# How important is symmetry to plane engines? **URL:** \ **Category:** Factual Questions\ **Created:** [March 12, 2010, 11:28pm UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307 "2010-03-12T23:28:23Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![rockypg](https://avatars.discourse-cdn.com/v4/letter/r/aeb1de/32.png) [@rockypg](https://boards.straightdope.com/u/rockypg)\ **Post date:** [March 12, 2010, 11:28pm UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/1 "2010-03-12T23:28:23Z")
All the plane designs I’ve seen including ones with 1 engine, 2 engines, 3 engines and 4 engines have the engines placed symmetrically about the long axis of the plane. ie, one engine is always in the middle, two engines on either side, 3 engines, one in the middle and two on either side, and 4 engines, 2 on each wing. Now I’ve heard of scenarios where one plane engine failed and the plane made it safely back to ground, how are the engines used when one of them fails? If one engine in a two engine plane fails, the symmetry is lost, the thrust is generated on one side and the rest of the plane drags on the other side. What compensation is required to keep the plane flying in a straight line? banking? ailerons? Tail flaps? ---
**Author:** ![Waffle\_Decider](https://avatars.discourse-cdn.com/v4/letter/w/77aa72/32.png) [@Waffle\_Decider](https://boards.straightdope.com/u/Waffle_Decider)\ **Post date:** [March 12, 2010, 11:54pm UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/2 "2010-03-12T23:54:56Z")
Very important. An engine failure at a critical moment, such as around or immediately after lift-off, is catastrophic if not promptly and correctly dealt with. The training for a multi-engine rating is pretty much devoted to the skills necessary to manage such a failure. The pilot can correct for the asymmetry by applying opposite rudder, and also by slightly raising the side with the dead engine carefully. You do need to be very careful with those ailerons though, because it tends to create additional drags and yaws at the oppsite direction that you want. Also, because the aircraft is no longer flying straight, its drag has also increased significantly. For example, a twin-engined aircraft will loss a LOT more than 50% of its performance after an engine failure. In fact, some light piston twins have extremely meager or non-existent climb performance after an engine failure. The pilot will need to promptly clean up the aircraft by raising the gear and flaps, and by securing the prop on the failed engine if applicable, if they want to keep flying. ---
**Author:** ![garygnu](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garygnu/32/2924_2.png) [@garygnu](https://boards.straightdope.com/u/garygnu)\ **Post date:** [March 13, 2010, 12:23am UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/3 "2010-03-13T00:23:08Z")
The reason behind Burt Rutan designing and building the [Boomerang](http://www.popularmechanics.com/technology/transportation/1289166.html?page=all) was to reduce the problems stemming from losing an engine in twin-engine craft. > [@](#): > > Popular Mechanics article linked above: > “He has placed a 210-hp engine on the nose of the fuselage and a smaller 200-hp engine on a boom connected to the left wing, separating the two puller-prop tips by a scant 12 in. Positioning the engines so close together shifts the airplane’s center of gravity 5 in. to the left of the fuselage. That means **the Boomerang can fly nearly straight ahead on a single engine with almost no need to touch the rudders, regardless of which engine quits**.” (emphasis mine) ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [March 13, 2010, 12:46am UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/4 "2010-03-13T00:46:52Z")
> [@Waffle\_Decider](#): > > Very important. An engine failure at a critical moment, such as around or immediately after lift-off, is catastrophic if not promptly and correctly dealt with. The training for a multi-engine rating is pretty much devoted to the skills necessary to manage such a failure. The pilot can correct for the asymmetry by applying opposite rudder, and also by slightly raising the side with the dead engine carefully. You do need to be very careful with those ailerons though, because it tends to create additional drags and yaws at the oppsite direction that you want. Also, because the aircraft is no longer flying straight, its drag has also increased significantly. For example, a twin-engined aircraft will loss a LOT more than 50% of its performance after an engine failure. In fact, some light piston twins have extremely meager or non-existent climb performance after an engine failure. The pilot will need to promptly clean up the aircraft by raising the gear and flaps, and by securing the prop on the failed engine if applicable, if they want to keep flying. Having been in a friends twin that could climb 200 fpm on one engine I can attest to the drag of a windmilling prop on takeoff. ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [March 13, 2010, 1:32am UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/5 "2010-03-13T01:32:08Z")
Since the OP has been answered, I thought I’d comment that symmetry is not important just to the engines. The A-10 Warthog’s 30mm Gatling has such a kick that putting it even slightly off-center would throw the plane into a spin when it fired. Even more modest guns are generally centered or paired for this reason. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [March 13, 2010, 2:19am UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/6 "2010-03-13T02:19:19Z")
The nice thing about flying a twin heavier than 5700kg as opposed to a light twin is that the larger twin has “guaranteed performance” with one engine out. This isn’t a real guarantee, it just means that the aircraft manufacturer has demonstrated that it can meet specific climb performance requirements on one engine in the take-off phase of flight. It basically means that an ordinary pilot in such an aircraft can safely depart on one engine as long as they follow correct procedure. A light piston engined twin doesn’t need to demonstrate the same performance and so if you have an engine failure in one of those you have a lot more working against you. Another factor in twin engine aircraft is that if the engines and props rotate in the same direction as each other, a failure of one of the engines, the “critical engine”, will result in worse performance than failure of the other engine. The reason for this is that, particularly at high angles of attack, there is more thrust produced from the side of the engine where the prop blades are going down compared to the other side where the blades are going up. This results in a thrust line that is offset from the centre of the engine. The engine with the thrust line offset toward the wingtip will produce more yaw if the other engine failed. This isn’t a big difference but in aircraft that already have marginal single engine performance it can be the difference between climbing slightly and not climbing at all. ---
**Author:** ![kdeus](https://avatars.discourse-cdn.com/v4/letter/k/b487fb/32.png) [@kdeus](https://boards.straightdope.com/u/kdeus)\ **Post date:** [March 13, 2010, 2:29am UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/7 "2010-03-13T02:29:54Z")
My second favorite line from the flight manual of the old Beech King Air C200 (Navy’s T-44A): “There is no configuration which results in a positive rate of climb with one engine windmilling” Which means, in the event of an engine failure, get the bad prop feathered immediately. ---
**Author:** ![lazybratsche](https://avatars.discourse-cdn.com/v4/letter/l/ba8739/32.png) [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)\ **Post date:** [March 13, 2010, 3:02am UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/8 "2010-03-13T03:02:11Z")
There’s a very revealing quote in the Popular Mechanics piece linked above: > [@](#): > > “The engine-out safety of most twins is so bad that even though they provide engine-failure redundancy, they kill more people per mile than singles,” says Rutan. **kdeus** : What’s your first favorite line? ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [March 13, 2010, 3:07am UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/9 "2010-03-13T03:07:37Z")
> [@lazybratsche](#): > > **kdeus** : What’s your first favorite line? My favourite line is from the supplemental pages in the Robinson R22 POH: > [@](#): > > LOW ROTOR RPM STALL CAN BE FATAL > > \ > > The resulting boom chop, however, is somewhat acedemic, as the aircraft and it occupants are already doomed by the stalled rotor before the chop occurs. ---
**Author:** ![BrightNShiny](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@BrightNShiny](https://boards.straightdope.com/u/BrightNShiny)\ **Post date:** [March 13, 2010, 3:08am UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/10 "2010-03-13T03:08:22Z")
Didn’t the Nazis or the Soviets have a bunch of asymmetrical designs? I thought I’d read that somewhere. ---
**Author:** ![Gorsnak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gorsnak/32/7587_2.png) [@Gorsnak](https://boards.straightdope.com/u/Gorsnak)\ **Post date:** [March 13, 2010, 4:31am UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/11 "2010-03-13T04:31:21Z")
You’ll be thinking of some aircraft designs by [Blohm & Voss](http://en.wikipedia.org/wiki/Blohm_%26_Voss_BV_141), I think. ---
**Author:** ![rockypg](https://avatars.discourse-cdn.com/v4/letter/r/aeb1de/32.png) [@rockypg](https://boards.straightdope.com/u/rockypg)\ **Post date:** [March 14, 2010, 4:54am UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/12 "2010-03-14T04:54:34Z")
> [@Johnny\_L.A](#): > > My favourite line is from the supplemental pages in the Robinson R22 POH: What is a boom chop? ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [March 14, 2010, 5:05am UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/13 "2010-03-14T05:05:38Z")
> [@rockypg](#): > > What is a boom chop? Boom chop occurs when a rotor blade in a helicopter strikes the tail boom. This is suboptimal. [R22 Helicopter Safety Notice SN-24](http://www.robinsonheli.com/srvclib/rchsn24.pdf) is the safety notice from which I got the line. The link is a ‘translated’ .pdf, so there are some spelling errors that are artifacts of the machine-transcription. It details low rotor RPM, which is one thing that can cause boom chop. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [March 14, 2010, 6:45am UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/14 "2010-03-14T06:45:05Z")
> [@Johnny\_L.A](#): > > Boom chop occurs when a rotor blade in a helicopter strikes the tail boom. This is suboptimal. That would be suboptimal in the same way that having someone chop your head off with an axe is suboptimal? :D. Can that not also happen when you exceed Vne, as the retreating blade flaps too much? ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [March 14, 2010, 7:19am UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/15 "2010-03-14T07:19:22Z")
Yes. In retreating blade stall, the retreating blade… well, stalls. The advancing blade continues to generate lift. You have blow-back, and the rotor disc tilts aft. (This is because of precession; the lift generated by the advancing blade manifests itself 90º in the direction of rotation.) Blow-back brings the retreating blade into proximity with the boom. ---
**Author:** ![Icerigger](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/icerigger/32/432_2.png) [@Icerigger](https://boards.straightdope.com/u/Icerigger)\ **Post date:** [March 14, 2010, 10:03am UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/16 "2010-03-14T10:03:37Z")
> [@Gorsnak](#): > > You’ll be thinking of some aircraft designs by Blohm & Voss, I think. That thing really looks like it shouldn’t fly! ---
**Author:** ![N9IWP](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/n9iwp/32/3154_2.png) [@N9IWP](https://boards.straightdope.com/u/N9IWP)\ **Post date:** [March 14, 2010, 12:21pm UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/17 "2010-03-14T12:21:05Z")
Some twin engine planes have the engines inline (one in front, the other in back) The example that comes to mind in The Cessna 337 Skymaster [Cessna Skymaster - Wikipedia](http://en.wikipedia.org/wiki/Cessna_337) Brian ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [March 14, 2010, 12:30pm UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/18 "2010-03-14T12:30:45Z")
Yes. And because the C337 has inline engines, I believe hours flown in it are not to be logged as multi-engine. ---
**Author:** ![StrangerThanFiction](https://avatars.discourse-cdn.com/v4/letter/s/f05b48/32.png) [@StrangerThanFiction](https://boards.straightdope.com/u/StrangerThanFiction)\ **Post date:** [March 14, 2010, 12:55pm UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/19 "2010-03-14T12:55:44Z")
That’s not true. However, if you do your training and receive your multi-engine rating in a 337 or other similarly-configured airplane, your rating will have a limitation for centerline thrust. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [March 14, 2010, 1:54pm UTC](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307/20 "2010-03-14T13:54:03Z")
As far as I know, it’s not possible to get a multi-engine rating in a C337 here in Australia. But you seem to be right about logging, it should still be logged as multi-engine. [Next page](https://boards.straightdope.com/t/how-important-is-symmetry-to-plane-engines/532307.md?page=2)