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?