Tip driven rotors II

January 26, 2013 by Tim McAdams

Placing a small jet engine (such as a pulsejet, turbojet and ramjet) on a helicopter’s main rotor blade tips for propulsion never developed into a major commercial success. Despite the advantages of no heavy transmission or anti-torque rotor, several major problems could not be solved. One was the high centrifugal loads acting on the engines. However, from a design standpoint, driving the rotor system from the blade tips was so attractive that during the 1940s engineers came up with another concept called the pressure-jet rotor. 

The pressure-jet rotor operates by forcing compressed air out aft facing nozzles at the blade tips. An engine driven air compressor located in the fuselage pumps air through a rotating seal and into hollow rotor blades. Initially, this solved the noise issue that was associated with engines at the blade tips. However, compressed air alone did not provide enough thrust for flight, so fuel was added and then ignited at the blade tips. This added more thrust, but resulted in higher noise levels. Several prototypes were built as compound aircraft. This design uses the noisy rotor for take-off and landing, and then a propeller system with a small wing for forward flight. In an attempt to solve the noise issue without adding complexity, the French built a small helicopter with a large compressor that was successful without burning fuel at the rotor tips. It was quieter and worked well enough that the French Army ordered about 200 of them. However, using just compressed air was not powerful enough for a larger design. 

The final attempt at a tip driven design directed exhaust gasses from a turbine engine through the rotor blades. Hughes Helicopters built a working prototype during the 1960s; however, pumping 800 plus degrees (F) air through a seal in the rotor was problematic. Eventually, the tip driven concept was abandoned in favor of the engine driven main and tail rotor design that is used on the vast majority of helicopters flying today.

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Ramjet powered helicopters

January 15, 2013 by Tim McAdams

Early helicopter engineers were looking for ways to increase power available and decrease weight. Placing the propulsion system on the tips of the rotor blades eliminated the need for a power consuming anti-torque rotor (tail rotor) and a heavy transmission. Although, propellers mounted on the blade were tried it was the ramjet developed during World War II that launched a major effort to build a successful rotor tip driven helicopter. 

In 1946, McDonnell developed a single-seat, 285 pound (empty) helicopter for the U.S. Air Force. Called Little Henry, it used two ramjets (one on each blade tip) producing about 10 pounds of thrust each. A fuselage mounted tank supplies fuel to each engine through a rotating seal in the rotor hub. After a long flight test program, the Air Force decided not to purchase the helicopter. 

Hiller Aircraft also developed a ramjet powered helicopter. The XHJ-1 Hornet’s larger engines developed 40 pounds of thrust each. Flight test began in 1950 and the U.S. Military expressed interest provided the helicopter and its engines received FAA certification. Hiller worked hard to overcome several design challenges like excessive centrifugal loads acting on the engines and high fuel consumption. Ramjets are also noisy and need a substantial airflow to start, requiring a small ground engine to spin the rotor system. Although Hiller eventually received certification for the engines, the FAA would not sign off on the high level of drag caused by the engines during autorotation. Without the Military, Hiller ended the project.   

A Dutch company solved the drag issue during autorotation with a larger high inertia rotor system. The company’s NHI H-3 Kolibrie (Hummingbird) received certification in 1958.

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CFIT

January 1, 2013 by Tim McAdams

Pilots can learn a lot from reviewing accident reports. The idea is to understand the mistakes that led up to a particular accident so as not to repeat them. Yet, several types of preventable accidents occur over and over. One of these is Controlled Flight Into Terrain (CFIT) in IMC conditions. According to the NTSB, October 2012 was a bad month with three fatal helicopter CFIT accidents in poor weather. 

On October 5, 2012, about 0758 central daylight time, a Bell407 helicopter was substantially damaged when it collided with terrain shortly after takeoff from CentralIndustriesAirport(2LA0), near Intracoastal City, Louisiana. The commercial pilot, who was the sole occupant, was fatally injured. Day instrument meteorological conditions prevailed for the post-maintenance flight. One witness reported that she saw the helicopter depart on the runway heading and disappear into fog or a low cloud ceiling. Several witnesses reported hearing a sound consistent with a ground impact shortly after the helicopter had departed toward the southwest.

The closest weather observing station was located at the Abbeville Chris Crusta Memorial Airport (KIYA), about 13.6 miles north-northeast of the departure airstrip. At 0755, the KIYA automated surface observing system reported the following weather conditions: calm wind, visibility 1/4 mile with fog, overcast ceiling 200 feet, temperature 20 degrees Celsius, dew point 20 degrees Celsius, and altimeter setting 30.14 inches of mercury. 

OnOctober 9, 2012, about 2000 eastern daylight time, aBell407, was substantially damaged when it impacted trees and terrain inCoolbaugh Township,Pennsylvania. The airline transport pilot and one passenger were fatally injured, and one passenger was seriously injured. 

According to a limousine driver who was supposed to pick up one of the passengers at HPN, at 1938 he received a text from the passenger stating that they were running late. Then at 1953, he received another text instructing him to go back to MMU to pick up the passenger. After arriving at MMU, the driver waited but the helicopter never arrived.

A search by Federal, State, and Local authorities was initiated. On October 10, 2012 at approximately 0230 the helicopter was discovered in a heavily wooded area approximately 1.3 miles northwest of Pocono Mountains Municipal Airport (MPO), Mount Pocono, Pennsylvania. The recorded weather at MPO, at 2003, included: wind 100 degrees at 6 knots, visibility 1 1/4 miles, light rain, mist, overcast ceiling of 200 feet, temperature 09 degrees C, dew point 09 degrees C, and an altimeter setting of 30.10 inches of mercury.

According to Federal Aviation Administration (FAA) records, the pilot held an airline transport pilot certificate with ratings for airplane multi-engine land, with commercial privileges for airplane single-engine land, and rotorcraft-helicopter. His most recent application for an FAA first-class medical certificate was dated June 1, 2012. On that date, he reported 19,000 hours of flight time.

 On October 17, 2012, about 0640 eastern daylight time, an Aerospatiale AS 355 F2, was substantially damaged when it impacted trees and terrain shortly after takeoff from Brigham Heliport (4PN5), Erwinna, Pennsylvania. The certificated airline transport pilot was fatally injured. Dark night instrument meteorological conditions prevailed, and no flight plan was filed. Several witnesses reported hearing the helicopter as it over flew the residential neighborhood about the time of the accident, and one witness observed two lights that she presumed to be the accident helicopter as they descended into trees behind her home. Each of the witnesses described the lighting conditions at the time of the accident as dark, and reported that visibility in the immediate area was restricted due to fog.

No pilot takes off thinking that it’s highly likely they will have an accident. In these kinds of accidents most pilots know it is not ideal conditions, but probably believe it is manageable. However, what is missing is the realization that an unpleasant condition can quickly escalate into an unrecoverable and fatal situation. These kinds of accidents are worth thinking about every time you are faced with a decision regarding continued VFR flight in IMC.

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Flapping

December 13, 2012 by Tim McAdams

In the early 1900s, Juan de la Cierva, a Spanish aviator who built airplanes and gliders, unknowingly helped with the development of the helicopter. When one of his airplane prototypes crashed on its second flight during a low speed stall he decided to try to find a way to allow airplanes to fly slower. Windmills got him thinking that a rotating wing could produce lift without the need for forward airspeed. This led him to build the first autogyro (an aircraft that uses a propeller for thrust, but replaces the wing with a free-wheeling rotor for lift).

His first design lifted off the ground and immediately rolled over and crashed. He rebuilt the aircraft and tried again only to see the same result. This perplexed Cierva because the small model he built first as a proof-of-concept did not roll over. What was becoming clear to him was the concept of dissymmetry of lift – that is the difference in relative wind (and as a consequence lift) seen by the advancing and retreating sides of a rotor flown edgewise through the air. After much thought, the difference between his model and the full scale aircraft became clear. The model’s rotors were small and did not need supports which allowed them to flex, while the full scale rotors were heavy and required wire bracing making them stiff. The flexible rotors on the model could flap up and down which compensates for dissymmetry of lift. He then added hinges to his full scale aircraft to allow flapping and was able to proceed with development. The autogyro could not hover, but did meet his goal of slower flight. Over the next several years, various manufactures developed and sold autogyros.

The autogyro went on to achieve limited success until the 1930s, which saw the development of helicopters that could hover. As helicopter designs continued to mature, the autogyro faded out as a commercial aircraft. However, it was the autogyro that solved one of the biggest aerodynamic problems for rotary wing flight.

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Moral Courage Award

November 30, 2012 by Tim McAdams

Moral Courage can be thought of as the willingness to make sound safety decisions even when they are difficult or unpopular. Many times the pilot that exercises the courage to turn a flight down is making a tough decision, and in many cases their actions go unnoticed. Although it is impossible to know about the accident that didn’t happen, it is a reasonable assumption that a certain percentage of no-go decisions have prevented accidents and possibly saved lives.  Mr. D Smith, Senior Air Safety Investigator with the DOT/Transportation Safety Institute, believes that although these types of decisions are ones without fanfare, they are no less heroic than the more visible actions of other aviators. As such, he is looking for nominations for a new Moral Courage Safety Award. The award is aimed at recognizing individuals and organizations in the helicopter industry that make operational decisions based on sound safety risk management principles.  

According to Mr. Smith, “The award was inspired from the true story of an EMS pilot who told of his decision to abort a critical neonatal transport after encountering un-forecasted bad weather.  It was a very tough call; he had to weigh the safety of the crew with the life of a patient.  In the end he aborted the transport knowing it was the right decision for the safety of everyone.  His organization supported the decision and even went so far as to recognize him for making that tough call.  Sometimes choosing the safest course of action can cost time or money, but in the long run it saves time, money, reputation, and possibly lives.  It takes moral courage to do the right thing.  We believe there are many individuals and organizations making these tough calls every day.  We want to recognize them for their contribution to promoting a positive safety culture in the rotorcraft community.” 

Helicopter crew members, maintenance personnel, managers, and their organizations are eligible for the award. If you are interested in nominating someone please draft a short narrative of the event(s) and send it to d.smith@dot.gov . The award will be presented during the annual HAI Heli-Expo.

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Flight controls and passengers

November 20, 2012 by Tim McAdams

I was at a state fair several years ago where a pilot was giving helicopter rides in a Bell 206 JetRanger (a 5-seat single engine turbine powered helicopter). What caught my attention was that the dual controls were installed and passengers were being loaded into the left front seat. Allowing strangers access to the flight controls, like when giving rides, is very risky. Even when a pilot knows the passenger, they need to be extremely cautious and give serious consideration as to whether someone is provided access to the flight controls. For example consider the following accident that happened February 14th, 2010.

According to the NTSB, a ranch foreman who observed the flight preparations saw the helicopter owner board the helicopter through the left forward cockpit door and occupy the left front cockpit seat. The helicopter owner’s 5-year old daughter also boarded the helicopter through the left forward cockpit door and sat on her father’s lap. The pilot, who had 11,045 hours of total flight time, all in rotorcraft-helicopters, 824 hours of which were in the EC135 T1, was already seated in the right front cockpit seat. Both the left and right front cockpit seats were equipped with dual flight controls. Operator personnel revealed that the helicopter owner’s daughter had sat on her father’s lap occasionally during flights, that the owner liked to fly the helicopter, and that it was common for him to fly. Although the owner held a certificate for airplane single-engine land, he was not a rated helicopter pilot. However, it could not be determined who was flying the helicopter at the time of the accident.

About 35 minutes after departing the ranch, radar data revealed that the helicopter was about 2,000 feet above ground level when witnesses on the ground stated they heard unusual popping or banging noises. Several witnesses also stated that they saw parts separate from the helicopter before it circled and dove to the ground. The helicopter impacted a river wash area north of the destination airport in a slightly nose-down and slightly left-bank attitude. The helicopter was subsequently consumed by a post crash fire. The accident was not survivable.

A post accident examination of the helicopter revealed that the yellow blade had impacted the left horizontal endplate and the tail rotor drive shaft in the area of the sixth hangar bearing, which resulted in the loss of control and subsequent impact with terrain. No pre-impact failures or material anomalies were found in the wreckage and component examinations that could explain the divergence of the yellow blade from the plane of main rotor rotation. Flight simulation indicated that the only way that this condition could have occurred was as a result of a sudden lowering of the collective to near the lower stop, followed by a simultaneous reaction of nearly full-up collective and near full-aft cyclic control inputs. A helicopter pilot would not intentionally make such control movements.

A biomechanical study determined that it was feasible that the child passenger was seated on the helicopter owner’s lap in the left front cockpit seat during the flight and that the child could fully depress the left-side collective control by stepping on it with her left foot. The study also found that the collective lever’s full range of motion was 9.5 inches from full up to full down and that the spacing between the left edge of the seat, the collective, and the door are sufficient such that a child’s foot could rest on the collective and depress it. The study noted that the cyclic control could be moved to the full-aft position even with a small child of this size seated on the lap of an adult male in various positions.

Considering that the child was sitting on the owner’s lap in the left front cockpit seat, it is highly likely that the child inadvertently stepped on the collective with her left foot and displaced it to the full down position. This condition would have then resulted in either the pilot or the helicopter owner raising the collective, followed by a full-aft input pull of the cyclic control and the subsequent main rotor departing the normal plane of rotation and striking the left endplate and the aft end of the tail rotor drive shaft.

The National Transportation Safety Board determined the probable causes of this accident are:

The sudden and inadvertent lowering of the collective to near the lower stop, followed by a simultaneous movement of the collective back up and the cyclic control to a nearly full-aft position, which resulted in the main rotor disc diverging from its normal plane of rotation and striking the tail rotor drive shaft and culminated in a loss of control and subsequent impact with terrain. Contributing to the accident was absence of proper cockpit discipline from the pilot.

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Off airport landings

November 5, 2012 by Tim McAdams

One of the big advantages of helicopters is the ability to land off airport. However, deciding where and when to land a helicopter deserves considerable thought as the consequences of a bad decision can be very serious.

A case in point happened to a pilot in New York on October 27th.  According to news reports, police were called to the Coliseum after receiving several 911 calls complaining of many intoxicated youths at a rave concert there. While they were at the scene, police said a pilot attempted to land a Bell 407 on a grassy area on the side of the Coliseum. The first landing had to be aborted due to pedestrians walking in the area. The pilot returned and landed on the grassy area where at least 20 pedestrians were walking. The pilot was arrested, his helicopter was seized and he was charged with first-degree reckless endangerment.

The Federal Aviation Regulations also have a rule (14 CFR Part 91.13) prohibiting the careless and reckless operation of an aircraft; however, they do not address the legality of landing on someone else’s property. Failing to receive permission from the land owner could be just simple trespassing, nevertheless; zoning laws can prohibit the landing of aircraft even with owner consent. Some municipalities have specific ordnances that require a permit to land an aircraft and even if proper security measures are taken a fine can result if one is not obtained. In any event, landing in an unsecured area requires trained ground personnel (police, fire personnel etc…) to secure the area and prevent unauthorized persons from approaching the helicopter until it is safe. Moreover, as happened in New York, criminal charges can be filed if persons or property are placed at risk during a landing. It also would not be a surprise to see the FAA attempt to violate this pilot under 91.13.

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Running landings

October 24, 2012 by Tim McAdams

A running landing is used when there might not be enough available power to hover. This maneuver is typically used at high gross weight, high density altitude and for some emergency procedures involving the tail rotor. A twin engine helicopter might use this type of landing when making a single engine landing. As such, pilots normally practice this type of landing.

Performing the maneuver is fairly straight forward. The pilot would typically use a shallower approach; align the helicopter with the touchdown area and touchdown at or above effective translational lift. Depending on the reason, the touchdown airspeed is normally between 20 and 40 knots, however, it should be at the lowest airspeed for the situation to minimize ground run. As the helicopter slides to a stop the pilot should use the cyclic control to maintain ground track, the pedals for heading (important not to allow the helicopter to slide sideways), and the collective control to apply braking force.

When practicing this maneuver, extreme caution should be taken to insure a level and unobstructed landing surface. The follow NTSB account illustrates just what can go wrong even when care is taken.

On May 24, 2011, at 1503 eastern daylight time, a Schweizer 269C, N7505Y, sustained substantial damage during a practice run-on landing at Asheville Regional Airport (AVL), Asheville, North Carolina. The certificated flight instructor (CFI) and private pilot receiving instruction were not injured.

According to the pilot receiving instruction, who was also the owner of the helicopter, the purpose of the flight was to conduct a flight review. Approximately 50 minutes into the flight, the CFI asked the pilot to demonstrate a run-on landing to runway 16. The pilot conducted the approach for landing at about 40 knots and touched down left of the runway centerline on both skids. As he lowered the collective, the helicopter’s right skid contacted a runway centerline light, shearing off the right skid and its support arms.

The pilot raised the collective, picked the helicopter up to a hover and turned towards the taxiway. Shortly after, the engine and rotor RPM began to drop, the pilot opened the throttle and lowered the collective, setting the helicopter on the left skid. The helicopter rolled over and came to rest on its right side, resulting in substantial damage to the main rotor blades.

A post-accident examination by the pilot revealed that during the right skid’s impact with the centerline light the front landing gear crossbeam was pushed aft, crimping the fuel supply line.

The pilot later made these comments:

“From the skid tube marks on the runway, it became evident that the right, central skid shoe (attached to the bottom of the skid) had contacted the recessed centerline runway light straight-on. As such, the shoe jammed firmly against the flat surface of the runway light housing at the lens face of the light (north end of the light housing). Here is my main concern: Had my run-on landing been a true emergency landing without the benefit of power (that was instantly available after impact, though short-lived), the outcome would have been much different. The right under-carriage, now minus all skid landing gear, would have contacted the asphalt surface spinning us clock-wise, the left skid would have dug into the surface, and we would have cart-wheeled down runway 16 with a certain likelihood of serious injury or death. The threat of a helicopter fire would have been very likely.

No one would expect a 1.5 inch wide skid shoe would ever jam into an approximately 2 inch wide light port on this recessed light. But it did, and my helicopter is destroyed as a result.”

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Rotor blade icing

October 12, 2012 by Tim McAdams

Flying in conditions conducive to ice formation is problematic for virtually all helicopters. Moreover, many twin engine IFR helicopters are not certified for flight in known icing conditions. As such, helicopter pilots should understand the problems an encounter with icing can create for the rotor system.

Ice buildup on rotor blades will change the shape of the airfoil and consequentially, its ability to produce lift while increasing drag. The increased drag will slow the main rotor requiring the pilot to add power – which in some cases might not be available. Ice accumulation on the airframe can increase the helicopter’s gross weight requiring more power as well. Ice buildup is rarely, if ever, symmetrical causing an imbalance that produces vibrations in the rotor system. These vibrations can cause shedding of the ice and if all the ice comes off, vibration levels, lift and drag will return to normal. Asymmetrical shedding, however, can make the vibrations worse. Hopefully, the increased vibration will shed the remaining ice before any damage can occur. Ice accumulation is less on the outboard section of the rotor blade which is helpful because this area produces a larger amount of lift. However, an autorotation could be more difficult as the driving region is closer to the blade’s center.

Deicing refers to removing ice that has accumulated, while anti-icing is the prevention of ice formation. The few helicopters that having ice protection on the main rotor system use a de-icing system as the power required to anti-ice a main rotor system is extremely high. One of these is the Sikorsky S92 and it uses heater mats in the rotor blades to melt a thin layer of ice in contact with the blade surface causing the remaining ice to shed from the blade. According to Sikorsky, heat is applied to the mats to melt the ice in specific zones at precisely the right time for controlled shedding. Opposite main rotor blades are deiced simultaneously in order to prevent rotor imbalance and small sections of the rotor blades are deiced alternately to reducing the amount of electrical power required at any given time. The tail rotor ice protection system can be set to de-icing mode, which applies power in a scheduled manner or anti-icing mode in which heat is continuously applied to tail rotor heating mats.

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Slope landings

October 4, 2012 by Tim McAdams

Not every surface a helicopter lands on is perfectly level. So a slope landing is a maneuver that helicopter pilots need to know how to perform. The first step is bringing the helicopter to a stabilized hover into the wind and insure the ground is stable (for example, no loose gravel). Care must be taken when making pedal turns to avoid getting the tail rotor too close to ground. In the case of the ground sloping laterally, the pilot should slowly lower the collective until the upslope skid contacts the ground. At this point, apply lateral cyclic to firmly seat the skid into the slope. Maintain heading control with the pedals to prevent the skid from pivoting. Holding the upslope skid against the slope with cyclic, continue slowly lowering the other skid a little at a time with the collective. As the pilot continues lowering the collective, more lateral cyclic is required to hold the upslope skid firmly against the ground. If the pilot runs out of lateral cyclic prior to the downslope skid becoming firmly seated on the ground, then the slope is too steep and the landing should be aborted. When performing slope landings pilots need to be aware of the increased risk of dynamic roll over and, with a semi-rigid rotor system, mast bumping.

Once both skids are securely down, some instructors recommend centering the cyclic after the collective reaches flat pitch in order to have more clearance with the rotor system on the upslope side, others recommend keeping it displaced into the slope for the duration of the landing to prevent any sliding.

Lifting off a slope is essentially the reverse procedure. Raising the downslope skid with collective while moving the cyclic back neutral. Once the helicopter is level, lift off the slope. The pilot should keep in mind if a lot of weight is off loaded the CG might have changed enough to shift the cyclic neutral point, which could compromise a safe lift off.

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