Archive for January, 2012

Gyroscopic Precession

Sunday, January 29th, 2012

When reading about helicopter aerodynamics you will see the term gyroscopic precession. It refers to a principle of gyroscopes that states when a force is applied to a spinning object, the maximum reaction occurs approximately 90 degrees later in the direction of rotation. Since a helicopter’s main rotor acts like a gyroscope, this principle applies to the rotor disc.

 

To understand what this means for a helicopter we need to know the basics of how a rotor disc tilts in the direction commanded by the pilot. Fundamentally, the pitch of each blade is controlled by a swash plate, and each blade is connected to the upper ring (also called the rotating part) of the swash plate via a pitch link. The cyclic flight control is connected to the lower ring (also called the non-rotating part). When control inputs are made it tilts the swash plate changing the pitch of each blade independently, according to the blade’s position, to tilt the rotor disk in the desired direction. 

This can be observed during a pre-flight. If you move the cyclic control forward you will see the swash plate tilt forward. While the swash plate actually tilts in the direction that the cyclic control moves, each blade’s pitch must change approximately 90 degrees prior to get the rotor disk to also tilt in that direction. This is accomplished by the blades’ pitch horns which are offset approximately 90 degrees. Therefore, to tilt the rotor disc forward in-flight (in a rotor system that spins counter-clock-wise) the pitch of the blade on the right decreases and the one on the left increases. Since maximum deflection takes place approximately 90 degrees later, the disc tilts forward.

Helicopter maneuvers manual

Monday, January 16th, 2012

I recently had the opportunity to read a new book on helicopter maneuvers by author Ryan Dale. The book is titled Helicopter Maneuvers Manual and he wrote it to give students more resources for understanding how to perform required helicopter maneuvers. I liked the full color illustrations he used to help students visualize what the maneuvers should look like. Although, all the drawings use the Robinson R22 (probably because it is still the most popular basic training helicopter) the information is applicable to other models as well.

Each maneuver or task is organized into 6 chapters:

1)      Ground Operations

2)      Basic Maneuvers

3)      Airport Operations

4)      Performance Operations

5)      Off-Airport Operations

6)      Emergency Operations

Each chapter contains the appropriate maneuvers with one or more color illustrations. I also liked the way it is organized with each maneuver or task having a clearly defined purpose followed by a detailed description with references to points on the illustrations. Also included is a section on common errors, tips and a reference to the Private and Commercial PTS. The appendix contains a Private, Commercial and Flight Instructor checklist of maneuvers from the associated PTS.

For new students this book is worth reading, and one of the better sources for getting a clear understanding of all the maneuvers required for helicopter flying.

Ryan Dale holds both a helicopter and airplane instructor ratings and works for a regional flight training company. He also wrote the Helicopter Oral Exam Guide. His books are available from ASA at www.asa2fly.com

Click on these to enlarge 

Sample Illustration

Sample Text

Special VFR

Wednesday, January 4th, 2012

On a sectional map, many large airports have “NO SVFR” printed near the airport information. SVFR refers to Special VFR, which allows a pilot to fly in lower visibility in controlled airspace. When giving flight reviews to helicopter pilots, I ask what that means. Occasionally, I am told that SVFR is not permitted at that airport. The correct answer is SVFR is not permitted for fixed-wing aircraft. FAR 91.157 states the requirements for SVFR, and appendix D, section 3 contains the verbiage that prohibits SVFR for fixed-wing only.

In fact, many of the requirements for SVFR are different for helicopters. A helicopter pilot still needs an ATC clearance and must remain clear of clouds; however they are exempted from the 1 statute mile restriction and the requirements for night time operations. Additionally, helicopters are excluded from the takeoff and landing requirements outlined in 91.157 (c). However, as with fixed-wing aircraft, the controller cannot suggest SVFR, a helicopter pilot must still request it.

In Canada the requirements for helicopters are more stringent than in the US. For example, a pilot must have at least 500 hours as pilot-in-command, have completed an approved pilot decision making course and received ground/flight instruction on issues related to reduced visibility. More details on additional requirements can be found in Canadian Aviation Regulation 602.117.