Home Content About Contact
Yellow Jacket Hovercraft - Kirk's Story
My latest project, (the one that distracted me from the Yellow Jacket...)

My first discovery of the Yellow Jacket design

In 1991, while searching for that "just right" hovercraft design I was destined to build, I discovered the Yellow Jacket on microfilm in a (Popular Mechanics Magazine article) in the Corvallis, Oregon public library. Unfortunately, the year, month etc., of the magazine issue was not included on the incomplete article image. I had to live off the low quality, microfilm print for some time.

Here's a Yellow Jacket "like" Frame design I started based on the glimpses of the Yellow Jacket I had from the microfilm library article. I was brain storming... that attitude adjustments and mild banking might be possible with the horizontal stabilizer/aileron like wing at higher speeds. Also, shooting to be more aerodynamically more stable than that of a round nosed craft (less prone to an aerodynamic stall). -other problems reside... I began to understand why most recreational hovercraft take the shape and form which they commonly do.

My 1992 trip to see the original "Yellow Jacket" featured in the article for the first time.

I called Popular Mechanics and talked to the fellow who wrote the artical on the Yellow Jacket, and from him I found the name of the designer and that he was a helicopter designer who at least used to work for Sikorsky. They managed to give me his current wherebouts, and I went for a trip to talk with him. On arrival, I took videotape, measurements & talked design/performance improvement ideas. I brought prints of 2D and 3D images to express my concepts. Yellow Jacket was a shocking, rusting, rotting mess with the engines stolen and the paint a peelin. Below is picture of the Yellow Jacket which I took on that trip. Eugene said it had been parked for 13 years (at that time).
Snapshot of the Yellow Jacket where it sat for years
Whew, so much for taking a ride.

Eugene had other interesting projects going on, including a helicopter kit. He also designed and sold propane jet engines as kits or assembled.

Collecting statistics/Specifications for design alterations to build my own.

i.e.. prop pitch, foil, sizes, weights, etc.,.

I'm linking this part of the story to my "statistics/Specifications" information because you probably don't want to sift it out of my story telling style. But I thought I'd note that the airfoil appeared incredibly complex when I first observed it in person during this 1994 trip. Knowing that a helicopter designer put this thing together, I felt very small in the way of knowledge. I hesitated, then asked naively "why does the trailing edge swoop sharply upward over the rear 1/3rd of the foil?". He laughed..., so I laughed as though I knew why. He then replied "Because it's so badly warped!. In Eugene's words "It's supposed to be a standard Clark Y airfoil".

Some of my designs considered, BASED on the Yellow Jacket design:

Composites, wood, aluminum, steel, sheet, combo's of all the above.
*Steel frame design (29kb)
Notes: Modeled with a 25hp marine engine.

*Foam and composite Hull design (45kb)

Really starting my hovercraft build:

During a business trip to San Diego, I drove up to Eugene's place with a friend to take some last minute measurements from the hovercraft, prior to starting my own. I thought to myself "Why not make an offer on the old hull?". I did, we haggled, we laughed, I bought it.

How in the world am I going to get this thing home? -to Corvallis Oregon!:

Now I'm frantically trying to find a friend in San Diego who will keep the hull for me until I can make it back to pick it up. But then, I thought perhaps I could ship it North. I called around and got quotes from $400 to $500. Wow! I had never shipped anything like this before. Finally I spoke with Reddaway, who's nearest shipping dock was in Fontana, closer to L.A. than Hesperia, where I'd picked up the hull. They indicated the other quotes were probably considering crating up. Reddaway said they could "Top Load" it, therefore not requiring crating. Below is a photo of the Yellow Jacket on it's side in the truck (at about 10pm) as we began to unload it at Reddaway's Fontana shipping dock in March 1994.
Snapshot of tattered Yellow Jacket in truck
When my friend and I unloaded the hovercraft onto their shipping dock however, they became incredibly nervous, afraid that I may accuse them of the damaged appearance of the Yellow Jacket. I reassured them I wouldn't and reluctantly, they took Polaroid's and shipped it to Albany, Oregon for me charging only $120.
Thank you Reddaway!

Home at last!:

Images of the Yellow Jacket, tired & worn, but IN MY GARAGE!
Show's what a mess it was during disassembly.

The photo above shows the skirt, falling apart from age and more than likely an overdose of fun over the years. Notice the steel bar sewn into the bottom of the skirt. More on this later.

Can you say... RUST! This photo illustrates the condition of the frame in some portions where foam was previously used in the hull for water flotation. It has trapped water and contributed to the corrosion. The rusty parts on the floor were dangling from the aluminum skin edge, also quite corroded. I'm sure electrolysis (contact between dissimilar metals) is partly to blame too. Digging out the foam was NOT a fun job. I'll be sure to find a better way to do this.

This is the prop shaft removed, with Torrington needle bearings seized to the shaft due to corrosion.

Making that list of "Oh, I never thought of that" stuff, to ask Eugene:

Composed a 12 page letter & mailed it off. Eugene's daughter received my letter and called me -- Eugene had passed on three months after I obtained the hull. An incredible loss of many things. Though I'm proud to share the Yellow Jacket dream.

Information about EMG Engineering products like Jet Engines, Kit helicopter, etc. are now available through Robert Q. Riley's website

Progress in Strip Down & Rebuild

A June 1995 rear view, with the puff ports and back wall removed. Taken right after I sandblasted the hull & frame.

A June 1995 image with some annotation of work to be done at that time.

Deciding go with a closed canopy means work...

I have built a fiberglass cowl that encloses the pilot and engine. Check out these pic's.

Let me start by saying I'm no pro with this kind of stuff. I believe you can learn the most by asking around, until you feel comfortable taking it on. When you're ready to do any fiber glass work, hopefully in asking around you found someone who buys resin and fabric bulk, so you don't pay a ridiculous price for it. The things I have done as described below, was on my own, but not without being coached from friends who have been there. For the mold and two bodies described below, I used 2 gallons of resin and about 40-50 square feet of mat and cloth.
I know you may have already seen this, but just in case, check out the image of the electric car body compared to my hovercraft body . The body on the car has a positive draft and is quite simple as compared to the contours of the one I ended up with. The body on the car is made of ABS plastic. Not good after much sun (w/out paint), not good considering the vibrations and stress a hovercraft may dish out and not good for heat as it is a low temp thermoplastic. The Canopy bubble itself is identical. The unmodified ABS parts are made by : Mark Murphy at Blue Sky Design. This link will get you to his Phone, mailing address, etc.

Assuming you have seen those two pages, here is an over view:

The overall plan was to modify the simple ABS body to mate smoothly to the Yellow Jacket's surface while maintaining the upper contour (as is) to mate with the incredibly cool looking bubble.

Then, to make a fiberglass mold off the ABS "plug" so I could produce a stronger, more robust body, i.e.: heat, vibration, UV. Not to mention the fact that I can pop out more body's from the mold if needed. And I did need to...

A more detailed explanation:
I bought an ABS body (top shell only) from Mark, cut it 's base off. About 14" from the bottom of nose, upward. Tapering to 0"(cut) in the rear, because the pilot's cockpit is sunk into the hull of my craft. The rear is intended to cowl the cooling air which flows over the air-cooled engine, so heat resistance and proper clearance (why rear is raised more than the front) is important. I then took the rear base of the body (which is over the fans) and used duct tape to hold the left and right sides in together (about 7" wide at the narrowest spot). This is significantly narrower than the normal width of the ABS body (approx. 24 inches). Next, a bit of crude trimming to get a reasonable fit to the (far form imperfect or symmetrical) surface of the Yellow Jacket. Once it generally looks good, the next step I took was to begin the blend to smoothly transition the ABS body to the aluminum hull. I wanted to add some tapering blends ranging from none (in the duct area) to ~ 2" and larger radii, most obvious just in front of the ducts and all the way forward to the nose and back to the duct on the other side. This was done with spray foam, then hand carved. I started this by laying a $2.00 sheet of thin mylar across the YJ hull and then laying the body on top of it. This way the foam will settle to the contour of the YJ (but not stick!) while it cures against the ABS body too . Next, I used strips of 2" dry wall repair tape (like a net w/ 1/8" spacing) to create a sunken) blend in the really big gaps that would otherwise be filled inches deep of foam. This tape supported the foam well when sprayed over it. Next I sprayed polyurethane foam (from a can) to roughly add enough material all around the edges to later carve the desired blends from. The first surprise was how much foam it took. I used about 7 cans, (~40.00) and I thought this was a fairly small job, only running a perimeter of about 16 feet. The next surprise was discovering the foam that comes in a can, does not have a catalyst (or it would cure in the can) so it more or less air dries. Which means the thick spots (greater than 1.5") took longer to cure and was more dense, making it a challenge to carve. I'd recommend adding tape, wood or anything to fill areas requiring foam, keeping the foam layer to 3/4" or less where possible. This is also assuming were talking about a plug for mold purposes, not a functional body. I don't know how you could do the job with a 2-part (catalyst type) foam and still direct the foam into a crevasse where you want your blends, but I'd give it consideration if I had to do it again.

On to the next step, carving. I found the best way to do the first pass, rough blends was to use a drill (I used an air drill).

I used different radii of wire wheels in the chuck. They rip through the foam like it was dust, conveniently in radius's. It raises a lot of it into the air too. Once close (but over sized) I used a body working tool called ?. It is common, 2-3 inches wide and about 20" long. and flat. It LOOKS some what like a hand plainer that is normally used on wood. It worked very well for most of the blends I needed. Always keep moving you sanding tool in random directions. Much of the inside (bowl like) blends however, were carved with sand paper wrapped around similar shapes I could find laying around the garage. If you haven't done any sculpting or body work, the time it takes is a function of your learning curve, multiplied by 10. I believe one can get educated on this, but the rest is art. I needed to reapply more foam where I goofed and to fill some thumbnail sized bubble holes here and there but at this point didn't worry about anything smaller. Watch out, the density of the new foam is usually a bit different than the foam in the previous layer. Don't worry too much about feathering the foam into the ABS either.

After the foam job is roughly the way I wanted the final pass to be (or slightly smaller). I Began the Bondo (body filler) work. This was the surface which was to be near perfect in general shape(opinion). I applied it with a typical, cheap plastic Bondo applicator. If you haven't used Bondo before, don't worry. Read the directions and mix it well prior to starting each batch. Oh, and if your job is compatible to mine, buy 1 gallon. I applied it just thick enough to cover the foam leaving a 1/16th to 1/8th typical thickness. Of course some gaps and bubbles required more. I sanded, using 40 grit and contour appropriate tools. More Bondo on the shallow spots, sand, apply, sand, apply, sand... You get the idea. As the large dips and woweee's became satisfactory, I moved to finer sand paper until I got to surface scratches of 80 grit only. (No 40 grit scratches).

Once it was perfect (the definition evolved as I burned out), I was ready to spray on the primer and fill those 80 grit scratches. I carefully moved the mold plug/body as not to crack it, (but did anyway) to a blocked up position for final primer and mold lay-up.

I used polyurethane primer first because other paints may attack the ABS. But don't use it out of a rattle can. Out of the spray can polyurethane paint never seems to cure, again no catalyst in this type. Spray it from a cheap paint gun if you have to, you'll be sanding in anyway.

In my case, I sprayed it with 3 thick coats. Let it cure well, then found it quite nice to sand. It filled all the 80 grit scratch marks well and even many of those 40 grit scratches I missed. 2 more coats and a light scuff and I was ready to squirt it with polyester primer.

The idea of the polyester primer was to protect from out-gassing that might occur during the lay-up. Again, a light sanding with 120 grit, and it was ready for the next step, waxing.

The wax had the consistency of bar soap. Rubbing it in shined up the surface and even filled some oversights nicely. Much like polishing a car.

Next, was spraying on the mold release. A water soluble, alcohol based solution which looks just like Palm Olive liquid dish washing soap. I learned the hard way that you must dust on the first coat, or it does some strange ugly things. I had to wash it off with a sponge and water exclaiming words I'd rather not write. After dusting on the first coat however, and the second and the third... the fourth thin coat left me an appealing outer surface which was to become the inner surface of my mold.

I brushed on the inner surface of the mold, the gel coat. It was recommended to me to do a gel coat surface because otherwise, the resin shrink that occurs is will reveal the cloth pattern in the surface contour. A gel coat (just resin with pigment -no cloth or fiber) only surface is allowed to mostly cure prior to adding the first layer of cloth or mat. Next I used 2 layers of random mat to make the mold surface laid up on the entire surface (one at a time). I did it in manageable resin batch sizes, mixed to cure in approx. 20 minutes, I was able to thoroughly soak the mat with resin for an area of about 4 square feet before the resin began to cure. Be patient. Ask your body shop dealer for a couple of tools appropriate for your job. I don't know what they're called but they are aluminum rollers of different diameters with small ridges and are used for working the resin into the fabric and the air bubbles out. If you have more time than money, a paint brush or two will do (appropriate sizes for your job). Using only brushes in other jobs, I found it best to slop the resin where you want it, then poke and dab to work out the bubbles. It works, it's just a bit slower.

After the surface was covered with 2 layers cured, it was time to add some wood. Not for structure, the mold was pretty stout with the 2 layers. So why the wood then?, By curving the bases of the ABS body in for the fan duct radii, I created negative draft angles, so the part eventually made by this mold I was making would not just pull out of the open end. I planned to split the mold down a centerline front to rear, allowing a mold half on the left and right sides to exit outward off a potential plug and parts. This meant the mold needed a mating face on the centerline in which to bolt together. I added enough random mat strips to build up some flat surfaces along the centerline to be split (parting line) and laid pieces of ~12"x 2"x 2" wood along each flat spot, wrapping mat sheets up the sides from the mold surface to permanently adhere them to the mold. Once that was done, I drilled holes through them so that after being cut on the parting line (later) they would have a bolt pattern to align and clamp them together as one. Next I used an electric grinder with a flexible grinding disk to trim the excess fiberglass and resin which dripped off the base while I was doing the lay-up. Then, finally, I used an orbital saw (only to get the depth required) to cut the wood, mold and yes, that plug I spent hours on, down the parting line. For most cases a skill saw would be better and more likely to produce a straight line.

Excited, edges trimmed and parting line cut, I squirted a garden hose between the exposed edges of the left mold and plug. In minuets, the water had dissolved the mold release and was forcing it's way between the mold and plug. With a bit of persuasion, the plug fell out of the mold half. The other side went the same. The plug ejected, I put the mold halves together with the bolts in the holes previously drilled and I had a parting line that was roughly 0" to 1/16" in the worst case. Success!

Whew!, That was a couple of weeks worth of evenings. And still no part.

I wanted the body to be light, so after viewing other similar light structures, such as canoes, I thought I had the solution. I used a strip of scotch tape to prevent the parting line from filling with resin during the body lay-up. Then, after waxing the mold and spraying it with mold release as was done for the plug/mold lay-up described above, I put on the gel coat and one layer of 3oz cloth against the mold. Then for structure, I laid strips of 1/4" x 1/4" foam tape running top to bottom spaced about every 12". I laid a strip of random mat (thicker than cloth) about 2" wide and as long as the foam tape, over the foam tape to produce an air filled tube shape. This worked great!

My part weighed less than 5 pounds. It was questionable however if it was strong enough to take the vibrations. It took me the better part of a week end to do the part lay-up. About one week later, the difference in thickness' of the walls due to the structural tubes I added, caused uneven shrinkage to occur and added undesirable ripples in the thin walled sections between them. Yes I was depressed, but glad I had a mold :-)

I made another body, using a single layer of mat, which provided ridged walls, will require minimal structural additions and still only weighs less than10 lb. (Shell only). It will require some structural walls inside, but they will double as cooling air ducting for the engine and a separation wall between the engine compartment and the driver.

I added wood blocks into the fiberglass body at points where it is to be mounted to the YJ hull. This allows the stress to be distributed more equally over the body piece.

That's it for now on the body and canopy.

Forging ahead, more progress...

This is a picture from September, 95, showing the assembled fans in place. I have started the engine and tacked it up a bit. Still need to rebuild the rear wall, puff ports, belt tensioners and put the skirt on to be ready for a hover test. Well, maybe not the skirt... In addition, a great deal of cosmetic work lies ahead before a practical hovercraft ride is possible.

Starting up the Yellow Jacket for the first time in decades?

At least 15 years, was an exciting moment...

A new engine where there once was two, new enclosure, drive system and fans too. I think it's more like a mutant than a rebirth. At this moment I could relate to Dr. Frankenstein. The clutches smoothly engage at 2000 rpm. At an idle, the props remain at a stop. The "V" belt's 90 degree twist from the horizontal drive axis (v belt go-kart clutches) to the vertical prop shafts seem to handle low prop rpm's just fine (this was pushing the rules according to the book with my center to center distances and pulley diameters). The backside belt tensioners seem to bounce a bit due to the 2 stroke engines rough idle, but smooth out after gaining a little speed. With the prop rpm of aprox. 1000, the carpet, loose rags, etc., are blowing out into my driveway, I decided this is not the place to test higher rpm's.

Been side tracked for too long, getting back to the Yellow Jacket project...

Side tracked by (fun) Electrathon activity

I've decided to try and get hovering by the July 97 West Coast Hover-Inn. At the end of April (97) I summarized what was left to do:
  • Determine and build maneuvering method: Puff ports, deflection vanes or just leaning.
  • Wiring: kill switch('s), gauges, etc. -Done 5/97
  • Fab rear wall: currently open where puff ports once existed. -temp wall done 5/97
  • Engine: already mounted and operational, needs adequate cooling airflow duct work.
  • Cowl completion: Structural support where needed, design & build.
  • Canopy attach and opening method.
  • Interior: Design & mount seat, carpet?, gauges, etc.
  • Fuel Tank: Mount & plumb w/filter and accessible shut off valve, vent to air filter intake, accessible filling. -Done 5/97
  • Cosmetics: Body work, paint, trim? etc.
  • 5/97 added tuned exhaust pipe to this list. Faxing Dyno-Port.
  • Fab & mount skirt
  • Land surface testing & tuning
  • Design, add & test floatation method.

May 97 progress report: Hover ready?...

I bought one of those aluminum tanks you see in the dune buggy or hot rod magazines. Very light and simple to mount. Done!
Carb is all that hangs out. Cable and fuel have since been routed internally.
Minimal clearance required for exhaust.

Ignition switches are wired and I've built a temporary rear wall which completes the hull seal with exception to a skirt. Next, I'll attempt a skirtless hover to confirm that my neutral C.G. is where I want it to be. I'll post pictures here. -Need tuned pipe first.

PS Current all-up-weight (with out canopy) is currently 218 lb with 1/2 a tank of fuel on board. The canopy and mounting hardware is expected to weigh approx. 20 lbs.

Engine off, I'm sitting in the position which, if I planned my center of gravity correctly, I will be at a neutral or slightly reversed hover.

1st run up test. I'm thinking to myself WOW! those fans seem to be moving fast and that I should NOT have a hat on. The hull is sitting on it's wheels, not hovering. Test proved I need a tuned pipe and perhaps less pitch. I'll try the tuned pipe first...

-4/98 update:
* I haven't done much all winter with the Yellow Jacket.
* A friend is going to assist in making my skirt.
* Having trouble with the rope starter reliability -hope to add simple electric start but I must fabricate it.

Another piece of history, PM plans donated to me

On February 10th, at 6:14pm, I got the email I've waited for since 1996, when I started this website. Here it is:
	Hi Kirk,
		I have had the plans and magazine in a folder, in my closet for the last 		
		twenty some years and I'm not sure why.  If you are willing to pay the 		
		postage, you can have it all.  I might ask that you e-mail me a picture 		
		of your progress once in a while.  I'll get my pleasure vicariously.  $3		
		or $4 should cover the postage.		
I've tried many sources for these plans, not because I need them, but because they are an important piece of the Yellow Jacket history.

Thank You John!