# Jetliners - down angle on rear engines? **URL:** \ **Category:** Factual Questions\ **Created:** [June 25, 2004, 12:39pm UTC](https://boards.straightdope.com/t/jetliners-down-angle-on-rear-engines/252168 "2004-06-25T12:39:59Z")\ **Posts on this page:** 8\ **Page:** 2
**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:** [June 26, 2004, 5:11pm UTC](https://boards.straightdope.com/t/jetliners-down-angle-on-rear-engines/252168/21 "2004-06-26T17:11:31Z")
> [@](#): > > I think you are contradicting yourself, Johnny. I see what you are saying. I definitely mis-spoke when I said: > [@](#): > > In a larger aircraft, the flying surface can provide an aerodynamic downward pressure and you can put engines back there that are “pointed down” so that they also tend to keep the nose down. As you say, the engines will tend to push the nose down while the stab will tend to push it up. I stand corrected. > [@](#): > > The reason the engines are tilted is to align them with the airflow where they are mounted. Right. You’ll notice that most jets fly with a nose-up attitude. This would put the engines in line with the airflow. Then angle of incidence is built into the airframe, so the wings will be at a set angle of attack in cruise. The engines would have an angle of incidence such that the airflow is “straight in” at cruise. The thrust line and CG are two different issues. Weight and balance is the aerodynamic balancing of forces and moments of an airframe within a specified envelope. The placement of engines or the length of the fuselage are design decisions that result in a balanced aircraft. So an aircraft with a long nose will have to have a heavy tail, and it’s much more efficient to put the heavy engines on the aft fuselage than it is to stow hundreds of pounds of unproductive ballast there. As **Q.E.D.** points out, the aircraft needs to be able to fly even if both (all) engines are lost. So the placement of the engines are a design factor that is considered in the weight and balance. The _angle_ of the engines – where they point their thrust – is something else. > [@](#): > > The reason the engines are tilted is to align them with the airflow where they are mounted. Right. You’ll notice that most jets fly with a nose-up attitude. This would put the engines in line with the airflow. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [June 26, 2004, 5:16pm UTC](https://boards.straightdope.com/t/jetliners-down-angle-on-rear-engines/252168/22 "2004-06-26T17:16:47Z")
> [@Berkut](#): > > If you rely on an upward component of the engine’s thrust to balance the plane fore/aft and then lose that thrust, the airplane’s balance will be off. I see what you’re saying, and I have to agree. OTOH, since the rear-mounted engines _are_ angled downward, there _is_ an upward thrust component. Just to use round numbers, let’s say the total engine thrust on the MD-90 is 100,000 pounds, and the engine is tilted down by 5[sup]o[/sup] WRT the long axis, this means that there are about 5,555 pounds of downward thrust that must be compensated for (assuming I’ve done my math correctly). Given this, I’d say it’s likely soem combination of thwo prevailing theories here. That is, the engines are tilted to align them with the local airflow, and are mounted slightly off-axis to compensate for the downward thrust component. Make sense? ---
**Author:** ![Berkut](https://avatars.discourse-cdn.com/v4/letter/b/a88e57/32.png) [@Berkut](https://boards.straightdope.com/u/Berkut)\ **Post date:** [June 26, 2004, 5:45pm UTC](https://boards.straightdope.com/t/jetliners-down-angle-on-rear-engines/252168/23 "2004-06-26T17:45:32Z")
> [@Q.E.D.](#): > > Make sense? Yep, that’s exactly what I was trying to say. I think you put it better than I did. > [@](#): > > Given this, I’d say it’s likely soem combination of thwo prevailing theories here. Actually, there are three theories here; 1. Using some vertical component of the thrust to balance the aircraft fore/aft. 2. Tilting the engines so that the thrustline goes through the CG. 3. Tilting the engines so they are aligned with the local airflow. (This isn’t a theory, BTW 🙂 ) For #1, you are relying on a force that may go away at any minute to balance the aircraft. The engines were put back there as part of the aircrafts design, not added as an afterthought. #2 is possible, although you can make the thrustline pass through the CG by just placing the engines at the correct height on the fuselage. Design concerns may prevent this, though. #3 is the main reason. The airflow comes over the top of the wing and starts to angle downward. The engines are placed right in this airflow, and are tilted aft slightly so the inlets are square into the wind. Like I said before, you can see this same thing on wing mounted engines also, but you don’t notice it because the engines are canted inwards, not tilted up/down. They are angled inward slighty into the spanwise airflow around the wing. ---
**Author:** ![cabdude](https://avatars.discourse-cdn.com/v4/letter/c/58956e/32.png) [@cabdude](https://boards.straightdope.com/u/cabdude)\ **Post date:** [June 26, 2004, 6:14pm UTC](https://boards.straightdope.com/t/jetliners-down-angle-on-rear-engines/252168/24 "2004-06-26T18:14:40Z")
> [@Berkut](#): > > 1. Tilting the engines so they are aligned with the local airflow. (This isn’t a theory, BTW 🙂 ) > … > #3 is the main reason. The airflow comes over the top of the wing and starts to angle downward. The engines are placed right in this airflow, and are tilted aft slightly so the inlets are square into the wind. Like I said before, you can see this same thing on wing mounted engines also, but you don’t notice it because the engines are canted inwards, not tilted up/down. They are angled inward slighty into the spanwise airflow around the wing. I like it - cite? ---
**Author:** ![Berkut](https://avatars.discourse-cdn.com/v4/letter/b/a88e57/32.png) [@Berkut](https://boards.straightdope.com/u/Berkut)\ **Post date:** [June 26, 2004, 10:01pm UTC](https://boards.straightdope.com/t/jetliners-down-angle-on-rear-engines/252168/25 "2004-06-26T22:01:04Z")
I can find you something Monday, but it’s gonna be from a book, not the Internet. Apallingly stone age, I know. ---
**Author:** ![cabdude](https://avatars.discourse-cdn.com/v4/letter/c/58956e/32.png) [@cabdude](https://boards.straightdope.com/u/cabdude)\ **Post date:** [June 27, 2004, 12:17am UTC](https://boards.straightdope.com/t/jetliners-down-angle-on-rear-engines/252168/26 "2004-06-27T00:17:43Z")
> [@Berkut](#): > > I can find you something Monday, but it’s gonna be from a book, not the Internet. > > Apallingly stone age, I know. Books? They still have those? 🙂 NP dude - and thanks. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [June 27, 2004, 6:40pm UTC](https://boards.straightdope.com/t/jetliners-down-angle-on-rear-engines/252168/27 "2004-06-27T18:40:08Z")
Airline pilot here … I’m not an engineer, but I do fly these things. I believe you’re on the right track with thrust line vs CG considerations. Airplanes with engines under the wings (eg 737 / 747 / 757 / 767 / 777) have a pronounced pitch-up tendency as you add power. It’s not enough to produce a dangerous change in trim, but it does need to be corrected for every time you change power. The loss of power, whether due to throttling back back to idle or engine failure, produces a corresponding trim change nose down. Airplanes with high mounted engines (717/ 727 / DC9 / MD80 / MD90) have the reverse tendency, all else equal. Naturally, any particular thrust line offset will only be perfect under one combination of speed, power setting, and aircraft CG. But by setting the engine at some offset, the engineers are able to get the heart of that envelope near the common cruise design point. As well, for drag considerations, you want the overall cowling structure aligned with the local airflow. Back in the tail area that’ll tend to have soem downward cant moving rearward. It’s also possible that the engine centerline does not track exactly with the cowling centerline. This is particularly apparent on the 727 and DC9 (not MD80), where the tops of the engine cowlings are essentially flat, while the bottom has a pronouced bulge or belly. That bulge’s purpose is to house the engine accessories (fuel pump, electrical generator, etc), but it also serve to camoflage the actual engine centerline for the casual observer. ---
**Author:** ![Berkut](https://avatars.discourse-cdn.com/v4/letter/b/a88e57/32.png) [@Berkut](https://boards.straightdope.com/u/Berkut)\ **Post date:** [June 29, 2004, 1:20am UTC](https://boards.straightdope.com/t/jetliners-down-angle-on-rear-engines/252168/28 "2004-06-29T01:20:46Z")
Man. I went through my college textbooks and couldn’t find any reference to it, but I’m sure that’s where I learned it. You may have to settle with what you can find online and make up your own mind. I found a halfway decent online webpage that you might want to look over: [http://adg.stanford.edu/aa241/propulsion/engineplacement.html](http://adg.stanford.edu/aa241/propulsion/engineplacement.html) Look under the heading “Aft-engine placement” and the diagram that goes with it showing the 3 degree tilt. It’s not very plainly explained, but you can get the general idea. [Previous page](https://boards.straightdope.com/t/jetliners-down-angle-on-rear-engines/252168.md?page=1)