Category: Techspot

From the archives: Propeller Inertia and Why It Matters

Propeller Hub
With our website refreshment in-process, we decided to go back through the archives and select some of the most popular articles for re-posting. Enjoy this “classic” on prop inertia from the Fall of 2009.
In recent years, there has been substantial confusion in the light aircraft and ultralight industry regarding propeller inertia.

Rotax specifies the following propeller mass moment of inertia limits for their various gearboxes in order to “not overstrain the propeller shaft and gearbox and to avoid problems with the shock absorber installed in the gearbox”:

  • Model “A” or “B” gearbox–less than or equal to 3000 kg cm2
  • Model “C” gearbox–less than or equal to 6000 kg cm2

So what exactly is “mass moment of inertia”, and what does it mean?

In a nutshell, inertia is defined as the amount of resistance to a change in motion. The more resistant an object is to changes in motion (whether that is slowing down or speeding up), the greater its inertia. One example is that a heavy book is harder to get sliding than a lightweight book (ignoring friction effects). The heavy book has more resistance to changes in motion, so it has more inertia.

The definition of inertia is based on Newton’s First Law: objects in motion tend to stay in motion and objects at rest tend to stay at rest. Imagine a book sitting on a table. The book will not slide across the table unless something pushes it. Another example is the feeling of being pushed back into your seat when your car accelerates. Your body tends to stay at rest and the car moves forward. The difference you feel is the seat pressing into your back.

Specifically, engine manufacturers are concerned with the rotational mass moment of inertia of the propeller. Rotational mass moment of inertia is defined as the sum of the product of mass times radius squared, where the radius is the distance of the mass from the axis of rotation.

Rotational Mass Moment of Inertia = mass * radius2

For a propeller, the mass in this definition includes the mass of the propeller hub and the propeller blades. Notice that the units on Rotax’s limits match the definition [kg (mass) * cm2 (radius, or distance, squared)]. Rotational mass moment of inertia can be thought of as the amount of resistance to changes in rotational speed. Here are a few everyday examples of rotational mass moment of inertia:

  • It is easier to get a merry-go-round turning when children are at the platform’s edges than when the same merry-go-round has adults at its edges. The one with the adults on it has the higher rotational mass moment of inertia because it has greater mass.
  • A person sitting in an office desk chair is easier to spin with her arms tucked in than when she has her arms extended. Because the distance of mass (her arms) from the axis of rotation is greater with her arms extended, she is more resistant to being spun or stopped, and she has more rotational inertia in this position.

Notice that rotational inertia is not only determined by mass, but also the distance of the mass from the axis of rotation (also called the moment).

Every object has a unique equation for calculating mass moments of inertia. The more complex the geometry, the more complex the equation gets. Just like the merry-go-round and office chair, the rotational mass moment of inertia of a propeller is directly related to its geometry, the amount of mass and its distance from the axis of rotation. Every time a cylinder fires in your engine, it exerts a rotational force on the shaft and accelerates the propeller. The rotational inertia of the propeller causes it to resist the acceleration, putting torsional loads on the gears and shafting. The higher the mass moment of inertia, the higher the loads. These loads are transmitted through the gearbox, engine, and airframe thousands of times per minute in a typical light aircraft setup. Unless the loads are reduced or dissipated, the associated vibration can cause instrument failure, engine and airframe damage.

Engines and gearboxes are designed to carry specific loads. Engine and gearbox manufacturers specify mass moment of inertia limits so these design loads are not exceeded. Exceeding the mass moment of inertia limit can put your powerplant and airframe at risk for rapid or excessive wear or even failure.

It’s easy to go to AirVenture and pick up a snazzy new composite prop or to want to get that same 3-bladed big prop your buddy raves about. But whenever purchasing a new propeller, it is always a good idea to contact the manufacturer to be sure the prop does not exceed the mass moment of inertia limitations of your engine and gearbox combination. You could be saving yourself a lot of headaches or even avoiding a premature engine failure.

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Self-Regulation of the Light Sport Aircraft Industry: The Role of Third-Party Independent Audits

A Google search for the term “industry self-regulation” quickly turns up over 87,000 hits, with much of it being bad news: from papers on how self-regulation fails to keep dangerous internet advertising from children, to articles on salmonella in peanut butter. In a culture of instant news and visual media, whole companies and even the livelihood of entire towns (e.g. Peanut City, Georgia or Toy Paint City, China) wax and wane based on two things: industry sustainability, and the media. Today most of the general public hasn’t heard about LSA or Sport Pilot, despite the fact that our own spouses suffer from information overload on the topic. And while the goal is to revitalize and provide new cost-effective entryways to aviation, the idea is to do it without having LSA show up on the 11 o’clock news.
After working with regulating and advocacy organizations for at least a decade, the LSA industry in its pre-adolescent state has gotten the car keys from mom and dad. The question is, will we chin up to responsibility, get legitimate, and enjoy our newfound flexibility — or will we wreck with the freedom we have received?

According to Harvard Business author Michael Toffel “there are four angles [to industry self-regulation]: how the rules are designed, who adopts them, whether and how compliance is monitored, and whether these rules actually achieve what they purport to achieve.”1 How has the LSA industry stepped up in these four areas? First, the ASTM standards process has received accolades and has moved the standards development process at a lightning-quick pace, enabling the industry to even exist. 2Next, the “who” of adaptation includes over 60 manufacturers of thousands of registered LSA at the time of authoring this paper. The third area — compliance monitoring — is where we will linger for the rest of this white paper.

Imagine you are getting ready to engage in combat. Your product and savvy are your offensive weapons and your compliance to regulatory expectations, as legal protection, is your defense. Historically, the ASTM consensus standard approach provides very good protection, if the product and manufacturer are truly compliant. Despite the popular notion that auditors are “the bad guy,” auditors and their audits provide needed testing and proving for sustained industry legitimacy and public confidence. Just as a team’s defense or soldier’s defense is tested either with “drills” or with real games and combat, audits test a company’s defense and adds confidence that under fire, a manufacturer’s statement of compliance will hold. According to ASTM legal counsel, in cases where independent, third-party verification of compliance was completed, this protection resulted in some cases never even making it to court.3

So what are the characteristics of a sound, third-party audit program that will gain the respect of both legislative and judicial branches of the government? In other industries dealing with public safety such as food or amusement rides, water-tightness is not necessarily the expectation. Humans are clearly imperfect and the industries we create and sustain are done so imperfectly. One characteristic of effective compliance monitoring is that problems are identified and corrective action is taken. Another is that a healthy pattern with response to change in the industry and standards is established. Finally, a third characteristic of a successful oversight program is the fostering of a general attitude of responsibility and due diligence which is publicly exhibited.

The state of the LSA industry will clearly impact the way a penetrating audit program is designed. Today’s oversight has different needs and requirements than the oversight for more established industries, or for what the LSA industry will need ten years from now. The idea is to show that we can be responsible: that we are trustworthy to declare compliance, willing to be examined, and willing to correct any shortcomings.

The LAMA audit program, an effort recognized by the FAA, has been designed with these requirements in mind. The approach itself is two-pronged, consisting of a Compliance Check and an On-Site Check. The Compliance Check is typically the initial focus, involving a comprehensive check of verifiable compliance materials. An On-Site Check follows, which involves a spot-check of the facilities, including compliance to quality systems requirements and continued airworthiness standards.

Upon signing up for a LAMA audit, a manufacturer is asked to provide verifiable evidence for each and every requirement of the standards against which the audit is being conducted. This doesn’t mean that a long-time employee says to the auditor, “we have a fleet of 100 planes and haven’t seen a problem yet,” or “that landing gear member is so thick, there is no way it will bend.” It means that each requirement is addressed objectively. And the flexibility of the standards today makes this possible and affordable. Passing the LAMA audit not only puts the manufacturer in an improved legal position, but is just another pixel in the collective picture of good standing in the eyes of the consumer and our governing agencies.

In the case that major findings are determined during an audit, LAMA has structured tools and programs in place to help solve manufacturers’ compliance issues. Whether that means coaching the writing of a missing report or conducting additional testing or analysis, the audit team has cost effective resources at its fingertips. The aim of the LAMA audit program is to develop a healthy pattern of problem identification and cost-effective and efficient correction.

Recertification audits are completed periodically and cover areas of question, or of change to either the product or the standards. A focused, third party organization like LAMA can keep up with a dynamic and rapidly developing industry, helping manufacturers keep verifiable evidence for their compliance current. Just as a flight review completed every other year by pilots can help sharpen skills, develop new ones, or awaken a pilot to unknown dullness, these recertification audits add confidence to manufacturers who have already passed the initial two-part LAMA audit.

So we arrive at the fourth aspect of LSA self-regulation: is the industry achieving what the rules and rule-makers purport to achieve? Long, long ago in a land not too far from where you are now, many men and women started the work of dreaming and incubating the idea of an affordable way to fly simple aircraft. The LSA industry self-regulation journey walks a fine line between industry legitimacy, public reputation, and the flexibility, affordability, and accessibility of the aircraft. The answer to whether LSA will successfully and sustainably achieve this high calling is up to you. If you are an aircraft manufacturer or importer, will you advance public and governmental confidence by engaging the LAMA audit program?

If this article was helpful to you or for more information on this or other topics, please contact us.

3. ASTM International Legal Counsel, “Standards and Liability” presented in Miami, FL on 2 October 2008 by Tom O’Brien.