Hughes TH-55A Osage

The TH-55A was developed from the Hughes Model 269A light helicopter, development of which started in September, 1955. Originally five pre-production helicopters were purchased by the U.S. Army to be evaluated as possible replacements for the OH-13 Sioux and OH-23 Raven helicopters. Although the U.S. Army turned down the Model 269A (designated as the YHO-2HU) in 1959 as light observation helicopter, the evaluation elicited positive comments in several areas. In 1964 the U.S. chose the Model 269A as its next primary training helicopter - to be designated as the TH-55A.

TH-55-1520-233-10 was the Operator's Manual which covered the relevant flight characteristics and technical details of the TH-55A. To view the complete "dash-10" manual, click on the cover image to the left While the Operator's Manual described how to operate the TH-55A and its systems, it was more useful for rated pilots, and student pilots who were learning to fly would more frequently refer to the "Primary Flight Training Manual" to reinforce the airmanship lessons they received from their Instructor Pilots.

Variants History
Specifications Performance
Cockpit Procedures Shutdown Procedure
Before Takeoff Reminisces
   

 

The initial contract was awarded by the Army Materiel Command in St. Louis, Missouri and was for an initial batch of 20 TH-55As at a total contract cost of $517,500 ($25,875 each), with the first group to be delivered by December, 1964. The contract also contained an option for the purchase of 215 more TH-55As in fiscal year 1965 for $5,563,125. The TH-55A was purchased to replace and supplement the OH-13 and the OH-23 in the primary training role at at the U.S. Army Primary Helicopter School at Fort Wolters. Both the OH-13 and OH-23 were used in Vietnam, and attrition and age eventually reduced the numbers available for training. New production of the TH-55A expanded its role and eventually it became the pre-eminent training helicopter with the U.S. Army eventually purchasing 792 . When Fort Wolters closed in 1973 the TH-55A continued to train new student pilots at Fort Rucker until 1988.

The only variant used by the U.S. Army was the TH-55A which was powered by a Lycoming HIO-360-BIA of 180 hp. Refined versions of the original Model 269A design evolved into the Model 300 and were produced for the civilian market, as well as the military forces of other nations. Hughes Helicopters was purchased by McDonnell Douglas Helicopters in 1983, and production was transferred to Schweizer Aircraft Corporation. In 1986 Schweizer purchased the entire Model 300 program. By late 1999 nearly 3,500 Hughes/Schweizer 300's of all models had been sold - making it one of the most successful helicopters ever produced.

Specifications:     Performance:  
Diameter of main rotor
25 ft. 3 1/2 in
  Max level speed at S/L
86 mph
Diameter of tail rotor
3 ft. 10 in
  Max cruising speed at S/L
75 mph
Length overall, rotors fore and aft
28 ft 10 3/4 in
  Max rate of climb at S/L
1,450 ft min
Width, rotors for and aft
10 ft 0 in
  Vertical rate of climb
740 ft min
Height
8 ft 2 5/8 in
  Service Ceiling
15,200 ft
Skid Track
6 ft 6 1/2 in
  Hovering ceiling in ground effect
9,500 ft
Weight empty
913 lb
  Hovering ceiling out of ground effect
5,800 ft
Max normal T-O
1,600 lb
  Range
200 miles

The "TH-55A Cockpit Procedure" card was on laminated stock and gave the instructions on how to get the aircraft started. Initially a copy was issued to each student pilot but were frequently lost or misplaced. Additional copies could be purchased at the Bookstore on the post for a nominal amount.

On the flip side of the card was the "Before Takeoff" procedure and the "Shutdown Procedure." The "Before Takeoff" procedure was used after the TH-55A was cranked, warmed-up and all of the cockpit checks had been made. After clearing the aircraft visually, the engine was throttled up to 2900 rpm, collective added to bring it light on the skids and then up to a 3 foot hover. Another visual check and the TH-55A was hovered down the takeoff lane to the takeoff panel.

The "Shutdown Procedure" was followed at the end of the period of flying. It was important to leave the aircraft in a uniform state for the next student pilot - the more so because the same aircraft could be flown in the morning, afternoon and even at night on the same day.

Operating Instructions - TH-55A

Airspeeds

Cruise 50 Kts
Climb & Descent 40 Kts
High Cruise 60 Kts
Autorotations 45 - 50 Kts
 
Engine rpm Settings
Hover, Take-Off and Climb 2900 rpm
Cruise and Descent above 500 feet 2700 rpm
Ground Reconnaissance 850 rpm
Autorotations (Entry) 2900 rpm
Simulated Forced Landings and Autorotations 2400 rpm
Descents Below 500 feet 2900 rpm
Rotor rpm Operating Range (green Area) 400 - 530 rpm
 

Power Settings - Manifold Pressure

Hover Power necessary to hover at an altitude of 3 feet into the direction of take-off, using 2900 rpm.
Normal Take-Off and Climb Hover Power.
Descent and Approach Power as necessary
Cruise Power necessary to maintain 50 Kts or 60 Kts during high cruise.
Maximum Performance Take-Off 2 inches above hover power or full throttle, whichever occurs first.

Reminisces of the TH-55A at Fort Wolters

The OH-23 Raven was the first training helicopter to be used for primary flight training at Fort Wolters, first appearing there in 1957. It also had other uses in the Army serving also in utility, observation and Medevac roles during the Korean War, and also as an armed observation helicopter in the earlier part of the Vietnam war. As the war in Vietnam expanded so did the need for additional Helicopter Pilots and in 1964 the Army placed the first of a series of orders for the Hughes TH-55A which eventually totaled 792 aircraft. It was unique as it served in no other role except as a primary trainer.

I'm not sure if there was any logic as to how the Army decided if the student pilots were assigned to an OH-23, TH-55A or OH-13 aircraft for training. Most of us had wandered over to the Main Heliport before we started flight training and examined the different trainers to which we would be assigned. My first impression was how small theTH-55A appeared compared to the others. Watching them as they landed and took off left me with the impression that they were just more "twitchy" than the two other types. As luck would have it, our flight was the only one in our class (ORWAC 68-24) to train on the OH-23. My roommate was assigned to one of theTH-55A flights and to be expected, we often discussed the relative merits of our assigned aircraft. Because the TH-55A used eight V-belts to transfer power from the engine crankshaft to the main transmission input shaft and tail rotor drive shaft, the OH-23 drivers would resort to calling it "rinky-dink." Plus, having a fully articulated rotor system and oleo skids, the TH-55A could be subject to the phenomena of ground resonance and had a known nose tuck problem upon entering power-off flight - if the pilot was extremely "ham-handed." Our technical knowledge in primary training was limited and usually any rational discourse quickly denigrated into less-than-mature name calling. The truth of the matter is that the TH-55A was a very good training aircraft and served the Army well for many years.

After returning from Vietnam, many pilots were assigned as Instructor Pilots, imparting their flying skills to the new students. The remaining Officers and Warrant Officers served in the myriad of Staff, Training, or Academic Instructor positions vital for the day-to-day functioning of the Helicopter School and Center. They had to maintain Flight Proficiency and were required to fly a certain amount of hours each year ("minimums") to stay on Flight Status, and of course, to receive Flight Pay. A small number of each type aircraft were kept to allow them to schedule and then fly the required hours in each category (i.e. daytime, nighttime, cross-country, etc.). I returned from Vietnam and enjoyed my periodic flights as a purgative from the stress of a demanding staff position.

In about 1970 or so, the OH-13s and OH-23s started to be removed from the training fleet, and it was mandated that all permanent pilots become rated in the TH-55A. I was less than thrilled with this news, but dutifully started studying the flight manuals and scheduled the required instruction with a designated Flight Instructor. The instruction was basic as we were already rated pilots and after all, the TH-55A was a very simple aircraft. We practiced a number of auto-rotations, the adverse characteristics of the aircraft were pointed out and certain "to be avoided" situations were demonstrated. After a few hours we were given a check-ride and then signed-off as qualified TH-55A pilots. During my check-ride I remember that we performed the required autorotations at a nearby stage-field that was not being used by students, and I noticed that the check-pilot was very smooth and proficient when he was on the controls. Before heading back he asked me if I wanted to see some "backward autorotations." demonstrated. This maneuver was not normally practiced and consisted of splitting the needles to enter a normal autorotation, applying aft cyclic and right pedal to offset the nose pitching down and to the left. Then, Instead of completing the autorotation normally, cyclic was then moved aft until the aircraft was in a extreme nose-high position and it started to slide backwards, lose altitude, and drop downwards. Recovery was made by pushing the cyclic forward, the degree proportional to ensure the main rotor rpm was in the green, and the maneuver was terminated in a (hopefully) normal autorotation to the ground.

I was impressed with how easily the Check-Pilot performed what was a confidence building maneuver. He explained the aerodynamics of what he had performed and the necessity of always closely monitoring that the rotor rpm and not be allowing it to decay greatly. I was already impressed with the way the TH-55A autorotated, but remembered the warnings that the TH-55A had a low inertia rotor system, which when subject to excessive abrupt upward collective movements without proper throttle coordination, could cause serious rotor rpm decay. This normally wasn't a problem for experienced pilots, but low-inertia (as opposed to high-inertia) rotor systems would tend to decay rpm more rapidly while transitioning from powered flight to non-powered flight. Not so good, but conversely rotor rpm could be regained more quickly in a low-inertia system - which was a better characteristic.

He performed a couple more autorotations and then let me have the TH-55A to perform the same maneuver. It wasn't too bad, and I felt I was doing well, except for one attempt where I started the recovery too close to the ground. The sun was starting to set and the Check-Pilot complemented me on my air work and asked if I would like to shoot some autorotations "for a beer." This meant of course, that a spot would be picked out on the ground and we would take turns to see who could autorotate the aircraft closest to that spot. I smiled and told him that the contest wouldn't really be fair as he flew the TH-55A every day while all I normally flew was my gray metal government-issued desk. The check-ride over, we landed at Downing Heliport and then drove over to the Officer's Club where I bought him the beer and thanked him for check-ride and the additional instruction. I felt more confident in my abilities and appreciated the flying characteristics of the TH-55A.

As my time in the TH-55A grew so did my confidence. The "Mattel Messerschmitt" was like a little nimble sports-car in the air and could be flung around easily. Easily yes, with impunity no. Accidents during the years reminded us that our chosen profession was always dangerous - in combat or not. All of the different type of training helicopters suffered crashes and even the occasional mid-air collisions in the crowded skies around Fort Wolters. But it appeared that the TH-55A was subject to some of the more different accidents because of some of its design features. During the late Fall and Winter months the doors were installed back on the aircraft to provide a modicum of heat for the pilots - supplemented of course by the Cabin Heating System. The Operator's Manual describes this simple system as consisting of the heater blower, the heat exchanger mounted on the engine exhaust manifolds, a heater valve, two plenum outlets, and flexible interconnecting air ducts. It usually worked well in all but the coldest of days, but then accidents started where pilots had been subject to carbon-monoxide poisoning in flight. The heat exchanger was basically a metal shroud fitting around the exhaust pipes which would absorb heat and transfer it to the plenums (ducts) in the cabin. As the TH-55A fleet aged, cracks had appeared in the exhaust manifolds, allowing deadly exhaust fumes directly into the shrouds - with predictable results. Before a more permanent solution could be fitted to all the TH-55As, a more expedient "work around" was to install a couple of large tablets on the cabin floor which would turn black in the presence of carbon-monoxide. I'm not sure if any fatalities had been caused by this problem, but the tablets did add some assurance against what was known as a silent killer.

Another undesirable trait that the TH-55As possessed that the OH-23s and OH-13s didn't, was its susceptibility to a condition known as Ground Resonance. This was a condition applicable only to helicopters with articulated rotor systems which also have oleo damper struts incorporated in their landing gear. When an oleo damper strut is not correctly maintained an out-of-balance vibration can quickly feed from the oleo strut up to the rotor head, there to be reinforced and sent back down to the strut where it is repeated quickly and often violently, and which ultimately often results in the helicopter being totally destroyed. The problem is only manifested when the helicopter is on the ground and stops when the aircraft is taken off to a hover. To do this is easier said then done as often the vibration only begins during shutdown or startup and there just isn't enough time to crank the engine up to the operating rpm. A young WOC (Warrant Officer Candidate) was assigned to our office for several weeks pending his medical recovery from such an accident. Ground Resonance had violently started when shutting down his aircraft, and the TH-55A immediately started to thrash itself to destruction. He tried to fling himself clear, but as he evacuated the aircraft he was hit in the head by one of the rotor blades. His flight helmet saved his life but he was left with a severe skull fracture, which I suppose ended his Army career,

The TH-55A did have to be given a measure of respect but it was a great break from the staff work at the office when we were able to schedule one to whittle down our annual flight minimum requirements. It was on once such occasion that myself and another Captain from the office had scheduled a TH-55A to get some night cross-country time. The springtime weather around central North Texas can often turn violent quickly, and what had started as a bright sunny day began to cloud-up in the afternoon. After work we drove down to Fixed Wing Operations at Downing Heliport to fill out the required paperwork and to receive the assigned aircraft's tail number and the spot where it was parked. It was also the place that the latest weather forecasts could be reviewed. Our Air Force friends from Detachment 20, 16th Weather Squadron, operated their weather station 24 hours a day, seven days a week at Fort Wolters. Their forecasts for the local area were usually very accurate and were trusted implicitly. However the standing forecast for that evening stated a strong line of thunderstorms was expected to pass just to the west of Mineral Wells. Stephenville was to the south of Mineral Wells, so our plan was to fly to Stephenville Municipal Airport just before dusk, park the aircraft and walk down the road to a local motel and eat supper in the small restaurant before returning and taking off to finish our night time trip. Yes, the bad weather was considered, but in the true spirit of Army Aviation we reassured ourselves with "what the heck, we should be alright, let's go - if it gets bad we'll fly around the crud and scoot back."

When we examined the logbook of our assigned aircraft we noticed that a notated "Red-X" condition had been cleared by replacing a faulty starter micro-switch. No problem, we completed the pre-flight, cranked it up, and departed southwards. Our arrival at Stephenville was a non-event, and we finished our meal and returned to complete our evening's flight. There were strong lightning flashes in the distance which we were given pause to think when an airport worker told us that we better get out of there as "it was going to turn nasty in a couple of hours - we are under a tornado warning!" We started to worry when we hopped in the TH-55A and began the startup procedures. There was the normal hum from the fuel pump, we had opened the throttle slightly to prime the fuel system, closed the throttle and then engaged the starter for the required 3 seconds. Nothing happened - absolutely nothing. ... and absolutely nothing on our subsequent attempts. We brilliantly concluded that maybe the previous starting problem hadn't been fixed completely.

A telephone call to the maintenance folks at the heliport wasn't too encouraging. They would round up some parts and would send down a mechanic in a pick-up truck to fix our aircraft, but it would take another couple or more hours. We explained that there was bad weather approaching and it was suggested that we try to elicit some local help to pull the aircraft under shelter. The worker returned to tell us he was leaving as the airport closed down each day after dark. When asked he told us the hanger was already completely full with aircraft parked there as protection against the expected rain. Before he left he did hook up our aircraft to a length of rope and non-too-gently pulled it closer to the side of the hanger for a little protection. It was now completely dark and after an hour or so it started to rain, lightly at first, but then with the savage intensity common to the season in central Texas. The wind howled and we were quickly soaked as it drove right through removed doors of our aircraft. The small TH-55A rocked badly in the strong winds and we thought there was a good chance that it would get blown over - like the infamous incident of April, 1967 when a severe storm had damaged many TH-55s at Downing Heliport.

Now and then the winds died down and we would dash out to the pay-phone to check status on our expected mechanic. Still another couple of hours before he would arrive, but some of the parts had been rounded up and things were moving better. There was a pick-up truck parked nearby and luckily the passenger's door was unlocked. We jumped in but by then we were totally soaked through and pretty miserable. There was nothing we could do but wait and it was after midnight when the Southern Airways mechanic drove up. The wind and rain by then had almost stopped and he quickly got to work as we held the necessary flashlights over the engine bay. After a period of time he said he thought he knew what the problem was but didn't have the part with him to fix it completely. Pressed for an explanation, he said his instructions were to let us have the Southern Airways' pick-up and he was to stay with the helicopter overnight until someone was sent to relieve him in the morning to fix and retrieve the aircraft. Well at least the couple of soaked aviators would be able to get home before dawn. We felt sorry for him especially as he said that he had already worked a full day and really wanted to get back to his family - his expectations were he would be there for at least another twelve hours. The TH-55A wasn't cleared for normal flight, but he said that he could get it started temporarily and if "we didn't mind" could fly it back so he could go home to his family. It was our decision and it wouldn't be a problem as long as we didn't have to stop anywhere. The mechanic was able to get the engine to crank, but it ran very rough from its prolonged soaking. After a few minutes we pulled pitched and headed northwards, the skies were clearing as the airstream drove through the open cockpit and through our wet flight suits.

Of course our wives had been very worried until we returned, but the incident had become humorous to the others in the office. And, later that day I had to attend a meeting at the School Headquarters and overheard a conversation about "how a couple of dumb Captains had managed to get caught in the severe weather the previous night." I kept quiet and didn't add anything - but perhaps they were correct about the dumb description.

The TH-55A was a useful training aircraft and did well in its training role. That was its only purpose and I believe that most pilots tolerated some of its known characteristics but were more comfortable flying the OH-23 or the OH-13.