An argument in favor of multi-engine design

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by blane.c, Aug 29, 2016.

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  1. Aug 29, 2016 #1

    blane.c

    blane.c

    blane.c

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    First lets address some of the obvious shortcomings of multiple engine design.

    I would think the first problem to multi-engine design is not with design at all, it is the pilot. The pilot must be properly trained and/or educated for multi-engine aircraft.

    Complexity is a second issue of multi-engine design and is much of the reason for the first problem. Complexity can by modern design be reduced from the contemporary norm. Others thoughts and ideas on this matter is one of the main reasons I am posting this argument.

    Efficiency is a third issue of multi-engine design more engines and the related components create more drag.

    So why multi-engines? Well lose an engine over an expanse of water, mountainous or otherwise hazardous terrain and you may wish you had at least one more and maybe two or three additional. So despite the additional workload on the pilot due to the complexity of the machine it can become safer to have an additional engine or engines under certain conditions. Also it can be more efficient in regards to operation, if you have three or more engines and have a engine failure your options are greatly increased as to were you terminate your trip. You may wish to choose an airport more cost friendly in regards to getting repairs for instance, or perhaps you would prefer someplace with a longer runway and or better weather.

    My personal belief/opinion is that one of the biggest problems of most small multi-engine airplanes is that they only have two engines and that small light multi engine aircraft with three or more engines would be safer than two engine aircraft. One of the drawbacks of a two engine design is that after considering drag you end up with something less than 50% power with an engine loss and with a large number of designs this is inadequate. With three engines you would end up with something around and possibly more than 60% power with an engine loss and also less losses to drag because the yaw will be less pronounced even on the critical engine (three engines 33 1/3% power each, two engines 50% power each, 50% minus 33 1/3% equals 16 2/3%, so a reduction of 16 2/3% of the power effecting yaw). Because of the reduced yaw a three engine airplane is easier to control and is more efficient with an engine out. Also it may be possible for a three engine design to be lighter and have less drag than a two engine design because the nacelle's and propellers would be lighter and smaller and part of the middle engines drag component could be married with the fuselage.


    Three engines in the 50hp to 60hp range would make a nice two or three seat cruising airplane, and three 80hp to 100hp engines would make a nice four or five seat cruising airplane. Considering the cost and maintenance expense of 150hp to 180hp engines or of 240hp to 300hp engines the actual cost and maintenance of a nice three engine design may defy conventional thinking.



    I am looking forward to your thoughts and comments on the subject, thank you.
     
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  2. Aug 29, 2016 #2

    BJC

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    One can make somewhat similar arguments in favor of six cylinder singles over four cylinder singles.

    A downside of 2, 3 or 4 engine HBA is the complexity of the systems involved. That also translates to greater risk of pilot error.


    BJC
     
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  3. Aug 29, 2016 #3

    Tiger Tim

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    So this but with VWs?
    [​IMG]
     
  4. Aug 29, 2016 #4

    BJC

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    OK, what is it?


    BJC
     
  5. Aug 29, 2016 #5

    cheapracer

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  6. Aug 29, 2016 #6

    BJC

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    It needs a triple tail like a Connie.


    BJC
     
  7. Aug 29, 2016 #7

    Turd Ferguson

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    The problem with light twins is that they have the requisite power divided into two separate packages. Under most conditions, a single power package on the light twin is woefully inadequate to provide the necessary performance.

    Shutting down one engine on a twin is mathematically a 50% reduction in power available. However, depending on the type plane, performance is reduced on average of 80% (for the reasons you listed). What I said above, when performance was barely adequate with both engines, there's a problem when one of those quits and performance is reduced by 80%.

    This is how the problem is solved on Part 25 airplanes:

    Simply install way more power than necessary. Not necessarily more power units as that complicates things (notice 3+ engine transports have all but disappeared). If you need 100hp to make the plane perform sufficiently, then install 2-100hp (or larger) powerplants instead of 2-50 hp powerplants. The first doubles (or more) the necessary power, the second is dividing the necessary power into two power units. The latter is a recipe for disaster - it's how light twins are designed - and it requires superhuman performance from the pilot when things go wrong. On a Part 25 airplane, an engine takes a dump, it's noted and you continue on your way. There is huge loss of performance but it's not a factor because you started off with way more performance than necessary. No instantaneous action need to clean up and manage paltry double digit climb numbers to clear the next hedgerow (Heaven forbid you encounter a downdraft).

    More power is the solution (Tim Taylor), not more powerplants. Or you can do like Rutan and work on reducing that 80% performance loss with aerodynamic breakthroughs.
     
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  8. Aug 29, 2016 #8

    nerobro

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    Assuming engine weight divides equally, it's a pretty good argument.

    The engineering downside of that is weight. Replicated crankcase area, carbs, ignitions, engine mounts, airframe hardpoints.. it starts to add up fast.

    Pilot workload ends up being a big thing. More than say.. four engines.. and I'd want FADEC and a single lever. Heck, that might even be a good idea with two engines. In the old days, they'd have a dedicated engineer monitoring engines. For six engines, that's a ~need~ not a want.

    Now there are some other things that don't scale well too. Multiple smaller engines, would typically mean multiple smaller props. Smaller props have less "useful" area, and you end up with the tip losses of more propellors. You also end up with the slipstream losses if they're tractor. And then you need to consider interference drag between the nacelles and the wing, fuselage, support pylons, whatever.

    I can think of ways around each of these concerns, but they all seem to end up rolling more complexity in.

    Lets say we want one, or two big props with controllable pitch. To do that, we'd need a ring gear, or belts to gang engines to the same prop. This would not be light, but we'd then be able to stuff the engines in one or two cowls. This arrangement would allow catastrophic failures of individual engines and to be able to continue flight. But we're dangerously close to "why not just make them one engine".

    The engines are smaller, there's some good chance that there won't need to be a "cowling" per-se. and we could spread engines through the wings. We could get around control weight, by using servos at each engine to control mixture and throttle position. (the control wires would need to be something like 22ga) Perhaps we could do controllable ptich with a servo? I don't know if that would work. And I think controllable pitch might be more of a problem, if a servo failed. The big benefit here is we could mount the engine perhaps directly to the spar, saving engine mount weight, and by having it "in" the wing, we save cowling drag. By blowing along the wing, we might even see takeoff improvments... maybe enough to compensate for the lack for controlled pitch?

    In all seriousness, a three engine setup would probably be safer than a twin. Even with the increased pilot workload. The change in thrust line would be much smaller no matter which engine cut out. You'd have a lot more time to figure out what was going on, and with the center engine, VMC would be much lower, probally below stall speed.

    .... I've put more than passing thought into this. Mostly because some r/c engines have hilarious power to weight ratios. And a gang of 50 .21ci nitro powered beasts would get some serious power in a low weight package.
     
  9. Aug 29, 2016 #9

    homebuilderfan

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    I have always thought a multi engines was a goos choice. But: more engines=more weight and fuel consumption; also need a stronger airframe.
    It is true that light twins often are in big troubles when an engine quit. Obviously the power is 50 % less; but the ratio weight=power? Is worst.
    It comes from an american way of thinking to add more and more power. Not always a good choice. Is it better a biplane, full of wires and so on, or an advanced glider able to fly hours (or hundreds of km) without a drop of fuel? Rutan understood the right way to solve.
    Flying over a water in single engine. Should not be permitted. Wow, I can imagine the reaction when you will read this sentence... :)
    Well, it means that if you are going to use in such conditions, buy a multiengines or an amphibian. Anyway, in my short flight activity I have very very often seen that a single engine failure would have been very very "sad"...
    Usually the ground is used in other ways than airfield. No need to have a large water under the main gear to be worried: "normal" aircraft aren't very good at landing on trees or building either...
     
  10. Aug 29, 2016 #10

    Rockiedog2

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    With a twin we got 2x the liklihood of an engine related emergency. I still like a lotta motors tho unless I'm designing/building it.
    If I needed max reliability for max simple/min weight I would look at a single proven turboprop.
    Maybe somebody said that already...I skimmed it pretty quick
     
  11. Aug 29, 2016 #11

    Hot Wings

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    Thread drift - yes on the first page even: :emb:

    If we ever get practical hybrid systems this practice will become obsolete.
     
  12. Aug 29, 2016 #12

    blane.c

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    basically yes that is a tractor alternative.
     
  13. Aug 29, 2016 #13

    blane.c

    blane.c

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    Basically yes that is a tractor version. You could have three pushers also or any combination of pushers and tractors.
     
  14. Aug 29, 2016 #14

    blane.c

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    Because I believe systems can be made simpler today than those historically used, and because smaller engines will be less expensive more efficient and demand less effort from the pilot to control, I think the "two way bigger" argument is outdated in regards to light private aircraft.
     
  15. Aug 29, 2016 #15

    TFF

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    Years ago I believe in Flying Magazine there was an article on the perfect pair of planes. The Beech Bonanza and the Barron. Except engine configuration the same airplane. Both nice airplanes and both have good and bad history. It was found that you were more likely to crash the Barron under the same emergencies. You were more likely to survive an engine out in the Bonanza. Why? One training. Most private owners do not fly to airline standards of multiple engines. They get biannuals in 172s cause it's cheaper and they don't train emergencies cause it cost money to practice. You are more likely to focus on getting the Bonanza down while you try to save the Barron. I here students pull the wrong engine on engine outs. Someone who has not practiced will likely do the same. How often do you practice flying around on one engine? Training and practice.
     
  16. Aug 29, 2016 #16

    blane.c

    blane.c

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    I don't see three engines requiring a significant increase in pilot workload over a twin engine design if of course proper thought and modern elements are incorporated into the design.

    While a multi-engine airplane will probably always weigh more than a single engine airplane, it is unlikely a three engine airplane will weigh much more than a twin percentage wise in comparison to a single. In some cases a three engine plane may be made to be lighter than a twin because engine supporting wing structure can be smaller and thereby lighter.

    Another thing about three or more engines is the luxury of being able to "baby" a problem engine to "nurse" it home or to shut it down before failure thereby reducing the expense of repairing it.
     
  17. Aug 29, 2016 #17

    blane.c

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    Since the pilot bill of rights has been signed into law you may find a significant number of pilots with lots of multi time are now going to be flying again. While most will go with certified aircraft there will be a significant number looking for experimental rides and of those some will want multiple engines. They will consider an aircraft along the lines of what I am discussing "easy peasy" and actually "fun" to fly.

    Dead foot dead engine.
     
  18. Aug 29, 2016 #18

    nerobro

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    On a high performance twin, if an engine goes out, you have seconds to reconfigure for single engine flight. I suspect, on a triple, you just "lose climb rate and maybe put a gentle push on the rudder." It's "not an emergency" which is definitely easier on pilot workload.
     
  19. Aug 29, 2016 #19

    Aesquire

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  20. Aug 29, 2016 #20

    nerobro

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    I was waiting for the DO-X to show up.
     

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