Back Lockheed Model 186 / CL-575 / XH-51

Lockheed XH-51

The Lockheed Model 186 was designed in response to a 1960 joint Army/Navy requirement for a high-speed, highly manoeuvrable research helicopter. Two examples were ordered in early 1962, with the first of the two making its maiden flight in November of that year. The aircraft, designated XH-51A, were operated by both Army and Navy pilots and carried Navy Bureau numbers (151262 and 151263) in addition to markings indicating their dual-service status.

In keeping with its intended role of high-speed research vehicle the XH-51A was far more streamlined than most contemporary helicopters. The craft's aluminum skin was flush-riveted to the extensively flush-sealed fuselage frame, the engine air intake scoops were faired into the upper fuselage decking, the skid landing gear was retractable, and the main rotor plane was tilted a few degrees forward of the fuselage datum line. All these features combined gave the XH-51A an extremely low aerodynamic drag coefficient, which in turn resulted in a maximum level speed of 180kph. The craft's rigid main rotor, a system pioneered by Lockheed on its earlier CL-475 research helicopter, made the XH-51A extremely agile and allowed it to perform flight manoeuvres previously possible only in fixed-wing aircraft.

The XH-51A's quite impressive performance significantly improved, however, following the second prototype's 1964 conversion into a compound rotorcraft.

S.Harding "U.S.Army Aircraft since 1947", 1990

Lockheed Model 186 / CL-575 / XH-51

Satisfactory results obtained with the CL-475 rigid-rotor research vehicle encouraged Lockheed to proceed with the design of a more advanced helicopter combining the novel rotor system with turbine power. Bearing the Temporary Design Designation CL-595, the new helicopter was conceived as a four-seater, with a very aerodynamically-clean, flush-sealed fuselage and retractable aluminium-alloy landing skids to minimize drag, and achieve speeds in excess of 320km/h. Further attention to drag reduction led to the installation of the rotor swashplate within the fuselage and the control rods were brought up through the main rotor shaft. Moreover, the control gyroscope ring of the CL-475 was replaced by three weighted arms mounted above the three-blade rotor.

The anticipated performance of the CL-595 and the demonstrated advantages of the CL-475 attracted the attention of both the US Army and Navy and these Services joined in ordering from Lockheed prototypes of a high-performance research helicopter, with the Navy issuing Contract NOw 62-0665 in February 1962. Bearing the military designation XH-51A and assigned BuNos 151262 and 151263, the two Model 186s differed from the CL-595 design in being fitted as two-seaters to provide space for test instrumentation. Both XH-51As were powered by the 550shp Pratt & Whitney (Canada) PT6B-9 shaft-turbine and carried 303 litres of fuel. First flight was made by BuNo 151262 on 2 November, 1962, and, after replacement of the original three-blade rotor with a four-blade unit, the type proved reliable and easy to fly during eighteen months of joint Army-Navy evaluation. In particular, the XH-51As were very stable, without the use of either autopilot or artificial stabilization device, and pilots without rotorcraft experience could convert more rapidly than to standard helicopters.

With its top speed of 280km/h the XH-51A was already quite fast, yet its rotor system had the potential for operation at substantially higher speed.

Accordingly, the Army funded the modification of the second machine as a compound helicopter with mid-mounted 5.18m-span wing and a 1,315kg thrust Pratt & Whitney J60-P-2 turbojet on the port side of the fuselage. These modifications resulted in an increase in gross weight from 1,860 to 2,041kg. First flown in September 1964 without its turbojet, the XH-51A Compound reached a speed of 438km/h in May 1965 after its J60 had been installed. Its performance envelope was then progressively expanded until 29 June 1967, when it set an unofficial speed record for its class of 486.9km/h. Both this compound helicopter and the original XH-51A are now preserved in the collection of the US Army Aviation Museum at Fort Rucker, Alabama.

Another Model 186 was built in 1964 and, designated XH-51N, was delivered to NASA at the end of that year. As NASA 531, this five-seater was operated by the Langley Research Center at Hampton, Virginia, for testing advanced helicopter systems. Finally, Lockheed built two Model 286s as five-seat commercial demonstrators. FAA certification was obtained in June 1966 and the Model 286s flew more than 240,000km and carried more than 5000 guests on demonstrations in the United States and Europe. Unfortunately, and in spite of having demonstrated exceptional manoeuvrability and performance (for example, its top speed of 331km/h comfortably exceeded the 220km/h maximum speed of the contemporary Aerospatiale Alouette III, which was in the same size and power class), the Model 286 failed to attract civil customers. In particular, without the benefit of military orders over which to spread part of the development cost, the price could not be made competitive. Moreover, a tentative Netherlands order for twelve derivatives of the Model 286, which were to have been fitted for anti-submarine operations, proved insufficient to warrant production of the type.

R.Francillon "Lockheed Aircraft Since 1913", 1987

Lockheed Model 186 / CL-575 / XH-51

Technical data for XH-51A

Engine: 1 x Pratt & Whitney Canada PT6B-9 turboshaft, rated at 410kW, main rotor diameter: 10.67m, length: 12.83m, height: 2.48m, fuselage length: 9.69m, empty weight: 1197kg loaded weight: 1860kg max speed at sea level: 280km/h, cruising speed: 257km/h, initial rate of climb: 610m/min, hover ceiling IGE: 4875m, range: 565km,

Sharlet, e-mail, 19.02.2023reply

I invested 32,000 Pounds, and later aggravated to 218,000 Pounds. I requested to place Withdrawal of my funds to be paid to my Bank account. But nothing happened I was subjected to pay more fee until I can get my funds on my account, i got in-touched with MORRIS GRAY here on this platform who had helped a lot of people similar problems concerning crypto, I followed all instructions and he legally got back my withheld funds, I got threatened by the company that if I don't pay they will get my account frozen, But all thanks to MORRIS GRAY, I highly recommend him to anyone dealing with an unregulated broker company… contact him on his
Gmail- MORRISGRAY830 (at) or
WhatsApp +1 607 698 0239.>>

Sharlet, e-mail, 19.02.2023reply

I invested 32,000 Pounds, and later aggravated to 218,000 Pounds. I requested to place Withdrawal of my funds to be paid to my Bank account. But nothing happened I was subjected to pay more fee until I can get my funds on my account, i got in-touched with MORRIS GRAY here on this platform who had helped a lot of people similar problems concerning crypto, I followed all instructions and he legally got back my withheld funds, I got threatened by the company that if I don't pay they will get my account frozen, But all thanks to MORRIS GRAY, I highly recommend him to anyone dealing with an unregulated broker company… contact him on his
Gmail- MORRISGRAY830 (at) or
WhatsApp +1 607 698 0239.>>>

Denice Raven, e-mail, 03.02.2023reply


I hope you're having a great day. I'm a representative here at Copy.AI and I'm contacting businesses, creators, and marketing teams to introduce them to the most powerful AI-driven content platforms out there.

This AI-powered tool will save you oodles of time and effort by writing your content for you in a matter of seconds, that's right, SECONDS! Think of all the extra time you'd have to focus on other important tasks like networking, marketing, and growing your business.

Copy.AI is an easy-to-use and intuitive tool that helps you generate content quickly and accurately. Think of a topic and the AI will generate content with just a click of a button. It can also be used to write many different types of copy, such as sales emails, blog articles, video scripts, social media ads, product descriptions, website content, captions, create unique art and images based on your imagination and so much more. Even write in over 25+ different languages.

It's designed with content writers in mind, so you can trust that it will provide you with top-notch content every time. Plus, the AI takes into account your target audience so that the content will be tailored to their needs. And guess what - all the content generated is plagiarism-free! How about that?

All these features and benefits are just the tip of the iceberg; there are so many more things you can do with it that it will simply blow you away.

So if you're looking for a way to save time and effort without compromising on the quality of your content, give Copy.AI a try - we do a free trial.

Thanks for your time and I hope you find Copy.AI a useful companion for your content-writing needs.


P.S. This was written by Copy.AI ;) Here's the link to the free trial if you would love to give it a go and see what I'm talking about for yourself:

Ree Timmer, e-mail, 22.02.2015reply

It was a modification of his design. Some don't want to give him credit for the design. But the patent (which can be accessed at the U.S. Patent office clearly indicates that his patent was filed in 1960 on the Model 186 (XH-151). The XH-151A which was a modification of Paul E. Williams' original design.

Arthur Moss, e-mail, 09.10.2010reply

The two L-286's were lost in a hanger fire. I was the Quality Engineer on their build. The PT-6 was very expensive. The AH-56A Cheyene was Fantastic. It was better in 1969 than the Apache is today. A Commercial transport version was on the drawing boards.

Jim Maloy, e-mail, 20.10.2010reply

In the 1960's I was in the California National Guard, 140th Aviation Battalion, operating out of the Oxnard, CA airport. (This was the home base for testing the Lockheed Model 186). So, I witnessed some of it's first test flights. One day while I was at an Oxnard beach a Model 186 came out to do some test flights. I watched it do several mock strafing runs on a structure near by. I don't remember how many loops it did, one after another, but I was sure impressed. After it finished the strafing runs it climbed up a few thousand feet and made a run from south to north and performed some rolls. This was by far the best air show I have ever seen. At the time it was the only helicopter that could perform these maneuvers.

A few years later, while I was still in the 140th but stationed in Long Beach, CA, I was invited to watch a film that was taken that day on the beach. The camera was mounted behind the pilot and copilot and pointed out the front widow. I watched the whole flight over again, only from inside the Model 186 this time. The camera was focused on one of the hills north of Ventura and when the craft did it's roll, I could detect no yaw or pitch whatsoever in the craft. The camera stayed focused on one spot on the hill. A perfect roll! The other men watching the film were mostly single engine fixed wing and small helicopter pilots. The Model 186 was more suited for fixed wing pilots than for helicopter pilots. The pilots were all very excited about the possibility of getting to fly one of these someday. This film is out there somewhere but I have no idea how to find it. Sorry!

All of the above mentioned pilots were activated soon after this and sent to Vietnam.

One of my uncles was a mechanic who worked on the Model 186 and it's predecessor the XH-51A-1. The following is one of his stories: one day while working on the engine, he needed a part from the parts room so he climbed down off the craft and went to get it. When he returned with new part, the craft was gone. He went back into the dispatch office to find out where it had been moved to. While waiting to talk to the dispatch person, he over heard a radio report from the craft. The pilot was reporting that he had just made an emergency landing on the beach at Point Mugu Naval Air Station. He reported he saw something fall off of his craft and wanted to know what it might be. My uncle spoke up and said he knew what it was. It was a make-shift stool he had built to sit on while working on the engine. My uncle then wanted to know who had released the craft for flight because he had the craft tagged out of service for maintenance.

Don Hillberg, e-mail, 07.01.2011reply

Looking for Data, old prints ,drawings,reports on this Helicopter,anything will help-call 805-658-8245 thank you.P.S. its for a special project...

Paul Law, e-mail, 20.01.2012reply

The AH-56 was awesome and had extremely advanced capabilities. I had the privilege of being a structures engineer on that project (fresh out of College). Its cancellation by the Army and down fall was related to at least two major issues. One is it had a number of vibration harmonic problems associated with the fuselage and I think secondly and maybe more importantly the Army's resistance to allow fly by wire control system which was leading edge technology at the time. One flight test article was lost and the pilot killed due to control system harmonics when the main rotor deflected sufficiently to strike the canopy. The other flight test / qualification article was then subsequently modified to accommodate a early F-104 downward ejection set so that the pilot did not have to pass synchronized thru the rotor system if he decided to eject. Much of the subsequent flight test focus then was redesign of the flight control system and applying vibration absorbers band aides to combat the fuselage vibration harmonic problems (which many helicopter designs had issues with as well). Anyway, that my recollection.

Don Hillberg, e-mail, 30.03.2012reply

Have lots of data still need more. new # 805-832-4248
I have lots of time on my hands. Back injury.

Rick Koehnen, e-mail, 29.10.2012reply

Both the XH-51A and XH-51B survived and I last saw them (15 years ago) in the US Army Aviation Museum at Ft Rucker, Alabama. Same place the base commander buldozed the Boeing HLH ten years ago. At the time they were both in an old WWII wooden hanger which I hear no longer stands. I don't know the status of the aircraft inside which included another AH-56, numerous prototype helicopters and vtol platforms. Unfortunately for lack of funds and help the staff had to utilize 'woc candidates' from the Warrant Officer School to move things around. Ropes were put around the horizontal stabilizers of the XH-51B and it was towed on its skids into the hanger until the tip of the right wing broke and protruded into the nose and cockpit of the McDonnell XV-1. Its a shame they don't have the funds and manpower to restore and maintain their collection as its probably the best collection of original prototype helicopter and vtol platforms in the world. What would help is since 9 /11 access is restricted as the museum is on the military base and the general public can no longer just drive up and visit.

Rick Koehnen, e-mail, 29.10.2012reply

P.s. the ropes wrapped around the horizontal stabilizer of the XH-51B tore through the skin and ribs and only stopped against the spars.

Phil Salvatore, e-mail, 06.11.2009reply

I grew up in the 1960's in the San Fernando Valley. Back then the Valley soundtrack was Clay Lacy taking hot laps in his racing P-51 ( quite the treat for a kid outside for recess ), rocket tests at nearby Rocketdyne, and almost daily sonic booms.
Even as a kid I was aware of the Lockheed AH-56 and the 286. I knew it was special. One day at recess I vividly recall the Lockheed 286 pulling a loop right over our elementary school. It was a riveting sight, and it remains burned into my mind today. Later in life I flew tandem rotor Boeing Vertols and consider them to be the best operational helo design, period. But still, I wish I could have gotten my hands on something like a Cheyenne or 286.

Mike Gray, e-mail, 23.09.2008reply

I came across one of the 286s in central California around 1986, stored in the hanger of a local rancher /pilot. Tried to get the money together for a restoration to flying condition, hope someone with the financing finds it someday.

Art Bliss, e-mail, 04.04.2008reply

I was fortunate enough to get a one hour demo ride in the 286 in Aug 1967 with Lockheed test pilot Herman (Fish) Salmon. I do not understand why this machine didn't sell. It was superior to anything at the time and was ahead of some machines flying today.

Cathy Edwards, e-mail, 11.02.2008reply

Is this the same helicopter patented by Paul e Williams? Patent #3065933.

Do you have any comments ?

Name   E-mail

Virtual Aircraft Museum

All the World's Rotorcraft