Breaking The Propeller Performance Barrier

A Study Of Existing Propellers And Methods For Circumventing Their Limitations.

Christopher L. Williams

Paul L. Whelan

Whelan & Williams Industries Inc., Dallas, TX 75254

whelanwilliamsinc.com

August 4, 2017

Abstract

Reducing fuel consumption, expanding the useful speed range, increasing payload, reducing time to

climb, prolonging engine life and reduction/elimination of lead emissions are goals that every aircraft

operator is always in search of. We are interested in determining if a new type of propeller can meet all

of these goals plus several others. This paper will explore the problems associated with existing

propellers, how we intend to improve on them and the overall benefits of this technology.

Introduction

Propellers have often been overlooked in the quest for better aircraft performance with most attention

being directed at better engines, engine accessories and fuel types. While we agree that enhancements

to the standard horizontally-opposed, air-cooled engines used in general aviation are needed, vast

improvements to performance can be attained for far less expense by installing the correct propeller.

The use of horsepower as a measure of aircraft performance is misguided. Aircraft do not move as a

result of power, they move as a result of thrust. Without a high-performance propeller to generate

thrust, all the horsepower in the world will not make an airplane fly faster or climb steeper. Since there

is a tendency to focus on horsepower when building or improving aircraft, it is not a surprise that the

performance of light aircraft has not improved significantly in the last 60 years. It may be possible to

achieve very high level flight speeds with existing powerplants, thus negating the perceived need for a

technology breakthrough in engine design.

I. Existing Propellers

1. Background

The first true aerospace propellers were designed for airships in the 1800s. These were not optimized

for flight in the atmosphere and were very inefficient. It was not until the Wright Brothers discovered

that propellers should be designed as a rotating wing that performance really began to increase.

Subsequent development hit a peak during World War II and plateaued until roughly the mid-1970s

with a renewed interest in high performance propellers for turboprop and propfan aircraft. Today, light

general aviation (FIG 1-2) enjoy advanced airfoils, Q-tips, cuffs and scimitar shapes that enhance

efficiency and improve performance. Yet despite all these advancements, few aircraft can point to gains

in performance on the order of 30% or more from simply changing their propeller.

2. Tip Speed Relationship To Top Speed

Propeller driven aircraft have always had the disadvantage of exponential horsepower requirements as

speed increases. Approximately 33% more horsepower is required to fly 10% faster. This has always

been accepted as unavoidable and most manufacturers focus on airframe drag cleanup to boost

performance. These fixes do not usually result in significant increases to the aircraft's top speed unless

Fig 1: Enhanced twist and scimitar curvature of modern propeller.

Fig 2: Q-Tip modification to reduce noise and increase efficiency

the aircraft was especially draggy to begin with. After studying the relationship between propeller

design and transonic drag, it is clear that the sharp increase in horsepower required is caused by the

propeller's inherent limitations rather than the airframe.

If one calculates the tip speed and forward velocity of a given propeller at a given velocity, there is a

strong correlation between the beginning of drag divergence for that specific airfoil section (Fig 3) and

the max level speed of the aircraft. Propeller airfoils for light aircraft are subsonic in nature and by

definition, not optimized for transonic operation. Therefore they will begin to create localized

supersonic flow near the tips at rotational speeds well below Mach 1. As the propeller begins to create

supersonic shock waves, the efficiency drops rapidly as well (Fig 4).

Fig 3: Chart of drag divergence force with increasing Mach.

Fig 4: Chart of propeller efficiency drop with increasing forward velocity.

Fig 5: Power required with respect to increasing airspeed.

A close look shows that the spike in drag created by exceeding the critical Mach number is inversely

related to the drop in propeller efficiency with increasing speed. It follows that the exponential increase

in horsepower required is due to the loss of propeller efficiency, not just the predicted increase in

airframe parasite drag (Fig 5). This relationship also illustrates why simply adding horsepower to a

given aircraft type does not translate into higher cruise speeds. If the propeller is already suffering

compressibility losses, adding 10, 20 or even 50 extra horsepower will only result in a fraction of those

being useful. Not only has the propeller lost significant levels of efficiency, but it is producing ever-

increasing amounts of drag. We can thus state that top speed of a propeller driven aircraft is limited by

its tip speed rather than airframe design and engine output. While we are not claiming discovery of this

fact, we did not find any reference material during our research that directly connects maximum speed

of light planes with the onset of compressibility phenomena.

For example, the Cessna 172S equipped with the Lycoming IO-360-L2A and McCauley 2 blade, 76

inch propeller may be viewed as a low-speed aircraft, but when calculating tip speed, it is clear that the

blades are operating deep into the transonic regime (Fig 6). If one plots tip speed of a given aircraft in

high speed cruise, they will find similar values for nearly all propeller driven general aviation aircraft.

Of course the actual Mach number at which drag divergence occurs varies with blade airfoil selection

and this will be reflected in the maximum level speed of the aircraft. The tip speed of the propeller can

be expressed via the following equation:

Vr = ( ∏ x D x RPM ) / 60

Vt = √ (Vr² + Va²)

Vt = Resultant tip speed in feet per secondVr = Rotational tip speed in feet per second∏= Pi 3.14159D = Diameter in feetVa = Aircraft speed in feet per secondRPM = Revolutions Per Minute

This phenomenon is not prevalent in turbofan engines which experience a much smaller decrease in

thrust and in some cases, actually increase in thrust depending on inlet/intake setup and bypass ratio.

Turbofan engines are essentially a shrouded, high blade-count propeller driven by a turbine engine. In

many engine models, 80-90% of the total thrust produced comes from the cold fan flow rather than the

hot core flow. These cold/hot flow ratios are on par with many turboprops, which can generate roughly

10% of their thrust from pure jet exhaust. Yet

due to specific differences, turbofans cruise at

least 200 knots faster than their propellered

brethren.

A turbofan's blades differ from that of a

propeller in that they have severe twist, sharp

transonic airfoils and very high blade solidity

Fig 6: Comparison of tip speed and power required as compared to RPM.

2200 2300 2400 2500 2600 2650 27000

10

20

30

40

50

60

70

80

90

Power Required As Related To Tip Speed

Cessna 172S @ 8000ft, ISA

Tip Velocity (% Mach)

%HP Req

RPM

Fig 7: Severe twist and sharp airfoil sections on the blades of a Rolls Royce UltraFan.

(Fig 7). This is in contrast to the limited twist, classic airfoils and very low blade solidity of standard

propellers. While classic sections may be found on some turbofans, it's only near the hub where

rotational velocities do not approach the compressibility region. On the outer portions of the blade, a

circular-arc airfoil is industry standard.. A classic airfoil such as a Clark-Y, even in a thin section is

poorly suited to work at significant fractions of the speed of sound. Additionally, the close-fitting

cowling of turbofans serves to reduce tip losses, especially since fan blade tips are often supersonic in

order to create a higher pressure differential. Such an arrangement is impractical for most general

aviation aircraft.

II. Improvements On Existing Propellers

1. Basic Overview

Our propeller research set out to address all of the aerodynamic issues that hinder standard propellers.

We focused on high-speed airfoils, enhanced blade twist, pronounced curvature and a high blade count

to create an efficient method of moving aircraft. What was required was a propeller that had a small

enough diameter to remain well outside of the transonic range, enough blade area to produce the

required amount of thrust and most importantly, the practicality to install on existing aircraft.

At first glance, our propeller design resembles the propfans of the late 1970s but with significant

differences to make them useful for small aircraft. Most importantly, propfans were turbine driven.

Turbine power meant that they were geared towards large commercial aircraft rather than general

aviation. Another difference is that the blades were adjustable pitch to allow for a speed range up to and

beyond Mach .75 along with reverse thrust for landing. These factors meant that a propfan powerplant

would have been comparable to a turbofan engine in acquisition cost. Some versions had contra-

rotating arrangements to eliminate torque and increase efficiency at the cost of complexity (Fig 8).

What we have done is taken the best parts of the propfan and scaled them down for use on existing

general aviation aircraft.

2. Blade Sweep

Sweeping the blades would allow us to keep the diameter low and reduce the amount and intensity of

drag rise experienced at higher speeds. A common way to reducing wave drag in transonic aircraft is to

sweep a wing with respect to the airflow (Fig 9). Since propellers are simply rotating wings, the same

principles apply. While NACA conducted 45 degree sweep tests of a propeller in the 1950s, the results

were not as significant as expected. This was in part due to the fact that the test prop only had two

blades and the sweep lowered the L/D of said blades. Their subsequent research focused on thinner

airfoils near the propeller tips which was easier to manufacture and was less stressful on the blades.

Currently, sweeping of propeller blades is done on a limited basis in the form of scimitar sections.

However most standard blades are unswept and encounter compressibility issues when operating at

high RPM and moderate forward velocity. This severely curtails thrust at the upper end of the speed

spectrum. We swept our blades aggressively near the midsection to achieve the required tip speed and

Fig 8: General Electric UDF Propfan demonstrator.

blade diameter. An added benefit is that as speed increases, thrust also increases up to the limit of the

engine's RPM range. This is in line with what is observed in turbine aircraft which do not lose thrust

over a significant portion of their operating speed range.

3. Blade Count

We also decided to go with a fairly high blade count to increase blade solidity and thrust/horsepower

ratios. Adding blades can also reduce the RPM required for a given thrust level, but this must be

balanced against the torque loads on the engine. With reduced RPM and proper design, an aircraft may

be able to attain a higher speed before the onset of propeller compressibility effects. High blade count

propellers have not been very popular in piston aircraft, possibly due to extra cost and possibly due to

misconceptions.

Fig 9: Chart showing the reduction in maximum drag with sweep.

Many sources state that having fewer blades increases total efficiency of a propeller. In reality, the

opposite is true. Increasing the number of propeller blades will actually increase the overall

effectiveness of the propeller even if the individual blade efficiency is reduced. An easy way to

conceptualize this is to imagine that a person has to move a very heavy object that weighs 450lbs. They

have the option of selecting either one helper who can lift 500lbs, a pair of people who can lift 300lbs

each or a team of 6 who can each lift 150lbs. While the man who can lift 500lbs will get the job done,

it's very strenuous for him (maximum efficiency). Things are easier for the team of 2 since they can

move a total of 600lbs and thus don't have to have as much individual “efficiency”. However, the best

solution is the team of 6 who can move 150lbs each. That's a total of 900lbs of lifting ability for a

demand of only 450lbs, demonstrating that even a very low individual efficiency still results in a high

system effectiveness with regards to applicable force.

Another myth that may limit the use of

high blade count propellers is the

“blade-wake” fallacy. An often cited

consequence of multiple blades is that

each blade is operating in the wake of

the previous blade. This is

demonstrably false except in very

specific circumstances that would not

be encountered in normal flight, such as

momentarily in a tailslide when rearward velocity equals propwash velocity. Evidence of this is

illustrated in the photos of aircraft shedding helical vortices from their propellers (Fig 10). No single

vortex interacts with any of the others. Further evidence is also present in the existence of advanced

turboprops and bypass turbofan engines that operate millions of hours per year with no issues related to

Fig 10: DHC-5 displaying tip condensation, confirming that there is no blade-vortex interaction with multi-blade props.

blade-vortex interaction. In fact high bypass turbofans regularly operate at speeds up to Mach .90

essentially on the thrust from a fixed pitch fan. While these speeds are highly unlikely for a propeller

driven aircraft due to the lack of inlet to tailor the airflow and prevent separation, it does demonstrate

that there higher performance potential with more blades versus fewer.

The added weight of the extra blades will be counteracted somewhat by reduced diameter and material

selection. Blade balance is also a critical part of maintaining any type of propeller, but becomes even

more important as the number of blades increases. Checking propeller balance and blade tracking

should be a regular part of any aircraft maintenance program as unbalanced blades can be

uncomfortable at best and catastrophic at worst. Steps to limit blade erosion such as enhanced ground

clearance and minimal use of unimproved fields will help reduce balance discrepancies. Use of non-

metallic materials in the blades will serve to reduce or eliminate resonance within the RPM range of

horizontally-opposed engines. and keep weight low.

In the end we ended up with a propeller that closely resembles a propfan but without variable pitch, a

turbine power section or the associated nacelle. The aerodynamic properties are well understood and at

this point we are moving away from simulations towards wind tunnel and vibration testing. Our

concerns are structural integrity at and beyond the operating range, resonance analysis, acoustic

characteristics and full scale thrust measurements. It is most likely that some modifications will be

made as that is the nature of refining a design. Our objective is to ensure that it can be used with the

vast majority of engine/airframe combinations without any special requirements.

4. Further Optimization

Effective pitch is at least 70 inches which under normal circumstances would be considered to be very

coarse. However due to the sweep of the blades and relatively small diameter, the ability of the

propeller to reach static RPM during takeoff is on par with existing propellers. This gives the propeller

a variable pitch effect without actually moving the blades. As it stands, the propeller is limited via the

engine's RPM range, as tip speeds will remain below the drag divergence speed up to roughly 350

KTAS.

In order to fly at these speeds and possibly beyond, one could “oversize” the propeller so that the

engine is several hundred RPM below the static range. While this is not an optimal situation with

regards to engine torque load, forward movement unloads the propeller and the engine reaches normal

operating speeds between 60 and 85 KIAS. This change allows the aircraft to reach much higher top

speeds than would be expected for the amount of installed horsepower. Another option is to utilize

engines that feature reduction gearboxes that allow both the engine and propeller to work within their

optimal ranges with fewer compromises.

5. Design Progression

Fig 11: First iteration. Starting with a 3 bladedprop, we swept the tips to reduce the local flowvelocity. Propeller overspeeds at static conditions and is in need of more blade area toabsorb horsepower.

Fig 12: Second iteration. More severe sweeping of tips to further reduce shock potential. Pitch is increased at root in attempt to reduce static RPM without losing windmill effect at higher speeds.

Fig 13: Third iteration. Two blades are added to increase thrust potential of propeller. Thrust decay with speed reduced substantially, but amount of horsepower required at static conditions has increased as well. Pitch reduced at root to reverse this trend.

Fig 14: Fourth iteration. Sixth blade added. Severe tip sweeping moves inboard to involve alarger portion of the blade. Efficiency increases with increasing forward velocity. Limiting factor is now RPM rather than horsepower with regards to maximum speed. Further sweep discouraged until stress analysis can be conducted, especially around the trailing edge radius.

6. Benefits

There will be significant benefits to existing general aviation aircraft. As our simulations are in the

early stages, we are not ready to officially quote any performance figures. However we can make some

statements about general characteristics:

• Marked improvement in rate of climb, especially at higher altitudes.

• Higher level flight speeds.

• Lower fuel consumption in cruise.

• Enhanced range.

• Lower engine RPM in cruise.

• Higher service ceiling.

• Quieter operation both inside and outside the cabin.

• Lower vibration for occupants and structure.

• Reduced engine wear.

• Lower hourly operating cost.

• Best rate of climb speed increased to higher value.

• Best angle of climb is at a steeper gradient.

• Better engine cooling during climb.

• Reduced output of lead.

• Enhanced glide ratio.

• Easily scalable for engines of different output levels.

Summary

Propellers in their current form encounter sonic airflow at rotational speeds well below Mach 1,

contributing to exponential drag rise and loss of efficiency. The airfoils being used are for the most part

inadequate for use at such velocities. As such, there is a need to either reduce the tip speed of the

propeller, use an airfoil variant that has a much higher critical Mach number or a combination of such.

Given that a high-speed airfoil will not have the same L/D characteristics as a low speed airfoil, extra

blades will be required to recover and in some cases exceed the baseline thrust levels. If designed

correctly, such a propeller can rival turboprop performance with the right airframe and engine

combination. More immediately, it can increase the utility of light general aviation aircraft so that it

becomes comparable cost-wise to commercial airline travel for a similar trip length. Additionally, due

to the lower horsepower requirements, it can counter the fuel and operation costs that are often a barrier

of entry to a lot of would-be and existing pilots.

We look forward to further developing, testing and eventually flying behind this propeller.

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