Asymmetric Aircraft

[mikegarrison]

Engines are primarily sized to provide enough power for takeoff. (Military or acrobatic planes are a little different.) But if you have a multi-engine airplane, you also have to meet requirements for one-engine-out situations, and those can end up meaning that for many twins the limiting condition for engine thrust is actually single-engine climb performance.

The choice of how many engines depends on what's available with the right thrust, how the design pencils out single v. two v. more, and cost. There are also regulatory requirements to consider -- I think Part 25 airplanes must all be multi-engine, while Part 23 airplanes can be single-engine.

I know a lot of people already understand this, but the point is that twin-engine airplanes are not selected to be twin-engine just because two engines are always better or safer.

Edited January 1, 2018 by mikegarrison

[Racescort666]

Regarding asymmetric aircraft, if there is a specific need for it sure, but being different to be different is ridiculous. In aerospace function always preceded form. That being said, Burt Rutan is kind of a hack and makes something conventional overly complicated.

For the case in point: why design a complicated asymmetric aircraft when you could design half of one and just mirror it and save yourself a lot of design time and headache? It makes everything from testing to manufacture easier and less expensive.

[Zeiss Ikon]

As far as I know, two engines are always better than one from the standpoint of safety.  Engine-out performance wasn't always a design criterion (for instance, a Ford Trimotor could barely climb on two, especially if the one out wasn't the center one).  Then again, some twins (notably the P-38) had very good one-engine performance.  I forget who it was, but there was an air show pilot who used to fly a full aerobatic pattern in a P-38, then shut down one engine and feather it's propeller and repeat the entire pattern on one engine (the fellow who did the same in a much newer business turboprop died a few years ago when a wing came off when pulling out of a loop).

FWIW, the "engine explodes, now you don't have a wing" is extremely rare, absent things like cannon fire or air-to-air missiles.  A bigger problem has been "engine mount fails, now you have 1200 lb more weight on one engine mount than the other" (few aircraft have enough roll trim authority to survive that), and even that is vanishingly rare.  In general, a single engine failure amounts to "engine has a lubrication, electrical, or fuel delivery problem, can no longer operate."  In this sort of failure, having another engine on the other wing changes "I'm going to make a dead stick landing, right now!" into "I need to divert to the nearest airport that can handle my aircraft."  Both are emergencies, technically, but the former is far more likely to result in loss of airframe, crew, and passengers or payload.

It's this difference between "going down, NOW" and "I need to land soon" that has made twin (and more) engine aircraft the darlings of commercial air travel.  Having a backup engine (in an aircraft designed to keep flying on one, if needed) has been the biggest single factor in bringing commercial air travel to the forefront of travel safety.  The DC-2 showed how it should be done (it could, if not overloaded or at extreme density altitude, take off and fly a normal leg on either engine, as was demonstrated when it was introduced); the DC-3 (longer, wider span, a few more horsepower on each wing) showed how to do it at a profit on a day to day basis.

BTW, I don't recall if anyone mentioned the Blohm und Voss BV-141 -- a German light bomber and recon aircraft from late WWII.  It had a single engine and tail boom (on the left), and next to it a fuselage with forward bombardier position, pilot/copilot seats, and rear tail gunner.  A bit like a Boomerang without the pilot-side engine.  Aside from turning a little differently to the right, as compared to leftward, it was said to be very conventional in its performance.  The original design goal was to give twin engine bomber forward visibility and tail defenses, with single engine cost.  I've heard that it was part of the original proposal to make them in right-engine form as well, so a formation of them could be equally defended on both sides, but the war went from offensive to defensive about then...

[Ultimate Steve]

On ‎12‎/‎31‎/‎2017 at 1:43 PM, DDE said:

Been waiting for this thread for a while. I've been trying to justify why a modern fighter jet would have to have two asymmetric fuselages. Came up with a semi-decent reason.

Space core?

[kerbiloid]

7 hours ago, Shpaget said:

In any case, if I have to choose between being on a plane with one or two engines when one fails, I'll take an engine failure on a two engine plane any day.

Sure.

Spoiler

 

Sure?

Spoiler

 

4 hours ago, Zeiss Ikon said:

As far as I know, two engines are always better than one from the standpoint of safety. 

See above.

[TaRebelSheep]

I've made a few asymmetrical designs before, here's a new one (I am not to be held responsible for any bad that comes out of the monstrosity i have made):

The AEG-5s Asymmetrical Flyer is a wonder born from the talented (and probably high) minds at Trifekta Aeronautics. It features... well not much really apart from its ugliness. It was designed with nothing but asymmetricality in mind, but has a decent passenger capacity, and makes a great conversation starter.

https://kerbalx.com/TaRebelSheep/AEG-5s-Asymmetrical-Flyer

SAS is a must, though most people automatically turn it on as a reflex (it really should turn itself on at launch) and afterburning with its 2 panthers is rather unstable so don't do it if you don't want to get hands on with the controls (always keep you hands on the controls. Trifekta Aeronautics is not responsible for any incidents related to pilots attempting to fly a craft without using their hands.)

Make sure the panther on the right is thrust limited to 70.

 

Edited January 2, 2018 by TaRebelSheep
Spellcheck

[Green Baron]

I'd like to see the weight & balance sheet of that Rutan-insect. Is it principally different from a symmetric aircraft ?

Also, does the asymmetry add a level of complexity to the procedures in case of an engine failure ? Like, fails the right engine: pray to Ra, fails the left engine: pray to Ishtar ?

Well, organisms aren't always symmetric as well, though all the flyers are, i think.

Edited January 2, 2018 by Green Baron

[Shpaget]

4 hours ago, kerbiloid said:

Sure.

Sure?

See above.

I don't think I understand what point you are trying to make. Are you saying that those particular designs are worse off with one engine failure that designs that have only one engine to begin with? Why?

If one engine fails on any of those designs you still have the other one that can provide (probably) enough thrust for continued level flight. I can't speak about military designs, since a good deal of that is classified and the rest I just don't bother with, but in commercial aviation, it is a requirement that airliners are capable of taking off with one engine failed. Yes, the climb rate suffers and required runway length is greater and an engine failure is considered an emergency, but they are certainly capable of level flight which can bring them to the nearest airport. Sure, one could argue that if a modern airliner suffers double engine failure while cruising at about 10 km it can still reach any airport withing 150 km radius (they have glide ratios of around 15:1), but if they fail on lower altitudes, it's much harder to find a good place to land.

When one engine fails, you get asymmetric thrust, but control surfaces are usually more than adequate to compensate, and if some particular design is not capable of compensating that asymmetric full thrust, simply lower the engine power to the point where it can. Even at half power of one engine you'll still get better glide ratio than with no engines. And you always have the choice to turn the functional engine off completely. You have that choice, compared to single engine planes where you do not.

46 minutes ago, Green Baron said:

organisms aren't always symmetric as well, though all the flyers are, i think.

That's because evolution does not have a goal (other than to make a better breeding machine), while asymmetric plane designs usually are a special purpose planes, or built around a component that dictated the placement.

Edited January 2, 2018 by Shpaget

[Green Baron]

9 minutes ago, Shpaget said:

That's because evolution does not have a goal ....

I know that, okay ?

It is because it is fitful. But i just realised that there are birds with a sideways bent beak.

"I want you an my left side, honey." :-)

Edited January 2, 2018 by Green Baron

[Shpaget]

Sorry if that sounded condescending, I did not think you didn't know that, it was just an introduction to the contrasting second part of my statement.

[Hotel26]

Quote

Unlike the preceding Pup and Triplane, the Camel was considered to be difficult to fly.[12] The type owed both its extreme manoeuvrability and its difficult handling to the close placement of the engine, pilot, guns and fuel tank (some 90% of the aircraft's weight) within the front seven feet of the aircraft, and to the strong gyroscopic effect of the rotating mass of the cylinders common to rotary engines.[Note 1] Aviation author Robert Jackson notes that: "in the hands of a novice it displayed vicious characteristics that could make it a killer; but under the firm touch of a skilled pilot, who knew how to turn its vices to his own advantage, it was one of the most superb fighting machines ever built".

from https://en.wikipedia.org/wiki/Sopwith_Camel

introduced by the British in 1917, in WW1.

The Camel had a rotary engine that produced a gyroscopic turn effect (right turn, IIRC) that gave the airpplane, in skilled hands, an advantage over conventional enemy aircraft

 

[As "asymmetric design" goes, I know this is so much on the borderline as likely yo get tossed out...]

Edited January 2, 2018 by Hotel26

[bigcalm]

10 hours ago, Zeiss Ikon said:

BTW, I don't recall if anyone mentioned the Blohm und Voss BV-141 -- a German light bomber and recon aircraft from late WWII.  It had a single engine and tail boom (on the left), and next to it a fuselage with forward bombardier position, pilot/copilot seats, and rear tail gunner.  A bit like a Boomerang without the pilot-side engine.  Aside from turning a little differently to the right, as compared to leftward, it was said to be very conventional in its performance.  The original design goal was to give twin engine bomber forward visibility and tail defenses, with single engine cost.  I've heard that it was part of the original proposal to make them in right-engine form as well, so a formation of them could be equally defended on both sides, but the war went from offensive to defensive about then...

Yeah, it was #2 in the original link .  Actually, most of those are fine, except for #8 which, once you see it, you can't not see it.

[PB666]

5 hours ago, Shpaget said:

That's because evolution does not have a goal (other than to make a better breeding machine), while asymmetric plane designs usually are a special purpose planes, or built around a component that dictated the placement.

Yes it does, its goal was to create a creature that build fictitious rockets and launched from fictitious planets. Whoops, I guess it doesn't have a goal anymore. Time to reboot.

 

Flying (gliding) snakes are bilaterally asymmetric in glide. This is sort of a faux analogy, because things that don't fly are mostly symmetric to begin with.   Thus they are retaining what they have.

When we talking about the evolution of animals, particularly predators there was selection to move the femur from the side of the hip to under the hip and basically compress the trunk of the creature in horizontally along the midline of the body.
A cheetah is an example of this in the extreme.

I just want to make the point that in an emergency you don't care how the plane is pulling when in flight because you can fix that in your approach, anyway. You would care much more about not having lift than not being true on the Yaw. Once you call an emergency in flight, center will clear space around you (hopefully) so all the pilot has to do is find a vector that crosses the glide slope of a runway and turn slightly before he intercepts, if he comes in at a higher approach angle he can kill the thrust on the second engine and glide into land. 

Secondarily, if your asymmetrically powered craft is pulling a bit in one direction because of differential drag or thrust . . . . . Just throttle down one engine a bit . . . . .if it tilting a bit just move one elevator a bit. If its yawing move the horizontal stabilizer a bit. If you think this is a problem, a commercial Aircraft traveling above 15,000 feet can experience winds aloft at 120 m/s, if these winds are 135 and you are traveling 225 at 200 m/s  . . . . . . . . . this is flying. Its why radionavigation was created.  There are on most aircraft 7 control surfaces that can be adjusted 9 on the larger ones, that's not including the engines on multiengine craft that can be deferentially throttled. 

So why do we have symmetrical aircraft?

You are on an inbound on the first of 2 stops between JFK and Denver at Cleveland then Chicago then Denver.  Upon departure weather conditions in Cleveland have deteriorated and ice is now potentially forming on the runway. There is a slight crosswind and heavy gusts. The controllers in the tower are considering closing the airport to deice the runway, you have already been given clearance to land, at which point you find that your thrust diverters are not working and as you apply more pressure on the breaks the aircraft begins to turn and you are off the runway and your wingtip has just anchored itself in the mud as you spin even more feverishly. Airlines are not particularly amused with runway acrobatics, particularly when it involves the words "loss" and "airframe" when used in the same sentence. Secondarily, in the modern age airlines that profit love consistency, look at Southwest and its devotion to the 737, you don't want odd aircraft in your livery. You want a plane that can be used on any route, land at any airport, have ground crew that are fully experienced to repair . . . . . Ford produced a three engine commercial airliner, as with DC10 and DC11 and Boeing 727 (which was one of the fastest not-wide body commercial airline ever produced). But think about it the craft are symmetrical but they have two types of engines. On a twin with two slightly larger engines they only have one type to deal with. Compare the Ford triplane with either the DC3 or DC4 (both still in active use). Look at those engines for the DC3 . . . . designed to survive . . .  Every airline knows that sometime in a jets life they will have to replace the engines, some of the earliest 727s have had two engine replacements, you want this process to be as trouble-free as possible.

If you want asymmetry, hire Picasso for a skin job.

[DDE]

23 minutes ago, PB666 said:

You are on an inbound on the first of 2 stops between JFK and Denver at Cleveland then Chicago then Denver.  Upon departure weather conditions in Cleveland have deteriorated and ice is now potentially forming on the runway. There is a slight crosswind and heavy gusts. The controllers in the tower are considering closing the airport to deice the runway, you have already been given clearance to land, at which point you find that your thrust diverters are not working and as you apply more pressure on the breaks the aircraft begins to turn and you are off the runway and your wingtip has just anchored itself in the mud as you spin even more feverishly. Airlines are not particularly amused with runway acrobatics, particularly when it involves the words "loss" and "airframe" when used in the same sentence. Secondarily, in the modern age airlines that profit love consistency, look at Southwest and its devotion to the 737, you don't want odd aircraft in your livery. You want a plane that can be used on any route, land at any airport, have ground crew that are fully experienced to repair . . . . . Ford produced a three engine commercial airliner, as with DC10 and DC11 and Boeing 727 (which was one of the fastest not-wide body commercial airline ever produced). But think about it the craft are symmetrical but they have two types of engines. On a twin with two slightly larger engines they only have one type to deal with. Compare the Ford triplane with either the DC3 or DC4 (both still in active use). Look at those engines for the DC3 . . . . designed to survive . . .  Every airline knows that sometime in a jets life they will have to replace the engines, some of the earliest 727s have had two engine replacements, you want this process to be as trouble-free as possible.

Actually gave a thought in that direction with the blended wing-body aircraft and their short fuselages.

Ended up having to make them taxi and approach the terminal sideways.

[wumpus]

15 hours ago, Zeiss Ikon said:

As far as I know, two engines are always better than one from the standpoint of safety.

Theoretically the chance of an engine failure in a two engine plane is twice that of a single engine plane.  If the plane isn't stable on a single engine (kerbloid had a couple, and I'm pretty sure that was true of the U2 as well), then two engines are twice as bad for safety.

My understanding was that Rutan's Boomerang was built to be stable using either (or both) engines, thus its weird shape.  Of course, I'm still wondering why you couldn't just use a pushme-pullyou (propellers at front and rear) configuration to get the same thing in a configuration far easier to sell.  I'm guessing Rutan's engineering ego got in the way.

There's also the issue of FAA licensing.  I'm pretty sure that it takes significantly more flight time to qualify to fly a plane with more than one engine than one with a single engine.  Things might get more interesting if various hybrid schemes (gas generators and electric motors) are used in planes (can a battery get you to a sufficiently better place to land?).

[PB666]

If you are building for personal use, probably, if you are building for commercial use. . . . even the best designed planes require lots of certification.
Just remember in the commercial aircraft business its NOT just being able to fly. Here are some of the qualifications
1. To fly in airspace to commercial airtraffice
2. To land at IFR approach only airports or runways in those airports
3. ETOPs.
. . . . .
We haven't even gotten to passenger ready certifications.

IF you look on the left hand side of every Boeing cockpit instrument panel you will find something like B723-021 and some more numbers. Those numbers the FAA tracks from the time the AC is built until it leaves service in US airspace, so if you forget to do anything or are neglectful in your care, and something happens, a passenger fails and breaks their ankle, whatever, they are going to go looking for a cause.

You can build an airplane from a kit (I wouldn't unless I was a certified mechanic), and with a minimal pilots license you can get it certified to fly, and as long as you stay out of restricted airspace (pretty much everything above a certain altitude and and around paved airports) you can take off from small asphault and dirt runways . . dirt country roads . . . . .VFR navigation (poor man's IFR . . . .i fly roads). They really don't care about you unless you crash near a populated area or enter restricted airspace. Again I can see the need for single engine craft .  . . .manueverability (crop dusters, bush pilots), light weight (landing in unimproved landing areas, landing on water). low takeoff speed (taking off from short runways) . . . . . BUt for everything else twin engine is the way to go . . . .single engine craft are not unsafe, there is just so little margin for error in the low end market. These bush pilots are experienced pilots, even the best of these have fatal accidents through no fault of their own, the only difference is there is a good need for their skills. You can say the same thing about a DC-3, its insane for people to still be flying aircraft that are approaching a century in age . . . .but for the fact they are sometimes the best at what they can do.

 

Edited January 2, 2018 by PB666
Add so eye candy

[Green Baron]

50 minutes ago, wumpus said:

Theoretically the chance of an engine failure in a two engine plane is twice that of a single engine plane. 

The math isn't always correct, since normally the two engines are of the same type and the same maintenance intervals and part exchange regulations apply the probability that two engines fail because of maintenance is higher than half.

There was a case of a 787 i think, one engine failed because of oil pressure problems in the middle over the Pacific, they diverted to Hawaii and after landing during the following checks it became obvious that the other one was just close to the exactly the same failure.

The certification process of commercial airliners are meant to exclude these sort of accidents.

Edited January 2, 2018 by Green Baron

[mikegarrison]

5 hours ago, Green Baron said:

The math isn't always correct, since normally the two engines are of the same type and the same maintenance intervals and part exchange regulations apply the probability that two engines fail because of maintenance is higher than half.

There was a case of a 787 i think, one engine failed because of oil pressure problems in the middle over the Pacific, they diverted to Hawaii and after landing during the following checks it became obvious that the other one was just close to the exactly the same failure.

The certification process of commercial airliners are meant to exclude these sort of accidents.

Do you have a source for this? Because I haven't found any source that agrees there was any problem with the other engine.

I assume you are talking about this incident: https://aviation-safety.net/wikibase/wiki.php?id=164573

[radonek]

5 hours ago, Green Baron said:

The math isn't always correct, since normally the two engines are of the same type and the same maintenance intervals and part exchange regulations apply the probability that two engines fail because of maintenance is higher than half.

There was a case of a 787 i think, one engine failed because of oil pressure problems in the middle over the Pacific, they diverted to Hawaii and after landing during the following checks it became obvious that the other one was just close to the exactly the same failure.

The certification process of commercial airliners are meant to exclude these sort of accidents.

This may happen in any redundant system. For example it is a well known trick of trade among RAID array operators to shuffle disk drives around to avoid having redundant array made of disks of same production series. I would expect aircraft mechanics to develop similar trick.

[PB666]

4 minutes ago, radonek said:

This may happen in any redundant system. For example it is a well known trick of trade among RAID array operators to shuffle disk drives around to avoid having redundant array made of disks of same production series. I would expect aircraft mechanics to develop similar trick.

Yeah I had bought 5 disk drives at once, it think they were Fujitsu and installed them in different computers, it was something like 4 years later most of the stopped working within a couple of months.

[Green Baron]

51 minutes ago, mikegarrison said:

Do you have a source for this? Because I haven't found any source that agrees there was any problem with the other engine.

I assume you are talking about this incident: https://aviation-safety.net/wikibase/wiki.php?id=164573

Probably, i sacrificed almost half an hour search now, i only found another 777 flight from New Zealand to the Americas, which was diverted to Kona in the longest 1 engine flight until now, in 2011. These things happen ...

I read about it when it happened a few years ago, if i recall it right, the failure was discussed in conjunction with maintenance problems on a part that showed use wear, and when the other engine was checked the exactly same problem showed up.

[radonek]

8 minutes ago, PB666 said:

Yeah I had bought 5 disk drives at once, it think they were Fujitsu and installed them in different computers, it was something like 4 years later most of the stopped working within a couple of months.

I've seen it happen within two weeks, I know people who were even less lucky.  It's much worse with RAID arrays, because such disks are also under similar stress all the time. Just like in airplane. So Green Barons story seems plausible to me.

[Van Disaster]

On 31/12/2017 at 5:02 PM, wumpus said:

I doubt it would work in Ferram, let along stock KSP.  Rutan level design probably works in X-plane, if you want to go there.

Assymetric stuff works fine in FAR - Ferram won a dogfight contest with one, even. The last BAD-T had a craft that could still fight with one wing shot off. As long as the forces all balance out somehow you can make practically anything fly. Rutan's plane there isn't really *that* assymetric anyway ( I'd be interested how the front prop interacts with the rear one though ).

Had to go way back to dig this up, but subsonic FAR flight hasn't changed that much.

Spoiler

 

 

Edited January 3, 2018 by Van Disaster

[kerbiloid]

19 hours ago, Shpaget said:

Are you saying that those particular designs are worse off with one engine failure that designs that have only one engine to begin with? Why?

Due to different arm length, different yaw torque.

Here you can see one plane with both engines sitting close to each other.
The axes of both of them are close to the plane axis.
If one fails, this just means you have to force the another one, but as the arm is short, your plane will be yawing just a little. It even can safely keep flying.

Another plane has two engines sitting far from each other and from the plane axis.
If one of them is off, the lever arm is long, so its torque will be higher, and you even probably can't force engine, maybe even have to set low thrust.
It's very possible that it would be safer to switch both engines off and fly/land like a sailplane.

P.S.
Talking about the animal symmetry probably it would be more relevant to speak about flying/swimming ones.
Crawlers/crippers don't have to be symmetric, they are happy enough even being spiralled.

Edited January 3, 2018 by kerbiloid

[YNM]

Here's probably the first asymmetric airplane.