The Hughes HK-1 Hercules

The birch framed HK-1 Hercules had a 320ft wingspan compared to the Antonov 225's 290ft, but the Hughes aircraft is almost 60ft shorter.

It was a shame what became of her, I mean that she never really proved herself. There would have been a lot of proud men had she done so.

I've been looking to find details of her construction, but so far haven't been too enlightened. From what I have read, she was built mainly of birch, 1/32" laminates of timber veneer were glued with epoxy, alternating grain direction as in typical plywood construction. I also read that steam was used to bend timber components, which I found surprising, as laminating wood with epoxy in my experience has never required the use of steam bending. Steam was probably only used in areas of complex curves like leading edges etc.

This is a link to an article where a person aboard the only flight the Hercules ever made gives an account through a third party.

http://www.findarticles.com/p/articles/mi_qa3897/is_200406/ai_n9455603#continue

According to that account, the "taxiing trials" turned into an impromptu flight at Hughes' whim.

Hughes was warned that there was a flap design flaw that needed correcting before flight over 130knots could be safely done.

It was a failure in the sense that it never went into production and never fulfilled it's intended role.

Howard Hughes himself is possibly to blame for this, due to his eccentricity, production was delayed beyond reason. But, who knows, maybe if he wasn't so meticulous, it may have never flown at all.

I have no doubt that the aircraft was a sound design, the US Navy wanted to fly it again in 1977 for research purposes.

Hughes continued working on the plane until his death, it was a 30 year project/hobby for him. I'll bet it was nearly perfect by the time he died.

I found the following info:

"The idea for a giant seaplane was initially championed by industrial magnate Henry Kaiser, who had masterminded the Liberty Ship construction program, which cranked out freighters in an unbelievable 48 days (record: five days). Kaiser wanted to transport war materiel overseas by air, where it would be safe from enemy torpedoes. But he knew nothing about airplane building and was happy to hook up with Hughes, who'd assembled a team of crack aeronautics engineers that, among other things, helped him design a plane that set a speed record in 1935. Despite opposition from the military and the aircraft industry, Kaiser and Hughes landed a government contract to build three prototype planes. The catch: the long-shot project could make only minimal use of strategic materials such as metals. That meant using wood, common in small aircraft but untested in one so large."


As production costs soared, Hughes absorbed the excess costs (which were substantial) rather than wait for the government to continue funding, in order to keep the project moving ahead without delay.

Many older resinous glues work just fine and are strong and durable. There was a problem with older airframes glued with cellulose based glues and one or two other types. You wouldn't expect very old airframes to stay perfectly preserved in some of the conditions the aircraft were stored in.

Epoxy was patented in 1939, a few years before construction of the HK-1 (Hughes - Kaiser - design No.1)

Most of the huge plane is actually made of birch, with only small amounts of maple, poplar, balsa, and, yes, spruce.

It's a well known engineering fact that epoxy-wood composites have superior strength/weight ratios than aluminium construction and they do not suffer stress fatigue like metal construction does.

The fact that the "Spruce Goose" was made of timber immediately conjures up images of "an old piece of junk" to most people who do not understand the importance or integrity of wood composite construction.

Anybody can easily test the virtues of this type of construction by puchasing a hobby store balsa sheet and coating it with a construction grade epoxy, (not the inferior 5 minute epoxy resin glue). You will be absolutely amazed at the transformation of a soft and supple piece of pulpy wood into a strong and durable construction material, completely impervious to rot or degradation.

I have used epoxies in wood composite construction my entire life in large model aircraft construction and boatbuilding. When I coat timber with epoxy, I know it will be around a lot longer than I will be. I know from experience that this type of construction is very labour intensive, and quality control is extremely important to the integrity of the resulting structure. There would have been very few people in the world at that time with skills using wood - epoxy composites, and it would be difficult to implement this construction method on a production line. I can easily see how the construction method alone would have hindered the HK-1 development.

As far as being under-powered? The Hercules had eight 3000 horsepower engines (24,000 HP total), each one swinging a 17 foot, 4 bladed  prop. A seaplane needs the most power when it accelerates through the water from displacement speed to semi planing speed then "over the hump" and up "onto the step" planing. Once "on the step" they easily gain airspeed and you can rotate for climbout.

Any seaplane that can get "on the step" can easily fly at reduced throttle settings.

The HK-1's useful design load was 750 soldiers or two Sherman tanks! Hughes was subsequently preoccupied with increasing the useful load!

I just read a report that said Long Beach city banned the aircraft from taking off and landing on it's harbour, and that the aircraft was actually confiscated by the authorities for Hughes' breach of this ruling by his impromptu flight! (I wonder what the fine was............ )

Epoxy was used on the Hercules, and why wouldn't it have been? Epoxy resin was the latest thing and the innovative Hughes actually bought the company that had patented the construction method used.

Until I started researching the Spruce Goose, I knew of the Glasfugel Libelle, which was a popular and record breaking sailplane with many fine examples still flying today, (a friend of mine who is visiting at the moment owns a record breaking Libelle). The Libelle is constructed of epoxy/glass over end grain balsa sandwich as were a few designs of the period, (1960's).



Urea formaldehyde glue was popularly marketed as Resourcinol and it was the of critical construction adhesive of choice for aircraft and boatbuilding uses where high strength is required. Resourcinol's disadvantages were that it had no gap filling properties and required perfect joinery work and surface preparation to bond properly. Epoxy has good gap filling properties and considerably less skill is required to achieve a good bond.

Timber/epoxy still represents the contruction method of choice for many high strength applications, with exotic materials such as carbon fibre and kevlar often being substituted or used in conjuction with wood in critical high strength/weight ratio applications.

That's probably what actually happened while constructing this airplane, Hughes was a known eccentric, imagine it, he was pouring heaps of his own money into the project, it was his model plane, he wanted it perfect.

Cold moulded is epoxy composite construction Woodlouse.

If an epoxy boat is falling apart it's not because of the epoxy, but rather other factors of design loading or lack of maintenance (epoxy is UV sensitive and must be kept painted if exposed to sunlight).

Here is an example of the durability of epoxy:

The Australian Gliding Federation was concerned about the aging Australian glider fleet. Most sailplanes were imported into Australia in the 1970's and were epoxy composite construction. These aircraft have been used privately and in clubs for 4 decades. No prior research had been done on the fatigue and possible lifespan of the material and it's implications on air safety. A matter of some realistic concern!

A common epoxy composite 2 seat sailplane, I believe it was a Janus, was set upon a specially designed stress testing rig at the Melbourne Institute of Technology.

The aircraft was shaken violently up and down for months, I believe several repairs had to be made to the test rig during the trials. Eventually, a small stress crack appeared near the spar root I believe, (I just can't remember the details accurately), and the cause was a  design flaw which resulted in an AD being issued on the type. The failure occured at a simulated 80,000 hours or some other ridiculously high number from memory.

Further testing was carried out for several months after the spar was reparied, and the test rig eventually broke down.

The result was inconclusive as far as the possible lifespan of epoxy composite contruction, the tests showed that hundreds of thousands of hours is probable.

As a result, the epoxy gliders in australia had their Certificates of Airworthiness extended INDEFINITELY subject to 20 year renewels and annual inspections, with the approval of Australia's Civil Aviation Safety Authority, one of the strictest safety organisations in the world, and responsible for Australia's outstanding air safety record.

How about that?  

This might have been pointed out earlier in the thread but I think the main advantage of epoxy resin adhesives over traditional glues is their gap filling properties. A perfectly formed joint using older conventional wood glues can be just as strong if not stronger than with one of the modern adhesives.  Most modern boats & glider wings using this form of laminated construction also use a glass fibre cloth covering to seal it from the elements. The glass fibre is bonded to the wood & becomes part of the whole structure making it extremely strong for its weight. I don't know when glass fibre cloth was first invented or how long it's been used for this purpose.

An extra consideration with boats or water planes is water saturation which will affect most forms of construction over a period of time to varying degrees. While epoxy resin is water resistant the wood isn't so unless the epoxy is bonded properly with the wood, deterioration will take place over a period of time. This would not have been considered so important during WWII as most aircraft only had a limited service life.

The question is a little unfair Woodlouse, considering the fact that production has only been happening for about 40 years on aircraft, and less on boats. (with the exception of some experimental craft).

I don't need converting, I've owned a traditional wooden vessel myself for twenty years, there's not a drop of epoxy in it. But I've also used epoxy extensively and I know it's strengths and weaknesses.

I really am only just realising that epoxy was around for quite a long time, and I was amazed at the spruce goose using the material. Until just recently, the oldest use I had heard of was in early FRP glider construction, around mid to late fifties, and I have seen those gliders still in regular service here in Australia.

As I said before, problems associated with epoxy construction are design and maintenance related, (though I should have included quality control in this list).

You will find the epoxy sailplane manuals state that ALL surfaces are to be painted WHITE and that a silicone free wax polish be applied at all times. Now, if someone paints the elevator blue or red, then he shouldn't be surprised when it starts to de-laminate. Silicones in polish penetrate the paint/gelcoat finish and actually permeate the laminate, this is dangerous, insidiously so, because if at some stage a repair is required, the repaired structure could have bond failure due to the introduced silicone.

If paint is allowed to wear thin, or colors are used, UV light breaks down the epoxy in no time, severely weaking the structure. These problems are known are they are merely a maintenance issue.

Quality control is important, and as an example I'll cite a well known problem in FRP boat manufacturing. I also have been wondering if this problem is what you may be mistakenly thinking of when you are bagging the use of epoxy.

FRP (Fibreglass Reinforced Plastic) boat construction predominately uses Polyester resins and not epoxies, Polyesters are cheaper and relatively easier for novices to use, (without critical mixing ratios).  Most FRP boats are built in production lines using un-skilled and semi-skilled labour, they get shown briefly how to do a glass layup and off they go building boats, often thinking of things back on the farm or whatever.............

A result of this practice is the phenomenon known as OSMOSIS in FRP boats, where the boat deteriorates at the glass laminate, rapidly and catastrophically. Without going into the exact bio-chemical process of osmosis in boats, suffice to say that inadequate care in the construction process allowed trapped air and unsaturated glass in the laminates, which allowed the osmosis to begin. As far as I'm aware this does not occur in epoxy constructed laminates, it is a product of polyester resins and improper layups. (Especially those boats built with or partially built with chopper gun layups). The chopper gun operator is semi-skilled, the guy rolling out the chopped glass laminate is usually unskilled and brain-dead from the fumes (wax in styrene added to the resin), or soon will be, and he really just wants to get away from the acetone so he can light a cigarette.

Now, I have seen such zombies clearly state that smoking around acetone is no big deal and to prove this fact, he promptly threw a lit cigarette into a bucket of pure acetone beside me, the cigarette fortunately extinguished itself, much to my bemusement at the time.

Now, consider the aircraft factory, where absolute precision manufacturing is being carried out with epoxy composites, I watched a friend of mine carefully and precisely make a laminate at the Jabiru factory in Bundaberg, the next day, the laminate was measured, weighed, and promptly discarded, again much to my amazement, as it was the most precise glass laminate I ever witnessed.    

Well, I guess epoxy in aircraft construction could be likened to aluminium in aircraft construction with the advent of WW2 fighter planes before alloy was 50 years old. Except perhaps that alloy has a known and relatively short fatigue life.