An argument in favor of multi-engine design

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BJC

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Having sufficient power to continue flying with an engine loss in a high drag configuration as after takeoff is one requirement of a multi engine plane to be useful, .....
Agree, but that has not been a certification requirement for light twins. At least one E-AB twin can't maintain altitude on one engine with the landing gear extended.


BJC
 

arj1

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Agree, but that has not been a certification requirement for light twins. At least one E-AB twin can't maintain altitude on one engine with the landing gear extended. BJC
BJC, I thought EASA 23.51 Take-off speeds, (b)(1)(i) is exactly about that, or am I missing something? (b) For normal, utility and aerobatic category aeroplanes, the speed at 15 m (50 ft) above the take-off surface level must not be less than – (1) For twin-engined aeroplanes, the highest of – (i) A speed that is shown to be safe for continued flight (or land-back, if applicable) under all reasonably expected conditions, including turbulence and complete failure of the critical engine; or (ii) 1·10 VMC; or (iii) 1·20 VS1 Alex.
 

BJC

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Having sufficient power to continue flying with an engine loss in a high drag configuration as after takeoff is one requirement of a multi engine plane to be useful, another requirement is to be able to continue flight to a safe destination after engine loss somewhere around the midway point of flight. Considering drop in airspeed with engine loss and/or additional power required on remaining engines can you still make it to (a) destination from midpoint of flight.
Agree, but that has not been a certification requirement for light twins. At least one E-AB twin can't maintain altitude on one engine with the landing gear extended.


BJC
BJC, I thought EASA 23.51 Take-off speeds, (b)(1)(i) is exactly about that, or am I missing something? (b) For normal, utility and aerobatic category aeroplanes, the speed at 15 m (50 ft) above the take-off surface level must not be less than – (1) For twin-engined aeroplanes, the highest of – (i) A speed that is shown to be safe for continued flight (or land-back, if applicable) under all reasonably expected conditions, including turbulence and complete failure of the critical engine; or (ii) 1·10 VMC; or (iii) 1·20 VS1 Alex.
EASA minimum RoC requirements are for the airplane in a "clean" configuration (i.e.feathered propeller, gear and flaps retracted) rather than "high drag" configuration under discussion.

Part 23 requirements in the USA (at least former requirements that many light twins were certificated under - don't know if they have been revised) for reciprocating engine aircraft with a GW of less than 6000 pounds have less stringent requirements than EASA.


BJC
 

Armchair Flying Ace

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Hmm... the fuselage seems to have kind of a stubby shape. Is it 2 seats side-by-side?

The design does look a bit awkward and the cockpit is pretty high up.

If all three engines are tractors, what's the benefit of a twin-boom design? Could that be changed to a single boom, possibly giving the rear of the fuselage a more aerodynamic shape? Alternately, how about moving the engines up onto the wing, lowering the cabin, and possibly turning it into a flying boat?

For that matter, if you are going to use twin booms, what about sticking the outboard engines on the fronts of the booms and then putting the central engine on the back of the fuselage? Kind of like this:

seaplane.PNG
 

blane.c

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It is 4 seats for short range missions and 2 seats for long range, that is trade people weight for fuel weight. It is my goal to design it to go from NY to the Caribbean. It is normal to put the props out of the water so they need to be up a little, and I am trying to put the cg somewhere near the pilots butt and also giving the pilot good angle of vision so that puts the pilot high and the engines near center-line. I like the look of the twin booms and the empennage may need to move up or back or both and I do not wish for a fat fuselage section to be exposed above the wing. The reason for going to floats instead of flying boat is twofold (1) I don't like sitting on my fuel and there is not enough room in the wings without going to elaborate measures so about 2/3 of the fuel supply (or more with bigger engines) will be in the floats and (2) I am trying to (cough) get some lifting body effect from the fuselage. The rear engine on this kind of design would be to susceptible to spray and more importantly in my opinion having people moving between both front and rear prop arcs is a recipe for tragedy.
 

Matt G.

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I would not want to be a backseat passenger in that. No room to move, and probably unable to escape in a crash.
 

blane.c

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Yes, the back seat is not designed to be comfortable. It is merely to allow the transport of additional persons, this is supposed to be a comfortable two seat with ability to haul four. For example maybe you want to take friends to dinner across the island. I plan for the rear to open up AKA a hatchback, which may alleviate some of the safety concerns.
 

Sockmonkey

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I was thinking about the carb icing issue and I think you might mitigate that with s conductive bimetallic strip. You know those alloys that flex when they're heated or cooled? Stick one on the carb positioned so that if it gets too cold the contraction of it brings a piece of copper connected to the carb into contact with a hot part of the engine to warm the carb back up. If it gets too warm the strip expands and stops delivering heat. It's cheap and pretty much idiot-proof.
 

nerobro

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Carbs are really quite chilly all the time. Where would you put the strip that it could detect icing? Also, ice is an insulator, and a huge thermal sink. The air might get colder, but the ice is going to pretty well stay at 32 deg until it stops depositing new water on it. ... heck the carb might warm up while the carb throat fills with ice.
 

nerobro

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Ice is a pretty good insulator. It doesn't "move" to transmit heat, like air, or water does, so you get no convection. It doesn't transmit heat quickly through itself, so conduction is fairly slow. That's a big reason ice cube crack when dropped in even a cold drink. It also has all the thermal mass that water does, so even if you do heat it up there's a lot of mass to heat up. And if you're going to try to heat things up, the ice is going to fight to stay at ~32 deg becuase you need to deal with the heat of phase change.

A chart to back it up the statment a bit:
http://inspectapedia.com/insulation/Insulation_Values_Table.php#Roof

Looks like snow (and other places have said ice) has an r-value of 1. Moving air, according to that chart, is .15. But I think those are missing some factors.
 

Sockmonkey

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The thing would presumably be set to kick in a little before the carb hits freezing temperatures so the ice doesn't form in the first place. It's tricky to figure out how to place it, but doable.
 
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