This guide discusses some of the primary considerations to be made in selecting a Hartzell constant-speed propeller. Much of this discussion will apply to other certified propellers as well. This information is provided to help aircraft builders make a safe propeller choice that will enhance the enjoyment of their aircraft.
It will be helpful to begin with a brief description of a Hartzell "compact" series constant-speed propeller. The most visible parts of the propeller are the blades. Propeller blades are complex airfoils which are constructed of forged aluminum alloy or molded Kevlar™ advanced composite material. The blades are retained by, and transfer load through, a hub. The propeller hub is also constructed of forged aluminum alloy. Contained within the hub is the pitch-change mechanism. The pitch-change mechanism converts governed engine oil pressure into rotational motion to change the blade's pitch angle. A spinner is used to cover the propeller hub and conform to the engine cowling. Spinners can be made from spun aluminum or various composite materials.
Constant-Speed vs.
Fixed Pitch
Constant-speed propellers offer higher efficiency over a wider speed range compared to
fixed-pitch propellers. With a fixed-pitch propeller, builders have to choose between a
"climb" pitch and a "cruise" pitch. Choosing one results in a
propeller system that compromises the other. The pilot has only a limited range of engine
speed variation available to control the thrust of an aircraft equipped with a fixed-pitch
prop. The ability to vary blade angle in a constant-speed propeller allows the engine to
develop more power since rpm and power can be controlled independently at any air speed.
This enables the operator to optimize both cruise and climb for maximum efficiency and
performance.
Optimized for the Airplane Flight
Envelope
From a performance standpoint, not all constant-speed propellers are created equal. While
volumes could and have been written on propeller performance, we will save that for
another forum. In the simplest sense, props for "fast" airplanes are designed to
different criteria than "relatively" slower airplanes. This allows the propeller
to deliver optimal performance throughout the individual aircraft's flight envelope. Using
a prop design that does not match the actual aircraft's flight envelope will compromise
overall performance. Hartzell has created propellers especially for many kit aircraft. If
there has not been one specified, choose one for an aircraft that closely matches the
performance characteristics of the plane you are building.
Propeller Systems Endure Tremendous Stresses
Metal propeller blades typically experience centrifugal forces of 30,000 to 50,000 pounds
each. This is in addition to the aerodynamic loading of the propeller pulling an aircraft
through the air, as well as the forced inputs of the engine cylinders firing. All this
adds up to tremendous stresses literally trying to rip the blades from the hub and the
propeller from the aircraft. Propeller failures involving blade separations are
catastrophic, potentially life-threatening events. Blade separations can induce enough
vibration to shake engines off mounts, resulting in loss of control of the aircraft.
To ensure adequate strength and to prevent fatigue problems, all manufacturers of certified propellers perform the work required to obtain certification. Although you may be thinking "my airplane is experimental; why should I care about certification?"... remember that even though your airworthiness certificate says "experimental," the laws of physics are still in effect. The certification process is your assurance that the propeller has been adequately engineered to resist failure.
Propeller certification is done in two stages: "Basic" approval per Federal Aviation Regulation (FAR) Part 35, and, "Specific" approval for use on a particular aircraft per FAR Part 23.
The Basic Approval Process
Basic approval confirms that the propeller
meets certain strength and durability criteria. The following chart lists the FAR 35
requirements that must be met in order for a propeller to be type certified:
- Must have installation, operation, and maintenance manuals.
- Must be constructed from approved materials.
- Must have all operating limits defined.
- Must be able to withstand a 41% overspeed (twice the rated centrifugal force) without failing.
- Fatigue tests of the hub and blades must be conducted.
- Must pass an endurance test while installed on an engine.
- Functional testing requires operation of over 1,500 pitch change cycles.
- Durability testing must be conducted for 1,000 hours of operation.
Special conditions may be assigned. For composite blades, bird strike and lightning strike tests are generally required before the propeller is certified.
Specific Approval Is Critical To Safety
Specific approval assures compatibility between the propeller, engine, and airframe. The
most significant aspect of specific approval is the vibration approval. This approval is
the most important safety consideration in selecting a propeller for an aircraft.
Propellers, like all structures, have certain natural frequencies at which they will vibrate excessively with very little coaxing. The example of a tuning fork comes to mind. The sound produced by a tuning fork is it resonating at the fork's natural frequency. If a propeller is installed on an engine which excites (shakes) it at or near a natural frequency, like the tuning fork, it resonates. When operating in or near a resonance condition, stresses in the propeller are very high.
Most of us have flexed a paper clip back and forth until it broke. A propeller blade or hub can behave like the paper clip on a larger scale. The lower the amount of bending stress, the more cycles it takes to break the part and the longer it will last. If the stresses are low enough, the propeller will last almost indefinitely.
In a resonance condition, however, high stresses can occur at frequencies of up to 10 or 12 cycles per crankshaft revolution. This can put a large number of cycles on a propeller in a short time. A little arithmetic shows that 2500 rpm x 10 cycles per revolution of the crankshaft x 60 minutes per hour = 1.5 million cycles per hour. In 667 hours of flying, this works out to 1 billion cycles. If the stresses at this condition are high enough, it may not be long before something sudden and unpleasant happens.
Vibration approval of the airplane/engine/propeller combination involves flight testing a strain-gauged propeller and measuring vibratory stresses in various areas of the propeller while the airplane is flown throughout its flight envelope.
Generally, the propeller is also tested at several different diameters to allow field repair for blade erosion and minor tip damage. The stress levels measured in flight are compared to the strength of the propeller at the measured locations. Occasionally, a resonant condition will be found to exist at a specific engine operating condition. Often, a change in the blade design will correct this. When redesign is impractical, however, these propellers require placards which warn the pilot not to operate at the resonant condition.
Many factors can change the natural frequency or vibrational characteristics of the propeller and have major effects on the longevity and safety of the installation. In addition to blade design criteria, such as thickness, chord dimensions and diameter, and engine operating conditions, such as rpm and manifold pressure, other variables that can affect vibration include hub extension length, crankshaft counterweight configuration, and sometimes the indexing of the propeller on the crankshaft.
A builder may assume that simply because the propeller installation is "smooth" (lack of instrument panel, cockpit, and control stick vibration), it is safe from destructive propeller vibration. This is definitely not true. It is possible to fly along in a perfectly smooth cockpit while the propeller is gradually destroying itself. On the other hand, a particular installation could be very annoying to fly, but could actually be quite safe (at least until the instruments vibrate out of the panel and land in your lap). The reason a good installation is not felt in the seat of your pants is because the frequencies which occur in the propeller are typically much higher than the airframe structural frequencies which are sometimes be felt by pilots.
It is also worth pointing out that an accurate tachometer is required to avoid inadvertent operation of the propeller in a placarded region or beyond its maximum certified speed. Mechanical tachometers are notoriously inaccurate and should be checked and calibrated periodically to prevent this from occurring.
A secondary part of the specific approval of the propeller and an important criteria for achieving acceptable performance for the installation are the blade angle limits. Proper low blade angle allows the engine to develop maximum power for takeoff, while the high angle needs to be set high enough to prevent the engine from over-speeding at high air speeds. The low angle is typically set to achieve an engine speed of 50 rpm below red line when the airplane is not moving. Hartzell Owner's Manual 115 will guide you in setting this properly.
As part of the certification process, all type certified aircraft, engines, and propellers have a Type Certificate Data Sheet (TCDS) developed for that product. TCDS's contain a wealth of helpful information regarding the proper selection and setup of a propeller. Your local IA mechanic will have access to these and, of course, they are always available directly from the FAA or on the internet at http://www.airweb.faa.gov.
For certified airplanes, the aircraft TCDS's contain the propeller blade angle limits for that airplane. They can serve as a reference for a similar home-built airplane. It's a good idea to check your installation against a similar production airplane (one having the same engine, propeller and similar cruise and climb speed) for blade angle settings. The first figure at right shows a typical example, the first page of the Cirrus SR20 data sheets which lists the approved propeller models and blade angle settings.
Engine data sheets include information relevant to the propeller, such as rated horsepower and rpm limits, compression ratio, and, if applicable, the crankshaft damper configuration.
A real "gold mine" of information can be found in the propeller type certificate data sheets. These sheets include not only the basic approval information, such as diameter, horsepower and rpm limits, but they also contain "Note 9" vibration approvals. The second figure at right is an example of a prop TCDS.
As stated earlier, vibration approval is the most important aspect in selecting a safe propeller for your aircraft. An additional benefit of selecting an approved engine/propeller combination is that this will usually allow you to obtain a 25 hour test period for your airplane rather than the usual 40 hours required for uncertified engine/propeller combinations. The installations listed under Note 9 contain vibration approvals for the entire diameter range. Any placard limitations are also listed. This approval covers all normal category single-engine aircraft in a tractor configuration.
Such broad approval is possible because the airframe does not interact with the propeller in this configuration and the engine/propeller combination is the only critical issue from a propeller vibration standpoint. Use of the same "factory airplane" shock mounts and a similar engine mount structure is strongly recommended. Also note that twins and pusher installations are not covered by this approval as these aircraft installations can affect the propeller stresses differently than singles. Aerobatic aircraft are also not included because of the extremes of their flight envelope. These excluded configurations require testing in order to achieve specific approval for that aircraft type.
With a very few exceptions, we do not endorse the use of Hartzell propellers on non-certified engines due to the lack of demonstrated engine/propeller vibrational compatibility. In a few cases, Hartzell has performed the necessary testing to verify the vibration characteristics of automotive engine/propeller combination and have approved the use of a particular propeller with specifically configured engines. Unfortunately, due to the wide variety of engine conversions available and the testing expense involved, it is not practical to test all of them. If you are considering using a non-certificated engine that has not had a propeller vibration survey performed, a wooden fixed-pitched propeller is the safest choice.
Used
Propellers
Often builders want to economize by purchasing a used propeller for their project. Some
builders are even fortunate enough to buy a "firewall forward" package from a
damaged factory-built airplane or one which is being upgraded to another engine. When
purchased alone, the builder should verify vibrational compatibility with the engine. In
both cases, it is also important that the proper design matches the flight characteristics
of the airplane to ensure optimal performance.
If you do choose a used propeller either by itself or as part of a "firewall forward" unit, it should definitely be overhauled. Even if it looks fine at first glance, it could have internal corrosion or may have suffered other abuse in the past. In the overhaul process, the propeller is completely disassembled, corrosion is identified and removed, parts are checked for wear and damage, blades are measured to ensure sufficient material is present and airfoils are restored, and seals and corrosion preventive coatings are replaced. The identification and removal of stress risers, which can cause fatigue, are also a significant part of the process. When the prop is reassembled and balanced, it should also be set up to the proper blade angle limits using data discussed earlier. Additionally, you will get a shiny new paint job to go with your shiny new airplane and, of course, peace of mind.
An
Example
Armed with all of this background information, consider
the following example to examine some important considerations in propeller selection.
Let's say a builder we know happens to have a flying buddy with a factory-built airplane.
The propeller on his buddy's airplane has eroded below minimum-approved diameter limits
for the airplane and, therefore, must be replaced. The builder asks his buddy for the
eroded prop since he needs to cut down the diameter anyway in order to get more ground
clearance. He also plans to run his engine over the maximum recommended rpm, install
high-compression pistons to get more power and does not want extra noise due to excessive
tip speed. So, out comes the hacksaw, off go a few inches from each tip, and it's ready
for a quick balance check. While he's at it, he decides he has always wanted to have a
sleek extended nosebowl for his engine cowling. He goes to a friend with a machine shop
and a nice long spacer is created to fit the engine and propeller with the nosebowl and
spinner.
The great day arrives and our builder makes his first flight. Back on the ground, he is ecstatic, and, rightfully so. He has completed his project and it performs and handles well. But what's wrong with this picture? Unfortunately, plenty! All of the decisions he has made regarding his propeller can have a profound effect on the longevity of the airplane and its builder.
Our builder friend could have several problems. Aside from neglecting to check that the propeller, at its reduced diameter, was vibrationally compatible with his engine, the changes in operating rpm; compression ratio; and inclusion of a custom spacer make this a very high-risk installation. Cutting down the diameter below approved limits, increasing the compression ratio, or adding the spacer can increase stresses in the propeller. It also changes the locations and rpm at which they occur. Even if he had used an approved propeller at its proper diameter for the engine, over-speeding may have exposed him to new resonance conditions which may exist past the maximum rated rpm.
While this was a hypothetical situation, there are some real life examples of improper propeller application as well. For instance, on August 17, 1997 a Pitts S1S crashed due to a propeller blade failure. Metallurgical analysis revealed that the blade failed at mid-span due to fatigue. The NTSB report listed the probable cause as: "the operation of the propeller beyond design rpm limitations and the resultant gyroscopic loads from aerobatics. Contributing to the accident was the lack of a required instrument panel rpm restriction for the propeller mandated by an airworthiness directive."
In Conclusion
Hartzell has worked with many of the popular kit
manufacturers to develop suitable propellers to be installed with the engines recommended
for the kit. If you are building a kit for which a Hartzell propeller is available from
the kit manufacturer, obtaining the propeller from that manufacturer is the best option.
Many kit manufacturers also offer builders special pricing on Hartzell propellers. So, not
only is this choice less risky, it may also be less expensive.
The creativity and originality exhibited by aircraft builders are some of the things that make "home building" so enjoyable. Modifying or mis-applying a propeller, however, is a poor place to express your individuality. Why not take advantage of the available information that someone else has spent the time and money to acquire? Operating your propeller within its approved diameter and rpm range when installed on the proper engine will go a long way towards ensuring a safe, dependable aircraft. All it takes is a little awareness and research.