An argument in favor of multi-engine design

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bmcj

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Wing mounted twins can benefit structurally (less g-induced root bending moment).

Wing mounted twins might tend to take more recovery effort (or go flat) in a spin (high mass moment of inertia).

Wing mounted twins might suffer in crash impact survivability (higher weight equals higher speed and no heavy engine in the nose for protection/momentum decrease).

I suspect a single larger engine is more fuel efficient than two smaller engines.
 

blane.c

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Wing mounted twins can benefit structurally (less g-induced root bending moment).

Wing mounted twins might tend to take more recovery effort (or go flat) in a spin (high mass moment of inertia).

Wing mounted twins might suffer in crash impact survivability (higher weight equals higher speed and no heavy engine in the nose for protection/momentum decrease).

I suspect a single larger engine is more fuel efficient than two smaller engines.
How do you feel about three engines?
 

bmcj

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How do you feel about three engines?
From my four points:

Point 1 - still benefits by having some of the engine weight on the wings.

Point 2 - still an issue, but to a lesser degree.

Point 3 - a third nose mounted engine might help here.

Point 4 - still an issue, but a full analysis will tell for sure. I think larger props tend to give better fuel economy, so 3 smaller props might suffer a bit.

Keep in mind that all of my points were suppositions made off-the-cuff, so YMMV.
 

Turd Ferguson

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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expensive more efficient and demand less effort from the pilot to control, I think the "two way bigger" argument is outdated in regards to light private aircraft.
I think with regard to performance, modern materials may allow a light twin to be built lighter which could be used to offset power required. Simplifying systems is not going to add anything to performance and it's still quite possible to run our of altitude and crash with simple systems. An engine that boast 4-5% more efficiency is not giving me a warm fuzzy feeling.

Regarding handling, the only multiengine planes I know of that come close to being idiot proof are the centerline thrust planes and Rutan's Boomerang. There is no critical engine, no yawing and rolling when either engine is shutdown. There is a huge loss of performance. If the performance was marginal to begin with the remaining engine(s) will only serve to fly the plane to the scene of the accident. Of course, no such light twin is being produced today because that is not what customers want, i.e. no market, no plane.

I've flown airplanes for a number of years. Lots of light twins. There is no substitute for power because that translates into performance. The last thing anybody wants is to be looking at an obstruction wondering whether or not they are going to clear it. That is simply not a concern in a Part 25 airplane. If the data doesn't show engine out performance will carry the plane and occupants to safety at any point during the flight, there is no takeoff.

As far as three engines not being any more difficult to manage than two, I can't fathom a logical argument for that. Of course its more difficult. Anything added to lessen the difficulty of managing three engines adds complexity and weight.
 

Turd Ferguson

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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.
Sounds speculative. What info are you using to make that prediction? As with Sport Pilot, I don't see a large number of pilots sitting on the sidelines just waiting for relaxation of medical standards as an opportunity to get back into flying.
 

blane.c

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As far as three engines not being any more difficult to manage than two, I can't fathom a logical argument for that. Of course its more difficult. Anything added to lessen the difficulty of managing three engines adds complexity and weight.
From a standpoint of flying behind four engines I can say that losing an engine is rather a non event, even the critical one doesn't yaw the plane much. Much less of a deal than a twin. Feather and checklist, feed in some rudder trim, adjust the nose trim for a slightly reduced airspeed maybe add a little power on the good'uns maybe not depends. Fuel management again depends on how far your going. Since it is less than a 50% power loss you don't even have to notify control but it is advisable if nothing else as a courtesy and besides you are going to be late to all your way-points flying at reduced airspeed. During normal flight it just doesn't seem that big a deal to me, multiple engines to adjust power during climb and descent and lean during cruse just doesn't take much time. What's the big deal?
 

blane.c

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I've flown airplanes for a number of years. Lots of light twins. There is no substitute for power because that translates into performance. The last thing anybody wants is to be looking at an obstruction wondering whether or not they are going to clear it. That is simply not a concern in a Part 25 airplane. If the data doesn't show engine out performance will carry the plane and occupants to safety at any point during the flight, there is no takeoff.
So if you need two 120hp engines to fly safely in a micro-light twin what would be wrong with three 80hp engines instead? Wouldn't you be better off all the way around?
 

blane.c

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Sounds speculative. What info are you using to make that prediction? As with Sport Pilot, I don't see a large number of pilots sitting on the sidelines just waiting for relaxation of medical standards as an opportunity to get back into flying.
Yes speculation, also curiosity.
 

Turd Ferguson

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From a standpoint of flying behind four engines I can say that losing an engine is rather a non event, even the critical one doesn't yaw the plane much. Much less of a deal than a twin. Feather and checklist, feed in some rudder trim, adjust the nose trim for a slightly reduced airspeed maybe add a little power on the good'uns maybe not depends. Fuel management again depends on how far your going. Since it is less than a 50% power loss you don't even have to notify control but it is advisable if nothing else as a courtesy and besides you are going to be late to all your way-points flying at reduced airspeed. During normal flight it just doesn't seem that big a deal to me, multiple engines to adjust power during climb and descent and lean during cruse just doesn't take much time. What's the big deal?
You are arguing in favor of the point I made earlier. If starting out with more power than needed, loss of 1 out of 4 engines (25%) is a non event. If an airplane has a power requirement of 100hp, and that is accomplished with 4 engines of 25 hp each, loss of one engine is a huge problem despite being only a 25% power loss.
 

Turd Ferguson

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So if you need two 120hp engines to fly safely in a micro-light twin what would be wrong with three 80hp engines instead? Wouldn't you be better off all the way around?
I'd choose two 120 hp engines. I think you'll find adding structure to support third engine is not free. You end up having horses that eat but don't pull.
 

BBerson

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For efficient flight at modest speed, I propose a new configuration:

Two engines. Main engine in nose, second engine retracts into aft fuselage (like a self launching glider)
Both engines used for takeoff. The front engine should be just large enough to climb alone if auxiliary engine fails on takeoff. The rear engine has a large climb prop that will sustain altitude if front engine quits on takeoff. With both for takeoff, it provides a normal nice climb rate.
The normal cruise is with the one efficient engine while the auxiliary engine and prop drag is completely stowed.
For low level or flight over Lake Michigan, the auxiliary engine is started and both engines are used.
For trans-Atlantic, the rear engine would be stowed, but the cruise height would allow enough time to unfold if needed. The rear engine prop has an extreme climb prop, but could sustain flight alone at minimum speed, in an emergency.
 
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blane.c

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For efficient flight at modest speed, I propose a new configuration:

Two engines. Main engine in nose, second engine retracts into aft fuselage (like a self launching glider)
Both engines used for takeoff. The front engine should be just large enough to climb solo. With the addition of the auxiliary engine for takeoff, it provides a normal nice climb rate.
The normal cruise is with the one efficient engine while the auxiliary engine and prop drag is completely stowed.
Forlorn level or flight over Lake Michigan, the auxiliary engine is started and both engines are used.
For trans-Atlantic, the rear engine would be stowed, but the cruise height would allow enough time to unfold if needed.
Keeping the stowed engine warm at altitude so it could be restarted in a reasonable time would have to be worked out. It is a good idea from an efficiency standpoint, kind of a cruising motor-glider with a spare?
 

gtae07

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For efficient flight at modest speed, I propose a new configuration:

Two engines. Main engine in nose, second engine retracts into aft fuselage (like a self launching glider)
Both engines used for takeoff. The front engine should be just large enough to climb solo. With the addition of the auxiliary engine for takeoff, it provides a normal nice climb rate.
The normal cruise is with the one efficient engine while the auxiliary engine and prop drag is completely stowed.
For low level or flight over Lake Michigan, the auxiliary engine is started and both engines are used.
For trans-Atlantic, the rear engine would be stowed, but the cruise height would allow enough time to unfold if needed. The rear engine prop has an extreme climb prop, but could sustain flight alone at minimum speed, in an emergency.
What happens if your main engine quits, or the aux quits with more than solo weight?

I'd rather see a battery and a couple electric motors with folding props. Do it right and you could get STOL performance without as many aerodynamic compromises for cruise, very good climb rates, and less stressful engine-out behavior.
 

Turd Ferguson

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Rutan Voyager style: Centerline thrust. Shutdown front engine at cruise and feather prop. Second engine provides extra performance during critical phases of flight. Readily available for emergency. Figure out a way to close off cooling air flow when engine not in use so there is no excess drag. I guess that would be easier with liquid cooling.
 

BBerson

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What happens if your main engine quits, or the aux quits with more than solo weight?

I'd rather see a battery and a couple electric motors with folding props. Do it right and you could get STOL performance without as many aerodynamic compromises for cruise, very good climb rates, and less stressful engine-out behavior.
When I said solo, I meant sole engine. Should have said single engine, will edit.
The point is all drag is eliminated, cooling and folded prop drag is eliminated. Fixed props are cheaper than folding, (ask Stemme :))
The rear engine is optimized for climb, but should be designed to slowly climb on either single engine if either fails on takeoff. The battery auxiliary won't cross the Atlantic if front engine fails.
 

Tiger Tim

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I would think if carrying on long distances in the event of a failure were the goal, I would want a centreline twin with full feathering props at both ends. For me each engine would have to be conventional (off the shelf Lycomings or Continentals) and capable of producing enough power to take off and climb at least 500fpm at max gross weight. Essentially a reliable single engine airplane with an extra engine on it. For long range cruise in sparse areas you could throttle both way back or if you're over endless farmland I suspect shutting one down would be a little more fuel efficient but you have to consider that you're lugging around the dead weight of an unused engine - that's the compromise here. Both engines wide open should make it quite the performer on departure or even in I-don't-care-what-it-costs fast cruise.
 

blane.c

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I would think if carrying on long distances in the event of a failure were the goal, I would want a centreline twin with full feathering props at both ends. For me each engine would have to be conventional (off the shelf Lycomings or Continentals) and capable of producing enough power to take off and climb at least 500fpm at max gross weight. Essentially a reliable single engine airplane with an extra engine on it. For long range cruise in sparse areas you could throttle both way back or if you're over endless farmland I suspect shutting one down would be a little more fuel efficient but you have to consider that you're lugging around the dead weight of an unused engine - that's the compromise here. Both engines wide open should make it quite the performer on departure or even in I-don't-care-what-it-costs fast cruise.
A modern style center-line twin is an idea I have liked for a while but I keep wanting to shove the engines up on a pod above the cabin which of course adds new thrust line problems. I would at least like the rear engine high enough that I don't have to wear it in a forced landing.
 
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