Technique Archive

The mysteries of the height-velocity curve

Wednesday, December 17th, 2014

Call it what you want; height-velocity curve, dead-man’s curve or even limiting height-speed envelope for those who like sophisticated phrases. The “dead-man’s curve” is probably a carryover from our fixed-wing brethren while the industry generally accepts the simple reference of “H/V curve”. The inside of the curve is the area from which it will be difficult or nearly impossible to make a safe landing following an engine failure if you are in the same conditions depicted with respect to airspeed and altitude.

The H/V (height-velocity) diagram is a staple in the helicopter arena but sadly is often misunderstood by student and instructors alike. So, let’s take a look at what it is and how it is developed.

What is it?

The Height-Velocity diagram (curve) is a chart showing various heights above the ground with a combination of a velocity (indicated airspeed) where successful autorotation and landing is or is not possible. This magical combination of numbers yields two major regions on the chart; the area above the knee and the area below the knee. These areas are what actually plot much of the “curve”.  During initial helicopter certification, test pilots evaluate several characteristics of the helicopter that help determine the H/V curve. These factors include the helicopters initial response to a power loss, steady-state descent performance and power-off landing characteristics and capabilities.

Unknown to many, the development of the H/V curve and its associated number combinations is based on “pilot minimum skill level”. So, in a perfect world this means if the engine fails while I’m going this fast (KIAS) and at this height a pilot at a “minimum skill level” should be able to make a successful autorotation and hopefully some resemblance of a landing.

How do we define the pilot “minimum skill level?” That is a question I and many others can’t answer. Many will agree the current practical test standards are somewhat lacking and aren’t necessarily cultivating the “minimum skill level” necessary.

HVDiagramR44

No cookies for me please

To delve into this quandary let us recap the typical sequence of an autorotation training exercise. The instructor has the student line the helicopter up with the runway so that the power-recovery phase of the autorotation will occur as closely to the runway numbers as possible. Sound familiar? You know what I’m talking about, the “3, 2, 1, roll-off power” etc.

It’s the same thing with a 180-degree autorotation where the student is taught where to “fail” the engine based on tailwind strength and land at the “spot” within the practical test standard. Is there really anything practical about it?  Just what, if an engine failure occurs in the real world and the only spot you have is 600 feet directly below you? Could you get there? Safely? What about engine failures at night time with the same situation, the only place to go is directly below you or just out in front of you. If you remember anything from this article remember this, autorotations are like fingerprints in that no two are exactly the same.

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Slaying the dragon

Tuesday, August 26th, 2014

Regardless of what helicopter you are flying, whether it’s the Robinson R22, Bell JetRanger, or any helicopter for that matter, you need to be comfortable with autorotations. At our flight school we have broken the auto in to three flights. If you’re a CFI reading this, try it. If you’re the student or certificated pilot looking to get proficient, ask for it.

Start with talking on the ground, sitting in the helicopter, and going through the physical motions. Move the controls the way you would actually respond. If you are the CFI, play the whole thing down (mentally) and don’t let the student get beaten before they even lift off. If you are the client/student try to put past bad experiences with autos behind you.

First Flight: Auto-rotative decent. Climb to at least 3,000 feet. I like even higher. The only thing you want at first is RPM control. There is plenty of time to adjust airspeed. RPM is the constant in most cases. Climb back up and then try adjusting the airspeed all the way through the decent from 30-70 knots, noting what cyclic control movements do to the RPMs. Get comfortable with controlling RPM with mostly cyclic movement. The ONLY thing you want to achieve by the end of this lesson is comfort with RPM and airspeed control in the decent.

Second Flight: I like to start with quick stops from 50 feet and 60 knots, which is very similar to the flare in an autorotation. End this lesson with auto-rotative descents, followed by a flare (quick stop). Join the needles (rotor and engine RPM) very early so it seems just like the two maneuvers put together. By doing this you’ve learned to join the needles at 300 feet AGL, and not in the flare where most over-speeds occur. End this lesson being comfortable with descents and the flare.

Third Flight: Go over all three maneuvers and then combine them all together. Join the needles a little further down the line each time. Don’t be crazy about that; the auto looks the same regardless of where you join the needles.

If you want to accomplish full down autorotations, add a fourth lesson of hovering autos and run-on landings, which will be the same as a touch down from zero ground speed or from 15-20 knots if you are unable to zero out the ground speed.

This should build your confidence and make it fun, regardless of what helicopter you are flying.

Heroism

Friday, December 10th, 2010

I learned to fly helicopters in the early 1980s. Back then I read a story about a helicopter pilot who rescued an airplane pilot who had crashed on the ice of Lake Erie. The helicopter pilot was flying along the shore of the lake in a Hughes 300C when he heard on the radio that the Coast Guard helicopter had turned back because of weather. He decided to head out over the lake and look for the downed pilot.

The traffic-reporting helicopter did not have any navigational radios so the pilot asked the Coast Guard for directional information. Ice had built up on the blades and airframe, and his skids contacted the water a few times when he lost depth perception because of fatigue. After several attempts, in total darkness, he found the pilot. After getting him onboard, he flew back and landed on the shoreline with less than five minutes of fuel remaining. The helicopter pilot was awarded the Avco/Aviation/Space Writers Association Heroism Award.

There is no doubt that his actions probably saved the downed pilot’s life. However, had he crashed while attempting this rescue would he have been viewed as an excessive risk-taker or a hero who tried despite the odds? I wonder in today’s environment of risk assessments and safety management systems if perceptions would have been different?

Eurocopter’s quest for speed

Monday, November 22nd, 2010

In the 1980s, Bell and Boeing Helicopters began developing a twin-turbo shaft military tilt rotor aircraft called the V22 Osprey. Bell then teamed with AgustaWestland to develop a commercial version known as the BA609 and it achieved its first flight in March 2003. During this time the helicopter industry was excited about VTOL aircraft reaching higher speeds. However, Eurocopter was quiet about its plans only saying it had no plans to develop a tilt-rotor aircraft.

On September 6, Eurocopter began test flights of its high-speed, long-range hybrid helicopter concept, which combines vertical takeoff and landing capabilities with fast cruise speeds of more than 220 knots. Called the X3, it is equipped with two turbo shaft engines that power a five-blade main rotor system and two propellers installed on short-span fixed wings. The engines are RTM322s, which power the company’s NH90 military transport. The main rotor gear box is a derivative of the yet-to-be certified EC175 medium size twin helicopter with a modification of two output drives for the propellers.

In cruise flight the rotor pitch is reduced to provide minimal drag and the small wings provide lift. Thrust comes from the propellers. There is no tail rotor so yaw and anti-torque are controlled by a standard pedal configuration that varies the thrust on each propeller separately. The aircraft can be flown like a traditional helicopter until 80 knots, then the main rotor pitch is reduced as the propeller thrust is increased.

According to Eurocopter, the hybrid aircraft will cost about 25 percent more per hour to operate than a conventional helicopter. However, with the increased speed the company points out that when measured in a per passenger/mile basis the operating costs will drop 20 percent. The X3 is currently a technology demonstrator, but Eurocopter says the concepts could be ready for production models in less than a decade.

Tail-boom strikes

Thursday, November 4th, 2010

Some helicopters, like the Robinson R66, have tall masts putting the rotor system high above the tail boom; others, like the MD500, have a more compact design. Engineers take into account flight characteristics of a design when considering the distance between the tail boom and the rotor disk. Even so, tail-boom strikes can and do happen.

One of the more common scenarios is when a helicopter makes a hard landing following an autorotation. Touching down too nose-high on the aft part of the skids can cause a nose-down pitch that instinctively causes the pilot to pull back on the cyclic to counteract it. This action in combination with the fact that the blades are still moving downward can result in the blades contacting the tail boom. An in-flight entry to an autorotation will also cause a nose-down pitch because of the advancing blade seeing a greater reduction in lift than the retreating blade. A pilot who overreacts with sudden aft cyclic will cause the rotor system to flap back while the tail boom is still rising, which can lead to the blades to come in contact with the tail boom. In-flight blade-to-tail boom strikes are normally fatal.

Strong wind gusts can also create a problem. A tail-boom strike can happen as rotor rpm gets lower and the centrifugal force holding the rotor stiff drops. A helicopter that is starting up or shutting down in high winds or near another hovering helicopter is particularly vulnerable. Manufacturers have used droop stops in teetering rotor systems to support the blades at slow speeds. One design uses spring-loaded droop stops with weights that pull them out of the way when the rotor speed–and related centrifugal force–gets high enough. Even so, many manufacturers and operators have maximum wind speeds for start up and shut down.

Max performance take-off

Friday, October 8th, 2010

A maximum performance takeoff is used to climb at a steep angle to clear barriers in the departure flight path. To perform this maneuver successfully a pilot must consider the wind velocity, temperature, altitude, gross weight, center-of-gravity location, and other factors affecting performance of the helicopter.

The textbook procedure is this: After performing a hover power check to determine if there is sufficient power available, position the helicopter into the wind and begin by getting the helicopter light on the skids. Pause to neutralize all aircraft movement. Slowly increase the collective and position the cyclic to break ground in a 40-knot attitude. This is normally about the same attitude as when the helicopter is light on the skids. Continue to slowly increase the collective until the maximum power available is reached. Keep in mind this large collective movement requires a substantial increase in pedal movement to maintain heading. Use the cyclic, as necessary, to control movement toward the desired flight path and, therefore, climb angle during the maneuver. Maintain rotor rpm at its maximum, and do not allow it to decrease since you would probably have to lower the collective to regain it. Maintain these inputs until the helicopter clears the obstacle then establish a normal climb attitude and reduce power. As in any maximum performance maneuver, the techniques you use affect the actual results. Smooth, coordinated inputs coupled with precise control allow the helicopter to attain its maximum performance. Also, the helicopter will most likely be inside the shaded area of the height-velocity diagram where there is very little energy available to perform an autorotation in the event of an engine failure. Using maximum power would decrease the time the helicopter is exposed to a low energy situation.

However, other pilots I have talked to prefer a slightly different technique. Instead of using maximum power available, use the minimum power necessary to safely climb and clear the obstacle. The theory being that in the event of an engine failure the less power the pilot is using the more rotor rpm will be recoverable to help in the autorotation. Also, demanding high power might increase the probability of a part failure at a critical time. I would be interested in hearing comments on the two theories.

Robinson R66—First flight

Monday, September 13th, 2010

I just spent the last two days flying Robinson Helicopter’s new light turbine helicopter, the R66. Although it is still in experimental category, FAA certification is expected in the next 30 days as Robinson and the FAA work out some final details.

Having a couple of thousand hours in the Bell 206 light turbine series helicopter made for an easy direct comparison. Last year Bell announced that it would cease production of the five-place Bell 206B JetRanger, citing the R66 as one reason. Company founder Frank Robinson’s design goals are not just well-engineered products, but cost effective as well. The R66, the company’s first turbine helicopter, exemplifies this objective extremely well, and after flying it, I think Bell made the right decision.

The R66 is powered by a Rolls-Royce RR300 (model number 250-C300/A1), a new engine based to the proven 250-series engine (same engine used in the 206B). It is mounted below the transmission deck at a 37-degree angle which gives easy access for maintenance. The engine produces 300 shaft horsepower and is derated to 270 shp for a five-minute take off rating and 224 shp for max continuous operation. Starting is simple; igniter switch to enable (a nice feature that allows you to motor the starter without firing the igniters–no more need to pull the igniter circuit breaker); press-and-release the start button (it’s latched so no need to hold it down), at 15 percent N1 push the fuel control in and monitor engine light off and acceleration. At 65- to 67-percent N1 the starter disengages and the generator is switched on.

Picking the R66 up to a hover is smooth and it feels a little bigger and a little heavier than the piston-powered R44, which it is. I flew with Doug Tompkins, Robinson’s chief pilot who did all the experimental test flying on the R66. We were hovering at 64-percent torque and as we approached 60 knots during the take off Doug suggested pulling 100-percent torque. I started raising the collective, before I got to 90 percent the VSI was pegged at 2,000 feet per minute and at 100 percent we were climbing like a banshee. It didn’t take long to feel comfortable with the helicopter and we moved on to autorotations. These were predictable and basically a lot of fun. I did 180-degree, 90-degree, and out-of-ground-effect hovering autorotations to a full touchdown. It is just like the R44, only easier.

Another noticeable feature is comfort; the cabin is eight inches wider than the R44. The cyclic flight control retains Robinson’s T-bar arrangement. Not only does this ease transitioning from the R44 to the R66, but the T-bar is exceedingly comfortable in flight.

There is not doubt this helicopter will do very well. Once again Frank Robinson has found a need and filled it. The agile and turbine-powered R66 will do the jobs that a piston engine simply can’t, such as high-altitude flying. It will also find great acceptance in parts of the world where avgas is hard to get or just not available. And for those operators and contracts that require a turbine engine, the R66 will fit perfectly.

There is a lot more to say about this helicopter so look for a full feature article in an upcoming issue of AOPA Pilot magazine.

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.