Flying Cars: Challenges In Design And Implementation

Whenever people find out that I’m involved in aerospace research, I get asked the question “Hey what do you think about flying cars?”. Invariably, my drawn out utterance of the word “Wellllllllll” serves as a precursor to me depressing the entire group with copious servings of aerodynamic reality.

Most of the current flying car designs are from startup companies with no history or focus in full-scale aviation. This isn’t necessarily a problem as many times a novel solution is discovered by firms outside of a given industry. Several companies have went with the “Let’s scale up a quad/octo/dodecacopter and stick a person inside it” method. Some others have taken the Star Wars pod racer look and gave it a more utilitarian slant. I will admit, several of these designs look pretty awesome. But unfortunately, the laws of physics don’t respond to awesome. It appears that many of the designs were made without consideration of aerodynamics, human factors, the atmosphere or the realities of being in a small flying vehicle.

Established aerospace companies such as Boeing, Cessna and Bell should theoretically be all over the flying car craze. Bell, as a large helicopter manufacturer is well versed in the techniques associated with vertical takeoff and landing. Cessna not only makes some of the fastest business jets available, but they are responsible for the best selling light aircraft type in history, the Cessna 172 Skyhawk. And as if you didn’t know, Boeing dominates the US commercial aircraft industry.

So far Bell has teamed with Uber to help accelerate the technological leaps required to enable the development of point-to-point, electric VTOL vehicles. This doesn’t mean they’re working on a prototype, it just means they’re helping guide a company that has little to no experience with anything that flies. Boeing for their part has had a few limited projects in the past to design flying cars but were canceled before ever getting to a flying prototype. Finally Cessna is not involved at all with any type of flying car construction or collaboration (at least not publicly).

What could be some of the reasons for wide-eyed startups to be chomping at the bit to build these vehicles while grizzled industry veterans keep a wary distance? Could it be that they’ve seen some things that the new companies haven’t yet? Do Bell, Boeing and Cessna have the thousand-regulation stare? In this piece, I’ll highlight some of the issues facing flying car designers and why it will probably take a lot longer than the public thinks to make them a safe and widespread mode of travel (notice I didn’t say impossible…just a longer wait).

Automation

There is a misconception today amongst the non-aviation public that all modern airplanes are flown by computers and the pilots just sit there eating chocolate cake and watching Top Gun on their tablets. The reality is that pilots still fly the airplane, there’s just a computer that stands as a gatekeeper to ensure that the aircraft stays within limitations. Even still there are regions of flight where human skill is faster and more accurate than the computers. Autopilots/FMS still have to have flight plans loaded in advance and these plans invariably change multiple times per flight. Pilots still monitor and adjust systems during flight. They regularly deal with failed systems, changing weather conditions, rerouting by ATC and a multitude of other tasks. The automation is there to help the pilots focus on things that the automation can’t do.

Yet when reading articles on flying cars, the answer to everything seems to be automation. How will these flying cars operate in congested cities with skyscrapers all around them? Automation. How will the vehicle transition from vertical to horizontal flight? Automation. What will happen if an engine fails or a blade gets thrown? Automation. No detailed discussion of thrust vectoring or ducted fans or control surfaces. No mentioning if the omnipotent automation will activate systems via pushrods, cables, hydraulics or electric servos. Just automation. This shows a disconnect with actual flying vehicle design and a mere concept.

In order for automation to work, it needs something to work with. And once it has something to work with, it has to know the static and dynamic stability margins for the vehicle to ensure it remains flyable (better hope the occupants understand weight and balance). It also has to know where all other vehicles are. And all obstructions. And all restricted airspace. And considering that the passenger-not-pilot won’t have any control, the automation also has to identify and handle any conceivable emergency situations. Given the amount of effort being put into self-driving cars that only operate in 2 dimensions, a 3 dimensional activity like flying will require automation that is going to be unlike anything ever coded.

Airspace

A few questions about how this system will integrate with the National Airspace System are probably in order: Will this system work with the upcoming ADS-B mandate in 2020 that most aircraft in the United States will have to conform to? Will there be an international standard to allow sales in other countries? Will the system communicate to local air traffic control? What about operations near airports? Will flying cars be banned within X miles of a runway, or will they be allowed to funnel into the airport parking lot via strictly monitored ingress routes? Will air traffic control need to monitor and separate commercial, private and military traffic from flying cars? What about military training routes? What about prohibited areas? How will flying cars integrate with news and medivac helicopters? What about sightseeing…at some point people are going to want to see the Grand Canyon, Niagara Falls and Yosemite from their own flying car. Will people on the ground have to hear the constant drone of, well, drones? What about Washington D.C.? The entire nation’s capital has a permanent TFR which, short of some landmark change in legislation would rule out any flying cars within its vast boundaries. The same issue arises at football and baseball games since they too are ensconced in TFRs during games. In all honesty, even if every technical issue is surmounted, just figuring out integration and getting FAA approval for the plan is going to take the wind out of the sails of all but the hardiest of companies.

Weather

So far most of the animations show the flying cars operating in clear skies or silhouetted against a fiery sunset. While it looks really nice, it is not the full story of where these vehicles are going to be operated. They have to consider that people are going to expect that they can jump in their flying car and head home even if it starts raining, snowing or blowing 30mph out of the northwest. Are these vehicles going to be certified for known ice? The fact that even the largest commercial airliners have to be serviced with de-icing fluid before takeoff demonstrates the seriousness of the threat. After takeoff, hot bleed air from their jet engines can be routed along the leading edge of the wing to melt ice off. Just as critical is the engine anti-ice (different than de-ice) system that keeps ice from collecting on the nacelle lip. Needless to say, a chunk of ice down the inlet of a jet engine spinning at several thousand RPM is not an ideal situation so they take steps to prevent it from happening in the first place.

Now imagine a small flying car with uninformed passengers riding in it on a cold night. They have no knowledge of the environment in which they’re in, no ability to affect the flight and no way to save themselves other than perhaps a land-now button or a ballistic recovery parachute. A flying car encountering ice will rapidly collect it on the windshield, flying surfaces and engine nacelles, robbing the rotors of thrust, disrupting smooth airflow over the wings and weighing the aircraft down. Since these aircraft are intended to be electric, its only means of protection is to draw a lot of current to heat the critical surfaces, or carry the extra weight of a fluid known as TKS that is pumped out of tiny holes to retard ice accumulation.

There are other weather threats besides ice. Most of the designs presented are not touting the use of aviation aluminum as the primary construction material. Thus we can assume that a composite such as carbon fiber will be used instead. While very strong for its weight, carbon fiber does not react well with electricity. In the event of lightning striking a flying car…and it will happen…the manufacturers must prove that no strike will damage the structure beyond airworthiness limits and just as importantly, that it will not disable any of the automatic flight systems. Exhaustive testing and shielding will be required before any design is certified. Some existing aircraft use lighting diverter bars on composite parts to give the current a path to follow. It is then able to follow these routes and discharge through static wicks rather than blow holes in structure. All parts of the airplane must have a conductive path to these wicks in order for them to function properly. Again these are the realities of the atmosphere in which we fly and why airplanes cost so much to design.

Passenger Comfort

I make it a point to ask people who have mentioned wanting to have a flying car if they have ever flown in a small plane before. The answer is invariably “No.” This is very telling as the fantasy of flight is sometimes more attractive than the reality. However, most of those same people have flown in commercial aircraft before and remember experiencing turbulence of some kind.

Turbulence can range from gentle rocking to quite literally bouncing people off the ceiling. Mind you, these are heavy aircraft with high wing loadings. They have a lot of inertia and they can still be thrown around like ragdolls in the right conditions. Small aircraft by obvious virtue of being lighter are affected to a greater degree than large aircraft. Imagine the effect of 5 foot waves on a cruise ship and then picture an inflatable raft in the same size waves and you get an idea of the large plane vs small plane dilemma.

I have personally been bounced around rather brusquely in small aircraft before. Friends of mine have hit their heads on the ceilings while wearing their seatbelt due to the severity of turbulence they’ve encountered. Since flying cars are going to be lighter than many general aviation aircraft, this does not bode well for the dyspeptic among travelers. Hot air, strong winds, wind tunnel effects near skyscrapers, outflow from rain showers, and even the wake from other vehicles can all be nauseating for the uninitiated. While manufacturers will probably try to say that computerized flight controls will smooth out the bumps, that’s going to be limited by the vehicle’s lack of inertia. All I can say is these flying cars better have a really good air conditioning system and a place to store used barf bags.

Preflight and General Safety

Before any aircraft takes to the sky, the flightcrew will perform a preflight inspection commonly referred to as a walkaround. This activity is standard for every airplane from Piper Cub all the way up to Airbus A380s. During this inspection, they are looking over the condition of the aircraft, verifying fluid levels and quantities and ensuring that there are no blatantly obvious deformations. So far, flying car concepts have intimated that a user just hops in, pushes a few buttons and flies away. While it may seem like a yeah-so-what detail, this has very important ramifications for who will actually have their butts in these machines. Are the sensors that detect buildings and other aircraft obscured by dirt or dead insects? Are the thrust vectoring vanes able to move freely? Did some idiot ram a shopping cart into my rear stabilizer?

Since a person can’t just pull over if there’s a problem, and more critically, there may not be time to land or deploy a parachute if certain items fail, inspecting a flying car before flight will be just as important as it is for real aircraft. This will require teaching non-pilots the importance of ensuring the airworthiness of their vehicle before every flight. Somehow I have a feeling that there will be a lot of lip service paid but very little attention. The allure of jumping in and flying away is just too strong.

As for general safety, some designs have serious issues with the placement of certain components. For example, when I see a 2 seat quadcopter with unprotected blades, I see a lawsuit because someone walked into a running rotor and was disfigured or killed. Therefore, regardless of the number or orientation of the blades, they would have to be recessed into ducts in order to keep people from getting maimed when entering, exiting or walking around the aircraft. This would also protect the blades from damage in the event of a bird strike (assuming none got sucked down into the duct) and allow for thrust vectoring without tilting the entire motor/rotor combination.

There should be a battery firewall feature since the risk of thermal runaway while remote, is still a possibility. Someone once suggested to me that the offending battery be jettisonable. I then mentioned that the people below who get crushed/burned by a 500lb lithium ion battery falling out of the sky would be pretty upset if they survived and he agreed the idea wouldn’t be socially acceptable.

Likewise, a land-now feature and ballistic recovery parachutes should be standard. If a passenger is not going to be allowed to be a pilot, they should at least have a way to save their own life if an emergency develops. With that in mind, occupant enclosures should have sufficient bracing and structural integrity to protect people in the event of a rollover or hard landing.

Building a flying car to a reduced power level is another good idea to help in the event of an engine failure (assuming multiple engines). Setting MTOW to correlate to 80% of total engine thrust gives the vehicle a margin in case of engine failure. If a powerplant takes an early retirement, the remaining engines can be brought up to 100%. This should be enough to help balance the reduction in power and prevent airborne rollover from the sudden loss of thrust in a given quadrant.

So those are some of the off-the-top-of-my-head observations about flying car design (we didn’t even bother talking about the motors and the energy density of chemical fuels versus batteries as that’s enough for another article). Mind you, these are considerations that anyone who builds an aircraft must incorporate into their design before cutting metal. Some people will complain and say “Well if you know so much why don’t you build one!”. To which I’ll respond “How do you know I’m not?”

In any case, I write this because I love aviation. I grew up involved in aviation and what I see is a collision course of people who have great imaginations and some wonderful ideas, but not enough grounding (punintentional) to know why some of those ideas are not wonderful. If flying cars are going to be commonplace and accepted, they have to be built right. People are not going to tolerate these things taking them to the wrong destinations, to say nothing of what would happen if they start falling out of the sky due to underestimating the laws of aerodynamics.

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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. WWIScimitar

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 worAvengerHubk 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.CF6

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 Natalie Flyinga 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.

Hangar Party

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 aftebike-vs-planer 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.

Supersonic Now

When the Concorde was gearing up to fight off environmentalists, Lockheed and Boeing were in the middle of trying to construct a supersonic transport for the United States. Due to budget issues and noise concerns, the projects both were abandoned and we’ve spent the rest of our lives tooling along at the universal atmospheric speed limit of Mach 0.80. While I have no problem with people lobbying against having to live with 15-30 booms per day (for those who live under jet routes, it would have been very disturbing), I do have an issue with the “oh well, forget it” mentality that plagues our society after the passing of regulations. We collectively said forget it, nobody needs to fly that fast because supersonic flight was banned via federal law.

Or was it?

FAR 91.817 states that:

(a) No person may operate a civil aircraft in the United States at a true flight Mach number greater than 1 except in compliance with conditions and limitations in an authorization to exceed Mach 1 issued to the operator under appendix B of this part.
(b) In addition, no person may operate a civil aircraft for which the maximum operating limit speed MM0exceeds a Mach number of 1, to or from an airport in the United States, unless—
(1) Information available to the flight crew includes flight limitations that ensure that flights entering or leaving the United States will not cause a sonic boom to reach the surface within the United States; and
(2) The operator complies with the flight limitations prescribed in paragraph (b)(1) of this section or complies with conditions and limitations in an authorization to exceed Mach 1 issued under appendix B of this part.

 

Now, I for one would like it if FAA regulations were written for regular people instead of law students, but it doesn’t take many mental cartwheels to understand this rule. Nowhere in the entire rule did it say that you could not fly past Mach 1 under any circumstance. Further more, subsection (b)(1) has given operators a stipulation that allows them to break Mach. What the FAA has stated is that supersonic flight that creates a sonic boom on the surface is prohibited. Technically, if one were able to fly supersonic without creating an audible boom, it is allowed. That means you can imitate Chuck Yeager all the way from TEB to DFW provided that your boom does not reach the surface.

If my decidedly non-legal interpretation is true, why have we as a nation stopped all development on civil supersonic flight? Other than the persistent efforts of Aerion Corporation and a few attempts by Gulfstream, Dassault and Lockheed Martin, there has been very little interest in going faster. I shudder to think that manufacturers were limiting their efforts based on misinterpretation of a rule (or maybe they had it right and it’s my misinterpretation). What is more likely is that faster speeds like Mach 2.0 and Mach 3.0 were what designers wanted to achieve. Engine efficiency goes way up due to ram compression and range can actually be improved with an increase in speed (to a certain limit). Going that fast in the stratosphere will always generate an audible boom, so there was no point in conducting research. The end result is that by taking a government rule and connecting it with a physical rule (flight beyond Mach 1 will always produce some sort of boom), manufacturers in effect put themselves out of the supercruise business.

But for every law of nature, there is a workaround (you may not get exactly what you wanted but what you get is better than nothing). There is a way to go fast without scaring the crap out of citizens. It takes advantage of the different temperatures in our atmosphere and uses them as a muffler. For years, scientists, engineers and pilots have known about a phenomenon called Mach Cutoff. In layman’s terms, it is a certain Mach speed that if exceeded, will result in an audible sonic boom on the ground. Below this speed, no sonic boom will be heard by people on the surface. This speed varies based on air temperature, weight of the aircraft and of course the elevation of the ground one happens to be flying over. For practical purposes, most aerodynamicists use the range between Mach 1.15 and Mach 1.2 as a standard cutoff.

Hidden by obscurity, Mach Cutoff did not do anything to spur development of faster aircraft until fairly recently. In those intervening years there was also very little research in how to fly with a quiet, muffled or ground-level silent sonic boom. The conventional wisdom said that all sonic booms were created equal and that you couldn’t avoid or mitigate them in any way. However, in the late 1990s and early 2000s there was a resurgence in high speed flight testing. NASA tested a modified F-5 with what appeared to be a boat hull fuselage. They also tested an extendable spike in the nose of an F-15 to see if creating a series of weak oblique shocks would be less offensive acoustically. Aerion tested a natural laminar flow wing section on the bottom of an F-15 to study the effects of supersonic forces on a straight wing. It appears that there is at least some interest in going fast if it can kept quiet.

What does this mean for a well-organized and equally well-funded designer? It means that there is no reason that designer can’t build something fast today. Of course this is very easy for me to say sitting here with exactly $17.81 in my pocket and not a single airplane built to my name. But a person doesn’t need to run an aerospace firm to know that there is a lot to be gained from building a proof-of-concept vehicle. Investors like something they can see, touch and sit in better than CAD generated images. Other developers become inspired and improve upon another firm’s work. Sometimes breakthroughs are made when idiosyncrasies that simply cannot be predicted in computer models are experienced and rectified in a flight test program.

It’s 2015 and we are well into the future that was predicted when most of us were children. I don’t have to say that while some things are an improvement, other things are complete letdowns. Flying at the same speeds that we attained in the 1950s is not a limitation of physics or economics, but of our own desires. We’ve become complacent and comfortable with what is essentially six decade old technology. Sure we’ve refined it and eked out far more efficiency than we ever imagined, but is that it? Are we supposed to be excited over another 1% fuel savings? Are we supposed to look at an aircraft with awe because it features a self contained 5G network?

The last 60 years were nice, but it’s time to go meet the future. Let’s move beyond where we are to where we should be.

CJ Sunset

Is this as fast as we’ll ever go?

 

Wing Loading: More Important Than You Think

In 2011 I said “We’ll continue soon with more on wing loading…”.

It’s 2015. I think you can see that I get distracted easily and persistently. In any case, the information I’m about to present to you has not and will not change. Use it to help understand why your airplane does what it does when you leave the boring confines of straight and level, or use it to help you design that superplane you’ve always wanted to build. Off we go into the underrated world of wing loading!

Wing loading and power/thrust loading are the two most telling specifications about an airplane. Most pilots go right to horsepower and start swelling with pride when numbers north of 300hp start appearing. The assumption is that a lot of horsepower equates to a lot of performance. This is a huge misconception. The total horsepower of an airplane is irrelevant unless you have another number to compare it with; the total weight. A 200hp plane that weighs 1500lbs is going to have way better acceleration than a 500hp airplane that grosses 8000lbs.

Likewise, wing size and thus the wing loading is critical for many of the aerodynamics qualities of a given airplane. A highly loaded wing doesn’t always equate to high speed. There is a delicate balance of wing loading and all performance parameters that matter to you. Absolute ceiling, stall speed, takeoff distance , landing distance, cruise speed, ride quality and turn radius are all tied to the wing loading of an aircraft. For our purposes, we’re going to ignore the effects of power/thrust loading and focus only on wing loading for the duration of this discussion. By pretending that adequate power is not an issue, we’ll have the ability to truly see the importance of the wing area to weight ratio.

ANGLE OF ATTACK

For any airplane to fly, there needs to be some positive angle between the wing and the relative wind. The higher the wing loading, the greater this angle of attack will be in 1G flight for the same wing at a particular speed (this is where the FAA safety advisory about heavy, clean and slow airplanes generating strong wakes comes from…more alpha = stronger vortex). One can think of wing loading as an energy budget for the wing. If the loading increases, level flight is going to cost more through either higher speed or increased angle of attack. No matter what, you’ve go to pay for weight.

 

HIGH SPEED

Conventional wisdom says that if I want to fly fast, I should have a small wing. This is not completely true; if I wish to fly fast, I need to have low drag. Wing size is related to speed only through skin friction drag, induced drag and in some cases, wave drag. The size of the wing itself is not the sole determining factor. That being said, it takes some creative airfoil design to allow for a big wing that does not produce large amounts of drag. The aft-loaded airfoils that first saw widespread use on the Boeing 757 and 767 series are examples of this type of design. Even with a relatively low sweep compared to earlier models, they produced far less drag per pound of aircraft even at high subsonic speeds.

 

SLOW SPEED

An assortment of flaps and slats allow this 757-200 to land at very slow speeds for an aircraft of its size.

An assortment of flaps and slats allow this 757-200 to land at very slow speeds for an aircraft of its size.

 

Conventional wisdom also says that if I want to fly slow, I should have a large wing. If the wing does not change shape, this is true. However, a small wing can become a large wing through the use of flaps, slats and slots. These devices change the camber and in the case of slotted flaps, the area of the original wing. Since nothing is free on this planet, the extra lift produced also produces extra drag. During takeoff and landing, the objective is to fly as slow as practical to reduce the amount of runway needed. For this reason, the flap/slat/slot solution is used on almost all airplanes to varying degrees. The best of both worlds is attained at the cost of complexity and cost.

 

RIDE

A small wing is advantageous for ride comfort and structural integrity at high speeds. With the increase in wing loading, gusts and turbulence have less impact on the aircraft due to its higher aerodynamic inertia. The best comparison is a ship cutting through 8 foot waves versus a little canoe being tossed around the same ocean. For an aircraft flying in turbulent regions or at high dynamic pressures (50 feet and the speed o’ heat), alleviating aerodynamic stress is a matter of keeping the plane in one piece. There may be reason to maintain a fairly large wing but create the same lift-curve effects through significant sweeping. An example of this requirement would include low level strike aircraft with a secondary air-to-air mission.

 

TURNING

Aircraft A vs Aircraft B and the difference in turning drag.

Aircraft A vs Aircraft B and the difference in turning drag.

Wing loading is very important during any type of maneuvering but for our discussion, we’ll focus on level turns. The moment an aircraft rolls into a turn, the angle of attack is increased. This creates more drag that has to be overcome with additional power/thrust. Obviously, the lower the angle of attack, the lower the turning drag generated. A low wing loading has the effect of reducing the induced and profile drag created during a turn. Using the chart, compare Aircraft A that requires 2 degrees alpha for level flight compared with Aircraft B that requires 5 degrees. As they both roll into a hard turn that demands an extra 10 degrees of alpha, Aircraft A will be turning with a total of 12 degrees alpha while Aircraft B will be at 15 degrees. It is quite clear which airplane will have the better turn performance with regards to drag.

 

CEILING

Yes, this high aspect ratio glider is efficient, but only at low relative speeds.

Yes, this high aspect ratio glider is efficient, but only at low relative speeds.

The oft repeated maxim is that a long thin wing is best for flying at high altitude. The real variable at play here is wing loading, not necessarily aspect ratio. If a given aircraft requires a large angle of attack in order to generate the necessary lift coefficient, excessive amounts of induced drag will result regardless of the speed. An aircraft designed for low indicated airspeeds will probably have a high aspect ratio wing with minimal sweep while one designed for higher speeds will have some level of sweep and a much lower aspect ratio. Either way, the wing has to have a low enough loading to allow for a reasonable angle of attack.

 

INERTIA

For visualization, imagine a dumbell like you’d find in a gym. Pick up a 5lb weight, hold it up about 2 feet over your other hand and let it go. Chances are you can probably catch it quite easily. If you attempt the same feat with a 25lb weight, the only way to catch it is to let your hand drop down to eliminate the shock of it impacting your hand, if you can hold onto it at all. This is the same inertia effect that wings experience as their loading goes up. For this reason wing loading can be thought of as “inertia loading”. You may experience some of these effects in certain aircraft with highly loaded wings in the form of post-stall gyrations or uncommanded rolls.

 

GA DESIGN

A lot of modern GA wings can thank the P-51 for leading the way for laminar flow airfoil sections and high speed planforms.

A lot of modern GA wings can thank the P-51 for leading the way for laminar flow airfoil sections and high speed planforms.

Modern high-speed general aviation designs often have partial laminar flow, medium to high aspect ratio wings. These wings are very efficient at low angles of attack, resulting in low drag at high cruise speeds. They do have the drawback of less than satisfactory behavior at high angles of attack and post-stall. Usually these foibles are corrected with the use of leading-edge cuffs, vortex generators, and washout. In the case that the problems cannot be rectified totally, the aircraft will have limitations such as “spins prohibited” in the manual. If the aircraft encounters turbulence or maneuvers too hard, the stall margin can easily be exceeded resulting in a departure from controlled flight. How bad this gets depends on the aircraft’s design, center of gravity location, altitude and true airspeed at the time.

 

TRANSPORT DESIGN

Aft loaded “supercritical” airfoils allow commercial aircraft to use mildly swept wings without sacrificing subsonic performance. The airfoils delay shockwave formation on the upper surfaces, effectively raising the critical Mach number to a higher value. For this reason, a large wing is feasible but not often used in practice. Ride comfort and the fact that massive flaps are industry standard all point towards ubiquitous use of a highly loaded, high aspect ratio wing.

If a designer wishes to take advantage of smaller airports, utilize lower V1 and Vref values, and produce less drag at high altitudes (eliminate step climbs), a larger wing may be something to consider. The current trend is to utilize effective aspect ratio devices such as winglets or sharklets. These artificially raise the aspect ratio without actually adding any span to the wing. The result is that aerodynamic performance is improved markedly over much of the envelope, although not by the same degree that a larger wing would provide.

 

FIGHTER/TACTICAL DESIGN

F-4, F-22 and F-15E all with highly swept, low aspect ratio wings.

F-4, F-22 and F-15E all with highly swept, low aspect ratio wings.

The advent of thrust vectoring has made nose-pointing an accurate exercise even down to zero airspeed. This does not negate the need for a well designed wing since not all maneuvers will be able to take advantage of vectoring. For aircraft with a low level strike mission, encountering turbulence at high speed can seriously fatigue the crew and at worst, damage the airframe. A high wing loading, significant sweep or a combination may be required to keep the stresses within limits. For all other regimes of combat, a large wing is desirable for maneuverability, payload carriage, and high altitude performance. Due to the forces sustained during combat maneuvering, a larger wing in this case entails additional chord rather than more span.

 

This is just a short highlight of the importance that wing loading carries (punintentional) for all airplanes. I hope you’re more aware of how critical this number is for everything that you do in the air.

Supersonic Engines

(This is not meant to be an exhaustive text on gas turbine theory and application, plus I’m trying to refrain from being my usual aero-nerd self. Some things are not mentioned like combustion chamber design and the varieties of turbine one could opt for. I know! This is meant to focus on the really critical parts of cobbling together a supercruise engine.)

 

An adequate powerplant for supersonic flight is a massive sticking point for anyone hoping to develop a civilian transport. Efficiency at high speed without excessive fuel consumption is a balancing act that becomes more difficult the less one is able to spend on exotic materials. For a civilian designer faced with a budget that resonates more with FBO than DOD, we should explore what is required with regards to creating a supersonic powerplant for non-military aircraft.

The most important thing for any aircraft flying faster than the Mach is to have an exhaust speed which is equal to (in actuality, slightly faster) than the true airspeed. The drawback is very high noise levels from exhaust shearing (like I care) and very high fuel consumption for a given thrust setting (this I really do care about). The first solution most people would consider is to increase the bypass ratio. The complicated reality is that there is not a single bypass ratio that is optimal for all phases of flight, especially for an aircraft with a 750+ knot speed range. What may help at Mach 0.60 will probably hurt at Mach 1.15. Furthermore, by increasing the fan size or fan rotational speed, the core of the engine has to work harder and either give up thrust or increase turbine temperatures to compensate. Any increase in fan diameter has to be weighed against these factors.

We can rule out large bypass ratios of greater than 5, such as those used for modern airliners. The fan diameter is too large and creates too much frontal area drag. In addition, the massive volume of cool exhaust flow is not able to move fast enough to push the aircraft to supersonic speeds. Theoretically one could take a large diameter fan and spin it faster (many large civilian turbofan engines spin their fans at speeds below 3000RPM) but this would require a lot of extra energy from the core. On the other hand, a military style bypass ratio of 0.2 to 0.8 is not enough to produce the required TSFC for a civilian aircraft that cannot refuel in-flight or carry external tanks. Thus somewhere between a bypass ratio of 1.0 and 5.0 is the optimal choice for our engine.

Way too wide for our purposes, but perfect for the C-17.

Way too wide for our purposes, but perfect for the C-17.

On the front end, the fan pressure ratio affects the specific thrust and indirectly, speed of the air through the engine. Specific thrust is the thrust divided by the inlet airflow in pounds per second. Low bypass engines tend to have very high specific thrust values while large high bypass turbofans have a very low specific thrust. Civilian turbofans usually use one large fan whereas high performance military turbofans typically use 3 or more fan stages for this exact reason (as a reference, the F100-PW-229 has a specific thrust of 71.77, the F101-GE-102 ranks at 48.84 and the GE90-B4 comes in at only 28.78). The now “hardened” bypass air is able to move through the engine with enough energy (combined with the core flow) to provide a choked nozzle condition at the exhaust orifice (Mach 1 flow at the narrowest point). The flow can then be exploited by a variable system aft of the choke point to accelerate the air to the required supersonic speed (I have simplified so much that it annoys me, but otherwise, this article would go on for days).

We also must mention overall pressure ratio, which is the amount of total compression achieved by the engine/inlet combination. A lower pressure ratio means fewer exotic materials required in the hot section, a lower engine weight, less extravagant methods of cooling and as a result, a lower manufacturing cost. On the downside, the thrust level is lower for a given engine as compared to the same engine with a higher pressure ratio. Another negative effect is that a lower overall pressure ratio raises TSFC for a given thrust setting. Depending on the aircraft budget and expected operating environment, trading extra fuel burn for lower initial cost may be acceptable. A word of note is that as Mach number increases, TSFC increases as well. This may be offset by ram pressure recovery (mentioned later) but it is important to know that the TSFC at Mach 0.80 is not going to be the same at Mach 1.3.

The intake setup is important even though strictly speaking it is a part of the airplane, not the engine. As an aircraft with the proper fan pressure ratio moves faster, it is able to delay thrust decay and in some cases, reverse the process thanks to ram recovery. But this is if and only if the intake is designed properly. At low supersonic velocities, a simple normal shock inlet is sufficient to allow adequate pressure recovery at the fan/compressor face. As velocities increase past roughly Mach 1.5, the losses mount exponentially and thrust will degrade accordingly. A multiple shock inlet can reduce these losses significantly, however there may be issues with shockwave placement at off-design speeds. In the quest for low cost and low weight, a fixed position normal shock inlet is probably the best choice for a civilian supersonic jet. If one wishes to engage in what the military terms “carefree handling”, then intake design must be given far more attention to ensure that certain flight conditions do not lead to disturbed flow. Turbulent air at the fan/compressor face can lead to surges and stalls of varying severity.

If it's good enough for the Viper, it should be good enough for us...fixed normal shock inlets are lightweight, inexpensive and have no moving parts.

If it’s good enough for the Viper, it should be good enough for us…fixed normal shock inlets are lightweight, inexpensive and have no moving parts.

Finally, the most important piece of the puzzle is the exhaust nozzle. Ideally, a jet engine exhausts air at ambient pressure to produce a stable column of thrust. A given engine can force air out at a higher pressure than ambient, but this flow will simply overexpand, collapse in upon its now low pressure core and possibly re-expand. This is very inefficient and can be hazardous to the aircraft’s operation. To allow the higher than ambient pressure flow to expand under control so that it’s energy is translated aft rather than radially, a divergent section of nozzle is required. Every angle made with respect to the convergent and divergent sections has a particular Mach number and pressure ratio associated with it. Knowing this, for an aircraft to have maximum efficiency across a wide range of Mach numbers, a variable convergent-divergent nozzle would be required. However, a variable exhaust nozzle is extremely complex to build and requires a system to activate it (oil or bleed air in most cases). A fixed nozzle will have far less efficiency but an attendant lower cost.

Controlled by bleed air, these F100-PW-220 nozzles are very complex.

To recap, we are in need of a low bypass turbofan probably between 1.0 and 5.0 with a high fan pressure ratio, moderate overall pressure ratio, adjustable exhaust and fixed inlet. Starting at the front of the engine, we can assume a bypass ratio of 2.5 for no other reason than it’s halfway (and back-of-the-napkin wit). With this we are assured of an acceptable frontal drag penalty, while still having a fan small enough to stage if required. To keep core requirements within a reasonable range, a specific thrust range of 50-70 allows us to move air fast enough without taxing the core too much. An added bonus is that multiple fan stages can eliminate the need for a low-pressure compressor altogether. At the rear of the engine, the exhaust nozzle should be adjustable to a certain extent. A dual-position nozzle may be the best alternative to a fully articulated iris nozzle when cost and complexity is considered.

As of now, there are several civilian engines that qualify with minimal modifications and many that could fit the bill with more extensive changes. In the interest of rapid development, low cost and minimal risk to the aircraft, a minimal-change option is the best choice for a civilian budget. The Williams FJ44-2 and Pratt & Whitney Canada JT15D are both contenders for small aircraft. The medium to large designs could be well served by the Rolls Royce Tay 611-8, Spey 511-8 or Pratt & Whitney JT8D with very few changes (certainly for less effort and expense than a cleansheet design). While some of these engines are no longer in production, there are enough examples to support a test program and even limited run manufacturing of aircraft.

This is a classic chicken vs egg issue. Engines will not be produced unless there is an airframe that requires them and airframes will not be built unless there is a reliable engine available to power it. Somebody has to blink first.

This Makes Me Happy

It’s about time.

http://www.generalaviationnews.com/2013/06/saker-personal-jet-introduced/

I’ll get on my soapbox later this week to expand on why this is a good thing.