Definition of throttle valve

1. a valve designed to regulate the supply of a fluid (as steam or gas and air) to an engine and operated by a handwheel, a lever, or automatically by a governor; especially  the valve in an internal combustion engine incorporated in or just outside the carburetor and controlling the volume of vaporized fuel charge delivered to the cylinders  

Throttle ball valves are based on the 1B valves and include modified ball ports to allow flow control. 

• Features on 1B valves apply.

• Designed to meet the demands of virtually all throttling applications. 

• Flow passage is engineered specifically for high-pressure drop throttling service with precise control throughout the operating range. 

• Delivers higher linear flow rates than any equivalent positive choke. 

• Flow indicator for precise flow control. 

• Ball design enables precise throttling with minimum erosion resistance.

Throttle valve

In general terms, the throttle valve must regulate the air or mixture supply for the combustion engine. Depending on the engine concept, this serves different purposes. In the case of petrol engines...

Function

In general terms, the throttle valve must regulate the air or mixture supply for the combustion engine. Depending on the engine concept, this serves different purposes. In the case of petrol engines, speed and power output are regulated by means of fresh air or mixture dosing. Diesel engines generally do not need a throttle valve. However, in modern diesel cars, throttling the amount of intake air facilitates precision control for exhaust gas recirculation and stops the engine from shaking when the ignition is switched off.

The throttle valve is installed in the intake air system of the combustion engine. The opening angle of the valve determines how much fresh air or air/fuel mixture flows into the cylinders (carburettor engines, for example). In older generation engines, the throttle valve is connected directly to the accelerator pedal and operated mechanically via a cable. For newer vehicles there are various principles of operation:

Electromotive throttle actuators:

With electromotive throttle actuators, the position of the throttle valve is regulated mechanically via the accelerator Bowden cable. The throttle valve electronics forward the position of the throttle valve to the engine control unit as an electrical signal. This information is compared with other up-to-date data from a variety of engine management sensors. The engine control unit permanently calculates the optimum throttle position for consumption and exhaust gas emissions and sends this information back to the throttle valve as an electrical control signal. The position of the throttle valve is then fine-tuned with the assistance of a servomotor.

Electronic throttle actuators:

With electronic throttle actuators, there is no direct connection to the accelerator pedal. The driver's desired load is captured by an electronic accelerator pedal (electromotive throttle actuator). The engine management permanently matches this signal to all other available data from the engine sensors, using the information obtained to calculated the optimum throttle position for the prevailing situation. The electronic throttle actuator is controlled exclusively using the control signal from the engine management and with the assistance of a servomotor.

Air management valves:

If throttle valves are used in diesel engines, they are generally referred to as air management valves. Air management valves can be with or without integrated control electronics. As indicated above, air management valves throttle the intake air in the intake air system of diesel engines via electromotive means in order to achieve precision controlled exhaust gas recirculation and prevent the inconvenient shaking that would otherwise occur when the engine is switched off.

Air flap servomotors:

Air flap servomotors are electrical actuators with integrated position sensor and optional integrated electronics. They facilitate the continuous adjustment of intake pipe flaps or turbocharger guide vanes, for example, and, by means of more precise control, are able to replace conventional pneumatic drives which are no longer sufficient for the advanced requirements that have to be met.

ELECTROMOTIVE THROTTLE ACTUATORS:

With electromotive throttle actuators, the position of the throttle valve is regulated mechanically via the accelerator Bowden cable. The throttle valve electronics forward the position of the throttle valve to the engine control unit as an electrical signal. This information is compared with other up-to-date data from a variety of engine management sensors. The engine control unit permanently calculates the optimum throttle position for consumption and exhaust gas emissions and sends this information back to the throttle valve as an electrical control signal. The position of the throttle valve is then fine-tuned with the assistance of a servomotor.

Safety

The perfect function of the throttle valve is the key to optimum power development of the vehicle in critical situations. As such, the throttle valves make an essential contribution to improved road safety.

Environmental protection

Optimum operation of the combustion engine and minimum pollutant emissions rely on precision control of the intake air. Throttle valve modules with integrated electronics enable the intake air quantity to be exactly matched to the prevailing operation conditions independently of the driver's performance requirements. As such they make an important contribution to effective fuel combustion and low pollutant emissions.

Depreciation

Throttle valves are maintenance-free. They are designed to last the entire service life of the vehicle. Poor maintenance (missing oil change intervals, for example) can lead to soiling of the throttle valve and cause deposits to build up, resulting in premature wear or even complete failure. For this reason, compliance with the maintenance intervals prescribed by the vehicle manufacturer is essential.

Propeller Damage, Direction of Engine and Propeller Rotation

Propeller Damage

It is an unfortunate fact of life in aviation that accidents occur. There are many forms of aircraft accidents which can inflict potentially serious damage to the propeller and, by extension, to all the internal components of the engine system.

The term "engine system" includes all the rotating and reciprocating parts of the engine proper, AND the PSRU (if applicable), AND all driven accessories (prop governor, distributors, magnetos, pumps, etc.)

The picture below shows an obvious example of severe propeller damage. However, you should be aware that serious damage to the entire engine system can occur during events far less serious than what is shown in this picture.

In the world of certificated aircraft, there are required procedures to be followed in the event of a prop strike. These procedures are well-established, and have been developed mainly from the hard school of experience.

In the experimental community, there appears to be a somewhat cavalier attitude about such events, apparently based on denial and wishful thinking. In fact, after the accident pictured above, there was a strong opinion among the people directly involved that no inspection to any engine system parts should be needed because the prop involved wasn't a metal prop!

The discovery and correction of potentially fatal damage to engine systems resulting from a prop strike incident is the subject of a recent EPI Service Bulletin (hyperlinked below).

A portion of this Service Bulletin is presented here in order to give the mose exposure possible to the seriousness with which ANY damage to a propeller should be treated. These definitions and procedures have evolved from the experience gained from post-accident investigations over the course of many, many years. Consider them to have been written in blood.

The following definition of PROPELLER STRIKE is taken from a blending of the requirements stated in Lycoming Service Bulletin SB 533-A, Continental Service Bulletin SB96-11, and AD-2004-10-14.

A propeller strike is defined as follows:

1. Any incident, whether or not the engine is operating, that requires repair to the propeller other than minor dressing of the blades;
2. Any incident during engine operation in which the propeller impacts a solid object which causes a drop in RPM and also requires structural repair of the propeller (incidents requiring only paint touch-up are not included). This is not restricted to propeller strikes against the ground, and although the propeller may continue to rotate, damage to the engine may result, possibly progressing to engine failure.
3. A sudden RPM drop while impacting water, tall grass, or similar non-solid medium, where propeller damage is not normally incurred.
4. Any propeller strike occurring at taxi speeds and during touch-and-go operations, involving any propeller tip-to-ground contact.
5. Any situation where an aircraft is stationary and the landing gear collapses causing one or more blades to be bent or substantially damaged, or where a hangar door (or other object) strikes the propeller blade. These cases should be handled as a sudden engine stoppage because of potentially severe side loadings on the propshaft flange, front bearing, and seal.

Whereas the definitions stated above are intended to be unambiguous, the reality of aircraft ownership is that those definitions are likely to form the genesis of endless quibbling about such arcane trivia as:

1. How sudden is "sudden",
2. How tall is tall grass?
3. How deep does the water have to be?
4. How much of an RPM drop? (50 RPM, 500 RPM) ?
   {Were you really watching the tach when the prop hit?}

These arguments are specious and merely constitute an attempt to rationalize out of doing the required inspections. And, even though a large number of such inspections find no damage, the consequences of undetected damage from a "prop strike" can be grave.

(If you doubt the susceptibility to such damage, take a look sometime at the tiny dowel that locates the accessory drive gear on the end of a Lycoming crankshaft.)

The information contained in the complete EPI Prop Strike SERVICE BULLETIN should be of interest to ALL operators of propeller-driven aircraft.

ONE EXAMPLE

The picture at the top of this page will, undoubtedly, bring some form of gut-wrenching to all builders. It was taken just after an accident which resulted from an unfortunate power loss in a brand-new, V8-powered Lancair 4P, on its way to OSHKOSH-2001. According to many people who saw this aircraft (prior to this event), it had the potential to capture the Grand Champion award.

The builder completed this aircraft in July, 2001. It was subsequently inspected and signed off by the FAA. The initial test flight program was conducted by Lancair Factory test pilot Don Goetz. During the initial flight hours, the builder received transition training from Don, and continued on to fly off the remaining test hours. The FAA then removed the restrictions, and the aircraft was prepped for the flight to Oshkosh.

In preparation for the trip to Oshkosh, the crew performed a final inspection of the aircraft, which included removal of the cowling and a thorough inspection of the engine system (EngineAir turbocharged V8 with EPI Mark-9 PSRU).

Early the next morning, the builder and a friend boarded the aircraft and departed for Oshkosh (on a flight plan). While climbing through 8,000 ft. on the way to the assigned cruising altitude, the engine began to gradually lose power, and ultimately stopped producing any power at all. The only indication of malfunction was an extremely low MAP, suggesting at first, a broken throttle linkage.

The pilot (and builder) handled the situation by the book. First and foremost, while initially analyzing the situation, he FLEW THE PLANE. Maintaining best-glide airspeed, he contacted ATC to declare an emergency, located potential landing sites, selected the best, and flew toward it.

Note that, during this adventure, the prop is windmilling and driving the engine through the PSRU, creating the effect of a large vacuum pump (MAP = 5" HG) and a very effective speed brake. In a normal power-off descent, this aircraft, with roughly 30 PSF wing loading, will manifest a descent rate unfamiliar to pilots accustomed to small GA certificated aircraft. In this situation (with the prop driving the engine at very low MAP), the descent rate necessary to maintain safe airspeed would be daunting to all but the most experienced pilots.

This situation has all the elements for a tragic outcome:

1. a total loss of power,
2. a significant increase in drag,
3. a pilot with very little experience in high-wing-loading aircraft,
4. the added emotional burden of a pilot who has just invested a significant fraction of his life in the construction of this potentially-prize-winning aircraft (not to mention the MONEY involved).

Fortunately, the weather was good.

To his great credit, this pilot overcame all these problems, maintained control of the aircraft and delayed deployment of flaps or gear until he was assured he could make his targeted runway.

Unfortunately, there was not quite enough energy in the system for him to reach the approach-end of the targeted runway. The pilot realized he would be a bit short, kept the aircraft configuration clean and fought off the strong impulse to try and milk a few more feet of glide. He maintained control through a premature touchdown, a few hundred feet short of the runway. The aircraft came to a controlled stop a few feet after sliding onto the pavement, and both occupants emerged unharmed (picture above).

The cause of the power loss was interesting. During the post-accident engine system was disassembly, the intercooler was found to be completely blocked by a large mass of ground paper particles, having the same color as the paper towels used during the cowl-off inspection. Further searching revealed a fault in the alternate-air system which, apparently, allowed a paper towel (probably left in the engine bay during the inspection) to be ingested by the turbocharger, pulverized by the compressor, and discharged into the intercooler. The sharp edges and small passages in the intercooler quickly collected most of these particles, and ultimately blocked it completely. There was no damage to the engine system.

The EPI Mark-9 PSRU and Woodward prop governor were inspected in accordance with EPI Service Bulletin 260002. After this complete disassembly and thorough inspection, both the PSRU and the governor were found to have suffered no detectable damage. The PSRU bearings and seals were replaced during the rebuild simply as a matter of good practice.

Direction of Engine and Propeller Rotation

This page presents the factors to consider when deciding on the direction of engine and propeller rotation. One of the main decisions is whether or not the aircraft designer is willing to build an aircraft with a propeller which rotates in the "wrong" (for American aircraft, but "right" for European aircraft) direction.

The aircraft in which most of us learned to fly have propellers which turn clockwise as viewed from the cockpit. Most pilots are unwilling to reverse all those hard-learned synapses which cause automatic application of the correct rudder pedal in climb, stall, and high-gyroscopic-moment maneuvers. (This reluctance, of course, does not apply to pilots who use the pedals only during taxi.)

Let’s assume that a counter-clockwise rotating propeller (on a single-engine airplane) is not acceptable. Most engines considered suitable for conversion turn clockwise as viewed from the cockpit. If one wishes to retain that direction of rotation on the propeller, one could select a two-mesh PSRU, or a PSRU using belts, chains, or certain planetary arrangements, in which the output shaft rotates in the same direction as the input.

The output of a single-mesh externally-toothed gear reduction is opposite of the input. Therefore, if one decides to use an externally-toothed offset gear-PSRU, one can: (1) reverse the engine rotation and use a single-mesh reduction attached to the flywheel-end of the engine, (2) retain the original direction of engine rotation and use a PSRU with an idler gear, or (3) retain the original engine rotation and drive a single-mesh PSRU off the wrong end of the crankshaft (a VERY bad choice). Here is an expanded discussion of each option.

1. Reversing the engine (covered on another page of this site) so that it turns in the opposite direction of the propeller has some interesting advantages. An obvious one is that, all other things being equal, a single mesh gearbox will weigh less than one with an idler gear. EPI has designed propeller reduction gearboxes in both configurations (with and without an idler). One model for an engine producing up to 600 lb.-ft. of torque weighs 73 pounds in the two-mesh (idler configuration), including an integrated gear-drive for the prop governor. The single mesh version of that same EPI PSRU weighs just 59 pounds including the same integrated prop-governor drive. A fourteen pound weight reduction can be hard to find on a 500 pound powerplant.
   
   A less obvious, but significant advantage is that the gyroscopic moment of the engine can reduce or cancel the gyroscopic moment of the propeller with respect to the loads the engine mount applies to the airframe. In fact, when EPI designed the engine mount system for the Orenda conversion of a popular high-performance twin, the calculations showed that the gyro-moment of the counterclockwise-rotating engine significantly reduced the gyro-moment of the large metal propeller.
2. A gearbox which contains a single idler gear requires special attention with respect to gear fatigue. The teeth on the PSRU input and output gears are subjected to severe fatigue loading: zero to maximum and back to zero, every revolution. However, the teeth of a single idler which meshes with both the input and output gears are subjected to the most severe form of fatigue loading: fully-reversing load. They are fully loaded in one direction when they mesh with the input gear and fully loaded in the opposite direction when they mesh with the output gear. In the single-idler case, the allowable working stress level is about 2/3 of the unidirectional-load value, and tooth load limits are usually established by the idler at that reduced working stress.
   
   Some gearboxes use an idler having two different gears on a common shaft, one meshing with the driver and one meshing with the driven gear. That arrangement has two advantages: (a) because of the compound reduction, it can implement a large reduction ratio with relatively compact gears, and (b) it eliminates the additional fatigue load experienced by a single idler gear. However, that arrangement adds weight and cost.
3. There are several good reasons not to drive off the wrong end of the crankshaft, including (a) lack of crankshaft structural strength to support the torque output stresses, and (b) torsional vibration issues potentially destructive to the crankshaft. The folly of this practice has been proven time and agiain in several classes of boat racing, but still it happens.... A further discussion of this particular example ofWishful Thinking Engineering is beyond the intended scope of this presentation, but be assured the arguments are both valid and persuading.

Propeller Vibration

There are two primary reasons why it is important to suppress engine torque pulses. The first (discussed in PSRU Technology) is to reduce the fatigue loadings on the gearbox components.

However, metal propeller blades are especially unforgiving of being excited near a resonant frequency. So the more important reason to suppress engine torsional excitation is to eliminate the pulse excitations fed through the gearbox, multiplied by the gear ratio, and applied to the propeller blades.

Each prop blade has more than one resonant frequency. The frequency excited by thrust vibrations is different (typically) from the one excited by torsional vibrations.

Metal prop blades are especially susceptible to destructive vibration if they are excited near a resonant frequency. One reason is that a metal blade contains very little internal damping, so resonant vibrations can build in amplitude very quickly. Another reason is that the blade material (typically aluminum) HAS NO ENDURANCE LIMIT in a fatigue environment. Therefore, the fact that an aluminum part has run for x-number of fatigue cycles is NO GUARANTEE that it will not fail during the next hundred similar cycles.

Manufacturers of certified props go through extensive analysis and testing to be sure that a particular prop will survive the fatigue environment produced by a particular engine on a particular airframe. Get it wrong, and blade pieces will be departing the aircraft. It’s just a matter of time.

In addition to being susceptible to engine vibration, a propeller produces torsional excitation which varies with rotational and translational speed, flight attitude, airframe characteristics, and the specific properties of the engine torsional excitation which are applied to the propeller.

For purposes of illustration, consider just one mode of propeller-blade vibration. Picture yourself in the pilot seat, looking forward over the cowl watching the rotating propeller, being driven by your 4-cylinder Lycoming. (For the purposes of this experiment, you will have acquired very-high-speed vision, which enables you to see fast-moving things in slow motion.)

In normal vision mode, you see the prop blades rotating clockwise across your field of vision. Now, switch into high-speed-vision mode. Here is the slow-motion picture you see: A prop blade rises into view just as one cylinder of the engine begins it’s compression stroke (remember the instantaneous torque curve for a 4-cylinder engine in the TORSIONAL EXCITATION BY PISTON ENGINES section). As that progresses, the instantaneous torque is at its minimum (actually negative in a 4-cylinder engine). Since the torque is minimized, the engine decelerates slightly and is not keeping up with the rotation of the prop. The blade, being a heavy, fast-moving thing, tries to maintain its speed, but the prop hub is slowing down. The blade, also being a springy thing, deflects clockwise in a nice curve as the result of the blade momentum being opposed by the decelerating hub.

Now, just as the blade reaches it’s maximum clockwise deflection, the cylinder fires and the crankshaft torque quickly reaches it’s maximum positive value, and the crankshaft now tries to accelerate the prop hub, which in turn, tries to accelerate the blade we are watching. The blade responds by starting to bend in the opposite direction (counterclockwise), but it was already in motion, springing back from the clockwise deflection we just saw. That motion, added to the motion induced by the new pulse, causes this blade deflection to be greater that the one which occurred last rotation.

This is an example of blade excitation at a frequency too close to resonance. Each cycle imparts energy into the system, and the spring-mass properties of the blades stores some of the energy and adds it back into the system in phase with the engine. The forces increase with each cycle. The deflections continue to increase as a result of the energy added during each cycle, until something breaks.

There are more complex modes of propeller blade vibration, but this simple one serves to illustrate the point.

The more likely scenario is that high blade stresses induced by vibratory bending near resonance exceed the endurance limit of the blade material, and the fatigue cycles pile up, and at some point, a piece of blade departs. That’s why certain direct-drive aircraft engines have a band (2200-2400 RPM, for example) where continuous operation is not allowed.

If a large enough section of blade breaks off, the resulting vibration could easily tear the engine off its mounts. Consider how the resulting CG might alter the rest of your day.

The propulsion systems of certificated aircraft are approved as a powerplant/propeller/airframe combination. Torsional interactions between engines and propellers are subject to close engineering scrutiny. In order for a new propeller to be approved for a certified aircraft, it must be shown that the engine-propeller-airframe combination does not experience dangerous levels of stress from vibration. That is accomplished by design and development engineering, and verified by an extensive vibration survey, performed in flight, using real-time data acquisition equipment.

Metal-blade propellers are especially critical because they have very little internal damping and closely resemble perfect springs. Wood and composite propellers have varying degrees of internal damping, so they tend to be more tolerant of torsional excitation. The point is this: each system is different, and needs to be investigated.

The need for such scrutiny is illustrated by a few examples.

1. A certain propeller which survives quite well on a Continental IO-520 (6-cylinder horizontally opposed, 520 cubic inches, 554 lb.-ft. mean torque, torsionally counterweighted) has unacceptable vibration problems on a Lycoming IO-540 (6-cylinder horizontally opposed, 540 cubic inches, 585 lb.-ft. mean torque, torsionally counterweighted).
2. A recent Hartzell vibration survey revealed an interesting situation. There is a certain Hartzell propeller which is certified on a Lycoming IO-360-A3B6D for a specific aircraft. This engine is rated at 200HP, and is equipped with 8.7:1 compression ratio pistons (LW-10207-S) and 6^th and 8^th order torsional counterweights. On that engine, the propeller blade stresses were acceptable.
   
   However, there is an STC for that aircraft which allows the installation of the Lycoming 10:1 pistons (LW-11487-S from the HIO-360-D1A) to increase the engine power output. Hartzell discovered that the installation of those STC’d pistons significantly altered the torsional characteristics of the IO-360-A engine, to the point that, on that same propeller, it drove the blade root stresses beyond the endurance limit of the material. That engine, with its pendulous torsional absorber counterweights, might be expected to be "torsionally tolerant" (for a 4-cylinder engine). The fact that a relatively small change in engine configuration caused such a significant change in vibratory stress gives some insight into just how critical this engine/propeller interaction can be.

While we are on the subject of propellers, an extremely dangerous practice in the experimental community is that of shortening the blades of a given propeller to fit a new situation. For example, a certain popular certificated Hartzell propeller has a minimum diameter of 72". If the blades are shortened below that limit, the resonant frequency of the blades will be increased to the point that engine torsionals can excite vibratory stresses which exceed the endurance limit of the blade alloy. What that means is that sooner or later, one of those blades is going to shed a piece, which could easily tear the entire engine off the airframe. Again, consider how the resulting CG might alter the rest of your day.

We have read reports on conversion projects in which propellers from turboprop applications have been shortened and attached to a V8-with-PSRU. That should give someone nightmares, since the vibratory modes which a turboprop propeller is designed to handle are significantly different from those generated by a high-output piston engine, and the PSRU's used in these projects had no torsional pulse suppression characteristicswhatsoever.

There are many complex modes of propeller blade vibration. These examples are given to emphasize the fact that making a propeller survive on a piston engine is a challenging task, demanding very specialized skills and equipment. The intent here is to discourage the practice of willy-nilly mix-&-match attachment of propellers to engines, with the compatibility of the bolt pattern being the only criterion.

A deeper insight into propeller considerations is presented in Sport Aviation Magazine (July, 1994) by Brian Meyer, a propeller engineer. (Access to the Sport Aviation online archive is restricted to EAA members.)

Propeller Technology

Propeller Performance Factors 

The performance of a propeller in flight involves several complex subjects, and the high performance propellers we have available today are the product of a huge amount of engineering, development, testing, and (unfortunately) a few mistakes. The selection of an appropriate propeller for a new aircraft should not be done without considering several factors which characterize the performance of a propeller. The following sections (EFFICIENCY, TIP SPEED, and PERFORMANCE MAPS) present a few basics regarding propeller performance.

EFFICIENCY

The purpose of a propeller is to convert power (delivered by a rotating shaft) into thrust. It does that by accelerating a large mass of air to a higher velocity. The effectiveness with which a propeller performs this conversion is known as "efficiency".

As you already know, a propeller blade is a sophisticated whirling airfoil. At a constant RPM and aircraft true airspeed, the speed of the air over any portion of the airfoil varies with the distance from the center of rotation. The maximum velocity occurs at the point of maximum thickness out near the tip.

Therefore, in an effort to provide an ideal angle of attack all along the blade, the blade has a "twist" to it which varies the pitch angle of the blade from root to tip. The pitch angle of a blade (β) is typically the angle measured at 75% of the radial distance from the center of rotation to the prop tip.

As aircraft velocity increases, the angle of attack seen by the prop blade of a fixed-pitch prop will decrease. That effect limits the maximum efficiency of a fixed pitch prop to a single airspeed at a given RPM, as shown by the following plot  of efficiency at different blade pitch angles (β) shows.

Figure 1

The curves in Figure 1 suggest that if the blade pitch could be varied in flight, the prop efficiency could be very high for a wide range of operating conditions. Therefore, many propellers contain a mechanism in the hub to change the overall pitch of the blades in response to a servo command from a control system. That control system is typically a propeller governor, which maintains prop RPM at a pilot-set value (within certain limits) regardless of aircraft speed or engine power setting.

Propeller efficiency is defined as:

eff = "K" * Thrust * Speed / Power

(where "K" is a constant to account for units).

If the system of units is Pounds (thrust), Horsepower (power) and Knots True Airspeed (KTAS), then the equation becomes:

eff = ( Thrust * KTAS ) / ( HP * 326 )

(if you prefer MPH instead of Knots, use 375 instead of 326)

The equation for efficiency has other useful forms. Rearranging the terms, the equation for the thrust produced at a known airspeed, engine power, and prop efficiency is:

Thrust = ( HP * eff * 326 ) / KTAS

To find the HP required to produce a known thrust at a known airspeed and prop efficiency:

HP = ( Thrust * KTAS ) / ( 326 * eff )

To find the speed which can be reached with a known engine HP, prop efficiency and airframe drag (thrust = drag in steady state level flight):

KTAS = ( HP * eff * 326 ) / Drag

It is clear from the relationship between power, thrust and speed, that if power and propeller efficiency are held constant, then propeller thrust decreases as true airspeed increases. Add to that the fact that aerodynamic drag increases with the square of speed, and it becomes clear why it takes 8 times the power to double the airspeed ( 8 = 2 ³ ) of a given airframe (oversimplified to make the point).

Figure 2 shows a plot of the thrust generated by a particular (variable pitch) propeller as a function of the airspeed (15 through 240 MPH) and power applied to it (250 through 500 HP).

Figure 2

In case you were wondering, the hump in those curves is due to the fact that at low airspeeds, prop efficiency is very low. As airspeed increases, so does efficiency, quickly at first, then more slowly, up to it's max (about 85-87%).

In general, the larger the prop diameter, the more efficient it will be. The following three equations (ref-4:9:219) provide an estimate of the recommended prop diameters (inches) as a function of the horsepower available to the prop. ("Fourth root" is the square root of the square root.)

Two-blade:      d = 22 x fourth root of (HP)
Three-blade:    d = 18 x fourth root of (HP)
Three-blade (agricultural application):  d = 20 x fourth root of (HP)

However, the maximum useful prop diameter will be limited by the speed of the prop tip.

TIP SPEED

Anytime the aircraft is in motion (and the propeller is turning, of course) the path of the tip of a prop blade through the air is a helix, and therefore, it's velocity (the "tip speed") is the vector sum of the rotational velocity plus the translational velocity, or the helical tip velocity (explained in detail below).

Maximum helical tip velocity is an important parameter for propeller selection. In the absence of specific data from the prop manufacturer, it is safe to assume that (a) the maximum prop efficiency will be about 87% (for any metal prop a non-governmental agency can afford), and (b) that the prop efficiency begins to decrease dramatically when the prop is operated at a helical tip velocity in excess of 0.85 Mach. That occurs because the local air velocity over the surface of the prop (near the point of maximum airfoil thickness) will reach Mach 1, and create a shock wave, separating the flow and dissipating prop energy.

That phenomenon is very easy to spot in a high speed aircraft which has the capacity to run the prop too fast. Here is an example. A few years ago, I was flying a Glasair-3 to an airshow. I was cruising at 13,000 feet, 2400 RPM, wide open throttle. I was running a bit behind schedule, so in pursuit of a few more knots, I decided to operate at max power (2700 RPM, WOT). It was something of a surprise when I lost about 15 knots of airspeed. I set the RPM back to 2400, and regained the lost 15 knots. Later I did the calculations to verify that the loss was due to the sudden loss of efficiency. It was.

It is actually quite simple to do the arithmetic necessary to determine the tip Mach of a prop at a given RPM and true airspeed. First, calculate the helical tip velocity components.

The rotational velocity is the diameter of the prop times the RPM times a conversion factor. Again using KTAS as the unit of speed, the rotational velocity in feet per second is:

Vr (ft / sec) = RPM * Prop Diameter (inches) * 3.1416 / (12 * 60),    or

Vr (ft / sec) = RPM * Prop Diameter (inches) / 229.2

The translational velocity is simply the aircraft TAS expressed in feet per second, or:

Vt (ft / sec) = KTAS * 6076 / 3600   or  Vt = KTAS * 1.688

With the rotational and translational speed (in the same units, of course) you can easily calculate the helical tip speed:

Vht = square root ( Vr² + Vt²)

Next, calculate the speed of sound (Mach 1.0). The speed of sound in air varies with the square root of absolute temperature ONLY, as defined by the following equation:

Vs = square root (k*g*R*T) 

where k, g and R are constants (1.4, 32.17 and 53.34 for air)
and T is the absolute temperature (°F + 460) of the surrounding air.

So, if you are at 13,000 feet on a standard day, the air temperature is 12.71 °F and the speed of sound (in feet per second) is:

Vs1 = square root ( 1.4 * 32.17 * 53.34 * (460 + 12.71) )
Vs1 = 49.013 * square root ( 472.71 )  = 1065.6 ft / sec.

The Mach number of a given speed is simply:

M =  speed / Vs1

Putting it all together in a specific example, suppose you are flying at 13,000 feet on a standard day at a true airspeed of 240 knots and an 84-inch prop turning at 2700 RPM. Here is how to calculate your prop-tip Mach (using the simple equations above):

Vr = 2700 * 84 / 229.2  =  989.5
Vt = 240 * 1.688  =  405.1
Vht = square root ( 405.1²  + 989.5² ) = 1069.2 feet per second
Tip Mach = 1069.2 / 1065.6 = 1.034

See why the Glasair slowed down?

Now, to calculate the RPM at which a known tip mach occurs on your propeller, perform a bit of simple algebra on those four equations (Vr, Vt, Vs1 and M) to solve for RPM, given Prop Diameter, TAS, and outside air temperature.

(Instead of doing the calculations by hand, it is very convenient to put the equations into an Excel spreadsheet and let your computer do the arithmetic. The computer is much faster at it that you are.)

PERFORMANCE MAPS

Propeller performance maps are 3-dimensional tables which list the efficiency of a propeller at various combinations of advance ratio and power loading for various altitude conditions. If you are fortunate to have access to a map for your prop, you can determine the operating efficiency accurately for most every condition (as long as map data represent the actual performance of the prop, which is not always the case ! ).

In order to use a performance map, you will need to calculate the advance ratio and power loading.

Advance Ratio (J) = TAS / ( N * D ) and

Power Loading (Cp) = ( Prop HP * 550 ) / (air density * N^3 * D^5)

where
TAS is true airspeed in feet-per-second;
N  is prop speed in Revs-per-second;
D is prop diameter in feet;
air density in is slugs-per-cubic foot ( sea level = 0.002376 )

Engine Conversions on Certified Aircraft

If you are contemplating swapping a liquid-cooled V8 into a certified aircraft, you better have a long, heart-to-heart talk with a highly-placed person in the FSDO responsible for your geographic area before committing any significant resources to the project.

Although the flexibility and willingness to work with engine conversions varies from district to district, the bottom line is this:

We are not aware of ANY LEGITIMATE WAY to obtain long-term authorization to fly a certificated aircraft which has been modified by the installation of an uncertified powerplant.

HERE are the reasons for that statement.

The Federal Aviation Regulations are quite specific regarding aircraft powerplants. FAR-Part 33 (14-CFR-33) defines the requirements for certified powerplants. FAR-33.1 states:

• (a) This part prescribes airworthiness standards for the issuance of type certificates and changes to those certificates for aircraft engines.
• (b) Each person who applies under Part 21 for such a certificate or change must show compliance with the applicable requirements of this part and the applicable requirements of Part 34 of this chapter.

In short, that means you can't get an STC for the installation of a non-certified powerplant in an aircraft which was originally certified in the Normal, Utility, Aerobatic or Transport categories. That leaves the Experimental category and the Restricted category.

The following list, excerpted from 14-CFR-21.191, explains the different types of Experimental certificates that are available and how they apply to the installation of a non-certified powerplant onto a certified airframe.

• (a) Research and Development: Testing new aircraft design concepts, new aircraft equipment, new aircraft equipment, new aircraft installations.
  
        NOTE: An R&D certificate must be renewed annually. Your FSDO contact will usually issue the first one fairly easily. You will have to prove some substantial reasons to justify the issuance for a second year. In my region, there has never been one issued for the third year.
        ALSO, after the expiration of your R&D certificates, you will find it extremely difficult to convert the aircraft back into its original form (ie, restore it to being a standard Cessna-185 or whatever) because now the burden is on you, the modifier, to prove that the aircraft complies with the original type certificate .
  
• (b) Showing Compliance with Regulations: Conducting flight tests and other operations to show compliance with the airworthiness regulations including flights to show compliance for issuance of type certificates and supplemental type certificates, flights to substantiate major design changes, and flights to show compliance with the function and reliability requirements of the regulations.
  
        This classification is what you would use for the flight test verification to obtain an STC for the installation of a different certified powerplant onto a certified airframe. The number of times this certificate can be renewed is also limited.
  
• (c) Crew Training: Training of the applicant's flight crews.
  
        This requires substantiation, periodic inspections, and ongoing renewals. The allowable operations under this classification are limited.
  
• (d) Exhibition: Exhibiting the aircraft's flight capabilities, performance, or unusual characteristics at air shows, motion picture, television and similar productions, and the maintenance of exhibition flight proficiency, including (for persons exhibiting aircraft) flying to and from such air shows and productions.
  
        The allowable aircraft operations under this classification are very limited.
  
• (e) Air Racing: Participating in air races, including (for such participants) practicing for such air races and flying to and from such events.
  
        The allowable aircraft operations under this classification are very limited.
  
• (f) Market Surveys: Use of aircraft for purposes of conducting market surveys, sales demonstrations, and customer crew training
  
       
  
• (g) Operating Amateur-built Aircraft: Operating an aircraft, the major portion of which has been fabricated and assembled by persons who undertook the construction project solely for their own education or recreation.
  
       
  
• (h) Operating Kit-built Aircraft: Operating a primary category aircraft that meets the criteria of paragraph 21.24(a)(1) that was assembled by a person from a kit manufactured by the holder of a production certificate for that kit, without the supervision and quality control of the production certificate holder, under paragraph 21.184.
  
  

The Restricted category offers at least the remote possibility, but we have not fully explored that avenue. BUT notice the name: RESTRICTED.

Another approach we frequently hear suggested is an approval by means of a One-Time STC. Again, STC's apply to certified engines in certified airframes.

Yet another often-suggested approach is an authorization by means of a Form-337. Realistically, I think it would be quite a challenge to find an A&P-IA who would be willing to risk losing his tickets by signing and submitting to the FAA a field approval which flies in the face of existing spirit, intent and practice. More important, since 337's must be approved by FSDO, it is unlikely that the field approval would be granted.

Bottom line: unless you are ready to obtain BOTH a type certificate and a production certificate for the powerplant, AND an STC for the installation into an existing certified airframe, you might as well forget it.

NOTE: We do know of a path which a few energetic builders have taken, with varying degrees of success. The steps are:

1. disassemble the certified aircraft down to bare pieces (de-riveting the skins in the case of monocoque airframes),
2. make a few new duplicates of existing parts along with the new pieces required for the desired modifications,
3. reassemble the aircraft, using the duplicated original parts and incorporating the desired modifications,
4. Generate copious photos and documentation of the rebuild process,
5. Apply for Experimental-Homebuilt { FAR 21.191(g) } certification of your new "from scratch" aircraft as the "Hermann Umschlagplatz Firebreather Which Looks A Lot Like A Bonanza With A V8 Engine".

The success of that strategy involves a fair bit of luck in convincing the FAA Inspector or Designee that your project really is a "homebuilt" under the 14-CFR-21.191(g) definitions, and it entails certain risks with respect to deception of Government Officials, but it has been done.

The best plan for this strategy includes (1) energetic invocation of the "...for the education and recreation of the builder..." clause, (2) having pounds of documentation, including detailed engineering drawings of most parts, which you are alleging to have "handmade", and (3) excellent salesmanship skills.

Whirl Mode

Whirl Mode is a divergent, low-frequency vibration of the engine and mount which can result in separation of the engine from the airframe. Whirl Mode is an issue which is often ignored when designing the engine mounting structures used to install V8 engines on aircraft.

As of this writing (2003) FAR Part 23 does not currently require whirl-mode analysis for piston powered aircraft. However, with a high-powered V8 installation, there is the potential for generating a Whirl Mode oscillation. This section explains why.

Whirl Mode is a divergent conical oscillation of the engine and mount about an axis more or less parallel with the powerplant static thrustline. It occurs when the application of a gyroscopic moment to the powerplant provides an excitation of a bending resonant frequency of the powerplant / mount system.

The divergent oscillation occurs because of a positive feedback loop created by the 90° phase offset between the airframe motion generating the gyro-moment and the bending response of the structure. It usually begins when a large, sudden gyroscopic moment (such as from a sharp gust when flying in "chop") is applied to the engine mount structure which, although it may be sufficiently strong, the structure is not sufficiently stiff. That sudden load causes a relatively quick bending deflection 90° from the triggering maneuver. For example, a propeller rotating clockwise (viewed from behind) when subjected to a sudden nose-right yaw, produces a pitch-down moment which is proportional to the velocity of the yaw.

The feedback loop occurs when:

1. the initial bending deflection is quick enough to generate a large, new gyroscopic moment, which again, is applied 90° from the generating deflection,
2. when the excitation frequency is sufficiently close to a resonant bending frequency of the engine-and-mount system, and
3. the horizontal and vertical resonant frequencies are insufficiently separated.

If this divergent oscillation begins, the prop-end of the powerplant can begin to whirl around the static prop centerline, describing a horizontal cone of ever-increasing base diameter until something breaks off.

For example, suppose that an aircraft has an engine mounting structure with insufficient STIFFNESS (as differentiated from strength). Suppose that aircraft encountered a violent gust which caused it to pitch-up rapidly. If the prop rotation is clockwise, the prop gyro-moment would try to bend the engine mount structure to the right. If the mount was sufficiently flexible, it would deflect (yaw) rapidly to the right, which would generate an upward gyroscopic moment on the engine mount. If the flexible mount deflects upward quickly, it causes a strong yaw-left moment. The 90°-out-of-phase excitation continues, and if these excitations and deflections occur at or near the natural frequency (or a harmonic thereof) of the engine-and-mount system, the deflections can develop into a whirling deflection of the engine structure of increasing amplitude until some disaster occurs.

Early in the days of turboprop-powered aircraft, there were several inflight breakups caused by engine nacelle departure from the aircraft because of whirl mode. Generally, it happened because the engines were much lighter and much longer than the radial engines of comparable (or less) power which they replaced. The longer mounting structures, which were designed to carry the g-loads appropriate for the powerplant weight were relatively more flexible (less stiff) than those designed to carry the heavier and shorter radial engines.

A similar scenario is possible with a high-powered, relatively light V8. An engine mount which has suitable strength to bear the g-loads might not have sufficient stiffness (resulting in a low resonant frequency) to prevent whirl. In addition to high stiffness, it is also essential that the horizontal and vertical stiffnesses of the mount be separated by an appropriate margin, in order to assure that the horizontal and vertical resonant frequencies are substantially different.

Whirl can be less likely to occur with low mass-moment-of-inertia (MMOI) composite props, but a higher probability exists with a large MMOI metal prop.

Whirl mode is an important flight-safety subject that demands analysis for each specific airframe-engine-propeller installation.