.


EDIT.............................................. .........................
Perhaps I've tried to find a universal solution that just does not apply.

Example of what I meant.

Weight Transfer-1
http://www.msgroup.org/TIP074.html

When you change speed (accelerate or decelerate) the weight of your motorcycle (including you) shifts in such a way as to put more or less load on your tires. You do not have to weigh the load on your tires to know this with certainty because you can see it happen by observing your front-end 'dive' when you brake.

Stern/rear hovercraft skirt drag might be a version of torque or braking power applied.

1. Car/Motorcyle; weight and center of gravity stay the same unless braking torque is applied or aerodynamic forces have an influenence.

2. Boat; weight and center of gravity are the same until displacement mode is replaced by planing mode, then there is a shift to the "center of lift". This is countered by tabs at the rear which induce drag counterforces and angle changes.

3. Airplane; weight and center of gravity are the same until speed increases lift on the wings causing a shift which is countered by the elevator (typically at the back).

4. Hovercraft; once it's over the "hump speed" which is also a boat term with a slightly diffenerent context, it is in level flight. Weight is centered until torque (paracitic skirt drag) is encountered in the form of wave impact and aerodynamic resistance.

[fastpat]

Drag moves the aerodynamic center of pressure rearward, acellerating rotates the vehicle around the center of gravity, the center of gravity depends on where on the vehicle the weight is located, and does not change with wind drag factored in. The aerodynamic center of pressure is affected by the shapes of everything on any vehicle moving through the air, and since you have plenty of "dragy" things hitting the air, that has the affect of rotating your craft around the center of gravity even in steady state movement, with no acelleration or decelleration.

[kach22i]

Originally posted by fastpat
Drag moves the aerodynamic center of pressure rearward, acellerating rotates the vehicle around the center of gravity, the center of gravity depends on where on the vehicle the weight is located, and does not change with wind drag factored in. The aerodynamic center of pressure is affected by the shapes of everything on any vehicle moving through the air, and since you have plenty of "dragy" things hitting the air, that has the affect of rotating your craft around the center of gravity even in steady state movement, with no acelleration or decelleration.

Pat, that is a very clear, simple and understandable description of what goes on, thank you very much. It's like I have to relearn everything I have not thought about since early college.

On a special note, apply all that Pat has said, and add in hull drag for a boat or hovercarft skirt in my case.

A friend e-mailed me this link, the early start of one of my favorite hovercraft designers.

http://members.aol.com/buftec/buf/red.html

[kach22i]

UPDATE: January 3rd 2007 - Test Flight 8

I took 20 lbs off the tail which helped.

The engine does not seem to be producing the power it should be, oil around the plugs. It was difficult to start too, maybe the plugs are loose again or I have to tighten the bolts that hold the engine together. I'm not an engine guy, but I don't like what I see, feel and hear so I'll just have to deal with it like I do with the old Porsche 911.

I'm so close to getting this thing to work - everything just needs tweeking. Flying back after making my turn it's right at the threshold of getting back over the hump - very fustrating.

1. I just have too much skirt, I first wanted 12 inches, going from the the original 5 inches. The fabric is still cut on the sides for this even though the front and back have been adjusted for 6-3/4" front and 5" rear. This means I'm draging the skirt around with no daylight clearance what so ever on parts of the side - the back 2/3rds.

2. 50 degrees in January, looks like I'l be able to make these tapering skirt adjustments and do water testing most of the winter. Who said climate change had no up side? :lol:

Video Link 8a:
http://www.youtube.com/watch?v=MG4HT17VQGY



With the new lower weights I can feel the craft move front to back when I move my body around, it also banks a little too so I can turn better. Like I said power is becoming the real problem but it will not solve poor fit and detailing.

[kach22i]

UPDATE: USA, Michigan, Gun Lake, March 10 2007

Test Flight 9 Video:
http://www.youtube.com/watch?v=N5tKir2Or9s

First meeting of the Hoverclub of America "Great lakes" chapter/group.

There was some water on the lake, at least four ice types: underwater, smooth, course and very rough.

I need to take about a foot of fabric out of the skirt, refit to the front and the tail to get rid of wrinkles and pinches (see arrows). Secure the sponson tails to the "curtain rod" to prevent side-way drift openings between the stern skirt and inflatable sponson skirt (see lines drawn in).

My made in China $15 boots are a similar material to the skirt, and the same color- I'm styling man.

[kach22i]

More pictures of the problem areas of the current skirt. When it gets warm enough I'll be fixing it.

[kach22i]

das boot.

[azasadny]

George,
Very cool!!

[fastpat]

One thing about this project, it demonstrates perseverance on your part; and is a lot of fun to read about.

[kach22i]

Thank you all for the comments.

A few more pictures from the last test flight.




From here:
http://www.hoverclubofamerica.org/forum/index.php?act=module&module=gallery

[kach22i]

UPDATE: 05/31/2007

I'm redoing the stern hull and stern skirt. I'll be replacing the useless on water "anti-drag" flap I called "the cape" because it caused a huge water wake from all the surface drag. It worked just fine on ice, ice is so forgiving with hovercraft.

I'll be trying some balloon fabric for the rear/sten skirt. If it works out I'll put it on the whole craft. The new fabric weighs about 1/10 of the heavy yellow material and I picked up six pieces which if laid end to end would be four foot wide by 60 foot long, more than enough to do the whole craft.

I have less than three weeks to finish this and test it, and I still have to put the gas tank back in the tow vehicle (1990 Geo Tracker).

I'll be busy for a while.

[kmhemi]

George,
Are you going to make it to Chillocothe again this year? I might be able to bring my HC if I get it done in time. I'll be there regardless. I'll try to track you down again. You aren't hard to find.

Keith

[kach22i]

I'm doing my best to try and make it. Money, time, rain, excessive heat.........................I'm fighting it all like a M-F'er.

Link below for anyone interested in going to the National hovercraft rally in southern Ohio.

June 21, 22, 23 & 24th.
http://hoverclubofamerica.org/Events/Chillicothe/index.html

[kach22i]

UPDATE: 06/14/2007

I'm going to try and fit the skirt today, the paper/cardboard study model has helped already. I found out what I did wrong last time in the process.