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.

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.

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.

F-111B Colossal Weight Improvement Program

This assessment is based on a 1/50th scale model in the Grumman History Archives on Long Island and a three-page summary of Grumman's CWIP (Colossal Weight Improvement Program) study provided to General Dynamics.

The empty weight proposed for the F-111 was even more optimistic than usual in winner-take-all paper competitions. As is customary, Grumman and General Dynamics initiated a two-pronged F-111B weight reduction study effort, the Super Weight Improvement Program and the Colossal Weight Improvement Program, even before first flight. Roughly speaking, the ground rules for the SWIP were to reduce the weight but not significantly depart from the design and mission requirements. The CWIP allowed a great deal more flexibility, basically tossing out anything imposed only by the Air Force low-altitude strike mission and preferences like the crew escape capsule.

For the CWIP configuration, Grumman engineers deleted the bomb bay and escape capsule and reduced the volume required for the main landing gear by not allowing for the large high-flotation tires required for operation from unprepared fields. That enabled them to shorten the forward fuselage by about five feet. The shorter forward fuselage presumably allowed them to delete the ventral fins, with the original vertical fin now adequate for directional stability even at high angles of attack. However, the horizontal stabilizers were slightly increased in size, presumably for improved low-speed handling qualities for the carrier approach.

All six Phoenix missiles were now carried on the fuselage, four semi-submerged and two on short pylons on the lower sides of the fuselage. This arrangement eliminated both the wing pylons and swivel mechanisms required to keep the missiles aligned when the wings were swept. I haven't yet found any information on the main landing gear configuration change required by putting two missiles on the centerline of the belly, but presumably it resembled that on either the Grumman F11F Tiger or the North American A3J Vigilante.

The center fuselage with the engine inlets and wing mounting structure were basically unchanged except for the main landing gear bay. The wings were also unchanged. The engines appear to have been moved forward by about two feet to restore the center of gravity after the nose was shortened.

Although the canopy appears to be bulged upwards, my preliminary assessment is that the visibility over the nose was no better than it was on the original F-111B, which was determined to be unsatisfactory. However, the lower weight would have resulted in a lower angle of attack for the same lift, possibly providing the same over-the-nose visibility improvement as the raised cockpit that was eventually required.

The government program team elected to incorporate most if not all of the SWIP changes in the 12th F-111A and the 4th F-111B. The CWIP specification changes stayed on the drawing board until Grumman was able to apply them to what became the F-14.

One if by Land, Two if by Sea

“One if by land, and two if by sea.” That line from Longfellow’s poem commemorating Paul Revere’s famous ride in 1775 was one of the justifications used by the Navy in 1976 to select the twin-engine McDonnell F-18 over the single-engine F-16 for its VFAX program. Some viewed it as dissembling on the Navy's part since the desirability, much less necessity, for twin-engine carrier-based aircraft had not been very evident up until then. In fact, although single-engine airplanes were in the minority in the air wings at the time, that was a relatively recent change from past practice. A year earlier there was still an air wing aboard Hancock (CV-19) that was almost entirely comprised of single-engine aircraft. Now, of course, there are no single-engine airplanes in the carrier air wings.

The major benefit of twins, the capability to return to base after an engine failure, was initially problematic for a carrier-based airplane. The pilots approached power-on in level flight at the lowest safe speed and minimum altitude, cutting the engine just when the airplane would settle onto the deck. This was essentially the same technique used for a short-field landing ashore, where it assured a touchdown very close to the approach end of the runway, allowing the maximum distance for stopping.

The pilot of a twin-engine propeller-driven airplane with one engine inoperative had to take into account the minimum control speed in that situation. Since the engines were almost always placed out on the wing, when one failed the other produced a significant turning moment which had to be counteracted by the rudder. Since rudder effectiveness varied with airspeed, at some point the pilot could no longer stop the airplane from turning with the engine at full power. What’s worse, the turning generated a roll because of the difference in the lift on the wing on the outside of the turn versus the one moving slower on the inside. Since the ailerons also lost effectiveness with decreasing airspeed, a loss of control in roll would result as well if the engine power was not immediately reduced.

Unfortunately, the minimum-control speed at the power required to climb with the gear and flaps down was almost certainly higher than the required approach speed dictated by arresting gear, which made a successful wave-off an iffy proposition.

The tyranny of minimum-control speed was also imposed on an airplane taking off, but it was more draconian at sea than ashore. If the pilot taking off from a runway lost an engine while still below minimum-control speed, he would simply close the throttle on the good engine and reject the takeoff. The outcome varied with the length of the runway and if it came to that, the landscape beyond its end, but was rarely as dire as faced by the pilot of an airplane less than one hundred feet above the sea after being launched from an aircraft carrier at less than the minimum-control speed with full power on the operating engine. If he reduced power on it engine to maintain control, he almost certainly would not have enough for level flight, much less to climb or accelerate to a speed at which he could use all the power available.

Having a second engine was therefore not as good a deal for a pilot flying from an aircraft carrier as it was for one flying from an airport. Although it enabled one to divert to a land base or get back to friendly ships and ditch if an engine was lost in flight, it doubled the risk of an engine failure during a critical, albeit short, time during takeoff and landing. Twin-engine airplanes also tended to be bigger than singles whereas compactness was a virtue on an aircraft carrier.

Nevertheless, there were benefits beyond the ability to continue flight after an engine failure. The easy way to improve the performance of fighter airplanes is to incorporate more powerful engines in new or existing designs. Increasing power in piston engines basically meant adding more and/or bigger cylinders and supercharging. By the late 1930s, the engine manufacturers were beginning to approach the limits of existing technology and incremental horsepower increases were resulting in increasingly smaller increases in speed and greater engine complexity. The obvious next step was the twin-engine fighter, a doubling of power available without requiring the time and expense of a new engine development.

The U.S. Navy solicited proposals for a twin-engine carrier-based fighter in 1937 but none of the submittals were deemed to be acceptable. In 1938, the Navy had Lockheed modify an Electra Junior to have a fixed tricycle landing gear and tail hook. It was designated XJO-3 and delivered in October 1938. On 30 August 1939, Navy pilots made 11 takeoffs and landings from Lexington (CV-2) to evaluate it from both twin engine and tricycle landing gear standpoints.

In parallel with this research program, the 1938 competition for a new fighter was opened to both single and twin-engine designs. This time, the Grumman design number G-34 was considered worthy of evaluation by the Navy as the XF5F along with single-engine designs from Vought, the XF4U-1 powered by the big new P&W R-2800; and Bell, offering a derivative of the Army Air Forces P-39, the XFL-1.

The XF5F, probably in consideration of the one-engine-inoperative requirement, had the engines mounted as far inboard as possible and twin vertical fins, one in each engine’s slipstream. One-engine-inoperative wave offs were evaluated at altitude: "(A wave-off) might be accomplished (on one engine) provided the airspeed is about 80 knots or more and no more the 1/2 power on the operative engine were used." The "proper" approach speed based on stall speed, however, was defined as about 74 knots.

In spite of having as much or a little more installed horsepower than the XF4U, the XF5F was slower and couldn’t climb as high although its rate of climb through 20,000 feet was essentially the same. As a result, the Navy elected to proceed with the F4U for development and production. Nevertheless, the Bureau of Aeronautics continued to be interested in a twin-engine carrier-based fighter. On 30 June 1941, Grumman received a contract for the two XF6Fs and two XF7Fs. The F7F program suffered from the priority on F6F Hellcat development but the prototype Tigercat finally flew for the first time on 3 November 1943.

As soon as Grumman test pilots flew the XF7F-1, they realized that it did not have a big enough fin and rudder for an acceptable minimum control speed in the event of an engine failure on takeoff or a wave-off. Design of a bigger fin and rudder was initiated and introduced on the F7F-3. Although all models of the F7F were carrier qualified, the likelihood of and/or concern about a successful single-engine wave-off must have been low as there was no description of the technique for a single-engine carrier landing in the flight manual. In any event, the Tigercat never deployed with an air group on a carrier, probably due to its size as much as anything else.

One of the Navy’s first carrier-based jets, the McDonnell FD-1 Phantom was a twin, mainly because the Westinghouse-provided engine wasn’t very big. It grew to become the F2H Banshee, the first twin-engine airplane to regularly deploy on carriers. Two of its contemporaries, the Douglas F3D Skyknight and the North American AJ Savage, were also multi-engined. The AJ had three engines, two turning propellers and a jet. Like the F7F Tigercat, the Skynight was primarily operated by the Marines and made very few deployments. The Savage did deploy because of its critical mission of long-range nuclear strike, but because of its size, it generally was held in readiness at nearby Naval air stations during a carrier’s deployment. These jets were less limited from a minimum control speed standpoint in the event of a one-engine-inoperative situation than previous twin-engine propeller-driven airplanes because the engines were located close to the centerline; the AJs were slightly better off if one of its piston engines failed because the jet engine was located on its centerline.

However, North American was concerned about minimum control speed as evidenced by the size of the AJ’s original fin and rudder, made even bigger because carrier basing necessitated a fairly short airplane. Unfortunately, the rudder proved to be too big for high speed flight and resulted in a fatal accident when it broke the tail off in a flight test maneuver. The empennage was redesigned to increase the size of the fin, reduce the size of the rudder, and delete the dihedral in the horizontal stabilizer.

The lack of U.S. Navy concern about engine failures in the late 1940s was evident by the initiation of single-engine airplane programs, the Douglas F4D Skyray and the McDonnell F3H Demon, to replace the twin-engine all-weather Banshee. It was still true in 1958, when the Navy had to choose between the single-engine Vought F8U-3 and the twin-engine McDonnell F4H. The safety record of twin versus single-engine airplanes was examined and determined to not be a deciding factor. In fact, the only twin-engine airplane in the deployed carrier air groups at the time was the Douglas A3D Skywarrior, which had two engines because it was too big to be powered by only one. The F4H was selected because it had a dedicated radar operator, not because it had two engines.

The Navy did regularly deploy one twin-engine propeller-driven airplane at sea for more than two decades beginning in the mid-1950s on axial deck carriers, in part because Grumman had learned a lot about operating twin-engine airplanes from aircraft carriers with the F7F program. Its S2F (S-2) was as short-coupled as carrier airplanes get, so in order to size the rudder both for the single engine takeoff and wave-off condition and—relatively speaking—high-speed flight, it had a two-piece rudder. Up and away, the forward portion of the rudder was just used for directional trim and only the aft portion of the rudder moved with the rudder pedals. For takeoffs and landings, the forward and aft portions of the rudder could be selected to move as a unit, doubling the width of the rudder and reducing the S2F's  minimum control speed to one suitable for carrier launches and wave-offs.

The introduction of steam catapults, angled decks, and descending, constant angle of attack approaches also reduced the degree of difficulty of one-engine-inoperative takeoffs and landings.

By the time Grumman engineers designed the F-14, they felt confident enough in their handling qualities analysis to widely separate its engines to provide a "tunnel" where two of the big Phoenix missiles could be carried side-by-side.

However, minimum control speed would still prove fatal to the unwary: Hultgreen Crash

Finally, click HERE for a great tale of how a second engine and a naval aviator saved an airplane...

Blog Index

See July 2010 for an index of blog entries 1-99.

Barriers and Barricades, One More Time

I recently read an excellent history of a carrier-based airplane but noted that the author, in the captions, did not bother to differentiate between barriers and barricades. It is a minor quibble, but the nomenclature is specific and illustrates a two-step set of changes to carrier-deck equipment forced by the introduction of jet airplanes, one element of which was retained on angled-deck carriers.

Barrier

Davis Barrier

The configuration of the Davis barrier changed over time but the principal remained the same.

Barricade

The original barrier was introduced at the very beginning of carrier operations to stop an airplane when its tail hook had missed all the arresting wires. First one steel cable and then two were strung across the deck about three feet high at each barrier station. They were attached to stanchions which could be folded down to place the cables on the deck so airplanes could taxi past the barriers. An operator was stationed at each barrier to raise and lower it.

The steel cable barriers were very effective.

Unfortunately, the original barriers were not safe to use to stop airplanes with nose landing gears and to some extent, with twin-engine airplanes. The steel cables would wipe out the nose landing gear, raising the potential for the cables on the next barrier forward to slice the canopy off the airplane, and with it the pilot's head.

There was also the potential on a twin-engine propeller-driven airplane for the nose gear to pull the cable forward, allowing a propeller to hit it an angle and cut it, rather than skip off of it and past it. A tightly stretched steel cable when cut could wreak all kinds of havoc, not to mention not stopping the airplane.

The Davis barrier solved those problems by having the cables laying flat on the deck. A canvas strap was strung across the deck about three feet up using the same stanchions used for the original barrier. When the airplane's nose gear (or a "retractable barrier guard" in front of the windscreen if the nose gear had collapsed) hit the horizontal strap, vertical straps between it and the cables pulled them up off the deck to engage the main landing gear, thereby stopping the airplane. There were about six or so barriers on a carrier, so some were rigged for props and some for jets. They could also be reconfigured or replaced fairly quickly. Four barriers are shown in the following picture, two prop (lying on the deck) and two jet/AJ, i.e. Davis, one that has been activated but didn't snag the main landing gear because the jet had hooked a late wire so was going too slowly (barrier operators were cautioned not to drop their barriers too quickly) and the other in the ready position.

The Davis barrier worked acceptably after some development, although it was recognized that if the airplane were going too fast when it hit the Davis barrier, the cables might not be pulled up high enough, fast enough so they didn't get above the main landing gear tires and snag the landing gear struts before the main landing gear had passed by. There was also a problem with the steel cables being cut by airplane appendages at the higher landing speed of jets as well as pilots defeating the purpose of the barrier with a late and unsuccessful wave off as pictured above. After a few incidents in the fleet with jets not being stopped by the Davis barrier, a really big canvas net hung from scaled-up barrier stanchions was introduced as the last-chance layer of protection for the men and aircraft forward of the landing area. This was the barricade.

With the advent of the angled deck, barriers were no longer required. However, the barricade was still necessary if a jet had a landing gear or tail hook problem and couldn't land ashore. It is only rigged when required and the deck crews periodically practice erecting it on short notice and in only a few minutes.