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.

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.

Delta Arrow Wings: Advantages For Civil Supersonic Flight

There is more than one way to skin a cat; this cat happens to be supersonic drag. There are many theories on how to achieve efficient civil supersonic flight, each with distinct advantages. One method that I have always favored is the simple and reliable delta wing. No complex construction, boundary layer control systems, or reliance on laminar flow. Just a big triangle without the need for high lift devices. There are tradeoffs but if your goal is to go supersonic without complexity, a good place to start is with the delta.

It is a fact that sweeping a wing delays supersonic drag rise and raises the critical mach number. It is also a fact that sweeping a wing with no special treatments will cause all kinds of hellacious stability problems at low speeds. While I could go into at least 4 pages of descriptions, case studies and NACA test data, I’m trying to write less like a mad scientist this year. So I’ll limit the following formulas to the basics in trying to get the point across.

An object moving faster than the speed of sound in air will produce a shockwave. The angle created by this shock cone (it’s a three dimensional wave) is dependent on the speed of the object. The faster the object, the smaller the angle created by the shock cone. With low supersonic Mach numbers, it is entirely possible to sweep a wing enough to contain it within the subsonic wake of the cone. The formula for determining the half-angle is:

1 / Mach#  = sin * cone angle

As stated, this is the half angle formula. To get what the entire cone would look like if drawn whole and not bisected, simply multiply the result by 2. For example, say that my aircraft is going Mach 1.3 and the wing is swept 54 degrees. With a cone half-angle of  50.3 degrees, my wing is definitely within the confines of the wake. This has a significant effect on reducing wave drag.

In addition to reducing wave drag, critical Mach number can also be reduced from sweep. Simply put, air is tricked into thinking that the wing has a longer chord and accelerates over the top of the wing at a slower rate. The formula for this effect is:

Vmach (cos * sweep angle) = Effective Vmach

This formula determines the Mach velocity over the wing when sweep is introduced. This may or may not be lower than the critical Mach number for that airfoil section. To determine what the new critical Mach number is when sweep is accounted for, the following formula is appropriate:

Mcrit / cos * sweep angle = Effective Mcrit

In effect, a highly swept wing can delay the critical Mach number to supersonic velocities. A straight wing with a relatively low Mcrit of Mach 0.7 would have an effective Mcrit of Mach 1.19 when swept to 54 degrees. This holds a lot of promise for reducing drag in the low supersonic speed range.

It’s not all free soda and candy for the swept wing. As mentioned before, there are very serious effects to consider aerodynamically. Swept wings stall at the tips first, creating unstable rolling and pitching moments during the stall. In other situations, pitch-up may occur when the horizontal stabilizer gets caught in the wing’s flowfield. Slow speed handling is degraded by spanwise flow, the same phenomenon that helps to reduce drag at higher speeds. Maximum lift coefficient for a given angle of attack is also reduced, leading to sometimes extreme pitch attitudes at slow speeds. There are other issues but these are the most critical to control and stability.

A straight wing that is swept may be troublesome, but delta wings have distinct advantages  that make them attractive for our purposes. The double delta design is a derivative that is configurable to nearly any range of speeds. The two main variants of the double delta are the “shovel”, with the low sweep segment in front, and the “arrow” with the high sweep segment in front. In either configuration, the forward segment produces low-pressure vortices that drift over the aft segment and delay the stall to a much higher angle of attack (non-linear lift). If properly balanced, a double delta will display highly favorable stability characteristics as well.  For this discussion, we will deal with the delta arrow variant since it is customized for supersonic speeds.

DSC_0260

LoFlyte test vehicle at USAF Museum in Dayton, OH. Designed for hypersonic waveriding flight, this basic design is applicable to low supersonic flight as well.

A delta arrow consists of a highly swept leading edge section and a less severely swept aft wing section. This ensures that the leading edge of the wing remains behind the shockwave at moderate supersonic speeds while retaining adequate lift reserves for slow speed operations. As mentioned earlier, swept wings have a reduced lift curve slope with the degree of sweep directly correlated to the reduction (provided airfoil section and thickness remain constant). This disadvantage becomes an advantage in supersonic flight. All aircraft experience a rearward shift in the center of lift which reduces maneuverability and increases drag. Rather than rely on large control surface deflections to correct this situation, the delta arrow’s forward wing segment provides the lift required with minimal drag.

Airfoil thickness is a strong modifier of total drag at high subsonic speeds. A thin airfoil has far less drag than a thick one, but in trade, it has a lower lift curve slope, and less space for structure and fuel. A way around this is to sweep a moderately thick airfoil so that the effective thickness is reduced while retaining actual space inside for the structure and fuel. This phenomenon is more pronounced with large amounts of sweep, so the volume inside the forward segment of a delta arrow is quite extensive.

Maneuvrability is closely tied to the wing loading and center of gravity location. A heavily loaded wing will have a larger angle of attack in 1G flight, reducing the amount of lift available for aggressive maneuvering, regardless of aspect ratio. A reduced angle of attack can be achieved by moving the CG aft but within certain limits. Locating the CG too far aft would render the aircraft uncontrollable, even with fly-by-wire. A forward CG will reduce the control response and increase static angle of attack, but enhance stability. Balance between the two extremes will be determined by the aircraft’s purpose and desired handling capabilities.

The reduction in lift coefficient for a given angle of attack is subject to the same cosine formula that was applied to the critical Mach number earlier. Therefore the formula is as follows:

Cl ( cos * sweep angle) = Effective Cl

Assume an aircraft requires a Cl of 0.2 to sustain level flight at a given speed. If the aft wing is swept at 54 degrees, the effective Cl would be 0.12, demanding the aircraft increase its angle of attack to create 0.2 Cl. An alternative is to reduce wing loading, reducing the required Cl and by association, angle of attack. This reduction in lift is beneficial for flying in turbulence as the reaction of the aircraft to disturbances will be markedly reduced. While some people may not consider turbulence reaction to be a reason to reduce Cl slope, those who fly low-level, high-speed profiles, especially over warm areas or near mountains may have differing viewpoints. Not all civilian designed turbine aircraft have to be business jets.

So what does this mean in plain English? It means that economical and safe civil supersonic flight is possible. The reason we have not been able to achieve this so far is that industry has been focused on improving efficiency of existing designs. To integrate these advantages, a radical departure in construction has to be undertaken. The wing must be blended with the fuselage to keep drag and weight to a minimum without sacrificing strength. Thrust to weight has to be increased to ensure adequate acceleration is available at high altitudes. Low wing loading will not only improve induced drag numbers while subsonic, but reduce the impact of sonic booms at higher speeds.

The aviation industry has been hearing for at least 15 years about proposed civil supersonic aircraft. In each case the designs were business jets. With the tumultuous world economy of the early 21st century, no builder or prospective buyer has seen fit to invest in such a jet for understandable reasons. To date, no one has proposed building a manned research aircraft of much smaller size to investigate actual performance, handling, environmental effects and integration with the current ATC environment. The cost of a purpose-built test aircraft would be far less than attempting to build a full size business jet requiring full Part 25 certification. Something to think about for the frugal mavericks among us.

P Over D

Lift to drag ratio or L/D is the figure that many crucial aerodynamic criteria are measured by. When soon-to-be pilots study for the FAA written exam, they pore over diagrams of L/D curves and where the most efficient speeds are. They see how glide ratio, climb speeds and best endurance speeds are determined. They see how L/D varies with angle of attack and altitude. They see all this information presented before them and yet are only getting half of what they need to know operationally. The number we’ll be exploring today is the P/D, or payload to drag ratio.

First, what does L/D mean aerodynamically? Quite simple; it means that you have the lowest amount of drag for your given weight. Hence, lift (holding up your weight) over drag. If your L/D went from 10 to 5, it would mean that you are producing more drag for your given weight (whether it is parasite or induced drag depends on the type of speed change). Speaking in terms of the airplane itself, L/D is the determining factor for efficiency. Sailplanes have extremely high lift to drag ratios and are often touted as being the most aerodynamically efficient winged craft on the planet. Sailplane-type aerodynamic efficiency means that making 180 degree turns with no engine at 300 feet above the ground and staying aloft in thermals for hours is completely normal. On the other hand (and there always is another hand), the downside to measuring an airplane’s worth by its L/D ican limit its usefulness. While a sailplane can play in ridge lift for the better part of a day, it’s doing it at the expense of two very important variables: payload and speed.

P/D is the transportation equivalent of the aerodynamic L/D. While everyone talks lovingly of L/D, they almost never ask the question I always ask: “What are you lifting?” It’s similar to me telling a big rig driver that I’m more efficient because I get 100mpg on my moped and he only gets 6mpg. Sure I’ll feel like a eco-hero, but the missing part of the equation is what is my moped can carry and how fast. I weigh around 200lbs, or 0.1 tons, so we can assume that as the moped’s payload. The 18 wheeler on the other hand has a payload of roughly 48,000 lbs or 24 tons. My moped (wide open throttle and downhill with a tailwind) can hit 40mph while the truck can easily cruise over 70mph. What would you rather have deliver 24 tons of groceries to your store from a warehouse site 200 miles away? One truck that takes just under 3 hours, or 240 mopeds that will take 7.5 hours?

The same principle applies to aircraft. Most people buy airplanes for speed. They want to save time over driving or taking the airlines. They want to be on their own schedule. They sometimes just want to be able to say “I cruise at Mach 9.5” to their friends who all cruise at Mach 9.2. Very few people buy powered airplanes for a distinct lack of speed. There is also another camp that values payload over blistering speed. The ability to lift a lot of people or a lot of cargo is critical for airlines and charters in order to make a profit. It counts for general aviation as well since quite frankly, many of the standard “4 place” airplanes are really “2 people plus a couple bags” craft with respect to payload and full tanks. Many planes have been wrecked and too many people killed because pilots failed to fully understand the significance of this nebulous limitation.

Time to compare L/D to P/D. The Schwiezer SGS 2-32 glider has a reported L/D somewhere between 32/1 and 36/1. For it’s gross weight of 1340 lbs, it will be producing between 38 and 42 lbs of drag. Incredibly low numbers thanks in part to it’s low drag fuselage and high aspect ratio wing. This max L/D is achieved at a speed of 45 knots, which is very slow for transportation considerations and totally unusable for any powered airplane’s cruising flight. Assuming 2 large men weighing 250lbs each are on board, the payload is 500 lbs. P/D is between 13.9 and 15.6, which are the real numbers we want. For each pound of drag, the aircraft is carrying an average of 14 lbs of payload.

P/D ratios that high will not be commonplace in powered aircraft simply because they are much heavier in structure and can’t carry as large payload with respect to their maximum weight. A very heavy-lifter, the Boeing 747-400F has an average L/D of 17.5 at 35,000 feet and normal cruise speed, while maximum payload is listed as 248,000 lbs. Assuming an late-in-cruise weight of 700,000 lbs, drag would be 40,000 lbs and P/D therefore is 6.2. Closer to home for many of us, the Cessna 172 has an L/D in the region of 9.0. At 2400 lbs gross weight, the resultant amount of drag is 267 lbs. Assuming 3 people are on board who weigh 200 lbs each, the P/D works out to 2.25. Most light aircraft will end up reasonably close to this number, varying by the number of seats on board.

Why is P/D important to designers? Because while purist pilots may love airplanes, airlines quite frankly couldn’t care less about the aircraft itself as long as it does 3 things:

  1. Doesn’t crash randomly.
  2. Is reliable.
  3. Turns a profit.

If you wish to build military fighter/attack aircraft, the desires are slightly different:

  1. Doesn’t crash randomly.
  2. Is reliable.
  3. Carries a lot of weapons.

And if you want to build a business jet…well, you get the idea. The only category of aircraft where payload can be ignored somewhat is the personal pleasure craft, such as certain experimental and ultralight aircraft. They aren’t made to make money, they’re made to enrich the builder/owner’s life. On the other hand, if an airliner has a beautiful shape that inspires poets worldwide, sets speed records and adds alleged prestige to the company, that won’t save it from being cast aside in favor of something more pedestrian but economical (and yes, I was referring to that supersonic work of art, the Concorde).

The more an aircraft weighs empty equates to more structure and materials which directly leads to more cost. The more an aircraft weighs empty, the less it can carry in fuel and payload if it’s maximum takeoff weight is not also adjusted upwards. So not only will it cost more to buy, but it produces less per trip in terms of revenue. From an economic standpoint, an airplane that has lower than average aerodynamic virtues but exceptional carrying capabilities will be more desirable than a craft with a high L/D but limited payload. This explains the ongoing interest with composites to reduce airframe weight without sacrificing strength.

P/D is just another number that can help you analyze performance in a practical manner. For instance, if you’re looking to buy an airplane for personal use, consider who and what you want to take with you. A super efficient powered glider that has only one seat is pointless if your goal is to carry your family on short trips. Likewise, if you find an airplane that has a very high cruise speed but it requires you to lay flat like a Mercury astronaut, that won’t make the kids very happy, especially since there are no windows to see out of. Like everything else in aviation, its a balance with your needs as the fulcrum.

Next time, we’ll talk about all the “best” speeds and why they aren’t speeds.

Hangar Talk 1: Of Boats, Planes and Flying Wings

Standing in the hangar today, I was deeply engaged in a discussion with another pilot about the pros and cons of current aircraft design. We both agreed that designers have pretty much maximized the efficiency of the fuselage/wing method. Any further increases will come from add-ons like winglets, sharklets, alligatorlets, or whatever other things they come up with to slap onto the end of a wing. Therefore any radical increase in payload or range will have to come from a radical change in construction.

Before everyone throws down their copy of Controller and spills coffee in a rage at my upstart attitude, consider that almost all respected designers as recent as the 1990s regarded winglets as a fashion statement added by the marketing department. In 2007, the last true defender of “build the wing right the first time”, Dassault Falcon has finally and begrudgingly added winglets to their aircraft. It took a massive increase in fuel prices to force a change in perspective but everyone eventually came around. It will be much the same with a new type of aircraft design. Don’t worry, you’ll get used to it.

Lifting body/flying wing/blended wing body designs are among the most efficient shapes in the sky. All fighter jets from the 4th generation onward have lifting body tendencies. That’s why an F-4 Phantom’s turn radius makes F/A-18 pilots start laughing uncontrollably…the F-4 was a brute force airplane: Fuselage, intake/exhaust, wing, stabilizers and engines. The F/A-18 had and still has leading edge root extensions (shortened to LERX to prevent pilots from having to say “My plane has ‘stensions!”). At higher angles of attack, they begin producing powerful vortices that drift over the top of the wing and delay the stall far beyond what would be expected from looking at the airfoil’s lift curve graph.

McDonnell Douglas F/A-18F Super Hornet pulling into a square loop, showing the powerful LERX vortex generated from the increase in angle of attack.
(I know Boeing bought out McAir but it just doesn't sound right even today)

But there is part of the problem. In order for the LERX to work, it needs to be at a fairly steep angle of attack. High angles of attack and normal landing gear aren’t friends. The result is a long, ungainly set of gear so the aircraft can rotate to a useful pitch angle, or a set of regular gear that will require delaying takeoff to a higher speed, which negates one of the reasons for generating more lift in the first place. Also little tidbits like being able to see over the nose and not scraping the tail on the runway are important as well. These will have to be addressed before any alternative design can be taken seriously.

In the air, the problems begin to vanish as the advantages outweigh the disadvantages (depending on how fast, high and aggressively you’re flying). One major disadvantage is the volume requirement for flying wings. A pure flying wing encompasses everything inside the wing. In order for this to happen, the wing must be very large. Think YB-49 large. Otherwise, it will be a not-so-pure flying wing, which is not a bad thing (blended wing body or BWB). The B-2 Spirit stealth bomber really is a BWB as the cockpit and intakes protrude above the upper surface of the wing. The thickness to chord ratio would have been prohibitively large in order to fit the cockpit inside which would have reduced maximum speed. The really nice thing is that the “fuselage” on the B-2 does not need the normal streamlining since only a portion of it is in the airflow. The part that’s buried in the wing can be whatever dimension the designer desires.

Okay, back to the hangar conversation. My stance was that normal airplanes waste a lot of energy lifting parts they don’t need to lift. If I’m on a boat, I sit on the deck which is covering the hull. The hull is the only part that moves through the water. There is no heavy, non-buoyant passenger cabin supported by hulls on the port and starboard like poorly designed pontoons. Even catamarans keep their center sections out of the water (it would cause an incredible penalty in drag to have it actually in the water). And yet that’s how we build airplanes, with non-contributing sections in the airflow. The other pilot (who occasionally goes out in boats) understood immediately. We have to be able to put almost everything inside the wing, or make most of the fuselage into a wing in order to make it work more efficiently for us.

Vincent Burnelli, John Northrup and Charles Zimmerman laid the ground work in the early 20th century. Designers that followed have taken bits and pieces from their discoveries but few if any have been able to put them together in a way that was commercially viable, useful and safe for the civilian market. There is no reason that people can’t be flying around in a 4 seat airplane that burns 8 gallons per hour, lands at 50mph and cruises at 200mph. There is no reason why people can’t be surrounded by a steel safety cage much like the one utilized in Barnaby Wainfan’s Facetmobile to protect them in the event of an accident. We just have to readjust our perspective as pilots, designers and as people and ask what can be instead of what will never be.