By Tommy H. Thomason

Wednesday, December 29, 2010

And Now For Something Completely Different

The Coast Guard was responsible for helicopter development during World War II. One of their concerns was the rescue of the crew from a ship that had been sunk by German submarines off the U.S. east coast. Existing helicopters were too small to carry more than one or two rescuees. The Coast Guard wanted one that could carry eight in addition to a crew of two.

At the time, based on experience with autogiros, it was believed that the empty weight of a single rotor helicopter as a percentage of gross weight would increase with rotor diameter so quickly that large single rotor helicopters would have no payload capability. Frank Piasecki, a young engineer who had just flown his first helicopter, convinced the Coast Guard and the Navy that the answer was a tandem-rotor helicopter. He got a contract for a full-scale demonstrator in January 1944.

The first flight of what was designated the XHRP-X, Bureau Number (BuNo) 37968, was made in March 1945, little more than a year after go-ahead. It was powered by a 450-hp Continental R-975 radial engine and subsequently referred to as the Dogship at Piasecki. Unique features in addition to the tandem rotors were the castering wheels intended to minimize side loads in a touchdown with sideward motion; the pilot sitting aft of the front pylon (the copilot sat directly behind him); and the bottom of the forward fuselage being "skinned" with clear plexiglass panels for downward vision. Unlike subsequent tandem-rotor helicopters, the rotors did not overlap.
The XHRP-X was featured in this late 1945 newsreel as the "World's Largest Helicopter!". Following its successful development and demonstration, Piasecki received approval to build a Static-Dynamic Test Article for qualification testing of the drive and rotor systems. It did not receive a BuNo and the fuselage was never covered with fabric.  It was powered by a 600-hp engine P&W R-1340 engine.

 The XHRP-1, BuNo 37969, was also authorized.

 Its first flight was accomplished in March 1947. It initially had a aft fuselage without as much side area as the XHRP-X, possibly to reduce weight. Stability problems resulted in the addition of a small horizontal stabilizer and vertical fins.

Piasecki also received a production order for 10 HRP-1s in June 1946 and another 10 in April 1947.

The first production HRP-1 flew in September 1947 and was delivered later that year. The last of the total order of 20 was delivered in 1949. These were initially used by the Marine Corps to develop tactics for vertical assault and by the Coast Guard for its ongoing helicopter search and rescue development.
Five of the HRPs initially assigned to the Marines Corps were then used by the Navy for the development of dipping sonar and airborne minesweeping. The fabric was taken off to reduce weight (testing was done off Key West, Florida) and flotation bags were added so the helicopter could be recovered if its engine quit over the water.

Eventually a portion of the small fleet was acquired by civil operators from military surplus.

The Dogship is currently stored at the Smithsonian's Paul E. Garber Preservation, Restoration, and Storage Facility located at Silver Hill, Maryland.

The first production HRP-1, BuNo 111809, that was at the New England Air Museum is currently being restored at Piasecki Aircraft Corporation in Essington, Pennsylvania for eventual display at the American Helicopter Museum in West Chester, Pennsylvania.

The HRP-2 utilized the same drive train and engine as the HRP-1 in an all-metal fuselage. The pilot and copilot now set side by side ahead of the front rotor mast. First flight was on 10 November 1949. Only five were produced. They were initially used by the Marines along with the HRP-1s and then all of the -2s were assigned to the Coast Guard. It was clear that they were underpowered with the same 600-hp engine installed in the HRP-1.

At least one surplus HRP-2 was procured and operated by Rick Helicopters, then the largest civil operator of rotorcraft, in the 1950s.

The Air Force and subsequently the Army recognized the value of the basic design for their emerging helicopter missions, however, and ordered modified versions of the HRP-2 as the H-21 with more powerful Wright engines with up to 1,425 horsepower.

Friday, December 10, 2010

The Davis Barrier, One More Time

For background and other information on the Davis barrier, see the following entries.
22 September 2008
1 October 2008
26 September 2009
4 October 2010
One of these days, I'll combine all this into one entry.

The FJ-2 and -3 Furies had a retractable barrier pickup device located aft of the nose gear wheel well. This insured that the activated barrier cable did not fall back down before it engaged the airplane's main landing gear struts. (It was subsequently removed from at least the -3s when the angled deck eliminated the need for the Davis barrier.)
There is a similar, albeit non-retractable, device on the belly of the Grumman S2F. I had assumed that it performed the same function, but it turns out that it corrected a different problem related to the Davis barrier function.

The initial S2F Davis barrier qualification at Naval Air Material Center (NAMC) had actually been accomplished with an F7F that had been modified with tubing shaped to simulate the S2F-1 belly. Surplus F7Fs were plentiful and surplus S2Fs, not. Moreover, in the interest of minimizing cost, nose-high, off-center, and lifter strap run-down by a dual nose wheel landing gear (the F7F had a single nose wheel) were not evaluated.

In early 1954, during one of the first S2F squadron carrier qualification periods aboard Siboney (CVE-112), a pilot failed to lower his tail hook, which wasn’t noticed by anyone on the LSO platform as it should have been. This guaranteed a trip into the barriers. The airplane also engaged nose high, which didn’t help matters. Things went badly after that. The cables didn't engage the left main landing gear as they should have. The nose gear failed when the airplane pitched down and the right wing was torn off when the airplane was yanked violently around by the engagement of only one landing gear strut by the barrier cable.

S2F-1 BuNo 129146 was therefore assigned to NAMC to provide a more representative test article for a series of more comprehensive tests. It was catapulted, unmanned, into a Davis barrier at speeds from 35 to 70 knots in five-knot increments. Normal, nose-high, and nose-low engagements were made as well as at 17 feet off of centerline. A total of 129 shots were accomplished between 30 May and 23 November 1954 to develop a configuration that provided a higher likelihood of a benign barrier encounter.

One of the problems encountered was that on a nose gear with two wheels, a lifter strap could conceivably be caught between them, preventing the lifting of the barrier cables. The S2F also had a relatively short wheelbase and a wide tread, which meant the cable might not always rise high enough across the width of the tread in time to engage both landing gear struts. If only one strut was engaged, a violent yaw resulted as it had in the Siboney incident.

S2F changes established during the testing included the addition of four pairs of small detents on a strengthened nose wheel door so the actuator strap would not slip down, affecting the rise of the barrier cable. The nose gear strut torque arms were modified and the tow fitting between the wheels was extended forward to reduce the likelihood that a lifter strap would be trapped between the nose wheels. A slanted fairing was also added ahead of the catapult hook so there was less likelihood that the cable would be deflected downward and not engage the main gear struts.

Most importantly, a cable scoop, the “Fosdick,” was added on the bottom of the fuselage. Unlike the one on the FJ-2/3, it was not just there to keep the barrier cable up off the deck. It was primarily there to reduce the drag loads of the barrier cables on the landing gear and the violent yaw that would result if only one main landing gear was engaged by the cables, assuming that the cable scoop was engaged as well.

Here is an early S2F, flying with one engine shut down and the propeller feathered. Note that the only protuberances on the belly ahead of the retractable radar dome are two radio antennas and the catapult hook.

This is the belly of the S2F after the addition of the detents on the nose gear door, the fairing ahead of the catapult hook, and the cable scoop.

The existing main landing gear "scoops" were also modified to be more effective at diverting the cables onto the struts.

Steve Ginter is working on an S2F-1 monograph for publication in his Naval Fighters series.

Wednesday, December 8, 2010

Once Upon A Time

I rediscovered an interesting report in the Vought archives this week, Notes on Comparison of Carrier and Land-based Fighter Airplanes Incorporating Folding Wingtips dated 21 March 1952 and authored by John H. Quinn Jr. I had previously copied it from the George Spangenberg collection in the National Archives but hadn't taken the time to examine it closely.

The folding wingtips aspect of it was only of passing interest to me because it was never incorporated on a produced design, at least not for the original purpose, which was to provide a high aspect ratio wing for cruise flight with the wingtips lowered and a low aspect ratio wing for combat maneuvering with the wing tips raised as shown in the following artists concept.
Vought submitted an unsolicited informal proposal to the Navy for the V-381 which incorporated the feature in September 1952. The Navy passed, having already given Douglas a contract for the A4D Skyhawk.

The report, however, addressed a more pressing issue, which was the Navy's need to achieve performance parity with land-based jet fighters. Three predesigns were accomplished for the study: a carrier-based airplane designed to the existing limitations, a land-based airplane, and a carrier-based airplane taking advantage of relaxation of the existing limitations.

"It was assumed the airplanes were powered with a single J67W-1 engine... and were designed to combat radius of 600 nautical miles, approximately." The J67 was the designation of the Curtiss-Wright license-built Bristol Aero Engines Olympus; it was projected to provide 21,500 lbs of thrust. The mission radius was, of course, selected to highlight the cruise benefit of the folding wingtip feature.

At the time, carrier airplanes had to be designed for launch from the existing hydraulic catapults and recovery using the Mk 7 arresting gear; tactical airplanes, as opposed to the big nuclear bombers, could not be any longer than 56 feet (for straight spotting on Essex-class elevators) or taller than 17 feet and more than 24.4* feet wide when folded. (The folded height and width were Essex-class hangar deck constraints.)

Although there was no dimensional restriction on the land-based airplane, it was required to operate from a 5,000-ft runway with the takeoff roll not to exceed 3,000 feet. (Sounds short, but according to the F-100A Standard Aircraft Characteristics chart, it had a ground roll of 2,970 feet at its maximum gross weight of 29,000 lbs.) It was also required to have a combat ceiling of 55,000 feet as another constraint on wing loading. (The F-100A was bit short on that and woefully short of the range, not having folding wingtips.)

Because of the lift and size limitations imposed by the existing carrier-basing ground rules, the first carrier-based design was predicted to be 200 knots slower than the land-based one at 35,000 feet, about Mach 1.7 and 2.0 respectively. Analysis indicated that only two of the carrier-basing imposed constraints, takeoff wing loading and overall length, were the cause of the difference. The newly invented steam catapult was projected to eliminate the wing loading penalty; diagonal spotting was suggested to allow for more length and provide a better fineness ratio for less drag. The result was parity of performance with land-based airplanes. (It was recognized that the longer airplane would result in fewer being accommodated aboard, a shortcoming for carrier-basing.)

Navy Length and Wing Area Constrained

Air Force Land-Based

Navy Diagonal Spotting and High Wing Loading

As it happened, the F8U-1 achieved parity (even superiority) with the F-100, also powered by the P&W J57, based on the benefit of the steam catapult alone. (It was just within the 56-foot length limit.) The F8U-3 exceeded 56 feet in length by a little less than three feet but it had Mach 2 performance in part due to the rediscovery of the area rule by Whitcomb in 1952; it too had performance parity with the land-based J75-powered fighters.

*The maximum folded width was in the process of being increased to 27.5 feet, in part perhaps to allow the Douglas A4D Skyhawk to go below without having to fold its wings.