Multi-Engine Challenge Guide
If you've ever looked up at the sky and seen an aircraft fly over, you've probably noticed that apart from the very smallest aircraft in the skies, a large number of designs use more than one engine. There are, of course, a number of advantages and disadvantages to a multi-engine aircraft, and a lot of design compromises that result from this.
Redunancy and Reliability
The ability to continue flight if an engine fails is one of the most important reasons why aircraft sprouted a second engine not long after the Wright Brothers flew for the first time. Just as important, it allowed for a dramatic improvement in safety considering the rather suspect reliability of the engines of the day. In fact, it has only been in the last 25 years or so that aircraft engines were reliable enough that single-engine aircraft would be afforded the same privileges as a multi-engine aircraft in commercial operations.
As such, the next challenge will include, in addition to Time Between Overhaul (TBO) calculations, a Mean Time Between Failure (MTBF) calculation. There will be no points scored on this number, but for certain aircraft, there will be a minimum acceptable value if you opt for a single-engine design - powerplants that do not meet this criteria will be disqualified. The calculation will be in the latest Powerplant Calculator, but the MTBF calculation will be as such:
e^((Reliability x 100)/Rated Horsepower (flat-rated or maximum))
The minimum value will be 100,000 - matching the generally accepted value of one engine failure per 100,000 flying hours for commercial IFR operations in a single-engine aircraft. As an example, a 500-horsepower engine would need to have a minimum reliability of 57.6 to meet this criteria. This shouldn't be tremendously difficult to achieve with 2016 technology levels, but it will become difficult, if not impossible, when or if we do a challenge in earlier years.
In this challenge, powerplants become considerably heavier and more expensive the larger and more powerful they become. For each challenge, there may be a point where the increased airframe cost, weight and drag is offset by the ability to use two or more cheaper, lighter engines than one large, expensive and heavy engine...even more so if the minimum MTBF is in play.
Speaking of cost, multi-engine aircraft will pay more than just a potential cost penalty in this challenge. As discussed in the original post, single-engine aircraft have no drag penalty associated with their ppwerplant other than that of the cooling system. In a multi-engine aircraft, there is a drag penalty from each engine nacelle in addition to the cooling system penalty, the calculations for which are detailed in the first post yet again. This means that engines with small frontal areas will produce less drag than an engine with a large frontal area. So, a comparably-sized inline-6 will perform better in this respect than a V6, which will tend to have the worst frontal area (and length, which affects the fineness ratio of the engine nacelle) for a given engine size. Also, a V8 will do better than a comparably-sized V6 because the engine will be physically smaller (due to the smaller bore/stroke needed for a given displacement) than the V6. Other things will affect engine frontal area as well, such as the type of intake and exhaust selected and the valvetrain layout. Additionally, multi-engine aircraft require larger vertical stabilisers and rudders to counteract asymmetric thrust in the event of an engine failure, which also comes with a slight drag penalty.
This is one of the biggest issues with a multi-engine aircraft; how it performs with one engine not functioning. Unfortunately, it isn't as simple as "two engines, lose one... 50 percent of maximum performance". The reality is that because the aircraft wants to yaw and roll into the inoperative engine, you need to use a coordinated application of rudder and aileron to hold a small, continuous bank angle into the good engine, as well as holding an input to the rudder.
This produces a fair bit of drag by itself, in addition to the drag associated with the propeller of the inoperative engine. The net result is that the loss of performance is far higher than just 50%...the rough rule of thumb is that you lose 85-90% of your climb and altitude performance with one engine inoperative in a twin engine aircraft (this proportion drops in aircraft with more than two engines, and changes based on which engine fails in those cases, but generally the worst-case scenario is assumed in any performance calculations). So all of a sudden that twin that climbs at 2000 feet per minute with all engines running is now wallowing along at 200-250 feet per minute with an engine out.
There are certification requirements for climb gradient with an engine inoperative in reality, and they will apply in this challenge as well. They are much more complicated in reality than what we will use, but how the challenge calculates performance means I only need to factor the worst case scenario for each group. This will all be worked into the calculator when it comes out, so you don't need to know numbers off by heart, but here they are for those who are interested:
Normal Category, <6000 pounds gross weight, stall speed <61 knots:
No climb gradient necessary
Normal Category, <6000 pounds gross weight, stall speed ≥61 knots:
1.5% (91 feet per nautical mile)
Normal Category, ≥6000 pounds gross weight:
0.75% (46 feet per nautical mile)
Two-engine - 2.4% (146 feet per nautical mile)
Three-engine - 2.7% (164 feet per nautical mile)
Four-engine - 3.0% (182 feet per nautical mile)
Normal and Transport categories are certification categories for aircraft; they will be specified in the design criteria of each challenge going forward.
This climb gradient calculation will be used as a scoring category, as will something known as the "drift-down altitude", which is the maximum altitude an aircraft can maintain with one engine inoperative and the remaining engine(s) at maximum continuous power (which for the sake of simplicity in this challenge is just maximum or maximum rated power). Since single-engine aircraft have no engine-out climb gradient or drift-down altitude, aircraft will be scored as a percentage of the maximum possible points in single versus multi-engine challenges, rather than as a plain raw point value as we have seen in both rounds so far.
On another note, I am taking steps to build in stability calculations, with the engine weight and position having an effect on the aircraft's handling characteristics...essentially, I am closing in on actually designing and building not just a functional aircraft simulation model, but simulated aircraft as well! It isn't hard to imagine the next step as throwing something together for Microsoft Flight Simulator or X-Plane and actually flying our creations "for real"!
And now I'm daydreaming of using this as a stepping stone to Aeronation: The Airplane Company Tycoon Game!