Piston Propfan Proposal

Traditional Propellers

After a recent bout of studying piston engine aircraft performance, I’ve become convinced that advanced engines are not really necessary for higher cruise speeds. All evidence points towards propeller design as the missing link in light aircraft capabilities. Propellers for the most part have not changed since World War I. The most recent developments have involved scimitar shapes, swept tips and lightweight composite materials. However the basic issue of thrust diminishing with increasing speed has remained an issue.

An aircraft with a fixed pitch propeller can be thought of as a car with only one gear. Fixed pitch props can be optimized for takeoff, cruise or a mixture of the two. The cruise prop will have lackluster takeoff and climb performance due to its higher pitch. The engine literally doesn’t have the power to spin the prop to its optimum speed at a static condition. Conversely, the takeoff or climb prop will have large amounts of thrust at low speed, but a limited top speed. The hybrid prop is a middle of the road compromise between the two.

High performance aircraft work around this problem by varying the pitch of their blades to allow for maximum thrust at high RPM situations like takeoff, and maximum efficiency at lower RPM conditions like long-range cruise. This is the aeronautical equivalent of shifting gears in a manual transmission car. A system of flyweights and a governor allows the system to seek the pitch that maintains a selected RPM.

As an aircraft accelerates, all propellers make progressively less thrust, even though the engine is producing increasing amounts of power as the prop unloads. This situation has been considered unavoidable, even though it is very possible that there is a way around this dilemma. In theory, there is a way to achieve this, that is not complex and fairly easy to manufacture. To solve this problem, we will look to high bypass turbofan engines.

 

Turbofan Advantages

In the quest for speed, the piston engine has long been abandoned for turbine engines. With an extremely high power to weight ratio, long times between overhauls and superb performance at high altitudes, any turbine engine can be viewed as superior to a piston engine with regards to speed. Where they do no fare as well is in terms of fuel economy and purchase cost. While a new mid-size piston engine for aircraft will be in the $30,000 range, a small turbine engine can easily run 10 times that price.

Fuel consumption is also high for a pure jet, thus almost all new engines are either turboprop or turbofan variants. In the turboprop, a propeller is driven by the turbine via a reduction gearbox. In a turbofan, a large ducted fan is driven by the turbine. While aerodynamically speaking, they are very similar in operation, there are specific features in design that allow for the turbofan to perform at high subsonic speeds with incredible efficiency. The fact that the massive fan does not require variable pitch, yet produces most of the engine’s thrust from zero airspeed all the way up to Mach 0.90 indicates that their design should be studied in further detail for our purposes.

A close look shows that a fan can be considered a nearly solid disk with slots for air to be pulled through (looking at a turbofan from the front, it is very hard to see behind the blades compared with the ease of looking past the 2 or 3 blades of a propeller). These slots are simply the spaces between the blades and vary based on the blade chord. The blades themselves can range in shape from simple twisted polygons to scimitar shaped with serrations for shockwave control. Blade twist is markedly more severe than the twist featured on a propeller due to the larger operational speed range. The cross-section is normally a circular-arc airfoil optimized for supersonic flow. These factors are important in creating a propeller derivative.

Propfan Ver 2.0?

Compare the fan on a turbofan engine with a traditional propeller. Whereas the fan may have over 30 blades, a propeller will have only 2 or 3. If properly balanced, the fan will operate with far less vibration than the propeller. It will also move a greater mass flow per second due provided that adequate horsepower is available to spin it. A propeller will have a larger diameter for a given thrust level than a similar turbofan. This allows a slower rotational speed, keeping tip speeds subsonic and providing higher propulsive efficiency. Supersonic tip speeds are not a concern for turbofans due to the duct eliminating tip losses. Finally, a fan can recover ram pressure as forward speed increases, whereas a propeller will not.

If we assume that our propeller will be a bolt-on replacement for standard 2 and 3 blade props, we cannot duct it. It also must not exceed the diameter or weight of the original prop and be simple to maintain. With these constraints in mind, a 6 to 8 blade design will provide a compromise between static thrust, ram recovery, and low enough noise without the issues of balancing that arise with increasing numbers of blades.

Starting with straight blades of roughly equal tip and root chord, we can introduce severe twist to allow the root to operate un-stalled at very high forward velocities at a rotational speed of around 2500 RPM, which is an average operating range for a piston engine. To ensure that noise is kept to acceptable levels, the blades must be curved and swept to reduce diameter without a commensurate loss of blade surface area. The sweep ideally would begin around the midspan of the blade, rather than near the tip as is typically done.

Those who remember the propfan studies of the late 1980s may make the connection that the aforementioned propeller sounds a lot like the UDF demonstrator. Aesthetically this is true but there are several significant differences. A propfan has much higher blade loading, variable pitch and is driven by a turbine. Our design has a relatively low loading, fixed pitch and is driven by a piston engine. The benefits are ease of construction, maintenance, and operation. For existing aircraft, the major advantages will be improved fuel consumption, faster climb rates, higher ceilings, lower noise and longer engine life. Obviously, cleansheet designs will have to be conceived in order to take full advantage of the possibilities.

Concerns

A potential issue with the use of these propellers is the relatively low RPM at takeoff. This reduces the amount of power that the engine can generate. Luckily, most existing aircraft have a narrow enough speed range that plenty of thrust can still be created even at reduced horsepower levels. As designs emerge that have a speed range greater than roughly 300mph, more work will have to be done on optimizing both ends of the speed spectrum.

With normally aspirated engines, a ram effect from the inner section of the propeller will delay loss of power at higher altitudes. Should forced induction through supercharging or turbocharging be used, care should be taken to ensure that dangerously high manifold pressures are not produced, especially at low RPM settings. The use of electronic ignition with variable spark timing is recommended to ensure that the engine remains out of detonation range at all times.

Structural integrity of the blades will be complicated by the compound curvatures. Materials that are resistant to impact, vibration and torsional stress are required for adequate durability and resistance to flight loads. Use of titanium or single crystal materials would provide the required strength at acceptable weights, but at the detriment of increased cost.

Conclusion

All of this is theoretical and has not been proven other than basic research and back-of-the-napkin sketches. Considering that experimental propfans were able to achieve a 30% reduction in fuel consumption over existing turbofans, the concept does hold merit. Our design may provide a similar boost to existing piston aircraft, but only if the thrust per horsepower ratio is improved over traditional propellers. Testing will begin with experimenting via computer simulations for the best designs and progress to physical models. Comparing our design to traditional propellers will indicate the potential efficiency gains. From there more complete analysis using the horsepower required and drag force of several common general aviation aircraft can be completed.

GA For The Masses

Like many others, I am acutely aware of the slow (and accelerating) death of general aviation in the United States. I won’t go into all the reasons for this as we’d end up with a 400 page article on everything from Baby Boomers to the aircraft certification process. I would like to bring to light some things that will help the public feel like general aviation is something they can be involved with. Hopefully with larger numbers of people who care about flying, our diagnosis will change to “critical but stable” rather than “who is the next of kin?”.

Don’t take these suggestions personally. If we aren’t honest with ourselves, we can’t help ourselves.

 

Stop trying to make everyone a pilot.

Guilty parties: Pilots, aviation advocacy groups.
Who can help: Pilots, aviation advocacy groups, FBOs, flight schools.

Just because a person likes football does not mean that they can or even should tryout for the Dallas Cowboys. Similarly, just because a person shows a passing interest in airplanes does not mean we should try to coerce them to become a pilot. There are people who love to photograph airplanes but hate being in the air. There are some who like being around fast machines but have no desire to spend thousands of dollars on the license (let alone currency and additional ratings). The enthusiast who enjoys paying for a sightseeing ride may not want instruction but is still helping to keep that aircraft and its operator in business. These people are valuable allies in the effort to keep general aviation a part of the fabric of America. One hundred thousand people who are passionate about aviation but aren’t rated are more effective than ten thousand pilots with similar passion. It’s all a numbers game, especially in Washington D.C..

Instead of telling people how great it is to be a pilot, we should understand that while anyone can like airplanes, taking that extra step to become a pilot for most people is a pretty significant leap. Invite those who are open to the idea for rides around the pattern. Don’t teach them anything, just let them enjoy and take in the unique perspective from 1000ft AGL. The experience should be something akin to cruising in a classic convertible on a sunny day. The ambiance would be ruined if the driver suddenly began explaining the construction method used for the valve lifters and the maximum cornering g-force.

For those who show no interest in going up, let them have fun on the ground. Sponsoring regular open-house BBQs or hangar hang-out events at local airports is a great way to get people to the airport. Take care to see that non-aviators aren’t made to feel like outsiders. Consider a country club or marina; not everyone who goes to those facilities knows how golf or sail. For them, the golf and the boats are a backdrop for social interaction. If we use aircraft as a backdrop to events rather than the centerpiece, it makes the concept of being around airplanes less foreign.

 

Make the airport accessible.

Guilty parties: DHS, airport management, people afraid of their own shadows
Who can help: DHS, airport management, local municipalities, aviation advocacy groups, FBOs, flight schools

After 9/11, many airports went from being a fun place to hang out to a glorified Supermax with runways. Trying to fence off an airport for anti-terrorism purposes is to be polite, pointless and insulting. Maybe lawmakers haven’t noticed but airplanes have a peculiar habit of rising far above the security fence once they take off. A two-dimensional solution for a three-dimensional vehicle leaves a spare dimension of uselessness. Furthermore, I doubt that anyone bent on creating havoc and killing innocent people is really going to be worried about a trespassing rap for jumping a six-foot fence.

The best defense is popularity. Rather than fence off airports, turn them into even more valuable places for commerce and recreation. Recreation? At an airport? Of course! Why wait for a municipality to close an airport and turn it into a park? Make it a park right now. Find regions outside the runway protection zone and install bike/jogging trails complete with mile markers and the occasional water fountain. Create a playground in an empty corner of the field safely away from any operations but close enough for kids to see airplanes. With a steady stream of people using the airport for recreation, it becomes much more difficult for the maladjusted to execute their plot. For those convinced that trails would attract ne’er-do-wells, random placement of security/safety cameras along the trail would allow for monitoring of the perimeter, probably to a higher degree than would be possible without such a park.

The idea of an airport as a commerce center is not radical, but actually a very low risk method to bring regular people in close proximity with aviation. With proximity, uneasiness and fear begin to vanish and understanding takes its place. If there is an abandoned building or hangar, there is little reason why the airport, FAA and governing town can’t come to an agreement to let a non-aviation business operate in that location. For that matter, undeveloped space on or near the airport should be considered for retail or commercial buildings. In an ideal world, any retail space would feature windows that face the runway, aviation artwork or even ATC piped in over the stereo system. But even without those nods to aerospace, it’s a far better solution than letting airport buildings sit in disrepair and disintegrate. Not to mention, the tax revenue generated would be a welcome addition to the governing municipality’s coffers (and thus secure the airport a more stable future).

 

Reduce The Elite Status of Aviation

Guilty parties: Pilots
Who can help: Pilots, aviation advocacy groups

Since the first airplane took to the skies, non-pilots have imagined that it takes nerves of steel, lightning fast reflexes and a better handle on math than Euclid. For the majority of flying, this is simply untrue. Judgment and planning are the difficult parts. Usually that’s where mistakes are made that manifest themselves later in flight. The actual act of flying is really easy provided that the proper motor skills and coordination have been learned. I liken it to throwing a perfect spiral in football. You may be able to explain it with physics and algebra but the best way to learn is to practice under the tutelage of someone experienced. After a while it becomes second nature.

The image that the public has of VFR general aviation flying is wrong on many counts. One thing that remains true however, is that flying is unavoidably expensive and that cannot be changed (at least in the current economic situation). We must acknowledge that barrier and not pretend that flying is an affordable activity for everyone. But in terms of operation, a person by no means has to be a steely eyed missile man in order to fly a Piper Cherokee. We won’t be able to impress people anymore about how hard it is to wrestle the controls on a 5 knot crosswind landing, but there will be many more people who will realize that they have the ability to become a pilot too.

 

Safety. Enough Already.

Guilty parties: All of aviation
Who can help: All of aviation

Aviation has a hazardous streak. There are a lot of things that can go wrong very quickly. Even with backups and training, accidents will happen. That being said, aviation as a culture is so safety obsessive that it frightens people away. Right now I’m looking at an general aviation magazine and a motorcycle magazine that are both sitting in my room. Guess which magazine has more articles on safety despite having a lower number of articles total?

Motorcycle riding has very real hazards associated with it, just like general aviation flying. Yet when you read their periodicals, you don’t see issue after issue featuring discussions about accidents and close calls. They focus on the fun aspects of the hobby while still encouraging responsible riding. Justifying our accident discussions as wanting others to learn from our mistakes is noble but selfish. If we think that pilots are the only ones who look at these magazines, we’re wrong. Many a spouse has seen one too many articles on accident rates and one too many features with the title “There I Was On A Dark And Stormy Night With An Engine On Fire” and decided that their mate was not going to engage in the apparently deadly act of flying small planes. Let’s do our best not to scare off people who want to fly or give fodder to the misinformed who think that “little airplanes are always crashing”. This is not to gloss over the risks involved, but to moderate the rate at which they are exposed to them.

 

 

 
These observations are based on spending time around regular people, pilots, then finding the average between the two. Thinking from the perspective of someone who knows nothing about general aviation, a lot of things about flying can be intimidating. Great strides have been made in making airports more accessible to people other than pilots and there are many cases of airports and cities working together rather than against each other. This is proof that reaching out is more effective than pulling back.

There are a lot of misconceptions about flying and many of them are self-inflicted due to our relative isolation from the general public. We need more people to support general aviation but they won’t show up until they feel welcome. Giant billboards and ad campaigns won’t change anything. Conversely, slightly altering our actions makes every pilot in America an ambassador and every airport a welcome center.