The Hutterer Hybrid

Hutterer Engineering has developed and patented the concept of what we are calling a “hybrid airplane.”  This concept is applicable to aircraft sized to carry 6 to 50 passengers, with or without pressurized cabins, and which are used primarily for commercial or corporate purposes.  The primary advantage of this concept is a 40% reduction in fuel consumption, The basic configuration includes a rear turbine engine with a propeller (turboprop) and a front-mounted auxiliary fan jet engine which can be retracted into the nose section to obtain an aerodynamically clean, low-drag, airframe in cruise configuration.  There are other important advantages relating to safety, passenger comfort and operational flexibility.  Exemplar designs, with engineering designations A-9 (unpressurized) and A-9P (pressurized), have been created to demonstrate the advantages of this concept in 10-seat versions (1 crew + up to 9 passengers).

The hybrid airplane has a single turboprop mounted in the rear fuselage in a “pusher” configuration.  The front of the design will therefore look somewhat like a business jet.  The most innovative feature of this design is the incorporation of a fan jet engine, mounted in the nose section, ahead of the cabin.  When the fan jet engine is not operating, it is stored inside the nose section (retracted position).  When it is to be used for takeoff and climb, or in the event of loss of power from the turboprop engine, it is deployed downward to its operational (extended) position, outside of the fuselage contour.  When the fan jet is no longer needed, it is retracted to within the nose section and the covering door is then closed, leaving an aerodynamically clean surface.  The hybrid airplane thus has multi-engine safety without the increased aerodynamic drag caused by wing mounted engines.  Because of the reduced fuel load required, the hybrid maximum takeoff weight is about 25% less than that of a conventional twin.  The net result is that the aircraft has the efficiency of a single-engine airplane with the back-up power and safety of a twin.

An efficiency analysis of a number of turboprop aircraft, expressed as “Passenger Miles per Gallon” (pm/g), demonstrates that at a 500 mile range, the exemplar designs reduce fuel consumption by approximately 40% compared to the average of five turboprops which have wing-mounted engines.  Reduction of fuel consumption of this magnitude also reduces exhaust emissions by a similar amount.  


The Case for the Turboprop

For about three decades, in both the General Aviation and airline markets, the jet airplane has gained market share at the expense of turboprops.  The premise of this project is that market conditions are changing and the hybrid design addresses the basic reasons for this market shift.  In both of these markets customers have preferred the jet because of lower cabin noise and vibration levels and the more modern image of the jet.  Although the high speed of the jet is obviously a big factor for longer trips, many jet airplane trips are less than 500 miles, where the jet spends much of its flight time climbing to its best (high) operating altitude and then descending from altitude.  We believe the market is changing to favor the resurgence of the turboprop for the following reasons:

  • A continued increase of fuel prices is expected and the turboprop is more fuel efficient.
  • There will be greater demand that all transportation modes reduce CO2 emissions
  • The desire to reduce oil imports to the US, especially from unfriendly countries
  • The public perception that business jets are an unjustified luxury.

Propulsive Efficiency

The chart above was developed by a turbine engine manufacturer.  The turbine engine is relatively simple in operation and has been developed to a high degree of reliability and efficiency since its introduction in WW-2.  The turbine engine can be configured to produce shaft horsepower (as in helicopter or industrial operations), or it can be configured to produce thrust (forward force) for an airplane application.

The chart shows the amount of thrust produced by the same turbine, at the same fuel consumption, when it is used in different configurations for airplane propulsion.  We are interested in the speed range below Mach 1.0, which is the speed of sound, which is 661 knots or 760 miles per hour at sea level.  Thrust is produced by accelerating a quantity of air, either by direct flow through the turbine or by using a portion of the engine power to accelerate some of the air around the core engine with a ducted fan or a propeller.  The more air that can be accelerated by the engine, the higher the thrust produced.

The thrust produced by the directing all air flow through the engine is shown on the bottom line on the chart, labeled “turbojet.”  This method is least efficient in terms of thrust produced at lower speeds, but has the advantage of higher efficiency at supersonic speed.  This is the type of jet engine used on supersonic military airplanes.  

The next line, labeled “turbofan”, shows the thrust produced (again the same fuel flow) with part of the turbine energy directed to a “fan” mounted on the front of the core engine, with a duct surrounding the fan.  This moves a greater mass of air around the engine (more thrust) and increases efficiency relative to the turbojet.  This is the preferred engine for business jets and airliners which operate in the speed range of Mach 0.65 to about 0.90.  

The top line on the chart, labeled “turboprop” shows the same basic turbine engine with a large portion of the energy produced absorbed by the propeller.  This configuration is able to accelerate even more air than the turbo fan, and the resultant thrust is seen to be much greater that the other two configurations.  However, the turboprop loses efficiency rapidly above Mach 0.75.  At 30,000 feet Mach 0.75 is 508 MPH, which is faster than some of the slower fanjet aircraft.  A turboprop flying at Mach 0.75 is competitive with the slower jets.

We see from the above that by using a propeller there is a lot of efficiency to be gained at speeds less than Mach 0.75.  We can be competitive in the light jet market assuming we can address market demand for a comfortable cabin with jet-type comfort levels.  At ranges of about 500 miles, there is very little door-to-door time difference between a high performance turboprop and a jet.

So we see that turboprops are more fuel efficient than turbojets or fan jets in the speed range we are targeting, and the following sections will show the hybrid design is more efficient than a conventional twin turboprop or a single engine turboprop.



General Hybrid Design Features

The hybrid design is applicable to aircraft with 6 to perhaps as many as 50 passengers.  Certificated turbine engines are not currently available for aircraft smaller than 6 seats and larger aircraft would be limited to the largest size turboprop engines available.  The following features apply to airplanes within this 6-50 size range:

The hybrid design features a fanjet installed at the front of the fuselage and a rear-mounted turboprop installed at the rear of the fuselage.  The nose section is therefore a bit larger than a conventional twin and the aft fuselage is slightly larger to accommodate the turboprop.  For balance reasons the wing is located further aft on the fuselage compared to conventional piston or turboprop airplanes.  The pilot’s compartment is therefore located ahead of the wing, providing excellent visibility.  The wing is near the center of the cabin which reduces center-of-gravity shift when the aircraft is loaded.  The main cabin entrance can be positioned at the front of the fuselage between the cockpit and the passenger cabin/cargo area.  The lavatory would be typically at the rear of the cabin and most baggage stored aft of the lavatory.  Location of the propeller at the very aft end of the aircraft essentially eliminates propeller noise radiated to the cabin under cruise conditions.  The wing is free of drag caused by engine nacelles and can be designed with modern low-drag airfoil sections without aerodynamic interruptions caused by nacelles and “prop wash” effects.

The fuel load of an airplane is a significant percent of the “all-up” or takeoff weight.  Therefore, lower fuel consumption allows us to design a lighter airplane for the same mission, and the lighter weight in turn leads to a still more efficient airplane.  Because of this efficiency multiplier, the hybrid is about 25% lighter than an equivalent conventional twin turboprop.  

For takeoff the fan jet engine is rotated 140 degrees downward around a longitudinal axis from a stored position in the nose section to the operating mode external to the fuselage.  The fan jet can then be used to boost takeoff and climb performance, or to continue safe flight in the event of a problem with the main turboprop engine.  In that case the jet engine, at maximum airplane weight and high altitudes, is fully capable of providing necessary power with the main turboprop inoperative.  The jet engine will be turned off and retracted after cruise altitude is attained.  It will again be extended and used for the approach and landing mode.  The additional power is not needed for the landing condition but the front engine would normally be on standby in the event a “go-around” is needed due to low visibility or a runway incursion.  The jet engine can also be used, together with the main turboprop, for faster cruise speed, at the expense of optimal fuel economy and range.  When used only for the takeoff and climb phases, the jet will normally be used only for 5 to 10 minutes, depending on cruise altitude desired.  The auxiliary jet and the turboprop use the same fuel supply.

The hybrid airplane can be more efficient than a single engine airplane due to a particular requirement of the federal safety regulations.  FAA Part 23 regulations specify that for safety reasons the stall speed of a single-engine aircraft in landing configuration must not exceed 70 MPH (61 knots).  The FAA considers the hybrid design to be a multi-engine airplane, and although it would operate most of the time on only the rear engine, the back-up engine will eliminate the 70 MPH limitation.  A higher stall speed reduces the required wing area and tail areas, so the hybrid design can use smaller wing and tail surfaces than a conventional single, resulting in lower drag and greater efficiency.

For example, raising the design stall speed from 70 MPH to 80 MPH will allow us to design a wing with 30% less area.  This increase in stall speed will not have a significant effect on takeoff runway length because of the high acceleration available with use of the auxiliary jet during takeoff.

Current technology, such as a natural laminar flow airfoil, large span flaps, spoiler roll assist, electro-impulse deicing and state-of-the-art electrical components, are compatible with the hybrid design.  The rear of the fuselage, ahead of the turboprop engine, will be optimized aerodynamically to take advantage of a low-drag fuselage design possible with the rear propeller location.  Such a rear fuselage design would incorporate compound curvature surfaces which would be easier to manufacture with advanced composite construction.  A preliminary meeting with the FAA indicated that no unusual certification issues would be anticipated.


Fuel Efficiency

Historically, about 50% of the direct operating cost of turboprops is attributed to the cost of fuel.  The airlines report that their fuel costs are about 25% to 30% of their total operating costs.  As the increases in fuel prices exceed the rate of inflation, aircraft operators, both corporate and airline, are purchasing more fuel efficient aircraft.  This phenomena is demonstrated by the marketing success of the Boeing 787, which offers 20% better fuel efficiency than the aircraft being replaced.  The manufacturers of regional and business turboprops are also experiencing more sales due to fuel concerns.

Although it has been estimated that aviation is responsible for only about 2% of global CO2 emissions, the aircraft industry is coming under greater scrutiny and political pressure to examine its fuel consumption and resultant CO2 emissions.  As the most fuel efficient turbine powered aircraft offered in the 6 to 50 seat categories, the hybrid airplane designs could legitimately be promoted as “green” products.

The table below presents fuel efficiency in terms of "Passenger Miles per Gallon."  The following graph presents the same data plotted against cruise speed for a 500 mile range.  This data is a sample of the many models  This data is a sample of the many models of private-corporate aircraft in service.  The data includes pressurized and unpressurized models of higher performance piston powered single-engine and twin engine airplanes, single-engine and twin-engine turboprops and three light jets.  The A-9 and A-9P exemplar designs are included for direct comparison.  To provide consistency for valid comparisons, the author has used the following guidelines to calculate and present the performance resulting in the “passenger miles per gallon” data.

  • To avoid the use of oxygen masks, performance for unpressurized aircraft is shown at 11,000 feet altitude or less.  For aircraft with pressurized cabins, performance is given at 20,000 feet or higher.
  • Where possible, data from Business and Commercial Aviation (B/CA), a respected trade journal, is used, including data from older issues.  Empty weights used are B/CA’s equipped weights, without crew.  Speed and distance data are converted from knots and nautical miles to miles per hour and statute miles.
  • The fuel required for each airplane assumes normal high speed cruise power (per B/CA) for a 500 mile trip plus a 45 minute reserve at full cruise power.
  • This analysis assumes commercial operation.  Most of the piston powered aircraft analyzed were marketed as owner-flown aircraft; however, many are being used commercially today.  The net payload available for all models investigated assumes one pilot plus personal equipment at 200 pounds.  We have assumed 200 pounds for each passenger (with baggage).  This is consistent with B/CA standards.



The “payload” that an aircraft can carry is the maximum operating weight minus the equipped empty weight and crew.  Most aircraft can not legally carry the full amount of fuel the tanks can hold when they have a full passenger load.  Some trade-off is therefore required between the passenger or cargo load carried and the desired trip length.  The “net payload” shown on in the table represents the number of passengers (or fractions) which can be carried at a 500 mile range, not including the pilot.  The data would be similar if we used cargo weight instead of passengers.  The graph illustrates the following general aerodynamic principles regarding aircraft efficiency:

  • The Beech A36 Bonanza, which has a single piston engine, has the same cabin as its twin engine counterpart, the 58 Baron.  The twin is faster than the single but there is a significant reduction in efficiency as shown by the arrow on the graph.
  • The Beech 58P Baron twin, which has a pressurized cabin the same size as its unpressurized counterpart, the 58 Baron, is faster than the 58 and more efficient due to its ability to operate in the less dense air at higher altitudes.  The Cessna models T210R (unpressurized) and P210R (pressurized) and also the exemplar A-9 (unpressurized) and A-9P (pressurized) show the same effect.

The Cessna 208B, a very popular single-engine unpressurized turboprop, is slower than the other turboprop singles because is has fixed landing gear (higher drag) and it operates at lower altitudes.

The exemplar A-9P can be compared to the 5 pressurized twin turboprops shown on the graph.  Fuel consumption of the A-9P is conservatively 40% less than the average of the Beech C90GT, Cessna 425 and 441, Piper PA31-T2 and the Piaggio Avanti II.  The Piaggio P180 Avanti II is a twin pusher turboprop which demonstrates that turboprops can have very high cruise speeds; in this case faster than the 3 jets shown.


Hybrid Safety

A new single engine turboprop market has developed since 1985, starting with the Cessna 208 Caravan.  The success of this market has been attributed in large part to the reliability of the turbine engine.  Turboprops are generally about twice as reliable as piston engines.  

The FAA regulation which requires that the stall speed of a single engine airplane must not exceed 70 MPH is meant to reduce risk in the event of a forced landing.  In a few cases, the FAA has relaxed this rule for turbine powered singles, recognizing their increased reliability.  In these cases FAA requires additional crashworthiness features such as stronger seats.  However, even with the higher reliability of turbine engines, there have been a number of “Loss-of-Power” incidents and accidents since 1985 which have required a forced landing.  

The single engine turboprop market share has been expanding since the introduction of the Cessna 208. The author has reviewed FAA and National Transportation Safety Board (NTSB) loss-of-power reports experienced by the 4 most popular single engine turboprop models since 1985, and included foreign data when it was available.  The four models reviewed were the Cessna 208 (1985), the Socata TBM 700 (1991), the Pilatus P-12 (1995) and the Piper PA46-500 (2001).  Over 3,000 of these models are now in service.  There were approximately 60 incidents or accidents due to loss of power, or about 2 to 3 per year.  Only 5 involved fatalities, but many involved serious injury and/or serious damage to the aircraft.  All four of these models use a version of the Pratt and Whitney PT6A turboprop engine.  The reliability of that engine has been responsible for the success of the single turboprop market.  However, in air taxi or other commercial activity, a power-off landing, even if it is concluded safely, rightfully causes great concern on the part of the passengers, crew, aircraft owners, insurance companies and the regulatory agencies.

The hybrid operates most of the time as a single engine airplane but has the back-up of the jet if power from the main engine is lost.  In addition to the loss of power issue, there are other operational advantages to having the auxiliary jet engine.  Among these are:

  • The availability of a second electrical source if the main turboprop generator becomes inoperative.
  • The availability of additional power should extreme airframe icing occur and additional climb performance be needed.
  • The availability of additional power for terrain clearance in mountainous areas and/or near small airports which may have nearby obstructions.
  • The availability of additional power for fast climb capability to higher altitude to clear adverse weather or comply with an Air Traffic Control request.
  • The ability for faster cruise speeds with the jet operating for a limited time as a supplement to the main turboprop.

An important safety feature of the hybrid design is that the auxiliary engine provides emergency power without unsymmetrical thrust.  Unsymmetrical thrust is the condition caused by loss of power on one side of a conventional twin engine aircraft.  Such power loss seriously decreases aircraft performance and makes the aircraft much more difficult to control.  If the pilot does lose control, the airplane will turn toward the inoperative engine and can start a steep uncontrolled dive.  Historically, the unsymmetrical thrust condition has been a factor in many accidents involving aircraft which have conventional wing-mounted engines.  During his career, the author has personally investigated 12 conventional piston twin accidents caused by loss of power.  Of these 12 accidents 6 involved loss of control, and 5 of these 6 caused fatalities.  This experience is in line with NTSB historical data.  These loss-of-control accidents occur even with experienced pilots.  It is clear that eliminating unsymmetrical thrust will improve safety.



Cabin Comfort

With the main engine / propeller positioned at the rear of the fuselage, the hybrid airplane will be as quiet as a business jet, and will not have the cabin noise and vibration levels typical of conventional twin turboprops.  The front fan jet noise will only be apparent during the short climb to altitude, and will not be a factor in cabin comfort.  A new design can incorporate a modern high-lift wing design, which will have higher than normal “wing loading” which will provide a smoother ride in rough air compared to that of many current airplanes.


Hybrid Performance

The engines on a conventional twin, or the primary engine in the case of the hybrid, are typically sized for the cruise power required.  When the hybrid airplane jet is used during takeoff and climb the aircraft has effectively more than 100% power than would be available to a conventional twin.  Therefore, the climb-out from the airport will be steeper (smaller airport noise footprint) and the aircraft will reach cruise altitude faster than the twin.