Archive for August, 2010

Low fuel

Tuesday, August 31st, 2010

I have studied and written about helicopter accidents for most of my career. I believe many accidents contain valuable lessons that can help all of us be safer pilots. However, one kind of accident that I find hard to believe–because it defies common sens–involves fuel exhaustion. Not a situation where something prevents fuel from getting to the engine, but rather when a pilot knows he is low on fuel, keeps flying anyway, and then experiences an in-flight engine shut down.

A helicopter pilot who allows his fuel to get too low has many more landing options than an airplane pilot. Maybe that contributes to a complacent mindset when in a low-fuel situation. I am sure it’s embarrassing, or maybe even places ones job in jeopardy, to land in a field low on fuel, but to me it sure beats the alternative. Moreover, some of the reasons pilots give for not stopping for fuel seem bizarre.

Case in point, according to the NSTB on October 15, 2002, a CFI was providing night VFR cross-country instruction to a student in a Schweizer 269C helicopter. They had discussed their low-fuel situation, but elected not to stop and refuel because neither had a credit card. On the last leg of their flight, the low-fuel light illuminated, followed a few minutes later by complete loss of engine power. During the autorotation the helicopter was substantially damaged when it struck trees and the tail boom separated from the airframe. Miraculously, neither pilot was injured.

This is not the first accident of this kind and, unfortunately, probably will not be the last.

Collective control

Thursday, August 19th, 2010

The T-bar cyclic control in the Robinson series helicopters is a departure from the typical flight-control design, however, its collective control conforms to industry standards. I know of only one manufacturer that has designed and installed a collective control that differs from the norm. Bell Helicopter certified the twin-engine model 222 in 1979 with a collective control that moves in more of a horizontal arc as opposed to up and down. The pilot pulls the collective rearward to increase pitch and pushes it forward to decrease. Bell used this arrangement on all subsequent variants including the model 430.

When designing the model 222 in the early 1970s, Bell was targeting the business jet community. Thinking the helicopter would appeal to corporate operators, the company believed the new collective design would feel more like a corporate jet. Additionally, Bell claimed the unique collective lever reduced pilot effort and enhanced safe operation. Also according to company documents, the near horizontal arc is perpendicular to any vertical rotor system vibrations which eliminate any pilot induced oscillations.

The twist-grip engine throttles are located on the collective, however, they are perpendicular (canted aft about 10 degrees to make for a comfortable grip) to the collective shaft as opposed to the standard in-line arrangement. This left/right design works well with the cockpit engine instruments making identification of the correct engine with its corresponding throttle very easy.

When I started flying a Bell 430, the collective movement was natural and instinctive from the beginning. Throttle friction is adjustable, so I never had an issue with inadvertently twisting the throttles while making collective changes. Although the standard up and down collective control works fine, I do like the arc motion better.

T-bar cyclic

Tuesday, August 10th, 2010

 

In 1978 Frank Robinson was granted a patent for a T-bar cyclic flight control system in a helicopter. His concept was a departure from the conventional helicopter flight control design where the cyclic control came up between the pilot’s legs. During the last 30 years the T-bar cyclic in Robinson helicopters has generated a lot of comments.

Conventional cyclic control in S300R66 with T-bar cyclic control
 

Robinson’s objective when designing the R22 helicopter was low cost and mechanical simplicity and the T-bar cyclic fits this design goal by reducing the complexity, weight, and cost of the conventional flight-control system. Other advantages include ease of getting in and out of the helicopter and a comfortable flying position as the pilot’s arm can rest on top of his leg. Moreover, not having the control between the pilot’s legs allows for a narrower cabin, which lends to a more aerodynamic fuselage design. Robinson used this arrangement on the larger R44 and the soon to be certified turbine- powered R66.

I have flown many different helicopters with the conventional cyclic design and do not see any difference in the T-bar’s ability to control the helicopter. Also, I agree with the claims of increased comfort while flying and find entering and leaving the helicopter much easier as well. I view it as simply different and believe with just a small amount flight time with the system most pilots will discover that it works quite well.

Still, like all systems, there are unique issues that need to be addressed. One is that the horizontal control bar pivots, allowing the pilot to lower the hand grip to his leg. I have seen pilots struggle with the control’s ability to move up and down while in flight. Once they get comfortable resting their hand on their leg, that goes away. Also, when giving dual instruction–especially to primary students–the instructor needs to pay close attention to the controls and keep his hand close to the cyclic grip, which can be fairly high up when the student is flying. Another point, even with the dual controls removed, is the access the front-seat passenger has to the flight controls. I have had pilots tell me that when giving rides they have had passengers grab or bump the center stick.

The T-bar cyclic works well, however, as with any aviation system, a complete understanding of limitations coupled with good student/passenger briefings is important.

Stored energy

Monday, August 2nd, 2010

 

One way to think of autorotation is the effective use of stored energy to safely land the helicopter. Like the slow and careful release of the energy stored in a wound spring as opposed to allowing a quick high-energy release.

 

 

 A helicopter sitting on the ground has no stored energy (battery excluded). After start up, the energy in the fuel is converted to motion via the engine. During lift off and climb out the engine continues to add energy to the system. Once established in cruise flight, the helicopter has three sources of stored energy: kinetic energy (from motion) in airspeed and rotor rpm, and potential energy (known as gravitational potential energy because of its position in a gravitational field) in altitude.

 

  

When an engine quits, the conversion of fuel to energy stops. When this happens the first step is to lower the collective control to reduce drag on the main rotor blades, which prevents rotor rpm from slowing down. This causes the helicopter to start descending and this begins the consuming of altitude energy to keep the rotor system spinning. In an autorotative descent, at a fixed airspeed, lowering the collective will increase rotor rpm. To spin faster, the rotor system requires energy, so energy is removed from stored altitude and the helicopter’s descent rate increases. In reverse, raising the collective takes energy from the rotor system (it slows down) and transfers it to altitude and the helicopter’s descent rate slows. Using the collective, the pilot can move energy between rotor and altitude to assist in maneuvering the helicopter to a landing spot.

 

 

However, it is extremely important to not let the rotor rpm get too slow. Allowing this to happen will cause the rotor blades to stall and completely eliminate the pilot’s ability to control and slowly use the stored energy. The helicopter will free-fall and release all its energy at impact–enough energy to destroy the helicopter and its occupants.

 

  

In an autoraotative glide the pilot can also control airspeed and the same energy transfer concepts apply. Increasing airspeed requires energy and it needs to come from somewhere. In this case from altitude and rotor rpm, so when increasing airspeed the helicopter will descend faster (loss of altitude energy) and rotor rpm will drop (loss of rpm energy). Basically, the pilot is transferring energy from altitude and rotor rpm to airspeed. Decreasing airspeed puts energy back into altitude and rotor rpm. It is the skillful manipulation of all this stored energy that will allow the pilot to make a successful power off landing.

 

 

As the pilot maneuvers to a landing spot the helicopter gets closer to the ground and is running out of stored altitude energy. That’s OK as the goal is to land. Maintaining approximately 60 knots airspeed leaves a healthy amount of energy in airspeed to stop the descent rate and this is done by flaring at a low altitude, normally less than 100 feet agl. During the flare, the rotor system will also absorb energy causing rpm to increase and can be controlled by raising the collective. Care must be taken to not flare too much or add too much collective as this can cause the helicopter to gain altitude. The objective is to bring the helicopter to about a 5- or 10-feet hover above the surface. Timed right, all or most of the airspeed energy will be consumed and the helicopter will momentarily be close to the ground with no descent rate and little or no forward speed. However, it will start descending again and here is where the pilot will raise the collective to provide a burst of lift to cushion the touchdown. Raising the collective uses the energy stored in the rotor system and rpm rapidly slows down. Done right the helicopter will once again be sitting on the ground with all of its stored energy depleted.

 

 

AS350 flaring during a practice autorotation

AS350 flaring during a practice autorotation