An argument in favor of multi-engine design

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Turd Ferguson

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Essentially a reliable single engine airplane with an extra engine on it.

Both engines wide open should make it quite the performer on departure or even in I-don't-care-what-it-costs fast cruise.
That's essentially what you have in a Part 25 twin. Either engine by itself can do the job; both engines running make for an e-ticket ride!
 

blane.c

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Typically we don't cruise airplane engines at 100% power it is more often 60% to 75% power for cruise. Additionally we don't on a multi-engine airplane want to have to use 100% power in advent of an engine loss, at least not in an otherwise normal situation. I mean you've already lost one engine then your going to fly with the other one(s) maxed out? That just doesn't give one the warm and tingly does it? So as a compromise we are going to except a lower cruise/flying speed and something less than 100% power. We want to keep the speed at a safe margin above VMC without taxing the engine(s) more than necessary.

I think one point (spelled out) that is being argued is as follows.

So if we have a twin engine airplane that is cruising at 75% power and 150hp and we lose an engine we only have 100hp at max power in the remaining engine. This is probably not a good situation and in fact is a more likely to be a bad situation. Therefore we may want to change the scenario. So we now have a twin engine aircraft safely flying at 50% power and 150hp and we lose an engine we now have 150hp left on the remaining engine in the advent of an engine loss but at 100% power. Further we now are carrying the extra drag of the dead engine so even tho our hp remains constant our drag increases and we are going to slow down in order to maintain altitude. Additionally we have the sound of one engine at full power weighing on our minds. So let's say we want to fly a little more comfortably with an engine loss? Altho it is not a cruising power setting we have a twin that will fly safely at 33 1/3% power at 150hp now with an engine loss we can keep the remaining engine at 75% power and have a little power in reserve to counter our losses due to drag and maybe not exactly but we can maintain our airspeed pretty closely to what it was prior to the engine loss. So we have gone from a 200 hp deathtrap to a 450hp airplane which is more or less what it is going to take with a twin.

This partially explains the cost of twins and why single engine airplanes are so popular.

My argument along this line of thinking is use three engines, we are not much better off in actuality at the 75% power and 150 hp level than we would be in the twin. We possibly go a little farther to the crash site or perhaps just get there a little quicker. But look at were we are at the 50% power and 150hp level. At this point we can maintain 150 hp at 75% power on the two remaining engines altho we are still obviously going to have some losses to drag and we will have to slow down a little a safe flight envelope can be designed around this criteria.

Now to have a safe airplane in regards to power reserves in a three engine airplane we need around one third less hp than a twin. And a corresponding reduction in weight and cost.

In light multi-engine aircraft were the cost of turbine engines is out of the question a third engine option seems to me to be a viable avenue for more discussion. I would like to explore ideas to make the arrangement less complex or more importantly or correctly said “less workload on the pilot” in all phases of flight especially those in an emergency or with an engine shutdown.
 

nerobro

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I think three engines has a lot going for it. The emergency on takeoff is the one that is the seller for me. If you're cruising, or even in climb, an engine out has some leeway. Having no Vmc would totally help things.
 

Tiger Tim

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Now to have a safe airplane in regards to power reserves in a three engine airplane we need around one third less hp than a twin. And a corresponding reduction in weight and cost.
Of course all of the pictures online seem to have disappeared when I need one, but I think the plane you want has already been built. IIRC once upon a time Piper decided they wanted to sell a plane to carry six people reliably and cheaply so they took the basic design of the Cherokee, widened the fuselage six inches as well as stretching it and put three O-235s(? Small-ish flat fours at any rate) on it. What they ended up with wasn't what they were hoping for so it was reconfigured to have one large engine and badged the Cherokee Six. The rest is history.

Information seems to be scarce but it would be interesting to see what they learned through the whole exercise.
 

blane.c

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I think three engines has a lot going for it. The emergency on takeoff is the one that is the seller for me. If you're cruising, or even in climb, an engine out has some leeway. Having no Vmc would totally help things.
One of the aspects I like about more engines is that you are not necessarily in an emergency because of an engine failure and in many cases can continue the flight except for airspeed normally.

In some designs it maybe possible to have a VMC lower than the stall. This would probably be some kind of STOL design. I don't foresee that it would be the case for most designs because multi-engine aircraft are more generally cruise orientated.
 

BJC

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Forgive me, blane.c, but everytime I read the thread title, I recall this joke:

A military pilot called for a priority landing because his single-engine jet fighter was running “a bit peaked.” Air Traffic Control told the fighter pilot that he was number two, behind a B-52 that had one engine shut down. “Ah,” the fighter pilot remarked, “The dreaded seven-engine approach.”


BJC
 

blane.c

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Forgive me, blane.c, but everytime I read the thread title, I recall this joke:

A military pilot called for a priority landing because his single-engine jet fighter was running “a bit peaked.” Air Traffic Control told the fighter pilot that he was number two, behind a B-52 that had one engine shut down. “Ah,” the fighter pilot remarked, “The dreaded seven-engine approach.”


BJC
Ah yes I heard that many years ago it was funny then too.

I got permission once to fly the Cesnna 152 close to a B-52 that was landing at Eielson (was going over the top of them in a 152) could see into the cockpit and wave at the pilots, it was cool.
 

blane.c

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Typically we don't cruise airplane engines at 100% power it is more often 60% to 75% power for cruise. Additionally we don't on a multi-engine airplane want to have to use 100% power in advent of an engine loss, at least not in an otherwise normal situation. I mean you've already lost one engine then your going to fly with the other one(s) maxed out? That just doesn't give one the warm and tingly does it? So as a compromise we are going to except a lower cruise/flying speed and something less than 100% power. We want to keep the speed at a safe margin above VMC without taxing the engine(s) more than necessary.

I think one point (spelled out) that is being argued is as follows.

So if we have a twin engine airplane that is cruising at 75% power and 150hp and we lose an engine we only have 100hp at max power in the remaining engine. This is probably not a good situation and in fact is a more likely to be a bad situation. Therefore we may want to change the scenario. So we now have a twin engine aircraft safely flying at 50% power and 150hp and we lose an engine we now have 150hp left on the remaining engine in the advent of an engine loss but at 100% power. Further we now are carrying the extra drag of the dead engine so even tho our hp remains constant our drag increases and we are going to slow down in order to maintain altitude. Additionally we have the sound of one engine at full power weighing on our minds. So let's say we want to fly a little more comfortably with an engine loss? Altho it is not a cruising power setting we have a twin that will fly safely at 33 1/3% power at 150hp now with an engine loss we can keep the remaining engine at 75% power and have a little power in reserve to counter our losses due to drag and maybe not exactly but we can maintain our airspeed pretty closely to what it was prior to the engine loss. So we have gone from a 200 hp deathtrap to a 450hp airplane which is more or less what it is going to take with a twin.

This partially explains the cost of twins and why single engine airplanes are so popular.

My argument along this line of thinking is use three engines, we are not much better off in actuality at the 75% power and 150 hp level than we would be in the twin. We possibly go a little farther to the crash site or perhaps just get there a little quicker. But look at were we are at the 50% power and 150hp level. At this point we can maintain 150 hp at 75% power on the two remaining engines altho we are still obviously going to have some losses to drag and we will have to slow down a little a safe flight envelope can be designed around this criteria.

Now to have a safe airplane in regards to power reserves in a three engine airplane we need around one third less hp than a twin. And a corresponding reduction in weight and cost.

In light multi-engine aircraft were the cost of turbine engines is out of the question a third engine option seems to me to be a viable avenue for more discussion. I would like to explore ideas to make the arrangement less complex or more importantly or correctly said “less workload on the pilot” in all phases of flight especially those in an emergency or with an engine shutdown.
So thinking about three 100hp 4 cylinder engines with two blade props vs two 225hp six cylinder engines with three blade props. 225hp engines weigh about 350lbs each dry on a quick search to Wikipedia and the 100hp engines weigh about 170lbs dry each also on a quick search at Wikipedia. so a savings of nearly 200lbs there. Twelve cylinders to feed and maintain either way, and six propeller blades to maintain either way.
 

Swampyankee

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One of the reasons that most commercial airliners are twins is noise abatement. Each engine on a twin has to be able to climb on one engine, after losing half of its thrust. A three or four engine aircraft has less surplus thrust on all engines than does a twin.
 

mcrae0104

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#1: Fuel starvation

if flying a twin, then both #1 & #2 are fuel starvation.
 

mcrae0104

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I think blockages are rare. Starvation could be any reason the gas doesn't get to the engine. Mismanaging the various tanks is one possibility, but plain ol' running out of gas would be the biggie.
 

blane.c

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I copied this reply from rv6ejguy to my question on FADEC over from another thread. Because I would like this discussion to go in another direction.

Re: FADEC?

With regards to piston engines, FADEC is usually used to describe a fully automatic integrated engine and propeller control system. Since a large number of Homebuilts don't have controllable pitch or C/S props, we often use the term EMS (Engine Management System) instead to describe an electronic system which controls just the fuel delivery and ignition timing.

The latter has been around 20ish years now for GA aircraft and the level of automation varies. Aerosance, Lycoming IE2, Eagle EMS and SDS have been around a number of years. The first 3 were sold in limited numbers due to high costs primarily. SDS has been around longer that the first 3 and is priced well below them as well, making it viable for at least the middle and high end Experimental aircraft market. The low production volumes generally do not permit pricing to be low enough to suit most sub 100hp users and I'm talking dedicated aviation EMS systems here which are specifically designed for the application.

The IE2 system is extremely sophisticated, similar in capability to modern OEM automotive EMS offerings and does integrate propeller control.

The Eagle and Aerosance systems do not control propellers to my knowledge. SDS also does not offer propeller control. These 3 systems do control air/fuel ratio and spark timing according to rpm, manifold pressure, temperature, altitude etc. SDS permits user intervention for running LOP and is also easily user programmable to suit it to a wide variety of engine types, modifications and applications. Over 1400 SDS units have been sold for Experimental and military UAV engine applications.

Rockwell developed the FADECs for the Rotax 9 series aviation engines and you can clearly see the economy benefits over the Bing carbs and fixed timing systems they replace.

Both the D-Motor and UL Engines also come with EMS as factory equipment.

Last edited by rv6ejguy; August 29th, 2016 at 08:21 PM.

How this kind of technology works and can be incorporated into multi-engine designs is more the direction I would like to turn.
 
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